Description of a Cranial Endocast from the Fossil Mammal Vincelestes neuquenianus (Theriiformes) and its Relevance to the Evolution of Endocranial Characters in Therians

Abstract

We generated a digital cranial endocast (infilling of the braincase) of Vincelestes neuquenianus, a Cretaceous theriiform mammal from Argentina, to achieve two goals. First, we described this endocast of Vincelestes to reconstruct the brain, associated soft-tissue structures, and internal osteological features. This report represents the first description of an endocast from a stem therian that is near crown group Theria (marsupials, placentals, and all descendants of that ancestor). Second, we examined 21 morphological characters related to the morphology of endocasts and endocranial osteology across 19 taxa (including Vincelestes) in the context of a current hypothesis about mammal phylogeny to identify potential synapomorphies for Theria. The digital endocast of Vincelestes is mostly complete, facilitating description in all views and allowing collection of accurate linear and volumetric measurements. However, it is unclear if the midbrain is exposed on the dorsal surface of the brain because of damage to this region of the endocast. Other portions of this specimen are extraordinarily well preserved, allowing identification of the accessory olfactory bulbs on the endocast, an ossified falx cerebri, and an osseous tentorium. The encephalization quotient (EQ) calculated for Vincelestes falls within the range of EQs of crown therians. Comparison of the endocranial characters across different mammalian taxa did not reveal any new synapomorphies for the clade Theria. Anat Rec, 290:875-892, 2007Rec, 290:875-892, . 2007 Wiley-Liss, Inc.Key words: computed tomography; encephalization quotient; Mammalia; fossil; paleoneurology; brain evolutionThere are over 5,400 species of extant mammals (Wilson and Reeder, 2005) that belong to two major clades, Theria and Monotremata (Fig. 1). Mammals are members of the clade Mammalia, which was defined phylogenetically as including the most recent common ancestor (hereafter ''MRCA'') between extant therians and extant monotremes, and all descendants of that ancestor (Rowe, 1988). Monotremata consists of the MRCA of all extant monotremes (four species of echidnas and the duckbill platypus; Wilson and Reeder, 2005) and all descendants of that ancestor. Theria contains the remaining overwhelming majority of extant mammalian species (5,412 therian species vs. 5 monotreme species;

Supplemental Material for 'Exceptional skull of Huayqueriana (Mammalia, Litopterna, Macraucheniidae) from the late Miocene of Argentina : anatomy, systematics, and paleobiological implications. (Bulletin of the American Museum of Natural History, no. 404)'

“…22B) than in Te. lucarius (Simpson, 1933), which is consistent with a negligible or absent tentorium cerebelli (Macrini et al, 2007b). The volume of the cerebellum c forms 22% of the total endocast, which is smaller than its contribution in Te.…”

“…Anatomical terminology generally conforms to, or is adapted from, nomina recognized in Terminologia Anatomica (e.g., Schaller, 1992); for more specialized terms, we utilized MacPhee (1981MacPhee ( , 2014 for the caudal cranium, Soria (2001) for the dentition, and Macrini et al (2007aMacrini et al ( , 2007b for the cerebral endocast. The description of the inner ear assumes that the lateral semicircular canal is oriented horizontally and centrally within the surrounding array of anatomical structures (Graf and Klam, 2006;Ekdale, 2013).…”

“…Mimics 18.0 (Materialise NV, Leuven, Belgium) was used for 3D reconstruction of the petrosal and VGStudio MAX (Volume Graphics GmbH, Heidelberg, Germany) for the osseous inner ear. Linear and volumetric measurements of the endocast were calculated with 3D Slicer following standard dimensions proposed by Macrini et al (2007b); angular measurements were calculated with the program ImageJ; linear measurements of the bony labyrinth were calculated with Mimics following the protocole of Schmelzle et al (2007) for the dimensions of the semicircular canals. Height and width of the cochlea were measured in profile.…”

“…Forebrain region: In life, the most anterior structures of the endocast would have housed the olfactory bulbs and associated meninges (Macrini et al, 2007b). The bulbar portion of the endocast is small in IANIGLA-PV 29, contributing only ~4% of the total volume (table 4).…”

“…Asterisk (*) indicates depression on right lateral piriform lobe. tory tracts projecting from the cortex (Macrini et al, 2007a(Macrini et al, , 2007b.…”