Elsa Panciroli, Julia A. Schultz & ZheâXi Luo (2018)
Morphology of the petrosal and stapes of Borealestes (Mammaliaformes, Docodonta) from the Middle Jurassic of Skye, Scotland.
Papers in Palaeontology (advance online publication)
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We describe, in unprecedented detail, the petrosals and stapes of the docodont Borealestes from the Middle Jurassic of Scotland, using high resolution ÎCT and phaseâcontrast synchrotron imaging. We describe the inner ear endocast and the vascularized interior structure of the petrosal, and provide the first endocranial view of a docodontan petrosal. Our study confirms some similarities in petrosal and stapedial morphology with the better known Haldanodon of the Late Jurassic of Portugal, including: (1) the degree of curvature of the cochlea; (2) multiple features related to the highly pneumatized paroccipital region; (3) the shape of lateral trough, the fossa of the M. tensor tympani, and the ridge on the promontorium; (4) the round shape of the fenestra vestibuli; and (5) overall morphology of the stapes. But Borealestes differs from Haldanodon in having a bony ridge that separates the tympanic opening of the prootic canal, the secondary facial foramen and the hiatus Fallopii, from the fenestra vestibuli. We identify two new vascular structures: the anterior and posterior transâcochlear sinuses, which traverse the pars cochlearis around the cochlear nerve (VIII). These transâcochlear sinuses have not been observed in previous docodont specimens, and could be an autapomorphy of Borealestes, or apomorphic for this clade. We also establish the anatomical relationship of the circumâpromontorium plexus to the inner endocast. The high quality of our scans has made these structures visible for the first time.
Pedro L. Godoy, Roger B. J. Benson, Mario Bronzati & Richard J. Butler (2018)
The multi-peak adaptive landscape of crocodylomorph body size evolution.
bioRxiv preprint (not peer-reviewed)
Background: Little is known about the long-term patterns of body size evolution in Crocodylomorpha, the > 200-million-year-old group that includes living crocodylians and their extinct relatives. Extant crocodylians include 24 species and span a narrow range of phenotypes, being mostly large-bodied (3-7 m) semi-aquatic predators. However, extinct crocodylomorphs exhibit a wider range of phenotypes, and many of the earliest taxa were much smaller (< 1.2 m). This suggests a pattern of size increase during the evolutionary history of Crocodylomorpha. Furthermore, the influence of abiotic and biotic factors on crocodylomorph body size evolution is currently poorly understood. In this study, we quantify patterns of body size disparity through the evolutionary history of crocodylomorphs. We use phylogenetic comparative methods to characterise the macroevolutionary patterns that gave rise to this disparity, and to quantitatively test hypotheses of abiotic (i.e., palaeotemperature and palaeolatitude) and intrinsic (habitat) factors as potential drivers. Results: Crocodylomorphs reached an early peak in body size disparity during the Late Jurassic, and underwent essentially continual decline since then. A multi-peak Ornstein-Uhlenbeck model outperforms all other evolutionary models fitted to our data (either single- or multi-regime models), indicating that the macroevolutionary dynamics of crocodylomorph body size evolution are better described within the concept of adaptive landscape, with most body size variation emerging after shifts to new adaptive zones (macroevolutionary regimes). We did not find support for a consistent evolutionary trend towards larger sizes among lineages (i.e., Cope's rule), or strong correlations of body size with climate. Instead, the intermediate to large body sizes of some crocodylomorphs are better explained by group-specific adaptations. In particular, the evolution of a more aquatic lifestyle (especially marine) correlates, in several clades, with increases in average body size, though not without exceptions. Conclusions: On large phylogenetic scales, shifts between macroevolutionary regimes (representing lineage-specific innovations) are more important than isolated climatic factors. Shifts leading to larger body sizes are commonly associated with predominantly aquatic or semi-aquatic groups. This, combined with the extinction of smaller-sized regimes (particularly during the Late Cretaceous and Cenozoic), gave rise to the upward-shifted body size distribution of extant crocodylomorphs compared to their terrestrial ancestors.