The scientists also found that embryo size does not influence the fossilization process. Sea urchin species whose animal embryos are small (a tenth of a millimeter) were just as likely to be fossilized as those of larger cousins (about half a millimeter).
"This is good news," Raff said. "Embryo size tells us a great deal about the evolution and development of these organisms."
The fertilization envelope, a semi-hard membrane that surrounds the urchin envelope, is crucial to the fossilization of the embryo. Without the membrane, or with a broken membrane, embryos degraded quickly. Likewise, soft-bodied larvae that hatch from the egg-like fertilization envelope did not fossilize, even under reducing conditions.
The scientists predict early animal fossils from 500 million years ago will be embryos encased in the fertilization envelope and will have been fossilized under highly reducing conditions. This suggests "biases in distribution of fossil embryos," the scientists write, since fossils of more mature animal forms are likely to be limited, maybe non-existent.
Much mystery surrounds the sudden appearance of animals in the fossil record, between 500 and 600 million years ago. Within a few million years, the fossil record goes from zero evidence of animals to great diversity in animal forms, including anomalocarids and trilobites. Harvard University biologist and historian Stephen Jay Gould brought this "Cambrian explosion" to the popular consciousness in 199