First virtual endocasts of adapiform primates

Abstract

Well-preserved crania of notharctine adapiforms from the Eocene of North America provide the best direct evidence available for inferring neuroanatomy and encephalization in early euprimates (crown primates). Virtual endocasts of the notharctines Notharctus tenebrosus (n = 3) and Smilodectes gracilis (n = 4) from the middle Eocene Bridger formation of Wyoming, and the late Eocene European adapid adapiform Adapis parisiensis (n = 1), were reconstructed from high-resolution X-ray computed tomography (CT) data. While the three species share many neuroanatomical similarities differentiating them from plesiadapiforms (stem primates) and extant euprimates, our sample of N. tenebrosus displays more variation than that of S. gracilis, possibly related to differences in the patterns of cranial sexual dimorphism or within-lineage evolution. Body masses predicted from associated teeth suggest that N. tenebrosus was larger and had a lower encephalization quotient (EQ) than S. gracilis, despite their close relationship and similar inferred ecologies. Meanwhile, body masses predicted from cranial length of the same specimens suggest that the two species were more similar, with overlapping body mass and EQ, although S. gracilis exhibits a range of EQs shifted upwards relative to that of N. tenebrosus. While associated data from other parts of the skeleton are mostly lacking for specimens included in this study, measurements for unassociated postcrania attributed to these species yield body mass and EQ estimates that are also more similar to each other than those based on teeth. Regardless of the body mass prediction method used, results suggest that the average EQ of adapiforms was similar to that of plesiadapiforms, only overlapped the lower quadrant for the range of extant strepsirrhines, and did not overlap with the range of extant haplorhines. However, structural changes evident in these endocasts suggest that early euprimates relied more on vision than olfaction relative to plesiadapiforms, despite having relatively small endocranial volumes compared to extant taxa.

The cranium of Proviverra typica (Mammalia, Hyaenodonta) and its impact on hyaenodont phylogeny and endocranial evolution

“…), diet specialization (Harrington et al . ) and nocturnal behaviour may be associated with a higher EQ score (Bauchot & Stephan , ).…”

Eocene Paleoecology of Adapis parisiensis (Primate, Adapidae): From Inner Ear to Lifestyle

“…Values available in the literature for Adapis parisiensis BM, are, most of the time, estimations based on few specimens, generalized to the species (Table a), thus leading to very approximate values. For example, the BM values proposed by Jerison (), Radinsky (), Gingerich et al (), and more recently by Long et al () or Harrington et al () are based on only one or two specimens. In this study, we sought to include as many specimens of the fossil species as possible, and our results suggest that intraspecific variation needs to be investigated more fully.…”

Endocranial shape variation in the squirrel‐related clade and their fossil relatives using 3D geometric morphometrics: contributions of locomotion and phylogeny to brain shape

“…Endocasts are the imprint of the brain against the inner part of the cranium and are the best proxy for investigating brain shape and size in fossil mammals because brain tissues are not preserved in the fossil record (e.g., Edinger, ; Jerison, ). With increased access to Micro‐CT scanners, it is now possible to create endocasts virtually, expanding considerably the range of specimens from which endocasts can be acquired, and leading to new insights into how the brain of modern groups evolved (e.g., Silcox, Benham & Bloch, ; Orliac & Gilissen, ; Dozo & Martínez, ; Harrington et al ., ; Bertrand et al ., ).…”