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

Convergent reduction of olfactory genes and olfactory bulb size in mammalian species at altitude

The invasion of specialized ecological niches can cause drastic changes to selection regimes, resulting in genomic and phenotypic transformation.1 High-altitude habitats offer an excellent opportunity to investigate the genetic basis of local adaptation,2,3 as the repeated specialization of multiple lineages for high altitude has produced striking examples of convergent evolution, adaptation, and changes in their underlying genes.4,5,6 Although enlightening, this focus on adaptation has left aspects of evolution in high-altitude locations understudied-including the role of gene loss and pseudogenization, maladaptation and trait loss, and physiological aspects outside of respiration and gas exchange. To characterize how mammals responded to high altitude in a new, unbiased way, we screened the genomes of 27 species living exclusively at high altitude (>1,000-1,500 m) and their lowland relatives for inactivated pseudogenes or lost genes.7 Genes that convergently lost function in high-altitude species were highly enriched for olfactory receptor (OR) genes, with an average reduction of ∼23% of OR repertoire in high-altitude species. No such trend was found for genes involved in pheromone detection and taste perception. In addition to OR loss, cranial endocasts show the brains of high-altitude species have on average ∼18% smaller olfactory bulbs relative to lowland relatives. Together, these repeated evolutionary outcomes suggest a general relaxation of constraint on olfaction at altitude, perhaps due to reduced odorant diversity in high-altitude environments or reduced effectiveness of mammalian olfactory physiology in thin, dry, or cold air.

Predicting hydric and thermic balance in caviomorph rodents through nasal turbinals morphometry: Impact of life habits

Nasal turbinals are key osseous structures for air conditioning and olfaction in mammals, with their morphology reflecting both ecological adaptations and evolutionary history. This study evaluates how climatic gradients and locomotor strategy (subterranean or surface dwelling species) influence turbinal complexity in caviomorph rodents. Using microCT imaging, we quantified respiratory (RZ) and olfactory (OZ) turbinal morphology across eight caviomorph rodents and two outgroups from xeric, mesic, and generalist habitats, including subterranean and surface-dwelling species. Our results revealed that xeric-adapted subterranean species exhibited significantly expanded RZ surface areas and greater structural complexity, consistent with enhanced water retention demands in arid environments. While surface-dwelling species showed larger absolute OZ areas compared to subterranean taxa, this difference became non-significant after accounting for body size, suggesting olfactory structures are less influenced by locomotor strategy than by allometric or phylogenetic factors. Respiratory turbinals appeared more variable across habitats, whereas olfactory turbinals showed comparatively conserved morphology among ecological groups. This pattern could reflect differing evolutionary pressures acting on thermoregulatory versus sensory systems in rodents. The observed trade-off between respiratory efficiency and olfactory capacity suggests how multiple selective forces may shape anatomical specialization in response to environmental challenges. These findings provide new insights into functional constraints governing nasal evolution, proposing a framework for interpreting ecological adaptations in caviomorphs. Our study illustrates how integrating quantitative morphometrics with ecological data can elucidate complex structure-function relationships in mammalian anatomy.

Ontogeny of a Brazilian Late Triassic Traversodontid (Cynodontia, Cynognathia): Anatomical and Paleoecological Implications

Investigating the developmental patterns of extinct species provides valuable insights into their anatomy, biology and ecomorphological adaptations. Research on the ontogeny of non-mammaliaform cynodonts has offered significant contributions to our understanding of these aspects. Here, we aim to describe and discuss the intraspecific and ontogenetic variation of the skull of the Brazilian traversodontid Siriusgnathus niemeyerorum (Candelária Sequence, Upper Triassic). We evaluated an ontogenetic series of the species through qualitative comparison and allometric analyses using cranial measures. Our findings reveal several trends during skull growth, including a relative increase in rostrum length, a relative decrease in orbit size, and changes in the zygomatic arch and temporal fenestra proportions. These patterns, when analyzed in the context of the adductor musculature, may be correlated with changes in feeding behaviour, similar to those described for the gomphodontosuchine Exaeretodon argentinus. We also report changes in cranial ornamentation, bone fusion, and suture complexity throughout ontogeny. Overall, this study provides a greater understanding of the cranial ontogenetic patterns of S. niemeyerorum, contributing to the knowledge of its intraspecific variation. The possible ecological implications of these findings highlight the importance of ontogenetic studies for elucidating the biology of extinct taxa.

Postnatal Skull Development Reveals a Conservative Pattern in Living and Fossil Vizcachas Genus Lagostomus (Rodentia, Chinchillidae)

The plains vizcacha, Lagostomus maximus, is the only living species in the genus, being notably larger than fossil congeneric species, such as Lagostomus incisus, from the Pliocene of Argentina and Uruguay. Here, we compare the skull growth allometric pattern and sexual dimorphism of L. maximus and L. incisus, relating shape and size changes with skull function. We also test whether the ontogenetic trajectories and allometric trends between both sexes of L. maximus follow the same pattern. A common allometric pattern between both species was the elongation of the skull, a product of the lengthening of rostrum, and chondrogenesis on the spheno-occipitalis synchondrosis and coronalis suture. We also detected a low proportion of skull suture fusion. In some variables, older male specimens did not represent a simple linear extension of female trajectory, and all dimorphic traits were related to the development of the masticatory muscles. Sexual dimorphism previously attributed to L. incisus would indicate that this phenomenon was present in the genus since the early Pliocene and suggests social behaviors such as polygyny and male-male competition. Ontogenetic changes in L. incisus were similar to L. maximus, showing a conservative condition of the genus. Only two changes were different in the ontogeny of both species, which appeared earlier in L. incisus compared to L. maximus: the development of the frontal process of the nasals in a square shape, and the straight shape of the occipital bone in lateral view. Juveniles of L. maximus were close to adult L. incisus in the morphospace, suggesting a peramorphic process. The sequence of suture and synchondroses fusion showed minor differences in temporozygomatica and frontonasalis sutures, indicating major mechanical stress in L. maximus related to size. We suggest a generalized growth path in Chinchillidae, but further analyses are necessary at an evolutionary level, including Lagidium and Chinchilla.

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.

Virtual endocasts of Clevosaurus brasiliensis and the tuatara: Rhynchocephalian neuroanatomy and the oldest endocranial record for Lepidosauria

Understanding the origins of the vertebrate brain is fundamental for uncovering evolutionary patterns in neuroanatomy. Regarding extinct species, the anatomy of the brain and other soft tissues housed in endocranial spaces can be approximated by casts of these cavities (endocasts). The neuroanatomical knowledge of Rhynchocephalia, a reptilian clade exceptionally diverse in the early Mesozoic, is restricted to the brain of its only living relative, Sphenodon punctatus, and unknown for fossil species. Here, we describe the endocast and the reptilian encephalization quotient (REQ) of the Triassic rhynchocephalian Clevosaurus brasiliensis and compare it with an ontogenetic series of S. punctatus. To better understand the informative potential of endocasts in Rhynchocephalia, we also examine the brain-endocast relationship in S. punctatus. We found that the brain occupies 30% of its cavity, but the latter recovers the general shape and length of the brain. The REQ of C. brasiliensis (0.27) is much lower than S. punctatus (0.84-1.16), with the tuatara being close to the mean for non-avian reptiles. The endocast of S. punctatus is dorsoventrally flexed and becomes more elongated throughout ontogeny. The endocast of C. brasiliensis is mostly unflexed and tubular, possibly representing a more plesiomorphic anatomy in relation to S. punctatus. Given the small size of C. brasiliensis, the main differences may result from allometric and heterochronic phenomena, consistent with suggestions that S. punctatus shows peramorphic anatomy compared to Mesozoic rhynchocephalians. Our results highlight a previously undocumented anatomical diversity among rhynchocephalians and provide a framework for future neuroanatomical comparisons among lepidosaurs.

Adaptation in brain structure and respiratory and olfactory structures across environmental gradients in African and North American muroid rodents

Morphometric studies of 3D micro CT-scanned images can provide insights into the evolution of the brain and sensory structures but such data are still scarce for the most diverse mammalian order of rodents. From reviewed and new data, we tested for convergence to extreme aridity and high elevation in the sensory and brain morphology of rodents, from morphometric data from micro-CT X-ray scans of 174 crania of 16 species of three distantly related African murid (soft-furred mice, Praomyini, laminate-toothed rats, Otomyini, and gerbils, Gerbillinae) clades and one North American cricetid (deer mice and white-footed mice, Peromyscus) clade. Recent studies demonstrated convergent evolution acting on the oval window area of the cochlea (enlarged in extremely arid-adapted species of Otomyini and Gerbillinae) and on endocranial volume (reduced in high elevation taxa of Otomyini and Peromyscus). However, contrary to our predictions, we did not find evidence of convergence in brain structure to aridity, or in the olfactory/respiratory system (turbinate bones) to high elevation. Brain structure differed, particularly in the petrosal lobules of the cerebellum and the olfactory bulbs, between Otomyini and Gerbillinae, with extreme arid-adapted species in each clade being highly divergent (not convergent) from other species in the same clade. We observed greater "packing" of the maxillary turbinate bones, which have important respiratory functions, in Peromyscus mice from high and low elevations compared to the high-elevation African Praomyini, but more complex patterns within Peromyscus, probably related to trade-offs in respiratory physiology and heat exchange in the nasal epithelium associated with high-elevation adaptation.

Anatomy and computed tomography of the nasal cavity, nasal conchae, and paranasal sinuses of the endangered Patagonian huemul deer (Hippocamelus bisulcus)

This study explores for the first time the shape, volume, and configuration of nasal cavity structures of the endangered Patagonian huemul deer via computed tomography (CT). Three-dimensional (3D) reconstructions derived from data sets obtained from five Patagonian huemul deer skulls were analyzed. Using semiautomatic segmentation, 3D models were created of all the sinus compartments and nasal conchae. Volumetric measurements were taken of seven sinus compartments. The Patagonian huemul deer has a wide, large nasal cavity, with an osseous nasal aperture typical of cervids and a choana with characteristics that differentiate it from the pudu and roe deer. It also has six nasal meatuses and three nasal conchae, with the ventral nasal concha having the greatest volume and surface, which given its extension ensures a greater ability to humidify and heat the air. Further analysis showed the complex system of paranasal sinuses to be characterized by a rostroventral and interconnected group, where communication with the nasal cavity is common through the nasomaxillary opening, and a caudodorsal group that communicates with the nasal cavity through openings in the nasal meatuses. Our study of the endangered Patagonian huemul deer documents an intricate, and in some nasal cavity structures, unique morphological construction which may predispose it to higher rates of sinonasal afflictions due largely to its nasal complex anatomy, thus affecting its high cultural value.

Anatomical correlates and nomenclature of the chiropteran endocranial cast

Bats form a diverse group of mammals that are highly specialized in active flight and ultrasound echolocation. These specializations rely on adaptations that reflect on their morphoanatomy and have been tentatively linked to brain morphology and volumetry. Despite their small size and fragility, bat crania and natural braincase casts ("endocasts") have been preserved in the fossil record, which allows for investigating brain evolution and inferring paleobiology. Advances in imaging techniques have allowed virtual extraction of internal structures, assuming that the shape of the endocast reflects soft organ morphology. However, there is no direct correspondence between the endocast and internal structures because meninges and vascular tissues mark the inner braincase together with the brain they surround, resulting in a mosaic morphology of the endocast. The hypothesis suggesting that the endocast reflects the brain in terms of both external shape and volume has drastic implications when addressing brain evolution, but it has been rarely discussed. To date, only a single study addressed the correspondence between the brain and braincase in bats. Taking advantage of the advent of imaging techniques, we reviewed the anatomical, neuroanatomical, and angiological literature and compare this knowledge available on bat's braincase anatomy with anatomical observations using a sample of endocranial casts representing most modern bat families. Such comparison allows to propose a Chiroptera-scale nomenclature for future descriptions and comparisons among bat endocasts. Describing the imprints of the tissues surrounding the brain also allows to address to what extent brain features can be blurred or hidden (e.g., hypophysis, epiphysis, colliculi, flocculus). Furthermore, this approach encourages further study to formally test the proposed hypotheses.

Counter-gradient variation and the expensive tissue hypothesis explain parallel brain size reductions at high elevation in cricetid and murid rodents

To better understand functional morphological adaptations to high elevation (> 3000 m above sea level) life in both North American and African mountain-associated rodents, we used microCT scanning to acquire 3D images and a 3D morphometric approach to calculate endocranial volumes and skull lengths. This was done on 113 crania of low-elevation and high-elevation populations in species of North American cricetid mice (two Peromyscus species, n = 53), and African murid rodents of two tribes, Otomyini (five species, n = 49) and Praomyini (four species, n = 11). We tested two distinct hypotheses for how endocranial volume might vary in high-elevation populations: the expensive tissue hypothesis, which predicts that brain and endocranial volumes will be reduced to lessen the costs of growing and maintaining a large brain; and the brain-swelling hypothesis, which predicts that endocranial volumes will be increased either as a direct phenotypic effect or as an adaptation to accommodate brain swelling and thus minimize pathological symptoms of altitude sickness. After correcting for general allometric variation in cranial size, we found that in both North American Peromyscus mice and African laminate-toothed (Otomys) rats, highland rodents had smaller endocranial volumes than lower-elevation rodents, consistent with the expensive tissue hypothesis. In the former group, Peromyscus mice, crania were obtained not just from wild-caught mice from high and low elevations but also from those bred in common-garden laboratory conditions from parents caught from either high or low elevations. Our results in these mice showed that brain size responses to elevation might have a strong genetic basis, which counters an opposite but weaker environmental effect on brain volume. These results potentially suggest that selection may act to reduce brain volume across small mammals at high elevations but further experiments are needed to assess the generality of this conclusion and the nature of underlying mechanisms.