Just about every organism in the animal kingdom develops from the same basic unit: an embryo. Though not all animals behave exactly like ‘the birds and the bees,’ the primary steps of sexual reproduction apply to all the major clades. Parents produce male and female gametes—they combine them—and two gametes fuse to yield a zygote that undergoes successive cell divisions in a series of increasingly complex embryonic stages. Eventually, depending on the developmental history of the taxon, the embryo will become a fetus, a larva, or a miniature adult. Arthropods, jellyfish, even sponges originate through this process.
It should not be surprising then—given that sexual modes of reproduction appear so low on the animal tree—that embryos might represent some of the oldest and most tantalizing fossil data for studying their evolution. Evolutionary developmental biology strives to understand how organisms relate to each other, in part, through comparisons of their ontogenies (e.g. how their zygotes transform into a mature adults). Whereas the embryos of recent taxa may help scientists to infer the characters possessed by that the last common ancestors of certain groups, fossilized embryos can provide them with direct evidence of primitive developmental programs. In effect, these ‘missing links’ can help bridge the gap between the ambiguous fossils that first appear in the Ediacaran (the oldest period on the geological timescale that includes Bona fide animal fossils, spanning from 635 to 542 million years ago) and all the diverse taxa that exist today.
However, all these questions depend on preservation. Can embryos become fossils? If so, how abundant are they? And what sort of detail might they possess? Twenty years ago, the promise of paleoembryology exceeded the practicality of pursuit. Literature published prior to this time contains scattered reports of small spherical and globular bodies, resembling eggs and embryos, as well as several Bona fide examples. However, studies of these microfossils yielded little in the way of true evolutionary insight. It took some time for paleontologists to recognize the significance of embryos in the fossil record, and more importantly, to interpret them in a phylogenetic context.
Though there are many ways to make a fossil, the most common embryo fossils result from a process known as phosphatization, in which (one at a time) the molecules that comprise soft tissues are replaced by the molecules of calcium phosphate (the mineral apatite). This natural process only occurs in systems with unique geochemical and biological conditions—such as those that existed roughly 580 to 500 million years ago—but it yields exquisite fossils preserved in three dimensions. Ocean chemistry around the time of the Ediacaran-Cambrian transition fostered marine environments in the area of present-day China that were especially conducive to the fossilization of microorganisms. Today, most (and indeed, the best) egg and embryo fossils come from the rocks of South China, dated from the Ediacaran and Cambrian (~541 to 485 million years ago) periods.
Rising awareness regarding soft tissue phosphatization led way to new research in the 1990s. After reexamining some of the problematic globular taxa from the Ediacaran and Cambrian, scientists finally recognized the tantalizing species that had previously eluded them and laid the groundwork for modern fossil embryology. In just a few short years of study, they announced three significant discoveries with big implications for early animal evolution.
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 email@example.com.
Interested to find out what these three discoveries were? Check back next week for Part Two of Evolution: This View of Life’s new Paleontology series Cambrian Caviar!
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