CONSERVATION OF SUBSTRATE AFFINITY WITHIN MARINE INVERTEBRATE GENERA DURING THE PHANEROZOIC

A considerable body of work has shown environmental patterning of diversification rates through time. Substrate affinity has a particularly strong impact on Phanerozoic-scale diversity dynamics. However, despite the common occurrence of habitat shifting and invasion in both fossil and recent taxa, substrate affinity has been assessed only for genera over their entire durations. Here we use data from the Paleobiology Database to investigate the frequency with which marine invertebrate genera maintain one substrate affinity throughout their durations. We use three different methods to determine the substrate affinity of each genus for each geological stage in which it has at least three occurrences.

Depending on the method used, 35-40% of genera spanning more than one stage maintain their affinities, showing affinity only for carbonates, only for clastics, or no affinity over their durations. Of the genera that are assigned an affinity in at least one stage, 83-88% are assigned to only one affinity. Shifting from one affinity to the other in adjacent intervals occurs at very low rates. Of the five best represented classes in the dataset, Gastropoda and Cephalopoda show similar rates of affinity conservation as marine invertebrates as a whole, Bivalvia and Rhynchonellata show lower rates and Anthozoa shows higher rates. In addition, these classes differ in the timing and extent of shifting over the Phanerozoic. Notably both gastropods and bivalves show relatively high rates of shifting between affinities around the Permo-Triassic boundary.

Overall these results suggest that substrate affinity is highly conserved within marine invertebrate genera. Genera may be able to survive environmental change by shifting far enough into other environments that they effectively show no affinity. However, the complete adaptation to new substrate types appears exceedingly difficult, which has implications for conservation biology.