Preservation and phylogeny of Cambrian ecdysozoans tested by experimental decay of Priapulus

Abstract

The exceptionally preserved Cambrian fossil record provides unique insight into the early evolutionary history of animals. Understanding of the mechanisms of exceptional soft tissue preservation frames all interpretations of the fauna and its evolutionary significance. This is especially true for recent interpretations of preserved nervous tissues in fossil ecdysozoans. However, models of soft tissue preservation lack empirical support from actualistic studies. Here experimental decay of the priapulid Priapulus reveal consistent bias towards rapid loss of internal non-cuticular anatomy compared with recalcitrant cuticular anatomy. This is consistent with models of Burgess Shale-type preservation and indicates that internal tissues are unlikely to be preserved with fidelity if organically preserved. This pattern, along with extreme body margin distortion, is consistent with onychophoran decay, and is therefore resolved as general for early ecdysozoans. Application of these patterns to phylogenetic data finds scalidophoran taxa to be very sensitive to taphonomically informed character coding, but not panarthropodan taxa. Priapulid decay also have unexpected relevance for interpretation of myomeres in fossil chordates. The decay data presented serve not only as a test of models of preservation but also a framework with which to interpret ecdysozoan fossil anatomies, and the subsequent evolutionary inferences drawn from them.

Soft‐Bodied Fossils Are Not Simply Rotten Carcasses – Toward a Holistic Understanding of Exceptional Fossil Preservation

“…Notable examples are the jaws and chaetae of nereid polychaetes, [48] the notochord and myomeres of chordates, [20] and the chitinous parts of non-arthropod ecdysozoans such as the claws of onychophorans [17] and scalids of priapulids. [18] Despite the apparent decay resistance of these structures, however, they are not always preserved in fossils. The jaws of nereid polychaetes, for example, do not survive diagenesis despite being heavily sclerotized: they only survive in recent sediments, [61] whereas the jaws of other polychaetes occur abundantly as fossils.…”

“…[51] The same decay experiments allowed the chevron-shaped structures in Conopiscius, a Carboniferous chordate, to be interpreted as myomeres rather than external scales, and also indicated that a decay-resistant cuticle was not necessarily present in Pikaia from the Burgess Shale. [51,55] Decay in seawater has now been monitored in a range of taxa in laboratory experiments (see Table S1, Supporting Information): anthozoans, [56] annelids, [48] chaetognaths, [57] priapulids, [18] onychophorans, [17] pterobranchs, [58] enteropneusts, [59] nonvertebrate chordates, [20] and cyclostomes. [60] Thus the sequence of character loss has been determined for taxa representing most clades of eumetazoans.…”

“…Gut, muscle, and nervous tissue, for example, are among the first to decay in a broad range of taxa in decay experiments. [17,18,48] The majority of recent experiments were carried out in the absence of sediment in order to facilitate observations of the sequence of decay stages and to reduce the number of variables involved in the experiments. The sedimentary environment in which a carcass is buried is an important control on decay.…”

“…Reports of organically preserved neural and circulatory [21] tissues in Cambrian panarthropods have proved particularly controversial as an interpretation based on stages of decay [16][17][18] implies that such decay-prone features should not persist and fossilize.…”

“…[15] More recently, however, there has emerged a tendency to apply the results of decay experiments more literally to the interpretation of soft-bodied fossils, using the relative susceptibility of morphological characters to decay as a measure of whether or not they could be preserved at all. [16][17][18] While an experimental approach is important to determining how exceptional fossils are formed [19] microbial decay is just one of many processes that can distort the original morphology of an organism. A variety of interlinked processes play a role in the preservation of different anatomical features.…”