Important albeit contentious groundwork generated in the 1990s on 580 to 485 million-year-old fossils from South China spawned countless inquiries into field of fossil embryology. Although the early work demonstrated that fossils of eggs and embryos exist, that they represent the earliest evidence for animals, and that they can provide extremely useful phylogenetic data, there were still many questions left to address. How did these fossils come to exist? Are there other taxa out there? And to what extent do they conserve original biological features?
Beyond the evolutionary insight they provide, the embryos from South China contain a fine level of detail that occurs only very rarely in the fossil record. Whereas the vast majority of all fossils are reflective of hard parts, like shells and bones, the globular microfossils capture the earliest and most precious steps in the formation of life. If these embryos exist, what organisms can’t become fossils (under the right conditions, of course)?
Consider some of the most common follow-up investigations that have been conducted: laboratory experiments designed to study the decay and fossilization potential of recent embryos. Scientists have subjected all sorts of makeshift caviar (lobster, crayfish, brine shrimp, sea urchin, and horseshoe crab eggs) to varying chemical, pH, and biological conditions, examining which features are preserved and under what conditions. Their results? Embryo fossilization becomes feasible in anoxic and reducing environments. In fact, under the right conditions, an egg can withstand decay for as long as a year, giving it plenty of time to become a fossil. Of course, processes that can occur in laboratories do not necessarily occur in nature, but paleontologists can’t ignore the possibility for similar paleontological discoveries.
With all the attention they have received, the microfossils have also garnered some skepticism, particularly the Doushantuo taxa. After all, if they are embryos, then where are their parents? The lower Doushantuo Formation includes no macroscopic fossils that could have filled that role. One simple explanation for this incongruity is that the adults were not preserved—phosphatization is generally known for exceptional preservation of microscopic (as opposed to macroscopic) fossils. The embryos may also have been transported away from their parents to their final resting place. Many animals disperse their gametes randomly into the water column during large broadcast spawning events; the taxa time the events so that all adults release their gametes at once, maximizing reproductive efficiency. Though these explanations fit, they haven’t settled the debate.
In recent years, international research groups have generated alternative hypotheses for the globular fossils, proposing that they might represent protistan eukaryotes or giant sulfur-oxidizing bacteria. However, fossilization experiments conducted on modern giant sulfur-oxidizing bacteria yielded observations inconsistent with the globular fossils, and interpretations of the material have not aligned unambiguously with the biology of recent protists. Although the exact phylogenetic affinities of the Doushantuo fossils remain ambiguous, the embryo interpretation has maintained the most traction.
Investigations have favorably advanced our knowledge regarding the younger Cambrian embryos, Olivooides and Markuelia. In addition to extending the geographic range of the taxa to parts of Australia and North America, these analyses have documented many distinct developmental stages and refined phylogenetic interpretations. For instance, Markuelia—once grouped ambiguously among multiple worm-like taxa—has been reasonably assigned to the Priapulida, a group of worms related to arthropods and nematodes in the clade Ecdysozoa (organisms that molt). In February, a team of international researchers led by Xi-Ping Dong presented new data concerning the ontogeny of Olivooides. The team used scanning electron microscopy and synchrotron radiation X-ray tomographic microscopy to study rarely preserved internal anatomical features that strongly affirm affinities with scyphozoan Cnidarians (the ‘true jellyfish’). They published their results in the February 27th issue of the Proceedings of the Royal Society B.
Twenty years on from the initial description of the South China taxa, fossil embryos remain one of the most promising resources for studying animal evolution. Most embryo fossils occur in the 100 to 200-million-year period that included the origin of animals, the end-Ediacaran mass extinction, and the intense diversification of phyla associated with the Cambrian Explosion. As direct snapshots of ontogeny, they capture the developmental diversity that must have existed during this critical time in the establishment of animal body plans. Indeed, as these important microfossils continue to spawn new work, interpretations of early animal evolution are certain to mature.
Drew Muscente is a Ph.D. student in the Department of Geosciences at Virginia Tech studying the taphonomy and paleobiology of Ediacaran microorganisms. Email Mr. Muscente at firstname.lastname@example.org.
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