Tuesday, May 01, 2007

There's No Such Thing as a "Mammal-Like Reptile"

Remember how people used to throw the word "thecodont" around to describe some generalized archosaurian ancestor of dinosaurs? The problem with the term was that it had no formal definition--it was just a loose amalgamation of archosaur taxa which had vaguely dinosaurian bauplans. The term "Ornithosuchians" was used for a little while until it was decided that ornithosuchids actually fell to the crurotarsian side of the archosaur family tree. Eventually, thanks to cladistics and common freaking sense, the word "thecodont" went out of style and was replaced by the much more accurate and exclusive name, "ornithodiran." "Ornithodira" has a formal diagnosis. You can tell if something is an ornithodir or not, but that was not the case with "thecodont." This is one of the reasons I love cladistics--it has really put the taxonomy of extinct animals into the limelight. Sure, there are times when you have an animal like Silesaurus or Dracorex where cladistics actually impedes forward momentum, but overall it's quite the system.
Anyway, there's another outdated term floating around the discourse right now: "Mammal-like reptile." The term is used to loosely describe the critters that aren't snakes, lizards, crocodiles, or Petrolacosaurus that eventually gave rise to mammals. Dimetrodon grandis (above) is an oft-cited "mammal-like reptile" that was, in truth, neither a mammal nor a reptile.
"Reptilia" is a formal clade name that includes anapsids and diapsids. The group is defined by features of the skull, mainly, and exactly how many holes are in said skull, and how those holes are arranged. After a long evolutionary history, anapsids are only now represented by turtles, which are themselves a very ancient group of anapsids. Diapsids include lizards, snakes, tuataras, crocodiles, plesiosaurs, non-avian dinosaurs, pterosaurs, Arizonasaurus, birds, and a bunch of other groups. The point is that both the small anapsid group and the huge diapsid group are reptiles. "Mammal-like reptiles" have nothing to do with reptiles.
You see, Timmy, when tetrapods conquered the land and abandoned the water, they became amniotes. Amniotes then split into two main factions--the reptiles, which focused on adaptability, and the synapsids, which focused on...um...their teeth. Both groups layed leathery eggs, and both groups had scaly skin. So why wouldn't they both be called reptiles? Because true reptiles modified their skulls in one way, and synapsids modified their skulls in a different way. So we can diagnose a fossil animal as either synapsid or reptile by looking at its skull. It's an ancient division, and it's the whole reason those labels exist in the first place.
So when people say "mammal-like reptile," what they really mean is "lower-tier synapsid." I'm not an expert on synapsid taxonomy, but I know that "reptile" never shows up in the formal clade definitions. Think about it this way: the word "synapsid" is the equivalent of saying "reptile," but on the mammal side of things. Mammals are synapsids, just like archosaurs are reptiles. It's not hard.
Now stop saying "mammal-like reptile."


Scott said...

Great essay. Minor points only:

"Sure, there are times when you have an animal like Silesaurus or Dracorex where cladistics actually impedes forward momentum,"

You know this how?

"and both groups had scaly skin."

Really? We have integument impressions for protomammals? I was unaware of this! Details, man! Spill me the details!

And this isn't a technical pick, but a grammatic pick:
"Because true reptiles modified their skulls in one way, and synapsids modified their skulls in a different way."

Say, rather, that reptile skulls were modified (or evolved) along one line, and synapsid skulls evolved along a different way.

I have the same response whenever workers use the term "design". There is no design in nature, naturally, and that's not what they mean, but they should say what they mean, rather than use lazy terminology like "design" where they mean "evolved."

Zach Miller said...

Harsh toke, man. Well, there are some skin impressions from critters like Scutosaurus, and they appear to be scaly. I guess I'm merely using PHYLOGENETIC BRACKETTING to guess that the first synapsids had scaly skin.

And as for the Silesaurus thing, as you're probably aware from my previous postings, I'm convinced that it's a primitive ornithischian. Because it seems to have a few primitive characters (closed acetabelum, two sacrals), Dzik's cladistics posit it as a late-blooming ornithodir. Never mind that Saturnalia has a closed acetabelum and Eoraptor (and Herrerasaurus maybe) have two sacrals. And Silesaurus has a freakin' predentary. What more do you need?
As for Dracorex, cladistics would position it as the most primitive pachycephalosaur, but as Bakker points out, its end-Cretaceous age as well as its many-spined noggin suggest a close affiliation with Stygimoloch. It's lack of dome confuses the cladistic analysis, when that feature is really just a reversal in response to sexual selection (horns over dome).

So that's what I meant by that.

Scott said...

Phylo bracketing:

I don't think you have a case, here. Bracketing (by implication) indicates by extents or extremes, and the feature being interpolated resides with an animal sitting between the brackets.

Or you've taken mammals with scaly skin (such as pangolins) and made their integument very distant and highly derived models of scaly skin of the kind seen in reptiles.

I'll concede that's possible, but the terms superficial similarities and analogous structure haunt me.

Generally, I've repudiated the scaly integument supposition for protomammals, favoring a more primitive-derived-from-an-ambiphious-ancestor-unscaly-skin skin.

Additionally, seems like this would be a good point to push if we'd like protomammals to stop being called mammal-like reptiles. (Unless, of course, an impression of integument comes to light. Dang. I was really excited there for a minute.)

"What more do you need?"

Something testable?

Again, since definitions are arbitrary, there will be taxa that darken the bright lines of taxonomic purity. For them, new categrories will have to be created. Close to, but not of, the established definitions.

Dracorex: There are many examples of taxa largely considered "primitive" which are alive and well today, from the extant aquatic examples of Sarcopterygiian fishes (e.g. Coelacanths) through proto-tetrapod lungfishes. Though in fact each of these groups possess derived characteristics that reveal some distance between their extinct coutnerparts, they are still considered "primitive." I see no reason not to apply this principle to taxa much more closely related to one another.

(Stegosaurs got it on like ducks. If that isn't nightmare fodder, I'm not doing my job right.)

Zach Miller said...

Hmm...unscaly, amphibian-like skin? That's just plain hard to imagine. I demand a graphical representation! By Saturday! And Dracorex IS a derived pachycephalosaur, dammit, it's not a coelocanth!

Oh, and stegosaurs totally got it on like ducks. And if you've been checking out Naish's blog lately, you know just how terrifying that prospect can be.

Anonymous said...

Hey i gotta question is there a such thing as a dog cat??
-Dani age 11

Zach said...

No, but there is a bear-dog (Amphicyon)!