Differentiating convergent pathologies in turtle shells using computed tomographic scanning of modern and fossil bone

Patterns of bone damage can provide insights into the paleobiology and paleoecology of extinct groups, but accessing these data relies on accurate identifications. Among turtles, there are persistent concerns that shell disease and vertebrate feeding traces are confoundingly convergent. Though extreme cases of shell disease and more penetrative bite marks are more easily differentiated, more subtle examples of each can exhibit very similar morphologies. Given the different ways these two types of bone damage form, one through compression and impact damage and the other through immune system response to foreign bodies or infection, internal textures of the bone should provide helpful data to differentiate these convergent pathologies. Here we use microcomputed tomographic (mu CT) scanning to visualize internal bone damage in known examples of modern bite marks and shell disease in Trachemys carapace and plastron elements. Observed patterns are then compared to fossil examples of putative bite marks and shell disease from Cenomanian turtle shell fragments from the Woodbine Group of north-central Texas, U.S.A. Modern and fossil examples of shell disease exhibit an irregular, bubble-like texture in cross section that erodes away the subsurface below the surficial marks, whereas the bite marks exhibited more regular U- or V-shaped indentations in cross section, underlain by fractured, compressed bone. Our study demonstrates that while different bone modifying processes can produce similar, surficial patterns, internal damage patterns can provide an alternate method for differentiating them. This suggests that internal visualization techniques have been underutilized when attempting to untangle surficial modifications affected by equifinality.