Placentals are by far the most diverse group of mammals today, with 6,111 species. They occupy a plethora of ecological niches worldwide and display a broad range of body masses. The vacant niches left by non-avian dinosaurs and other vertebrates after the end-Cretaceous mass extinction provided a crucial opportunity for placentals to diversify; however, intrinsic factors also may have played a role. The general neurosensory organization exhibited by extant mammals has been maintained since the early Mesozoic. Much later, early members of extant placental groups from the Eocene and Oligocene including rodents, primates and artiodactyls—display neurosensory innovations such as a proportionally larger neocortex and higher encephalization quotient compared to their Mesozoic ancestors. However, between these two well-known intervals of mammalian neurosensory evolution, there is a gap: few studies have focused on the brains of the oldest placentals living during the early Paleogene, in the Paleocene. We focus on the ‘Arctocyonidae’, a likely polyphyletic group of ‘condylarths’, including species potentially implicated in the origins of some extant orders. ‘Arctocyonids’ were among the first placentals to diversify after the end-Cretaceous extinction. They have been reconstructed as small-tomedium sized, mainly omnivorous and terrestrial. We obtained cranial and bony labyrinth endocasts for Chriacus baldwini and C. pelvidens from the lower Paleocene of the San Juan Basin, New Mexico, and Arctocyon primaevus from the upper Paleocene of the Paris Basin, France, via high resolution computed tomography. Both share plesiomorphic brain features with previously described early Paleocene mammals. They have small lissencephalic brains with an EQ range of 0.12-0.43 and 0.16-0.31, respectively. The olfactory bulbs and the paraflocculi represent 6% and less than 1% of the total endocranial volume, respectively and the neocortical height ratio represents ~25% of the total endocranial height. Based on cochlear measurements, both taxa had hearing capabilities similar to those of extant wild boars. Agility scores between 2 and 3 were obtained for both taxa, similar to the modern American badger and crab-eating raccoon, suggesting that C. pelvidens and A. primaevus were ambulatory. These results support growing evidence that early placentals had lower EQs and less expanded neocortices compared to Eocene and later taxa, potentially indicating that high intelligence was not key to the placental radiation after the End-Cretaceous extinction. Grant Information: Marie Sklodowska-Curie Actions: Individual Fellowship, European Research Council Starting Grant, National Science Foundation, and Belgian Science Policy Office.
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RBINS Staff Publications 2019
The climate of the early Paleogene is characterized by short-scale temperature variations which are superimposed on a general trend of rising temperatures culminating during the late early Eocene (early Eocene climatic optimum, EECO). These include several transient periods of abrupt climate warming or ‘hyperthermals’, such as the PETM (~55 Ma). Profound proxy development is needed to successfully extract shorter-scale variability from suitable records and unravel its underlying mechanisms. This study assesses and extends the use of fossil fish otolith O and C stable isotopes as a paleotemperature and seasonality proxy for early Paleogene marginal marine sedimentary environments. Well-known limitations include the lack of accurate estimates for the oxygen isotope composition of ambient water, and potential bias when applying paleotemperature equations. Moreover, taxon inconsistencies for both O and C were observed, complicating data interpretation (Vanhove et al., 2011). A single locality test case in the southern North Sea Basin has been performed to address this observation (Egem, Belgium, coastal sands). In each of four fossiliferous levels sampled, the same three demersal otolith species were analyzed (Platycephalus janeti, Paraconger papointi and “genus Neobythitinorum” subregularis). Cross-plots of δ18O and δ13C isotopes show three statistically different data clouds, corresponding to the three taxa. Several processes can cause such discrepancies. The most likely option is the influence of freshwater influx. According to this interpretation, Paraconger sp. and Platycephalus janeti lived in coastal areas prone to freshwater influx, while “genus Neobythitinorum” subregularis inhabited more distal realms. This is confirmed by similar analyses on Callista sp. and Venericardia sp. bivalves of the same locality, because these were deposited relatively in situ compared with otoliths, which predominantly arrive in the sediment after post-mortem predation-related transport. Taxon-sensitive differential diagenesis is disproved by SEM, cold cathodoluminescence and X-ray diffraction investigations, revealing the presence of pristine aragonite in all cases. Bias resulting from variability in the amount of summer or winter carbonate deposition is contradicted by visual inspection of growth ring thicknesses, and cyclical incremental stable isotope patterns of individual growth bands. Taxon inconsistencies were not described previously by authors working on the same taxa and in the same area, hence the paleoecological interpretation of this data could indicate enhanced runoff and freshwater influx during the EECO relative to later time intervals, or the presence of a large river mound close the investigated location. Temperature calculations based on “genus Neobythitinorum” subregularis reveal mean annual temperatures around 27.5 °C and a seasonality of 9 °C for the EECO interval. Given the mentioned assumptions, future directions should include other quantitative, preferably salinity-independent paleotemperature proxies to test these data interpretations. VANHOVE D., STASSEN P., SPEIJER R. P. and STEURBAUT E., 2011. Assessing paleotemperature and seasonality during the early Eocene climatic optimum (EECO) in the Belgian Basin by means of fish otolith stable O and C isotopes. Geologica Belgica, 14(3-4): 143-158.
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