STEM-WARD SLIPPAGE AND THE RELEVANCE OF ARTHROPOD DECAY FOR INTERPRETING FOSSIL MORPHOLOGY: A NEW LOOK AT STRUDIELLA AND EBULLITIOCARIS

The phrase ‘stem-ward slippage’ was coined by Sansom and colleagues (2010) to refer to the relationship between morphological decay and phylogenetic interpretation in chordates. Namely, derived characters tend to decay first, causing an organism to essentially recapitulate its phylogenetic history as decays. A derived chordate, after significant decay, would be interpreted as a basal chordate based on the remaining preserved characters.

Sansom and colleagues suggest that stem-ward slippage may represent a general rule for other organisms. Decay experiments on arthropods suggest that this is not the case. The decay-resistant arthropod exoskeleton masks the decay of the internal anatomy. Post-mortem character loss in arthropods is driven not by decay, but by fragmentation. This often means that multiple characters are lost in fragmentation events; instead of the step-wise pattern of character loss observed by Sanson and colleagues, arthropod decay yields a tiered pattern with organisms falling to increasingly basal polytomies. Progressive decay of arthropods does not yield predictable phylogenetic placements as in chordates.

Taphonomy and decay can be confounding effects on fossil interpretation, and the ambiguity they introduce is often intractable. Two recently published pancrustacean fossils serve as examples of this. Strudiella devonica, a putative Devonian insect from Belgium, and Ebullitiocaris elatus, a putative Carboniferous anomopod cladoceran from northern England, illustrate this point. Based on comparisons with notostracan decay experiments, a fossilized notostracan decay series from the Mesozoic of China, and notostracan fossils from the same Devonian locality in Belgium, the identity of Strudiella is consistent with an interpretation as the remains of a notostracan after significant decay. Ebullitiocaris is reinterpreted after comparison with branchiopod fragmentation patterns and is found to be inconsistent with a cladoceran identity.