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Dryptosaurus: A possible North American megaraptorid?

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This is an idea that I’ve had knocking around in my head for a while. A recent post by fellow paleo-blogger Chase (who has a special interest in eastern North American Mesozoic life) on Dryptosaurus has spurred me to action in terms of writing a short article as well as doing some much delayed artwork.

For those of you who are too lazy to read Chase’s excellent article about this animal, Dryptosaurus was a medium-sized theropod dinosaur, approximately 20-25 feet long which lived in eastern North America during the late Cretaceous Period. Unfortunately, our total knowledge of this dinosaur is known from only a few fragmentary remains, including a hand claw that seems way too big in proportion with the rest of this animal’s body.

For as far back as I can remember, Dryptosaurus was classified as a tyrannosaur. But recently, I have my doubts about this classification. Even very primitive tyrannosaurs such as Guanlong and Proceratosaurus don’t have some of the anatomical features that Dryptosaurus appears to possess.

My curiosity centered around that claw. It didn’t look like a tyrannosaur claw – to me, it looked more like an allosaur claw. An unusually large hand claw also reminded me of another animal – Megaraptor, from South America. Originally, this was thought to be a gigantic dromaeosaur, but then it was hypothesized to be more closely related to the allosauroids, like Neovenator, Giganotosaurus, and Carcharodontosaurus. The allosaur-like claw would make this classification a good fit. Then I saw a picture of the skeletal remains of Australovenator, a megaraptorid that was discovered in (you guessed it) Australia. I immediately noticed similarities in the hand and body structure between Australovenator and Dryptosaurus.

  • Massive thumb claws in comparison with the other finger claws.
  • Short muscular arms and huge hands
  • Slender lower jaws with small closely-packed hook-shaped teeth.

What I find really interesting is that in 2012, a hypothesis was put forward by Fernando Novas and other paleontologists that the megaraptorids might actually be extremely primitive members of the tyrannosaur family. In 2014, fragments from a juvenile Megaraptor were discovered, including part of the upper jaw. The structure of the juvenile Megaraptor’s maxilla was very similar to the structure of the dentary from Australovenator. That year, Juan Porfiri re-iterated Novas’ hypothesis that the megaraptorids might be primitive tyrannosaurs.

So, with all of that being said, I hypothesize that Dryptosaurus was a member of Megaraptora, which would make it the first of its kind found within North America.

How did Dryptosaurus feed? The large hook-shaped claws and the small hook-shaped closely-packed teeth seem to indicate that Dryptosaurus and other megaraptorids were fish-eaters. The fossils of Dryptosaurus were discovered in the New Egypt Formation and the Navesink Formation, the later of which is known for both dinosaur fossils as well as fossilized shells. Also, Australovenator was found in deposits that indicate a swampy still wetlands environment, full of bivalves, fish, and turtles. The fact that this megaraptorid was found in a water-rich environment full of aquatic life leads me to suspect that Dryptosaurus might have had a similar lifestyle.

To conclude this short article, I have a drawing of the enigmatic Dryptosaurus portrayed as a megaraptorid rather than as a typical often-illustrated tyrannosaurid. Who knows – maybe my less-than-scholarly idea about Dryptosaurus being a fish-eating megaraptorid will prove out to be right. Only time and the discovery of more specimens will tell.

Dryptosaurus

Keep your pencils (and minds) sharp, everybody.

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7 Comments

  1. Chase says:

    Hello, and thanks for the mention and nice comments on my article. However, I’d like to make a few points on why I disagree with this hypothesis (and am writing a bit in my book about why this is likely false). The morphology of some of the features of D. aquilunguis seem to be a case of convergence rather than defining traits. Firstly, the megaraptorids (if they are indeed tyrannosauroids or something very closely related) have been found to be a branch off of tyrannosauroidea (i.e. Novas et. al., 2012). However, Dryptosaurus aquilunguis seems to have been more derived and has been found to be a fairly close outgroup to tyrannosauridae (Brusatte et. al.,

    • jrabdale says:

      I knew that this invasion of mine into your area of specialization would prompt an immediate response from you (haha). I was thinking about including a paragraph hypothesizing that the traits that we see in Dryptosaurus might be an example of convergent evolution – a tyrannosaur that kind of looks like a megaraptorid. I was especially thinking about foot structure, since the foot bones of Dryptosaurus are more similar to later advanced tyrannosaurs. However, I felt that adding in too many hypotheses would be confusing for the reader, who might not know exactly what idea I’m trying to prove.

      Like I said, I’m not saying that my idea is the real answer. With such fragmentary remains that we have, I dare say that paleontologists and paleo-buffs alike will be postulating about where exactly Dryptosaurus fits into the theropod tree for quite a while. Look at how many times its phylogeny has been revised or at least questioned.

      I am intrigued, nevertheless, about its lifestyle. It generally follows that habitat determines behavior, and I’m wondering if Dryptosaurus did indeed live a largely piscevorous diet. To tell you the truth, the question of its behavior and ecology interests me far more than where it fits on the dinosaur family tree. Personally, I don’t really care if it’s a megaraptorid or not – it was just an idea that I wanted to throw out there, and I’m happy that it has generated a bit of discussion.

      • Chase says:

        Hello again. I pressed send accidentally, so I was cut off. Here is the rest:
        (Brusatte et. al., 2011)(Loewen et. al., 2013).

        Interestingly enough, Carpenter et. al., (1997) suggested that Dryptosaurus was a close relative of the European form Betasuchus (which might also be an abelisaur). This implies some stuff about faunal interchange between Europe and Appalachia. Furthermore, White et. al. (2015) finds that Australovenator, a well-known megaraptorid, had pretty substantial grasping ability. This contrasts with Brusatte et. al.’s finding of Dryptosaurus having lost some grasping ability. This suggests that even if Dryptosaurus was a megaraptoran, it likely didn’t hunt fish as you suggested. This is also supported by D. aquilunguis having ziphodont rather than conical dentition. The serrations mean that if Dryptosaurus ate fish it would be like you or I doing so with a steak knife. That tactic doesn’t work well.

        The overarching geographical gap between the location of Dryptosaurus and known megaraptorids (I believe the current consensus is that we are still unsure of Siats being one at the moment) also clashes with your argument. However, I’m glad you wrote a bit about D. aquilunguis. This post will hopefully get some people interested in Appalachian animals. Believe me, interest in the continent is desperately needed.

        I am doing some research on a certain theory of mine on Late Cretaceous theropods of the north that relates to this article. You’ll likely see the results of the research in my book. Weirdly enough, it seems to me that there were a couple of distinct “dynasties” of theropods in the north during the Late Cretaceous which rose and fell according to the presence of certain groups of large herbivorous dinosaurs. If Dryptosaurus was a megaraptorid, it does beg the questions- why on Appalachia and how did it get there it such a short amount of time?

      • jrabdale says:

        Well, all of this information certainly proves the negative effects of me trying to formulate a hypothesis without keeping up with the scientific literature. The information that you’ve provided me leads me to ask even more questions. For instance, how can anyone determine Dryptosaurus’ grasping ability when nobody’s found a complete hand? Unless somebody has, and I haven’t heard about it yet? Damn it, now I’m doubting everything.

        I also heard that Eotyrannus “might” be a megaraptorid. If this is true, then how did the megaraptorids, which are predominantly from the Southern Hemisphere, get to Europe? Probably using the same routes as the abelisaurids did, I imagine, but I can’t be certain.

        I fully realize that the time frame of a megaraptorid living in North America by the end of the Late Cretaceous is rather a stretch, since they seem to have lived during the Middle Cretaceous, and would have been long extinct by the time Dryptosaurus came around. Now that all of this has been addressed, where do you think Dryptosaurus fits in with Appalachian paleo-ecology?

      • Chase says:

        Information on the grasping ability of Dryptosaurus can be found in Brusatte et. al. (2011) “ANATOMY OF DRYPTOSAURUS AQUILUNGUIS…”. Among other things, the ungual flexor tubercles are smaller than what you’d find in more basal tyrannosaurs, limiting the range of movement. You can find the paper easily online.

        On the subject of Eotyrannus- I’ve heard that Darren Naish is writing a new paper on the taxon, so hopefully we get a new phylogenetic analysis. I do not think Eotyrannus is a megaraptorid simply because of the robust skull morphology which really goes against the “norms” of the group.

        The temporal issue you suggested might actually be invalid since Aerosteon is found in the Santonian of Argentina (85 MYA). If Dryptosaurus was a megaraptorid, I’m intrigued as to how its kind migrated north and then went around the southern remnants of the Western Interior Seaway in the short amount of time between the main drainage of the Western Interior Seaway and the K-T event.

        Your fourth question has a short and a long answer. The short answer is that I don’t know, and the long answer is that I think the tyrannosaurs of Appalachia were doing something very different. I didn’t discuss in my article that the New Egypt has actually been considered contemporaneous with another formation: the Navesink. The New Egypt Formation may just represent an inland deposit. If these two formations are contemporaneous, we have a lot more information to go on. For one, the Navesink has preserved the remains of two more theropods: Diplotomodon horrificus and “Dryptosaurus” macropus. The first one, although a tooth taxon, has a robust tooth morphology, and I’m searching for better scans of the tooth to further study it. The tooth *might* imply the presence of a tyrannosaurid in the formation, which would be really crazy . “Dryptosaurus” macropus has been listed as an indeterminate tyrannosauroid by Holtz (2004) in “The Dinosauria”. It might be a juvenile, but we really don’t know at the moment. If the type specimen was fully grown, it was really small which suggests that it was hunting smaller animals. So we have a possible tyrannosaurid, a smaller-game hunter, and a large predatory tyrannosauroid with really big hands. I should also note that according to Gallagher (1997), Cope thought the holotype of Dryptosaurus aquilunguis was a juvenile because of the incomplete fusion of some bones. Brusatte et. al. (2011) disagree with that, so the type specimen may or may not be fully grown.
        It seems to me that Dryptosaurus was exploiting a specific type of large prey. I don’t want to give too much away about my research, so I’ll just say that I think it was going after animals which didn’t really have defensive structures.

      • Chase says:

        I just looked over this comment and I feel I should clarify some things. Firstly, the Navesink and New Egypt Formations being contemporary is widely disputed (I’m leaning neither way). I more so wanted to show the contemporaneousness of D. macropus and D. horrificus. Sorry for the possible confusion

  2. […] January, I published a post in which I hypothesized that Dryptosaurus, a Late Cretaceous theropod found in Eastern North […]

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