Download On the structure of the lower jaw in dipnoans: with a description of

J . Lirm. SOC.(Zool.), 47, 311,pp. 155-183
With 6 plates and 9 text-jigures
Printed in Great Britain
October, 1967
On the structure of the lower jaw in dipnoans: with a description
of an early Devonian dipnoan from Canada, Melanognuthus
canadensis gen. et sp. nov.
BY ERIK JA4RVIK
Pn1Po:oological Section. Stuedieh Mu.seu,n
if
Ntrt ural History, Stockholm .jO,A’J’iiledeti
The loiver jaw in Melrmo(lnnthus gen. nov. d n d st!\-eral other Devonian dipnuatis is described
and compared with that, in Xeocerotodzts. It, is ti tiiost ronservat,ivc struct,ure, which apart from
the effect of the retrogressive developnierit of ilie slteleton has hardly rhanged since Devonian
times. A new iiiterpretation of ihe sensory canal hones is given. With respect to these elements
and the st.rurture of the lower jaw as a wholr, 1 lie dipnoans differ fundamentally from the
rhipidistid crosaopterypia.ns, tetrepods and artinopterygians. Several resemblances to the
lower jaw in holoceplialiaris are demonstrated. The results confirni the view that the Dipnoi
comprise an early specialized and isolated group, perhaps more closely related to the elasniobranchioinorphs than t o the te1eost)oinesand tet rnpods.
CONTESTS
PAGE
Introduction
.
Reillarks on the lower jaw in Xcorrrtifodits .
The endoskeleton
.
The exoskeleton .
.
Sensory lines
.
The. lower jaw in Devonian dipnoans: w i t h :I. Clt
General remarks
The meckeliaii bone and the adducator ”it
Tooth-bearing bones ilnd dentition
Sensory lines
.
,4lelatrognrcthus ctinritLiusia gen. et sp. niiv.
The canal bones in M e k ~ n o g n c ~ t h z r s . .
The canal bones in certain other dipnoail.:
Reinarks on t h e dentary
.
Evolution of the dipiioan lower jaw
.
Coniparisons and conclusions
.
Acknowledgeirierits .
References
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1.58
ip(ioii o f a new genus
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INTRODUCTION
Since 1923, when Watson and Gill published their excellent account on Palaeozoic
Dipnoi and presented the well-known restoration of the lower jaw in Dipterus platycephalus, or D. valenciennesi as it now is to he called (White, 1965), our knowledge of the
Devonian dipnoans has greatly increased. A large number of new genera and species has
been described and in several of them the lower jaw has been treated more or less coinpletely. Most writers have used the terminology of the dermal bones applied by Watson
and Gill, either without change, or with but small modifications, and also the statements
that the early dipnoans in the structure of the lower jaw and in some other characters agree
well with the contemporaneous rhipidistid crossopterygians and that the dipnoans and the
rhipidistids are closely related, have been widely accepted. According to Watson & Gill
(1923: 210) the most important resemblance in the lower jaw is ‘the presence of splenial
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ERIKJARVIK
and post-splenial bones’, and also Westoll (1949: 122) has claimed that ‘the lateral line
bones of the mandible in these early forms are easily comparable’. However, in connection
with the preparation of a restoration of Scaumenacia interesting new facts about the sensory canals and canal bones of the lower jaw were discovered. The studies were therefore
extended to include the lower jaw in various other Devonian dipnoans and on the basis of
these studies, and comparison with recent dipnoans, I have arrived a t interpretations of
the sensory canal bones, and of the jaw as a whole, which in important regards differ from
the previous ones. Ah has been found to be the case for the skull-roof and the cheek (see
White, 1965) so also it is difficult to compare the dermal bones of the lower jaw with those
in rhipidistid crossopterygians, other teleostomes and tetrapods and to establish secure
homologies. Under these circumstances I have found it most reasonable to use symbols
also for the dermal bones of the lower jam.
The fossil material at my disposal includes : several specimens of Scauinenacia curta
from the lowermost Upper Devonian of Escuminac Bay belonging t o this institute ; some
specimens in this institute of Rhinodipterus ulrichi from about contemporaneous strata
at Bergisch Gladbach ; three specimens of Dipterus ualenciennesi from the Middle Devonian
of Scotland, viz. the excellent jaw in the Manchester Museum (No. L. 10858) described by
Watson & Gill, a similar jaw belonging to the Geological Survey, London (No. 53370), and
the specimen, No. 1859.33.622,Royal Scottish Museum, Edinburgh, figured by Jarvik
(1964, fig. 14A) ; the holotype of Chirodipterus wildungensis, belonging to the Humboldt
University, Berlin; a specimen of an undetermined ‘dipterid ’ from Anderson river,
N.W.T., Canada (late Lower Devonian), belonging to the National Museum of Canada. The
latter specimen, which consists of an imperfect lower jaw, has been referred to a new genus
and will be described as illelanognathus canadensis gen. et sp. nov. Moreover some specimens of D. valenciennesi showing the palate have been used.
I n order to obtain a safe basis for the interpretation of the structural features
encountered in the fossil forms the lower jaw of a specimen of Neoceratodus forsteri has
been dissected. For comparative purposes I have also used some other material of Neoceratodus and Protopterus in this museum and a dried skull of N . forsteri belonging t o the
Museum of Natural History, Gothenburg.
REMARKS ON THE LOWER JAW IN IVEOC”ERA2’ODUS
The morphology and ontogenetic development of the lower jaw in extant dipnoans
is still iniperfectly known, both with regard to the skeletal elements and the soft parts, and
there is some disagreement between the figures and descriptions given by the various
authors. I n Neoceratodus judging from the specimens a t my disposal and the literature
(Giinther. 1871; Huxley, 1876; Miall, 1878; Briihl, 1880; Ruge, 1897; Semon, 1899;
Furbringer, 1904; Goodrich, 1909, 1930; Greil, 1913; Luther, 1913; Watson & Gill, 1923;
Edgeworth, 1935 ; Holmgren & Stensio, 1936; Holmgren, 1949 ; Holmgren & Pehrson,
1949; Pehrson, 1949; Jarvik, 1964; Fox, 1965; Bertmar, 1966) the conditions are briefly
as follows.
As in other dipnoans the jaws in Neoceratodus (Text-figs 1, B, ZA, B, 3.4 to C, 7D,
9 B ; P1. 1) have fusvd anteriorly and we may distinguish a symphysial portion and a
paired ramus. The fusion has occurred only in the cartilaginous and soft parts. In the
lateral side of the sym~~hysial
portion lies a distinct pit, the labial pit (Zab.p,Text-fig. 1 ,
B),occupied by the anterior flap-like part (f0.n) of the fold of the lower lip (Jarvik, 1964:
41-45). Dorsally t o the posterior part of the symphysial portion there is a wide median
opening (mdep, P1. 1 , fig. 1) bounded dorso-laterally by the tooth plates of both sides.
This opening is occupied by the acutely conical basihyal (see Miall, 1878, pl. 1 A : figs 2 , 3 ;
Greil, 1913, figs 527,628,578; pl. 69, figs 2 , 3, 1 1 ) and associated soft tissues (the ‘tongue’;
to, Text-fig. 9B).
Structure of thP diptioajb lower jaw
[email protected]'Ze Devoniun
157
Neoceratodus.Recent
Tcst-fig. 1. Represeiit.ations of the anterior par( of 1 . h head in ventral view of, A, 1)ipfwrc.s
r;rtkrrciermesi and B, Seoceratodus forsteri. Fn)ni ,Jarvik, 1964, fig. 14. Slightly modified.
Lj, Fused lower jaws; L'Zb, supporting dermal h n e n of upper lip: romp, pores of medial coniiriissural branch ofmandibular sensory r a n d ; ,$. r t l , flap of upper lip;fo.lZ, anterior flap-like part
of fold of lower lip; irrc, notch in upper lip leatliri o aiiferior nostril; Irrb.p, labial pit; I I , lower
lip; m,,7>,orrp,pores of mandibular and oral seiiso canals : ri.s6?1,suhiiasal ridge; 10.0, opening
of uiicertain irriportarice; ul, upper lip.
Thr c H doskrle f on
The ineckelian cartilage ( N e c k ,P1. 1; ccratotiiaiidibular) is well developed and fills the
interior of the jaw. There is thus in the jaw no \vide space cotnparable to that in rhipiclistid
crossopt,erygians (Jarvik, 1963, figs 8, 14 R ) and in early a'ctinopterygians (Nielsen, 1042,
fig. 39) and t>etrapods(Nilsson, 1944, figs 5,13)a i d no cleepadductorfossa (ad.fo,Text-fig.
8D, E. F). The articular portion is strong with a small, hardly distinguishable, retroarticular process. As in many other fishes t,he glenoid fossa (glen, P1.1, fig. 1) is divided by a
ridge int,o lateral anti medial divisions. The incdial division, which is larger and extends
fart,lier forwards and upwards than the lateral one, is situated on tlie postero-medial
surface of a pronounced process of the riicckelian cartilage. This process, the preglenoid
proccss (pr.pgZ, P1. 1, figs 1 , 3; 'coronoid' pro
. Gunther), has a roughdorsal area (ar.ad,
. 1)for the attachment' of a part of t'he acldrictJormuscle. I n front of the pregleiioid
forwards to the wide vertical canal (c.by. 1'1. 1, figs 1, 2) of the ramus, t>hedorsal
side of the nieckelian cartilage is exposetl i n a tiarrow slit situated in its posterior. part in
the bottom of the sniall adductor pit ( a t l . p . PI. 1, fig. 1) and anteriorly underneat,h the
liiiid1iiost cr&s of the tooth plate. In front. of thc said vertical canal the dorsal side of the
cartilage is exposed in a broad area which vxtcntls forwards into the symphysial portion.
111 the ranills of the jaw the meckelian cartilage also has a ventro-medial exposed area
((ir.im3P1. 1, fig. 2 ) , ~ h i c his the area of at t~arlinietitl
of t,he inusculus intermandibnlaris.
Anteriorly the meckelian cartilage expands into a plate, which together with that of the
other d e forms a broad thin symphysial plate (.~,tj.pl,
PI. 1).The anterior part of this plat,e,
n.liicli i3 situated in front of t,he dermal hoiics. slipports the fleshy semicircular lower lip.
Its lateral part is thickened and is bent iipLjar(1s forming a fairly sharp edge (e.iTic, 1'1. 1,
figs 1 . 3 ) which is concave on its external side arid forms part of the skeletal wall of t,he
labial pit. According t o Holmgren (1949)the lateral part of the symphysial plate is thickened also in tlie larva but a most remarkable fact, is that this thickened lateral part is cont inucd forwards by a long slender process (Znb.c, Text-fig. 2 A) which antero-tnedially,
in front of t)he larval symphysial plate. almost) meets the corresponding process of t>he
other side. Holmgren regards this intercsting paired process as a labial cartilage, but
158
ERIKJARVIK
whether it corresponds to the so-called anterior labial cartilage in Protopterus, as suggested
by Holmgren, or if it is equivalent to tlhe ‘premandibular cartilage’ in holocephalians
(Luther, 1909: 33) or to other premandibular cartilages in elasmobranchs (see also
Furbringer, 1903) is difficult to say. The importance of the ‘ intermandibular cartilage ’
recently described by Fox (1965: 482484, fig. 20) is also obscure. It soon ossifies and in the
34-5 mm stage it ‘comprises a matrix of spongy bone from which teeth emerge’.
Text-fig. 2. A, B, Neoceratodus ,forsteri. A, Lower jaw in ventral aspect of larva or young
fish (probably 8 t o 10 months). From Holmgren, 1949, fig. 7. B, Dent,ition of lower jaw in
larva (stage 48, Semon). From Semon, 1899, pl. 20, fig. 14 ( x 68). C, Protopterus unriecterw.
Semidiagrammatic transverse section of the mandibu1a.r tooth of a small specimen. From
Parker, 1892, pl. 8, fig. 4.
M d E , M d I , M d i l I c J , M d X , M d Y , Dermal bones of lower jaw; Meck, meckelian cartilage;
ep, epithelium; fo.muc, fold of mucoiis membrane; horn, horny cap of tooth; Zab.c, lateral
psocess of symplysial plate (‘labial cartilage’) ; od, odontoblasts ;pler, columnar pleromic hard
tissue (petrodentine);.sy.pZ,symphysial p k e ;01, 03,pi,p z , ql, teeth of future ma,ndibular tooth
plate; il-zg, teeth of toot,h plate MdX; y, t’eeth of tooth plate MdY.
The exoskeleton
The medial side of the rneckelian cartilage in Neoceratodus is covered by a strong bone
that has been variously interpreted (‘dentary’, Giinther ; ‘splenial ’, Goodrich, Luther;
’prearticular’, Holmgren; ‘angular’, Greil ; ’ coronoid’, de Beer, Fox). Posteriorly this
bone, which will be referred t o as bone MdI (PI. 1, fig. l ) ,is provided with a low ascending
Structure of the dipnoan lower jaw
159
process (prm.asc, PI. 1, fig. 1) behind which the prrglenoid process is exposed. Anteriorly,
in the symphysial portion, bone MdI has a thin medial expansion which dorsally to the
symphysial plate extends almost to the metlian line. Bone MdI carries the well-known
mandibular tooth plate, MdT (Pl. 1. figs 1 . 2 ) , sometimes regarded as an independent
structure (‘operculare’, Greil; ‘ coronoicl’, Holingren, 1949: 468). This plate is partly
made up of a special kind of very hard cicntine recently designated by Orvig (1967) as
columiiar pleroniic hard tissue (pler, Text-fig 2 C; petrodentine, Lison, 1941). As demonstrated by Semon (1899) the tooth plate arises from separate teeth (Text-figs 2 B , 9B)
whirh fuse and grow down into the untlerlying bone (MdI). Semon’s assumption (1899.
128) that the remarkable sinking of the tooth plate into the supporting bone is associated
with resorption due to action of osteoclasts has been confirmed by Lison (1941 : 306-308)
in Protopterus. I n this genus the ridges of the tooth plates may be covered by a peciiliar
horny cap (horn,Text-fig. 2‘2; Parker, I X W : 132-133; Semon, 1899: 129) formed (seasonally?)by cornification of the epithelial layer ( ~ pText-fig.
,
2C) of the curious cushion-like
folds of the mucous rnembranr (fo.vzuc.Text-fig. 2C) which surround and largely hide
these ridges.
In front of the principal mandibular tooth plates in Neoceratodus there are, in larval
stages (Text-figs 2B. 9 B ) , two other tooth plates, one paired and one unpaired. The paired
plate, MdX (‘Dentale ’, Greil ; ‘ lip-scale ’. H olrngren; ‘ splenial’, Bertmar), wliich carries
teeth X I - 2 5 of Semon (see also Greil, 1913. 111 lil), according to Holmgren (1949: 469) is
situated dorsally to the proximal thickcnr~lpart of the lateral process (‘labial cartilage ’)
of the syniphysial plate. The unpaired plate. M t l Y , which carries one large median tooth
(‘Symphysenzahn ’, Greil) or occasionally t n o teeth (y) occurs dorsally to the syniphysial
plate and postero-medially to the anterior end of plate MdX. As pointed out by Sernon
this small structure occupies the same position as the so-called presymphysial tooth
(Pmy, Text-fig. 9D. E; Wood~varrl.1891, p1 2 ; Dean, 1906: 143) in 11Iyriacunthu.u and
certain other fossil holocephalians.
The large bone, MdE (Text-fig. 7D; 1’1 1. fig. 2 ; ‘articulary’, Guntlier; ‘angular’,
Huxley, Goodrich ; ‘ Priiangulo-Angulare ’, Holmgren and Stensio ; ’supra-angular ’,
cle Beer, Fox; ‘dentary’, Holmgren), on the extcrnal and ventral sides of the meckelian
cartilage is bent and consists of a lateral almost vertical portion (wp,P1. 1 , figs 2, 3),
covered by the upper lip and a ventro-metlial almost horizontal portion (hp,P1.l, figs 2 , 3 ) .
The vertical portion posteriorly has a distinct notch (i.m.e.V I l , PI. 1, fig. l ) ,or, as in the
left ramus of the figured specimen, a sniall foramen, probably for the r. rnentalis externus
VII (Fox, 1965, fig 30; see also Ruge, 1897. fig. 23; Luther, 1913, fig. 27). In front of this
notch or foramen the vertical portion extends into a fairly large ascending process (prl.asc,
PI 1 , figs I , 3) on the lateral side of the preglenoid process. This ascending process is bent
medially and i t almost meets the ascentling process of bone MdI. I n front of the preglenoid
process the vertical portion of bone MtlE has a sharp dorsal margin. Inside the posterior
part ofthis margin, in the area between the preglenoid process and the tooth plate, there
is a dorso-medially facing rough area which together with a similar dorso-laterally facing
area of bone MdI forms an elongated pit, V-shaped as seen in transverse section. This pit
(ad.13,PI. 1, fig. l),inthebottom ofmhich the nieclieliancartilage is exposedin anarrow slit,
is the main area of insertion of the m. adductor Inandibulae ( m a d , P1. 1, fig. 1) and may
therefore be named the adductor pit. This pit is continued forwards by a narrow groove
situated betaeen the dorsal edge of bone MdE and the tooth plate and partly covered by
the overhanging lateral parts of the tooth ridges. This groove (grt, P1. 1, figs 1, 3), which
extends forwards to the wide vertical canal (c.ly, P1. l ) ,is occupied by a strong tendon of
the adductor muscle (Luther, 1913: 36). The anterior end (p.end. P1. 1) of the vertical
portion is pointed. This pointed part, which covers the lateral side of the symphysial plate,
is slightly incurved and ventrally to the concave edge (e.inc,P1. 1, figs 1 , 3 ) of that plate it
forms part of the skeletal wall of the labial pit. The ventro-medial, horizontal portion of
bone MdE forms a narrow plate ventrally to the meckelian cartilage. Along its rnedial
160
E R ~ JARVIK
K
Text-fig. 3. Sensorylinesof theheadin extant dipnoans. A-C, Xeoceratodus forsteri. A, Lateral
aspect. Previously unpublished figure made by G. Save-Sederbergh. Unfinished in the anterior part of the lower jaw. B, Vent’ralaspect. From Furbringer, 1904, pl. 40, fig. 30. C, Larva,
11.5 mm. Ventral view. From Pehrson, 1949, fig. 20. D, Protopterus uethiopicus. Ventral
view. Original ( x $1.
com, Medial commissural branch of mandibular canal; jl.uZ. flap of upper lip;fo.ZZ, ariterior
flap-like part of fold of lower lip; gr.nac, groove for mandibular sensory canal; ioc, infraorbital
sensor.>’line; 1nb.m lshial p i h ; m e , o n , pop, 8o0, mandibular, oral, prcopercular and supraorbital sensory lines; sy.pZ, syniphysial plate ; v.opl, ventral oral pit-line.
Structure of the d i p o a n lower j a w
161
margin there is a row of small ossicles, MdPulc, sitiiatcd in the dorsal wall of the mandibular
sensory canal (Text-figs 3B, 7 D ; Furbringer, 1904: 441442; Holmgren, 1949: 468).
The postero-medial part of the ventral side of the symphysial plate is covered by a
fairly large, paired. sensory canal bone, RIdXcl (Text-figs 2A, 3B, 7 D ; El. 1, figs 2 . 3 ;
‘dentary ’, Huxley, Goodrich; Submandibular, Firbringer; Praedentary, van Wihje ;
‘splenial-postsplenial ’, Holmgren). The bone, \\ hicli is separated from bone MdE by an
almost longitudinal suture running from the medial side of the anterior part of the ranius
t o the labialpit, hasa branching groove ( g r nic, Text-fig. 3B; P1.l, figs 2 , 3 ) for the mandibular sensory canal nith its medial cominissirral branch.
Seiisot y 1iues
The sensory lines (Text-figs 1 B, 3A-C‘, 5 D ; Goodrich, 1930, fig. 273) are represented
by a pit-line (v.opZ;ventral oral pit-line, Stenslo, 1947 ; primary mandibular line, Pehrson,
1949. fig. 12; secondary mandibular line, Holmgren & Pehrson, 1949, fig. 14) and two
canals, the oral canal and the mandibular canal. The oral canal (orc), nrhich posteriorly
joins the infraorbital canal antero-ventrally to the eye, runs foiv artls in the skin outside
bone MclE. Like the corresponding sensory line in Protopterus (Text-fig. 3D) it passes
close to the fold of the loner lip and, when the mouth is shut, close to the ventral margin
of the upper lip. I n its anterior part the canal runs in a gentle curve along the concave
ventro-medial margin of the labial pit and terminates near the anterior end of the jaw.
The mandibular canal ( m c )runs forwards along the medial margin of the ventro-medial
horizontal portion of bone RfdE and in this part of its course is associated with the small
ossicles, MdMc, beyond \$hich it continues in the groove on the ventral side of bone
ItZdMcl, where it branches. The branch I\ hicli according t o Pehrsori (1949) represcnts the
anterior part of the canal runs forwards t o join the oral canal in the skin a little behind the
anterior end of that canal. The other branrli (coin)runs medially and joins the corresponding canal of the other side forming a trarisverse commissure. This commissure which is
present also in Protopteras and is well dereloped in the Devonian forms is not shonn in
Goodrich’s figure and is not mentioned by Pelirson. However, as is evident froin Pehrson’s
figures i t is present in embryonic stager of both Protopterus and Neoceratodus and is
represented by the so-called middle gular lint (corn, Text-fig. 3C; Pehrson, 1949, figs 3, 6,
8. 10, 14, 19, 20).
THE LOWER JAN‘ IN DEVONIAX DIPNOA
\1 ITH A DESCRIPTION OF A NEW GENUS
G‘enmul rtwinrks
The lo\ver jam is known more or less complctc-ly in a great number of Devonian genera:
C‘hirodipterus (Gross, 1933; Jarvik, 1955. 1964). Diporhynchus (Hills, 1935; White, 1966),
Dipferus (Pander, 1858; Traquair, 1878; i2’ood\rard, 1891; Watson & Gill, 1923; ForsterCooper, 1937 ; Westoll, 1949; Gross, 1964 ; Jarvik, 1964), Pleurantiu (Graham-Smith &
Westoll, 1937), Griphognuthu.s (Gross, 1956), NoZodipterus (‘Holodus’, Pander, 1858;
Gorizdro-Kulczycka, 195@),Jfelanognathur gen. nov. (see below), Palaeduphus (Traquair,
1858; Fritsch, 1888; Woodwarcl, 1891), Rhiiiodipferus (Gross, 1956; Orvig, 1961) ; Scuuiuenacia (Stensio, 1945),Soederberghia (Lehman. 1959). The fossil material a t my disposal,
including specimens of Chirodipterus, Dtpferus, Melanogimthus, Rhynchodipterus and
Scau~ne)iacirc,is incomplete or damaged hy crushing and it was impossible to give a
satisfactory restoration of the lower jan in any of these forms. I n particular it has been
difficult, to make out safely the boundaries of the bones of the symphysial portion, and
the course of most canals for nerves and vcsscls is still obscure
The lower jaw in the Devonian dipnoaiis (Text-figs 1A, 4 to 7, SA, C, DA, C; PIS2 to 6)
has on the 11 hole a similar structure in all the forms in which it is reasonably well known
11
162
ERIEJARWK
and as emphasized long ago by Giinther (1871) and Traquair (1878) it is very suggestive
of that in Neoceratodus. As a matter of fact the mutual differences in jaw structure between
the various Devonian forms or between a Devonian form like Dipterus and the recent
Neocerafodus are hardly as great as those between the latter and Protopterus or Lepidosiren. The differences between the Devonian forms in the first hand concern the general
shape and proportions. Several Devonian forms (Dipterus,Xcaumenaciu) in these respects
agree fairly well with Neocerutodus, whereas in others the jaw is comparatively stout
(Chirodipterus, Dipnorhynchus, Holodipterus, Palaeduphus), or has become elongated.
However of considerable interest from a taxonomic point of view is that the elongation,
which is accompanied by a corresponding lengthening of the head, has taken place in two
different areas, either in the symphysial portion as in Rhin0dipteru.s or in the ramus as in
Griphognathus and probably in certain other ‘long-headed’ dipnoans (Fleurantiu,Soederberghia). This condition suggests that the ‘long-headed ’ dipnoans include a t least two
different groups. Other differences concern the dentition, the fusions of dermal bones and
the degree of ossification. I n Dipterus and other ‘dipterid ’ genera (Chirodipterus, Dipnorhynchus, Melanognuthus, Rhinodipterus) the ventral side of the jaw, its anterior semicircular margin and sometimes the mandibular tooth plate are covered by a shiny layer
of cosmine, and the meckelian cartilage is generally ossified forming a meckelian bone.
However, as early as in Xcaumenacia from the lowermost Upper Devonian the cosmine
has disappeared and the meckelian cartilage is weakly ossified or altogether cartilaginous,
as in Neoceratodus. Moreover the sensory canals which in the ‘dipterids’ are enclosed in
the dermal bones run in Scuumenacia partly in open grooves as they do in the symphysial
portion in Neoceratodm.
The meckelian bone and the adductor pit
The meckelian bone in the ‘dipterids ‘ seems usually to be a solid structure and although
there is some variation it is on the whole very similar to the meckelian cartilage in
Neoceratodus. This applies especially to the articular portion and to the whole posterior
part of the jaw. I n the Devonian forms there is a small or, as in Griphognathus and other
long-headed forms, a more or less elongated retroarticular process. The glenoid fossa (glen)
is divided into lateral and medial divisions. The medial division is situated on the posteromedial side of the strong preglenoid process (pr.pgl),which as in Neocerutodus has a fairly
large rough dorsal surface (ar.ad). The preglenoid process also in the Devonian forms is
PLATE
1
Neocerutodus forsteri (Krefft,)
Figs 1 to 3. Lower jaw in dorsal, ventral and antero-lateral aspects. Photographs in alcohol of specimen
stained in alizarin ( x I t ) .
MdE, External dermal bone of jaw composed of membranous portions of middle and posterior oral
and posterior mandibular canal bones ; M d I , tooth-bearing bone on inner side of meckelian cartilage ;
M d M c l , anterior mandibular canal bone; M d T , mandibular tooth plate; M e c k , meckelian cartilage;
ad.p, adductor pit; ar.ad, rough dorsal area of preglenoid process for attachment of portion of m.
adductor mandibulae ; ar.im, ventro-medial exposed area of meckelian cartilage for attachment, of m.
intermandibularis; c.Zy, canal for lymphatic vessel (Ruge); e.inc, incurved dorsal edge of symphysial
plate ;f.in. V ,foramen for r. intermandibularistrigemini ;f.md. V ,foramen for r. mandibularis t.rigemini;
f.m.e.V I I , foramen for r. mentalis externus facialis; glen, glenoid fossa; gr.mc, groove for mandibular
sensory canal; grt, groove for anterior tendon of adductor muscle; h p , horizontal portion of external
complex dermal bone ; i . m . e . V I I , notch for r. mentalis externus facialis; m a d , musculus adductor
mandibulae; m.dep, median space for basihyal and ‘tongue’; p.end, anterior pointed end of vertical
portion of external complex dermal bone ; prZ.asc, prm.asc, lateral and medial ascending processes of
articular portion ;pr.pgZ, preglenoid process of meckelian cartilage ;sy.pZ, symphysial plate ;vp,vertical
portion of external complex dermal bone.
Note. The photographs from which Plates 1 to G were made were retouched by the author.
Structure of the dipnoa?Llower j a w
163
covered laterally and medially by ascending processes of the adjoining dermal bones. The
lateral ascending process (prl.asc),shown also in Scaumenacia and Griphognathus (Gross,
1956), has in its posterior margin a deep notch (i.m.e.V I I ) probably corresponding
t,o a similar notch in Neoceratodus which conceivably transmitted the ramus mentalis
externus VII. The lateral ascending process is also in the fossil forms bent medially and
almost meets the medial ascending process in front of the preglenoid process. As is particularly well shown in a specimen of Rhinodipterus (Text-fig.4 C ; P1.2, fig. 2) the meckelian
bone is exposed in a narrow slit between t)hecxt)crnaland internal dermal bones and as in
Yeoceratodus the dorsal edges of these bones form a small elongated adductor pit (ad.p),
V-shaped as seen in transverse secttiorisand continued forwards by a narrow groove (yrt)
below t,he overhanging posbero-lateral part of t,he tooth plate. In Dipterus (Text-figs 6,
8 A , C ; Pls 4 , 5 ) the exposed dorsal area of the meckelian bone is broader, and the adductor
fossa arid the groove below the postero-lateral part of the dental plate are wider, than in
Rhinodipterus and Neoceratodus. Moreover bone MdI has a lateral expansion which in the
anterior part of the ramus covers the dorsal side of the meckelian bone. I n Chirodipterus
(Text.-figs4 A , B ; PI. 3, figs 1 to 4), in which the articular portion and t’headjoining dermal
bones are imperfect>lypreserved, the meckelian bone is also exposed in a fairly broad area.
Probably because of the great height of the jaw the groove below the postero-lateral part
of the tooth plate opens laterally, dorsally to the dorsal edge of the lateral dermal bone.
However, the variations in the structure of tdheposterior part of the jaw in the Devonian
forms are insignificant and in view of the great similarities it, is obvious that the ventra,l
part of the m. adducbor mandibulae was developed fundamentally as in Neoceratodus
(as regards the dorsal part see SBve-Sotlerbergh, 1952: 16, fig. 5, f.mt, f.f m ; cf. Luther,
1913: 33-36). No doubt i t was inserted on the rough surface of the preglenoid process and
into the adductor pit with a st.rong tendon extending forwards in the groove (grt)below the
postero-lateral part of the tooth plate.
In the ramus of t’hejaw the meckelian bone is exposed between the dermal bones also
ventro-medially and no doubt the muscnlns intermandibularis inserted on this narrow
exposed area (ar.im).I n Recent dipnoans (Ruge, 1897 ; Luther, 1913; Edgewort,h, 1935)
t,he intermandibular muscles, which are innervated by the n. trigeminus and therefore no
doubt, represent the persisting musculature of t,he ma,ndibular gill-cover (Jarvik, 1963),
extend between the rami of the lower jaws. It is to be assumed that the conditions were
about the same in the early fossil forms. l n this connection it may be of some interest to
mention that the space between the raini of the jaws, in all early clipnoans (Diptcrus,
Rhin.odipterus, Scuumenacia,)in which the dermal bones of the ventral side of the head are
reasonably well known, is occupied by a series of bones which judging from its position and
relat,ions to the overlying intermandibular musculature represents the submandibular
series (Jarvik, 1963). This series (Text-fig. 5 B ) , which is composed of the supporting
elements of the mandibular gill-cover, consists of an unpaired median (Sbmm),a principal
1’1,ATE
2
Fig. 1. Melanogtiathus cutindensis gen. et,. sp. i i o ~Holot
.
ype. Imperfect flattened lower jaw in ventral
aspect. From Anderson River, District of Ma.clienzie, N.W.T., Canada. Specimen NMC no. 11595,
h’a,tional Museum of Canada ( x 44).
Fig. 2. Rhinodipterus zrZrich,i 0rvig. Lower jaw in dorsal aspect. From Heiligenstock, Bergisch Gladba.cli,
Germany. Spccirnen No. P. 6238, Ealeozoologicnl Section, Swedish Museum of Natural History ( x 2q).
M d I , Tooth-bearing bone on inner side of rricckelian bone; I l f c l M c l , M d M c 2 , anterior and posterior
mandibular canal bones; M d O c I , MdOc2, anterior wild middle oral canal hones; M d T , mandibular tooth
plat?: J f d - Y , MtZY, elements corresponding to B o named dental plates in larvae of h’coceratodus;nd.p,
addiictor pit: nr.trd, dorsal rough area of preglonoitl process: nr.gc, area for attachment of m. geniot,endon of adductor muscle ; i.m.e. V I I , notch for
curacoideus; glen,, gleiioid fossa; grt, groove for a.rit>erior
r. ment,alis externus facialis; lab.p, labial pit: p.end, anterior pointed end of vertical portion of middle
oral canal bone ; prl.nsc, pr?n.anc, lateral and medid ascending processes of art,iciilar portion ; p~r.pgl,
preglenoid process of meckelian bone; t , teeth; vp,up2, vertical portion of middle oral canal bone.
164
ERIKJARVIU
-
Text-fig. 4. A, B, Chirodipterus wildungensis. A, Imperfect lower jaw in dorsal aspect. B, Left
half of the same jaw in medial aspect. After the specimen shown in PI. 3 ( x 2 t ) . C , Rhinodipterus ulrichi. Lower jaw in dorsal aspect. After the specimen shown in P1. 2, fig. 2 ( x 2 ) .
M d I , Tooth-bearing bone on medial side of meckelian bone; M d M c l , anterior element of
Structure of the dipnoan lower j a w
163
(Sbnip),and a postcro-lateral (Sbml) submandibular. It is situated in front of, and overlaps, the antero-ventral, gular, part of the operculogular series, which likewise includes
median ( G m ) ,principal ( G p )and postero-lateral (GI)elements.
In the symphysial portion the endoskeleton is rarely well exposed. The ventral side and
the anterior margin of that portion are a1v ays covered completely with dermal bones.
The dorsal side, too, is covered by dermal bones to a considerable extent but unfortunately
it is most often difficult to distinguish clearly betn een endo- and exoskeleton and to make
out the extent of the individual dermal bones in this part of the jaw. Howevcr, judging
from the shape of the symphysial portion, i t IS likely that there is a syniphysial plate
( s y . p l , Text-fig. 8 C ) similar to that in Neoc todus. That this is so is supported by the
conditions in Chirodipterus (Text-figs 4 A , 13; PI. 3). I n the single known specimen of this
genus the symphysial plate is well shoxn m a section passing almost in the median line.
As shown by this section the plate, at the postwior margin of the symphysial portion, is
exposed between bones MdI above and RLdRIcl below. This exposed part on each side is
continnous with the ventro-medial exposed area (ar.im)of the ramus. The most posterior
part of the plate is thin but i t gradually increases in thickness forwards. I n front of the
anterior margin of bone Rfdl there is a slight depression exposed in a gap between the
dermal bones. The anterior part of the symphysial plate, which carries bone MclX, forms
a rounded dorsally projecting margin. This margin is semicircular as seen in dorsal aspect
but it is bent slightly inwards ( e i n c , P1.3. fig. 1)a t the labial pit very much as is the corresponding thickened margin of the symphysial plate in Neocerntodira
Tooth-bearing hortes and dentition
The inner side of the meckelian bone in t lie ranius of the jaw is covered by a strong bone,
MdT, M hich as in Neoceratodus extends foru ards into the symphysial portion and covers
part of the dorsal side of the symphysial plate. The posterior part of the bone shares in the
formation of the adductor pit and behind that there is an ascending process (prm.asc).
Sometimes a notch occurs in the posterior margin of the bone behind which the medial side
of the meckelian bone is exposed in a similar way as in Neoceratodus. As in that form the
syniphysial part, of the bone has a medial expansion extending to or almost to the median
line. The anterior extent of the sympliysial part could be clearly made out only in
Chirodipterus in which this part is considerably thickened. The syniphysial parts of bones
MdI of both sides together form a median depression or there is a median opening bounded
laterally or dorso-laterally by the tooth plates. Like that of Neoceratodus this space ( m dep)
doubtless was occupied by the basihyal and probably it is this element that Gross (1964:
19-20, fig. 3 E ) found in Dipterus oervigi and suggested to be the urohyal.
As is well known, bone MdI also in the fossil forms generally carries a strong tooth plate,
MdT, often distinguished from the supporting hone by its darker colour and sometimes by
a suture-like line. As shown by the excellent sections presented a t an early date by Pander
(1858, pl. 5, fig. 6, pl. 6, fig. 8) this plate also in the Devonian dipnoans is composed of
columnar pleromic hard tissue which apparently grew downwards into the supporting
bone. In Dipnorhynchus the tooth plate semis to be situated within the jaw (White, 1966,
pl. 1. fig. 1) and in GTiphognathus it is lacking. In the latter (Gross, 1956: 32-37) bone &Id1
inandibular series of canal bones; N d T , princ.ipa1tooth plate: M d S , N d Y , eleriients corresponding to so named tooth-hearing hones i i i larvae of Neocercrtndus; Meck, meckeliaii bone:
rsd.p, adduct,or pit : rcr.ad, rough dorsal areti ( ~ prrglerioid
f
process; ar.gc, area for attaclirneiit
of in. genio-coracoideus; ar.im, area of mcckclian bone for attachmeut of in. intermandibularis: glen, glenoid fossa; grt, groove for portion of adductor muscle: i . w ~ . eVZZ,
.
notcli
probably for r. mentalis externus facialis; 1trb.p. labial pit; m.dep, median space for ‘tongue’:
p.etLd, ant,erior pointed end of vertical port ion of middle oral bone; prZ.asc, prni.nsc, lateral
and medial ascending processes of articultir portion ; pr.pgZ, preglenoid process : sy.pl, spmphysial plate; vp,vpI, vertical portions of hones of oral series.
166
ERIKJARVIK
(‘Praarticulare’) bears a great number of isolated teeth (as regards Fleurantia and Soederberghia, see Graham-Smith & Westoll, 1937: 248-249; Lehman, 1959: 31). These teeth,
which have the same histological structure as those of Dipterus (Gross, 1956: 132-133),
are armnged somewhat as the teeth in larval stages of Neoceratodus. A surprising condition
is that the tooth plates in early dipnoans sometimes have a more or less complete layer of a
cosmine-likesubstance (Jarvik, 1964,fig. 13;White, 1965,1966).I n the material of Devonian dipnoans a t my disposal there are no quite distinct traces of this layer in the lower jaw
but it is well shown in several specimens of Dipterus on the principal tooth plate of the
palate. As in the specimens figured by White (1965)in most cases it is restricted to the flat
medial area inside the innermost tooth ridge, but sometimes (PI.4, fig. 4) it extends a little
more laterally and surrounds the bases of the teeth of the innermost one or two tooth
ridges. This peculiar layer which, as far as I can find, in Dipterus never covers the tips of the
teeth, generally shows signs of disintegration in its medial part andis sometimescompletely
lacking. These conditions suggest a periodical (seasonal?)resorption and reformation of the
cosmine-like layer and possibly this phenomenon is in some way associated with the
periodical reformation of the horny caps (horn,Text-fig. 2 C) in Protopterus. In this connect,ion it may be added that one specimen in which the cosmine-like layer is lacking, shows
the ‘secondary denticles’ recently described by White (1965: 39, s.d, fig. 49; pl. 1, fig. 1)
I n front of bone MdT in Holodipterus Gorizdro-Kulczycka (1950) discovered a median
dermal bone (‘leprearticulaire anterieur ’) withtraces of two fairly large teeth. This median
element ( M d Y , Text-fig. 9A), which is situated dorsally to the symphysial plate and no
doubt corresponds to bone MdY in larvae of Neoceratodus, is well shown in Chirodipterus
and seems to be present also in Rhinodipterm (Text-fig.4C; P1.2, fig. 2). I n Chirodipterus
(Text-figs 4A, B, 9C) bone MdY forms a narrow vertical rod reminiscent of the ‘presymphysial’ tooth in IlIyriacanthus (Text-fig. 9D, E). It is found in the postero-medial
part of the depression on the dorsal side of the symphysial plate and it is not wedged in
between the anterior ends of bones MdI as i t is in Holodipterus and probably in Rhiywdipterus. Its dorsal side is dama,gedand no traces of teeth have been found.
The equivalent of bone MdX in larvae of Neoceratodus is in the Devonian forms clearly
represent’edby the seniicircular bone (* dentary ’, Watson & Gill, 1923) which forms the
anterior edge of the lower jaw and in the ‘dipterids’ is coated by a layer of cosmine. This
bone ( M d X ,Text-figs 4 to 7, 8A, C, 9A; PIS2 to 6), which is intimately connected with
adjoining bones and appears to be unpaired, sheaths the anterior and lateral margins of
the symphysial plate and forms part of the skeletal wall of the labial pit. Dorso-medially
to that pit the dorsal margin of the bone sometimes bears teeth. I n Holodipterus and
Dipterus oervigi (Gross, 1964) these teeth, which in the latter are peculiar complex structures composed of a cluster of small denticles, are arranged in a longitudinal row. Teeth
probably belonging to this row are present also in Melanognathus, Chirodipterus (see also
PLATE
3
Chirodiptertrs i d d u n g e m i s Gross. Holotype. Imperfect lower jaw. Figured by Gross, 1933, pl. 11,
figs 1, 3; Jarvik, 1955, fig. 7C; 1964, fig. 13D, E. Specimen belonging to the Humboldt University,
Berlin ( x 24).
Fig. 1. Dorsal view.
Fig. 2. Right half in medial view. Photographed in alcohol.
Fig. 3. Left half in medial view. Photographed in alcohol.
Fig. 4. Left half in antero-lateral view. Photographed in alcohol.
d l d l , Toot,h-bearing bone on medial side of meckelian bone; M d T , mandibular tooth plate; M d X ,
M d Y , elements corresponding t,o so named dental plates in Neoeeratodus; Meck, meckelian bone;
nr.i)n, area of meckelian bone for attachment of m. intermandibularis; e.ir,c, incurved dorsal edge of
symphysial plate; grt, groove for ant,erior portion of m. adductor mandibulae; b b . p , labial pit; m.dep,
median depression for ‘tongue ’ ; orc, oral sensory canal ; p.end, anterior pointed end of vertical portion
of external bone of jaw; s:y.pZ, symphysial plate; up, vert,ical portion of external dermal bone; vpl,
vertical portion of anterior oral canal bone.
Plate 1
Structure of the dipnoan lower jaw
167
Gross, 1933) and Scuumenacia. These teeth are situated dorsally to the thickened lateral
edge of the symphysial plate, that is they have exactly the same position as the teeth
found by Holmgren (1949) (21-25, Text-figs ZB, 9B) dorsally to the ‘labial cartilage’ in
Neoceratodus. I n Dipterus oervigi there is in addition an anterior, transverse row of small
denticles and teeth in a corresponding position have been observed by Traquair (1878: 8)
in a single specimen of Dipterm valencie?znesi.The longitudinal row seems to be absent in
D. valenciennesi and in Rhinodipterus teeth are lacking altogether.
Sensory lines
The lower jaw in the Devonian dipnoans bears two sensory lines : the oral canal and the
mandibular canal (Text-figs 5 to 7).No traces of pit-lines have been found. The bony canals
have been almost completely prepared in Xcaumenacia (PI. 6) and are in other forms
partly shown or their course may be followed by means of the external pores. It could not
be established if the oral canal posteriorly joins the infraorbital canal ventrally to the eye
as i t does in Recent adult dipnoans, or if it was connected with the preoperculo-mandibular
canal on the cheek as in embryonic stages of dipnoans (Pehrson, 1949, figs 1, 2, 15, 16)
and sometimes in holocephalians (Holmgren & Pehrson, 1949, fig. 44). I n other respects
the sensory canals, if not secondarily displaced, run very much as in Neoceratodus. The
oral canal ( o w ) thus passes forwards in the lateral part of the ventral side of the jaw and
runs in a gentle curve along the margin of the labial pit to the anterior end of the jaw. The
mandibular canal (rnc)in the ramus of the jaw is situated in the medial part of the ventral
side. In the symphysial portion it bends in an antero-lateral direction giving off a medial
commissural canal (corn) and continues to the oral canal which it probably joins a little
behind its anterior end.
Before the discussion of the sensory canal bones a brief description will be given of the
new Devonian dipnoan from Canada.
Melanognathus canadensis gen. et. sp. nov.
(Text-figs 5 A , T A ; P1. 2 >fig. 1)
This new species is represented only by an incomplet,ejaw. This jaw is the holotype of
Melanognathus canadensis sp. nov. which is the type and only known species of the new
genus Melanognathus. The generic name refers to the black colour of the jaw.
The specimen (NMC no. 11595,National Museum of Canada) was collected by B. Moore
of the Chevron Standard Limited, Calgary, Alberta, Canada. The locality of the new discovery is described by Moore as the Anderson River, immediately upstream from the
P L A T E4
Dipterus valeiicien?iesi Sedgw. & Murch.
Figs 1 t o 3. Lower jaw in ventral, dorsal and antero-lateralaspects. Specimen No. L. 10858, Manchester
Museum, used by Watson and Gill, 1923, fig. 34. Middle Devonian. Caithness ( x 2).
Fig. 4. Palatal tooth plates in ventral aspect, showing cosrnine-like layer. Middle Devonian, Caithness.
Specimen No. 53389, Geol. Surv., London ( x 2).
M d I , Tooth-bearing bone on medial side of meckelian bone; M d M c l a , M d M c l b , medial and lateral
independent parts of anterior mandibular canal bone; MdOcl, anterior oral canal bone; MdOc2Mc2,
complex bone including middle oral and posterior mandibular canal bones; MdOc3, posterior oral canal
bone; M d T , mandibular tooth plate; M d X , bone corresponding to so named dental plate in larvae of
Neoceratodus; o r a d , dorsal rough area of preglenoid process; ur.im, area for attachment of m. intermandibularis; glen, glenoid fossa; hp2, hp3, horizontal portions of middle and posterior oral canal bones ;
i.m.e. V I I , notch probably for r. mentalis externus facialis; lab.p, labial pit; mc, orc, mandibular and
oral sensory canals; p e n d , anterior pointed end of vertical portion of middle oral canal bone; prZ.asc,
lateral ascending process of articular portion ; pr.pgZ, preglenoid process of meckelian bone ; ri, ridge a t
suture between middle and posterior oral canal bones; v p l , vp2, vp3, vertical portions o f oral canal
bones.
168
ERIKJARVIK
Text-fig. 5 . A, Melurmgnnth?ts c t e w d e m i s gen. e t qp. nov. Holotype. Imperfect lower jaw in
ventral aspect (see also P1. 2, fig. 1) ( x 4 4 ) . B, Scaumeizaciu curtn. Restoration of head in
ventral view ( x 2).
GZ, Gm, Gp, Postero-lateral, median and principal gular plates; M d F , external compound
sensory canal bone of jam; M d M c l , MdMcZ, anterior and posterior elements of mandibular
series of canal bones; M d O c l , MdOcZ, anterior and middle elements of oral series of canal
bones; M d X , anterior tooth-bearing element ; S b m l , SbnLm, Sbmp, postero-lateral, median
Xtructure of the dipnoan lower jaw
169
junction of the Ross River (latitude 68" ll'N, longitude 125"49'W), District of Mackenzie,
Canada. M . canadensis occurs in a sequence of black shale and limestone, about 50 feet
thick, in which it is associated with palaeoniscid and Porolepis-like scales. The shale and
limestone sequence overlies 20 feet of cryptocrystalline limestone, which in turn overlies
50 feet of dolomite breccia and limestone. Covered intervals separate the above described
strata from underlying and overlying sediments. The vertebrate-bearing strata are dated
tentatively as Lower Devonian on the basis of marine invertebrates. The Chevron Standard
Company has donated the vertebrate fossils, including the holotype of 211.canadensis, to
the National Museum of Canada.
The specimen is an incomplete and flattened jaw exposed from the vental side. The main
part of the left ramus and the articular portion of the right ramus are missing. The endoskeleton is not shown and is probably not preserved. The symphysial portion is almost
complete and measures 11 mm in length along the median line. The main part of the right
ramus has separated from the rest of the jaw a t a nearly longitudinal suture which, as in
other dipnoans, runs from the anterior part of the medial margin of the ramus to the labial
pit, and has become displaced backwards. The dermal bones are covered by a shiny layer
of cosmine, which, however, is deficient anteriorly, and is completely lacking a t the posterior end of the preserved part of the right ramus and along its antero-lateral margin. As
in other Devonian dipnoans there is a distinct labial pit bounded dorsally by the lateral
part of bone MdX, which in this area carries a row of fairly large teeth. Three such teeth
are preserved on the left side of the jaw. The pores of the sensory canals are well shown on
the cosmine-covered surface. They are arranged mainly in single rows and as shown by
their distribution the sensory canals run very much as in Neoceratodus.
The data presented so far show plainly that the jaw is that of a 'dipterid ' dipnoan. Its
general shape is most similar to that of the jaw of Dipterus but it differs from the jaw in that
genus and in other dipnoans in several respects. One distinctive feature is that the rami
of the jaw meet anteriorly in a sharp angle and accordingly the posterior margin of the
symphysial portion is not so gently concave as in other dipnoans known in this respect.
Other. more important. distinctive features concern the sensory canals and canal bones.
The canal boners111 Melanognathus
A remarkable fact is that the ramus of the jaw in Melanognathus, in contrast to conditions in other dipnoans known in this respect, includes two large sensory canal bones, one
lateral, MdOc2, pierced by the oral canal and one medial, MdMc2, by the mandibular
canal. The lateral element consists of a medial, broad, horizontal portion (hp2) covered
by cosmine and a lateral cosmine-free portion. As preserved the lateral portion (vp2)is
bent a little upwards but no doubt it formed an almost vertical lamina which ventrally
met the horizontal portion at about a right angle. The lateral vertical portion has a free
dorsal margin and a pointed anterior end ( p ~ n da )liich enters into the formation of the wall
of the labial pit and joins the overlying postero-lateral tooth-bearing part of bone MdX.
The oral canal runs straight forwards in the lateral part of the horizontal cosmine-coated
portion of the bone. Medially to this portion, separated from it by a distinct suture, follows
bone MdMc2, which is also a horizontal plate covered by cosmine. The mandibular canal
runs straight forwards near the medial margin of the bone and is accordiiiglyrunning almost
parallel with, but far apart from, the oral canal. In this respect Melanognathus agrees with
and principal submandibulars; Sop, suboperrular ; cmap, pores of medial cominissural branch
of niandibular canal; qr.wzc, gr.oc, grooves for niaiidihular and oral canals ; h p l , hp2, liorizont,al
portions of anterior and middle oral canal bones ; Inb.p, labial pit; mc, mandibular canal;
m c p , pores of mandibular canal; orc, oral r a n d ; orcp, pores of oral canal; p.end, ant,erior
pointed end of vertical portion of middle oral canal bone; p o p , preopercular canal; t, teeth;
op2, vertical portion of middle oral canal bone.
170
ERIKJARVIK
Neoceratodus but differs from the other early dipnoans (Dipterus,Scaumenacia) in which
the sensory canals of the jaw are well known.
In front of bones MdOc2 and MdMc2, separated from them by the longitudinal suture
running to the labial pit, there are in Melanognathus two other cosmine-coated canal bones,
one antero-lateral, MdOcl, housing the anterior part of the oral canal and one posteromedial, MdMcl, developed in relation to the mandibular canal and its medial commissural
branch. These bones, which occupy the main part of the ventral side of the symphysial
portion are separated by a distinct suture-an anterior continuation of the suture between
the canal bones of the ramus of the jaw. With respect to its position and relations to the
sensory canals bone MdMcl agrees well with, and is no doubt homologous to, the so
named bone in Neoceratodus. Bone MdOcl, which has no equivalent in the latter form
(see below), extends to, and forms the concave margin of, the labial pit. Most likely it is
provided with a lateral vertical cosmine-free portion (vpl)which forms part of the wall
of that pit. The pores of the oral canal are in a row along the margin of the labial pit suggesting that the oral canal in this part runs in a gentle curve as in Neoceratodus and many other
dipnoans. Anteriorly no suture is discernible separating the bone from bone MdX. Nor are
there any traces of a median suture between bone MdOcl and the corresponding element
of the other side. Possibly these elements are wholly or partly separated by a median
element.
T'lie canal bones in certain other dipnoans
In Dipterus (Text-figs6,7B, 8A, C; Pls 4,5) the area of the ramus of the jaw corresponding to that which in Melanognathus is covered by bones MdOc2 and MdMc2 is occupied by
a single large bone. This bone, MdOc2Mc2, consists of a lateral vertical portion devoid
of cosmine and a medial, horizontal portion. The vertical portion (vp2), which covers the
lateral side of the meckelian bone and has a free dorsal margin, corresponds to the vertical
portion of bone MdOc2 in Melanognathus and as in that form its anterior end (p.end) is
pointed. This pointed part shares in the formation of the wall of the labial pit and is
dorsally connected with the overlying postero-lateral part of bone MdX. The horizontal
portion (hp2)of bone MdOc2Mc2, which covers the ventral side of the meckelian bone, is
coated with cosmine like the horizontal plate of bone MdOc2 in Melanognathus. However,
it is broader than that portion extending to the medial margin of the ramus. Moreover it is
pierced by both the oral (orc)and the mandibular (mc)sensory canals. Within the bone
these canals bend towards each other and in the middle part of t,he bone, a t the centre of
radiation, they lie very close together. an example of displacement of sensory canals due
PLATE
5
Dipterm valencieiinesi Sedgw. & Murch. Flattened, inromplete lower jaw. Middle Devonian, Caithness.
Specimen No. 53370, Geol. Surv., London ( x 2).
Figs 1, 2. Ventral and dorsal views.
Fig. 3. Posterior part of left ramus men from the dorsal side.
Fig. 4. Left half in antero-dorso-lateral view.
MdZ, Tooth-bearing bone on medial side of meckelian bone; MdMcZa, MdMclb, medial and lateral
independent parts of anterior mandibular canal bone; MdOcl, anterior oral canal bone; MdOc2Mc2,
complex bone including middle oral and posterior mandibular canal bones; MdOc3, posterior oral canal
bone; M d T , mandibular tooth plate; M d X , bone corresponding to so named dental plate in larvae of
Neoceratodus; Meek, meckelian bone ;ad.p, adductor pit ;ur.ad, dorsal rough area of preglenoid process:
ar.int, ventro-medial exposed area of meckelian bone for attachment of m. intermandibularis; glen,
glenoid fossa; grt, groove for anterior portion of adductor muscle ;hp2, hp3, horizontal portions of middle
and posterior oral canal bones; i.m.e.BIZ, notch probably for r. mentalis externus facialis; Zab.p, labial
pit; m,o w , mandibular and oral sensory canals (posterior openings) ;prZ.asc,prm.asc, lateral and medial
ascending processes of articular portion; pr.pgZ, preglenoid process of meckelian bone; ri, ridge at
suture between middle and posterior oral canal bones; v p 2 , vp3, vertical portions of middle and posterior
oral canal bones.
Plate t i
Structure of the dipnoun lower jaw
171
t o fusion of bones (see Jarvik, 1967).Such a fusion is suggested also by the extent of bone
MdOc2Mc2, which would be a compound structure including the equivalents of bones
MdOc2 and MdMc2 in Melanognathus.
Post,eriorly to this compound bone in DiptPrus follows another sensory canal bone. This
bone, MdOc3, which was overlooked by Wat)son& Gill, also consists of a vertical cosminefree portion (wp3)and a ventral horizontal porttion(hp3)covered with cosmine. The vertical
portion, which is separated from the adjoining vertical portion of bone MdOcSMc2 by a
suture situated on a conspicuous ridge ( r i ) ,forms the lateral ascending process (prl.asc)
covering the lateral side of the preglenoid process of the meckelian bone. The narrow
horizontal portion is pierced by the oral canal and accordingly the bone belongs to the oral
series of canal bones. As indicated in Text-fig. 7 A this element (MdOc3) is probably
present also in Melanognathus.
The number and extent, of the dermal bones on the ventral side of the symphysial
portion in Dipterus are still obscure. The suture between the two bones, ‘splenial’ and
‘postsplenial’ described by Watson & Gill (1923) is discernible in the specimen studied by
them and in two other specimens a t my disposal. The medial element, MdMcla (’splenial’),
which hitherto has not been found independent in any other dipnoan, is pierced by the
medial commissural branch of the mandibular canal. The lateral element, MdMclb (‘postsplenial’) contains the lateral part of t’hat,branch and an anterior part of the mandibular
canal. but if i t reaches the labial pit and is pierced also by the oral canal is difficult to say.
Most likely there is an independent oral element corresponding to bone MdOcl in
Melanognathus. Like the ot,her bones in t)licoral series this bone consists of a horizontal
cosmine-covered portion ( h p l )pierced by t’heoral canal, and a lateral, vertical cosminefree portion (vpl).The latter forms the main part of the wall of the labial pit. Whether
t,here is a median element between bones MdOcl of both sides I have been unable to
ascertain.
In Xcuumenucia (Text-figs 5B, 7C; P I . 6) the ventral and latera,l sides of the main
posterior part of the ramus of the jaw are covered by a single large bone which carried the
ascending process ( p r h s c ) laterally to t lit. preglenoid process and, in addition to bone
MdOc2Mc2, doubtless includes also the equivalent of bone MdOc3 in Bipterus. This highly
complex bone, MdE, also consists of vert>icaland horizontal portions. The vertical portion
(vp),
which is composed of the vertical port.ionsof bones MdOcd and MdOc3, extends from
t,he posterior end of the jaw, where it shows a distinct notch (i.m,.e.V I I ) ,to t,he lahial pit.
Exactly as in Melanognathus and Dipterus its anterior end (p.end)is pointed and is joined
to t,he postero-lateral part of bone MdX. The horizontal portion (hp)in contrast to condit>ionsin the ‘dipterids ’ lacks cosmine and has an ornamented external surface. It contains
both the oral and mandibular canals. which as in Dipterus bend towards each ot,her and
I’I.ATE
0
Pcnumeruacia curta (Whiteaves). Incomplet,e lower jaws and the vomer. Lowermost. Upper Devonian,
Escuminac Bay, Canada. Specimens Nos. P. 3187, P. 4157, P. 766, P. 778, P. 3188, Palrozoological
Section, Swedish Museum of Natural History.
Fig. 1. Right side in lateral view. Same specimen as figured by Stensio, 1947, fig. 33.4 ( x 2).
Fig. 2 . Flattened left half in inner view ( x 4).
Fig. 3. Right half in external view ( x 2 % ) .
Fig. 4. Left half in lateral view ( x 4).
Fig. 5. Right half in lateral view. Vomer in ventral view ( x 2).
M d E , complex external dermal bone of jaw; i l f d . W c / , anterior mandibular canal b o n ;~JZrlT. mandibular toot,h plate; M d X , tooth-bearing bone c,orresponding to so named dentd plate in larvae of
Neocerntodus; Sbmp, principal snbmandibular plate; V o , vomer; com, medial commissural branch of
mandibular sensory canal : yr.mc, yr.oc, grooves for mandibular and oral sensory canals: hp, horizontal
portion of complex external dermal bone ; i.m.e.VZZ, notch probably for r. ment,alis exterr~usfacialis;
mc, orc, mandibular and oral sensory canals; p.erid, anterior pointed end of vertical portion of middle
oral canal bone; prZ.asc, lateral ascending process of articular portion; t , t,ooth of bone A41r/S;zy~,vertical
portion of complex external bone.
172
ERIKJARVIK
Text-fig. 6. Diptcrus ualcncicnnesi. Restorations of the lower jaw in, A, dorsal and, B, ventral
aspeck Based mainly on the specimens shown in Pls 4 and 5 ( x 3 ) .
M d I , Tooth-bearingbone on medial side of nieckelian bone; MdMclci, M d M c l b , independent parts of anterior mandibular canal bone ; M d O c l , anterior oral canal bone; MdOc2McZ.
complex bone including middle oral and posterior mandibular canal bones ; McZOc3, posterior
oral canal bone; M d T , principal mandibular tooth plate; M d X , bone corresponding to so
named tooth plate in larvae of Ncoceratodus; Meck, meckelian bone; ad.p, adductor pit; ar.ad,
rough dorsal area of preglenoid process; ar.im, area for attachment of in. iutermandibularis;
corn, medial commissural branch of mandibular canal; glen, glenoid fossa; grt, groove for
portion of adductor muscle; h p l , hp2, hp3, horizontal portions of anterior, middle and
posterior oral canal bones: i.m.e. V I I , notch probably for r. mentalis externus facialis; h b . p ,
labial pit; mc, mandibular sensory canal; m.dcp, median space for ‘tongue ’; orc, oral sensory
canal; ?’.end, anterior pointed end of horizontal portion of middle oral canal bone; prZ.asc,
prm.asc, lateral and medial ascending processes of articular portion ; pr.pgZ, preglenoid
process; ri, ridge a t suture between middle and posterior oral canal bones; sy.pZ, symphysial
plate; v p l , vp2, np3, vertical portions of ant,erior, middle and post>eriororal canal bones.
Structure of the dipnoan lozcer jaw
173
pass close together through the centre of iatliation. However, the canals are enclosed in
the bone only for a fairly short distance. A4ntrriorlythe oral canal soon emerges from the
bone and continues as an open groove ( g r o( ) extending to the anterior margin of the bone.
The mandibular canal, too, secms to be rvpresented anteriorly by a similar although
shorter groove (gr.mc).Posteriorly the nianrl I bular canal probably continues in some small
bones which are situated in the anterior continuation of the chain of bones of the cheek
housing the preopercular canal ( p o p ;cf. Biptrrus, Westoll, 1949, fig. 1B. C ; Neoceratodus,
Text-fig. 3 B ; Holmgren & Stensiii, 1936, fig 287, Sq.pop).
Bone MdE in Scaumenacia in most respects agrees uell with the so named bone in
Neoceratodus. In the latter (Text-fig. 7 D ; 1
1' 1) the bone is also composed of vertical and
horizontal portions and as in Scaun~enaczathe vertical portion posteriorly is provided with
a n ascending process (prl.asc) and sornetimes a notch (i.m.e. V I I ) and its anterior end
( p end) is pointed and forms part of the wall of the labial pit. However the horizontal
portion is a little narrower than in ~S'cantn(~tiacia;
moreover the sensory canals have a
straight course and run far apart in the hkni These differences are no doubt due to tlie
general retrogressive development of the slicileton (see Jarvik, 1960: 57-61 ; 1964: 8), a
trend M hich has resulted also in the loss oft h r cosrnine layer in the advanced dipnoans. As is
well known (see Jarvik, 1967) the canal bones in fishes are composed of two kinds of components : membranous, which generally form bony plates, and lateral sensory, which are
more or less complete cylindrical pieces. IThat apparently has happened in the rarnus of the
jaw in the phyletic development of Neocet atodus from an ancestral form like S'caunieriacia
is that the lateral sensory components in connection with the general reduction of the bone
have failed to fuse with the underlying inmibraneous components, allowing the oral and
mandibular canals to return to their original htraight course (cf. Melanognathus).Moreover
the lateral sensory Components have either become completely reduced (oral canal) or
retaiiicd in a vestigial state (bones i l f d M c of the niandibular canal). Accordingly, bone
MdE in Neocerafodus is composed of thc. membraneom components of bones RldOc2,
MdOc3 and MdMcB.
The main part of the ventral side of the synipliysial portion in Scaunbenacia IScovered by
a fairly large paired bone ( M d M c l ,Text-fig 7C; PI. 6) which in shape and extent as well
as itr relations t o the sensory canals agrecs well ~ i t bone
h
M d M c l in Neoceratodus (Textfig. 7D; PI. 1) and is no doubt homologous to that bone. The only difference a o r t h mentioning is that the soft sensory canals in Scaumenacia were enclosed in bony canals,
1%hereas in h'eoceratodus they run in open grooves. Antero-laterally, in tlie area corresponding to that which in MeZanognathus IS occupied by bone MdOcl, the conditions in
Scauinenacia are obscure, but judging from several specimens this area also in Scaunienacia
was probably occupied by a separate bone, MdOcl, which formed the concave margin of
the labial pit and part of its mall. In h'pocptatodus the meckelian cartilage is exposed in the
corresponding area and bone MdOcl in thib form has become reduced altogether
REMARKS ON THE DENTARY
The absence of an unquestionable dentary in dipnoans has been a serious stumblingblock in the attempts to derive tetrapods from dipnoans. This difficulty is reflected in the
terminology of dermal bones and in ordc,r t o give an example of the confusion among
authors in this respect it may be mentionrt1 that at one time or another each of the main
bones in the jaw of Neoceratodus has been I cgarded as the dentary.
Gunther in the first description of Neoceratodus (1871) thus considered the large bone
(MdI) on the medial side of the rneckelian cartilage to be the dentary, whereas Holmgren
more recently (1949: 472, fig. 7 ) claimed that it is represented by the large lateral bone
(MdE) generally called 'angular'. Several writers (Huxley, 1876: 34; Miall, 1878; Goodrich, 1909, fig. 206; 1930, fig. 315) were of the opinion that the sensory canal bone (RZdMcl)
on the ventral side of the symphysial plate ISthe dentary while others (Semon, 1899; Greil,
174
ERIKJARVIK
Text-fig. 5. Dia,gramrnatic representations of the flattened riglit half of the lower jaw to show
variat,ions in t,he sensory ca,nal bones. Verltml views. A, 1TfeZanognnthiLs cnnndeiisis gen. et
sp.nov., B, Diptems v a b n c i e w i e s i , C, Scuuii?enricin curtrr and D, Neoceratodus j o r s t e r i .
M d E , Complex bone including middle ancl post,erior oral and posterior mandibular canal
bones (in D membranous components only) : iMdMc, independent lateral sensory component's
of posterior mandibular canal bone : X d M c l , anterior mandibular cana,l bone; M d M c 2 ,
posterior mandibular canal bone ; M d O c l , M d O c 2 , MdOc3, anterior, middle and posterior
oral canal bones; MdOcZMcZ, bone formed by fusion of M d O c 2 and 1WdMc2; J l d X , toot,Iibearing bone corresponding t o so named tooth pla,t,ein larvae of Neoceratodus; corn, medial
commissural branch of ma.ndibular canal; hp, Iipl, hp2, h,p3, horizontal portions of oral canal
bones; i . m . e . V I I , notch probably for r. menta,lis externus facialis; Za6.p, labial pit; m e , orc,
mandibular and oral sensory canals; p.end, anterior point'ed end of vertical portion of middle
oral canal bone; p r h s c , lateral ssceriding process of articular portion (part of vertical portion
of posterior oral canal bone); sy.pZ, symphysial plate; t , teeth; wp, c p l , vp2, rp3, vertical
portions of oral canal bones.
Structure of t h dipnomi lower jaw
175
1913 ;Fox, 1965) thought that it is represented by the tooth plate (MdX) found in embryos
dorsally to the lateral thickened part (‘labial cartilage ’) of the symphysial plate.
As demonstrated above the large external bone (MdE)in Neoceratodus, which Holmgren
considered to be the dentary, has arisen by fusion of three originally independent sensory
canal bones (MdOc2, MdOc3, MdNc2). Because, in addition, it lacks teeth, is covered
externally by the fold of the lower lip, and. when the mouth is closed, is situated inside
the upper lip, it is evident that this bone cannot be a dentary. Nor can the large bone
(MdI) on the medial side of the meckelian cartilage, nor the sensory canal bone (MdMc1)
on the ventral side of the symphysial plate. be a dentary, which in the early teleostomian
fishes and tetrapods is a large tooth-bearing non-canal bone on the outside of the meckelian
bone or cartilage and opposed to the external tooth-bearing bones (maxillary, premaxillary) in the upper jaw. Thus there remains only bone MdX which in some fossil
forms and in larvae of Neoceratodus is tootlied along its oral margin. However, before we
are able to decide if this element is a true dei1tar.y homologous to that in other fishes and
tetrapods a number of intricate questions must be answered. To begin with we may ask
what is a true dentary? With regard to the inner arcade of tooth-bearing elements in the
loner and upper jau s (coronoids, vorner, dermopalatine, ectopterygoid) it is almost
certain that they represent modified dental plates of visceral arches (Jarvik, 1954),but the
origin of the elements (dentary,maxillary, premaxillary) of the outer dental arcade is still
unknown. We know, however, that the outer arcade a t the transition from fish to tetrapod
increases in importance, whereas the inner arcade becomes reduced (Jarvik, 1964 : 66).
I n dipnoans, on the other hand, the supposd dcntary (MdX) is obviously a bone under
reduction and it has disappeared in all recent adult dipnoans. Another intricate problem
concerns the ‘labial cartilage’ which according to Holmgren (1949) carries this element
in larvae of Neoceratodus. Is that cartilage, which no doubt in the Devonian forms is
represented by the marginal part of the symphysial plate underlying bone MdX, a part of
the mandibular arch; is it a vestige of a ventral element of thepremandibular arch; is it a
new aquisition, or does it possibly correspond to the ‘premandibular cartilage’ in holocephalians? No reliable answer can be given to any of these questions. A remarkable fact
a t present hard to explain is also that bone MdX in larvae of Neoceratodus (Semon, 1899:
122 ; Greil, 1913, pl. 69, fig. 4),and obviously in the Devonian dipnoans as well. is opposed
to the so-called vomer. I n Dipterus (White, 1965, fig. 50) the two vomers together form a
fan-shaped structure, as a whole rather suggestive of the corresponding structures in
Chirodipterus (SBve-Soderbergh, 1952, . 10) and Neoceratodus (Semon, 1899, fig. A ;
Bertmar, 1966, figs 6 , 9 , 1 1 ) .InScaumen
, too, there is a similar vorner (Vo,P1.6, fig. 5)
although i t is unpaired, as is the opposed element (MdX) in the lower jaw. If the vomer in
dipnoans is correctly identified. that is, if it does represent modified dental plates of the
infrapharyngopremandibular (Jarvik, 1954), then it is unlikely that bone MdX is a true
dentary which is opposed to the maxillary and premaxillary. Or, as maintained by Greil
(1913) is the dipnoan ‘vomer’ really the premaxillary which has shifted backwards into
the mouth cavity in connection with the migration of the nostrils and other transformations in the anterior part of the snout (Jarvik. 1942: 270-280)? If so, what about the
‘premaxillary teeth’ of the upper lip in Dipterus oervigi which according to Gross (1964)
are opposed to the anterior small teeth of bone MdX? There are thus it number of intricate
problems to be solved before we can decide if a dentary homologous to that in other fishes
and tetrapods is present in the dipnoans and further investigations are necessary.
EVOLUTION O F THE DIPNOAN LOWER JAW
The lower jaw in the Dipnoi illustrates in a most striking way the remarkable conservatism of this group of fishes. Important resemblances between Devonian lung-fishes and
the modern forms, in particular Neoceratodus, were demonstrated a t a n early date by
Giinther (1871) and Traquair (1878) and later Save-Soderbergh (1952) found striking
176
ERIKJARVIK
similarities in the braincase. We know now that there are detailed resemblances in the
lower jaw, and, disregarding the variations in general shape and proportions among the
Devonian forms and the effect of the retrogressive development of the skeleton in phylogeny, there are hardly any differences of importance in jaw structure between the various
Devonian forms or between these early representatives of the group and the extant
Neoceratodus.
When the lung-fishes first appear in the geological record, probably in the late Lower
Devonian, the lower jaws had fused and in all lung-fishes a symphysial portion and paired
rami may be distinguished. On the lateral side of the symphysial portion there is a characteristic pit, the labial pit, housing a flap-like portion of the fold of the lower lip. Posteriorly the labial pit emerges into the smooth lateral face of the jaw. No doubt this face.
which in the ‘dipterids’ lacks cosmine, in the early forms was also covered by the fleshy
posterior part of the lower lip. The characteristic flap of the upper lip, which, when the
mouth is shut, in the modern forms extends don-nwards outside the lower jaw almost to
the level of the oral sensory line, was almost certainly developed in the Devonian forms as
well. In some forms (Dipterus)i t seems to have been stiffened by a series of cosmine-coated
dermal bones (Ulb,Text-fig. 1 A ; Jarvik, 1964: 4 1 4 5 ) . The similarities in the development
of the lips and the jaw apparatus as a whole support the view advanced by White (1965 :41)
that the feeding mechanism characteristic of the modern forms was developed in their
Devonian forerunners and that the food while being chewed was repeatedly extruded
from the mouth. Most likely also the characteristic mode of breathing arose early in the
phylogeny of the group.
The muscles associated with the lower jaw were certainly also developed very much as in
Neoceratodus. The striking similarities in the structure of the posterior part of the jaw
demonstrate that the m. adductor mandibulae was inserted into the rough dorsal area of
the preglenoid process and the adductor pit, and that a strong tendon extended forwards
in the groove between the dorsal margin of the lateral dermal bone and the posterior more
or less overhanging part of the principal tooth plate. Moreover the posterior face of the
symphysial portion sometimes has a paired area for the attachment of the m. genioeoracoideus and no doubt the m. intermandibularis arose from the ventro-medial exposed
area of the meckelian bone in the same way as in Neoceratodus. According to views advanced in a previous paper (Jarvik, 1963) this muscle represents the persisting musculature
of the mandibular gill-cover, which in the early dipnoans, as in the rhipidistids, was
supported by elements of the submandibular series. However, in the crossopterygians,
brachiopterygiam and actinopterygians the paired operculo-gular membranes which
include the mandibular gill-covers are generally separate forwards to or almost to the
symphysis of the jaws (Jarvik, 1963: 6), whereas in recent dipnoans, sharks, rays and
holocephalians they have fused completely. The fact that there is not only a median
submandibular but also a median gular (Urn, Text-fig. 5B) must mean that these membranes in the Devonian dipnoans had fused a t any rate as far back as to the posterior end
of the median gular plate.
With regard to the tooth-bearing elements, too, the Devonian forms agree well with
Neoceratodus, but a remarkable fact is that two of these elements, MdX and MdY, are
retained only in larvae. The main tooth-bearing bone, MdI, has been preserved practically
without change since the Devonian and in the early forms as in Neoceratodus i t forms part
of the adductor pit and has a medial expansion dorsally to the symphysial plate. The mandibular tooth plate, MdT, carried by and generally intimately connected with bone MdI,
in the Devonian forms most often shows rows of teeth converging towards the posteromedial part of the plate. As in Recent dipnoans it is composed of columnar pleromic hard
tissue and apparently i t grew ventrally into the supporting bone. With respect to isolated
teeth, as in Griphognuthus, they agree in their histological structure with those in other
dipnoans and they seem to be arranged somewhat as in larvae of Neoceratodus. As is best
shown in the palate, the tooth plates in the Devonian forms may be covered by a cosmine-
Structure of the dipnouiL lower jaw
177
like substance which seems to be subject tmo
a periodical reformation. This plienornenon is
possibly in some way related to t’he periodical formation of horny caps on the teeth in
extant dipnoans (Protopterus). The tooth-hearing element MdY corresponding t o the
sympliysial tooth plat,e in larvae of Neocerutodus is a small median structure situated
on the dorsal side of the symphysial plate, eit8herclose in front of bones MdI (Chirodipterus)
or partly between their anterior ends (Ilolodipferus, Rhinodipterus). The other toothbearing element, MdX, reta,ined in larvac of Neoceratodus, is in all Devonian forms in
which it is known (Chirodipterus, Dipterus, Holodipterus, Melanognathu.s, Palneda,phus,
Rhinodipterus, Xcaumenacia) a fairly strong semicircular bone which occupies about
the same position as the anterior part of t,he fleshy lower lip in Neocerutodus arid like
that sheaths the anterior convex margin of the symphysial plate. According to Holmgren
(1942) this element (‘the lipscalc ’) in larvae of Neoceratodus arises on t,he dorsal side
of the ‘labial cartilage’ which in the adult, forms the marginal part of t)he symphysial
plate.
Disregarding displacement in connection with fusion of bones the sensory canals of the
lower jaw in the Devonian forms run almost exactly as in Neoceratodus. The sensory canal
bones are arranged in t’wooriginally quite scparate series, one lateral pierced k)y the oral
canal and one medial by the mandibular ca,nal.
The oral series includes a t least three eleriients. Each of these elements is bent) arid consists of a vent,ro-medial horizontal portion and a lateral vert>icalportion. The horizontal
portion is situated on the vent>ralside of t l w nicckelian cartilage, in the ‘dipteritls’ i t is
covered by cosmine and if not displaced the oral canal passes through its lateral part. The
vertical portions form the lateral side of thc jaw. They always lack cosmine and have a
smoot,li external surface covered by the lolver lip. The anterior oral element, h3dOc1, is
situated in the symphysial portion. Its horizontal portion has a concave antero-lateral
margin which is the margin of t,he labial pit,. Its vertical portion forms tJhemain part, of the
wall of that pit. Bone MdOcl is independent, in Melanognuthus and seems to be separate
also in Dipterus and Scaumenacia. I n Nwcoratodus it has disappeared altogether. The
middle oral element, MdOc2, which has been found independent only in JIclan.oynathus,
is sit,uated mainly in the anterior part of the ramus. However, the vertical lamina extends
into thc symphysial portion and covers the posterior part of the lateral side of the symphysial plate. It has a charact,eristic point>ctlanterior end joined dorsally t o the posterolateral part’ of bone MdX and forming pa.rt>of the wall of the labial pit. The posterior oral
element, NdOc3, separate in Dipterus, is s i t J u a t din the articular portion. Its vertical
portion, which generally has a notch in its posterior margin, forms the lateral ascending
process and the lateral wall of the adductor pit.
The mandibular series includes two main elements. These elements, which in the
‘dipterids’ are coated with cosmine, form horizontal plates on the ventral side of the
meckeliaii bone or cartilage inside the oral series. The anterior element, IVIdMcl. which
according to the conditions in Dipterus possibly comprises two bones (RldMcla, MtlMclb),
occupies the postero-medial part of the vtlntral side of the symphysial plate. As to its
shape, position and relations to the mand ihiilar sensory canal and t,he medial commissural
branch it is very much as in Neoceratodics, in which however the sensory canals. due to
superficial reduction, run in open grooves. The posterior mandibular element, MdMc2,
has been found separate only in &Ieluno~nuthus.in which the mandibular sensory canal
runs forward in the medial part of the bone. In Dipterus bone MdMc2 has fused with the
adjoining bone of the oral series forming a complex bone, MdOc2Mc2. As a consequence
of this fusion the oral and mandibular sensory canals have become displaced and run close
toget’herthrough the centre of radiation of the complex bone. A similar displaccrnent has
occurred in Xcaumenacia. However, in this form the posterior element of t,he oral series,
MdOc3, has become incorporat.ed, too, and accordingly the large external bone of the jaw in
Smumenacia is composed of three originally independent canal bones (MdOc2, MdOc3,
MdMc2).This highly complex bone, MdE, with only insignificant changes, confined mainly
12
178
ERIKJARVIK
Text-fig. 8. Representations to demonstrate some of the fundamental differences in the
structure of the lower jaw between (A, C) Dipnoi, on the one hand, and (B, D, F) Teleostomi
and (E) Tetrapoda, on the other. A, C, Dipterus valenciennesi. Lower jaw in antero-lateral
and dorsal aspects (cf. Text-fig. 6). I n C bone MdY and marginal teeth of bone MdX have been
added and the left half of the symphysial plate has been tentatively restored. B, D, Holoptychius sp. Restorations based on material from the upper Upper Devonian of East Greenland.
E, Eryops megacephulus. From Sawin, 1941, pl. 5C. F, Pteronisculus magnus. From Nielsen,
1942, fig. 38.
C o l , C02, 0 0 3 , Coronoids; De, dentary; I d l , Id2, I d 3 , I d 4 , infradentaries; M d I , toothbearing bone on medial side of meckelian bone; MdOcl, anterior oral canal bone; MdOc2Mc2,
complex bone formed by fusion of middle oral and posterior mandibular canal bones; MdOc3,
posterior oral canal bone; M d T , principal mandibular tooth plate; M d X , M d Y , toothbearing bones corresponding to so named dental plates in larvae of Neoceratodus; Prart,
prearticular; ad.fo, adductor fossa; ad.p, adductor pit; ar.ad, rough dorsal area of preglenoid
Structure of t h P dipnoaiL lower jaw
179
to the reduction of the lateral sensory components of the canal bones and straightening
out of the sensory canals, has been retaiiird in Neoceratodus.
COMPARISO?: S A N D CONCLUSIONS
The lower jaw of the Dipnoi is an early specialized, on the whole uniform, st,ructure
which has changed very lit,tle during the known history of the group. Among ‘bony fish’
(‘Osteichthyes’) i t clearly represents a type of its own. It differs widely from that in
a,ctinopterygians (Text-fig. 8 F), brachiopterygians, crossopterygians (Text-figs 8 B, D),
and lower tetrapods (Text,-fig.8 E ) and the statement (Wat,son & Gill. 1923; Holnigren &
Stensio, 1936; Westoll, 1949; antl ot.hcm) that in the lower jaw there are significant
resemblances indicating a close relatior iship between t,he early dipnoans and tlie early
rhipidistid crossopt.erygians lacks fountlat ion.
In the rhipidist,ids, both in the porolepiforms and the ost’eolepiforms,the j a w (Text-fig.
8R, D), in sharp contrast, to cor~tlit~ions
ill t,lie contemporaneous dipnoans, are always
independent and there is no sympliysial plate and no ‘labial cartilage ’. The labial pit)has no
equivalent in the rhipidistids. There is no preglenoid process and the deep adtluctor fossa
differs considerably from the small atlduvt c)r pit’generally found in dipnoans. With regard
to the both-bearing bones and the dcntit ion the differences are so great that a comparison
is tlificult. The mandibular tooth plate and the peculiar tooth-bearing elemcmt’s (MdX,
JltlT) i n t>heanterior part, of the jaw in d i p o a n s have no equivalents in t’herlzipitlistids,
in v.liich there is a dentary (Be)lioniologoris t,ot,liat in tetrapods, a series of three coronoids
(C’ol, Co2, Co3) with marginal teeth antl prominent tusks, a prearticular (P~ai?)
and sometimes (porolepiforms) a paraspphysial tlont,al plate with tusks ( t . Y s y ) .The columnar
pleromic hard tissue characteristic of tlw tceth in dipnoans is lacking in the rhipidistids
and tlie mode of growth of tbe teeth is different,. Finally the oral series of canal bones,
which in the dipnoans is well developed and forms the main part of the exoslicleton of the
lateral and ventral sides of the jaw, is la.ckiiig in the rhipidistids.
The fact, that the dipnoans in the strucl w e of the lower jaw differ fundainentally from
the rhipidistids and other crossopterygians is quite in accordance with the results gained
by studies of the snout and other partsof the sliull, the vertebral column, the ribs, the finscales and the dermal fin-rays (Jarvik, 1912, 1952, 1954, 1959, 1964; White, 1966, 1966)
and it is a t present impossible t’o find :L single character indicating a close relationship
bctween Dipnoi and Crossopterygii.
The untenability of bringing these two groups into a common systematic unit. (‘Sarcoptprygii’ ; see Jarvik, 1964 : 40) has recently been strongly emphasized also by White (1965,
1966)and is further ilIustJratedbg the fa,.et t>Iiatthe rhipidistids in the structure of the lower
jaw agree fundamentally not only with t,etrapods but also with actinopterygians (see
Text-fig. 9). I n many other respects, too, the rhipidistids agree better with actinopterygians
than with dipnoans, and as I have repeat’tdly pointed out and expressed in ‘family trees’
(1955, 1960, 1964) one is more justified in classifying the crossopt,erygians together with
actinopt,erygians (and brachiopterygians) than with dipnoans. That is actually what
Goodrich (1909, 1930) did in excluding the Dipnoi from the Teleostorni, but, it remains
uncertain whether the dipnoans should be placed together with arthrodires as was customary at the beginning of this century (Woodward, 1891 ; Goodrich, 1909).
It is true that dipnoans in certain regards, e.g. in the dentition (Gross, 1956: 133 ; White,
process for attachment of portion of addnct or musrle ;nr.Psy, modified part of dentar) carrying
parasymphysial dental plate; cot, coronoid tusk ; e . i n c , incurved dorsal edge of sl-mphysial
plate; h p l , h p 2 , hp3, horizontal portions of oral canal bones; Zuh.p, labial pit ; Tar, o r , niandibular arid oral sensory canals; pZ.Id2, pZ.Id$, pit>-linesof infradentaries 2 and 4: p r . p y l , preglenoid process; sy.pZ, symphysial plate; 1. Psy, tusks of parasymphysial dental plate ; u p l ,
wp2, wp3, vertical portions of oral canal bones.
180
ERIKJARVIK
1966: 7), agree with arthrodires and as I have recently pointed out (1964: 40)in the structure of the palate there are certain resemblances to holocephalians, which possibly are
surviving arthrodires. However, the similarities between dipnoans and holocephalians
are also distinct in the lower jaw. As in dipnoans the meckelian cartilages in holocephalians
have fused anteriorly forming a symphysial plate. I n front of that plate lies the paired
‘premandibular cartilage ’ which may be compared to the paired ‘labial cartilage’ in
Neocerutodus. On the antero-lateral side of the symphysial portion there is a flap-like
lip-fold suggestive of that in dipnoans and between the tooth plates of both sides there
Text-fig. 9. A, Holodipterus scrntacraccensis. Lower jaw in dorsal aspect. From GorizdroKulczycka, 1950, fig. 1. B, Neoceratodusforsteri. Lower jaw of larva (stage 48, Semon) in dorsal
aspect. From Semon, 1899, fig. B. C, Chirodipterus wildungensis. Parts of the jaw shown in
Fig. 4B. D, E, Myrimanthus pczradoxzis. Dental plates of lower jaw in medial and dorsal
aspects. From Dean, 1906; figs. 119, 130.
M d I , Tooth-bearing bone on medial side of meckelian bone ; MdT, principal mandibular
tooth plate; M d X , M d Y , tooth-bearing bones corresponding to so named dental plates in
larvae of Neoceratodzm; Prsy, ‘ pre-symphysial ’ dental plate; Zab.p, labial pit; m.dep, median
depression for ‘tongue’: ‘ t o ’ , ‘tongue’; 0 1 ~ 0 3 ,pi,pz, pl, larval teeth offuture principal mandibular tooth plate; q,q,teeth of tooth plate M d X ; y. teeth of tooth plate M d Y .
is a median space occupied by the ‘tongue ’ very much as in dipnoans. The dentary and the
coronoid series of the teleostomes and tetrapods have no equivalents in the holocephalians
either. I n them the main biting element, as in dipnoans, is the mandibular tooth plate
which also in holocephalians is composed of columnar pleromic hard tissue (Orvig, 1967).
I n front of the mandibular tooth plates ( M d T )some fossil holocephalians (Myriacanthus)
present a median element (Prsy, Text-fig. 9D, E) suggestive of bone MdY in Chirodipterus (Text-fig. 9C). Finally it may be added that a well-developed oral sensory canal
is present also in holocephalians and if the exoskeletal elements developed in relation to the
sensory canals are sensory canal bones, as suggested by Holmgren (1942: 217), an oral
series of canal bones is represented in holocephalians as well.
No doubt the dipnoans in the structure of the lower jaw agree better with holocephalians
than with teleostomes and tetrapods. However, in many important respects, e.g. in the
Structure of tkP dipman lower jair
181
presence of ‘lungs’ (cf. Brien, 1963; Jarvilc. 1965: 117-118) there are considerable differences and although probably more closely related to holoceplialians than to teleostomes
they cannot be classified together ith tlw former or any other elasinobrancliioniorphs.
As recently maintained also by White (1965. 19ciG) the Dipnoi represent an early specialized isolated group and their origins and relationships are wrapped in obscurity.
ACKKOM 1,EDCEMENTS
The present paper n as prepared in the f’;tleozoological Section of the Swedish Museum
of Katural History. For technical assistaiicr I am indebted to Miss A. Brasch (preparation
of fossils), Mr U. Sainuelsson (photography) and Mr C. Salguerio (retouch u ork and
drav ings). For the generous loan of niaterinl I wish to express my sincere gratitude to the
authorities of the National n’luseuni of C‘anada, the Rlanchester Museum, the Geological
Survey and Museum, London, the Royal Scottish Museum, Edinburgh, the l’aleontological Museum of the Hurnboldt University. Berlin and the Museum of Natural History,
Gothenburg.
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