Two New Leptoceratopsids Push the Boundaries of Artistic Inferrence

Have you heard? Two new basal neoceratopsids were discovered in Canada. Their names? Unescoceratops koppelhusae and Grphyoceratops morrisonii. They’re leptoceratopsids, a somewhat hazy-but-currently-monophyletic group of small-bodied, small-frill horned dinosaurs from (mostly) North America. Unescoceratops was named based on a fragment of the left mandible. Originally regaled into the genus Leptoceratops, Michael Ryan realized it was unique. Gryphoceratops is known from a piece of lower right mandible. Among its interesting features is size: an adult would not have grown two feet long, making it one of the smallest—if not the smallest—adult dinosaur known.

This is all well and good. It increases the diversity of the group and gives us information about the initial dispersal into North America. It’s also nice to find small dinosaurs, period: fossils of anything smaller than, say, a troodontid, is pretty hard to come by. Tiny things just don’t fossilize very well. Here are the mandibular fragments from both taxa!



Wow, there's just not a whole lot there, but it hasn't stopped Julius Csotonyi from painting that beautiful life reconstruction of both animals (above)!

Gorgeous thought it might be (and it is), one wonders what the utility of such a painting is, given that these two animals are known from...say it with me...fragments of the mandible in both cases. This painting is begging to be invalidated down the road. Oh, sure, phylogenetic inference can tell you something about the general form of these animals, but nothing specific: there's considerable morphological distance between, say, Udanoceratops and Cerasinops. It's a beautiful picture, but I have a hard time with the idea of painting an entire animal based on the most fragmentary of material.

Toroceratops: Revengeance

WARNING:

The following blog post is about "Toroceratops." I've taken heat for my views on "Toroceratops" in the past, and I expect this time to be no different. You should know that it is EXTREMELY ranty and, at times, kind of pissed off. I am FULLY aware that my frothing guile may have caused me to misunderstand or misconstrue certain things. If that's the case, please correct me in the comments. This rant is not so much an actual, serious rebuttal of "Toroceratops" as much as it is my personal problems with how the discussion is being carried out.

THAT SAID, please continue.

As Quilong said over on his blog, the "Toroceratops" controversy continues, but in an excellent way: each new paper that comes out not only attempts to rebut the previous one, but also provides priceless new information on Triceratops and Torosaurus in the meantime. Perhaps the best part is that this back-and-forth is available (mostly) freely to the public thanks to that bastion of our Shiny Digital Future, PLoS One:

The initial paper, unfortunately published in JVP;
Andrew Farke's redescription of Nedoceratops;
Scannella & Horner's reassertion that Nedoceratops is a transitional Trike;
Longrich & Field's attempt to suss out age based on skull suture fusion.

But here's what bugs me: In both rebuttals to "Toroceratops" (by Farke and now Longrich & Field), Scannella and/or Horner come back and basically say "those variables you tested scientifically in an attempt to create some kind of growth rubrik for Triceratops? That shit's way too variable, yo. Can't be used. Not phylogenetically significant."

"Epioccipitals? Please. We've found Trike skulls with asymmetrical numbers of epioccipitals. There is an insane amount of variation there. Orbital horn core angle? Dude, are you kidding me? Nasal horn size? Well, Nedoceratops might be clear at one end of the spectrum, but let me tell you--there's a spectrum. I've seen it!

"Skull suture fusion? Allow me to break up the party: we've got Triceratops skulls that are from big adults who don't have all their skull bones fused up. And the opposite, too: small Triceratops skulls with fused skull sutures! See what I did there? I blew your effing mind. Tatankaceratops, baby. Think about it."

Now, look: I am fully ready to accept that there are some freaking wierdos out there, but you haven't shown your work. You have given me one transitional morph: Nedoceratops--one of the most controversial ceratopsid skulls in history. It has three names. Out of hundreds of Triceratops skulls, this is the only one you can point to that has parietal fenestrae? And even then, these particular fenestrae are in wierd places that don't match up with Torosaurus really at all. And it's got that irritating squamosal fenestra that just doesn't look healthy.

Please dig through your massive collection of Triceratops skulls (over 100, apparently, in Montana alone!) and pull out another contender. I know you've got one. Don't hold out on us.

Oh, and the epioccipital thing? You'd like to think that the reason Torosaurus has more epioccipitals than Triceratops is due to two factors: epioccipital count is apparently extremely variable in that basement full of Triceratops skulls you have (thus influencing how many the eventual Torosaurus morph would have); maybe--now work with me here--the epioccipitals in Triceratops split, like an amoeba, into two distinct epioccipitals. As evidence, you're point me toward...

The episquamosal of MOR 2975. The point has been worn down, which you folks suggest is because of "splitting." Yes, good. That's the most likely answer. Has epiocciptal splitting been demonstrated in any other ceratopsid? Hell, the fact that you can't find more than one potential example of epioccipital splitting--what with your baseball stadium filled with Triceratops skulls--is just a hair troubling.

In fact, remind me which ceratopsid currently known from a good growth series (like Pachyrhinosaurus, Centrosaurus, or Ajugaceratops) demonstrates such a spectacular morphological change late in life. Now Pachyrhinosaurus, man, he goes through one helluva puberty phase. But it happens surprisingly early, and at a constant rate. And it seems like lil' Ajugaceratops provides a damn good basis for adult Ajugaceratops. And in all three examples (Centrosaurus included), the juveniles have parietal fenestrae!

For Triceratops to transition into Torosaurus requires some pretty heavy special pleading. I'm not comfortable with that. I need more evidence. I need people to show their work. You've got a boatload of Triceratops specimens? Great. Publish some kind of photoessay, either in a print or online journal or, hell, Ye Olde Internet, showing me and everybody else the full goddamn range of Triceratops variability, which as you keep saying, is insane. Prove that there is not a single effing skull variable that cannot be explained away as either age-related, and therefore not phylogenetically informative, or individual variation, which must be staggeringly huge. Midline epioccipital? Nope. Number of epiossifications? Sorry. Horn size/angle? No dice. Basic things like timing of age-related characters? Not gonna happen.

Look, Pachyrhinosaurus lakustai displays some pretty widespread individual variation, too, but at least we know it's Pachyrhinosaurus lakustai! It's not like Einiosaurus has a mid-life crisis and transforms overnight (thus hiding all transitional evidence) into Pachyrhinosaurus. That's not really how this works. Pachyrhinosaurus is diagnosable across the spectrum of individual variation. And hey, Triceratops clearly is, too. The question is whether all that individual variation has anything to do with what Torosaurus looks like.

I mean, it's one thing to say that Triceratops occupies a wide range of individual morphological variation. I can buy that. But it's tough for me when you start saying that Triceratops exhibits an incredibly wide range of individual growth timing variation. I know, I know, you've got that warehouse full of Triceratops specimens. One might be a big individual with little skull suture fusion, and one might be a small individual with lots of skull fusion. So, in theory, ANSP 15192 could just be a Trike that hit its growth spurt way too early, and Tatankaceratops is the Trike equivalent of Benjamin Button. But here's where my problem is: given that Triceratops apparently ages as fast as it goddamn pleases and exhibits more variation than Varanus, how can we adequately test the "Toroceratops" hypothesis?

Is nothing sacred? What kind of rubrik can you use when there is no rubrik? Scannella continues to argue that bone histology and microstructure is the only real way to figure out who's who, but we've already seen that the growth dynamics of Triceratops are apparently not set in stone. All you can really tell is whether Triceratops (or Torosaurus) is still growing or not. Just because your Triceratops is still growing does NOT mean that Torosaurus is the obvious next step. Can we get some postcranial, long-bone histology done? If all the Torosaurus skeletons are older than Triceratops skeletons, then slap my mouth wide open--THAT is good evidence.

Or wait, maybe it's not. After all, Triceratops wasn't keeping a firm growth schedule. ANSP 15192 might just be a Triceratops that started its transition really early, while the Triceratops individuals who appear to be older than ANSP 15192 just decided they liked having short frills. It's hard for me to believe that Triceratops figured out how to avoid the age-related growth dynamics that shackle the rest of us.

You can't sit there and tell me that, out of all the ceratopsids known and studied, and in fact most animals in the world, Triceratops was unique in its growth timing and morphology. Tyrannosaurus rex has a wierd growth curve for a tyrannosaur (or, indeed, any big theropod) but guess what? It's consistent! You can age a tyrannosaur. You apparently can't age a Triceratops. There must be certain morphological characteristics that appear at certain age ranges. Hell, it's been demonstrated for Triceratops by Horner & Goodwin! Are we abandoning that research now? The full range of variability in Triceratops apparently wipes out the morphological characters that define each age class, so are we just fucked?

I need consistency. I cannot abide it when Farke or Longrich & Field come up with testable cranial characteristics and they are basically brushed aside with this "that variable is too variable" comment. Meanwhile, Torosaurus must be Triceratops. Because THAT variable is not up for debate. It's clear as mud.

And what about all the strange variations on Triceratops that have cropped up lately? Tatankaceratops, Ojoceratops, and Eotriceratops? Are they all just somewhere on the incredibly generous bell curve of individual variation on Triceratops? I mean, they probably are! Shit, you could probably find other chasmosaurine genera that fit in that range of variation. Let's get Arrhinoceratops in that line! Aside from the slightly squared-off frill, he doesn't look too horribly different. And there are probably plenty of Triceratops specimens with slightly squared-off frills.

My point is that there needs to be a testable rubrik for "Toroceratops" to work or even not work. You can't just say "Triceratops is really variable, therefore Torosaurus." There have to be established baseline growth trajectories. I will say this: I am NOT opposed to the "Toroceratops" hypothesis. If it's true, it's intruiging and, apparently, unique among ceratopsids. But there is SO much more than needs to be done, and I don't like how the conversation is going. I don't care how much data you have if you're not sharing it with the rest of the class.

That is all. Rant over. I have a PowerPoint to work on.

Random Alvarezsaur

Trying to get back into the drawing habit. This was the result. Maybe it's Linhenykus? It's not clear. He's cute though, right?

All Your Boss Are Belong to Us

If you ask me (you did, didn’t you?), it’s well past the time for a new blog post. Additionally, it’s been far too long since I wrote about my favorite group of dinosaurs: the ceratopsids. This is supposed to be a paleontology-focused blog, after all. You may recall previous series on horns
‘n’ spikes (Part 1, Part 2, Part 3, Part 4), my back-and-forth with Tracy Ford (Part 1, Part 2), and my love-it-or-hate-it critique of the “Toroceratops” hypothesis (Part 1 and Part 2) (I think I’m off Denver Fowler’s Christmas card list). Let’s do something different this time: let’s just talk about the animals. So consider this post to be the beginning of series of posts about my favorite ceratopsids, period. We’ll start with my second-favorite: Pachyrhinosaurus.

The partial, holotype skull of Pachyrhinosaurus canadensis was discovered in 1946 and named four years later by the great Charles Sternberg. He dug up a big ol’ skull that was missing the frill and rostral, and some other bits of skull from different individuals, in Alberta’s Scabby Butte formation. So different was this “thick-nosed lizard” than its horned colleagues (seeing as it lacked a horn) that Sternberg erected a new subfamily, the Pachyrhinosaurinae, to be held to the same taxonomic level as the Centrosaurinae and the Ceratopsinae (Chasmosaurinae). His life restoration, shown below, looks something like a Protoceratops with a drink coaster on its nose. More material was discovered at the nearby Little Bow River Formation, and Wann Langston, Jr. unearthed a second good skull Pachyrhinosaurus from Scabby Butte in 1955.

Langston continued working on the genus through the 1970’s, and determined that it was a member of the well-established Centrosaurinae, or short-frilled ceratopsids. Though to this day, no complete frills exist for the species, the 1955 skull does preserve some proximal aspects of the frill, and an associated bit of the parietal’s outer rim. During the 1970’s, somebody (anybody out there know?) suggested that the characteristic, though bizarre, nasal boss that defined Pachyrhinosaurus actually represented the base of a massive, broken-off nasal horn. The idea actually gained some traction, and in fact a DinoRiders toy of a particularly well-endowed Pachyrhinosaurus was produced and released to an unsuspecting public. However, the hypothesis was quickly overridden by the unfortunate fact that no Pachyrhinosaurus skulls had anything taller than a big, molar-shaped boss (and besides, the recently-described Rubeosaurus ovatus fulfilled the “giant nasal horn” dream quite nicely).

The morphology of the boss became especially clear in 1972, when Alberta school teacher Al Lakusta stumbled across a massive Pachyrhinosaurus bonebed in that province’s Pipestone Creek Formation. Dozens, if not hundreds, of individuals died there most likely due to an unfortunate river crossing. Plenty of skull material and postcranial remains gave paleontologists previously unthinkable clarity into the morphology and, importantly, ontogeny of this hornless horned dinosaur. Decades of work were put into the bonebed and, in 2008, Currie, Langston, Jr., and Tanke published the results: a new species of Pachyrhinosaurus called P. lakustai, named after the bonebed’s discoverer.

Apart from their temporal separation, P. lakustai is distinct from P. canadensis in a number of ways. The shape and structure of the nasal boss, and that boss’ relationship to the postorbital bosses, differs considerably. While P. canadensis has a molar-shaped boss that rises more or less vertically from its base, P. lakustai has a more rounded boss that tips in back and rises up from before ending in a “spout” (in some individuals) that overhangs the rostral comb. The nasal boss is clearly separated from the postorbital bosses, even in old individuals, whereas in P. canadensis, the three bosses come together late in life. As in most horned dinosaurs, however, the most telling differences are in the frill. Aside from the significant differences in P3 morphology, P. lakustai differs from P. canadensis in having a large “unicorn horn” growing from the parietal bar. This does not occur in all individuals and could be a sign of age or sex.

The growth of Pachyrhinosaurus is also bizarre. Juveniles start life with a narrow, though proportionately large, nasal horn, and small postorbital horns. As they grow, the postorbital horns are reabsorbed and the nasal horn’s base elongates (back-to-front). It assumes a pyramidal shape; then the weird stuff starts. In every other ceratopsid with a nasal horn, the horn grows more or less vertically. In Pachyrhinosaurus, that pyramidal horn grows laterally, its base becoming wider and its upper surface becoming rugose and honeycombed. With age, the boss’ structure changes further—in some individuals, it becomes concave as bone continues to be reabsorbed and remodeled. While the overall form of the parietal’s outer rim remained consistent in each example of that bone, a surprising amount of individual variation is present.

While the authors state that almost every bone in the skeleton is represented, they do not provide a description of the post-crania (I can only assume that’s being saved for a later publication). I assume it’s similar to other related centrosaurines for whom relatively complete post-crania are known (Centrosaurus comes to mind). Of course, that old generalization may not be valid—-among the chasmosaurines, Anchiceratops has very different proportions from big bruisers like Triceratops.

Wonderful illustration of our new Alaskan species by Karen Carr.

As it turns out, Pachyrhinosaurus is not restricted to Alberta. No, sir, this ceratopsid lived all the way up on the North Slope of Alaska. A nice, but obliquely crushed, skull was discovered up there and provisionally described for Fiorillo in 2010’s big ceratopsid volume published by Indiana University Press. There’s actually a lot more unprepared material, just sitting in field jackets, and I can’t wait for that stuff to be described. Anyway, after extensive preparation, the holotype skull was written up and awaits publication in Acta Palaeontologica Polonica (where you can read the in-press draft). The authors of that paper have given this species a distinct name: Pachyrhinosaurus perotorum. It is the northern-most ceratopsid in the world, and also the youngest occurrence of the genus.

It differs from its more southerly cousins in not too many ways. In fact, it looks kind of like P. lakustai, but, again, it's the structure of the parietal that makes this guy different. The two P3 spikes are there, but this species expresses P1 spikes that grow down and overhang the parietal fenestrae, similar to the situation in Centrosaurus apertus and Centrosaurus brinkmani. This big guy would have lived in some cold temperatures in the winter--it probably got down below freezing for a few months a year, and believe it or not, the North Slope of Alaska was further north than it is today back in the Late Cretaceous, so that means even longer periods of darkness.

Exactly how Pachyrhinosaurus and its neighboring frozen north dinosaurs survived in these extreme conditions is unknown, but it certainly speaks to their hardy nature. Living alongside our boy here was a large form of Troodon formosus, good old Dromaeosaurus, Edmontosaurus, and one or possibly two tyrannosaurs--one of which was Albertasaurus.

UPDATE: I actually wrote this post a week ago, but only now found the motivation to post it. I may actually add to it later, including references. So look for that!

Velafrons coahuilensis & IV Status

My wife didn't make a color copy of this, but here it is! I draw this for a friend. It was between Velafrons and Coahuiluaceratops and the decision was made by asking her whether she liked Duckie or Sara more in "Land Before Time." I'm also starting a new paleo-artist internet meme:

"No Greg Paul skeletals were referenced for the production of this illustration."

Also, I had the I.V. taken out today. Only two weeks! I'm back to full power. I'll post a picture of my mangled arm later. It's actually not mangled at all. There's a large scab over the hole in my arm and that's about it. Whatever. Velafrons!

Ooh! Piece o' Arty

Here's an unusually short-bodied ornithomimosaur that I drew last night while Scott, Raven, and I all lambasted a certain egomaniacal paleoartist and had a good laugh while doing so.

Sketches & Scribbles

Scott sat me down and demanded that I draw something last night, so I did. And I came up with some pretty nice stuff. Above is a cute lil' diplodocid. It might not be the most accurate dinosaur, but that wasn't the point. I was just trying to give the animal some character.


And here's a cute little stegosaur, with massive parascapular spines. Maybe it's Gigantspinosaurus (awful name). I really like this drawing, especially the legs. Notice that the hands are like a sauropod--a new paper by Phil Senter suggests that stegosaurs and ankylosaurs had tubular feets.


And here's another version of Cthulhu. I'll get there eventually. I gave it too many appendages here.

I'm gonna keep up with this cartoony dinosaur thing. I like 'em.

First "New" Ceratopsid of 2011

The journal Cretaceous Research published an "accepted manuscript" by Nick Longrich in which he suggests that the McLargeHuge Pentaceratops specimen shown above (OMNH 10165) actually belongs to a separate genus, Titanoceratops, and that said genus is the earliest reprentative of the Triceratopsini, an end-Cretaceous group of giant chasmosaurine ceratopsids comprised of Eotriceratops and "Toroceratops."* Among other issues, Bill Parker has voiced his concern over the practice of journals posting unpublished manuscripts on their websites, a sentiment I share.

I'm going to hold off on my critique until Titanoceratops is actually published, but I'm a little iffy about the lines of evidence brought up in the manuscript. I had a few of the same problems with Mojoceratops, but I certainly wouldn't mind if the Austin Powers dinosaur remained viable, if only for it's awesome name. One positive is that the original skull of OMNH 10165 is lacking most of the frill--the reconstruction above is largely extrapolated. So, you know...there's that.

Godspeed, Titanoceratops. It looks like 2011 may be shaping up to be another good year for horned dinosaurs, valid or otherwise.

*Still waiting to see how this turns out. So sue me.

People "Don't Get" Pachyrhinosaurus

Not long ago, I was visiting the Alaska Museum of Natural History for reasons that are forgotten to me now. I try not to go over there, in fact. It's an active aversion--every time I get the urge to see "how they're doing," I just remember that my name, Scott's name, and Raven's name are still not anywhere near the big Tyrannosaurus rex skull cast that we restored over a year's time and put our backs into and got NO support and there's NO acknowledgement of our contribution to that particular project. In fact, I doubt our names are anywhere IN the museum, even though all three of us (but mostly Scott and I) have been heavily invested in exhibit prep, tours, etc. in the past.

Makes me mad.

Anyway, the museum's "director," Katch Batchelor, told me that they were getting a Diabloceratops skull cast (this was before that genus was published). She was very excited. I asked why she's wasting the museum's money on a taxon from Utah that lived millions and millions of years before Alaska's Pachyrhinosaurus. In fact, why isn't she getting a Pachyrhinosaurus skull cast? The terrible Fairbanks museum has one. If they can get one, it can't be all that difficult to get ahold of.

Her answer was "People don't 'get' Pachyrhinosaurus." I took this to mean that SHE did not "get" Pachyrhinosaurus. Here's my question: What's to GET?"


There's very little to misunderstand here. Pachyrhinosaurus is a derived member of the Centrosaurinae, which itself is one of the two branches of the Ceratopsidae. The other branch is called the Chasmosaurinae, and it features such well-known taxa as Triceratops, Chasmosaurus (go figure), and Kosmoceratops. The Centrosaurinae is just as diverse, and from there you get good old Centrosaurus, Rubeosaurus, and Diabloceratops. At the upper end of the Centrosaurinae is a monophyletic group of ceratopsids with big nasofrontal bosses instead of horns. They're called pachyrhinosaurines. The group includes two genera comprising three species (Achelousaurus horneri, Pachyrhinosaurus canadensis, and Pachyrhinosaurus lakustai), and several specimens that may represent more distinct species. The group extends from Montana up the west coast of the United States going all the way up to...you guessed it...Alaska's North Slope.

The holotype pachyrhinosaur skull from the Prince Creek Formation on the Colville River is not in great shape, but clearly represents a pachyrhinosaurine.

Right now, the Alaska Museum of Natural History looks more like an odds-and-ends collection of stuff, completely lacking any sort of context or reason for being. When you have a monitor lizard skeleton with an Ornithocheirus skull and a Basilosaurus skull just sitting on the same shelf, but without any sort of text...and in fact, you have a beluga whale skeleton just around the corner, WHAT DOES IT MEAN? And why aren't there any Alaskan dinosaurs? I could go on and on about my misgivings with the AKMNH, but it's useless. They don't even want to use the world "evolution" in fear of scaring people away.

Jesus.

Meet the Ceratopsian Class of 2010

As I've said before, 2010 has really been the Year of the Ceratopsian. Here are the new guys, in no particular order. Give 'em a round of applause--they've basically doubled the group's diversity, and they're JUST SO COOL.

Psittacosaurus gobiensis ("Yet another Psittacosaurus)
Claim to Fame: Supposedly shows evidence for how the jaw worked in the genus
Favorite Movie: Multiplicity
Image Credit: The Internet


Archaeoceratops yungjingziensis ("Bits and pieces of a horned face")
Claim to Fame: Hey, it's another species of Archaeoceratops!
Favorite Movie: Disney's Dinosaur!
Image Credit: I have no idea via the Internet


Ajkaceratops ("Part of a horned face")
Claim to Fame: Europe's first ceratopsian.
Favorite Movie: National Lampoon's European Vacation
Image credit: The paper


Sinoceratops ("Fung-Fu horned face")
Claim to Fame: China's first ceratopsid...much less centrosaurine ceratopsid!
Favorite Movie: Big Trouble in Little China
Image Credit: Olorotitan on DeviantArt



Tatankaceratops ("Like a corgi version of Triceratops")
Claim to Fame: Adorable miniature version of Triceratops
Favorite Movie: Twins
Image Credit: The paper


Diabloceratops ("Fiddle-playing horned face")
Claim to Fame: Very basal centrosaurine, narrow frill, odd nasal decorations
Favorite Movie: End of Days
Image Credit: Olorotitan on DeviantArt



Ojoceratops ("Triceratops 8 - THE OJO")
Claim to Fame: Like Triceratops with a rounded (instead of squared-off) squamosal
Favorite Movie: Bowfinger
Image Credit: The Internet


Medusaceratops ("Snake-haired horned face")
Caim to Fame: Basically a chasmosaurine version of Albertaceratops
Favorite Movie: Clash of the Titans
Image Credit: D. Sloan via the paper



Rubeosaurus ("Country bumpkin lizard")
Claim to Fame: Formerly known as Styracosaurus ovatus, has a ridiculously large nasal horn
Favorite Movie: Anything with Adrian Brody
Image Credit: Lukas Panzarin



Coahuilaceratops ("Spring Break horned face")
Claim to Fame: Mexico's first ceratopian; most difficult name to pronounce in all the Ceratopsidae; ridiculously long brow horns
Favorite Movie: Traffic
Image Credit: Lukas Panzarin



Kosmoceratops ("Show-off horned face")
Claim to Fame: Most complicated combination of spikes and hors of any known ceratopsid; brow horns point laterally and arc down along their length
Favorite Movie: Hellraiser
Image Credit: Lukas Panzarin



Utahceratops ("Creatively bankrupt horned face")
Claim to Fame: Small brow horns point laterally, nasal horn sits in front of nasal opening
Favorite Movie: Raising Arizona
Image Credit: Lukas Panzarin



Mojoceratops ("Groovy horned face")
Claim to Fame: Formerly Chasmosaurus kaiseni, named in a pub
Favorite Movie: Austin Powers 2: The Spy Who Shagged Me
Image Credit: Nick Longrich via the Internet



Vagaceratops ("Took a year off before college horned face")
Claim to Fame: Formerly Chasmosaurus irvinensis, lacks brow horns entirely, epiparietals fold down over parietal fenestrae
Favorite Movie: Road Trip
Image Credit: The Internet

Say Hello to the New Guys

The Year of Ceratopsians continues with two new additions to the Chasmosaurinae: Utahceratops gettyi and Kosmoceratops richardsoni. I will give you two guesses as to where the former was discovered. Here's one hint: It's in the name. There was absolutely nothing about this animal that warranted a better, more imaginative moniker. Meanwhile, Kosmoceratops is heavily airbrushed, full of fashion advice, and knows the top ten ways to make your bull go wild in the bedroom. Another surprise is that Chasmosaurus irvinensis is renamed Vagaceratops irvinensis, as it turns out to be closer to Kosmoceratops than Chasmosaurus.

But seriously, this is fantastically exciting. The paper was published in PLoS One and is thus freely available for all. Check out the paper by clicking this lengthy link, but don't be surprised if you can't see the pictures or download the paper as a PDF. It would seem that PLoS One went on hiatus the second this paper was announced, thus going from "awesome free-access journal" to "cocktease."

But don't cry about it--go to The Internets, where people have been blogging about these new dinosaurs all morning. Do you want to know more? Click these links to do just that!

Archosaur Musings
The Open Source Paleontologist
Dinosaur Tracking
Dinochick Blogs

Don't get too excited: they're probably all just ontogenetic stages of each other.

Last Thing on Toroceratops

Okay, look, it's clear that I'm not the BEST guy to go to when it comes to this subject. I don't have the same access to primary literature that many of you do, I certainly haven't seen many actual specimens of Triceratops OR Torosaurus, my knowledge of stratigraphy is poor, and bone histeology is still very new to me.

I'm not trying to overturn anybody or make a serious case against Scannella and/or Horner. I'm not trying to bogart anybody's future or current research. I'm sorry if I've hit any nerves or didn't read the right papers...well enough.

That said, on morphological grounds alone, I think that known specimens of Torosaurus differ significantly from known specimens of Triceratops, either T. prorsus or T. horridus (although T. horridus looks a bit more like Torosaurus). Several things lack decent explaination. Among them:

Why don't any specimens of Torosaurs have "adult" nasal horn morphology of Triceratops? What other ceratopsid goes through such a radical late-stage morphological change?
How do we know that a solid frill is a juvenile trait, and not a derived trait?
Why aren't there any (AFAIK) Triceratops skulls with small fenestrae besides Nedoceratops, which itself is a contentious specimen?

I liked Haikaru's example in the comments for the 2nd Toroceratops post: if you found a humpback whale and a blue whale as fossils, you might assume that the latter is the adult of the former. Again, I'm not saying that Scannella and/or Horner are WRONG, I'm saying that their conclusion is not completely solid based on that paper alone. And you know what? Few papers are. Further research into post-cranial anatomy, histeology, stratigraphy, and morphology will doubtlessly illuminate this topic further, and I welcome that research.

I didn't mean to step on any toes or insult anybody. My posts were meant to suggest that further research was needed, and that alternatives to Toroceratops are possible.

And that's the last I'll say on the subject...at least until more data is published.

Torosaurus latus is not Triceratops sp.

Earlier this year, Scannella & Horner hypothesized that skulls attributed to the large chamosaurine Torosaurus latus represented a “fully adult” ontogenetic stage of Triceratops, thus invalidating Marsh’s pierced lizard as a distinct taxon. This theory was based in part on a growth series for Triceratops published by Horner & Goodwin in 2006. In neither paper do the authors consider Triceratops at the species level. Prior to 1996, as many as sixteen separate species of Triceratops had been considered valid, but in that year Catherine Forster published a revision of the genus which greatly reduced that number to just two: Triceratops horridus and Triceratops prorsus. Actually, in 1986, Ostrom & Wellnhofer went as far as suggesting that only Triceratops horridus is the only valid species. Neither Scannella & Horner nor Horner & Goodwin address the issue of species-level ontogenetic change in Triceratops. Is the Horner & Goodwin growth series for T. horridus or T. prorsus? Is Torosaurus latus the “fully adult” form of T. horridus, T. prorsus, or both? While not stated outright, the Horner & Goodwin growth series implies that features used by Forster to distinguish T. prorsus from T. horridus may simply be ontogenetic markers, including size and shape of epioccipitals, size and shape of the nasal horn, and curvature of the postorbital horns. Certain abstracts that I’ve read but don’t know if I can actually divulge have suggested stratigraphic or anagenetic reasons for the changes between Triceratops species.

In Scannella & Horner’s “Toroceratops” hypothesis, the authors suggest that, very late in life, Triceratops goes through radical morphological change: the length and shape of the frill increases, and the parietal becomes rapidly fenestrated as bone is reabsorbed on either side of the parietal bar. The authors point to a number of Triceratops skulls with extremely thin areas of the parietal where fenestrae would be expected to appear. However, none of the specimens investigated by the authors have even incipient parietal perforations (except Nedoceratops, but we’ll get into that in a minute). All investigated Triceratops skulls retain a solid frill. That is, the authors cannot point to any Triceratops skulls that would form a transition between the "no fenestrae" condition and the "big fenestrae" condition of Torosaurus. With the often-stated abundance of Triceratops material, I believe this is an important consideration.

According to the Horner & Goodwin growth series for Triceratops, specimen MOR 004 is regarded as an “adult” form. It has thick, forwardly-curved postorbital horns; elongate, highly reabsorbed epiparietals; a large, thick, forwardly-directed nasal horn; and a relatively short, deep rostrum below and anterior to the nasal horn. If MOR 004 is the model adult form of Triceratops, then no currently recognized specimen of Torosaurus latus conforms to the Horner & Goodwin growth series. All currently recognized specimens of Torosaurus latus (ANSP 15192, MOR 1122, YPM 1830, and YPM 1831) have short, upwardly-directed nasal horns with an elongate, shallow rostrum anterior to the nasal horn. Additionally, the snout has a distinctive “stepped-up” morphology wherein the dorsal margin of the rostrum anterior to the nasal horn is lower than the dorsal margin of the rostrum behind the nasal horn. This feature is similar to the “large juvenile” and “subadult” examples of Triceratops in Horner & Goodwin’s growth series. However, even those Triceratops specimens lack the elongate, shallow rostrum anterior to the nasal horn.

Diagonally from top to bottom, the proposed Horner & Goodwin growth series for Triceratops showing overall trends in morphology. To the lower left, Torosaurus latus.

The overall shape of the frill is quite distinct in both taxa. In MOR 004, the frill more or less frames the rest of the skull in anterior view: the parietal bar forms the apex of the frill, and the left and right halves of the frill slope down and out from there. This is especially obvious in two other specimens of Triceratops: YPM 1822 (Forster 1996) and YUM 1822 (Hatcher 1903). In lateral view, the frill attains a distinctive upward curve. Additionally, the squamosals of Triceratops are D-shaped, or perhaps axeblade-shaped. In many individuals, the squamosal-parietal suture is not visible for most of its posterior length. In contrast, Torosaurus’ frill is broad and largely flat, and does not frame the face in anterior view. Rather, it grows away from the rest of the skull and does not retain a distinct upward curve. Additionally, the squamosals in all recognized specimens of Torosaurus have a distinct shape reminiscent of a chef’s onion or paring knife, and the suture between the parietal and the squamosals is surprisingly clear along its entire length.



The largely complete frill of Torosaurus latus, after Hatcher 1903.

According to Scannella & Horner, the underlying geometry of the frill of MOR 004 would have been significantly altered—and quickly—to produce a frill attributable to Torosaurus latus. This sort of morphological change is wholly unknown in other ceratopsians where juveniles are known. In centrosaurines, for instance, the adult frill is, by and large, simply a larger version of the juvenile skull with the addition of unique spikes on the parietal margin or parietal bar (Pachyrhinosaurus lakusai and Centrosaurus apertus is particularly illustrative of this). In addition, the morphology of the snout and nasal horn would essentially reverse from the adult condition to a subadult form. These sorts of radical morphological changes have no basis in close relatives of “Toroceratops.”

Because all of the known specimens of Torosaurus latus share certain morphological features regardless of size or, presumably, age, we are forced to conclude that they are taxonomically valid characters that differentiate it from its closest relatives. For reference, its closest relatives have been consistently shown to be Triceratops and Nedoceratops. Whereas skulls attributed to Triceratops are far more numerous and individualized, those recognized as Torosaurus are much more uniform (ANSP 15192 does differ significantly in the length of the frill and size of the postorbital horns, but this may be an age-related character). While there is obviously a sampling bias at work here, it is instructive to list characters shared by the best-known specimens of Torosaurus latus—ANSP 15192, MOR 1122, YPM 1830, and YPM 1831:

1) Relatively small, upwardly-directed nasal horn that is triangular and develops about halfway up the snout. The horn never grows into the impressive forward-pointing thick horn you see in many Triceratops skulls.

2) An elongate, shallow rostrum anterior to the nasal horn. There is a clear height differentiation between this region and the dorsal margin of the skull behind the nasal horn in the adult stage. This may be termed a “stepped-up” condition.

3) An elongate nasal passage retained into the adult stage. In Torosaurus, the extent of the nasal system resembles the juvenile condition in Triceratops. In that taxon, however, the nasal passage compresses and rounds out as the subadult and adult stages are reached. Torosaurus, by contrast, retains an elongate nasal passage.

4) Blade-like squamosals which remain distinct from the parietal even in the adult stage.

5) An elongate frill that is “swept back” rather than “swept up,” is relatively flat and broad, and contains large parietal fenestrae. In adults, the margins of the frill may be almost completely smooth due to the absorption of the epioccipitals.

Before we go too much farther into Torosaurus, let’s turn our attention to Triceratops. In fact, let’s go back to Forster’s paper regarding species diversity in that genus. She references two individual specimens as models for the two species of Triceratops: YPM 1822 for T. prorsus and SDSM 2760 for T. horridus. I note that the latter displays several features in common with Torosaurus, including the structure of the snout and nasal horn (though not to the same extent), and possibly the structure of the squamosals. It also seems to lack distinct epioccipitals, and the frill is broader than its sister species, T. prorsus. That species, represented by YPM 1822, shows very different features: the nasal horn is large and directed forward, the rostrum in front of the nasal horn is short and deep, and the nasal area is rounded. The epoccipitals are fairly large and triangular.

Neither skull displays a singular suite of ontologic characters consistent with any one growth stage according to Horner & Goodwin. In SDSM 2760, the morphology of the snout and nasal horn are juvenile characters according to the suggested growth series, whereas the thickness and orientation of the brow horns, as well as the loss of epioccipitals, are indicative of adult status. By contrast, YPM 1822 shows a clearly adult snout and nasal horn, but the ends of the brow horns point a bit upwards (a subadult trait) and the frill retains distinct, fairly large epioccipitals (a juvenile or subadult trait). Are these isolated incidents? I’m afraid not. Just ask John Hatcher. In 1903, he included several beautiful illustrations of skulls attributed to Triceratops in his wonderful monograph on the horned dinosaurs. Exactly none of them conform to the Horner & Goodwin growth series to a "T." Going forward, where I reference "juvenile," "subadult," and "adult" features, I'm talking about those ontogenetic stages as defined in Horner & Goodwin 2006.




This is USNM 2100, identified by Hatcher as Triceratops prorsus(?). Although the anterior portion of the snout is missing, one may notice the underdeveloped nasal horn (juvenile-subadult), low-angled brow horns (adult) with sloped tips (subadult), and small but distinctive epioccipitals (subadult). The parietal bar is interesting in that it’s quite bumpy. Where would USNM 2100 fit on Horner & Goodwin’s growth series?




This is Triceratops brevicornus, ”YPM” 1834. It also shows a curious mix of characters: the nasal horn is distinct and forwardly-directed (adult), but the anterior portion of the snout is quite long (subadult). The brow horns are directed forward (adult) and are unusually short. The parietal-squamosal suture appears to be lost (adult), but the epoccipitals are reasonably distinct and not entirely rounded (subadult). Where would YPM 1834 fit on Horner & Goodwin’s growth series?




This is Triceratops elatus, USNM 1201. It also displays some curious features. The nasal horn’s growth seems to have stalled: the underlying nasal is indeed reaching forward (subadult), but the epinasal is still apparent (juvenile). The brow horns are large and directed forward (adult). The frill’s margins are bumpy—the epoccipitals have largely been absorbed (adult). Notice the distinct upward bend to the squamosals. While it gives the bones a superficially onion-knife appearance, the parietal curves distinctly upward rather than being directed back. Where would USNM 1201 fit on Horner & Goodwin’s growth curve?




Then there’s this famously odd duck: USNM 2412, Nedoceratops hatcheri. The nasal horn isn’t really a horn so much as a bump in the road, giving the snout a very pronounced “stepped-up” profile. The brow horns are quite large and directed almost straight upward. The frill is riddled with accessory fenestrae, some of which are probably the result of pathologic or natural re-absorption. The skull retains distinct epioccipitals, and the squamosal has a bizarre shape. Nedoceratops may be too much of a wildcard to include in this analysis, but Scannella & Horner consider it to be a transitional form that would exist between MOR 004 and Torosaurus latus. But what happened to the nasal and brow horns? Where did all these accessory fenestrae come from? Surely, Nedoceratops is either an incredibly abarrant individual or a distinct taxon, and will not be considered further here.




This is where things start getting interesting. This is Triceratops calicornis, USNM 4928, and it displays a lot of features that I listed for Torosaurus latus, above. In other words, its morphology conforms nicely to known morphologies that are consistent across currently-recognized specimens of Torosaurus. These include: a small, upwardly-directed nasal horn that is roughly halfway down the snout; a stepped-up snout profile with an elongate anterior portion; an elongate nasal passage; and squamosals that are onion-knife shaped. Most of the epoccipitals are completely re-absorbed. There is one distinct epoccipital capping the parietal-squamosal contact, and half of one above it. The parietal was not preserved in USNM 4928—it may have been fenestrated.



AMNH 5116 (top) compared to YPM 1830 (bottom). The overall similarities are striking.

Based on these proposed characters, I could make an argument that the most famous, well-known specimen of Triceratops in the world—AMNH 5116—is actually a Torosaurus. It has a short, upwardly-directed nasal horn, an elongate rostrum anterior to the nasal horn, an elongate nasal passage, onion-knife shaped squamosals, and a lack of epioccipitals. Like USNM 4928, it was also missing portions of its parietal, although major portions were apparently recovered and plastered back on. The beast was restored with a solid frill (maybe it had one), the assumption being that this is Triceratops. However, when you take the parietal out of the equation, AMNH 5116 is strikingly similar to Torosaurus skulls, particularly YPM 1830. Sorry about the lack of fenestrae in the illustration for YPM 1830—I just now noticed that they’re not there. *facepalm*
MNHN 1912.20, housed in Paris, is strikingly similar to USNM 4928, although its “Torosaurus” features are even more obvious: the snout has a more pronounced “step-up,” the nasal horn is small and retains the epinasal. The anterior portion of the snout is elongate, as is the nasal passage. The frill is elongate and broad. Epoccipitals are nearly absent, and the squamosals are onion-knife shaped. Like AMNH 5116, the frill of MNHN 1912.20 has been heavily restored, although exactly how much of the parietal was “touched up” is not specified in this skull’s description (Goussard, 2006). It may very well be that MNHN 1912.20 is a Torosaurus skull.



AMNH 5116 (top) compared to MNHN 1912.20. Again, note the many similarities.

Go back and look at the growth series picture at the top of the post. Diagonally from top to bottom, this is Horner & Goodwin’s proposal for ontogenetic change in Triceratops from small juvenile (MOR 1199) to adult (MOR 004). If Torosaurus latus were to follow MOR 004, the underlying geometry of the frill would have to change radically, the snout would significantly elongate, and the nasal horn would regress to an earlier developmental stage. Additionally, two giant holes would suddenly open up in the parietal! Scannella & Horner expect us to believe that all of these large-scale changes would happen in the dinosaur’s final years, and in stark contrast to the direction of growth that Triceratops had been experiencing up to that point. No other ceratopsian goes through this sort of late-stage transformation, and there is no reason to think that Triceratops is any different. Additionally, as pointed out by many readers, the rarity of “fully adult” individuals of Triceratops (Torosaurus) compared to the incredible abundance of earlier growth stages is a bizarre and unrealistic preservation bias that does not occur in other fossil animals. If anything, the opposite tends to be true: juvenile and subadults are rare while adults and “full adults” are more common. That is certainly the case in other ceratopsids, even among bonebeds. The bottom line is this: assuming the Horner & Goodwin growth series is generally accurate, YPM 1830 and MOR 1122 would NOT follow MOR 004. It really is that simple.

However, Torosaurus material might not be nearly as rare as everyone seems to think. I believe that the genus can be distinguished based on more than just the presence or absence of parietal fenestrae. It’s possible that specimens once referred to as Triceratops are, in fact, Torosaurus. If nothing else, I hope this post has convinced some of you out there in Readerland that the Scannella & Horner paper presents interesting ideas but serious flaws as well, and needs to be given a second look.

I offer an enormous, Pentaceratops-skull-sized thanks to Andrew Farke for taking the time to double-check my claims and correct others. Readers, notice that I don’t really talk about histeology or stratigraphy. This is largely because I am ill-informed to do so intelligently. I do think that Torosaurus is morphologically distinct from Triceratops based on skull anatomy alone, however, so those factors may not necessarily play a part. On the point of stratigraphy, however, I do wonder whether T. prorsus or T. horridus is older, and whether the similarities between the latter and Torosaurus may represent a close relationship. That is, perhaps T. horridus is closer to the common ancestor of Triceratops and Torosaurus, and that Triceratops prorsus is derived, anagenetically or otherwise, from T. horridus?

Toroceratops Part 2 Teaser

I'll be revisiting the Toroceratops theory again soon, because I'm a glutton for punishment. I believe I've found a good way to distinguish Triceratops from Torosaurus without relying on parietal fenestrae, although frill morphology does come into play. I'm just making the text sound good, and make sure my argument is sound before I post the whole thing (it's a whopper). Anyway, here's an illustration of AMNH 1201, simplified from Hatcher (1903). It displays a unique combination of juvenile and adult traits, as do many specimens of Triceratops.

Stay tuned...

Toroceratops

We’ve been hearing rumbling of this paper for awhile now, but it became big news at SVP last year (in Bristol, England, a place I’ll never go back to): Jack Horner and John Scannella proposed a very radical taxonomic synonymy: Torosaurus, that famous long-frilled chasmosaurine, is actually just a very old Triceratops. This isn’t the first time Horner & Co. have applied ontogenetic lumping. Just last year, Horner & Goodwin suggested that Dracorex and Stygimoloch represent teenage and subadult stages, respectively, of Pachycephalosaurus. The paper has been met with mixed support, but this new effort has really split the community. This all seems to be an effort by Horner to show that the Maastrichtian period of the Cretaceous was a barren wasteland, taxonomically speaking, for the dinosaurs, and that diversity was in the crapper well before that giant meteor wiped the last few stragglers out.

And there’s nothing wrong with that. If you really can convincingly show that only one pachycephalosaur, one ceratopsid, one duckbill, and one tyrannosaur existed at the close of the Cretaceous, then yeah, the future looks grim for the Hell Creek dinosaur community. But that can be a tough pill to swallow. Any beefs I have with the Pachycephalosaurus paper have been addressed in an earlier post, so I won’t go too far into detail here. Mainly, I think the authors did a great job is showing that Dracorex is a teenager, Stygimoloch is a subadult, and Pachycephlosaurus is an old man. Those facts alone do not suggest synonymy. A larger sample size, more complete material, and better knowledge of the ontogenetic development of other pachycephalosaurs is necessary to make any kind of truly informed decision about the growth and development of a single poorly-represented genus.

Anyway, let’s talk about Triceratops and Torosaurus. The former is an extremely well-known, well-represented chasmosaurine ceratopsid from all over North America at the close of the Cretaceous. Torosaurus is known from the same deposits, but is extremely rare. Two species are tentatively known: T. latus and T. utahensis. The latter species was originally regarded as a species of Arrhinocertops by Gilmore, and, in fact, Horner & Scannella suggest that original classification may well bear out.

The authors based most of their claim on a paper from a few years ago by Horner & Goodwin which proposes an ontogenetic series for Triceratops. Key features of this growth series are the changing orientation of the brow horns and the eventual blunting and reabsorption of the epiparietals. The Horner & Goodwin paper details five distinct growth stages, though I do wonder about individual variation. Do all teenage Triceratops have slightly recurved brow horns? Would the brow horns really change orientation from basically straight as a baby to recurved as a teenager to forward-pointing as an adult? It just seems like a stretch. I’m all for horn growth, but I also know, from studies on centrosaurine ontogeny and bone beds, that ceratopsian exhibit a large degree of individual variation. Horner & Goodwin even acknowledge this when discussing Triceratops’ epiparietals, but we’ll get to that in a minute.

The point is that Torosaurus represents a sixth distinct growth stage, wherein the frill changes shape considerably—from wave-shaped in cross section to basically billboard-shaped and elongate, with squared-off upper corners instead of a nice round shape. Additionally, two large fenestrae appear as parietal bone is reabsorbed. Only as a full-grown adult does Triceratops come to resemble most of its chasmosaurine relatives. Apparently this all happens very quickly during the animal’s life, and there are no “in-between” specimens, although Horner & Scannella suggest that the oft-neglected Nedoceratops (Diceratops), with its incipient parietal fenestrae, may represent just such a “transitional” form.

The authors are quick to point out that the structure of the frill is the only skeletal feature that separates Triceratops from Torosaurus, but even this isn’t actually evidence for synonymy. If that’s your only justification for lumping two ceratopsids together, you may as well toss the entire Chasmosaurinae (except maybe Chasmosaurus) under one genus. It’s been repeatedly demonstrated that the parietal bone is the single most important bone in the ceratopsid body for providing taxonomic identification. Arrhinoceratops, Anchiceratops, Torosaurus, Mojoceratops, and Ajugaceratops all look pretty much the same beyond the parietal, although that last genus does have distinct brow horns. If anything, history has shown the safe bet—when it comes to horned dinosaurs—has been to split rather than lump.

Scott Sampson agonized over the decision to place Einiosaurus and Achelousaurus in different genera, since their parietals are so similar, or take Einiosaurus out of Styracosaurus in the first place, which is where it had informally sat for so long. Just recently, another “species” of Styracosaurus has been renamed as Rubeosaurus ovatus. A critical part of each of these decisions has been that the structure of the parietal bone. In Einosaurus and Achelousaurus, Scott eventually came to conclusion that, based on the fact that the former had a big hooked nasal horn while the latter had a roughened pachyrhinosaurine boss was enough to separate them at the generic level, although he briefly toyed with the idea that they merely represented sexual dimorphs of each other! This is how much the parietal bone means in ceratopsid taxonomy!

So, stating heroically that the structure of the frill is the only thing separating Triceratops from Torosaurus, as if that means something, is superfluous and misleading because you could say the same about any two other chasmosaurines or any two centrosaurines. It’s a good thing those two subfamilies differ in more than just parietal anatomy or we’d all be screwed.

Horner & Scannella also point out that Triceratops skulls are ridiculously common, and in fact so many are known that a convincing growth series has been established for the genus. By contrast, Torosaurus is amazingly rare, but it is known from the same time and places that one finds Triceratops. They take this to mean that Torosaurus must represent a very aged Triceratops, but that living such a long and fruitful life must have been a miracle given the rarity of that genus vs. the dozens of Triceratops specimens. I have problems with this for two reasons. First, just because an animal is uncommon doesn’t mean it doesn’t actually exist (taxonomically speaking). Nobody is accusing Bagaceratops or Udanoceratops to be a juvenile or full adult of Protoceratops, even though they’re ridiculously rare by comparison. I’m actually surprised Horner hasn’t tried synonymizing the two species of Protoceratops. Anyway, it’s entirely possible that Torosaurus preferred different environments than Triceratops, which would make sense if they’re going to avoid direct competition, and that environment might not be prone to fossilization. I’ve heard it argued that ceratopsids have such a great fossil record because they liked near-shore environments that increase their chances of being fossilized. Surely they didn’t all live the exact same lifestyle, and it’s possible that Torosaurus habitually lived in a different area than Triceratops. It’s also not fair to say that “no juveniles of this genus exist, therefore this genus must not exist.” I can name dozens of dinosaurs that aren’t known from juvenile forms, yet their validity is not doubted!

The second reason is that I doubt Triceratops would put a ton of effort into suddenly expanding its frill when death occurred so close to that period. As the authors acknowledge, Torosaurus skulls are very rare, yet juvenile, subadult, and adult Triceratops skulls are unbelievably common. Why would “old adult” Triceratops (Torosaurus) be so poorly represented? Horner & Scannella suggest that the mortality rate was higher among non-Torosaurus-stage Triceratops, but I have a hard time believing that. Such a strange survival separation isn’t seen in other ceratopsids—why would it be any different for Triceratops? That is to say, other ceratopsids have pretty equal survival rates, no matter what their growth stage. Triceratops doesn’t?


While the authors focus on Torosaurus latus, one may wonder about how they rectify Torosaurus utahensis. They question how many specimens of T. utahensis are diagnosable to the genus level, and even if it is valid, they throw up their hands and say that it’s a southern species of Triceratops, a different genus, or Arrhinoceratops—which it was originally referred to. Time will tell, I suppose, although Hunt & Lehman (2008) have stated that T. latus and T. utahensis are nearly indistinguishable, and in fact can only be discriminated based on the structure of the squamosal/parietal suture. Is Triceratops also present in Utah? I’m not sure, myself, but it might say something about Toroceratops.

Let’s talk about ontogeny. Horner & Scannella posit that Triceratops retains a paedomorphic condition of a solid frill well into adulthood, but what is the basis of determining that a solid frill is a juvenile trait? In centrosaurines, even the youngest animals have small parietal fenestrae. What about chasmosaurines? Are juvenile and subadult animals known chasmosaurines? Certainly, that Ajugaceratops bonebed can shed some light on this, but I haven’t read much on it. Apart from that concern, Horner & Scannella make a good case for the possible development of fenestrae in Triceratops over time. They discuss several specimens that have thin or sunken areas of the parietal where the bone is uniquely textured. I think this is good positive evidence that actually does make a correlation between Triceratops and Torosaurus. An alternate explanation is that they are actually describing juveniles or subadults of Torosaurus! Despite pointing out a lot of Triceratops skulls that have thinning areas of the parietal, the authors do not or cannot show a “transitional” frill, where the parietal fenestrae is present, but very small, and surrounded by thinning bone and a rim. If the bone was reabsorbed (as it must have been), it would have been reabsorbed from the inside out, so skulls should exist that have circular thinning sections with a small perforation at their center.

Next, the authors discuss squamosal elongation in Triceratops. A series of squamosal bones are shown: almost all of them are recognizably Triceratops: L-shaped. Figure I is interpreted as being from Triceratops, but it looks very similar to figures J and K, which are labeled Torosaurus. The authors are trying to show a purported growth series, but to my eye, the Triceratops squamosals are very obviously different from the Torosaurus squamosals. Worse, a line graph showing squamosal elongation is shown (Figure 4). Triceratops subadults and “young adults” cluster around the center, as expected, but the Torosaurus individuals fall way to the right of everything else. There are no Triceratops or Torosaurus specimens straddling the gap between the two genera. It’s very obvious that Torosaurus squamosals are very different than Triceratops. Plus, one Torosaurus squamosal (ANSP 15192) sits above the cluster of Triceratops squamosals. It’s recognizably Torosaurus but is similar in length to subadult and “young adult” Triceratops? To my mind, that means Torosaurus is fundamentally different than Triceratops at a similar age.

The authors also discuss epiparietal and episquamosal morphology. In the Horner & Goodwin Triceratops paper, the authors observe that Triceratops’ epiparietals and episquamosals become incorporated into the edge of the frill over time, and that older adults no longer have spiky frill edges, but more scalloped edges where the epiparietals and episquamosals are tab-like and rounded. In every known specimen of Torosaurus where epiparietals and episquamosals are preserved, they are also heavily absorbed. However, there are some key differences. Whereas the epiparietals and episquamosals of adult Triceratops are tab-like and rounded, those of Torosaurus are elongate and somewhat flattened, with distinct squared-off edges instead of rounded edges. Did the episquamosals and epiparietals elongate along with the frill bones? I doubt it. Their distinct morphology really does seem distinct.

Torosaurus specimen ANSP 15192 is particularly interesting because it appears to represent a subadult Torosaurus: its nasal horn morphology pertains to an early growth stage based on Horner & Goodwin’s Triceratops paper, and its frill is not as long as MOR 1122 or YPM 1831, yet it still has parietal fenestrae. Of course, Horner & Scannella recognize the analogy with Triceratops but ignore the nasal horn morphology: “No ‘Torosaurus’ specimen has posteriorly curving postorbital horn cores, which would be indicative of immaturity.” However, ANSP 15192’s postorbitals do conform very well to figure (d) of that paper—what you might call a young adult. And guess what? That’s where its squamosal sits on Figure 4 of Horner & Scannella’s paper. In that same growth stage (d), the epiparietals and episquamosals of Triceratops are starting to fuse and become rounded.

I’d talk about the bone histeology of the frill, as that’s one of the more interesting aspects of the paper, but I don’t know enough about bone histeology to do so intelligently.

Horner also seems married to the idea that the dinosaurs were dwindling in diversity at the end of the Cretaceous, and he’s been whining that paleontologists have been overstating the diversity of the Hell Creek Formation for years. That very well may be, but his solution is to synonymize everything that comes out of the ground. Look at what we used to have: Nanotyrannus, Tyrannosaurus, Dracorex, Stygimoloch, Pachycephalosaurus, Triceratops, and Torosaurus. Diceratops has always been questionable. Now we might just have Tyrannosaurus, Pachycephalosaurus, and Triceratops. I’m not against that idea, I’m just saying that extraordinary claims require extraordinary evidence. While the idea that Torosaurus represents the “silverback” stage in Triceratops’ lifecycle may hold merit for future study, I don’t think that Horner & Scannella proved it beyond a reasonable doubt.

Discuss!

EDIT: Can I stop pressing Control-I now?

Gigantic Diversification

2010 has been the Year of the Ceratopsian, certainly, but the lion's share of new named taxa have belonged to the Ceratopsidae--the large-bodied horned dinosaurs. Just for funsies, I'm going to list all the species known in 1995, then list all the species named since then. I think you'll see a massive increase in ceratopsid diversity. Ready? HERE WE GO! I've marked species of uncertain validity with a question-mark.

As of 1995:

Chasmosaurinae

Chasmosaurus belli
Chasmosaurus russelli
"Chasmosaurus mariscalensis"
Pentaceratops sternbergi
Anchiceratops ornatus
Arrhinoceratops brachyops
"Diceratops" hatcheri (?)
Torosaurus latus
Torosaurus utahensis
Triceratops horridus
Triceratops prorsus

Centrosaurinae

Centrosaurus apertus
Styracosaurus albertensis
"Styracosaurus" ovatus
Einiosaurus procurvicornis
Achelousaurus horneri
"Monoclonius flexus"
"Monoclonius nasicornis"
Pachyrhinosaurus canadensis
Avaceratops lammersi
"Brachyceratops montanensis"

Since 1995, as of this date:

Chasmosaurinae
Agujaceratops mariscalensis
Chasmosaurus irvinensis
Coahuilaceratops magnacuerna
Eotriceratops xerinsularis
Medusaceratops lokki
Mojoceratops perifania
Ojoceratops fowleri
Tatankaceratops sacrisonorum

Centrosaurinae
Pachyrhinosaurus lakusai
Albertaceratops nesmoi
Centrosaurus brinkmani
Diabloceratops eatoni
Rubeosaurus ovatus
Sinoceratops zhuchengensis

Am I forgetting anyone? Even though we've actually lost a few species, and sometimes even genera, since 1995 (Monoclonius, Brachyceratops), and a few have been renamed (Agujaceratops, Rubeosaurus), the Ceratopsidae has, overall, exploded in diversity. Currently, there are 24 more-or-less valid genera (I'm lookin' at you, Nedoceratops) containing a whopping 30 species, unless I'm missing anybody, in which case there are actually MORE. And we're not done yet. There are several yet-to-be-published taxa that I'm personally aware of, and I'm sure my more informed colleagues know of even more. And the year is only half over. A lot more publishing can happen between now and 2011. Let's hope that 2010 continues to deliver more delicious horned dinosaur goodness!