The Cambrian Period is a time when most phyla of marine invertebrates first appeared in the fossil record. Also dubbed the “Cambrian explosion,” fossilized records from this time provide glimpses into evolutionary biology when the world’s ecosystems rapidly changed and diversified. Most fossils show the organisms’ skeletal structure, which may or may not give researchers accurate pictures of these prehistoric organisms. Now, researchers at the University of Missouri have found rare, fossilized embryos they believe were undiscovered previously. Their methods of study may help with future interpretation of evolutionary history.
Any guesses what they’ll find?
“Something obviously went wrong in these fossils,” Schiffbauer said. “Our Earth has a pretty good way of cleaning up after things die. Here, the cells’ self-destructive mechanisms didn’t happen, and these soft tissues could be preserved. While studying the fossils we collected, we found over 140 spherically shaped fossils, some of which include features that are reminiscent of division stage embryos, essentially frozen in time.”
The fossilized embryos the researchers found were significantly smaller than other fossil embryos from the same time period, suggesting they represent a yet undescribed organism. Additional research will focus on identifying the parents of these embryos, and their evolutionary position. More.
Here’s the abstract (paywall):
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. This paper reports possible animal embryos, along with sponge spicules, hyolithelminths, and linguliformean brachiopods, from the upper Shuijingtuo Formation limestone (Cambrian Stage 3) at Changyang, Hubei Province, South China. This limestone unit has carbonate carbon and oxygen isotopic compositions similar to the upper Shuijingtuo limestone in the Yangtze Gorges area. The Shuijingtuo embryo fossils were exposed by physical fracturing, extracted with acetic acid maceration, and observed in thin sections. They were examined using light microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopic elemental mapping, and micro-focus X-ray computed tomography. Most of them are poorly preserved, with a phosphatic envelope (interpreted as a chorion or fertilization envelope) surrounding sparitic calcite. In some specimens, a polygonal pattern is present on the surface, and these are interpreted as multicelled blastula embryos. In others, sets of grooves are present on the surface of a calcitic spheroidal structure, presumably representing the calcitic interior within the chorion; these grooves are superficially similar to annulations of Markuelia embryos, but their biological significance is unknown. Although their phylogenetic and taxonomic placement is largely unconstrained, the Shuijingtuo animal embryos indicate that chorions are taphonomically more robust and are selectively phosphatized. Embryos within the chorions, on the other hand, can be completely lost or entirely replaced by calcite, with only poorly preserved surficial structures. This style of preservation can be explained by a taphonomic switch from early phosphatization to later calcitization. This study illustrates the importance of combining physical fracturing with widely used acid digestion methods in the study of calcitized animal embryos, and it alludes to the possibility that many empty phosphatic vesicles recovered by acid digestion from Cambrian carbonates may be fossilized chorions. – Jesse Broce, James D. Schiffbauer, Kriti Sen Sharma, Ge Wang, and Shuhai Xiao. Possible Animal Embryos from the Lower Cambrian (Stage 3) Shuijingtuo Formation, Hubei Province, South China. Journal of Paleontology, April 2014
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