DOI: 10.5061/dryad.47d7wm39n
title: Data from: Ontogeny of the trilobite Elrathia kingii (Meek, 1870), and comparison of growth rates between Elrathia kingii and Aulacopleura koninckii (Barrande, 1846)

description: 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.
description: 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 <em>Elrathia kingii</em>.
 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 <em>Elrathia kingii</em> 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.