Ontogeny of the trilobite Elrathia kingii (Meek) and comparison of growth rates between Elrathia kingii and Aulacopleura koninckii (Barrande)

Abstract

AbstractTrilobites offer almost unparalleled insight into the growth and development of fossil ecdysozoans. Here I use newly collected material of Elrathia kingii (Meek) to estimate growth rates and describe shape change over the ontogeny of E. kingii. Well‐preserved, articulated specimens from all post‐embryonic stages were collected from a 1.5 m interval of the upper Wheeler Formation (Miaolingian Series, Cambrian) in western Utah (USA), and size and landmark‐based shape data were digitized from photographs. Growth rates were estimated and compared with previously published data on the Silurian trilobite, Aulacopleura koninckii (Barrande). Like A. koninckii, the cephalic growth rate in E. kingii was constant and of similar magnitude to the minimum growth rate along the trunk, and growth rates in the trunk were lower during the holaspid (‘adult’) period than during the meraspid (‘juvenile’) period. However, body length at the onset of meraspis was smaller, the growth gradient along the trunk during meraspis was shallower, and the terminal number of thoracic tergites was smaller in E. kingii than in A. koninckii. Despite these differences, these two species had similar maximum body lengths, because higher overall growth rates in E. kingii compensated for other differences. The rate of cephalic shape change in E. kingii decreased at the transition from meraspis to holaspis, while the pygidium became more morphologically distinct from the thorax during holaspis. I also provide an emended diagnosis for E. kingii, descriptions of the ontogeny and ventral morphology, and evidence that E. kingii holaspids had an invariant number of tergites.