There has been a couple bits of news about comb jellies recently. For background, these are kind of jellyfish-like animals, but they don’t have any stings or do the whole pulsating thing. Most swim with special combs and either have sticky tentacles or eat things whole. Here are some examples:
Like jellyfish and their relatives, comb jellies plainly split off back when animals were relatively simple, but placing them has been difficult. There were even some studies which showed they diverged before sponges, which would mean that either comb jellies invented things like nerves on their own, or that sponges actually had and lost them. I’m not sure that’s held up though.
Anyway, it’s tricky is knowing what features are actually inherited in common, and what might be convergence or superficial similarity. For instance, whereas jellyfish have a single gut opening, comb jellies are supposed to have a one-way gut like us; but that might have a separate origin. And for that, there’s an interesting new finding that at least some comb jellies are intermediate, with one or two openings as needed:
The biggest problem with comparing comb jellies, though, is that what we have isn’t actually an ancient lineage. All the living types are part of a single group that only goes back to the end of the Mesozoic, so they could be a very specialized group. You can imagine trying to work out vertebrate evolution knowing about mammals but not fish.
This week there is a new study on Cambrian fossils from China, though, that identifies some of the fossils as having ciliate structures akin to combs. They interpret them as some of the earliest members of the comb jelly lineage, and provide a sketch of how they changed from anemone-like things to their modern forms:
https://www.cell.com/current-biology/fulltext/S0960-9822(19)30206-4
If it’s true, I think that’s not just a good step toward settling early animal relationships, but also the first time I’ve seen a story for how these strange creatures actually came about.
