Or my favorite example, there’s no such group as bacteria, either. Or rather, the clade of bacteria is the clade of all living things.
That said, if we’re going to be all cladistic about it, then the clade consisting of gorillas, ourangutangs, humans, etc. could reasonably be called “apes”, and the group consisting of those plus the Old and New World monkeys could reasonably be called “monkeys”. By this standard, all humans are apes and all apes are monkeys, but not all monkeys are apes and not all apes are humans.
No. There are creatures that live in water and have gills, but are not fish.
No again. Fish have scales, and even birds do (on their legs).
No again. There are other equines that have stripes but aren’t zebras.
As soon as you try to make those descriptive terms scientific (which is what you were doing if you don’t think you were), you are going to be wrong. There is nothing you can say about a reptile that you can’t also say about a bird. That’s the problem.
Let me rephrase that last bit. There is nothing you can say about reptiles, as a group, that doesn’t also apply to birds. The only way you can define reptiles, as a group, in such a way as to exclude birds is to say: “Any animal, except a bird, which has…”
Like starfish and jellyfish? They are fish! By vernacular definition, the “fish” part of their name being a clue.
Thing is, anything that lives in the water might be called a “fish”, just like anything that flies might be called a bird, even if it’s a bat or a pterodactyl, or anything that doesn’t have legs is a “worm”, or any animal we eat used to be called “deer”, or any animal we herd used to be called “cattle”.
Of course these categories aren’t scientific or rigorous, you can’t construct definitions of them that hold up, except through special pleading, like “humans are defined not to be apes”, or “apes are this species, this species, this species, and this species, and no others”, or “dinosaurs are all members of the saurischia or ornithiscia, except birds”. So categories like “reptile”, “mammal”, "ape: and “bird” make sense until you dig up a Therapsid or Australopithecus or Archeopteryx or Compsognathus.
Originally, the category of “fish” included whales, as well as seals (which could be eaten on fast days, according to the Catholic Church) and sea turtles (as indicated in the title of Archie Carr’s book on the Green Turtle, So Excellente a Fishe).
As in the case of apes and monkeys, here scientists took a vernacular term with no original scientific significance and gave it one. This has caused a shift in popular usage, so that most people no longer would consider whales, seals, or sea turtles to be fish, even though this was once a valid usage. And although the term is still used as part of the name of shellfish, jellyfish, starfish, etc., most people would not consider these to be “real” fish.
But fish itself doesn’t have a real taxonomic meaning under current standards, since it includes members of four different classes, and many more lineages. Trout are much more closely related to whales than they are to sharks, so to call a whale a fish is no more or less inaccurate, from a cladist’s point of view, than calling a shark one.
Interestingly, Linneaus’ taxon of Vermes (“worms”) contained not only jellyfish and starfish, but also the hagfish, which would still be considered a “fish” in popular usage.
I’m sorry, I have too many Simpson’s quotes in my mind for being a sane person. I guess the problem is when I was really young, I was as smart as a monkey, but now I’m just dumb as an ape.
OK, I retract the “gills” part. Anything that lives in the water but doesn’t have legs is a fish. Including whales, seals, walruses (whale-horses), sea turtles, octopus (“devilfish”), jellyfish, shellfish and starfish, but not otters and hippopotamuses.
Everyone knows hippos are really horses that just happen to live in rivers, and aren’t fish. So if a hippo had stripes, it would be a zebra. But to call it a fish would be ridiculous!
Couldn’t you define a “reptile” as a cold-blooded (yes, I know, not the correct term, but I can’t recall the Greek) amniotic vertibrate? Or do the crocodilians have good enough temperature control to make this fuzzy?
I let **Colibri **answer this one, but I don’t think you can draw a distinct line between cold and warm bloodedness. We also suspect that some non-avian dinosaurs were warm blooded like birds, so unless you want to call them “birds”, your definition wouldn’t work.
There are two sets of terms that can be used to mean warm vs cold blooded.
Endothermic vs Exothermic. Endothermic means they get most of their heat from “inside” by burning food. Exothermic means they get most of their heat from “outside”, by sitting in the sun and such.
Homeothermic vs Poikilothermic. Homeothermic means they keep a constant body temperature, Poikilothermic meaning they allow their body temperature to vary.
Most birds and mammals are endothermic and homeothermic, most turtles and lizards and snakes are exothermic and poikilothermic. But it is theorized that sauropods for instance were exothermic but homeothermic due to their large size.
Also, fish are creatures that live in the water that normally have either less than 3 legs or more than 5.
However, there are some mammals, such as sloths and echidnas, which are relatively poikilothermic (although endotherms). And when hummingbirds go into torpor, they are also poikilothermic, as are hibernating mammals. And many reptiles, such as lizards, although exothermic, are behavioral homeotherms, keeping their temperatures fairly constant by basking. Large crocodilians in the tropics, because they gain and lose heat slowly, are probably fairly homeothermic just by virtue of their size.
Overall, temperature regulation systems are a bit too vague a characteristic to serve as a basis for major taxonimic characters.
Well, I’m not sure if I’d say they were birds specifically (if anything, birds are a subset of dinosaurs, not the reverse), but I’m quite comfortable saying that dinosaurs are not reptiles. As I recall, their skeletal structures, particularly their hips, are rather different from all (other) reptiles as well.
But I see the point about temperature regulation systems being a bit blurry. One could get around the hibernation issue, for instance, by adding “…while active” to the end of the definitions, but then, I guess a basking lizard isn’t particularly active, either.
And the tuataras. Everyone always forgets about the poor tuataras.