Download ORIGIN OF EXTANT COELACANTHS

Part : ZOOGEOGRAPHY OF COELACANTH
ORIGIN OF EXTANT COELACANTHS
Teruya Uyeno
Yoshitaka Yabumoto
Curator Emeritus
National Museum of Nature and
Science, Japan
Chief Curator / Researcher
Kitakyushu Museum of
Natural History & Human History
At present, about 80 fossil species of coelacanths and
two extant species, Latimeria chalumnae from the western
Indian Ocean and L. menadoensis from Indonesia have been
known. he oldest fossil is Miguashaia from Late Devoinan
(about 365 million years ago) and the peak of diversity was
Early Triassic (from about 248 million years ago to 242
million years ago). The latest fossil record is Macropoma
from Late Cretaceous (about 90 million years ago). Extant
coelacanths live in deep sea, but fossil species lived in both
freshwater and marine.
Suborder Latimeroidei, which the extant coelacanths
belong to, consists of two families, Mawsoniidae and
Latimeriidae. New coelacanths, Mawsonia brasiliensis
Yabumoto, 2002 and an undescribed species, in the family
Mawsoniidae have recently been found from Brazil.
The undescribed species probably bridges between the
Triassic coelacanth in North America and the Cretaceous
coelacanths in South America and Africa.
It has been considered that extant coelacanths, which
belong to the family Latimeriidae, are closely related to
the Late Cretaceous Macropoma. Clément (2005; 2006)
described, however, Swenzia latimerae from the Late
Jurassic in France and suggested that it is closest to extant
coelacanths. If this is the case, extant coelacanths might
have been derived from the older group of the Late Jurassic
than the group of the Late Cretaceous.
We will present recent paleontological studies of
coelacanths and discuss the origin of extant coelacanths.
Among extant ishes, the coelacanths and lungishes
are considered closest to tetrapods. Key diagnostic
features of the coelacanth include a hinged cranium
(articulated anterior and posterior sections); a bony
forelimbs and hindlimbs supporting the pectoral and
pelvic inrays; paired gular plates; abbreviated diphycercal
tail; and a tubular vertebral column (coela = tube; acanth =
spine).
Extant coelacanths are known to inhabit open marine
waters from about 100 to 500 meters depth and caves at
about 200 m depth. Viviparous, the coelacanth gives birth
to up to 30 young as far as we know.
To date, two species of extant coelacanths have been
discovered: Latimeria chalumnae from southeastern Africa
(Indian Ocean) and L. menadoensis from Indonesia; and of
about 120 fossil species reported, about 85 are currently
recognized as valid (Forey, 1998). One of the earliest
24
T h e C o e l a c a nt h , Fa t h o m t h e My s t e r y 2 0 0 7
Latimeria chalumnae
L. menadoensis 2
80
3
6500
Miguashaia
2
4800
Macropoma
2
4200
Mawsonia brasiliensis Yabumoto, 2002
2005
Macropoma
Macropoma
Swenzia latimerae
Swenzia
Clément, 2005
2006
fossils is Miguashaia (Fig. 1) from the Canadian Late
Devonian (ca 365 ma), and peak diversity (26 species),
Early Triassic (from ca 248 – 242 ma). Macropoma from
the Late Cretaceous (ca 90 ma) is the latest coelacanth
fossil genus. About 20 species have been found from the
Cretaceous Period. The fossil record ends at this period.
Although extant species live in marine waters, the fossil
record shows species from freshwaters, as well.
Fig. 1. Miguashaia sp. from the Canadian Late Devonian (ca 365 ma).
To best analyze the origin of the extant species it is
necessary to investigate the systematic relationships of
the fossil coelacanths. Forey (1998) analyzed the fossil
record and extant species based on 108 characters. Of
these, 65 % of the characters from 30 species could be
used, and Porolepiformes was used as the outgroup to
form a phylogenetic tree (Fig. 2).
Fig. 4. The five alternative parsimonious cladograms of the
suborder Latimeroidei ater Clément (2005).
Fig. 2. he phylogenetic tree compared to stratigraphy, simpliied
from Forey (1998).
In this tre e, ex tant Latime ria is close st to the
Cretaceous Macropoma, form a clade with Early Jurassic
Holophagus and Late Jurassic Undina, and this clade
composes the family Latimeriidae. Another clade includes
Triassic Garnbergia, Triassic to Jurassic Diplurus, Late
Triassic Chinlea, Late Jurassic Libys, and Late Cretaceous
Mawsonia and Axelrodichthys. This latter clade forms the
family Mawsoniidae. These two families form the suborder
Latimeroidei (Forey, 1998).
Clément (2005; 2006) described Swenzia latimerae
from the late Jurassic of France, and on the basis of his
discovery, revised Forey’s (1998) data matrix adding this
species. Based on this analysis, Clément devised a new
tree with the result that Diplurus, Chinlea, Mawsonia and
Axelrodichthys form a clade, and the extant coelacanths
and Swenzia have the closest relationship (Figs. 3, 4).
Other coelacanths
Other latimerioids
Latimeria
Swenzia
Diplurus
Chinlea
Mawsonia
Axelrodichthys
Fig. 3. he phylogenetic tree, simpliied from Clément (2005; 2006).
T h e h a b i t at of M a ws o n i a a n d A xe l r o d i c ht hy s
(Cretaceous South America and Africa) probably were
marine, while that of Diplurus and Chinlea ( Triassic
North America) were freshwater. Santana Basin fossils
have been determined to come from a shallow sea or
freshwater beds (probably a shallow sea bed). Mawsonia
brasiliensis has been found to reach about 1.5m TL. M.
lavocati from Morocco, Africa reaches more than 3.5m
TL. Freshwater North American Triassic Mawsoniidae
predated the appearance of marine South American /
African members of the family prior to division of the latter
two continents. About the end of the Early Cretaceous,
Axelrodichthys araripensis, M. brasiliensis and M. gigas
appeared in South America while several species of
Mawsonia arose in Africa. Looking at the age of Mawsonia
coelacanths, there is a gap between Triassic Chinlea and
Early Cretaceous Mawsonia + Axelrodichthys. A newly
identiied coelacanth from the Jurassic of Brazil appears
to ill this gap (Yabumoto, in press).
On the other hand, the ancestor of extant latimeriid
coelacanths probably appeared in the Late Jurassic,
more precisely, toward the Late Cretaceous northwestern
Tethys Sea (Present day Europe) from which a number of
coelacanth fossils have been found. To date, no latimeriid
fossils have been found in Mesozoic and Cenozoic
deposits in other regions. According to Clément (2005),
the closest relative to extant coelacanths is not the
genus Macropoma, but rather, Jurassic Swenzia. If we
accept this idea, the coelacanth fossils of Cretaceous
Macropoma are not related to extant coelacanths and
no related fossils of this extant genus are found from the
Cretaceous Period.
I
The suborder Latimeroidei probably derived from
marine Whiteia. Mawsoniidae adapted to the freshwaters
of each continent after they drifted apart, and in the Late
Cretaceous, returned to the sea.
Alternatively, all the latimeriid coelacanths are
marine. Early Jurassic Holophagus and Late Jurassic
Undina + Swenzia lived in the western Tethys Sea. In
the Cretaceous, Macropoma lived in the same location,
but there is no fossil record related to extant species.
25
Part : ZOOGEOGRAPHY OF COELACANTH
A
B
This may indicate either no pertinent fossils have been
unearthed as yet, or that by the Cretaceous, the relatives
of extant species had already migrated to open ocean
depths. In the Late Jurassic, the great depths of the
Tethys Sea extended from its western edge (eastern
Africa) to its northern side. The ancestor of the extant
coelacanths probably arose in the western Tethys Sea at
the end of the Jurassic along the eastern coast of Africa,
including around Madagascar, and migrated to Indonesian
waters (Fig. 5A). During the Cretaceous, the Mozambique
Channel opened, extending the distribution to South
Africa (Fig. 5B). Following this, during the Eocene Period,
the Indian subcontinent moved close to Eurasia, and the
extant coelacanths were probably inhabiting great depths
of the sea (Fig. 5C). This geographic separation probably
led to the speciation of Latimeria chalumnae along the
eastern African coast and L. menadoensis in Indonesian
waters (Fig. 5D). This corresponds to the biogeographic
hypothesis for the Eocene isolation of the two extant
coelacanths (Springer, 1999; Inoue et al., 2005).
References
C
D
Fig. 5. A - C, coelacanth fossil localities and supposed distribution
of the ancestor of extant coelacanths (red) on paleogeographic
maps; D, distribution of the extant coelacanths. (after Plate
tectonic maps and Continental drit animations by C. R. Scotese,
PALEOMAP Project (www.scotese.com)).
26
T h e C o e l a c a nt h , Fa t h o m t h e My s t e r y 2 0 0 7
(1) Clément, G. 2005. A new coelacanth (Actinistia,
Sarcopterygii) from the Jurassic of France, and the
question of the closest relative fossil to Latimeria.
Journal of Vertebrate Paleontology, 25(3): 481 – 491.
(2) Clément, G. 2006. Swenzia, n. nov., a replacement
name for the preoccupied coelacanth genus Swenzia
Clément. Journal of Vertebrate Paleontology, 26(2):
461.
(3) Forey, P. L., 1998. History of the coelacanth fishes.
xiii+419pp. Chapman and Hall, London.
(4) Inoue, J. G., M. Miya, B. Venkatesh and M. Nishida.
2005. The mitochondrial genome of Indonesian
coelacanth Latimeria menadoensis (Sarcopterygii:
Coelacanthiformes) and divergence time estimation
between the two coelacanths. Gene, 349: 227 - 235.
(5) Scotese, C. R., 2001. Atlas of Earth History, Volume 1,
Paleogeography, PALEOMAP Project, Arlington, Texas,
52 pp.
(6) Springer, V.G., 1999. Are the Indonesian and western
Indian Ocean coelacanths conspecific: a prediction.
Environmental Biology of Fishes, 54: 453 - 456.
(7) Yabumoto, Y. 2002. A new coelacanth from the
Early Cretaceous of Brazil (Sarcopterygii, Actinistia).
Paleontological Research, 6(4): 343-350.
(8) Yabumoto, Y. (in press) A new Mesozoic coelacanth
from Brazil (Sarcopterygii, Actinistia). Paleontological
Research.