Half-a-billion-year-old fossil reveals the origins of comb jellies

After researchers observed a definite similarity between several sea-floor inhabitants, a small-known carnivore species of the ocean has been assigned a new spot in the evolutionary tree of life.

In the development of animals, comb jellies have an essential role, with some thinking they were the first to emerge. An international team of paleontologists has now found fossil evidence to support a link between comb jellies and their prior ancestors, which were polyp-like creatures that lived on the seafloor.

The holotype specimen of Daihua sanqiong.
The holotype specimen of Daihua sanqiong. Yang Zhao / University of Bristol

Current Biology reported the findings of a joint research effort between the University of Bristol, Yunnan University in China, and London’s Natural History Museum, who compared a 520 million-year-old fossil to those of similar skeletons. Results showed that these fossils derived from the same ancestors.

Professor Hou Xianguang, co-author of the study, discovered the fossil in the outcrops south of Kunming in the Yunnan Province, South China. It was embedded in a yellow and olive mudstone, and its shape resembled that of a flower.

In the last three decades, numerous fossils that had been remarkably preserved were discovered in the outcrops situated in between rice fields and farmlands in the tropical region of China.

This peculiar organism, given the name ‘Daihua’ in tribute to the ‘Dai’ tribe of Yunnan and the Chinese word for flower ‘Hua’, has a cup-like shape and 18 tentacles that encircle its mouth. Additionally, each tentacle has delicate, feather-like branches with large ciliary hairs preserved.

A close-up of the rows of cilia on Daihua, which helped the authors place the fossils on the comb jelly stem lineage.
A close-up of the rows of cilia on Daihua, which helped the authors place the fossils on the comb jelly stem lineage. Jakob Vinther / University of Bristol

Dr. Jakob Vinther, a molecular palaeobiologist from the University of Bristol, remarked upon first seeing the fossil that he noticed some characteristics that were similar to comb jellies. He mentioned seeing repeating dark patches along each tentacle, which is similar to how comb jellies fossilize. The fossil also displayed rows of cilia, which were visible due to their size; these large ciliary structures can only be found in comb jellies across the entirety of the Tree of Life.

In our oceans, comb jellies are present and they are carnivorous. They are considered to be pests, as some of them have become invasive. The jellies move around with the help of bands of rainbow-colored comb rows that line their bodies. These rows are made of densely packed cellular protrusions called cilia, and are the largest of their kind in the entire tree of life.

The researchers noticed a similarity between Daihua and another fossil from the Burgess Shale (508 million years old) known as Dinomischus. This creature possessed 18 tentacles and an organic skeleton which had been classified as an entoproct.

Prof Cong Peiyun, one of the co-authors, pointed out that a fossil, Xianguangia, which had been assumed to be a sea anemone, is actually a part of the comb jelly branch.

The trend that was becoming evident caused scholars to recognize a seamless evolution from the fossil records to comb jellies.

Artists' reconstruction of Daihua sanqiong.
Artists’ reconstruction of Daihua sanqiong. Xiaodong Wang / University of Bristol

Dr. Vinther exclaimed that it was a particularly thrilling experience. “We pulled out a zoology textbook and tried to wrap our head around the various differences and similarities, and then, bam! – here is another fossil that fills this gap.”

This research demonstrated the development of comb jellies from antecedents that had an organic skeleton, of which some still had and utilized to move during the Cambrian period. The comb feature evolved from tentacles of polyp-like progenitors that were attached to the ocean floor. Their mouths then progressed into balloon-like shapes while the primary body decreased in size to the point that the tentacles that were initially around the mouth now sprout from the back of the organism.

According to Dr. Luke Parry, a co-author of the study, the body transformations of comb jellies can help us to understand why they have lost so many genes and have a morphology similar to that of other animals.

Approximately 150 years ago, zoologists believed that comb jellies and cnidarians had a connection. However, recent genetic data has shown that comb jellies may be a distant ancestor to all living creatures, excluding sponges which are quite plain in appearance.

In the opinion of the authors of this research paper, their results strongly suggest that the comb jelly should be returned to its place with corals, sea anemones, and jellyfish.

The research originally published in the journal Current Biology. March 21, 2019.