Field of Science

New Diminutive Heterodontosaur from North America

Good to see this out. Available online today is a new paper describing a new heterodontosaurid, Fruitadens haagarorum, from the Late Jurassic on Colorado, U.S.A.

There are lots of cool things about this critter:

1) Small size. Specimens of Fruitadens have a maximum body length of about 75 cm, making it the smallest known ornithischian. Histological work was done to determine the relative age of the specimens. It is estimated that the largest individual was about 4-5 years old at death, but had already slowed down its growth as is seen in sub-adult and adult non-avian dinosaurs.

2) Age. The specimen is Late Jurassic, making it one of the youngest known heterodontosaurids. Only Echinodon from the Early Cretaceous of England is younger. Interestingly both Echinodon and Fruitadens have small adult sizes, but it is possible to differentiate the two.

3) Plesiomorphic dentition. Fruitadens differs from older heterodontosaurs in possessing less sophisticated craniodental adaptations, thus was probably more of a generalist feeder. The authors believe that this adaptation, as well as small size, allowed the group to survive into the Late Triassic and Early Cretaceous. Also cool is that it seems to have regularly undergone tooth replacement, which is unknown in most other heterodontosaurs.

4) Theropod-like femur. One feature of Fruitadens that I find interesting is that the femur closely resembles that of theropods like Dilophosaurus and is very unlike that of ornithischians. Richard Butler assures me that the material represents a single taxon so this is an interesting characteristic. I often like to play devil's advocate and claim that there are no Triassic ornithischians (not even Pisanosaurus) despite a wealth of saurischians, and although I've been assured over and over by dinosaur workers that the saurischian line could not have given rise to the ornithischians, I always find synapomorphies like this (basal ornithischian with saurischian-like characters) of great interest. Hey, someones got to keep stirring the pot or we'll get too complacent.

4) Biogeography. Fruitadens is from the Upper Jurassic Morrison Formation of Colorado. This is the first described heterodontosaurid from North America. As it is from the Brushy Basin Member it was a contemporary of the classic dinosaurs such as Allosaurus, Camarasaurus, Stegosaurus, and Diplodocus.

Overall another great find. They just keep coming. Read more here and here.

Butler, R. J., Galton, P. M., Porro, L. B., Chiappe, L. M., Henderson, D. M., and G. M. Erickson. 2009. Lower limits of ornithischian dinosaur body size inferred from a new Upper Jurassic heterodontosaurid from North America. Proceedings of the Royal Society B. doi:10.1098/rspb.2009.1494.

Abstract-The extremes of dinosaur body size have long fascinated scientists. The smallest (<1>Fruitadens haagarorum, from the Late Jurassic of western North America that rivals the smallest theropods in size. The largest specimens of Fruitadens represent young adults in their fifth year of development and are estimated at just 65–75 cm in total body length and 0.5–0.75 kg body mass. They are thus the smallest known ornithischians. Fruitadens is a late-surviving member of the basal dinosaur clade Heterodontosauridae, and is the first member of this clade to be described from North America. The craniodental anatomy and diminutive body size of Fruitadens suggest that this taxon was an ecological generalist with an omnivorous diet, thus providing new insights into morphological and palaeoecological diversity within Dinosauria. Late-surviving (Late Jurassic and Early Cretaceous) heterodontosaurids are smaller and less ecologically specialized than Early (Late Triassic and Early Jurassic) heterodontosaurids, and this ecological generalization may account in part for the remarkable 100-million-year-long longevity of the clade.


  1. Tianyulong is another heterodontosaur that is younger than Fruitadens

  2. Maybe not. Tianyulong s from the same beds (Tiaojishan Formation) that produced Darwinopterus, which were dated as Middle Jurassic in that paper (and Late Jurassic in the Anchiornis paper). Clearly the date of the Tiaojishan Formation is unsettled.

  3. Oh, I remember seeing this at SVP! What a cute little heterodontosaur. I like the name, too.

  4. Hey Bill - thanks for the post. I would just note that the hindlimb morphology of Fruitadens is very similar to that of Heterodontosaurus (as far as we can tell - the situation is complicated somewhat in the latter taxon by extensive fusion of the ankle elements). So the odd hindlimb anatomy is probably characteristic of the whole clade, and it is sometimes spookily reminiscent of basal theropods (particularly coelophysoids). Probably convergence...right?

  5. 'Probably convergence...right?'

    of course! ;)

  6. Tianyulong is from Tiaojishan Formation? Zheng et al. (2009) says only that it is from Jehol Group. Witmer - in the same Nature's issue - states that it is from Yixian - the same is in Butler et al Fruitadens' paper. Is Tiaojishan Frm. part of Jehol Group?

  7. My understanding is that Tianyulong is from the Yixian Formation, which is part of the Jehol Group, and thus hypothesized to be early Cretaceous. Thus it would be younger than Fruitadens.

    I should have noticed this before but although Butler et al. (2009) include Tianyulong in their strict component consensus tree, it is absent from their maximum agreement subtree and thus from their figure calibrating the phylogeny to the stratigraphic record. They claim to have removed Abrictosaurus because it was acting as a wildcard in the analysis, but they do not say why Tianyulong is not included in the subtree (or eight other taxa in the SCC for that matter).

  8. I forgot to say that I don't really find the femur of Fruitadens to be that similar to Dilophosaurus or theropods in general. It looks like a perfectly good basal ornithischian femur to me.

  9. Whoops. I meant tarsus instead of femur. In particular, some characteristics of the ascending process of the astragalus as was discussed in the text. My mistake.


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