NOT TOO LONG ago the idea of studying fossilized embryos half a billion years old would have seemed ridiculous, but advances in technology have made this practical. This week in Science Dr. Hagadorn and coworkers present the results of their studies of Precambrian embryos from the Doushantuo region of China using a battery of methods including X-ray computed tomography (CT), electron microscopy (SEM), transmission electron microscopy (TEM), and thin-layer petrography.

The study was hampered by the presence of various artifacts in many embryos, but the authors isolated 162 embryos that were spherical, enveloped, and containing only easily distinguished inorganic artifacts. The embryos consistently contained internal structures, one type described as “spheroidal-to-reniform”, the other small, scattered spheres. The first appeared in one four-cell embryo in mirror image in all four cells. The size, shape, and position of these structures is similar to that of spindle bundles or divided nuclei in modern dividing cells. The other structure seems to be the remains of membrane-bound vesicles not unlike the yolk granules in cleaving blastospheres of some modern metazoans.

The gross physical structure of the embryos is perhaps most interesting. The final results show some interesting differences from modern metazoan embryos. For instance, they found no sign of epithelization in any embryos, casting doubt on some previous reports of signs of tissue formation in Doushantuo embryos. Some of the embryos diverged from the modern 2^n doubling pattern, having odd numbers of cells. This was observed often enough to determine it must be a normal pattern, and possible explanations include developmental processes similar to modern unequal cleaving or asyncronous division.

However, there were many similarities to modern embryos:

Despite hypotheses that Doushantuo embryos are unusual in comparison to most known metazoans (25), the patterns of cleavage and cell topology are compatible with a range of animal groups. For instance, in embryos composed of eight or more cells, the offset arrangement of successive tiers of cells, strong cell cohesion, and a stereoblastic cell topology are comparable to early cleavage embryos of many arthropod groups. Stereoblastulae are also particularly common among sponges (26) and scyphozoan cnidarians (27). Doushantuo embryos composed of many hundreds of cells resemble the purported gastrulae of demosponges, before the development of parenchymella larvae, although at this stage demosponges exhibit evidence of gastrulation, with a differentiated superficial layer of micromeres surrounding a core of macromeres (19).

So while these embryos show features of modern metazoans, they also have features that are definitely not modern. We observe the same type of pattern in later Cambrian metazoans, most of which is it impossible to place into modern phyla. This difficulty of classification of primitive metazoans has (somewhat puzzlingly) been used as an argument against the theory of evolution by some creationists. However, this is just what we would expect if these organisms had evolved. Many Creationists give the impression that they think that the fossil record of the Cambrian explosion shows all modern phyla springing suddenly into existence in their modern forms. Previous attempts by some paleontologists to force classifications on some Cambrian fossils that do not really fit probably has contributed to this. In actuality we should not expect Cambrian organisms to neatly fall into modern phyla, as Graham Budd explains so well:

Extant monophyletic groupings are always morphologically distinct from their extant sister-group, and that distinctness is brought about by subsequent extinction of the lineages (plus its offshoots) that led to each of them, away from their last common ancestor. As random extinctions through time slowly remove lineages, the most basal taxon of a clade will sometimes be the victim, thus widening the path-length between the surviving most basal members of extant sister clades (Fig. 3). The bases of clades are therefore eroded by extinction, and, as only living members of the clade can rediversify, this is a permanent loss. These extinct basal taxa will not possess all of the apomorphies that define the basal node of the surviving clade. It should be noted that this process will occur whether or not basal members of clades are particularly prone to extinction or not; there does not have to be anything ‘‘special’’ about basal taxa. One further aspect about these now extinct basal taxa is that they would have accumulated their own autapomorphies not possessed by the extant taxa. As a result, these basal fossil taxa are bound to differ from the extant clades: they will not be diagnosable as members of those clades; and they will show a confusing mixture of some but not all features of those clades, together with a set of features absent from them. It should be noted that this characteristic mix has been repeatedly noted in Cambrian fossils. For example, Hughes (1975) said of the Cambrian arthropod Burgessia: ‘‘what is apparent from this restudy is that Burgessia did possess a mixture of characters . . . many of which are to be found in modern arthropods of various groups’’ (Hughes, 1975, p. 434).

This seems obvious if we consider more recent examples. For instance, cats and dogs descended from a common ancestor, the miacoids. If we look at the miacoids, we see they had some traits similar to those of dogs, some similar to those of cats, and others that are dissimilar from both. We would certainly not try to classify a miacoid among the felines or canines! The hitch is that while we can push the miacoids back to share the same order as felines or canines but have their own family, we can’t push the classification of these Cambrian fossils back any further. The phylum is the highest division except for kingdom, and these metazoans certainly belong in the animal kingdom. Since every living organism fits neatly into an extant phylum, there is a tendency to think that every extinct organism should fit neatly as well.

So should we create new phyla for these organisms? I think not. In a modern taxonomic scheme it makes more sense to determine the phylum a Cambrian organism most resembles and assign that creature as a stem-group member of that phylum. Moreover, since each phylum does not appear uniquely formed in the fossil record but instead its stem-group members have characteristics of earlier stem-groups, this allows us to piece together the puzzle to determine the evolutionary origin of each stem-group and, eventually, the origin of each modern phylum.

“Cellular and subcellular structure of neoproterozoic animal embryos.” Hagadorn, J., et al. Science. 2006, 314, 291-294.

“The Cambrian fossil record and the origin of the phyla.” Budd, G. Integrated Comprehensive Biology. 2003, 43, 157-165.