Download Megaladapids of Madagascar

Megaladapids of Madagascar
Cody Raskin
ASM 394
Certainly, the more interesting primates of study are those closest in form
to the basil primate, the strepsirrhines of Southern Asia and Madagascar. And
the oddest of these primates were the extinct primate group Megaladapidae. For
reasons which are the subject of much debate, this group of Madagascar
primates diverged from the common primate form, into animals that only barely
resembled
primates.
Species
include
the
‘pig-like’
M.
edwardsi,
M.
madagascariensis, and M. grandidieri.
Megaladapis fossils are found in southern and central Madagascar, and
living representatives would have ranged in size from 40 to 80 kg. These
primates arrived comparatively late in the phylogenetic history, after the other
large Lemuriformes (Hadropithecus, Archaeolemur, and Paleopropithecus). They
were entirely unique among primates, and no primate before or since has
possessed the range of peculiar traits unique to Megaladapids.
It has been suggested that Megaladapids were closely related to the
extant family Lepilemuridae, which today contains only one living genus
(Lepilemurs, also called Sportive Lemurs). And indeed, many have placed
Megaladapids into a subfamily (Megaladapinae) of the larger Lepilemuridae
family (Vuillaume-Randriamanantena, 1992). Evidence of this relationship is
found in the dental remains of fossil Megaladapids. Megaladapids shared with the
Lepilemurs the long, narrow molars and large molar crests, and most strikingly of
all, the lack of upper incisors (Megaladapids had a dental formula of 0133/2133).
These dental features are indicative of a predominately folivorous diet. The lack
of upper incisors is consistent with folivorous eating patterns, which include
‘cropping’. However, there is no direct evidence of a pre-maxillary pad for this
function. Since it is unlikely the loss of the upper incisors occurred for any other
reason, it may be assumed that Megaladapis did indeed have this feature, as
each of its extant analogues do. Megaladapids also possessed a large mandibular
condyle, which is not entirely unusual for a large folivorous animal, but is unique
among strepsirrhines, and shared only by Lepilemurs (Fleagle, 1999).
Due to their size, it was necessary for Megaladapids to possess certain
functional characteristics that no other Madagascar primates have. One such
feature is the fused mandibular symphasis. While this feature is present in all
New World and Old World primates, including Apes and Humans, it is not so in
the extant primates of Madagascar, which includes the Lemurs and Lepilemurs,
nor in the Lorisids and Galagos of Southern Asia or the Tarsiers. This is clearly a
derived trait among Megaladapids, which having been absent, would have
certainly posed a difficulty for these large primates during mastication.
Among the traits that are unique to
Megaladapids is the position of the foramen
magnum. In all primates, extant or otherwise,
the foramen magnum is directly underneath the
skull (with the exception of howler monkeys
where it is near 45º), and in Megaladapids, it is
directly behind the skull. This creates a very
‘un-primate-like’
Furthermore,
skull
position
(below).
Megaladapis orbits were not
forward facing as in all other primates, but
instead faced outward as in other, non-primate
herbivores. The comparisons at right of M. edwardsi to a modern gorilla and
lemur respectively illustrate these peculiar differences. What is also evident from
these
remains
is
that
Megaladapis
possessed very large teeth for its skull
size, which is indicative of a leaf diet, and
a very small brain case with enormous
sagital and nuchal crests, atop a much
elongated snout (the cranial length for M. edwardsi is 277-317 mm, 235-44 mm
for M. madagascariensis, and 273-300 mm for M. grandidieri) (Jenkins,
Preuschoft, Tattersall, 1995). The nasal region at the end of this long snout is
grossly oversized, and this suggests that Megaladapis may have had large,
prehensile lips – a feature which is not found in any other primate (Fleagle,
1999). The zygomatic arches as well, are enlarged in this skull.
Unique
characteristics
of
the
post-cranial
remains
include
the
extraordinarily long hands and feet, coupled with short, stubby legs and arms of
approximately equal length. Thus, it was likely to practice quadrupedalism when
on the ground (humeri and femurs of the edwardsi species are more equal in
length, suggesting it was the most quadrupedal of the 3 Megaladapinae species),
but would have been more adept at climbing. However, it would not have
practiced the same sort of vertical clinging and leaping as other strepsirrhines
(Simmons). Many have come to label its peculiar post-cranial form as Koala-like.
Indeed, this animal may very well have had the same positional and feeding
behavior as that of the Australian Koala. Some of the bones in this image of M.
Edwardsi below have been reconstructed, including the mandible, a complete
fossil of which has not yet been found.
One thing that is evident in this image of M. edwardsi, though, is that the
phalanges on the hands and feet are extremely curved. While this animal may
have been a quadruped on the ground, it would not have been very well suited
for ground travel. And since it has been established that Megaladapis did not
leap from tree to tree, trips between trees on the ground would have been
especially perilous for this animal due to predation. Perhaps this is the reason for
the outward-facing orbits, which would have given it earlier warning of a
predatory attack. However, it has been suggested that Megaladapids evolved at
a time when Madagascar was relatively devoid of predators, and perhaps the
extreme elongation of its snout ‘pushed’ the orbits into an outward-facing
position.
Megaladapis seems to have become extinct at about the same time as
another large pro-simian Paleopropithecus, at a time which coincides with human
colonization of Madagascar. It is likely that a combination of environmental
changes brought by the arriving humans (foreign, feral species, livestock
overpopulation, and deforestation) along with the possibility of having been
hunted by humans brought this animal to extinction. As professor Kaye Reed is
keen to point out, often when humans arrive, large animals disappear.
The presence of many of Megaladapis features alone, such as outwardfacing eyes, neglecting an evolutionary context, is intriguing, since this is a
derived trait from the familiar primate condition of forward-facing eyes. The
posterior foramen magnum, too, is derived, and these traits that represent a
return to the ‘pre-primate’ condition, are reminiscent of types of changes
experienced by Mesonichids. While there were other large pro-simians living in
Madagascar, none possessed such strange characteristics. Megaladapids were
entirely unique in the order of primates, and their particular features nearly
exclude them from being labeled primates. Whatever the environmental
conditions that drove their evolution into such a strange direction, they definitely
illustrate the potential variability inherent in such a generalized order as
primates.
Sources
Fleagle, John G. (1999). Primate Adaptation and
Evolution. New York: Academic Press
Simmons, Elwyn L. (unk.). Fossil Collection at the DUPC.
Internet Resource
Jenkins; Preuschoft; Tattersall (1995). Koala Lemurs.
Johns Hopkins University Press