A comb jelly fossil from some 500 million years ago shows a previously unknown species of these ancient sea animals that had a more complex nervous system than their modern descendants.
Evolutionary theory doesn’t preclude the possibility of organisms becoming simpler over geological time, but it’s a relatively uncommon phenomenon. Examples are mostly limited to ancient arthropods, sea lilies, and brachiopods – also known as lamp shells.
“Comb jellies occupy a much earlier position in the animal tree of life [than arthropods and brachiopods], so it is filling an important gap,” says team member at Harvard University.
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Comb jellies, or ctenophores – whose see-through bodies scatter light, creating a rainbow effect – have long been seen as potentially the most ancient branch of the animal evolutionary tree still existing today. Sponges may be even more ancient, though, particularly in light of the recent discovery of 890-million-year-old sponge fossils.
The latest work offers a unique glimpse into ctenophore evolution. Little was known because their soft, gelatinous bodies don’t fossilise easily. Rarer still is finding preserved organs and nervous systems of these small animals, which are generally no more than a few millimetres to several centimetres in size.
Ortega-Hernández and his colleagues at the University of Oxford looked at fossil specimens held at the Natural History Museum of Utah that came from a renowned Cambrian fossil site in Utah’s mountainous desert. Animals there were compressed in a way that immortalised their tissues as black carbon outlines.
Among the fossils, the team identified two new species of ctenophore. One of them (Ctenorhabdotus campanelliformis) looked like a tiny upturned bobble-hat and revealed a more complex nervous system than any ctenophore species alive today.
The team discovered a ring-shaped nerve around the animal’s mouth, which may have helped it engulf prey. Connected to this were long nerve fibres running underneath its body’s comb-like structures to a balance-sensing organ opposite the mouth. Most existing comb jellies lack distinct nerves but have a simpler nerve net for sensing. They use sticky tentacles to catch prey.
Why comb jellies ultimately lost the distinct nerves seen in C. campanelliformis and replaced them with a simpler nerve net isn’t yet known says the team, but speculates that it might have been due to distant comb jelly-like relatives that lived on the sea floor. These may have gradually adapted to a free-floating life where a simpler nerve net might have provided a better way to sense the space around them, like in ctenophores in the oceans today.
“The nervous system could have been modified in light of that,” says at London’s Natural History Museum.
However, little is known about the lifestyle of C. campanelliformis, so this idea can’t be thoroughly tested yet.
iScience
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