DISCOVERING ANIMAL FOSSILS IN THE NEOPROTEROZOIC DOUSHANTUO FORMATION OF SOUTHWEST CHINA

Molecular biology can tell one how modern organisms are built, and molecular clocks can broadly indicate when these organisms have their evolutionary roots. But these approaches, when searching to understand the evolutionary origin of animal life, are vastly strengthened if direct evidence through fossils can be found. One of the exceptional discoveries of the past few years has been the recognition that the Doushantuo phosphorites in southwestern China, which are as old as 600 million years, contain fossils that represent the oldest known direct evidence of animals. The Doushantuo Formation is a marine fossil Lagerstätten that exhibits a variety of modes of preservation, other than in phosphorites, including cherts and compressions in black shales. The phosphorites, however, have generated the most interest to date, because they contain phosphatized microfossils, including eggs and embryos which are most probably of sponges or cnidarians, as well as adult sponges, a variety of adult cnidarians, and putative embryos of bilaterians.

Like all evidence that potentially represents the oldest occurrence of some biological object or process, study of the Doushantuo microfossils has proceeded cautiously. Three-dimensional specimens recovered from acid residues have been studied extensively using SEM, and approaches utilizing X-ray computed tomography as well as synchrotron X-ray microscopy are being developed to delineate their internal details. Examination of thin sections typically reveals internal morphological characters necessary for assigning biological identity. Determination of primary biological objects from artifacts is crucial, as is the discrimination of animal fossils vs. those of other organisms. Unique biological characters in multiple specimens are necessary to make these determinations. We are developing a broad interdisciplinary approach towards understanding these fossils, which includes specimen-based analyses as well as integration of research involving geochemistry, sedimentology, taphonomy, paleoecology and evolutionary theory in our studies. Thus, the promise of the Doushantuo fossils remains great, as we attempt to illuminate the earliest annals of the history of animal life on Earth.