We all know the Burgess Shale fauna from Steve Gould’s book Wonderful Life, which described it as a group of fantastic animals lacking any affinity with modern-day animals. Gould suggested that they went extinct without issue in the Cambrian, about 505 million years ago, but could easily have given rise to modern animals if the “tape of life” had been rewound.
Later work by Simon Conway Morris and others, of course, showed that the Burgess Shale fauna, although weird, did have affinities with modern groups like arthropods, lobopods (Onychophora) and annelids. Indeed, some of the fauna could have been ancestors of modern groups, although there still seem to be bizarre forms—called the “problematica”—without affinities to modern animals.
Related to the Burgess Shale fauna is the even better collection of animals in the Chengjiang Fauna of China, also in the Cambrian but earlier—about 525 mya. Here we see clear representatives of echinoderms, arthropods, annelids, sponges, and even what seems to be a chordate.
The Burgess Shale fauna did disappear from the fossil record about 505 mya. Maybe they suddenly went extinct, or perhaps they lived on but just weren’t preserved, since many of them were soft bodied and wouldn’t have fossilized easily. The next major fauna we see is in the Ordovician period (488-443 million years ago), whose beginning shows mostly shelly or hard-bodied creatures: trilobites, echinoderms, gastropods, brachiopods, and, hard-shelled arthropods like barnacles. What happened to the Burgess shale animals?
A new paper in Nature by Peter Van Roy et al. gives the answer: those animals were there all along. They didn’t go extinct in the Cambrian, as long thought, but hung on for at least twenty million years more, perhaps giving rise to more forms that were ancestral to modern life. Their disappearance in the later Cambrian reflected not extinction but poor preservation.
Van Roy et al. describe a remarkably preserved fauna from a valley in southeastern Morocco (the “lower Fezouata Formation”) that dates to the lower Ordovician. Remarkably, the Fezouata fauna contains not only Burgess-Shale type creatures (Fig. 1), but also later and typical Ordovician creatures (Fig. 2) Clearly, the Burgess fauna persisted for millions of years after they were supposed to have gone extinct.
Here are figures from the Nature paper showing both the Burgess fauna and more typical Ordovician fauna. They’re lovely fossils, colored from the oxidation of the pyrite that infused them:
Fig. 2. Ordovician elements of the Fezouata fauna. a and b, arthropods; c, stalked barnacle?, d, Xiphoshuran (chilcerate resembling modern horseshoe crabs); e, another Xiphosuran.
Now this finding is not “revolutionary” in the sense of telling us more about the ancestry of modern phyla. That revolution occurred back when Simon Conway Morris et al. realized that, contrary to Gould’s—and Conway Morris’s own initial— suggestion, the Burgess creatures were not one-offs with no affinities to modern life, but belonged to extant groups. What the new finds show is simply that there wasn’t a mass extinction that destroyed Cambrian life, but a more gradual replacement that lasted millions of years. And it leaves the Burgess Shale creatures as good candidates for ancestors (or at least relatives) of many modern groups.
Fig. 3. Workers in the Draa Valley. Left to right: Jakob Vinther, Peter Van Roy, and Derek Briggs. Photo copyright Patrick Orr.
P. Van Roy, P. J. Orr, J. P. Botting, L. A. Muir, Jacob Vinther, B. Lefebvre, K. el Hariri, and D. E. G. Briggs. 2010. Ordovician faunas of Burgess Shale type. Nature 465:215-218.