Endocranial morphology and paleoneurology in notoungulates: Braincast, auditory region and adjacent intracranial spaces

Abstract

The endocranial spaces of notoungulates (especially the endocranial casts) have been of special interest since the pioneer contributions of Serres (1867) and Gervais (1872). Despite a couple of features shared by notoungulates that differentiate their braincasts from that of others SANUs (e.g. presence of a suprasylvian sulcus, a well-developed temporal lobe, and the orbitotemporal canal being close to, or even concealing, the rhinal fissure), there is an interesting morphological diversity within the order. Such diversity ranges from almost lissencephalic and anteriorly narrow braincasts with cerebellum projecting posteriorly (e.g. Notostylops murinus) that grossly resemble that of some condylarths, to anteriorly wide and dorsoventrally developed braincasts showing a well-developed neocortex and a relatively complex gyrification pattern (e.g. later diverging toxodonts). Other endocranial spaces have also been extensively studied, such as the tympanic cavity proper, the hypotympanic cavity (quite conspicuous in several representatives with large auditory bullae), and the epitympanic sinuses (posterodorsally located on the lateral sidewalls of the skull in all but some early diverging representatives). These spaces configure a heavily pneumatized middle ear, although morphofunctional interpretations concerning this condition have been scarcely addressed in the literature.The relative brain size (quantified by means of the EQ) does not seem to increase or decrease according to our data. However, methodological artifact (e.g. those derived from body mass estimation methods or from an oversimplification of the allometry) or biases (e.g. single specimen estimates) led us to be cautious on this assertion. Forthcoming contributions should provide brain size/body size allometric parameters specific for notoungulates and take into consideration the phylogenetic signal. This would allow better-supported comparisons and more solid inferences about the major forces driving the brain size evolution of the group.The advent of computed tomography technology not only revitalized the study of the above-mentioned issues but also stimulated research on other areas, such as the caudodorsal and basicranial vasculature. Of special interest is the presence and course of the internal carotid artery. Although an intratympanic course was first proposed (and later followed by some researchers), recently reported data suggest that the pre¬vailing condition among notoungulates would be the opposite (i.e. an extratympanic course). As for the venous vasculature, the relative development of the transverse, temporal and sigmoid sinuses (and their emissary veins and connectors) show that, despite a shared general pattern, there are some differences within notoungulates, even between relatively close related representatives. Such variability deserves to be addressed taking advantage of a growing sample of taxa for which data is available.Regarding the inferred sensory and locomotor capabilities, the well-developed olfactory bulbs and large piriform lobes observed in most notoungulates suggest high reliance on olfaction. On the contrary, inferences about auditory capabilities are not straightforward. Although bulged temporal lobes could reflect the expansion of the auditory cortex, the inability to perform cortical mapping studies precludes strongly supported assertions. An elaborated middle ear chamber composed of tym¬panic and paratympanic spaces could be associated to a hearing-related function, but such relationship needs yet to be demonstrated. The only morphofunctional-based inference about notoungulate auditory capabilities is based on cochlear mea¬surements that suggest low-frequency hearing limits for Notostylops, Altitypotherium, Pachyrukhos and Cochilius. Finally, the locomotion capabilities derived from the vestibular system dimensions (reported for a handful of notoungulates) is consistent with the postcranial-based inference. However, it is important to highlight that inner ear-based inferences must be considered as a complementary approach, avoiding conclusions based solely on this evidence (i.e. when no postcranial data is available).