The cranial endocast of the largest rodent and a comparative analysis of the encephalization in caviomorphs

FERREIRA, JOSÉ D.; RINDERKNECHT, ANDRÉS; DOZO, MARÍA TERESA; SANCHEZ VILLAGRA, MARCELO; KERBER, LEONARDO

Congreso; 3rd Virtual Palaeontological Congress; 2021

The high ecomorphological diversity of caviomorph rodents in South America includes extinct forms that have reached giant dimensions. The largest known rodent was the Dinomyidae Josephoartigasia monesi from the Pliocene of Uruguay. This work aims to study the neuroanatomy and encephalization of J. monesi (MNHN 921, San José For-mation, Uruguay) using virtual endocranial models. For this study 69 specimens of most caviomorph families were also scanned (CT-Scan). To estimate the encephalization quo-tient (EQ) of each specimen, we employed the endocast volume converted to brain mass and estimated the body mass based on the average of linear regressions from eight cranial linear measurements. The EQ was estimated based on an equation designed specifically for rodents. Our preliminary results demonstrate that the cranial endocast of J. monesi shows: a long olfactory tract, expansion of the frontal lobes, gyrencephaly, superior sagittal sinus forming a longitudinal ridge on the endocast surface, marked rhinal fissure, and absence of evident paraflocculi. The estimated body mass is 845 kg, and the volume of the endocast is 192,144 mm³. The estimated EQ of J. monesi is 0.31. This is a low encephalization coefficient when compared to extant caviomorphs which we analyzed here (Erethizontidae: 1.04-1.77, Caviidae: 0.75-1.18, Cuniculidae: 1.04-1.44, Dasyproctidae: 1.13-1.33, Chinchillidae: 0.85-1.49, Dinomyidae: 0.97, Echimyidae: 0.76-1.13, Abrocomyidae: 0.94, Octodontidae: 0.88-0.99), a pattern also found recently in neoepiblemids (0.36), another lineage of giant rodents. It has been hypothesized that the adaptive value of a low energetic cost and other ecological factors could explain the pres-ence of relative small brains in giant Neogene rodents.