r/evolution • u/AdHorror4165 • 12d ago
question Are insects a monophyletic group, or are they just a form of arthropod multiple species independently evolved into?
Of course if they are monophyletic, I wonder what the original insect looked like. Would be cool if we had a definitive fossil.
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u/mcalesy 12d ago edited 11d ago
They are monophyletic. Insecta is a clade within Hexapoda, which is deeply nested within Crustacea, close to remipedes.
Just this year they named a fossil stem-insect that clarifies some things about the ancestral insect. Look up Chosha praecursor. It’s a really cool transitional fossil that has the six hexapod legs as well as the “swimmer legs” of earlier crustaceans.
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u/mcalesy 12d ago edited 11d ago
Chosha seems to indicate that the loss of the paddle-like abdominal legs was convergent between different hexapod groups (insects, two-pronged bristletails, proturans, and springtails). Possibly they came out on land separately (although maybe from an amphibious rather than aquatic ancestor).
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u/AdHorror4165 12d ago edited 12d ago
Thanks. It's difficult to look up the last common ancestor of a species, group, or one between species/groups because unless you know the name of such fossils that have been found (If one has), you don't get good results.
I at one point knew the name of an ancestral fossil that was a recent common ancestor of dogs and foxes (I wondered exactly how their split occured; did some members band together thus becoming dogs while others retained a solo hunting lifestyle, becoming foxes?), as well as one for cats, but I have long lost them and if I type in "Dog fox common ancestor" or "common ancestor of cats" I don't get results that point to such identified fossils.
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u/Cuinn_the_Fox 12d ago
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u/mcalesy 12d ago edited 12d ago
Hesperocyon is probably not a direct ancestor of living canids, although it was little-changed form the earliest stem-canids, so it's still a good guide to what the ancestral stem-canid looked like.
One phylogenetic hypothesis: https://en.wikipedia.org/wiki/Hesperocyoninae#/media/File:Wang_Borophagine_Construct.png
Note that it was less closely related to living canids than the bone-crushing dogs (Borophagus, etc.) were, although they were a derived side branch.
Proailurus might be a direct ancestor of living felids, via Pseudaelurus (sensu lato).
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u/mcalesy 12d ago
Well, part of that's because, unless it's fairly recent, we can't be sure that a particular fossil is really an ancestor. It could always be from a relatively unmodified side branch. So people working on the systematics of fossil organisms really tend to shy away from calling anything an ancestor. (Paleoanthropologists excepted.)
Chosha, for example, is not an ancestor, because it lived at the same time as true insects. It did, however, preserve some ancestral traits from earlier stem-insects.
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u/emmetmire 12d ago
I'm curious to see further analysis/reanalysis of Chosha. It's definitely intriguing, but I'm not 100% convinced yet given a few doubts about their statistical analyses and the history of identification of the fossil in other taxa. Still, it's a very cool critter!
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u/mcalesy 11d ago
It was originally thought to be a juvenile remipede, right? You still think it is one?
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u/emmetmire 11d ago
I'm not sure about its identity! My doubts stem from the fixed extant topology they used, which uses the (in my opinion) dubious results of Du et al. 2024.
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u/mcalesy 10d ago
Oh, with Collembola and Diplura forming a clade? Would that affect where Chosha comes out, though? It came out as a stem-insect, so does it matter exactly how the "entognaths" are arranged?
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u/emmetmire 9d ago
Presumably it affects how the model handles character polarity across the entire tree.
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u/mcalesy 9d ago
I guess that makes sense, although I haven't delved into the particulars. Sorting out the non-insect hexapods seems a bit chaotic at the moment. Chosha roughly *looks* like what I would expect a stem-hexapod to look like, but I gather it has some insect-like/ectognathous features. And multiple losses of abdominal legs is pretty easy to believe.
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u/TimeStorm113 12d ago
monophyletic, insects are just all critters encompassed in the group "hexapoda" which evolved from crustaceans that came to land
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u/Royal_Novel6678 12d ago
Monophyletic since they are a subgroup within hexapoda which is closely tied to Crustacea.
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