Larvae and relationships of the Calymenina (Trilobita)

Abstract

Up to four discrete protaspid larval stages are described for calymenid trilobites of Ordovician to Devonian age. The earliest growth stages are nonadult-like planktonic protaspides; later protaspides are adult-like and benthonic. In contrast, the related homalonotid trilobites apparently lack planktonic protaspides, but have up to two large benthonic protaspid stages that are similar in form to calymenid benthonic protaspides. These differences in life history patterns between these families are reflected in their paleobiogeographic distributions. Calymenids werre widely dispersed from Ordovician to Devonian times, both being common in warm, low latitude provinces (particularly from the Late Ordovician onwards) and well represented in cooler, higher latitude regions. The paleogeographic distribution of the homalonotids during the Ordovician (Arenig to the Ashgill) was concentrated in high paleolatitudes, with only a few forms occurring at low paleolatitudes (often in deeper, cooler environments?). Both families survived the Ordovician–Silurian mass extinction, with the calymenids again being widely dispersed but the homalonotids being best represented in the cool-water Malvinokaffric Province and in other regions where they are largely restricted to clastic facies.So few complete growth series of calymenine trilobites are known that it is unlikely that the ontogenies of taxa that form parts of ancestor–descendant clades can be identified. However, some evidence for heterochronic, particularly paedomorphic (neotenic), evolution is suggested for larval stages of members of both the Calymenidae and the Homalonotidae. Such possible neotenic evolution leading to very large planktonic larval stages of calymenid trilobites during the Devonian could have enhanced dispersal during a period of widespread warm and equable climates. Comparisons of homalonotid protaspides with equivalent stages of calymenids support the close relationship of these families within the Calymenina. A data matrix based upon characters of protaspides of two calymenine trilobites (Flexicalymene Shirley, 1936, and Brongniartella Reed, 1918) and eight other trilobites, belonging to the Phacopina (Calyptaulax), Cheirurina (Physemataspis and Hyrokybe), Proetida (Scharyia), Lichida (Acanthopyge), Odontopleurida (Diacanthaspis), Corynexochida (Bathyuriscus), and Ptychopariida (Crassifimbra) was subjected to cladistic analysis using the parsimony program “Hennig 86.” The shortest length cladogram produced is consistent with the inclusion of the Homalonotidae in the Calymenina, and inclusion of the Calymenina in the order Phacopida. “Cheirurina” is the paraphyletic “stem group” of Phacopina. The hypothesis that Lonchocephalidae is most closely related to part of post-Cambrian Phacopida is poorly supported by protaspid characters.