If it’s any comfort, the displays at the Museum of the Rockies (Jack Horner’s home institution) are very careful to say things like “All the non-feathered dinosaurs are extinct”, and the last display in the dinosaur section of the museum has a bunch of pictures of magpies, gulls, and ostriches. Personally, I think that with the fascination all kids have with dinosaurs, and the even greater fascination they would have in knowing that not all dinosaurs are extinct, after all, the battle is far from lost, and will probably be won within a generation.
Still, though, any taxonomic scheme must have some tolerance for paraphyletic groups, lest “archae” become synonymous with “living thing”. But sometimes, it’s really useful to say things as a group about those single-celled organisms without organelles which live in places like the Yellowstone hot springs.
This is making my head hurt. Is it possible to say definitively that *any animals * are fish?
I don’t understand what the average Joe’s opinion has to do with it, though. I daresay that the average Joe probably doesn’t think of himself as an ape. Joe probably also thinks that he can easily distinguish between birds and reptiles, based on the fact that one group has feathers and the other doesn’t.
Again, I am not clear on why this is significant. Early on, the relationship between birds and dinosaurs wasn’t clear either. Now that it is, why should the term “dinosaur” be any more legitimate than “fish?”
I’ve been trying to apply this to your earlier cited uniting characters of Reptilia:
Small Tabular bone
Presence of suborbital foramen
Presence of supraoccipital anterior crista
Narrow supraoccipital plate
Birds don’t appear to have a tabular bone at all, and I’ve so far been unable to confirm whether any of them have a suborbital foramen. Do they?
There’s nothing wrong with a paraphyletic group, like “fish” provided that you do take into account that it is paraphyletic.
The problem is that many people mishear paraphyletic as polyphyletic – a group constructed from convergent evolution. “Anteaters” is polyphyletic, including an Austrodelphian marsupial, three edentates, at least two monotremes, the pangolins, and the lone tubulidentate. “Dinosaurs” is paraphyletic, including all archosaurs with the ornithodire metatarsals, the spurred astragalus, a tendency to large size, erect stance (legs beneath rather than to the side of hip and shoulder girdles), etc. – but excluding the large radiation of those forms which evolved feathers, wings, hollow bones, the ability to fly, etc., which we find it useful to distinguish from the other dinosaurs as “birds.”
“Fish” is useful in that it groups together all vertebrates which are obligate aquatic forms breathing by gills throughout their life (except those few amphibians where reversion to this habit is a secondary adaptation) and with paired fins rather than limbs. As a clade though, it necessarily includes the descendants of the sarcopterygian fish which adapted to life on land, i.e., the tetrapods – and therefore does not have the meaning which the layman attaches to “fish.”
Sure, just define “fish” to mean “these animals on this list”. The real question is, does that definition help us in classifying living organisms? And that depends on what you want your classification scheme to accomplish. Linneaus set out to group things together based on shared structures, and that actually worked pretty well. He wasn’t concerned with the idea of common descent because that wasn’t understood to be important at the time. Now, we think of common descent as the defining characteristic and usually (though not always) give that precedence over superficial structural or behavioral features.
At the most fundamental level, we try to decide how many species there are. If we discover what appears to be a new organism we ask if it belongs in its own species. After that, we have to decide how we want to place that species in relation to other species. But it’s important to keep in mind that nature doesn’t care one way or another how we classify things. Darwin teaches us that the biological world is one big continuum-- not a series of discrete boxes. And anywhere we try to draw lines is ultimately going to be an arbitrary decision. That wasn’t much of a problem when we were only concerned with classifying extant species-- most of them look pretty darn well defined. But as we consider extinct species, everything starst to blur together at some point, and the old labels just don’t make sense.
What used to be the ladder of life became the tree of life, and now we think of it as more like a bush. All we see in the present world are the outermost tips of the smallest branches. When we try to group those tips together, do we include those tips that just happen to be near each other or do we trace the tips back and try to find where they originated from, grouping together those that join together first? Given the importance of common descent in modern biology, I think the latter approach makes more sense-- that’s the cladistic approach. The problem is that you sometimes have to rewrite the whole thing just because you discover one new species. Or, you just don’t have enough data about where the lines of descent are, so certain species (or whole branches) end up in limbo. Hence the resistance.
Darwin’s Finch: Opps… my apologies for this impertinent question. Given the topic at hand, and considering your username, I somehow arrived at the conclusion that you would have access to enormous quantities of information regarding the suborbital foramen of birds. In hindsight I recognize that this might not necessarily be the case. Sorry about that.
If tetrapods were disregarded entirely, would the term “fish” make any more sense as a clade?
Yes. If tetrapods didn’t exist, then “fish” as it is commonly used would be a pretty good clade, although there still might be some argument about hagfish and a few other extant species.
You’re still going to run into the problem of defining what the first “fish” was, though. That’ll have to be someone arbitrary, since things get blurry as you go back in time.
It’s no different than if I were to ask you to list your relatives. At some point you have to stop (5th cousins?), but wherever you stop is completely arbitrary. OTOH, what value is there in saying that you’re related to everyone-- that contains no information. So, you end up listing only the relatives you’ve met in your life or those listed in a family Bible or whatever. Instead of “calde”, think “clan”. Once you define the clan founder, then it’s easy to say who’s in or out. But then you’re left with the weird idea that the founder’s parents aren’t in the clan.
The problem is that Average Joe is the one who tends to muddy the taxonomic waters. “Dinosaur” has a specific taxonomic meaning in scientific circles (one which, contraPolycarp, does include birds – though, again, only for those paleontologists who accept a dinosaurian origin for birds. There are those who believe, based on scant evidence, that birds arose from more basal archosaurs; such folks would not, of course, consider birds to be dinosaurs at all). But in the vernacular, all sorts of critters, including pterosaurs, plesiosaurs, and just about any other reptilian citter that lived during th Mesozoic, are dumped into the “dinosaur” bucket. And all too often, even some Permian critters, such as Dimetrodon, are thrown in as well (Dimetrodon being one of the aforementioned Synapsids, who was really neither mammal nor reptile, and certainly not a dinosaur by any means). After many years of this sort of lumping of disparate forms, the technical definition becomes lost to non-scientific folks, and it becomes a chore to clean up the terms so that everyone can once again be on the same page.
As for fish, we can certainly agree that any aquatic critter that looks more like a trout than, say, an eel, is a “fish”, but as far as actual taxonomic terms go, “fish” doesn’t get us very far. We have jawless fishes and armored fishes and cartilaginous fishes and boney fishes and fleshy-finned fishes, and these are all understood to be more or less “fish” shaped and share various adaptations as a result of their aquatic natures. But they are not all united by a single, common ancestry (at least, not one wherein we can create a definitive Fish clade). And some of them are more closely related to non-fishy things than they are to other fishy things.
I merely brought that point up as an indicator that, while “fleshy-finned fish” may have once meant something (e.g., a fish-like critter that had tetrapod-like bones in its fins, rather than the “rays” found in fish like trout, etc.), once an affinity with Tetrapods was recognized, and tetrapods were made a subgroup of the larger Sarcopterygian group, the descriptor “fish” no longer worked to describe the group as a whole, again as far as the average Joe might be concerned).
The suborbital foramen is a small hole on the palate, through which usually passes a blood vessel or some such. Birds do possess this, as far as I know.
As for the “small tabular”, this could include anything from a small tabular to a completely absent one. In the case of birds, the tabular is, indeed, absent. The main point being that all synapsids (and other vaguely reptilian critters) have rather large tabular bones. A more precise version of the character would be “small tabular bone, when present”.
I get that if we want to call both ornithischian and sauropods “dinosaurs” then acording to cladistics we have to include aves. But why do we want to lump those two rather disimilar ancient reptiloids together? They have removed the Pterosaurs and the sea-going giant reptiles even those were originally considered “dinosaurs” (and still are by “the Average Joe”). But just because Marsh & Cope lumped them as “dinosaurs”, why should we?
To be honest, unless we are just going to use “Dinosaur” as a non-scientific term to refer to all of the ancient reptiloids, do we need the term if by using it- it includes three groups that are clearly widely disparate?
Because the groups really aren’t as disparate as they appear. There are several characters which unite Saurischia and Ornithischia, and their status as a monophyletic clade has been re-affirmed several times in the literature. I included some of the uniting characters in my thread Why Pterosaurs aren’t Dinosaurs from a few years ago.
Terrifel: I think you’d really enjoy reading The Ancestor’s Tale by Richard Dawkins. He does a terrific job of explaining this issue. I find myself going back to that book again and again as I think about these things.
Okay, I think maybe a bit of light is beginning to dawn here (of course I’ve already thought that three or four times during the thread, but hope springs eternal). The term “fish,” rather than having any meaningful taxonomic significance, is a descriptor more analogous to “quadruped.” Therefore, humans are descended from Sarcopterygii, and are also descended from fish; but while humans are included among the Sarcopterygii, they are not considered fish. Likewise, humans are Tetrapods but not automatically quadrupeds; the one term denotes a specific pattern of interrelationship, whereas the other does not.
Just for the record, DF, I’m perfectly well aware that a properly constituted Clade Dinosauria most likely includes Aves; I was speaking of “dinosaur” in semi-informed popular conception, as being a paraphyletic group with a single common ancestor but omitting the crown group Aves. Of course, I may be giving just a bit of asperity and unclarity in my posts on the subuject today: I was awakened before dawn by the dinosaurs squabbling outside my window.
Wouldn’t something like Appendiculae work here? When I studied anatomy and was taught the distinction between the axial skeleton (skull, spine) and the appendicular skelton (limbs), the inference seemed obvious to me: the axial skeleton is derived from the ancestral skeleton common to all vertebrates (well, Osteichthyes) and the appendicular skeleton derives from the additions common to all Sarcopterygii.
Gotcha. I was sure you did know that, but I probably just misunderstood the “Dinosaurs” in your post as being a reference to the idea that dinosaurs weren’t a valid clade (as DrDeth suggested). Kind of like how I used to always write “fish” (with quotes) instead of fish (without quotes) in my undergrad evolution class, wherein the professor and TA were not cladists…used to drive the TA crazy
Well, swoggle my horn and call me Gabby. Taxonomy really doesn’t acknowledge a classification for fish? There’s a definition of “reptile” that encompasses sauropods, thecodonts, penguins, spitting cobras, hummingbirds, drepanosaurs, sea turtles, false gharials, hyacinth macaws, Namibian sand geckos and pterodactyls… but no workable definition for “fish?”
That blows my mind. How the heck have I managed to live this long without realizing this fact? I’ve read books about fish, and somehow managed to come away with the impression that they were categorized in the same manner as “reptiles” or “mollusks.” Instead I now learn that fish are a random assortment of unrelated creatures with no more evolutionary commonality than “animals with knees” or “green things.” My post-secondary school education was evidently even lousier than I’d ever imagined.
To get back to Terrifel’s OP, as messed around with, yes, it is possible to create a taxonomic definition of “fish” that corresponds to the intelligent layman’s view. (“Intelligent layman” means no background in taxonomic biology but aware that, e.g., “whales and dolphins are not fish” and similar concepts.)
In old traditional taxonomy, it would be something like “Superclass Pisces” with a listing of the Classes which fall into the taxon – Osteichthyes, Chondrichthyes, Placodermi, Agnatha, Acanthodii…
In modern cladistic terms, “Fish” might be defined as vertebrates bearing spines and heads with sense organs and a brain enclosed in a skull or parallel bone or cartiliginous structure, which have not developed walking limbs or secondary structures derived from them. It would then be a paraphyletic group, defined at the broad meaning by those characteristics that distinguish vertebrates from lancelets, and at the narrow meaning by the exclusion of the crown group Tetrapoda.
As noted earlier, there are valid reasons for knowingly distinguishing paraphyletic groups – when one group within a clade has undergone extensive radiation and adaptation, so that it can be clearly set apart from the other elements of the clade, but the common characters of the clade without that group make it worthwhile to discuss them together excluding the group set apart. Fish, for example, have in common a unique pharyngeal gill structure, with wide variants among the multiplicity of organisms that qualify – and only larval and neotenic amphibians (tetrapods) share that unique structure with them. Fish may or may not have particular fins, but there’s a clear set of fins from which they “choose one from column A and two from column B” – paired pectoral and pelvic fins, contrasted with midline dorsal, anal, and caudal fins, with possible reduplication in some cases (particularly the dorsal). This particular set of fin structures is present across virtually every taxon of fishlike vertebrate, though many members within each taxon do not have the full set and a few have only caudal fin or none at all, and variants are endless – multiple dorsals in some early sharks and the polypterids, for example, or armored pectorals in the antiarchs, one group of placoderms. But if a fin is present, it belongs to that list, and in the form noted – e.g., nobody has a midline pectoral fin, or paired anal fins.
