For about the first 3.4 billion years that life existed on Earth, creatures were very simple and their evolution was fairly slow. Then an 80-million-year timespan came along that sparked countless new marine species, many of which became increasingly complex. The rapid expansion of biodiversity is reflected in the name of the time period: the Cambrian explosion. Over 140 fossilized embryos of soft-bodied marine creatures from this time period were discovered by a team of researchers led by James Schiffbauer from the University of Missouri. Analyzing the fossils could provide a wealth of information about the anatomy of these early complex organisms. The team published two papers on the topic which will appear in a special issue of the Journal of Paleontology.

Fossilization is a difficult process, but hard tissues have a considerably advantage. Soft tissue is much more difficult to preserve, because the decomposition rate is much faster. For this reason, there isn’t a lot of of fossils of the earliest soft-bodied animals and not much is known of their anatomy. “The Cambrian Period, which occurred between 540 million and 485 million years ago, ushered in the advent of shells,” Schiffbauer explained in a press release. “Over time, shells and exoskeletons can be fossilized, giving scientists clues into how organisms existed millions of years ago. This adaptation provided protection and structural integrity for organisms. My work focuses on those harder-to-find, soft-tissue organisms that weren’t preserved quite as easily and aren’t quite as plentiful.”

The spherical fossils were retrieved from the lower Cambrian Shuijingtuo formation in South China. Because there are different chemical processes at play when soft tissue is preserved versus hard tissue, the researchers now have a unique opportunity to investigate the animal’s body plan.

“Something obviously went wrong in these fossils,” Schiffbauer. “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.”

Although fossilized embryos from that time period have been discovered before, these are much smaller and could possibly represent a yet-unidentified species. The team is currently analyzing these fossils and hopes to have conclusive results to announce in the future not only of their identity, but also of their evolutionary position in the grand scheme of life.