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I think my previous questions may have been misinterpreted (I honestly was too tired to formulate what exactly I was confused about), so I’d like to elaborate on my confusion about the phylogeny of Osteichthyes.

Ray finned fish (actinopterygians) and lobe finned fish (sarcoptertgii) are classified by the shape of their fins. Their common ancestor obviously had some type of fins, so either one of the groups is paraphyletic (in the sense that their common ancestor belonged to or evolved from one of the groups), or the common ancestor had fins that didn’t resemble either rays or lobes

Despite my efforts I can’t find a straightforward source saying the latter is true, so how are the 2 groups sister clades?

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    $\begingroup$ Or Osteichthyes have intermediate fins with characteristics of both. there are actually quite a variety of fin types in extinct fish. just because their descendants have evolved drastically different fins does not mean the basal forms did. Imagine if the two surviving group of mammals were ungulates and bats and you asked did there ancestor have wings or hooves? $\endgroup$
    – John
    Jan 6, 2023 at 22:56
  • $\begingroup$ Please re-read the title of your question and remove one of the instances of “are”. Also take out the first paragraph: I have no idea what your previous question was, and it is irrelevant to this question, which should stand by itself. If you need to explain anything of this sort do it in a comment. $\endgroup$
    – David
    May 17, 2023 at 21:52

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The misunderstanding here is the difference between phylogenetic classification and character evolution. Essentially, just because an organism has one certain trait it doesn't automatically belong to some group.

Words like paraphyletic, monophyletic, and clade, are descriptions of groupings by ancestor-descendant relationships. Clade and monophyletic, for example, refer to an ancestor and all of its descendants. Paraphyletic generally refers to an ancestor and most of its descendants, but excluding some portion. Talking about "bony fish" but excluding tetrapods would make bony fish a paraphyletic group.

Character evolution is the study of how characters have evolved among a group of organisms in light of their phylogenetic relationships. Textbooks often talk about clades having synapomorphies, or characters that have evolved in some ancestor and are present in its descendants. Some clades have clear synapomorphies that do indeed strongly match a clade (e.g. all the members of the clade have the character, and other organisms do not). However, it's not uncommon for synapomorphies to only be mostly true. Perhaps some members of the clade lost the new trait, or some species outside the clade independently evolved the same trait. These exceptions, however, don't change the group membership of those organism.

It's not the character that defines a species' clade, but the ancestor-descendant relationships. It's not exactly true that Actinopterygii and Sarcopterygii are "classified by the shape of their fins." It's just that fin shape is a really good synapomorphy for those clades because it's both consistent and probably has important evolutionary consequences. Because of this people adopted the common names ray-finned and lobe-finned fishes, giving the implication that that's what defines those groups.

The misconception is understandable because we use characters (previously morphological characters, recently increasingly genomic characters) to craft our hypotheses about what we think are the true ancestor-descendant relationships.

All that is to say that the most recent common ancestor of Osteichthyes is just that—it is an ancestor of both Actinopterygii and Sarcopterygii, but doesn't belong to either clade, regardless of its fin morphology. Actinopterygii and Sarcopterygii share that common ancestor to the exclusion of other groups, which makes them sister clades.


Here's a pretty nice resource from Berkeley relevant to this topic. I'll also recommend the Understanding Evolution site that it came from.

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  • $\begingroup$ to be fair those groups are also both clades and taxon, as groups they are robust enough to have survived the transition in methodology. so you also get a confusion of cenvention. $\endgroup$
    – John
    Jan 7, 2023 at 1:10

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