The Development of the Silurian Trilobite Aulacopleura koninckii Reconstructed by Applying Inferred Growth and Segmentation Dynamics: A Case Study in Paleo-Evo-Devo

Abstract

Fossilized growth series provide rare glimpses into the development of ancient organisms, illustrating descriptively how size and shape changed through ontogeny. Occasionally fossil preservation is such that it is feasible to test alternative possibilities about how ancient development was regulated. Here we apply inferred developmental parameters pertaining to size, shape, and segmentation in the abundant and well-preserved 429 Myr old trilobite Aulacopleura koninckii that we have investigated previously to reconstruct the post-embryonic ontogeny of this ancient arthropod. Our published morphometric analyses associated with model testing have shown that: specification of the adult number of trunk segments (polymorphic in this species) was determined precociously in ontogeny; that growth regulation was targeted (i.e., compensatory), such that each developmental stage exhibited comparable variance in size and shape; and that growth gradients operating along the main body axis, both during juvenile and adult ontogeny, resulted from a form of growth control based on positional specification. While such developmental features are common among extant organisms, our results represent the oldest evidence for them within Metazoa. Herein, the novel reconstruction of the development of Aulacopleura koninckii permits visualization of patterns of relative and absolute growth and segmentation as never before possible for a fossilized arthropod ontogeny. By conducting morphometric analysis of appropriate data sets it is thus possible to move beyond descriptive ontogenetic studies and to address questions of high interest for evolutionary developmental biology using data from fossils, which can help elucidate both how developmental processes themselves evolve and how they affect the evolution of organismal body patterning. By extending similar analyses to other cases of exceptional preservation of fossilized ontogeny, we can anticipate beginning to realize the research program of "paleo-evo-devo."

Anamorphic development and extended parental care in a 520 million-year-old stem-group euarthropod from China

“…The growth trajectory of F. protensa as recorded by the morphometric data ( Fig. s9b) also evokes parallels with that of the Silurian proetid trilobite Aulacopleura konincki, in which body size is somewhat constrained among juveniles, but is more variable during late ontogeny resulting in adults with a polymorphic trunk segment count [34,35]. The prevailing interpretation for the growth pattern of A. konincki favours a developmental model where the total number of thoracic segments was determined during early ontogeny, and once this maximum was reached, growth continued epimorphically [34,35].…”

“…s9b) also evokes parallels with that of the Silurian proetid trilobite Aulacopleura konincki, in which body size is somewhat constrained among juveniles, but is more variable during late ontogeny resulting in adults with a polymorphic trunk segment count [34,35]. The prevailing interpretation for the growth pattern of A. konincki favours a developmental model where the total number of thoracic segments was determined during early ontogeny, and once this maximum was reached, growth continued epimorphically [34,35]. Unfortunately the small sample size of measurable juveniles (stage 8 and 9, n = 9; Supplementary Table S1) prevents us from drawing a comparable interpretation for the regulatory mechanisms responsible for [31,32].…”

Anamorphic development and extended parental care in a 520 million-year-old stem-group euarthropod from China

“…12b ) also evokes parallels with that of the intensively studied Silurian proetid trilobite Aulacopleura konincki . Here, the range of body size is somewhat constrained among juvenile stages (meraspids), but becomes much more variable during late ontogeny resulting in mature adults (holaspids) with a polymorphic trunk segment count [ 37 , 38 ]. The prevailing interpretation for the growth pattern of A. konincki favours a developmental model where the total number of thoracic segments was determined during early ontogeny, and once this maximum was reached, growth continued epimorphically in the holaspid stage.…”

A simple generative model of trilobite segmentation and growth

“…I used a data set of measurements from Aulacopleura koninckii specimens (Hughes et al, 2017) to estimate values for each model parameter. A representative holaspid specimen of A. koninckii is shown in Supplementary Figure S1A; for additional specimens, including meraspid stages, see Hong et al (2014).…”

“…Segmentation and growth patterns in trilobites are now sufficiently well understood (e.g., Hughes, 2003a,b;Minelli et al, 2003;Hughes, 2005;Hughes et al, 2006;Hughes, 2007;Minelli and Fusco, 2013;Fusco et al, 2014;Hong et al, 2014;Fusco et al, 2016) that generative growth models are within reach. Further, measurements of body size and body proportions for a large sample of specimens are now available for one of the best studied trilobite species in this regard, Aulacopleura koninckii (Barrande, 1846) (Hughes et al, 2017). In this paper, I use these studies as a basis for developing a simple generative model for trilobites and explore the impact of varying different parameters on body size and body proportions.…”