HITCHIN' A RIDE: EPIBIONTS ON ARCHAEOCIDARIS ECHINOIDS

Epibiosis provides a unique opportunity to study interactions between contemporaneous organisms in the fossil record. Study of epibiont-host interactions through time provides insight into the evolutionary response of the host organisms to epibiosis.

Spines of many Pennsylvanian and Permian Archaeocidaris echinoids from north-central Texas bear epibionts. These attaching organisms vary in diversity and abundance, depending on echinoid species, spine morphology, and preservation of the echinoid. Epibionts include several bryozoans, Crurithyris brachiopods, worm tubes, Phosphannulus -like borings, and possibly gastropods. Spines on all echinoids are long relative to test diameter and contain abundant thorns and other protrusions, providing ample microhabitats for settling organisms. On some echinoid species, epibionts are preferentially located proximal to the test. Spine ornament on Texas archaeocidarids increases in complexity through time with a corresponding increase in epibiont diversity and abundance.

These epibiotic associations provide insight into the relationship between the attaching organisms and echinoid hosts. In one species, articulated archaeocidarids with and without epibionts do not differ significantly in test sizes, indicating that epibonts may have had little effect on echinoid growth. Increase in epibiont cover through time and corresponding microhabitat availability on spines may indicate a favorable response to epibiont settlement or may simply be a side effect from increased spine ornament. Benefits to the epibionts may include available substrate in soft-sediment environments, increased water current from spine and echinoid movement, and decreased spatial competition with other epibionts because of the high availability of settlement spots.