u/ReasonablePrimate

Here's a working definition of a crustacean that I think would be intuitive for a lot of people: a crustacean is any animal more closely related to a crab than to a centipede or a dragonfly.

So what does that include? Crustacea is now widely understood to be a paraphyletic taxon, wikipedia explains, because about three of its classes are more closely related to hexapods than to any other crustaceans, and one of its classes is an outgroup that is less closely related to hexapods than the other crustaceans.

(Those three classes that form a clade with hexapods are about 39 species of remipede, about 13 species of cephalocarida horseshoe shrimp, and about 2,476 species of plankton-like branchiopods, not to be confused with the mollusc-adjacent brachiopods. The one class that is an outgroup is about 7,909 species of seed shrimp, tongue worms, and fish lice. These numbers are from opentreeoflife.)

But here's the thing: about 50,910 species do in fact seem to be part of a single monophyletic clade, including just about every animal you might think of as a crustacean: crabs and hermit crabs, lobsters and crayfish, prawns and shrimp, krill, mantis shrimp, barnacles. Another 15,774 species of copepods might belong here, too.

So why have researchers from 2005-2023 sought to describe this clade (and various different formulations of it in each new study) with new titles (e.g., multicrustacea, vericrustacea, communostraca) and taken pains in the meantime to reeducate the public that crustaceans aren't a valid clade?

Wouldn't it be clearer to just call this large clade "Crustacea" and instead argue over whether copepods and remipedes and fish lice are or aren't crustaceans?

In a more general sense, I'm asking whether the practice of using new names for each new cladistic hypothesis in order to preserve the definitional continuity of taxonomic grades is actually better for public understanding than just updating the definition of old taxa as phylogenetic research advances.

I’m an amateur who enjoys reading about phylogeny and evolutionary history, and I’m trying to understand whether there are formal concepts the experts use to differentiate between “shapes” of clades based on their characteristics.

Some clades seem relatively balanced and easy to summarize. For example, all extant vertebrates can be described as Agnatha (jawless fish), Chondrichthyes (cartilaginous fish), Actinopterygii (ray-finned fish), or Sarcopterygii (lobe-finned fish). Likewise, all extant tetrapods divide into Amphibia, Mammalia, and Reptilia (including birds). These clades are all species-rich, morphologically distinctive, and fully resolved in phylogenetic studies.

By contrast, other clades seem asymmetric compared to the outgroup and sister clades from which they diverged. Sarcopterygii contains tens of thousands of tetrapod species, alongside a clade of six extant lungfish species and a clade of two coelacanths. Lepidosauria contains thousands of squamates, plus tuatara, the one surviving rhynchocephalian. Xenacoelomorpha is a proposed clade of about 400 species that seems difficult to place phylogenetically, but is somewhere near the base of Bilateria or Deuterostomia.

I realize these are all equally valid clades - they describe heredity as it happened to the best of our knowledge. But intuitively it feels like there are different evolutionary patterns involved that are worthy of study, including different patterns of speciation, morphological diversification, and extinction that sometimes result in clades marked by adaptive radiation, sometimes in the isolation of low-diversity lineages over long time periods, and sometimes in relictual survivors of once-diverse clades.

So that's my question: are there any characteristics of clades (maybe branch length or symmetry with outgroups or measures of internal diversity) that are used to study and teach these evolutionary patterns, or am I just barking up the wrong Gingko tree? I'd be grateful for any recommended terminology or conceptual frameworks that would help me think more clearly about these patterns. Thanks!

Crustacea actually seems like a pretty reasonable name for a clade, so why is there an effort to break it apart as a paraphyletic taxon?

Does the "shape" of a clade matter?