Are fish as taxonomically (or genetically, or whatever) diverse/separated as tetrapods?

What I mean is this: Are there types of fish that have a most recent common ancestor with other types of fish that is less recent (farther back in time) than transitional forms like Tiktaalik?

If so, does that mean that humans (and therefore all amphibians, reptiles, birds, and mammals) are more closely related to a particular type of fish (for example, the Coelocanth) than that fish is to some other type of fish?

I think this is true- because I think fish had already started to diversify into forms like sharks, rays, bony fish, etc. before they crawled onto the land. But if it’s true, then why do we commonly seem to treat fish as a big group akin to reptiles, amphibians, or mammals, when they’re really made up of multiple such groups? It seems like we should separate the big (vertebrate) animal groups like this: sharks/rays, bony fish, jawless fish, and lobe-finned fish/tetrapods (or something like that). It doesn’t seem like the accepted taxonomic breakdown reflects this (for example, this).

Yes. Humans are fish, in a sense.

Oh, Link.

Taxonomy is a classification scheme, and was not developed based on genetic similarity.

Then is there a classification scheme in which one could crawl up the “tree of life” to see relative Most Recent Common Ancestors for different species? It seems like there should be, if there isn’t.

But it is now.


Yes. This requires some twisting of the taxonomic grades. Look at a clading diagram of Amniota where Class Mammalia is shown as a descendant of Order Therapsida, Class Aves is shown as a descendant of an Order within Dinosaurs. … And this whole Amniota Superclass is a descendant of the Subclass Sarcopterygii (lobe-finned fishes). :smack:

Well, this all goes back to conventions developed long before we understood the genetics and evolutionary history. We see this everywhere. Are birds dinosaurs? Are humans monkeys? African monkeys (and others in the Old World) are more closely related to us than they are to New World monkeys.

Cladistics takes this into account even if our lay language hasn’t caught up.

Just for the record, “fish” are considered a pseudo-taxon (e.g. a historic common name) which are operationally defined as “all non-tetrapod craniates.”

It might be interesting to make a list of all the pseudo-taxa there are out there.

Whales v dolphins, eagles v hawks, monkeys v apes, apes v humans, fish v tetrapods, just off the top of my head.


Cladistically, “fish” is synonymous with vertebrates" – even discounting the broader use of the term in “shellfish” and “cuttlefish” and the like, there is no way tp set up a category of "all backboned animals termed ‘fish’ and their descendants that does not also include the tetrapods (=all land vertebrates).

In a cascading series of outlines guaranteed to give Opal Cat screaming hysterics, here’s a rough cladistic outline. Note that C is a major subgroup of B, B one of A, and so on.

A. Vertebrata
– 1. The hagfish
– 2. Osteostracans, one major sugroup of the jawless armored fish of the Paleozoic
– B. Craniata

B. Craniata
– 1. Heterostracans, a second major sugroup of jawless fish
– 2. The conodont animals
– 3. The lamprey
– 4. Anaspids, the third jawless-fish group (spelled correctly, Anapsids are turtles)
– C. Gnathostomes

C. Gnathostomes
– 1. Acanthodians, the small ‘spiny sharks’ of the Paleozoic
– 2. Tghe arthrodires, the giant jointed-armor fish of the Devonian
– 3. The antiarchs, another group of 'placoderms, with armored pectoral fins
– 4-6. Three smaller groups of ‘placoderms’
– 7. Chondrichthyes, the sharks, rays, skates, etc.
– 8. Holocephalians, the chimeras or ratfish
– D. Osteichthyes, the bony fish

D. Osteichthyes
– 1. Actinopterygians, the ray-frinned fish (including most modern fish)
– E. Sarcopterygians, the fleshy-finned fish

E. Sarcopterygians
– 1. Lungfish
– F. Crossopterygians

F. Crossopterygians
– 1. Porolepiformes
– 2. Onychodontida
– 3. Actinistia
– 4. Rhizodontida
– 5; Osteolepidida
– 6. Osteolepiformes
– 7. Panderichthyida
– G. Tetrapoda (all land-dwelling and secondarily aquatic vertebrates)