A modern sea goosebery, which the newly discovered 600-million-year old animal is thought to have resembled (Image:Ron Offermans, Buiten-beeld/FLPA)
Dickinsonia, a 558-million-year-old creature, was named the earliest known animal last year, but New Scientist has revealed evidence for an animal that existed more than 40 million years earlier. This previously unknown animal comes from 600-million-year-old rocks in China and doesn’t have a name yet. While Dickinsonia was an Ediacaran – a primitive group of organisms that went extinct about 541 million years ago – the unnamed animal seems to have belonged to a group of animals that still exists today: comb jellies.
It was discovered by Zhenbing She at the China University of Geosciences and announced at a meeting of the Geological Society of London in January 2019. The fossils were found in a drill core taken from the Doushantuo Formation in southern China. These beds have already yielded exquisitely preserved fossils from as far back as 631 million years ago. These mysterious fossils are only visible through microscopes, and may be algal cells, developing animal embryos or something else entirely. Among these rocks, She’s team has found structures visible to the naked eye, measuring about 0.7 millimetres across, that he claims are fossilised animals. The first clue to their identity was their jellyfish-like shape. Microscopic analysis revealed what appear to be tentacles, muscle tissue, nerve cells, gonads, mucous layers and clusters of hairlike-structures called cilia.
True jellyfish belong to the Cnidaria phylum of animals, but She said these features are reminiscent of the comb jelly phylum Ctenophora. The cilia clusters in particular look like structures called ctenes that comb jellies use to swim. The fossils most closely resemble the living genus of comb jellies called Pleurobrachia, or sea gooseberries, She told the meeting. As a rule, comb jellies are more primitive than jellyfish, says Dominic Papineau of the London Centre for Nanotechnology, whose laboratory helped to analyse the fossils. They have a simpler life cycle and are anatomically less advanced. Nonetheless, comb jellies would be remarkably advanced for 600 million years ago, says Tim Lenton at the University of Exeter, one of the meeting organisers. Living comb jellies have a gut connecting their mouth and anus, a feature not present in many other primitive groups including the Cnidarians.
If the new fossils are comb jellies, more discoveries may yet be to come. The vast majority of living comb jellies today are predators, feeding on small marine organisms. If the 600-million-year-old fossil was a carnivore too, then it must have been part of a food web and hence a surprisingly complex ecosystem. “There are many other creatures in the deposit, but we are not sure what they are,” She told New Scientist. Papineau confirmed that the deposits contain other, unidentified fossils that the comb jellies could have eaten. They resemble algae, he says.
The discovery has yet to be peer-reviewed or published in a journal but has been seen by other scientists, said She.
Some have doubts about the discovery. Maoyan Zhu of Nanjing Institute of Geology and Palaeontology, who was also at the meeting, says he doesn’t accept She’s interpretation. The comb-jelly-like structures could be artefacts or bacterial contamination, he says. If it is genuine, the discovery won’t radically alter our knowledge of life before the Cambrian explosion – the sudden appearance of many kinds of animal fossils about 541 million years ago. We know that the ancestors of modern species must have appeared long before this time, says Zhu – it is just that nobody has found them yet. Zhu presented his own evidence of early, non-Ediacaran animal life from Doushantuo at the same conference, but revealed that his work has been rejected by some leading journals.
This story is based on an article in New Scientist.