Virtual endocast of Ignacius graybullianus (Paromomyidae, Primates) and brain evolution in early primates

Abstract

Extant primates are distinctive among mammals in having relatively large brains. As stem primates, Paleogene plesiadapiforms provide direct information relevant to the earliest stages in the evolution of this characteristic. Here we describe a virtual endocast reconstructed from ultra high resolution X-ray computed tomography data for the paromomyid plesiadapiform Ignacius graybullianus (USNM 421608) from the early Eocene of Wyoming. This represents the most complete endocast known for a stem primate, allowing for an unprecedented study of both size and fine details of anatomy. Relative to fossil and extant euprimates, I. graybullianus had large olfactory lobes, but less caudal development of the cerebrum and a poorly demarcated temporal lobe, suggesting more emphasis on olfaction and a less well developed visual system. Although its brain was small compared to those of extant primates, the encephalization quotient of I. graybullianus is higher than that calculated for Paleocene Plesiadapis cookei and overlaps the lower portion of the range documented for fossil euprimates. Comparison to the basal gliroid Rhombomylus suggests that early primates exhibited some expansion of the cerebrum compared to their ancestors. The relatively small brain size of I. graybullianus, an arboreal frugivore, implies that neither arboreality nor frugivory was primarily responsible for the expanded brains of modern primates. However, the contrasts in features related to the visual system between I. graybullianus and fossil and extant euprimates suggest that improvements to these portions of the brain contributed to increases in brain size within Euprimates.computed tomography ͉ Eocene ͉ plesiadapiforms ͉ vertebrate paleontology ͉ Wyoming

Endocranial morphology of PalaeocenePlesiadapis tricuspidensand evolution of the early primate brain

“…Contrary to other known plesiadapiforms, the olfactory bulbs of P. tricuspidens are only slightly inflated dorsally, and in distinction from most primates, they are overlain by a thick roof of bone (figure 1a,b; as in P. cookei; electronic supplementary material, figure S2). The crushed bulbs represent 4.9% of the overall deformed braincase volume (table 1), which is congruent with the observation that plesiadapiforms have relatively large olfactory bulbs (5.53% for I. graybullianus; 5.10% for M. annectens [9,10]) compared with Euprimates (e.g. 3% for Adapis parisiensis; 3.4% for Tetonius homunculus [10]).…”

“…Lateral to the partial cast of the hypophyseal fossa is the cast of the foramen ovale (cranial nerve V3). As observed in I. graybullianus [9], a wide canal received the postglenoid vein. This communicates dorsally with a parietal foramen, which probably transmitted the capsuloparietal emissary vein [31].…”

“…The small anterior inflection of the rhinal fissure may represent the presylvia. The absence of a sylvian fissure, and, concurrently, of any demarcation of the temporal lobe, contrasts with the condition found in most Euprimates [2,16,20,24], but is found in other living and fossil euarchontoglires and in all plesiadapiforms known from the relevant part of the endocast [9,10]. The neocortical part of the cerebrum of P. tricuspidens is homogeneously smooth, except for a very slight inflection corresponding to the lateral sulcus, observed parallel to the sagittal sinus in the anterior half of the cerebral hemisphere (figure 2a).…”

“…The large inflection located in the first third of the hemisphere is here interpreted as a widely open sylvian fossa (figure 1b). As described for Ignacius and Microsyops [9]), (e) P. tricuspidens and (f ) Tetonius homunculus (after [16]). bsps, basisphenoid paratympanic sinus; cev, capsuloparietal emissary vein canal; cf, circular fissure; fp, fissura prima; ls, lateral sulcus; ob, olfactory bulb; on, optic nerve; ot, orbitotemporal canal; ov, ophthalmic vein; pf, paraflocculus; pgv, postglenoid vein canal; ps, presylvia; pss, presphenoid sinus; rf, rhinal fissure; sf, sylvian fossa; sov, suboptic vein canal; vm, vermis.…”

“…Just ventral to the parafloccular lobe, casts of the internal auditory meatus (cranial nerves VII and VIII) are preserved. On the caudal aspect of the endocast, the jugular foramen most probably transmitted cranial nerves IX, X and XI, as well as the internal jugular vein [9]. The cast of the hypoglossal foramen (cranial nerve XII) is visible on the brainstem.…”