Basicranial morphology and ontogeny of the extinct giant lemur Megaladapis



The extinct Holocene giant lemurs of Madagascar—Magaladapis, Palaeopropithecus, and Archaeoindris—possess a suite of basicranial features not found in any other lemuriforms: (1) ectotympanic not “free,” (2) external acoustic canal elongated, (3) tympanic floors not swollen, and (4) reduced internal carotid and stapedial arteries. According to some authors, the first three traits are strongly convergent on morphological conditions in catarrhine primates. Study of juvenile and adult stages of Megaladapis indicates that the actual degree of convergence, from an ontogentic perspective, is rather slight. Giant lemurs, as a consequence of their massive body size, exhibit more exaggerated versions of traits characteristic of all members of infraorder Lemuriformes. (1) Although the ectotympanic is expanded and fused to the bulla, fusion results from complete obliteration of the recessus dehiscence; anthropoids have a different ectotympanic-petrosal relationship and never exhibit a recessus dehiscence. (2) Although the external acoustic canal is elongated, its exterior portion is formed by the petrosal, and not by the ectotympanic as in anthropoids. (3) The volume of the middle ear is negatively allometric with respect to body size; in the absence of specific sorts of auditory specialization, mammals having the projected body sizes of the giant lemurs would be expected to have relatively small tympanic cavities, and therefore to lack prominent bullae. (4) The internal carotid systems of palaeopropithecines and Megaladapis are absolutely and relatively reduced; to compensate for this, these lemurs apparently relied on the vertebrobasilar system for most of their cerebro-orbital supply, and on divisions of the external carotid for meningeal supply. It is important to note that the basicrania of giant lemurs display the same essential combination of traits that serve to distinguish lemurs from other euprimates (e.g., aphaneric ectotympanic, importance of the vertebrobasilar system, entry point of internal carotid stem into middle ear). This implies that the ontogenetic basis for these features is not easily modified and provides a justification for the heavy weight traditionally placed upon this region by paleoprimatologists.