Global Size Trends in Ordovician and Silurian Brachiopod Lineages

Despite the emphasis on diversity trends of marine invertebrates in the early Paleozoic there are few comprehensive studies on size, especially at lower taxonomic levels. Trends in body size across related taxa can offer key insights into evolutionary and long-term ecological processes. This study looks at body size trends and changes in variance within and across groups through the Ordovician and into the Silurian.

Body size of mature specimens (>2500 specimens; >270 genera.) was estimated with linear shell dimensions measured using photographs in taxonomic monographs. To estimate size trends the median log length was plotted at temporal resolutions of the stage level. The initial analysis was limited to six total families in the orders of Orthida and Strophomenida which sample sizes were deemed adequate (n>20).

Global brachiopod size fluctuated throughout the Ordovician. Orthids and strophomenids exhibit an abrupt size increase in the Soudleyan stage (Late Ordovician), but this is not attributable to the introduction of new large-sized genera. By the Hirnantian stage (Late Ordovican) size decreased to values seen originally in the Early Ordovician. In the Early Silurian, the strophomenids' median size increased to Middle Ordovician values, but decreased to half that size fifteen million year later in the late Telychian (Early Silurian). The median size of orthid brachiopods remained relatively static in the Silurian with minimal fluctuations in median size and variance. The overall increase in brachiopod size from the Ordovician to the Silurian is comparable to global size trends reported for other marine invertebrates. However at finer temporal and taxonomic resolution, brachiopod lineages experienced two dramatic reductions in median size in the Late Ordovician (Hirtnantian) and Early Silurian (Telychian), with only the strophomenid lineages increasing in size in the Silurian.