Lionel Hautier

Rodents can be divided into three major morphological groups (myomorphy, sciuromorphy, and hystricomorphy; additionally, protogomorphy can be found in only one recent species) based on the configuration of the skull and jaw muscles. These three morphotypes have been suggested to be adapted to different food types and allow for different chewing movements. Previous authors have postulated that the masticatory grinding movements should occur mostly in the antero‐posterior direction, but other studies have suggested that different degrees of antero‐posterior movement may occur across the rodent tree of life and in response to differences in food properties. Here, we use 3D reconstructed movements from X‐Ray videos to show that both morphology and food type influence masticatory movements in two myomorphous and two hystricomorphous rodents. Yet, while hystricomorphs do not change the frequency of their masticatory movements in response to different food items, myomorphs do so. Additionally, myomorphs and hystricomorphs that exhibit propalinal chewing movements tend to possess a relatively lower mandibular condyle. Overall, our data suggest that molar morphology, in connection with dietary habits, directly influences masticatory movements. These results show that relatively small differences in a relatively conserved bauplan can have a large impact on jaw movements.

The trigeminal nerve (CN V) is a key cranial nerve responsible for sensory input from the face and motor control of the masticatory muscles, playing a central role in mandibular movement and oral cavity function across tetrapods. Despite its importance and its link with the masticatory musculature, no comprehensive comparative study of the trigeminal nerve across rodent groups has yet been conducted. This is partly due to the difficulty of visualizing nerves using conventional dissection techniques, and partly due to the complex, three-dimensional organization of the nervous system. Here, we combined diffusible iodine-based contrast-enhanced CT imaging (diceCT) with traditional dissection to characterize the trigeminal nerve and its relationship to masticatory muscles in three representative rodents: the rat, the red squirrel, and the Guinea pig. Our findings demonstrate that the broad architecture of the nervous system is highly constrained but exhibits fine-scale plasticity in nerve branching and muscle innervation. While the gross nerve organization is largely conserved, we identified species-specific differences, particularly in Guinea pigs, linked to skeletal and muscular modifications. The study underscores the value of integrating 3D imaging with dissection to accurately capture nerve morphology and discuss muscle homologies, with implications for comparative anatomy, functional morphology, and paleontological reconstruction.

This contribution contains the 3D models described and figured in: Phylogenetic signal in anteater snout morphology: implications for interpreting rare vermilinguan fossils. Palaeobiodiversity and Palaeoenvironments.

Olfaction is a critical sense for tetrapods, playing a key role in survival and reproduction by aiding in food detection, predator avoidance, and social interactions. Olfactory performance has been experimentally tested in only a few taxa, so comparative analyses rely on anatomical and genomic proxies. Among anatomical proxies, the olfactory bulb endocast is widely used, particularly in extinct species, where it is often the only preserved proxy and can be reconstructed even in million-year-old fossils. While the functional significance of chemoreceptor genes has received attention, the extent to which the olfactory bulb endocast correlates with genomic proxies remains unclear. Using brain endocasts across all mammalian orders, we investigated the relationship between the absolute (absOB) and relative (relatOB) volumes of the olfactory bulb endocast and the number of intact chemoreceptor genes. While no clear correlations were found between absOB and the genomic proxies tested, we identified a significant correlation between relatOB and the total number of combined intact chemoreceptor genes (CombChemo), primarily driven by olfactory receptor genes (OR). Leveraging this correlation, and aiming to infer olfactory capabilities in taxa for which only the skull is available, we estimated OR numbers for three mammalian orders lacking genomic data, as well as for five extinct mammals. Building on studies that have established a link between intact OR and olfactory sensitivity and discrimination, we conclude that relatOB enables inference of olfactory capabilities in mammals. This provides a basis to investigate sensory evolution and opens perspectives for interpreting paleoecology and behavior of extinct mammals.

Pangolins are the most heavily trafficked mammals in the world, largely because of the high demand for their keratinous scales from the traditional Chinese medicine market. While seizures of pangolin material are largely composed of isolated scales, efficient approaches to reach species‐level identification are missing. This mostly originates from the lack of comparative studies on the shape of pangolin scales, resulting in knowledge gaps on the imbricated effects of serial, ontogenetic, and evolutionary variations. Using a large sample of micro‐CT scan data, we performed the first quantitative 3D analysis of scale shape variation among the eight species of extant pangolins. Our geometric morphometric approach suggests that pangolins grossly share similar trends of serial variation, with scale shape and size being similarly distinct across the different body regions. Relative elongation is by far the main component of scale shape variation at the intraspecific level, although degree of asymmetry and shape of scale bed area also allow distinction among different body areas, especially in adults. At the evolutionary level, Phataginus is the most distinctive genus, with P. tricuspis having the most elongated scales overall. Scales of the back (dorsum) appear to be the best discriminator between species, providing one of the few scale shape differences recorded between the genera Smutsia and Manis. Our results provide an unprecedented, upstream understanding of broad differentiation patterns across the scaled body of pangolins. Together with other yet‐to‐be‐explored morphological traits (e.g., scale size, ornamentation, and thickness), scale shape could provide a valuable matrix of information for forensic applications.