Osteohistological investigations of hyperelongate vertebral spinous processes (neural spines) are presented to elucidate previously unknown aspects of dorsal sail form and function in two, closely related genera of "sail-backed" synapsids: Sphenacodon and Dimetrodon. Although recent and classic surveys of bone histology in extinct vertebrates have sampled the genus Dimetrodon, new sectioning of Sphenacodon material allows a comparative analysis of these structures among Sphenacodontidae for the first time. Variability within the histological organization of the neural spine is assessed by examining multiple regions along its length, and implications for soft tissue correlates, growth and mechanics are considered here. Both genera exhibit extensive parallel-fibered and fibrolamellar bone, in addition to lamellar bone. Several features vary along the length of the spine in each species. Muscle scars and extensive Sharpey's fibers are present at the base of the spine; no scars and fewer fibers are manifested ∼55-60 mm above the zygapophyses in mature individuals. The distal cortex of the spine does not exhibit greater vascularity than the proximal region in either genus. However, both genera manifest distinct vascular grooves of variable size along the distal periosteal surface, some of which become incorporated into the distal cortex. The observed histovariability appears to record the transition from the proximal (epaxial muscle embedded) to the distally protruding portion of the spine. These observations and independent pathological evidence support the existence of a short dorsal crest in Sphenacodon and possibly other basal sphenacodontids. Although the thermoregulatory capacity of such a crest remains uncertain, developmental and mechanical features are readily interpretable and are discussed with respect to the origins and early evolution of the dorsal sail in sphenacodontid synapsids.

This study presents evidence of pre-mortem traumatic injury and its sequalae on multiple Edmontosaurus annectens skeletal elements recovered from a largely monodominant Cretaceous (Maastrichtian) bonebed. The sample consists of 3013 specimens excavated and prepared from two quarries, of which 96 elements manifest one or more macroscopic bone abnormalities and 55 specimens display pathology attributable to physical trauma. Evidence of traumatic pathology is strongly associated (P < .05) with body region, occurring disproportionately in the caudal vertebrae. Pre-mortem fractures with subsequent bone remodeling and hypertrophic ossification of caudal neural spines are present principally in the middle and mid-distal regions of the tail, while fractures of the vertebral centra are present primarily in the distal tail region. Other skeletal regions, such as chevrons, phalanges of the manus and ribs display unambiguous evidence of healed trauma, but with less frequency than the tail. These findings, in combination with current understanding of hadrosaurian tail biomechanics, indicate that intervertebral flexibility within the middle and mid-distal region of the tail likely rendered these caudal vertebrae more susceptible to the deleterious effects of repeated mechanical stress and subsequent trauma, potentially accompanying running locomotion and other high-impact herd interactions. Healed fractures within the region are also suggestive of accumulated injuries due to a combination of tail usage in defense and possibly accidental bumping/trampling associated with gregarious behavior.

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While only distantly related to mammals, the anatomy of Permian gorgonopsians has shed light on the functional biology of non-mammalian synapsids and on the origins of iconic ‘mammal-like’ anatomical traits. However, little is known of gorgonopsian behaviour or physiology, which would aid in reconstructing the paleobiological context in which familiar mammalian features arose. Using multi-modal imaging, we report a discrete osseous lesion in the forelimb of a late Permian-aged gorgonopsian synapsid, recording reactive periosteal bone deposition and providing insights into the origins and diversity of skeletal healing responses in premammalian synapsids. We suggest that the localized lesion on the anterolateral (preaxial) shaft of the left radius represents acute periostitis and, conservatively, most likely developed as a subperiosteal haematoma with subsequent bone deposition and limited internal remodelling. The site records an inner zone of reactive cortical bone forming irregular to radial bony spicules and an outer, denser zone of slowed subperiosteal bone apposition, all of which likely occurred within a single growing season. In surveys of modern reptiles—crocodylians, varanids—such haematomas are rare compared to other documented osteopathologies. The extent and rapidity of the healing response is reminiscent of mammalian and dinosaurian bone pathologies, and may indicate differing behaviour or bone physiology compared to non-dinosaurian reptiles. This report adds to a growing list of putative disease entities recognized in early synapsids and broadens comparative baselines for pathologies and the evolution of bone response to disease in mammalian forebears. This article is part of the theme issue ‘Vertebrate palaeophysiology’.

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