ROUND UP THE USUAL SUSPECTS: UBIQUITOUS TAXA AND SYSTEMATIC INERTIA

Certain fossil taxa, such as Phacops, Turritella, and Gryphea are familiar to every North American paleontologist. They are described in all of our textbooks and found in our introductory teaching laboratories. Why are these taxa so ubiquitous? They may be common in a particular locale; e.g., Knightia from the Green River Fm. Perhaps they are widely distributed in time and space and thus commonly encountered. The underlying factors should be similar to those that control diversity patterns. Ecologically, these taxa could have been abundant in the life assemblage or had wide environmental or biogeographic ranges. The cause may be taphonomic; they are forms with unusually high preservation potential. Ubiquity may also be due to their morphology being highly plesiomorphic or otherwise highly generalized (i.e., "form taxa"). Finally, there may be a role played by the systematic equivalent of Paleontological Interest Units; e.g., Systematics Interest Units - some groups have not been recently revised and thus may contain many paraphyletic taxa. One potential influence we term "systematic inertia": specimens in field collections could tend to be assigned to taxa included in well-known, widely available sources and not reflect subsequent taxonomic revisions. This concept predicts that ubiquitous genera should have early dates of description. The Paleobiology Database [PBDB] was queried to determine the 100 genera with the highest number of occurrences within bivalves, brachiopods, trilobites, echinoids, cephalopods, crinoids and anthozoa, for the intervals Ordovician-Carboniferous and Jurassic-Paleogene. For each of these genera, we determined the date of original naming. Overall, the average year of first description for all taxa was ca. 1870. For genera with living representatives (many molluscs), the average year was ca. 1830. This suggests another aspect of the Pull of the Recent; i.e., fossil forms tend to be assigned to living groups of similar morphology. It is also possible that these forms are truly ubiquitous; they were the first encountered and thus named. If true, this further supports the robustness of many paleobiological patterns. [Contribution of the PBDB Taphonomy group].