Wednesday, January 30, 2008

Centrosaurines vs. Chasmosaurines

When I visited the AMNH this past December, I was particularly taken with the Triceratops on display. Having recently painted a skeleton in detail, each bone seemed extremely familiar and identifiable. In “person,” the Triceratops skeleton was a beautiful testament to the power and presence of the animal. I found myself with a new appreciation for Ceratopsids.

I don’t know much about Ceratopsids. It seems they’re lost in a sea of Theropods…overshadowed by the gigantic Sauropods. In movies, T-rex, Velociraptor and other carnivorous dinosaurs are given the spotlight. Jurassic Park featured a Triceratops, but it was sick, on its side and hardly depicted as exciting.

Most of what I do know about Ceratopsids comes from my visit to AMNH. In fact, during that visit, Brian taught me one of the most basic of facts: the difference between Chasmosaurines and Centrosaurines.

Chasmosaurinae includes Anchiceratops, Arrhinoceratops, Chasmosaurus, Eotriceratops Pentaceratops, Torosaurus and Triceratops. Chasmosaurines are characterized by huge triangular frills with large fenestrae to minimize the weight, large eye horns, long skulls and short-to-absent nasal horns.



Pentaceratops (top) and Chasmosaurus (middle) images from weatherenthusiast.com and skulls-skeletons.com, respectively. Triceratops (bottom) image from mnh.si.edu.

Centrosaurinae includes Achelosaurus, Avaceratops, Brachyceratops, Centrosaurus,, Einiosaurus, Pachyrhinosaurus and Styracosaurus. Centrosaurines are characterized by highly ornamented, small frills, short skulls, absent eye horns and large nose horns.



Centrosaurus (top) and Pachyrhinosaurus (middle) images from uoregon.edu and tyrrellmuseum.com, respectively. Styracosaurus (bottom)image from biology-blog.com.

You can see, particularly well, the general differences in the skulls by comparing the Triceratops and Centrosaurus images.

Also, you'll notice, in the photo of Chasmosaurus, a round opening in the skull slightly above and between the orbital sockets. From what I know, this opening is called the pineal foramen, frontoparietal foramen or postfrontal foramen, though I believe the latter is an out-of-date term. I haven't been able to find much information on this particular part of the skull, nor do I know if it is specific to certain families of Ceratopsians, though I do remember Brian telling me something about it at AMNH.

Anyone know more about this?

References:

Zach Miller, When Pigs Fly, Evolution of Ceratopsia Part 2.

Wikipedia. Ceratopsidae.

Dodson, P., Forster, C., and Sampson, S. 2004. Ceratopsidae, p. 494-513. In Weishampel, D., Dodson, P., and Osmólska, H. (eds.), The Dinosauria (second edition), University of California Press, Los Angeles.

10 comments:

Jerry D. Harris said...

Also, you'll notice, in the photo of Chasmosaurus, a round opening in the skull slightly above and between the orbital sockets. From what I know, this opening is called the pineal foramen, frontoparietal foramen or postfrontal foramen, though I believe the latter is an out-of-date term. I haven't been able to find much information on this particular part of the skull, nor do I know if it is specific to certain families of Ceratopsians, though I do remember Brian telling me something about it at AMNH.

Actually, what you're seeing in the photo isn't the opening of which you speak, although it's related to it. What you're seeing there is a kind of notch that leads to a compartment called the supracranial cavity. The opening you're referring to (the frontoparietal foramen) is on the floor of this supracranial cavity. The foramen is, apparently, nonhomologous with the parietal foramen (the correct name for what is often called the pineal foramen or pineal "eye"), and its function is unknown. For that matter, I'm not sure if there's a decent explanation for the supracranial cavity, either, but then again, I'm one of those Saurischia snobs*... ;-D

*Actually, I like ornithischians just fine -- well, except maybe hadrosaurs -- I've just not had any good opportunity to work on any short of sculpting missing parts for and mounting a bunch of Othnielosaurus juveniles, and mounting an Edmontosaurus, at the Denver Museum.

Amanda said...

Thanks Jerry :)

I'm not sure what you mean by nonhomologous. Is that the same as analogous? Thanks!

Jerry D. Harris said...

I'm not sure what you mean by nonhomologous. Is that the same as analogous?

Essentially, yes. It's a bit harsher (in the sense of drawing extra attention to the evolutionary implications) to say "nonhomologous" as opposed to "analogous," but yeah, the two are basically synonyms. But as I like to tell my students, "If everyone knew and used all the technical jargon, then getting a Ph.D. wouldn't be nearly as impressive, now, would it?" ;-D

Zach Miller said...

Non-homologous means it is not the same as. For example, let's say that bugs suddenly evolved backbones. If the backbones were homologous, that would mean that their spines are in some way related to OURS, but because they evolved the spine independantly, it is NONhomologous.

Let's use a dinosaurian example. Ceratopsians, as you probably noticed, have enormous parrot-like beaks. However, the beaks are not homologous to those of actual parrots, or turtles for that matter. The structures evolved independantly.

Also, thanks for using me as a reference! I'm glad people still find that old blog useful! I really need to do a ceratopsian post with my own picture...

Traumador said...

Too bad you missed the BIG ceratopsian symposium at the Tyrrell last year (I missed it too *tear*)...

There's a ton of new stuff on them in the last few years. They've nearly doubled the number of known ceratopsians in this small window of time.

Can't help you on your skull opening question though :(

Jerry D. Harris said...

Non-homologous means it is not the same as.

...in an evolutionary sense, of course. I'm not the same as my car, but that doesn't mean my car and I are nonhomologous, since homology (evolutionary identity) doesn't come into play in such a comparison. (Not correcting Zach, here, since I know that this is what he meant; just putting in a clarification for any other readers that might not be as familiar with the terms "homology" or "homologous" -- you never know; this could be the entry that comes up in some Google search for the term!) Specifically, homologous structures (anatomical, but also behaviors, gene sequences, etc. -- the term applies equally well to genotypic and phenotypic studies) are ones that share an evolutionary ancestry -- homologous structures are those that exist in different taxa because the structure evolved once in a common ancestor of all the organisms that have the structure, and was simply retained (though it can be heavily modified!) in all the descendants of that ancestor.

An analogous structure (or gene sequence, behavior, etc.) is, instead, one that arose separately in taxa that don't share a recent common ancestor. It's usually relatively easy to tell whether or not this is true by looking at a broad swath of descendants of an ancestor -- if most of them lack the structure, odds are better that it evolved convergently, but separately (analagously), in the few that have it rather than saying it evolved in a common ancestor but was lost a large number of times. This is because of the "rule" of parsimony -- convergent evolution a few times is a much more simple explanation than loss a large number of times, though the latter is certainly possible.

My favorite examples for discussing homology vs. analogy involve things like patellae (knee caps) -- neornithean birds have them and mammals have them, but the taxa that are ancestral to birds and the taxa that are ancestral to mammals lack them. Mammals and birds share a common ancestor, of course (the first amniote), but it's most parsimonious to perceive that the knee cap evolved twice (once in derived birds, once in mammals). It leads nicely to discussions about evolutionary constraints and how they select for particular features in only distantly related organisms -- convergence (analogy) can run rampant!

Zach Miller said...

Traumador, I'm only aware of Albertaceratops, Kirkland's new "octoceratops" (still doesn't have a name), a known species of Chasmosaurus getting its own genus (Agujaceratops), and Eotriceratops. Okay, that's a lot, but technically that's only three new animals. Are there more? Am I missing something? I MUST KNOW!

Traumador said...

Centrosaur Brinkmani, DPP Pachyrhino (which as you'll recall I thought was Albertoceratops... having JUST visited the Tyrrell for the first time in over a year I've got that a little straightened out) there is a rumoured new ceratopsian in Texas (according to the technicans at the Tyrrell mind you)...

I'll get the definative list from my buddy who claimed this stat as fact, but that's 2 more for sure, and one possible extra.

Traumador said...
This comment has been removed by the author.
Laelaps said...

Again I'm late to the game; I feel like such a slouch. Thanks for clearing up the issue about the supracranial cavity opening, Jerry. Every time I saw that mount I couldn't help thinking that someone had taken a drill to the skull to try to get at the brains of the animal and couldn't seem to find why that hole was there.

Thanks for the kind words, too, Amanda. That trip was a lot of fun, and I'm glad I was able to inspire one of your blog posts! I didn't really know about the difference (other than there was a difference) myself until I read Dodson's horned-dinosaurs book, which is a good introduction if you get the time to read it. (Of course a ton of new horned dinosaurs have been added since then so an expanded/updated edition is definitely needed.)

And if anyone really wants to confuse themselves about homology/analogy, read some of Cuvier's work and that of other naturalists of his day. He used "analogous" to mean what we mean by "homologous" today, which can be a bit confusing.