The functional morphology of xenarthrous vertebrae in the armadillo Dasypus novemcinctus (Mammalia, Xenarthra).

In order to assess the mechanical properties of xenarthrous vertebrae, and to evaluate the role of xenarthrae as fossorial adaptations, in vitro bending tests were performed on posterior thoracic and lumbar vertebral segments excised from specimens of the armadillo Dasypus novemcinctus and the opossum Didelphis virginiana, the latter being used to represent the primitive mammalian condition. The columns of the two species were subjected to dorsal, ventral, and lateral bending, as well as torsion, in order to determine their stiffness in each of these directions. During these tests, bone strains in the centra of selected vertebrae were determined using rosette strain gages. Overall stiffness of the armadillo backbone at physiologically relevant displacement levels was significantly higher than that of the opossum for both dorsal and lateral bending. The two species also exhibited significant differences in angular displacement of individual vertebrae and in vertebral strain magnitudes and orientations in these two directions. No significant differences were observed when the columns of the two species were subjected to torsion or to ventral bending. Our results suggest that some, but not all, of the mechanical differences between the two species are due to the presence of xenarthrae. For example, removal of the xenarthrae from selected vertebrae (L2-L4) changes strain orientation and shear, but not strain magnitudes. Comparisons with functional data from other digging mammals indicate that the modified mechanical properties of the Dasypus column are consistent with an interpretation of xenarthrae as digging adaptations and lend support to the idea that the order Xenarthra represents an early offshoot of placental mammals specialized for fossoriality.     

Histology and histochemistry of the gekkotan notochord and their bearing on the development of notochordal cartilage

The persistence of the notochord into the skeletally mature life stage is characteristic of gekkotans, but is otherwise of rare occurrence among amniotes. The taxonomic diversity of Gekkota affords the opportunity to investigate the structure and development of this phylogenetically ancestral component of the skeleton, and to determine its basic characteristics. The gekkotan notochord spans almost the entire postcranial long axis and is characterized by a moniliform morphology with regularly alternating zones of chordoid and chondroid tissue. Chordoid tissue persists in the region of intervertebral articulations and occupies the cavitations that lie between the centra of the amphicoelous vertebrae. Chondroid tissue is restricted to zones in which the diameter of the notochord is reduced, corresponding to mid‐vertebral locations. In the tail, these zones of chondroid tissue are associated with the autotomic fracture planes. Chondroid tissue first manifests during late embryogenesis, appears to differentiate from pre‐existing chordoid tissue, and has the histological and histochemical characteristics of cartilage. Our observations lend support to the hypothesis that cartilage can be derived directly from notochordal tissue, and suggest that the latter may be an evolutionary and developmental precursor to chordate cartilage. The persistence of chordoid tissue in the intervertebral regions of amphicoelous vertebrae is consistent with a suite of paedomorphic traits exhibited by gekkotans and suggests that the typical hydrostatic nature of notochordal tissue may play a role in mechanically governing patterns of displacement between adjacent amphicoelous vertebrae that lack extensive centrum‐to‐centrum contact. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Lumbar anomalies in the Shanidar 3 Neandertal

Recent examination of the Shanidar 3 remains revealed the presence of anomalous bilateral arthroses in the lumbar region. This paper describes this developmental anomaly, as well as several degenerative changes and offers potential etiologies.The Shanidar 3 remains represent an adult male Neandertal, approximately 35–50 years of age, dating to the Last Glacial. Although the partial skeleton is fragmentary, preserved elements include an almost complete set of ribs, portions of all thoracic vertebrae, all lumbar vertebrae, and the sacrum. Vertebral articulations from S1–T1 can be confidently assigned. The vertebra designated L1 is well preserved but lacks transverse processes. Instead, well defined bilateral articular surfaces, rather than transverse processes, are located on the pedicles. The skeletal elements associated with the anomalous L1 articulations were not recovered.The most likely interpretation is that the arthroses in question represent the facets for a 13th pair of ribs, a rare condition in modern hominid populations. Such lumbar developmental anomalies are an infrequent expression of a larger complex of cranial-caudal border shifting seen in the vertebral column. These shifts result in a change in the usual boundaries between the distinctive vertebral regions and are responsible for the majority of variability present in the vertebral column.     

Vertebral and rib anatomy in Caperea marginata: Implications for evolutionary patterning of the mammalian vertebral column

The pygmy right whale, Caperea marginata, is a rare mysticete cetacean with an unusual suite of axial skeletal characters. Distally expanded first ribs, a long thorax with broadly overlapping vertebral transverse processes, plate‐like posterior ribs, and a short tail contrast with other cetaceans and suggest unique developmental patterning. Twenty‐four individuals of diverse ontogenetic age were available for analysis. Multiple, variable examples of incomplete rib fusion in dependent calves indicate that the first rib of adults is an ontogenetic fusion product of ribs 1 and 2. The composite rib articulates by way of its anterior (Rib1) component to the sternum and by way of its posterior (Rib2) component with thoracic vertebra 2. Thoracic vertebra 1 lacks rib articulations. When rib fusion is taken into account, the most frequent column formulas are C7T18L1Cd16–17 = 42–43 and C7T17L1Cd16–18 = 41–43. Thoracic and lumbar series are not reciprocal in count, arguing against their developmental linkage. Instead, parallel reduction in both lumbar and caudal counts supports the existence of neocete patterning in Caperea, as in all other living cetaceans. Ontogenetic vertebral column elongation is most marked in the posterior thorax, lumbos, and anterior tail. Vertebrae in these column regions are excellent predictors of total body length.     

Testing an inference of function from structure: Snake vertebrae do the twist

The zygapophyses and zygosphene–zygantrum articulations of snake vertebrae are hypothesized to restrict or eliminate vertebral torsion. This hypothesis is apparently based solely on the inference of function from structure, despite the limitations of such inferences, as well as contradictory observations and measurements. In this study, I observed and measured axial torsion in gopher snakes, Pituophis melanoleucus. To examine the structural basis of axial torsion, I measured the vertebral articulation angles along the body and the insertion angles of five epaxial muscles. To examine torsion in a natural behavior, I digitized video images and measured the degree of apparent axial torsion during terrestrial lateral undulation. Finally, I measured the mechanical capacity of the vertebral joints for actual torsion over intervals of 10 vertebrae in fresh, skinned segments of the trunk. Vertebral articulation angles vary up to 30° and are associated with variation in torsional capacity along the trunk. The freely crawling P. melanoleucus twisted up to 2.19° per vertebra, which produced substantial overall torsion when added over several vertebrae. The vertebral joints are mechanically capable of torsion up to 2.89° per joint. Therefore, despite the mechanical restriction imposed by the complex articulations, vertebral torsion occurs in snakes and appears to be functionally important in several natural behaviors. Even in cases in which mechanical function appears to be narrowly constrained by morphology, specific functions should not be inferred solely from structural analyses. J. Morphol. 241:217–225, 1999. © 1999 Wiley‐Liss, Inc.     

Evo-Devo of the Human Vertebral Column: On Homeotic Transformations, Pathologies and Prenatal Selection

Homeotic transformations of vertebrae are particularly common in humans and tend to come associated with malformations in a wide variety of organ systems. In a dataset of 1,389 deceased human foetuses and infants a majority had cervical ribs and approximately half of these individuals also had missing twelfth ribs or lumbar ribs. In?~10?% of all cases there was an additional shift of the lumbo-sacral boundary and, hence, homeotic transformations resulted in shifts of at least three vertebral boundaries. We found a strong coupling between the abnormality of the vertebral patterns and the amount and strength of associated malformations, i.e., the longer the disturbance of the vertebral patterning has lasted, the more associated malformations have developed and the more organ systems are affected. The germ layer of origin of the malformations was not significantly associated with the frequency of vertebral patterns. In contrast, we find significant associations with the different developmental mechanisms that are involved in the causation of the malformations, that is, segmentation, neural crest development, left-right patterning, etc. Our results, thus, suggest that locally perceived developmental signals are more important for the developmental outcome than the origin of the cells. The low robustness of vertebral A-P patterning apparent from the large number of homeotic transformations is probably caused by the strong interactivity of developmental processes and the low redundancy of involved morphogens during early organogenesis. Additionally, the early irreversibility of the specification of the A-P identity of vertebrae probably adds to the vulnerability of the process by limiting the possibility for recovery from developmental disturbances. The low developmental robustness of vertebral A-P patterning contrasts with a high robustness of the A-P patterning of the vertebral regions. Not only the order is invariable, also the variation in the number of vertebrae per region is small. This robustness is in agreement with the evolutionary stability of vertebral regions in tetrapods. Finally, we propose a new hypothesis regarding the constancy of the presacral number of vertebrae in mammals.     

Developmental mechanisms of macroevolutionary change in the tetrapod axis: A case study of Sauropterygia

Understanding how developmental processes change on macroevolutionary timescales to generate body plan disparity is fundamental to the study of vertebrate evolution. Adult morphology of the vertebral column directly reflects the mechanisms that generate vertebral counts (somitogenesis) and their regionalisation (homeotic effects) during embryonic development. Sauropterygians were a group of Mesozoic marine reptiles that exhibited an extremely high disparity of presacral vertebral/somite counts. Using phylogenetic comparative methods, we demonstrate that somitogenesis and homeotic effects evolved in a co‐ordinated way among sauropterygians, contrasting with the wider pattern in tetrapods, in which somitogenetic and homeotic shifts are uncorrelated. Changes in sauropterygian body proportions were primarily enabled by homeotic shifts, with a lesser, but important, contribution from differences in postpatterning growth among somites. High body plan plasticity was present in Triassic sauropterygians and was maintained among their Jurassic and Cretaceous descendants. The extreme disparity in the body plan of plesiosaurian sauropterygians did not result from accelerated rates of evolutionary change in neck length, but instead reflect this ancestral versatility of sauropterygian axial development. Our results highlight variation in modes of axial development among tetrapods, and show that heterogeneous statistical models can uncover novel macroevolutionary patterns for animal body plans and the developmental mechanisms that control them.     

The Nerve Supply to the Second Metamere Basicranial Muscle in Osteolepiform Vertebrates,with Some Remarks on the Basic Composition of the Endocranium

The endocranium of Devonian osteolepiforms includes four minor structures, called zygals, which probably represent cephalic vertebral elements belonging to the second, third, and fourth metameres. Also, the endocranium exhibits a partial segmentation in the form of vestiges of intrametameric articulations attributable to the anterior five metameres. Finally, the basicranial muscle of osteolepiforms, extending across the lower part of the endocranial articulation within the second metamere, is possibly supplied by a separate cranial nerve, for which the term nervus rarus is proposed. This hypothetical cranial nerve, which in osteolepiforms possibly emerges from the endocranium through a canal in the fourth metamere, is suggested to constitute, together with the trochlear nerve, the somatic motor component of the second cranionerval segment.     

Linking vertebral number to performance of aquatic escape responses in the axolotl (Ambystoma mexicanum)

Environmental conditions during early development in ectothermic vertebrates can lead to variation in vertebral number among individuals of the same species. It is often seen that individuals of a species raised at cooler temperatures have more vertebrae than individuals raised at warmer temperatures, although the functional consequences of this variation in vertebral number on swimming performance are relatively unclear. To investigate this relationship, we tested how vertebral number in axolotls (Ambystoma mexicanum) affected performance of aquatic escape responses (C-starts). Axolotls were reared at four temperatures (12–24 °C) encompassing their natural thermal range and then transitioned to a mean temperature (18 °C) three months before C-starts were recorded. Our results showed variation in vertebral number, but that variation was not significantly affected by developmental temperature. C-start performance among axolotls was significantly correlated with caudal vertebral number, and individuals with more caudal vertebrae were able to achieve greater curvature more quickly during their responses than individuals with fewer vertebrae. However, our results show that these individuals did not achieve greater displacements or velocities, and that developmental temperature did not have any effect on C-start performance. We highlight that the most important aspects of escape swim performance (i.e., how far individuals get from a threat and how quickly they move the most important parts of the body away from that threat) are consistent across individuals regardless of developmental temperature and morphological variation.     

PATTERNS OF OSSIFICATION IN SOUTHERN VERSUS NORTHERN PLACENTAL MAMMALS

Consensus on placental mammal phylogeny is fairly recent compared to that for vertebrates as a whole. A stable phylogenetic hypothesis enables investigation into the possibility that placental clades differ from one another in terms of their development. Here, we focus on the sequence of skeletal ossification as a possible source of developmental distinctiveness in “northern” (Laurasiatheria and Euarchontoglires) versus “southern” (Afrotheria and Xenarthra) placental clades. We contribute data on cranial and postcranial ossification events during growth in Afrotheria, including elephants, hyraxes, golden moles, tenrecs, sengis, and aardvarks. We use three different techniques to quantify sequence heterochrony: continuous method, sequence‐ANOVA (analysis of variance) and event‐paring/Parsimov. We show that afrotherians significantly differ from other placentals by an early ossification of the orbitosphenoid and caudal vertebrae. Our analysis also suggests that both southern placental groups show a greater degree of developmental variability; however, they rarely seem to vary in the same direction, especially regarding the shifts that differ statistically. The latter observation is inconsistent with the Atlantogenata hypothesis in which afrotherians are considered as the sister clade of xenarthrans. Interestingly, ancestral nodes for Laurasiatheria and Euarchontoglires show very similar trends and our results suggest that developmental homogeneity in some ossification sequences may be restricted to northern placental mammals (Boreoeutheria).     

Development of the Synarcual in the Elephant Sharks (Holocephali; Chondrichthyes): Implications for Vertebral Formation and Fusion

The synarcual is a structure incorporating multiple elements of two or more anterior vertebrae of the axial skeleton, forming immediately posterior to the cranium. It has been convergently acquired in the fossil group ‘Placodermi’, in Chondrichthyes (Holocephali, Batoidea), within the teleost group Syngnathiformes, and to varying degrees in a range of mammalian taxa. In addition, cervical vertebral fusion presents as an abnormal pathology in a variety of human disorders. Vertebrae develop from axially arranged somites, so that fusion could result from a failure of somite segmentation early in development, or from later heterotopic development of intervertebral bone or cartilage. Examination of early developmental stages indicates that in the Batoidea and the ‘Placodermi’, individual vertebrae developed normally and only later become incorporated into the synarcual, implying regular somite segmentation and vertebral development. Here we show that in the holocephalan Callorhinchus milii, uniform and regular vertebral segmentation also occurs, with anterior individual vertebra developing separately with subsequent fusion into a synarcual. Vertebral elements forming directly behind the synarcual continue to be incorporated into the synarcual through growth. This appears to be a common pattern through the Vertebrata. Research into human disorders, presenting as cervical fusion at birth, focuses on gene misexpression studies in humans and other mammals such as the mouse. However, in chondrichthyans, vertebral fusion represents the normal morphology, moreover, taxa such Leucoraja (Batoidea) and Callorhinchus (Holocephali) are increasingly used as laboratory animals, and the Callorhinchus genome has been sequenced and is available for study. Our observations on synarcual development in three major groups of early jawed vertebrates indicate that fusion involves heterotopic cartilage and perichondral bone/mineralised cartilage developing outside the regular skeleton. We suggest that chondrichthyans have potential as ideal extant models for identifying the genes involved in these processes, for application to human skeletal heterotopic disorders.     

Morphofunctional peculiarities of the transverse processes of the axis, considerations on their pathogenetic significance.

The transverse process C2 plays an important role in the stabilization of the vertebral unit C2/C3 and the protection of the vertebral artery and the cervical spinal cord. The anatomical disposition of the transverse processes C2 and especially the lateral processes may be different from one subject to another. Certain anatomical variants of the transverse process can exert a negative effect on the stabilizing and protective function of the same. Our observations suggest that a maldevelopment or a lesion of the transverse process of the axis may induce extra stresses in the vertebral unit C2/C3 with subsequent development of a zygapophyseal arthrosis at this level. Any zygapophyseal arthrosis (uni- or bilaterally) should lead us to think of this possibility too.     

Segmental relationship between somites and vertebral column in zebrafish

The segmental heritage of all vertebrates is evident in the character of the vertebral column. And yet, the extent to which direct translation of pattern from the somitic mesoderm and de novo cell and tissue interactions pattern the vertebral column remains a fundamental, unresolved issue. The elements of vertebral column pattern under debate include both segmental pattern and anteroposterior regional specificity. Understanding how vertebral segmentation and anteroposterior positional identity are patterned requires understanding vertebral column cellular and developmental biology. In this study, we characterized alignment of somites and vertebrae, distribution of individual sclerotome progeny along the anteroposterior axis and development of the axial skeleton in zebrafish. Our clonal analysis of zebrafish sclerotome shows that anterior and posterior somite domains are not lineage-restricted compartments with respect to distribution along the anteroposterior axis but support a 'leaky' resegmentation in development from somite to vertebral column. Alignment of somites with vertebrae suggests that the first two somites do not contribute to the vertebral column. Characterization of vertebral column development allowed examination of the relationship between vertebral formula and expression patterns of zebrafish Hox genes. Our results support co-localization of the anterior expression boundaries of zebrafish hoxc6 homologs with a cervical/thoracic transition and also suggest Hox-independent patterning of regionally specific posterior vertebrae.     

Comparative anatomy and histology of xenarthran osteoderms

Reconstruction of soft tissues in fossil vertebrates is an enduring challenge for paleontologists. Because inferences must be based on evidence from hard tissues (typically bones or teeth), even the most complete fossils provide only limited information about certain organ systems. Osteoderms ("dermal armor") are integumentary bones with high fossilization potential that hold information about the anatomy of the skin in many extant and fossil amniotes. Their importance for functional morphology and phylogenetic research has recently been recognized, but studies have focused largely upon reptiles, in which osteoderms are most common. Among mammals, osteoderms occur only in members of the clade Xenarthra, which includes armadillos and their extinct relatives: glyptodonts, pampatheres, and, more distantly, ground sloths. Here, I present new information on the comparative morphology and histology of osteoderms and their associated soft tissues in 11 extant and fossil xenarthrans. Extinct mylodontid sloths possessed simple, isolated ossicles, the presence of which is likely plesiomorphic for Xenarthra. More highly derived osteoderms of glyptodonts, pampatheres, and armadillos feature complex articulations and surface ornamentation. Osteoderms of modern armadillos are physically associated with a variety of soft tissues, including nerve, muscle, gland, and connective tissue. In some cases, similar osteological features may be caused by two or more different tissue types, rendering soft-tissue inferences for fossil osteoderms equivocal. Certain osteological structures, however, are consistently associated with specific soft-tissue complexes and therefore represent a relatively robust foundation upon which to base soft-tissue reconstructions of extinct xenarthrans.     

Paleoepidemiology of a central California prehistoric population from Ca-Ala-329: II. Degenerative disease

Degenerative lesions are scored and frequencies of involvement are computed for a skeletal collection from Ca-Ala-329, a prehistoric site on the southeastern side of San Francisco Bay, dating from 500 A.D. up to European contact. A large earthmound site, excavations conducted there by San Jose State University retrieved close to 300 burials. For this epidemiological analysis, reasonably complete and aged skeletons representing 77 adult females and 90 adult males are available. Degenerative changes are scored macroscopically in an ordinal fashion for the large fibro-cartilagenous joints between adjacent vertebral bodies (vertebral osteophytosis) as well as the small apophyseal articulations of the spine. In addition, in the peripheral skeleton degenerative changes are scored in the temporo-mandibular, shoulder, elbow, hip, and knee joints as well as the small articulations of the hands and feet. The most common degenerative changes in the spine are seen between the vertebral bodies of the lower lumbar region. In the peripheral skeleton the highest involvement of degenerative disease is seen in the hands and feet. Compared to other relevant osteological samples, this group of hunting-gathering California Indians shows more degenerative changes than settled agriculturists (from Pecos Pueblo, New Mexico) but substantially less frequent involvement than in arctic hunters (Alaskan Eskimos).     

Female reproductive tract of the lesser anteater (Tamandua tetradactyla,myrmecophagidae, Xenarthra). Anatomy and histology

The morphological and histological features of the unusual reproductive tract of the female lesser anteater, Tamandua tetradactyla (Myrmecophagidae, Xenarthra), are described for the first time. The present study aimed to establish the main similarities and differences between this species and other xenarthrans. The populations of this species are declining rapidly for a number of reasons and our study is relevant to diverse programs related to its conservation. Studies were carried out on five female genital tracts of adult specimens. Ovaries were ovoid, presenting a medulla completely surrounded by the cortex, differently from that described in other xenarthans. Like in Dasypus but different from all other armadillos studied, single oocyte follicles were observed and a simple the uterus. The uterovaginal canal connects the uterus with the urogenital sinus. The simple columnar epithelium of the uterovaginal canal ends abruptly at a septum which resembles a hymen, where the transitional epithelium of the urogenital sinus appears. This ancestral feature is shared with that of other armadillos, except Tolypeutes matacus, which has a true vagina. Characteristics of the reproductive tract and sperm morphology of other Xenarthra are comparatively discussed. These observations suggest that important reproductive features are shared between the family Myrmecophagidae and the genus Dasypus, a basal group in the phylogeny of Xenarthra. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Evolution of axial patterning in elongate fishes

Within the ray-finned fishes, eel-like (extremely elongate) body forms have evolved multiple times from deeper-bodied forms. Previous studies have shown that elongation of the vertebral column may be associated with an increase in the number of vertebrae, an increase in the length of the vertebral centra, or a combination of both. Because the vertebral column of fishes has at least two anatomically distinct regions (i.e. abdominal and caudal), an increase in the number and relative length of the vertebrae could be region-specific or occur globally across the length of the vertebral column. In the present study, we recorded vertebral counts and measurements of vertebral aspect ratio (vertebral length/width) from museum specimens for 54 species representing seven groups of actinopterygian fishes. We also collected, from published literature, vertebral counts for 813 species from 14 orders of actinopterygian and elasmobranch fishes. We found that the number of vertebrae can increase independently in the abdominal and caudal regions of the vertebral column, but changes in aspect ratio occur similarly in both regions. These findings suggest that abdominal vertebral number, caudal vertebral number, and vertebral aspect ratio are controlled by separate developmental modules. Based on these findings, we suggest some candidate developmental mechanisms that may contribute to vertebral column patterning in fishes. Our study is an example of how comparative anatomical studies of adults can generate testable hypotheses of evolutionary changes in developmental mechanisms.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 97–116.     

The occipital region in the basal bony fish Erpetoichthys calabaricus (Actinopterygii: Cladistia)

Erpetoichthys calabaricus has unusual cranio‐vertebral anatomy, with an occipital centrum forming a component part of the compound basiexoccipital bone, and a ‘free‐floating’ occipital neural arch that differs from accessory arches found in some teleosts. The occipital neural arch bears autapomorphic lateral projections that articulate with small rod‐like bones resembling the spatial relationship of parapophyses and ribs, a feature normally restricted to vertebral centra. Based on analyses of cleared and stained specimens, computed tomography and histology, it is hypothesized that the lateral projections and associated rod‐shaped bones are structures that share developmental homologies to the unique ‘dorsal ribs’ of Polypteridae.     

Molecular phylogeny of living xenarthrans and the impact of character and taxon sampling on the placental tree rooting

Extant xenarthrans (armadillos, anteaters and sloths) are among the most derived placental mammals ever evolved. South America was the cradle of their evolutionary history. During the Tertiary, xenarthrans experienced an extraordinary radiation, whereas South America remained isolated from other continents. The 13 living genera are relics of this earlier diversification and represent one of the four major clades of placental mammals. Sequences of the three independent protein-coding nuclear markers alpha2B adrenergic receptor (ADRA2B), breast cancer susceptibility (BRCA1), and von Willebrand Factor (VWF) were determined for 12 of the 13 living xenarthran genera. Comparative evolutionary dynamics of these nuclear exons using a likelihood framework revealed contrasting patterns of molecular evolution. All codon positions of BRCA1 were shown to evolve in a strikingly similar manner, and third codon positions appeared less saturated within placentals than those of ADRA2B and VWF. Maximum likelihood and Bayesian phylogenetic analyses of a 47 placental taxa data set rooted by three marsupial outgroups resolved the phylogeny of Xenarthra with some evidence for two radiation events in armadillos and provided a strongly supported picture of placental interordinal relationships. This topology was fully compatible with recent studies, dividing placentals into the Southern Hemisphere clades Afrotheria and Xenarthra and a monophyletic Northern Hemisphere clade (Boreoeutheria) composed of Laurasiatheria and Euarchontoglires. Partitioned likelihood statistical tests of the position of the root, under different character partition schemes, identified three almost equally likely hypotheses for early placental divergences: a basal Afrotheria, an Afrotheria + Xenarthra clade, or a basal Xenarthra (Epitheria hypothesis). We took advantage of the extensive sampling realized within Xenarthra to assess its impact on the location of the root on the placental tree. By resampling taxa within Xenarthra, the conservative Shimodaira-Hasegawa likelihood-based test of alternative topologies was shown to be sensitive to both character and taxon sampling.     

Correlation between Hox code and vertebral morphology in archosaurs

The relationship between developmental genes and phenotypic variation is of central interest in evolutionary biology. An excellent example is the role of Hox genes in the anteroposterior regionalization of the vertebral column in vertebrates. Archosaurs (crocodiles, dinosaurs including birds) are highly variable both in vertebral morphology and number. Nevertheless, functionally equivalent Hox genes are active in the axial skeleton during embryonic development, indicating that the morphological variation across taxa is likely owing to modifications in the pattern of Hox gene expression. By using geometric morphometrics, we demonstrate a correlation between vertebral Hox code and quantifiable vertebral morphology in modern archosaurs, in which the boundaries between morphological subgroups of vertebrae can be linked to anterior Hox gene expression boundaries. Our findings reveal homologous units of cervical vertebrae in modern archosaurs, each with their specific Hox gene pattern, enabling us to trace these homologies in the extinct sauropodomorph dinosaurs, a group with highly variable vertebral counts. Based on the quantifiable vertebral morphology, this allows us to infer the underlying genetic mechanisms in vertebral evolution in fossils, which represents not only an important case study, but will lead to a better understanding of the origin of morphological disparity in recent archosaur vertebral columns.