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

Paul S. Hong,Giuseppe Fusco,Nigel C. Hughes,Jinbo Hou

doi:10.3389/fevo.2017.00037

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 A. 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”.

Highlights

The central idea of evolutionary developmental biology is that including information about developmental processes provides for a more complete explanation of observed evolutionary patterns, because evolutionary change is shaped by different processes of sorting of standing variation, such as natural selection, and by the variation that arises at each generation, which depends on developmental systems (Fusco, 2001, 2015)
In Hong et al (2014) we investigated the ontogenetic allometry of A. koninckii through geometric morphometrics (Klingenberg, 2010, 2016), by analyzing 22 landmarks positioned on the cephalon, thorax, and pygidium (Figure 3)
Papers describing trilobite ontogeny are commonly accompanied by drawings that represent ontogenetic series, the reconstruction presented (Figures 4–7) differs in that morphometric character estimates were derived by applying previously inferred growth and segmentation dynamics, as discussed in sections 4 and 5 above

Summary

Introduction

The central idea of evolutionary developmental biology (evodevo) is that including information about developmental processes provides for a more complete explanation of observed evolutionary patterns, because evolutionary change is shaped by different processes of sorting of standing variation, such as natural selection, and by the variation that arises at each generation, which depends on developmental systems (Fusco, 2001, 2015).The comparative approach is one of the main instruments of analysis in evo-devo studies (Müller, 2007), serving in both reconstructing the evolution of developmental systems and in exploring how developmental processes can themselves affect organismal evolution (Moczek, 2015). Like many extant arthropods (including several myriapod lineages, Minelli and Fusco, 2013), trilobites displayed hemianamorphic development (Minelli et al, 2003), i.e., they added new trunk segments during post-embryonic life until a fixed adult number was reached at a given stage, after which growth and molting continued but without any further increase in segment number.

Results

Conclusion