The Brain and Inner Ear of the Early Paleocene “Condylarth” Carsioptychus coarctatus: Implications for Early Placental Mammal Neurosensory Biology and Behavior

Abstract

Mammals underwent a profound diversification after the end-Cretaceous mass extinction, with placentals rapidly expanding in body size and diversity to fill new niches vacated by dinosaurs. Little is known, however, about the brains and senses of these earliest placentals, and how neurosensory features may have promoted their survival and diversification. We here use computed tomography (CT) to describe the brain, inner ear, sinuses, and endocranial nerves and vessels of Carsioptychus coarctatus, a periptychid "condylarth" that was among the first placentals to blossom during the few million years after the extinction, in the Paleocene. Carsioptychus has a generally primitive brain and inner ear that is similar to the inferred ancestral eutherian/placental condition. Notable "primitive" features include the large, anteriorly expanded, and conjoined olfactory bulbs, proportionally small neocortex, lissencephalic cerebrum, and large hindbrain compared to the cerebrum. An encephalization quotient (EQ) cannot be confidently calculated because of specimen crushing but was likely very small, and comparisons with other extinct placentals reveal that many Paleocene "archaic" mammals had EQ values below the hallmark threshold of modern placentals but within the zone of nonmammalian cynodonts, indicative of small brains and low intelligence. Carsioptychus did, however, have a "conventional" hearing range for a placental, but was not particularly agile, with semicircular canal dimensions similar to modern pigs. This information fleshes out the biology of a keystone Paleocene "archaic" placental, but more comparative work is needed to test hypotheses of how neurosensory evolution was related to the placental radiation. Anat Rec, 302:306-324, 2019.

Virtual endocranial and inner ear endocasts of the Paleocene ‘condylarth’ Chriacus: new insight into the neurosensory system and evolution of early placental mammals

“… Data for comparative sample are from Cameron et al. (), Orliac & O'Leary (), Ravel & Orliac (), Muizon et al. (), Orliac & Gilissen () and Orliac et al.…”

“…), Ca. coarctatus (AMNH 27601; Cameron et al., : fig. 2d) and Onychodectes tisonensis (AMNH 785; Napoli et al., : fig.1a).…”

“…Data for Protungulatum sp. (AMNH VP 118359), Hyopsodus lepidus (AMNH FM 143783) and Diacodexis ilicis (AMNH VP 16141) are from Orliac & O'Leary (); data for Carsioptychus coarctatus (AMNH 27601) are from Cameron et al ().…”

“…pelvidens ( NMMNH P‐62258); (E–H) Diacodexis ilicis ( AMNHVP 16141; Orliac et al., 2012b); (I–L) Hyopsodus lepidus ( AMNHFM 143783; Ravel & Orliac, ); (M–P) Protungulatum sp. ( AMNHVP 118359; Orliac & O'Leary, ); (Q–T) Alcidedorbignya inopinata ( MHNC 8399; Muizon et al., ); (U–X) Carsioptychus coarctatus ( AMNH 27601; Cameron et al., ). Scale bars: 1 mm.…”

“…Cameron et al. () hypothesized that Paleocene ‘archaic’ placentals may have had abnormally low EQ values compared with extant placentals, and perhaps even to their Mesozoic antecedents. The low EQ results for Ch .…”