Browsing by Author "Anderson, Jason S"

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    Feeding height stratification among the herbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta, Canada
    (BioMed Central, 2013-04-04) Mallon, Jordan C; Evans, David C; Ryan, Michael J; Anderson, Jason S
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    Hidden morphological diversity among early tetrapods
    (Nature, 2017-06-21) Pardo, Jason D; Szostakiwskyj, Matt; Ahlberg, Per E; Anderson, Jason S
    Phylogenetic analysis of early tetrapod evolution has resulted in a consensus across diverse data sets in which the tetrapod stem group is a relatively homogenous collection of medium- to large-sized animals showing a progressive loss of 'fish' characters as they become increasingly terrestrial, whereas the crown group demonstrates marked morphological diversity and disparity. The oldest fossil attributed to the tetrapod crown group is the highly specialized aïstopod Lethiscus stocki, which shows a small size, extreme axial elongation, loss of limbs, spool-shaped vertebral centra, and a skull with reduced centres of ossification, in common with an otherwise disparate group of small animals known as lepospondyls. Here we use micro-computed tomography of the only known specimen of Lethiscus to provide new information that strongly challenges this consensus. Digital dissection reveals extremely primitive cranial morphology, including a spiracular notch, a large remnant of the notochord within the braincase, an open ventral cranial fissure, an anteriorly restricted parasphenoid element, and Meckelian ossifications. The braincase is elongate and lies atop a dorsally projecting septum of the parasphenoid bone, similar to stem tetrapods such as embolomeres. This morphology is consistent in a second aïstopod, Coloraderpeton, although the details differ. Phylogenetic analysis, including critical new braincase data, places aïstopods deep on the tetrapod stem, whereas another major lepospondyl lineage is displaced into the amniotes. These results show that stem group tetrapods were much more diverse in their body plans than previously thought. Our study requires a change in commonly used calibration dates for molecular analyses, and emphasizes the importance of character sampling for early tetrapod evolutionary relationships.
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    Morphology, Ontogeny, and Phylogenetic Relationships of the Permo-Carboniferous tetrapod Brachydectes newberryi from the Council Grove Group, Nebraska, USA
    (2014-10-07) Pardo, Jason Daniel; Anderson, Jason S
    Lysorophia is a poorly-understood group of fossil tetrapods known from the Late Carboniferous and Early Permian of North America. Some prior workers have noted similarities between lysorophians and modern amphibians, suggesting that lissamphibians may have evolved from lysorophian-like ancestors. I used high resolution x-ray micro-computed tomography (HR-XCT) to study skulls of the lysorophian Brachydectes newberryi from the Early Permian of Kansas and Nebraska, USA. I present a detailed description of the skeletal morphology of these skulls, including fine structure of the braincase. With reference to the skeletal morphology described here, I present a list of new, phylogenetically-informative characters from the braincase and incorporate these into phylogenetic analysis of early tetrapods. Lysorophians are found to be microsaurs, a diverse group of early tetrapods. The data presented here suggest that lysorophians and microsaurs may be early reptiles and thus not relevant to the discussion of lissamphibian origins.
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    A multimethod approach to the differentiation of enthesis bone microstructure based on soft tissue type
    (Wiley, 2021-06) Whitebone, S Amber; Bari, A S M Hossain; Gavrilova, Marina L; Anderson, Jason S
    Whereas there is a wealth of research studying the nature of various soft tissues that attach to bone, comparatively little research focuses on the bone's microscopic properties in the area where these tissues attach. Using scanning electron microscopy to generate a dataset of 1600 images of soft tissue attachment sites, an image classification program with novel convolutional neural network architecture can categorize images of attachment areas by soft tissue type based on observed patterns in microstructure morphology. Using stained histological thin section and liquid crystal cross-polarized microscopy, it is determined that soft tissue type can be quantitatively determined from the microstructure. The primary diagnostic characters are the orientation of collagen fibers and heterogeneity of collagen density throughout the attachment area thickness. These determinations are made across broad taxonomic sampling and multiple skeletal elements.