In the 1990s, rising awareness regarding the consequences of soft tissue phosphatization led way to new research on globular microfossils from the Ediacaran and Cambrian (spanning in age from 635 to 485 million years ago) rocks of South China. Could they be eggs and embryos? The fossils—once relegated to the wastebasket—received new attention from the entire international scientific community, yielding the promising field of paleoembryology. After just a few years of studying the fossils, paleontologists recognized the long-term stasis of the embryo paradigm among animals; they described the developmental sequences of several widespread Cambrian taxa; and they announced that embryos represented the oldest evidence of animals in the fossil record.
Researchers from the University of Saskatchewan took the first leap, describing five spherical specimens from the Middle Cambrian of South China. Compared to material collected since then, the specimens were poorly preserved. Nonetheless, the fossils implied that the organisms originally possessed flexible ‘shells’ and tightly packed cells, arranged in patterns consistent with the cleavage stages of invertebrates embryos. The scientists hypothesized that their fossils represented the offspring of ancient arthropods, possibly trilobites. It was a reasonable hypothesis—arthropods are extremely diverse today, including crustaceans, spiders, and insects—but it necessitated further investigation.
Studies followed up on the most well-known globular fossils from the early Cambrian: Olivooides and Markuelia. Previous researchers had compared Olivooides to unfertilized eggs as early as 1977, as its name implies, but this resemblance garnered no serious consideration until 1997. After examining thousands of specimens from China and Siberia, researchers were able to separate individuals from the two species into distinct developmental stages consistent with ontogenies of extant taxa. These cleavage stages supported grouping Olivooides with scyphozoan jellyfish and Markuelia with one of several worm-like groups.
Finally, as the promise of fossil embryos was becoming a scientific reality, attention shifted to the next big horizon—bridging the gap between the molecular and fossil records. By the mid-1990s, phylogenetic studies had noted a problematic discrepancy between various estimates of animal diversification dates. Whereas the fossil record indicated that most animal phyla evolved very rapidly 540 million years ago (during the ‘Cambrian Explosion’), molecular clocks considering genetic sequence variability implied that the earliest branching of animals began, perhaps, as much as 500 million years earlier (approximately 1 billion years ago).
Paleontologists hoped to close the gap. If they couldn’t find any adult fossils, maybe they could locate some of their children. In 1998, their sights turned again to the globular fossils. On February 5th, scientists from Harvard University announced the discovery of eggs and early cleavage stage embryos in 570 million year old rocks from the Doushantuo Formation of South China; the very next day, on February 6th, Chinese researchers reported comparable microfossils from slightly older strata (580 million years old) in the same formation. Since then, animals have been reported from older rocks—in the form of biomarkers, traces, even some putative sponge bodies—but none have received as much attention. The Doushantuo embryos remain the best ‘earliest’ animal fossils in the record, and in recent years, improved molecular clock techniques have yielded divergence dates for the animal kingdom in close agreement with the fossil data.
Missed last week’s Cambrian Caviar? Read it here! And make sure to come back next week for Part Three of Evolution: This View of Life’s new Paleontology series Cambrian Caviar!