Dryosaurus, here pursued by the theropod Ceratosaurus. From wikipedia.

For no particular reason, I decided today to tweak text that originally appeared in the 2001 book Dinosaurs of the Isle of Wight (Naish & Martill 2001). Which dinosaurs did I choose to publish on? The most exciting of them all: the dryosaurids. I was kidding about the exciting part. The following text is really basic, and only intended for people who've never heard of these dinosaurs before. NB - don't confuse Dryosauridae with Dyrosauridae. The latter is a clade of fossil longirostrine crocodilians, known from the Cretaceous, Paleocene and Eocene.

Pedal skeletons of Dysalotosaurus, from Galton (1981). The two images on the left show the same foot as seen from its dorsal (l) and ventral (r) side. These are surprisingly long, slender, narrow feet for an ornithopod. Digit I was absent in these animals. The foot at far right only has the third and fourth digits preserved. Scale bars = 50 mm.

Dryosaurids were medium-sized (c. 3-4 m total length) long-legged ornithopod dinosaurs with small forelimbs and proportionally small, short-snouted skulls. Because they're superficially similar to ornithopods like Hypsilophodon, dryosaurids were once thought to be 'hypsilophodonts', and hence regarded as somehow ancestral or antecedent to the group that includes Iguanodon and the hadrosaurs. We now think that dryosaurids are members of Iguanodontia (though outside Ankylopollexia: the camptosaurid + Iguanodon + hadrosaur clade), and hence not close to Hypsilophodon. Undoubted dryosaurid records are from the Upper Jurassic and Lower Cretaceous (though read on). Dryosaurids were first reported in the 1870s from North America, were discovered in Africa early on in the 20th century, and were recognised in the European fossil record during the 1970s (the fossils concerned had been found much earlier, but misidentified as those of other kinds of ornithischian).

The type species for the group is Dryosaurus altus (Marsh, 1878) from the Morrison Formation of the USA's Western Interior. A similar Tanzanian species (known from thousands of remains) was originally named Dysalotosaurus lettowvorbecki Virchow, 1919: some authors regard it as congeneric with Dryosaurus while others argue that obvious differences better support genus-level distinction (Hbner & Rauhut 2010). A lot of ink has been spilt over the similarity evident between the American and Tanzanian species and what this might mean for Jurassic biogeography (e.g., Galton 1977), but it now seems that the dinosaurs of these two regions aren’t as similar as once thought.

Reconstructed skeleton of Dysalotosaurus, as displayed at the Museum fr Naturkunde, Berlin. Wow - was the tail skeleton really that long? Image by Masur, from wikipedia.

Lower Cretaceous English dryosaurid remains represent the species Valdosaurus canaliculatus (Galton, 1975). A Nigerian dryosaurid was originally named Valdosaurus nigeriensis Galton & Taquet, 1982 but has more recently been renamed Elrhazosaurus Galton, 2009. Kangnasaurus coetzeei Haughton, 1915 from the Lower Cretaceous of South Africa also seems to be a dryosaurid (McDonald et al. 2010). Callovosaurus leedsi (Lydekker, 1889), known only from a femur from the Middle Jurassic Oxford Clay Formation of England, has been identified as a dryosaurid (Ruiz-Omeaca et al. 2007), in which case it's the oldest iguanodontian. Because this identification pulls the origin of Iguanodontia down into the Callovian of the Middle Jurassic, the ghost lineages of many ornithopod taxa (including Hypsilophodon, Rhabdodontidae and Tenontosaurus) must be even longer than previously proposed (Weishampel et al. 2003, Naish & Martill 2008).

[PS - just to be clear on the citations and what I do, and do not, provide full references for... sometimes, when I mention an animal’s scientific name, I also list the name’s authorship. In order to distinguish this from a citation that I provide in the references, I include a comma. So, the full name for our species is Homo sapiens Linnaeus, 1758: when I mention ‘Linnaeus, 1758’ there, I don’t feel the need to add the full reference for that publication to the article’s bibliography. When binomial names become changed over time, we denote this by putting the taxon’s authorship in brackets. Example: Felis leo Linnaeus, 1758 is now Panthera leo (Linnaeus, 1758). This is a standard I’ve applied throughout Tet Zoo (I maintain it, where possible, across technical publications too), but I thought I should take time here to explain it.]

Given that dryosaurids appear both in the northern, Laurasian continents and the southern, Gondwanan ones, and originated prior to the fragmentation of these landmasses, it seems likely that they will eventually prove extremely widespread. Indeed, various fragmentary remains from the Upper Cretaceous of New Zealand, Antarctica and Argentina have all been suggested to belong to dryosaurids (see Ruiz-Omeaca et al. 2007).

Skull of Dryosaurus altus, from Galton (1983).

Dryosaurids were bipedal with cursorial hindlimb proportions. Three long toes are present on each foot and a first toe, aka hallux, is absent (the toes thus represent digits II, III and IV). The dryosaurid skull is short and tall with large orbits shaded above by elongate, rod-like palpebral bones. In life, the palpebrals would have supported soft-tissue ‘shelves’ jutting out above the animal's eyes, giving them a frowning demeanour much like than seen in living birds of prey. A peculiar feature of the dryosaurid skull is that the nostrils are open dorsally: there is no bar from the rostralmost portion of the premaxilla that joins the nasal over the top of the nostril.

Unlike Hypsilophodon and more primitive ornithopods, dryosaurids lacked premaxillary teeth, meaning that all cropping of food was performed solely by the beak - something common to nearly all iguanodontian ornithopods (there are annoying exceptions, like Tenontosaurus dossi). Dryosaurid maxillae and dentaries house serrated, leaf-shaped teeth which possess prominent ridges on the vertical midline. These teeth indicate that dryosaurids were leaf-eating herbivores.

While preparing this article I was surprised to find how few good life restorations there are of these dinosaurs. There are Greg Paul’s (but they’re not really available online)... and that’s about it. The other stuff is mostly terrible.

Anyway, for previous Tet Zoo articles on iguanodontian ornithopods, please see...

And for the Scientific American series on the ‘explosion of Iguanodon’, see...

Refs - -

Galton, P. M. 1977. The ornithopod dinosaur Dryosaurus and a Laurasia-Gondwanaland connection in the Upper Jurassic. Nature 268, 230-232.

- . 1981. Dryosaurus, a hypsilophodontid dinosaur from the Upper Jurassic of North America and Africa. Postcranial skeleton. Palontologische Zeitschrift 55, 271-312.

- . 1983. The cranial anatomy of Dryosaurus, a hypsilophodontid dinosaur from the Upper Jurassic of North America and East Africa, with a review of hypsilophodontids from the Upper Jurassic of North America. Geologica et Palaeontologica 17, 207-243.

Hbner, T. R & Rauhut, O. W. M. 2010. A juvenile skull of Dysalotosaurus lettowvorbecki (Ornithischia: Iguanodontia), and implications for cranial ontogeny, phylogeny, and taxonomy in ornithopod dinosaurs. Zoological Journal of the Linnean Society 160, 366–396.

McDonald, A. T., Kirkland, J. I., DeBlieux, D. D., Madsen, S. K., Cavin, J., Milner, A. R. C. & Panzarin, K. 2010. New basal iguanodonts from the Cedar Mountain Formation of Utah and the evolution of thumb-spiked dinosaurs. PLoS ONE 5(11): e14075. doi:10.1371/journal.pone.0014075

Naish, D. & Martill, D. M. 2001. Ornithopod dinosaurs. In Martill, D. M. & Naish, D. (eds) Dinosaurs of the Isle of Wight. The Palaeontological Association (London), pp. 60-132.

- . & Martill, D. M. 2008. Dinosaurs of Great Britain and the role of the Geological Society of London in their discovery: Ornithischia. Journal of the Geological Society, London 165, 613-623.

Ruiz-Omeaca, J. I., Pereda-Suberbiola, X. & Galton, P. M. 2007. Callovosaurus leedsi, the earliest dryosaurid dinosaur (Ornithischia: Euornithopoda) from the Middle Jurassic of England. In Carpenter, K. (ed) Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs. Indiana University Press (Bloomington & Indianapolis), pp. 3-16.

Weishampel, D. B., Jianu, C.-M., Csiki, Z. & Norman, D. B. 2003. Osteology and phylogeny of Zalmoxes (n. g.), an unusual euornithopod dinosaur from the latest Cretaceous of Romania. Journal of Systematic Palaeontology 1, 65-123.