ARKANSAURUS FRIDAYI DISCUSSION PAGE III

This page is reserved for discussion, rebuttal and comment on my article concerning Arkansaurus fridayi, in which I raised doubts about the authenticity of the dinosaur cast exhibits which have been on display in various museums around the state for over 25 years. Check the first batch of feedback at this URL and the second batch of feedback at this URL.

Here's an edited-for-space summary of correspondence I've been having with Mickey Mortimer. His words are indented.

You might be interested to know ReBecca Hunt of the University of Arkansas is in the process of describing "Arkansaurus fridayi". It's currently though to be a basal coelurosaur, like Ornitholestes or Nedcolbertia. You can download her abstract from last year's Society of Vertebrate Paleontology meeting here- http://www.vertpaleo.org/meetings/Abstracts02.pdf

Thanks. I'll check it out. I'm a chemist with a biology degree, not a vertebrate paleontologist, but (and please correct me if I'm wrong) the designation "basal coelurosaur" is another way of saying "miscellaneous early theropod."

In the 80's or earlier, it might have meant that, but today we have a strict definition of Coelurosauria. Coelurosaurs are all theropods more closely related to birds than to Allosaurus. Basal (or primitive) coelurosaurs include Proceratosaurus, Ornitholestes, Compsognathus, Sinosauropteryx, Scipionyx, Coelurus, Nedcolbertia, and several others.

I know you don't think size is an appropriate taxonomical divider, but that list does seem to favor the chicken, turkey, goose range.

Comparisons with Ornitholestes is a problem for me. According to my Dinosauria, the find was mainly skull and manus. The foot was not well preserved and the specimen had not been fully described. She's still having trouble finding good comparative specimens in the strata she wants.

The (more recently discovered) specimen of Ornitholestes consists of a nearly complete skeleton.It's going to be fully described very soon, probably within the year. You can see photos of the various bones here- http://paleo.amnh.org/fossil/find.html?A=Ornitholestes&B=&C=&X=genus&Y=catal ogue_number&Z=catalogue_number&K=OR&L=OR&img= The manus has turned out to be from an undescribed basal coelurosaur known as "Tanycolagreus", though some manual bones are preserved in the original Ornitholestes skeleton.
Primitive ornithomimosaurs like Harpymimus, "Grusimimus", "Ginnareemimus" and Shapiro et al.'s (2003) unnamed form have unconstricted third metatarsals like "Arkansaurus". The constricted arrangement has been named "arctometatarsaly" by Thomas Holtz. So advanced ornithomimosaurs are arctometatarsalian, and primitive ones are non-arctometatarsalian.

I think you're wrong. Metatarsal III on Harpymimus is pinched at the top (and twisted, too, baby), just not so much as to obscure it when viewed from the front. If you look at a cross section of the anterior metatarsus it should be a lot clearer. Like so: O\O. Like II is trying to elbow its way in front. Could it be one of the elusive "intermediate forms?" "Grusimimus" and "Ginnareemimus?" Must be new guys. Where can I get pictures of the fossils? Do the quotes denote they haven't been published yet?

"Arctometatarsalian." Cool. Glad to know what it's called.

Metatarsal III needs to be pinched enough to let metatarsals II and IV contact in front for the metatarsus to be called arctometatarsalian. So Harpymimus is close to being arctometatarsalian, but isn't quite. "Grusimimus" and "Ginnareemimus" are both undescribed....
We now know ceratosaurs lived until the very end of the Cretaceous, in the form of abelisaurids and noasaurids. There are small Early Cretaceous ceratosaurs like Ligabueino and Genusaurus, so it's not as unlikely as it sounds to find a small ceratosaur in Early Cretaceous Arkansas.

Again, you've sprung some new ones on me in Ligabueino and Genusaurus. I found Abelisaurus under Problematic Theropoda, (skull only, no foot) not under ceratosaur. Can't compare Abelisaurus itself without a foot. I'll see if the other two finds you mentioned have relevant comparative materials.

Incidentally, Liliensternus and other coelophysoids are no longer thought to be ceratosaurs, but are thought to be more primitive instead.

To diagnose ceratosaur you need vertebrae, hindlimb and pelvis. Seven other characteristics say what kind of ceratosaur you are. That's my understanding (speaking as an amateur part-timer, of course). What changed with Liliensternus? That find had lots of material that should have made for a real solid ID.

The problem is that those "ceratosaurian" characters turned out to be primitive for theropods, or hard to define, or missing in some "ceratosaurs". Rauhut (2003) was the first person to explicitly critique the characters in The Dinosauria....
Finally, Elaphrosaurus isn't thought to be an ornithomimosaur anymore, and is instead a ceratosaur.

Elaphrosaurus is the find which I think most closely resembles Arkansaurus. If you classify Arkansaurus as a ceratosaur as you imply two paragraphs above, the reclassification of Elaphrosaurus as a ceratosaur makes my conclusions seem much more credible.

It's possible "Arkansaurus" is a ceratosaur, but I doubt it because the third metatarsal is much narrower in proximal view. This is a tetanurine character. But I'd have to see good photos of "Arkansaurus"' real remains, in multiple views, and spend a lot of time comparing it to different taxa, to have a strong opinion.
Anyway, it's Nedcolbertia, not Nedcolbertisaurus.

Oops. I guess I should change that in case somebody wants to look it up.

You've already read my objections to Nedcolbertia. The bone fragments are so small and pieces from many individuals are so intermixed that nearly any reconstruction is as valid as the one in the article I saw.

Yes, there are three Nedcolbertia individuals known. I don't think anyone's arguing "Arkansaurus" and Nedcolbertia are especially close relatives, just that they are both primitive coelurosaurs from about the same time and place. The third metatarsal does look similar in both, when viewed proximally. If you need a proximal view of Nedcolbertia's metatarsus, I can scan it for you.
Also, nearly every theropod has four toes. Harpymimus probably did too. Advanced ornithomimids and maybe Avimimus are the only exceptions (not including some birds). Oftentimes the first toe isn't preserved because it's so small. So the "wrong number of toes" argument wouldn't work against any professional paleontologist.

Nearly? Probably?

Well, every time we've found a skeleton with the entire foot preserved, it has a first toe. Except in advanced ornithomimids and the one Avimimus skeleton with feet.

My argument is about finding the comparative material that most closely resembles Arkansaurus. When you do that, the best matches are in the old world and come from the wrong epochs.

I can't argue that metatarsal I did or didn't exist on Arkansaurus, but does it exist as a claw, an internal vestigial structure, a thumb, a turkey spur or not at all or what? The Nedcolbertia article simply assumes metatarsal I on Arkansaurus is LIKE that on Nedcolbertia in order to justify using Arkansaurus as a model for reconstruction.

I could probably make it clearer by saying the "wrong number of recovered toebone fossils." Let's compare bones we have to bones we have. We DO know Ned had a turkey spur, we DON'T know if Arkie did. The "wrong number of toes" objection is based on the Nedcolbertia article using an imaginary toe as evidence. I have to say, "Stop! You can't use that toe as evidence of similarity. You made that toe up."

Size really doesn't mean much. There are 10 meter long ornithomimosaurs (Deinocheirus) and there are 3 meter long ones. The three known specimens of Nedcolbertia aren't fully grown anyway.

I do have to admit that a housecat is related to a tiger. However, if I found tiger bones and wanted comparative materials to convince people with, more convincing comparative material would come from a lion or couger. And since you brought it up, isn't it preferable to compare two sets of fully developed adult bones? Doesn't it weaken further the choice of Nedcolbertia as comparative material that the only examples are juveniles?

That's true. But some species aren't known from adult material (Scipionyx, Nedcolbertia, Nqwebasaurus, Santanaraptor, the newly described Huaxiagnathus) and others were fairly large (Proceratosaurus ~4 m; Bahariasaurus ~12 m; "Alashansaurus" ~5 m). Also, tyrannosauroids are basal coelurosaurs too, and some of them got to be huge (~13 m).

From my talk with ReBecca Hunt, I know she thinks the world and all of James Kirkland and she respects his opinions and his work, and with him being state paleontologist of Utah (when I last checked) I'm sure she's eager to impress him, but her insistence on relating Arkansaurus to Nedcolbertia might be distracting her from better choices.

...Here's something I made mention of in comparing Arkansaurus, Elaphrosaurus and Liliensternus in my original article. On the front face of the proximal end of metatarsal III is a bump. I guess you'd call it a process. I found this process on all three Arkansaurus, Liliensternus and Elaphrosaurus but rarely in similar theropod feet. Are these bumps just artifacts of the imaging processes, or have I found a characteristic shared by these three specimens. I've attatched a graphic combining details of the three specimens.

Hmm. I can find similar bumps in Dilophosaurus and Sinraptor. But they may be hard to see in an illustration, as the original description of Elaphrosaurus doesn't include the bump in its drawing. I'm not sure what significance they have.
Okay, I made an image in Photoshop with all taxa that have bumps I can find (it's one megabyte though, so I didn't include it in this e-mail). This includes- Chuandongocoelurus, Elaphrosaurus, Liliensternus, Dilophosaurus, Sinraptor, Fukuiraptor, Allosaurus, "Arkansaurus", Nedcolbertia and Rahonavis. Here's what I think about it- In most of these taxa, the "bump" seems to be the entire proximal tip of metatarsal III being raised anteriorly compared to the shaft. This is what I see in "Arkansaurus" too. The third metatarsal is very narrow proximally, but there's no non-raised area to the right or left of the bump. Nedcolbertia has the same thing. This seems to be a common feature in theropods, although it is not always shaded in illustrations. But if you look at just about any theropod third metatarsal in side view, you'll see they have this raised proximal lip anteriorly. So, I don't think this tells us anything about what "Arkansaurus" is related to. In Elaphrosaurus, Liliensternus and maybe Sinraptor, there is a smaller bump that does not cover the whole proximal end. It's placed below the proximal lip in Liliensternus, on the distal edge of the lip in Elaphrosaurus, and on the proximal edge in Sinraptor. I don't know if these bumps are homologous. Note that the bump in Liliensternus was exaggerated in The Dinosauria, when they copied from the original description (which I've included a scan of). Note also the bump in Elaphrosaurus is not visible in the original description or The Dinosauria, the feature illustrated there is the proximal lip, as I mentioned earlier.

One more question: Since so many anarctometatarsic cretaceous dinos are small AND are juveniles, has anybody ever considered that among more advanced dinosaurs arctometatarsy might be a juvenile trait... that maybe Nedcolbertia or Ornitholestes is actually a juvenile Ornithomimus or something like that.

Sorry to raise the spectre of recapitulation, but anarctometatarsy might be a trait retained from more primitive ancestors and recapitulated in the development of later animals.

An interesting thought, but it turns out not to be the case. We have juvenile Sinornithomimus, Gallimimus and Saurornithoides. Varricchio et al. (2002) even described Troodon embryos with arctometatarsalian feet.

to be continued

If you have feedback on Arkansaurus, send email here.

RTJ--3/15/2004


Arkansas Travelogue home page | Arkansas Monster Index | Back to Arkansaurus article | Back to First Feedback Page