Ontogeny and shape change of the phacopid trilobite Calyptaulax

Abstract

Major transitions in trilobite ontogeny have historically been defined based on the number and distribution of trunk segments, and articulation between the trunk and cephalon. This study documents additional morphological change across the meraspid-holaspid transition on the Ordovician phacopid trilobite Calyptaulax strasburgensis. An extensive dataset of silicified cranidia and pygidia collected from the mid-Ordovician Edinburg Formation of Virginia was subjected to a series of multivariate analyses, with a primary focus on the intersections and termini of furrows. Multivariate regression of partial warp scores demonstrates statistically significant change in allometric growth patterns over the course of development. These changes are concentrated in earlier instars, but are coincident in cranidia and pygidia. This sharp decrease in the rate of allometry, present in both tagmata, is expressed as significant breakpoints derived from a segmented regression, with the largest portion of allometric change found in the pre-breakpoint individuals. The term holeidos is proposed to describe the completion of form during trilobite development, independent of the completion of the thorax. The most dramatic change in shape during this period of ontogeny includes lateral glabellar expansion through deflection of the axial and palpebral furrows, possibly reflecting a change in the feeding habit during later development. Other morphological changes include the development of a more angular appearance to the anterior portion of the glabella, and anterior migration of the pygidial anterior margin. The appearance of these growth patterns in Calyptaulax extends the temporal range of these changes, some of which have only been documented in Devonian phacopids.

Rise and fall of the phacopids: the morphological history of a successful trilobite family

“…Hence, the assumption that Pterygometopidae is a sister taxon of phacopids seems relevant (Ludvigsen & Chatterton 1982). Similarities between ontogenetic trajectories of this genus and some phacopids (Jacobs & Carlucci 2019) reinforce this hypothesis. Calyptaulax had a facial suture (and therefore the eyes) near to the posterior border furrow, and a vaulted cephalon (Clarkson & Tripp 1982).…”

Morphological disparity trends in Devonian trilobites from North Africa

“…Ontogenetic studies underlined a progressive enlargement (tr.) of the glabella during the development, especially in Phacopida, due to a change in the food niche (Crônier & Feist 1997; Crônier & Fortey 2006; Jacobs & Carlucci 2019). Thus, feeding constraints during development probably led to the absence of trilobites with a very narrow (tr.)…”

“…2021). Morphological disparity of trilobites has been extensively documented in the Cambrian and the Ordovician (Foote 1991 a ; Smith & Lieberman 1999; Webster 2007; Hopkins 2013; Jacobs & Carlucci 2019). However, Devonian studies are rare and often dedicated to developmental and ontogenetic topics exploring the intrinsic factors of morphological change (e.g.…”

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

“…Despite this, it has been shown that threshold models are often better at explaining overall shape change across ontogeny than simple linear regression models and that, where known, these changes appear to occur close to the meraspid/holaspid transition (e.g. [21,24]). The interplay between overall shape and the more specific axial growth patterns identified here will be explored further in a future contribution using geometric morphometrics to quantify allometric shape change in E. bilobata .…”