Unveiling the neuroanatomy of Josephoartigasia monesi and the evolution of encephalization in caviomorph rodents.

Caviomorph rodents are an exceptional model for studying the effects of ecological factors and size relations on brain evolution. These mammals are not only speciose and ecologically diverse but also present wide body size disparity, especially when considering their fossil relatives. Here, we described the brain anatomy of the largest known rodent, Josephoartigasia monesi, uncovering distinctive features within this species regarding other taxa. Albeit resembling extant pacarana Dinomys branickii, J. monesi stands out due to its longer olfactory tract and well-developed sagittal sinus. Challenging the previous hypothesis that giant rodents possessed comparatively smaller brains, we found that J. monesi and another giant extinct rodent, Neoepiblema acreensis, are within the encephalization range of extant caviomorphs. This was unraveled while developing the a Phylogenetic Encephalization Quotient (PEQ) for Caviomorpha. With PEQ, we were able to trace brain-size predictions more accurately, accounting for species-shared ancestry while adding the extinct taxa phenotypic diversity into the prediction model. According to our results, caviomorphs encephalization patterns are not the product of ecological adaptations, and brain allometry is highly conservative within the clade. We challenge future studies to investigate caviomorphs encephalization within different taxonomic ranks while increasing the sampled taxa diversity, especially of extinct forms, in order to fully comprehend the magnitude of this evolutionary stasis.

Morphology and postnatal ontogeny of the cranial endocast and paranasal sinuses of capybara (Hydrochoerus hydrochaeris), the largest living rodent.

Recent studies have analyzed and described the endocranial cavities of caviomorph rodents. However, no study has documented the changes in the morphology and relative size of such cavities during ontogeny. Expecting to contribute to the discussion of the endocranial spaces of extinct caviomorphs, we aimed to characterize the cranial endocast morphology and paranasal sinuses of the largest living rodent, Hydrochoerus hydrochaeris, by focusing on its ontogenetic growth patterns. We analyzed 12 specimens of different ontogenetic stages and provided a comparison with other cavioids. Our study demonstrates that the adult cranial endocast of H. hydrochaeris is characterized by olfactory bulbs with an irregular shape, showing an elongated olfactory tract without a clear circular fissure, a marked temporal region that makes the endocast with rhombus outline, and gyrencephaly. Some of these traits change as the brain grows. The cranial pneumatization is present in the frontal and lacrimal bones. We identified two recesses (frontal and lacrimal) and one sinus (frontal). These pneumatic cavities increase their volume as the cranium grows, covering the cranial region of the cranial endocast. The encephalization quotient was calculated for each specimen, demonstrating that it decreases as the individual grows, being much higher in younger specimens than in adults. Our results show that the ontogenetic stage can be a confounding factor when it comes to the general patterns of encephalization of extinct rodents, reinforcing the need for paleobiologists to take the age of the specimens into account in future studies on this subject to avoid age-related biases.

Cranial Morphology of the Late Oligocene Patagonian Notohippid Rhynchippus equinus Ameghino, 1897 (Mammalia, Notoungulata) with Emphases in Basicranial and Auditory Region.

"Notohippidae" is a probably paraphyletic family of medium sized notoungulates with complete dentition and early tendency to hypsodonty. They have been recorded from early Eocene to early Miocene, being particularly diverse by the late Oligocene. Although Rhynchippus equinus Ameghino is one of the most frequent notohippids in the fossil record, there are scarce data about cranial osteology other than the classical descriptions which date back to the early last century. In this context, we describe the exceptionally preserved specimen MPEF PV 695 (based on CT scanning technique and 3D reconstruction) with the aim of improving our knowledge of the species, especially regarding auditory region (petrosal, tympanic and surrounding elements), sphenoidal and occipital complexes. Besides a modular description of the whole skull, osteological correlates identified on the basicranium are used to infer some soft-tissue elements, especially those associated with vessels that supply the head, mainly intracranially. One of the most informative elements was the petrosal bone, whose general morphology matches that expected for a toxodont. The endocranial surface, together with the surrounding parietal, basisphenoid, occipital, and squamosal, enabled us to propose the location and communication of main venous sinuses of the lateral head wall (temporal, inferior and sigmoid sinuses), whereas the tympanic aspect and the identification of a posterior carotid artery canal provided strong evidence in support of an intratympanic course of the internal carotid artery, a controversial issue among notoungulates. Regarding the arrangement of tympanic and paratympanic spaces, the preservation of the specimen allowed us to appreciate the three connected spaces that constitute a heavily pneumatized middle ear; the epitympanic sinus, the tympanic cavity itself, and the ventral expansion of the tympanic cavity through the notably inflated bullae. We hope this study stimulates further inquires and provides potentially informative data for future research involving other representatives of the order.