The lower jaw of an unidentified Pennsylvanian (Late Carboniferous) tetrapod from Nova Scotia – the “Parrsboro jaw”- is redescribed in the light of recent tetrapod discoveries and work on evolution of tetrapod mandibular morphology and placed for the first time in a numerical cladistics analysis. All phylogenetic analyses place the jaw in a crownward polytomy of baphetids, temnospondyls, and embolomeres. Several features resemble baphetids and temnospondyls including dermal ornamentation, absence of coronoid teeth, and presence of coronoid shagreen. Dentary dentition is most similar to Baphetes. An adsymphysial toothplate may not preclude temnospondyl affinity. An apparent large exomeckelian fenestra, with the dorsal foraminal margins formed by an unossified element, echoes the morphology of the stem tetrapod Sigournea and is unusually primitive given the other features of the jaw. The jaw may thus provide an example of an intermediate stage in Meckelian element evolution.

The relationship between developmental genes and phenotypic variation is of central interest in evolutionary biology. An excellent example is the role ofHoxgenes 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 equivalentHoxgenes are active in the axial skeleton during embryonic development, indicating that the morphological variation across taxa is likely owing to modifications in the pattern ofHoxgene expression. By using geometric morphometrics, we demonstrate a correlation between vertebralHoxcode and quantifiable vertebral morphology in modern archosaurs, in which the boundaries between morphological subgroups of vertebrae can be linked to anteriorHoxgene expression boundaries. Our findings reveal homologous units of cervical vertebrae in modern archosaurs, each with their specificHoxgene 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.

Sloths are one of only two exceptions to the mammalian ‘rule of seven’ vertebrae in the neck. As a striking case of breaking the evolutionary constraint, the explanation for the exceptional number of cervical vertebrae in sloths is still under debate. Two diverging hypotheses, both ultimately linked to the low metabolic rate of sloths, have been proposed: hypothesis 1 involves morphological transformation of vertebrae due to changes in the Hox gene expression pattern and hypothesis 2 assumes that the Hox gene expression pattern is not altered and the identity of the vertebrae is not changed. Direct evidence supporting either hypothesis would involve knowledge of the vertebral Hox code in sloths, but the realization of such studies is extremely limited. Here, on the basis of the previously established correlation between anterior Hox gene expression and the quantifiable vertebral shape, we present the morphological regionalization of the neck in three different species of sloths with aberrant cervical count providing indirect insight into the vertebral Hox code. Shape differences within the cervical vertebral column suggest a mouse-like Hox code in the neck of sloths. We infer an anterior shift of HoxC-6 expression in association with the first thoracic vertebra in short-necked sloths with decreased cervical count, and a posterior shift of HoxC-5 and HoxC-6 expression in long-necked sloths with increased cervical count. Although only future developmental analyses in non-model organisms, such as sloths, will yield direct evidence for the evolutionary mechanism responsible for the aberrant number of cervical vertebrae, our observations lend support to hypothesis 1 indicating that the number of modules is retained but their boundaries are displaced. Our approach based on quantified morphological differences also provides a reliable basis for further research including fossil taxa such as extinct ‘ground sloths’ in order to trace the pattern and the underlying genetic mechanisms in the evolution of the vertebral column in mammals.

Atlas of Terrestrial Mammal Limbs is the first comprehensive and detailed anatomy book on a broad phylogenetic and ecological range of mammals. This extraordinary new work features more than 400 photographs and illustrations visualizing the limb musculature of 28 different species. Standardized views of the dissected bodies and concise text descriptions make it easy to compare the anatomy across different taxa. It provides tables of nomenclature and comparative muscle maps (schematic drawings on the origins and insertions of the muscles onto bones) in a diversity of animals. Atlas of Terrestrial Mammal Limbs is a reliable reference and an indispensable volume for all students and professional researchers in biology, paleontology, and veterinary medicine. Key Features: Provides an overview of the anatomy of the mammalian limb Includes osteological correlates of the limb muscles Illustrates anatomy in 2D Guides dissection Documents anatomical diversity in mammalian limbs Related Titles: D. L. France. Human and Nonhuman Bone Identification: A Color Atlas. (ISBN 978-1-4200-6286-1) S. N. Byers. Forensic Anthropology Laboratory Manual, 4th Edition (ISBN 978-1-1386-9073-8) S. N. Byers. Introduction to Forensic Anthropology, 5th Edition (ISBN 978-1-1381-8884-6) R. Diogo, et al. Muscles of Chordates: Development, Homologies, and Evolution (ISBN 978-1-1385-7116-7)