On embroyos and ancestors: Fossils of tiny embryos 570 million years old may well be the greatest paleontological discovery of our time

Natural History, July-August, 1998 by Stephen Jay Gould

The first hints that phosphatization of tiny creatures might resolve key issues in the early evolution of animals dates to a discovery made in the mid-1970s and then researched and reported in one of the most elegant (but rather sadly under-appreciated) series of papers ever published in the history of paleontology: the work of German scientists Klaus J. Muller and Dieter Walossek on the fauna of distinctive Upper Cambrian rocks in Sweden known as Orsten beds. In these layers of limestone concretions, tiny arthropods (mostly larvae of crustaceans) have been preserved by phosphatization in exquisite, three-dimensional detail. The photography and drawings of Walossek and Muller have rarely been equaled in clarity and aesthetic brilliance, and their papers are a delight both to read and see. (For a good early summary, consult Muller and Walossek: "A remarkable arthropod fauna from the Upper Cambrian `Orsten' of Sweden," 1985, Transactions of the Royal Society of Edinburgh, vol. 76, pp. 161-172; for a recent review, see Walossek and Muller: "Cambrian `Orsten'-type arthropods and the phylogeny of Crustacea," in R. A. Fortey and R. H. Thomas [eds.] Arthropod Relationships, London: Chapman and Hall, 1997.)

By dissolving the limestone in acetic acid, Walossek and Muller can recover the tiny, phosphatized arthropods intact. They have collected more than 100,000 specimens following this procedure and have summarized their findings in their paper of 1997 cited above:

The cuticular surface of these arthropods is

still present in full detail, revealing eyes

and limbs, hairs and minute bristles...

gland openings, and even cellular patterns

and grooves of muscle attachments

underneath .... The maximum size of

specimens recovered in this type of

preservation does not exceed 2 mm.

From this beginning, other paleontologists have proceeded backward in time, and downward in growth from larvae to early embryonic stages containing just a few cells. In 1994, Xi-guang Zhang and Brian R. Pratt found bails of presumably embryonic cells measuring 0.30 to 0.35 millimeters in diameter and representing, perhaps, the earliest stages of adult trilobites, which are also found in the same Middle Cambrian strata (see Zhang and Pratt: "Middle Cambrian arthropod embryos with blastomeres," 1994, Science, vol. 266, pp. 637-38). Just last year, Stefan Bengston and Yue Zhao reported even earlier phosphatized embryos from basal Cambrian strata in China and Siberia. In an exciting addition to this growing literature, these authors traced a probable growth series--from embryos to tiny near adults--for two entirely different animals: a species from an enigmatic extinct group, the conulariids; and a probable segmented worm (see Bengston and Zhao, "Fossilized metazoan embryos from the earliest Cambrian," 1997, Science, vol. 277, pp. 1645-48).

When such novel techniques first encounter materials from a truly unknown or unsuspected world, genuinely revolutionary conclusions often emerge. In what may well go down in history as the greatest paleontological discovery of the late twentieth century, Shuhai Xiao, a postdoctoral student in our paleontological program; Yun Zhang, of Beijing University; and my colleague (and Shuhai Xiao's mentor) Andrew H. Knoll, have just reported their discovery of the oldest triploblastic animals, preserved as phosphatized embryos in rocks from southern China estimated at 570 million years of age (and thus even older than the richest Ediacaran faunas found in strata about 10 million years younger [see Xiao, Zhang, and Knoll, "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite," 1998, Nature, vol. 391, pp. 553-58]). These phosphatized fossils include a rich variety of multicellular algae, showing, according to the authors, that "by the time large animals enter the fossil record, the three principal groups of multicellular algae had not only diverged from other protistan [unicellular] stocks but had evolved a surprising degree of the morphological complexity exhibited by living algae."