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

Abstract

Trilobites 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.

Absolute axial growth and trunk segmentation in the early Cambrian trilobite Oryctocarella duyunensis

“…Initial work in this area focused on a ~429-Myr-old Ma aulacopleurid trilobite, Aulacopleura koninckii , from one locality in the Czech Republic (Fusco et al 2004, 2014, 2016). Recently, similar studies have been conducted on other species and at other sites, including some of Cambrian age (e.g., Du et al 2019, 2020; Holmes et al 2020; Hopkins 2020). Although all such cases have limitations, including the rather specific set of paleoenvironments represented, these studies do offer important insight into how ancient developmental processes were regulated in this early euarthropod clade.…”

Complex axial growth patterns in an early Cambrian trilobite from South Australia

“…In O. duyunensis, TS2 is the longest for much of the meraspid period, shifting to TS3 at stage 11 [10]. In Elrathia kingii, TS2 is the longest in the early meraspid period, shifting to TS3-4 in later stages [9]. In royalsocietypublishing.org/journal/rspb Proc.…”

“…Many articulated trilobite ontogenies have been published, particularly in recent years (e.g. [6][7][8][9]). However, even with their exemplary record, obtaining the data required for detailed morphometric studies, such as those relating to segmental growth, is problematic.…”

Articulated trilobite ontogeny: suggestions for a methodological standard

“…Mixed cross-sectional data are those for which a criterion of stage assignment is not available, which is often the case in trilobite specimens with the mature number of segments, or when distinct sizes classes are not evident. Both types of data can be used in studies of fossil ontogeny (e.g., Fusco et al, 2016;Hopkins, 2020), but each requires different processing. When a criterion for stage assignment is available, both relative (allometric) and absolute (stage-based) growth analyses are possible.…”