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 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 cranidial 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.

This dataset comprises three* files. The first is a ReadMe file with usage notes for the other two files. The second is an R script (par.est.R) for estimating growth rates and other parameters from an empirical dataset of length measurements from different sclerites of a trilobite. The third is a text file (Elrathia-kingii-length.txt) with empirical data collected from specimens of the trilobite Elrathia kingii.

Specimens were collected from a 1.5-m thick interval of thin-bedded mud limestone from the Wheeler Formation, about 35 meters below the base of the Marjum Formation, at North Antelope, in the House Range east of Dome Canyon, Millard County, Utah, USA (AMNH locality 3952). Specimens were mechanically prepared from the matrix, whitened with ammonium chloride vapor and photographed. Lengths of sclerites (= cephalon, thoracic tergites, and pygidum) were measured from the photographs either directly as length measurements for partially disarticulated specimens or calculated as inter-landmark distances. True tergite lengths are sometimes obscured along the sagittal axis because the occipital ring overlaps the axial ring of the first tergite and because not all specimens are perfectly prone. To circumvent this, the length of each tergite was estimated along both axial furrows and averaged.

*On 12/17/2020, I added two additional datafiles which had been excluded unintentionally.  One datafile contains the landmark data for the 160 specimens of Elrathia kingii used for the shape analysis. This is a subset of the specimens used for length measurements reported in "Elrathia-kingii-length.txt".  The second datafile is a list of the number of thoracic tergites for each specimen. I have also included a second ReadMe file with full description of the landmark data.