PRECAMBRIAN MEROBLASTIC EMBRYOS FROM THE DOUSHANTUO PHOSPHORITE, CHINA

Phosphatized embryos from the terminal Neoproterozoic Doushantuo Formation (635-551Myr ago) of south China that display preserved three-dimensional (3D) cellular and sub-cellular structures provide a unique Precambrian window on the early evolution of animal development. These exquisite Precambrian animal embryos have elicited various interpretations, partially because the first to be described were comparatively simple forms resembling early cleavage embryos which in general lacked much phylogenetic signal. But more recently a variety of more advanced holoblastic cleavage forms were studied by high resolution X-ray microtomography, and shown to exhibit morphologically differentiated cell types and diverse, complex, multicellular structures. Here we report new fossil embryo forms from the Doushantuo phosphorite that utilized meroblastic cleavage, and that also display distinct cell differentiation. Sub-micron resolution 3D images, taken by improved propagation phase contrast-based synchrotron radiation X-ray microtomography (PPC-SR-μCT), demonstrate that these fossils preserve features directly comparable to modern bilaterian embryos that utilize discoidal meroblastic cleavage. Meroblastic as well as holoblastic cleavage forms were thus already present in Doushantuo times, substantiating the conclusion that a variety of eumetazoan lineages had evolved by the early Ediacaran after termination of Snowball Earth, if not even earlier.