An embryological perspective on the early arthropod fossil record

Abstract

BackgroundOur understanding of the early evolution of the arthropod body plan has recently improved significantly through advances in phylogeny and developmental biology and through new interpretations of the fossil record. However, there has been limited effort to synthesize data from these different sources. Bringing an embryological perspective into the fossil record is a useful way to integrate knowledge from different disciplines into a single coherent view of arthropod evolution.ResultsI have used current knowledge on the development of extant arthropods, together with published descriptions of fossils, to reconstruct the germband stages of a series of key taxa leading from the arthropod lower stem group to crown group taxa. These reconstruction highlight the main evolutionary transitions that have occurred during early arthropod evolution, provide new insights into the types of mechanisms that could have been active and suggest new questions and research directions.ConclusionsThe reconstructions suggest several novel homology hypotheses – e.g. the lower stem group head shield and head capsules in the crown group are all hypothesized to derive from the embryonic head lobes. The homology of anterior segments in different groups is resolved consistently. The transition between “lower-stem” and “upper-stem” arthropods is highlighted as a major transition with a concentration of novelties and innovations, suggesting a gap in the fossil record. A close relationship between chelicerates and megacheirans is supported by the embryonic reconstructions, and I suggest that the depth of the mandibulate-chelicerate split should be reexamined.

Homeosis in a scorpion supports a telopodal origin of pectines and components of the book lungs

“…The common ancestor of crown group arthropods is widely reconstructed as bearing a homonomous trunk with paired appendages on each segment [ 28 ]. Paralleling this hypothesis, during embryonic development of extant scorpions, opisthosomal organs first take the form of limb buds on the embryonic ventral mesosoma (Fig.…”

Fossils and the Evolution of the Arthropod Brain

“…Other than some exceptional studies that reconstruct ancestral brains from fossils or endocasts [72], evidence from the fossil record is at best indirect, such as from trace fossils [28,[73][74][75]. In the context of arthropod brain evolution, segmental homologies that reflect brain segmentation, either extrapolating from gene expression [76] or from fossilized head sclerites, with reference to traces of fossil brain beneath them [77], can suggest ancestral neural ground patterns; and comparisons spanning hundreds of millions of years [78,79] can also suggest that, despite the evolution of sensory elaborations, deviations from the ancestral ground pattern are less than might be expected.…”

New opabiniid diversifies the weirdest wonders of the euarthropod lower stem group

“…A possible developmental framework would entail the single anterior protocerebral pair of arthropodized limbs in radiodonts becoming co-opted posteriorly to enable the arthropodization of all limbs (Jockusch, 2017; Chipman and Edgecombe, 2019). This scenario would require the convergent fusion of presumed protocerebral appendages in opabiniids to form a single proboscis, and of protocerebral limb buds in deuteropods to form the labrum (Chipman, 2015; Jockusch, 2017; Ortega-Hernández, Janssen and Budd, 2017; Park et al , 2018). Evolutionary reversals or convergences are also required by these topologies ( Supplementary Figs.…”