Browsing by Author "Anderson, Jason Scott"

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    Salamander braincase morphology as revealed by micro‐computed tomography
    (Wiley, 2022-01-25) Szostakiwskyj, Matt; Anderson, Jason Scott
    Morphological data sets are misleading in salamander (Caudata) phylogeny due to the relative homoplasy of the dermal skull observed in paedomorphic forms, leading to the trend of excluding morphology when exploring questions of salamander phylogeny. Investigations in caecilians (Gymnophiona) have demonstrated that the inclusion of braincase morphology can rescue morphological phylogenetic analyses and produce topologies congruent with molecular data sets. We scanned 28 species (25 genera) of salamander, representing all 10 families, with high-resolution micro-computed tomography to investigate braincase variation. We describe the morphology of the braincase for all 10 families and distinguish between paedomorphic and metamorphic morphologies. Our results demonstrate a general uniformity amongst metamorphic species with variation largely restricted to the occipito-otic region. A greater range of variation is observed within paedomorphic forms than would be expected when considering the homoplasy of the dermal skull. Obligate paedomorphic forms demonstrate considerably more variation in the anterior braincase than do facultative paedomorphs, which we suggest is evidence of a greater complexity in the evolution and development of these forms than neoteny alone would produce. This raises the question of character independence within morphological data sets and warrants further investigation into the correlation of other characters before morphological data are omitted.
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    The Rhizodonts of Blue Beach, Nova Scotia, Canada (Horton Bluff Formation, Tournaisian Age): New Data on Letognathus hardingi, Other Genera Occurrences, and Their Phylogenetic Implications within the Devonian-Carboniferous Faunal Transition
    (2024-05-28) Heusinkveld, Holly Tegan; Anderson, Jason Scott; Jamniczky, Heather; Theodor, Jessica Madeleine; Dutchak, Alex
    Twenty-three Tournaisian-aged fossils from the Horton Bluff Formation of Blue Beach, Nova Scotia, Canada, were identified as consisting mainly of rhizodont material. Collected from the stratigraphic fossil hiatus “Romer’s Gap”, rhizodont specimens in the sample include humeri, skull bones, shoulder bones, a dermopalatine, and a portion of a braincase. The rhizodont genera identified were Letognathus, Strepsodus, and Rhizodus, which demonstrates a greater rhizodont diversity at Blue Beach than previously known. New morphological characters now identifiable to Letognathus from the sample include postparietals with an interdigitated interpostparietal suture, an ossified braincase, and a compact, semicircular humerus. Unlike the highly derived Strepsodus and Rhizodus, Letognathus displays a more equal mix of rhizodont characters that first appeared in Devonian genera and characters typical of derived Carboniferous genera. Letognathus shares the most in common with the Carboniferous genus Barameda, and is placed phylogenetically as a possible sister taxon to Barameda within the Rhizodontidae. The Strepsodus material from the sample consists of two examples of postparietals and a single humerus. The Rhizodus material consists of a scapulocoracoid. The possible cohabitation of Letognathus, Strepsodus, and Rhizodus at Blue Beach parallels the patterns of overlap previously found among Blue Beach actinopterygians and tetrapods, where related taxa with a greater number of ancestral Devonian characters overlapped taxa with more derived Carboniferous morphologies. This trend across families of closely related taxa with unique combinations of typical Devonian and Carboniferous morphologies overlapping in the Early Carboniferous argues against Romer’s initial proposal that the fossils of the layers composing Romer’s Gap would consist largely of intermediate forms between Devonian and Carboniferous morphologies.