The hidden anatomy of paranasal sinuses reveals biogeographically distinct morphotypes in the nine-banded armadillo (Dasypus novemcinctus)

Disparity of turbinal bones in placental mammals

Turbinals are key bony elements of the mammalian nasal cavity, involved in heat and moisture conservation as well as olfaction. While turbinals are well known in some groups, their diversity is poorly understood at the scale of placental mammals, which span 21 orders. Here, we investigated the turbinal bones and associated lamellae for one representative of each extant order of placental mammals. We segmented and isolated each independent turbinal and lamella and found an important diversity of variation in the number of turbinals, as well as their size, and shape. We found that the turbinal count varies widely, from zero in the La Plata dolphin, (Pontoporia blainvillei) to about 110 in the African bush elephant (Loxodonta africana). Multiple turbinal losses and additional gains took place along the phylogeny of placental mammals. Some changes are clearly attributed to ecological adaptation, while others are probably related to phylogenetic inertia. In addition, this work highlights the problem of turbinal nomenclature in some placental orders with numerous and highly complex turbinals, for which homologies are extremely difficult to resolve. Therefore, this work underscores the importance of developmental studies to better clarify turbinal homology and nomenclature and provides a standardized comparative framework for further research.

Correlation between Cross-Sectional Anatomy and Computed Tomography of the Normal Six-Banded Armadillo (Euphractus sexcintus) Nasal Cavity and Paranasal Sinuses

This research aimed to study the rostral part of the head of the six-banded armadillo, applying advanced imaging techniques such as CT. Furthermore, by combining the images obtained through this technique with anatomical cross-sections, an adequate description of the structures that constitute the rostral part of the head of this species is presented. This anatomical information could provide a valuable diagnostic tool for the clinical evaluation of different disorders in the six-banded armadillo's nasal cavity and paranasal sinuses.

Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia

The phylogenetic relationships within crown Crocodylia remain contentious due to conflicts between molecular and morphological hypotheses. However, morphology-based datasets are mostly constructed on external characters, overlooking internal structures. Here, we use 3D geometric morphometrics to study the shape of the intertympanic sinus system in crown crocodylians during ontogeny, in order to assess its significance in a taxonomic context. Intertympanic sinus shape was found to be highly correlated with size and modulated by cranial shape during development. Still, adult sinus morphology distinguishes specimens at the family, genus and species level. We observe a clear distinction between Alligatoridae and Longirostres, a separation of different Crocodylus species and the subfossil Malagasy genus Voay, and a distinction between the Tomistoma and Gavialis lineages. Our approach is independent of molecular methods but concurs with the molecular topologies. Therefore, sinus characters could add significantly to morphological datasets, offering an alternative viewpoint to resolve problems in crocodylian relationships.

Skeletal radiographic anatomy of echidnas: insights into unusual mammals

Long-beaked echidnas (Zaglossus spp.) have received less attention in the literature than Short-beaked echidnas (Tachyglossus aculeatus). Their natural history, anatomy, and physiology are poorly known. To improve our understanding of this unique group, we undertook a radiographic study of the Eastern long-beaked echidna (Zaglossus bartoni), and provide a comparative analysis of its radiographic skeletal anatomy with that of T. aculeatus. We examine conventional radiography and computed tomographic images of Zaglossus and Tachyglossus, describe the anatomical features of Zaglossus, and compare those with Tachyglossus. We provide evidence of epicoracoid overlap in echidnas, a feature not well documented in monotremes. The significance of epicoracoid overlap requires further investigation. Our study is intended as a reference for the radiographic anatomy of Z. bartoni.

Comparative anatomy and phylogenetic contribution of intracranial osseous canals and cavities in armadillos and glyptodonts (Xenarthra, Cingulata)

The evolutionary history of the Cingulata, as for many groups, remains a highly debated topic to this day, particularly for one of their most emblematic representatives: the glyptodonts. There is no consensus among morphological and molecular phylogenies regarding their position within Cingulata. As demonstrated by recent works, the study of the internal anatomy constitutes a promising path for enriching morphological matrices for the phylogenetic study of armadillos. However, internal cranial anatomy remains understudied in the Cingulata. Here we explored and compared the anatomy of intracranial osseous canals and cavities in a diverse sample of extant and extinct cingulates, including the earliest well-preserved glyptodont crania. The virtual 3D reconstruction (using X-ray microtomography) of selected canals, that is, the nasolacrimal canal, the palatine canal, the sphenopalatine canal, the canal for the frontal diploic vein, the transverse canal, the orbitotemporal canal, the canal for the capsuloparietal emissary vein and the posttemporal canal, and alveolar cavities related to cranial vascularization, innervation or tooth insertion allowed us to compare the locations, trajectories, and shape of these structures and to discuss their potential interest for cingulate systematics. We tentatively reconstructed evolutionary scenarios for eight selected traits related to these structures in which glyptodonts often showed a close resemblance to pampatheres, to the genus Proeutatus, and/or to chlamyphorines. This latter pattern was partly congruent with recent molecular hypotheses, but more research is needed on these resemblances and on the potential effects of development and allometry on the observed variations. Overall, these comparisons have enabled us to highlight new anatomical variation that may be of great interest to further explore the evolutionary history of cingulates and the origins of glyptodonts on a morphological basis.

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.

Ontogenetic and static allometry in the skull and cranial units of nine-banded armadillos (Cingulata: Dasypodidae: Dasypus novemcinctus)

A large part of extant and past mammalian morphological diversity is related to variation in size through allometric effects. Previous studies suggested that craniofacial allometry is the dominant pattern underlying mammalian skull shape variation, but cranial allometries were rarely characterized within cranial units such as individual bones. Here, we used 3D geometric morphometric methods to study allometric patterns of the whole skull (global) and of cranial units (local) in a postnatal developmental series of nine-banded armadillos (Dasypus novemcinctus ssp.). Analyses were conducted at the ontogenetic and static levels, and for successive developmental stages. Our results support craniofacial allometry as the global pattern along with more local allometric trends, such as the relative posterior elongation of the infraorbital canal, the tooth row reduction on the maxillary, and the marked development of nuchal crests on the supraoccipital with increasing skull size. Our study also reports allometric proportions of shape variation varying substantially among cranial units and across ontogenetic stages. The multi-scale approach advocated here allowed unveiling previously unnoticed allometric variations, indicating an untapped complexity of cranial allometric patterns to further explain mammalian morphological evolution.

Evolutionary adaptation to aquatic lifestyle in extinct sloths can lead to systemic alteration of bone structure

Through phenotypic plasticity, bones can change in structure and morphology, in response to physiological and biomechanical influences over the course of individual life. Changes in bones also occur in evolution as functional adaptations to the environment. In this study, we report on the evolution of bone mass increase (BMI) that occurred in the postcranium and skull of extinct aquatic sloths. Although non-pathological BMI in postcranial skeleton has been known in aquatic mammals, we here document general BMI in the skull for the first time. We present evidence of thickening of the nasal turbinates, nasal septum and cribriform plate, further thickening of the frontals, and infilling of sinus spaces by compact bone in the late and more aquatic species of the extinct sloth Thalassocnus Systemic bone mass increase occurred among the successively more aquatic species of Thalassocnus, as an evolutionary adaptation to the lineage's changing environment. The newly documented pachyostotic turbinates appear to have conferred little or no functional advantage and are here hypothesized as a correlation with or consequence of the systemic BMI among Thalassocnus species. This could, in turn, be consistent with a genetic accommodation of a physiological adjustment to a change of environment.

Phylogeny and molecular species delimitation of long-nosed armadillos (Dasypus: Cingulata) supports morphology-based taxonomy

The armadillo genus Dasypus is the most species-rich and widely distributed genus of the order Cingulata and it has a dynamic taxonomic history. Recent morphology-based studies have proposed new taxonomic arrangements, but these were not yet assessed with molecular data. The two comprehensive phylogenetic hypotheses available for the genus are conflicting and were each based on a subset of taxa, hampering a proper evaluation of species boundaries. Using a multilocus molecular dataset, based on the broadest geographic sampling of Dasypus to date, we inferred the phylogenetic relationships of all species of the genus, including the recently reinstated D. beniensis and D. pastasae. We tested recent taxonomic hypotheses using several species-delimitation approaches. Our phylogeny recovered three main lineages of long-nosed armadillos that we treat as subgenera (Hyperoambon, Muletia and Dasypus) and identified the majority of its diversification as having occurred during the Pliocene. Molecular species delimitation supported morphological evidence in assigning D. hybridus as a subspecies of D. septemcinctus and confirming the split of the D. kappleri complex into three species. Our results strongly support the recognition of Guiana Shield populations formerly assigned to D. novemcinctus as a distinct species. The phylogenetic positions of D. mazzai and D. sabanicola remain uncertain. Further investigation using faster-evolving genes and additional samples may help to clarify the relationships of these young species.

Evolutionary adaptation to aquatic lifestyle in extinct sloths can lead to systemic alteration of bone structure

Through phenotypic plasticity, bones can change in structure and morphology, in response to physiological and biomechanical influences over the course of individual life. Changes in bones also occur in evolution as functional adaptations to the environment. In this study, we report on the evolution of bone mass increase (BMI) that occurred in the postcranium and skull of extinct aquatic sloths. Although non-pathological BMI in postcranial skeleton has been known in aquatic mammals, we here document general BMI in the skull for the first time. We present evidence of thickening of the nasal turbinates, nasal septum and cribriform plate, further thickening of the frontals, and infilling of sinus spaces by compact bone in the late and more aquatic species of the extinct sloth Thalassocnus Systemic bone mass increase occurred among the successively more aquatic species of Thalassocnus, as an evolutionary adaptation to the lineage's changing environment. The newly documented pachyostotic turbinates appear to have conferred little or no functional advantage and are here hypothesized as a correlation with or consequence of the systemic BMI among Thalassocnus species. This could, in turn, be consistent with a genetic accommodation of a physiological adjustment to a change of environment.

Evolutionary adaptation to aquatic lifestyle in extinct sloths can lead to systemic alteration of bone structure

Through phenotypic plasticity, bones can change in structure and morphology, in response to physiological and biomechanical influences over the course of individual life. Changes in bones also occur in evolution as functional adaptations to the environment. In this study, we report on the evolution of bone mass increase (BMI) that occurred in the postcranium and skull of extinct aquatic sloths. Although non-pathological BMI in postcranial skeleton has been known in aquatic mammals, we here document general BMI in the skull for the first time. We present evidence of thickening of the nasal turbinates, nasal septum and cribriform plate, further thickening of the frontals, and infilling of sinus spaces by compact bone in the late and more aquatic species of the extinct sloth Thalassocnus Systemic bone mass increase occurred among the successively more aquatic species of Thalassocnus, as an evolutionary adaptation to the lineage's changing environment. The newly documented pachyostotic turbinates appear to have conferred little or no functional advantage and are here hypothesized as a correlation with or consequence of the systemic BMI among Thalassocnus species. This could, in turn, be consistent with a genetic accommodation of a physiological adjustment to a change of environment.

Taxonomic revision of the long-nosed armadillos, Genus Dasypus Linnaeus, 1758 (Mammalia, Cingulata)

Dasypus is the most speciose genus of the order Cingulata, including approximately 40% of known living armadillos. Nine species are currently recognized, although comprehensive analyses of the entire genus have never been done. Our aim is to revise the taxonomy of the long-nosed armadillos and properly define the taxa. We examined 2126 specimens of Dasypus preserved in 39 different museum collections, including 17 type specimens. Three complementary methods were applied to explore morphological datasets both qualitatively and quantitatively. Qualitative morphological variation in discrete characters was assessed by direct observations of specimens. Linear morphometric variation was based on external data and cranial measurements of 887 adult skulls. The shape and size of the skull was abstracted through two-dimensional geometric morphometric analyses of dorsal, lateral and ventral views of respectively 421, 211, and 220 adult specimens. Our results converge on the recognition of eight living species (D. beniensis, D. kappleri, D. mazzai, D. novemcinctus, D. pastasae, D. pilosus, D. sabanicola, and D. septemcinctus), and three subspecies of D. septemcinctus (D. s. septemcinctus, D. s. hybridus, and a new subspecies from Cordoba described here). Information on type material, diagnosis, distribution, and taxonomic comments for each taxon are provided. We designate a lectotype for D. novemcinctus; and a neotype for Loricatus hybridus (= D. septemcinctus hybridus).

Cranial osteology of the pampathere Holmesina floridanus (Xenarthra: Cingulata; Blancan NALMA), including a description of an isolated petrosal bone

The present study entails descriptions of several well-preserved skulls from the pampathere species Holmesina floridanus, recovered from Pliocene localities in central Florida and housed in the collections of the Florida Museum of Natural History. Bone by bone descriptions have allowed detailed reconstructions of cranial morphology. Cranial foramina are described and illustrated in detail, and their contents inferred. The first ever description of an isolated pampathere petrosal is also included. Cranial osteology of Holmesina floridanus is compared to that of Pleistocene species of Holmesina from both North and South America (Holmesina septentrionalis, Holmesina occidentalis), as well as to the other well-known pampathere genera, to closely related taxa among glyptodonts (Propalaehoplophorus), and to extinct and extant armadillos (Proeutatus, Euphractus). This study identifies a suite of apomorphic cranial features that serve to diagnose a putative, progressive series of more inclusive monophyletic groups, including the species Holmesina floridanus, the genus Holmesina, pampatheres, pampatheres plus glyptodonts, and a clade formed by pampatheres, glyptodonts, and Proeutatus. The study highlights the need for further anatomical investigations of pampathere cranial anatomy, especially those using modern scanning technology, and for analyses of pampathere phylogenetic relationships.

Beyond the carapace: skull shape variation and morphological systematics of long-nosed armadillos (genus Dasypus)

Background

The systematics of long-nosed armadillos (genus Dasypus) has been mainly based on a handful of external morphological characters and classical measurements. Here, we studied the pattern of morphological variation in the skull of long-nosed armadillos species, with a focus on the systematics of the widely distributed nine-banded armadillo (Dasypus novemcinctus).

Methods

We present the first exhaustive 3D comparison of the skull morphology within the genus Dasypus, based on micro-computed tomography. We used geometric morphometric approaches to explore the patterns of the intra- and interspecific morphological variation of the skull with regard to several factors such as taxonomy, geography, allometry, and sexual dimorphism.

Results

We show that the shape and size of the skull vary greatly among Dasypus species, with Dasypus pilosus representing a clear outlier compared to other long-nosed armadillos. The study of the cranial intraspecific variation in Dasypus novemcinctus evidences clear links to the geographic distribution and argues in favor of a revision of past taxonomic delimitations. Our detailed morphometric comparisons detected previously overlooked morphotypes of nine-banded armadillos, especially a very distinctive unit restricted to the Guiana Shield.

Discussion

As our results are congruent with recent molecular data and analyses of the structure of paranasal sinuses, we propose that Dasypus novemcinctus should be regarded either as a polytypic species (with three to four subspecies) or as a complex of several distinct species.