Ontogeny of the trilobite Elrathia kingii (Meek, 1870), and comparison of growth rates between Elrathia kingii and Aulacopleura koninckii (Barrande, 1846)

Trilobites offer almost unparalleled insight into the growth and development of fossil ecdysozoans. Here I use newly collected material of Elrathia kingii (Meek, 1870) 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-meter 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 to previously published data on the Silurian trilobite, Aulacopleura koninckii (Barrande, 1846). 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 lengths at the onset of meraspis were smaller, the growth gradient a...