Ediacaran Fossil Embryos
Sweeeet. Paleontologists have found late-stage fossil embryos in China’s Precambrian Doushantuo Formation. The things have a three-coiled spiral structure and look kind of like baseballs.
Less developed embryos (blastulas, for all you embryology nerds) colloquially known as Megasphaera ornata have already been found in the same rock (see Pharyngula for pictures). The new finds are about the same size as M. ornata, and the discoverers claim that there are transitional fossils, too, bearing faint spiral structures and a marked resemblance to the unspiralled blastulae. Which means, o my jellyfish, that we have assembled a good solid developmental sequence from an organism that juuuust predates the Cambrian Explosion.
The Doushantuo Formation is also home to tabular structures thought to be ancestral cnidarians, closely related to Paleozoic tabulate corals and vaguely related to modern corals and sea anemones. Because the partially uncoiled embryos are little tubes of about the same diameter as adult tabulate organisms, it is plausible that they are, indeed, the embryonic/larval form of these early anthozoans. For comparison, here’s the only remotely similar picture I could find of a modern coral embryo (from Ball et al., 2002):
I dunno, wee balls of cells kinda all look the same to me.
It appears that by the time the Doushantuo Formation was deposited ~550 million years ago, cnidarians had already split off from the lineage that evolved into cuter, fuzzier animals like nematodes and kitty-cats. So the discovery of a complete embryonic sequence of early cnidarian wouldn’t help much in the race to figure out what the hell was happening with bilateral symmetry and the general body plan confusion of the late Vendian and early Cambrian. In that respect, these findings are a little disappointing – what we really want are bilaterian embryos. But y’know, cnidarians are awesome too, and deserve love just as much as any other fossil embryos.
The fossils are reported by Xiao et al. in this month’s issue of Geology.
Bonus! While looking for pictures of cnidarian embryos, I ran across the best journal article title evar: G. H. Orions and D. H. Janzen, Why Are Embryos So Tasty?, The American Naturalist 108(963), 1974. Insert hilarious commentary on embryo-worship in modern politics here.