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Interactions with predators and parasites can result in traces found on Recent and fossil echinoids. However, identifying specific trace makers, particularly on fossil echinoids, remains contentious. To document the range of trace morphologies present on echinoids and improve our ability to identify and quantify biotic interactions affecting echinoids, we characterized traces found on fossil echinoids using museum collections and field sampling spanning the Jurassic to Recent worldwide. Using light microscopy, 8,564 individual echinoid specimens were examined including 130 species, and 516 traces of potential biotic interactions identified. Morphological characteristics were recorded for each trace, including the shape of the trace outline, maximum diameter and cross-section profile. Based on shared morphological characteristics, it was possible to classify all traces into eight categories: circular, subcircular, elongated, irregular, rectangular, figure-eight, notched, and linear. Cross-section characteristics provided additional insights into the identity of potential trace makers. To further evaluate the proposed biotic origins of these traces, trace diversity was examined through time and compared with anticipated ecological trends associated with the diversification of echinoids, and their predators and parasites. Trace diversity increased over time, starting in the late Eocene, coincident with the proliferation of echinoid-drilling gastropods, an indication that biotic interactions intensified through evolutionary time, as predicted by several macroevolutionary hypotheses previously tested using mollusks. The morphological descriptions provided here enhance our understanding of biotic traces on fossil echinoids, and the potential to identify temporal trends in the intensity and diversity of biotic interactions that have affected echinoids throughout their evolutionary history.
Trace fossils preserved in fossilized tissues provide a key resource for exploring the paleoecology of past ecosystems. Endobiont organisms are commonplace in modern ecosystems, but their trace fossils on vertebrates are rare as the organisms usually attack or attach to soft tissue. Here, we report the novel occurrence of flask-shaped boreholes representing the ichnotaxon Karethraichnus n. isp. in the carapace of the basal leatherback sea turtle Mesodermochelys sp. from the Upper Cretaceous of northern Japan. The distribution of the boreholes was determined by observing the carapace surface. Using X-ray computed tomography, we were also able to produce a 3D reconstruction of the whole carapace and examine a cross section of a borehole to analyze the histological aspects of the bone. In total, 43 holes were observed, 12 holes contained probable pholadoid bivalves, and 32 holes were not bored entirely through the carapace. Some of the bivalves found in the holes are larger than the aperture of the hole, suggesting that they continued to grow during boring. The holes are hemispherical to clavate in shape and developed on the exterior side of the carapace. Healing traces, i.e., repairing of bone, can be observed at the surface of the holes. Our observations strongly suggest that these pits were bored by pholadoid bivalves while the turtle was alive. This is the first report of the behavior of boring bivalves as sea-turtle endobionts boring into a unique free-living, i.e., “swimming substrate”.