Stromatoveris

The Cambrian vendobiont S. psygmoglena, gen.sp.nov., composite photo of part and counterpart to show both upper and lower surfaces.

From the pre-Cambrian and early Cambrian, we have a collection of enigmatic fossils: the small shellies appear to be bits and pieces of partially shelled animals; there are trace fossils, the tracks of small, soft-bodied wormlike animals; and there are the very peculiar Edicaran vendobionts, which look like fronds and fans and pleated or quilted sheets. In the Cambrian, of course, we find somewhat more familiar creatures—sure, they're weird and different, but we can at least tentatively see them as precursors to the modern members of their respective phyla. It's not surprising, though, that the farther back in time we go, the stranger animals appear, and the more difficult it is to place them in our phylogenies.

So here's something cool and helpful—it looks like a vendobiont, but it's been found in the Lower Cambrian fossil beds of Chengjiang. It's also very well preserved, and has features that suggest affinities to the ctenophores.

The animal has a tubelike stalk or base that was probably cylindrical in life, but is flattened in the fossils. Emanating from one end of the stalk is a narrow fan of branching fronds; the individual branches were probably also hollow tubes, as some at least are filled with sediment. The animal also has some bilateral symmetry, with a midline seam. Superficially, they resemble the Pennatulacea or sea pens, but the details are all wrong, and the similarities are probably an example of convergent evolution. The diagram below illustrates some of the major features of this beastie.

Camera lucida drawings of S. psygmoglena, gen. sp. nov.

So what is it? Shu et al. suggest that it is most closely related to ctenophores, or comb jellies.

The level of organization seen in Stromatoveris (and equivalent Ediacaran fossils) seems to transcend protistan complexity. It seems likely, therefore, that the vendobionts as currently recognized are not monophyletic. Taxa such as Ernietta and Pteridinium, built on simple modular units and apparently with an infaunal mode of life, may well be giant protistans. The frondlike fossils, however, are interpreted as metazoans, specifically diploblasts. The pronounced disparity within the diploblasts, notably between cnidarians and ctenophores, has made their early evolution highly speculative. Dzik, however, has hypothesized a link between Ediacaran fronds and Cambrian ctenophores. Although it is difficult to accommodate, for instance, taxa such as Dickinsonia and Thaumaptilon in this scheme, the fine transverse structures seen on the branches of Stromatoveris are similar to those seen in Cambrian ctenophores despite their otherwise disparate body plans. In Stromatoveris, the ciliated branches are closely spaced and attached to the frond. In Cambrian ctenophores, the branches became separated and the body more globular. Both were probably benthic, using the ciliated rows for suspension feeding, whereas in contrast extant ctenophores are highly derived. This evolutionary transition is marked by a shift to a pelagic existence, acquisition of a gelatinous body plan, and co-opting of the ciliary rows from feeding to locomotion.

We're seeing a lot of exciting work coming out of the study of the diploblastic grade of animal organization—there's a new molecular study I hope to describe in the next day or two, in between the stacks of grading I have to get done—so we can all look forward to learning more about animal origins from this kind of work. The paper closes with a very clean diagram illustrating the relationships of these various organisms. Memorize it, everyone! You're going to need it for the next decade of evolutionary biology!

(click for larger image)
Outline of metazoan phylogeny, showing proposed position of Stromatoveris and Thaumaptilon as primitive ctenophores and cnidarians, respectively, so implying convergent evolution of a frondlike habit. Metazoan phylogeny is still in a state of flux, but here sponges are taken to be basal, with the calcareans possibly a sister group of all other metazoans. The position of the placozoans is controversial, but here they are treated as primitive diploblasts, evolving before the invention of nerve cells. Ctenophores are monophyletic and are taken to be the sister group of cnidarians plus triploblasts. As argued in the text, ctenophores were primitively frondlike (vendobionts) before acquiring a globular body with separate comb rows that eventually were used in a pelagic existence. Although ctenophores have a biradial symmetry, this has a unique rotational element and may be derived and effectively unrelated to the biradial symmetry that may be primitive to cnidarians. Cnidarians are also monophyletic and are divided into anthozoans and medusozoans. Although previously ctenophores have been argued to be the sister group of all bilaterians, it is now widely accepted that cnidarians are the sister group. The triploblasts are composed of deuterostomes and protostomes.

Shu DG, Morris SC, Han J, Li Y, Zhang XL, Hua H, Zhang ZF, Liu JN, Guo JF, Yao Y, Yasui K (2006) Lower Cambrian vendobionts from china and early diploblast evolution. Science 312(5774):731-4.