Paleobiology of the Early Cambrian Yanjiahe Formation in Hubei Province of South China

Fossils recovered from limestones of the lower Cambrian (Stage 2-3) Yanjiahe Formation in Hubei Province, South China, recovered using acetic acid maceration, fracturing, and thin sectioning techniques were examined using a combination of analytical techniques, including energy dispersive spectroscopic (EDS) elemental mapping and micro-focus X-ray computed tomography (micro-CT). One important fossil recovered and analyzed with these techniques is a fossilized embryo. Fossilized animal embryos from lower Cambrian rocks provide a rare opportunity to study the ontogeny and developmental biology of early animals during the Cambrian explosion. The fossil embryos in this study exhibit a phosphatized outer envelope (interpreted as the chorion) that encloses a multicelled blastula-like embryo or a calcitized embryo marked by sets of grooves on its surface. The arrangement of these grooves resembles annulations found on the surface of the Cambrian-Ordovician fossil embryo Markuelia. Previously described late-stage Markuelia embryos exhibit annulations and an introvert ornamented by scalids, suggesting a scalidophoran affinity. In the Yanjiahe fossils illustrated herein, however, the phosphatized chorions and blastulas are not taxonomically or phylogenetically diagnostic, and the late-stage embryo is secondarily calcitized and thus poorly preserved, with only vague grooves indicative of Markuelia-type annulations. Consequently, their taxonomic assignment to the genus Markuelia is uncertain. If they indeed belong to the genus Markuelia, they are the oldest known Markuelia fossils from China, and represent both a new occurrence and possibly a new species.