Inside 530-million-year-old embryos, scientists uncover rare time-lapse of early animal growth

Think of a small fossil as the equivalent of a homemade video of prehistoric life. Fossils formed when sea organisms began their great proliferation more than half a billion years ago rarely contained anything but tough structures. However, occasionally we come across exquisitely preserved soft embryos that stopped the process of growth in its tracks.

Just recently, the oldest baby Octapyrgites elongatus, an ancient cousin of today's jellyfish, was discovered in South China. The tiny fossils, dating back to around 530 million years ago, provide the earliest evidence yet for the development of early jellyfish-like animals.

This exciting discovery is detailed in a new study, published in Palaeontology. Scientist Jiaxin Peng and colleagues examined 75 embryo specimens and 11 newly found hatchlings of O. elongatus from limestone in Hubei Province. By using advanced scanning techniques, they were able to look inside these ancient phosphate fossils.

What they found was quite a surprise: three exceptionally preserved embryos revealed not only the four obvious grooves at the mouth, but also a hidden set of eight developing lobes inside. This means each embryo started with four visible mouth parts and then developed a second set, totaling eight, as it matured.

Hidden lobes, revealed by X-rays

How could researchers tell? The CT slices of the very small fossils (average size of about 0.7mm in length) revealed the inner structures of the fossils. "The micro-CT scans of three embryos showed … stacked sets of four and eight embryonic oral lobes, with the four-lobed set positioned closest to the oral pole," according to Peng's team.

In simple words, in the process of developing a jellyfish embryo, two levels of lobes develop—the one consisting of four lobes develops first and is visible in the exterior part of the body of the embryo. The second four-lobed structures lie underneath the first four-lobed structure and are moving towards the mouth of the animal in the process of evagination.

These fossils thus reveal a sequence of development. The team's specimens span from late embryo stages up through small post-hatch juveniles with developing tubes. In fact, the authors note that "available specimens of O. elongatus record a mostly complete developmental cycle extending from the early organogenesis stage to hatchling specimens."

That means from when the first organs form all the way to a tiny "baby" just out of its egg. Even the juvenile fossils show 12 concentric ridges (annulations) on the calyx, indicating further growth. The fossils behave like frames of an animated movie of O. elongatus growing up.

The indicated pattern suggests that Octapyrgites had a much more direct life cycle and no radically different larval stage. The authors point out, "As in other olivooids, development was direct, with adoral migration of the embryonic oral lobes."

In living jellyfish (medusozoans), a typical life cycle has a swimming larval stage (planula) that later settles and metamorphoses. These fossils show that O. elongatus likely skipped any such free-swimming larva. Instead, the embryo developed its lobes in place and then hatched as a miniature version of the adult form. The lobes simply grew outward toward the mouth, rather than forming a separate "planula" body.

Extending a Cambrian family tree

The newly discovered embryos have also aided in establishing relationships between Cambrian cnidarians. Both Octapyrgites elongatus and Quadrapyrgites quadratacris exhibit four-fold or tetraradial symmetry, while other members of the family possess pentaradial symmetry. The similarity in embryology between these two species indicates that they are indeed related.

Researchers state that the early developmental pattern of O. elongatus is most similar to that of Q. quadratacris, indicating that the two tetraradials are related. In contrast, Olivooides and its relatives with five rays belong to another evolutionary line. Previously, the prevalence of pentaradial embryos and juveniles of Olivooides helped associate the genus with the jellyfish stem lineage.

A tiny fossil, big questions

There were some issues in finding this discovery. For the early cleavage and blastula stages, it was merely an interpretation, as these smallest spherical organisms found in the same sediment layers could not be conclusively associated with O. elongatus. Many of the embryos were deformed due to fossilization, having shrunk, caved in, or filled with minerals, and just three showed sufficient internal structure for micro-CT imaging.

Also, there are similar embryos found elsewhere, which could belong to a different species. Previous claims of four-lobed and eight-lobed embryos led to controversies regarding the taxonomy.

However, the findings open exciting avenues. Future fieldwork may unearth the missing link between eight- and 12-lobed embryos (known from related forms) or find more internal snapshots in other olivooids. More CT scanning of similar fossils could reveal if the hidden four-to-eight lobe progression is a common strategy.

The study even provides a framework to revisit other enigmatic embryo fossils from the Cambrian "small shelly" fauna of China. Ultimately, every new embryo fossil enriches our understanding of how early animal bodies evolved.

As the researchers emphasize, this speck of rock has become "a developmental time-lapse of early animal evolution," revealing that creatures half a billion years ago underwent body-changing stages just as animals do today.