Friday, December 05, 2008

Wherefore art thou, Pelycosauria?

I love pelycosaurs. They're probably the most famous non-dinosaurian prehistoric critters. They're not even reptiles, though--they're basal synapsids. They're more closely related to you and I than to lizards and crocodiles. Pelycosauria used to be a coherent group featuring a variety of Permian forms that all looked fairly similar. So similar, in fact, that in at least one case, one distinct genus was considered the female of another genus! Pelycosaurs were largish, lizard-like critters with large skulls and mean, bladed teeth. There was at least one herbivorous pelycosaur, Edaphosaurus, a small-headed form that lived alongside uber-carnivore Dimetrodon.

Some pelycosaurs developed sails on their backs. In Dimetrodon, the sail was tall and roughly symmetrical from front to back, as its tallest point was in the middle. In Edaphosaurus, though, the sail was much shorter on the neck, and the neural spines were swept back, giving the sail a ramp-like profile. Additionally, the neural spines had cross-pieces of bone along its length. The sail would have looked spikey! But most pelyosaurs were sail-less, including basal form Ophiacodon.

Lately I've been hearing that pelycosaurs are not a coherent group. That is, Dimetrodon is closer to stem mammals than Edaphosaurus is. Instead of forming their own family, it would seem that pelycosaurs form a stepwise progression toward stem-mammals, just like rhamphorhynchoids form a stepwise progression toward pterodactyloids. I'm perfectly willing to accept that, but I'd like to know where this idea came from. Additionally, are there any taxa within the traditional "Pelycosauria" that DO form monophyletic groups? There are such minor groupings among rhamphorhynchoids, after all. So while "Rhamphorhynchoidea" is not a monophyletic group, Anurognathidae IS, and so is Rhamphorhychidae (Rhamphorhynchus, Scaphognathus, a few others). So are there any pelycosaurs that form a monophyletic relationship?


Karl Zimmerman said...


My understanding is Pelycosauria is made up of the following groups:

Caseasauria: basal outgroup, containing mainly herbivorous synapsids lacking any sort of tooth differentation (after Diadectes, these are the first known herbivorous tetrapods)

And then everything else, which included the clades:

Ophiacodonts: Basal, lizard-like

Varanopseidae: Superficially monitor-like, sailless

Edaphosauridae: Contained at least three genera.

Sphenacodontia: The carnivorous sailbacks. All of them didn't have sails however, and the group is also partially paraphyletic in relation to therapsids.

Christopher Taylor said... has pretty good pelycosaur coverage. Start at the synapsid intro page and follow the links.

Mike Keesey said...

Point of order: "pelycosaurs" are stem-mammals. A "stem-mammal" is anything sharing closer ancestry with mammals than with anything else extant (except for mammals themselves).

Basically, a "stem-mammal" is pretty much what they used to call a "mammal-like reptile". The newer term is not only less misleading, but more succinct!

Neil said...

I know that Mike Keesey has deployed this use of stem-mammal (on tet-zoo and other places) before, and that it is in keeping with Phylocode conventions, and I recognize that it is in some sense convenient to have a straightforward and accurate replacement for "mammal-like reptile." Still, when I hear "stem-mammal" I intuitively think of something like Morganucodon (i.e. a mammaliform outside crown mammalia)...but certainly not something like say, Cotylorhynchus or Edaphosaurus as these are presumably well off the mainline of mammal evolution (all of which are "stem-mammals" by Keesey's definition). Then again, I realize no-one gives a flying eff about my intuition. I'd be curious to know what others think.

Neil said...

Or to put it more succinctly, do you all consider Diplodocus to be a "stem-bird"?...because I think it is according to this usage of the concept of stem-groups.

Christopher Taylor said...

Diplodocus is indeed a stem-bird. So is Triceratops. Similarly, Eusthenopteron is a stem-tetrapod, Diplocaulus is (possibly) a stem-amniote, Cooksonia (at least some species) is (possibly) a stem-lycopodiophyte. Though these may seem counter-intuitive, the thing is that "stem" and "crown" groups are only defined in reference to living taxa, so you don't get to pick and choose where your stem group begins.

The terms were introduced by Jefferies in 1979, so they well pre-date the PhyloCode.

Neil said...

Thanks for the reply Christopher. I think I'm straightened out now, after reading the original Jefferies reference, as well as Donoghue 2005 - from which I at least take solace that the concept has been used with an unhealthy measure of ambiguity, which partly accounts for my confusion.

I suppose if pterosaurs are closer to dinosaurs than crocodilians, that makes Pteranodon a stem-bird as well, huh? Crazy....

Christopher Taylor said...

Yep, pterosaurs are stem birds, and phytosaurs are stem crocodiles :-) .

I'll admit, the terms have been sorely abused. Giardia can not be a "stem eukaryote", nor Gerarus a "stem titanopteran" - the first would require Giardia to be extinct (which it isn't), while the second is meaningless because there's no crown group for Titanoptera, but I've seen both these used.

Mike Keesey said...

I've also seen "stem" used to refer to doubly-paraphyletic internal groups, i.e., Sereno uses "Stem Archosauria" to mean stem-crocodylians + stem-avians + the archosaurian ancestor., when the term should mean "non-archosaurian archosauromorphs" (e.g., Euparkeria, Trilophosaurus, etc.). Confusing, to say the least. But, used correctly, it's a powerful and succinct way to associate extinct organisms with their closest living relatives.