TRACKING TRACES OF PREDATION AND PARASITISM ON FOSSIL ECHINOIDS THROUGH GEOLOGIC TIME

Although a variety of traces are found on Recent and fossil echinoids, attributing traces to specific trace makers remains contentious due to a lack of diagnostic criteria. Rigorous characterizations and descriptions of a broad range of traces observed on echinoid tests could improve our ability to identify and interpret ecological interactions between echinoids and their predators and parasites in the fossil record.

Here we describe and characterize traces of biotic interactions observed in 8,693 specimens (from131 species) of fossil echinoids from Cretaceous to Recent. The 587 distinct biological traces observed were grouped into nine categories: circular, figure-eight, irregular, linear, notched circle, oblong, rectangular, slit, and subcircular. High-resolution images of exemplars of each were then taken using a SEM. To identify patterns in the diversity of ecological behaviors associated with echinoids through time, morphological trace diversity was modeled to determine whether observed trends could arise due to random chance. In each stage, trace morphologies were weighted based on their observed proportions, and randomly sampled with replacement based on the total number of traces present in that stage (1,000 iterations per stage).

Circular traces were most common (n=286), followed by subcircular (n=110) and oblong (n=78) traces, interpreted as being produced by either predation or parasitism. Traces surrounded by a dissolution halo (n=31) were interpreted as the product of acid etching during gastropod predation. Cumulative trace diversity increases in the late Eocene, much later than predicted by the null model. This suggests that biotic interactions may have intensified through evolutionary time, particularly during the Mesozoic, as predicted by several macroevolutionary hypotheses, previously only evaluated using mollusks.

These categorizations contribute to a growing literature on echinoid traces and ecology. A thorough classification of trace types may promote the identification of traces on fossil echinoids and enhance our ability to identify evolutionary trends in interaction intensity and diversity in the fossil record.