EXPERIMENTAL DECAY OF VELVET WORMS (ONYCHOPHORANS) AND TAPHONOMIC BIAS IN THE LOBOPODIAN FOSSIL RECORD

In terms of phylogenetic placement, fossil lobopodians are hard to pin down: recent evolutionary analyses suggest that they might include stem-arthropods, stem-onychophorans, and stem-panarthropods. Finding the correct place(s) for fossil lobopodians in the Tree of Life has the potential to reveal the sequence in which important characteristics of arthropods and onychophorans were acquired. But lobopodians are almost entirely soft-bodied, and although they include classic examples of exceptional preservation, the degree to which the anatomical content of the fossils is biased by processes of decay has yet to be tested. We have conducted a character-based experimental analysis of onychophoran rotting, providing a first test of the hypothesis that anatomical and phylogenetic interpretations of fossil lobopodians have been affected by non-random patterns of taphonomic character loss. Our results suggest that some features of fossil lobopodian anatomy are artefacts of decay, and that some characters are lost so quickly that their preservation in fossils is unlikely. Anatomically, character loss is not randomly distributed. All these factors have significant implications for interpreting the fossil record of lobopodians. Following from our recent work on chordate decay (Sansom et al. 2010), an obvious question to ask is 'do lobopodians exhibit stemward slippage?' The nature of their fossil record coupled with phylogenetic instability make this specific hypothesis difficult to test, but our results suggest that few if any fossil lobopodians could be the remains of badly rotted crown-group onychophorans.

Sansom, R. S., Gabbott, S. E., and Purnell, M. A., 2010, Non-random decay of chordate characters causes bias in fossil interpretation: Nature, v. 463, no. 7282, p. 797-800.