GENERA ARE EFFECTIVE SURROGATES FOR SPECIES RICHNESS IN DEEP TIME: COMPARISONS AT MULTIPLE SCALES

An extensive literature in modern ecology suggests that higher taxon richness is often an adequate surrogate for species richness. Higher taxa, particularly genera, have become the default proxy for species richness in deep time. The efficacy of this method has been questioned, with some claiming that issues such as sampling bias or the essentially arbitrary nature of taxonomic inclusivity may lead to secular biases in genus:species ratios. There are by now well documented examples of such problems, though they concern groups with modest total diversity. The ongoing development of a species-level database of Trilobita permits exploring the relationship between genus and species richness at multiple taxonomic and geographic scales.

The database currently includes 25,894 published species records, including 21,881 available and accepted named species, 1,763 available names considered junior subjective synonyms, 1, 559 taxa reported in open nomenclature that likely represent distinct species, 222 considered uninterpretable dubia, 194 identified as previously named species but which seem to represent distinct new species, and 22 junior homonyms. The genus table includes 5,171 records, of which 4,028 are considered valid, 776 junior subjective synonyms, 195 junior homonyms, 82 dubia, 64 objective synonyms, and 26 with other status (mostly suppressed by action of the ICZN).

Comparisons were carried out using richness in a fixed series of 37 temporal intervals spanning the documented range of trilobites. Diversity profiles were compared by calculating proportional diversity changes and their direction between intervals and applying nonparametric Wilcoxon signed-rank tests and Spearman’s rank correlation. Comparisons of genus and species richness were carried out: 1) globally using the entire dataset; 2) by major paleocontinent and terrane (ranging, e.g., from summed species richness of 5,397 for Laurentia to sizes in the hundreds for smaller terranes); and 3) within orders and families, both globally and parsed geographically. In virtually all comparisons, genus richness was significantly positively correlated with species richness. How representative trilobite patterns are is unknown, but the dataset is an order of magnitude larger than those explored previously.