Dryosaurus elderae and the revenge of Nanosaurus agilis

It's been a busy few days over at The Compact Thescelosaurus, with new alvarezsaurs, nodosaurs, and dryosaurs. For this post, I'm going to focus on Carpenter and Galton (2018), which not only describes new species Dryosaurus elderae, but also is quite important for previous subject Nanosaurus agilis, and in general ticks off several of my boxes anyway ("hypsilophodonts", Morrison Formation, National Park Service areas, etc.).

Figure 2 from Carpenter and Galton (2018), showing the distribution of bipedal Morrison ornithischians. A keen-eyed observer who's familiar with the Morrison fauna might notice the absence of Drinker nisti and Othnielosaurus consors, and an abundance of Nanosaurus agilis...

Dryosaurus elderae

Dryosaurus elderae is actually pretty familiar for a new species, because it is based on a skeleton that has long been the public face of Dryosaurus.

Clipped from Carpenter and Galton's Figure 27; A is CM 3392, the holotype of Dryosaurus elderae, in panel mount form; B is the same specimen in a new freestanding mount.

Genus and species: Dryosaurus elderae; the generic name is a combination of the Ancient Greek "drys" referring to "oak" or "forest", apparently for the inferred environment of the animal, and "sauros" for "lizard", and the species name honors the late Ann Elder, a National Park Service paleontologist and geologist who worked at Dinosaur National Monument. This gives us "Ann Elder's forest lizard".
Citation: Carpenter, K., and P. M. Galton. 2018. A photo documentation of bipedal ornithischian dinosaurs from the Upper Jurassic Morrison Formation, USA. Geology of the Intermountain West 5:167–207.
Stratigraphy and geography: The holotype and the four referred specimens all come from the Carnegie Quarry of Dinosaur National Monument in Uintah County of northeastern Utah. The quarry, in turn, is in the middle Brushy Basin Member of the Morrison Formation, and you probably don't need me to remind you that it's Late Jurassic in age.
Holotype: CM 3392 [Carnegie Museum of Natural History, Pittsburgh, Pennsylvania; occasionally the acronym is given in the literature as CMNH instead], a partial articulated skeleton including the skull and lower jaw, six cervicals, thirteen dorsals, the sacrum, rib pieces, the pelvis, the right shoulder girdle and humerus, pieces of the right femur and tibia, the articulated left foot, and three other complete or partial metatarsals. Most of the skeleton was discovered in March 1910, with the head and partial neck found later. The specimen was exhibited as a panel mount at the Carnegie from the 1940s to the early 2000s, when it was converted to a freestanding mount which went on display in 2006 (Carpenter and Galton 2018).

Four other specimens from the quarry have also been assigned to this species, the most significant being a partial juvenile (CM 11340), and a braincase (CM 87688) which had gotten into the holotype of Camptosaurus aphanoectes and made it seem strange enough to warrant a distinct genus for a while (Uteodon). D. elderae differs in various details from D. altus and Dysalatosaurus lettowvorbecki, such as a deeper maxillary ramus of the jugal below the orbit, relatively elongate cervicals, and a longer, lower ilium (Carpenter and Galton 2018). We spent some time with dryosaurids way back in 2014, and not a lot has changed since then, except I'd extend the Morrison species into the early Tithonian. Carpenter and Galton (2018) prefer including Dysalatosaurus lettowvorbecki in Dryosaurus, and also consider the generic-level distinctiveness of Eousdryosaurus nanohallucis to be questionable. They also note that CM 1949, a putative "Dryosaurus" specimen from Wyoming included in the Horner et al. (2009) growth rate paper, seems to be an undescribed form with a camptosaur-like ilium and a dryosaur-like leg, which would have something of an adverse effect on Horner et al.'s conclusions for Dryosaurus.

The revenge of Nanosaurus agilis

When we last checked in with Nanosaurus agilis, it was a historical curiosity known from a single specimen limited more or less to bone impressions. Carpenter and Galton (2018) come to a new conclusion that, if you accept it, instantly catapults N. agilis back into scientific relevance: that N. agilis is the same thing as the other attempts at describing small bipedal Morrison "hypsilophodont-grade"* ornithischians, namely Drinker nisti, Nanosaurus rex (aka Othnielia), and Othnielosaurus consors. Because N. agilis is the oldest name, it winds up the winner. You can think of N. agilis and Drinker as names given to the youngest individuals, and the Othnielosaurus/"Othnielia" specimens as older (but not necessarily fully grown).

*Yes, I'm well aware that "hypsilophodonts" are about as monophyletic as the clade of people named "Frank"; you can mentally replace "hypsilophodont" with "small bipedal ornithischian of nebulous classification" if you'd prefer, but my way's shorter.

Carpenter and Galton (2018) find that the various specimens assigned to Drinker nisti, Nanosaurus agilis, and Othnielosaurus consors share a set of characteristics that are unlike those found in the other bipedal Morrison ornithischians (Fruitadens haagarorum, Dryosaurus altus and elderae, and Camptosaurus dispar and aphanoectes). For example, in the dentary, there are:

• Dentary tapering anteriorly
• Gently convex ventral border of dentary
• Anterior dentary teeth slightly procumbent (lean forward)
• Widely spaced posterior dentary teeth
• Tooth roots well exposed
• Dentary tooth row continuing almost to the symphysis (the joint at the front)
• Triangular crowns wider than the roots
• Coronoid process (the part that sticks up) inline with tooth row rather than offset laterally

Figure 7 from Carpenter and Galton (2018), showing various jaws of the Morrison "hypsilophodont" (A through K) with Fruitadens (L), Dryosaurus (M), and Camptosaurus (N) for comparison.

In my opinion Carpenter and Galton have successfully demonstrated that there is basically one Morrison "hypsilophodont", although this doesn't necessarily exclude the possibility of anatomically similar species within the sample, as with the two Morrison species of Camptosaurus and Dryosaurus. The second part of their argument, that the Morrison "hypsilophodonts" should all be Nanosaurus agilis, may be harder to accept given the admittedly poor quality of the N. agilis holotype, and one could argue that it should be avoided. I'm not sure where that would leave the name of this taxon; it could be back to Othnielia rex, or maybe new combination Drinker consors if rex is also thought to not be up to snuff. However, the type of N. agilis does include a reasonably clear impression of a tooth-bearing dentary, which shows all of the features mentioned above. On that basis, it's hard to say that Nanosaurus agilis doesn't deserve serious consideration as the name of record.

Miscellaneous observations

The type material of Drinker nisti could not be located at two institutions where it was supposed to have been at one time or another. I suppose the other logical possibility would be in Bob Bakker's possession, as the original lead researcher.

In case you were wondering, no word on the other putative bipedal Morrison ornithischians Brachyrophus altarkansanus, Laosaurus celer, L. gracilis, Symphyrophus musculosus, Tichosteus aequifacies, or T. lucasanus, which is understandable considering that the sum total of fossils of those six species amounts to something like two to three dozen vertebral centra and some bits of limb bones. If someone else wants to take them on, they aren't going anywhere.

References

Carpenter, K., and P. M. Galton. 2018. A photo documentation of bipedal ornithischian dinosaurs from the Upper Jurassic Morrison Formation, USA. Geology of the Intermountain West 5:167–207.

Horner, J. R., A. D. Ricqlès, K. Padian, and R. D. Scheetz. 2009. Comparative long bone histology and growth of the “hypsilophodontid” dinosaurs Orodromeus makelai, Dryosaurus altus, and Tenontosaurus tillettii (Ornithischia: Euornithopoda). Journal of Vertebrate Paleontology 29(3):734–747.