Decay: A Roadmap

Decay of the branchial apparatus. A, amphioxus. B, ammocoete (upper row in situ, lower row dissected and extracted). C, adult lamprey (upper row external, lower row internal). D, hagfish (extracted and dissected). E, embryonic dogfish. F, pre-hatchling catshark (extracted and dissected). From Sampson, Gabbott & Purnell (2013), lined in text.

Decay of the branchial apparatus. A, amphioxus. B, ammocoete (upper row in situ, lower row dissected and extracted). C, adult lamprey (upper row external, lower row internal). D, hagfish (extracted and dissected). E, embryonic dogfish. F, pre-hatchling catshark (extracted and dissected). From Sampson, Gabbott & Purnell (2013), lined in text.

Mark Purnell must have spent an inordinate amount of time in rooms filled with rotting fish, sharks and various invertebrate chordates. The thought might nauseate some, but every time I think of his project, for some reason I have a craving for little neck clams with cocktail sauce.

When I first heard of the project he and his colleagues at the University of Leicester were undertaking, namely watching how various simple chordates and related animals rotted to give a better idea how to interpret fossil evidence, especially of animals that lacked “hard parts” like shells and bones, I thought it was superficially a plausible idea, probably unworkable, and largely driven by the need to publish or perish. Since that first ungenerous assumption I have read many of Purnell’s papers, about “primitive” chordates (he seems to specialize in conodonts (or now technically conodotophores), those mysterious early creatures which originally and for a long time were known only by very small tooth-like remains (technically now the “conodonts”) and, I must say, his writings have made me something of a fan of these and other early chordates, even though they were probably much like lampreys and hagfish of today. (Say what you will; be as broad-minded as you like. But you can probably assume a person is putting on a front if they say they “like” the slime-exuding hagfishes.)

So now I am fully convinced that a guide to how chordates decompose, in what order  they do so, and how various parts of the rotting corpse look at different stages would be a useful tool not only for taphonomists (although they certainly should find it useful) but for all paleontologists for guidance in determining whether features of rocks illustrate body parts or not. The question is whether anything definitive, a sort of Peterson’s Guide to Rotting Flesh, could actually be designed.

Well, there is no longer need to ponder what might be, because Purnell, Robert S. Sansom and Sarah E. Gabbott have put together “Atlas of vertebrate decay: a visual and taphonomic guide to fossil interpretation” in the May issue of Paleontology, which you can access for free here. The journal also allows you to download a PDF file here.

The authors make the case that it is possible to study comparative decay among chordates much in the same way that one can study comparative anatomy. And they claim that such a study has practical issues for identification of structures in fossilized organisms, significant enough to permit more certain classification.

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The holotype of Metaspriggina walcotti (USNM 198611). Lateral view clearly chowing myomeres (from Royal Ontario Museum’s online Burgess Shale fossil gallery: http://burgess-shale.rom.on.ca.)

They offer the example of Metaspriggina from the Cambrian Burgess Shale formation of British Columbia. Simon Conway Morris claimed in 2008 that the chevron-shaped blocks long considered to be myomeres (patterned blocks of muscles that generally identify a chordate) were separated by gaps not known in extent chordates, and therefore he proposed that they acted in a different way than in modern chordates. The authors of the Paleontology Atlas, however state that “decay-induced shrinkage of myomeres in amphioxus causes these structures to have the same appearance in terms of proportions and disposition as those in Metaspriggina” and therefore suggest that Metaspriggina is closer related to cephalochordates than vertebrates, and presumably Conway Morris’s proposed functional difference is unnecessary. 

I’m still not certain that decay observations are rigorous enough to make such fine distinctions in anatomy, but I suppose anything that might give more perspectives on the soft-body parts of those odd proto-chordates of the Cambrian Explosion is useful.

And of course if these purposes are of no use to you, it is still worth having the PDF if for no reason other than to literally show the way of all flesh.

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