Rhabdodontidae

French geologist Philippe Matheron named Rhabdodon priscus in 1869, making it among the oldest dinosaur names still in use. In terms of public interest, Rhabdodon and its close relatives have definitely been late bloomers. From 1869 through the 1990s, we've had a few papers, but the only person who seems to have put much energy into rhabdodontids during that time frame was the inimitable Baron Franz Nopcsa. The fortunes of the group have picked up in the past few decades; several new species have been described since 2000, old species have been reevaluated, and there was even an appearance in a segment of a TV documentary special. (Of course, there were the usual drama-related inaccuracies, and the rhabdodonts had to go in disguise as "dwarf Iguanodon", but at least they were there.) The latest news is a reevaluation of rhabdodontid paleobiology that makes them into something more than stock small ornithopods.

Rhabdodontidae to date is a small clade limited to the Cretaceous of Europe. Four genera and seven species are recognized as valid or at least debatable. Here's the thirty second tour:

• Matheronodon provincialis Godefroit et al. 2017, upper Campanian "Begudian" sandstones of southern France
• Mochlodon suessi Seeley 1881 (originally Iguanodon suessi Bunzel 1871), lower Campanian Grünbach Formation of eastern Austria
• Mochlodon vorosi Ősi et al. 2012, Santonian Csehbánya Formation of western Hungary
• Rhabdodon priscus (corrected from priscum) Matheron 1869, Campanian–Maastrichtian rocks of southern France and northeastern Spain, allegedly (see below)
• Rhabdodon septimanicus Buffetaut and Le Loeuff 1991, upper Campanian Gres de Saint-Chinian Formation of southwestern France
• Zalmoxes robustus Weishampel et al. 2003 (originally Mochlodon robustum Nopcsa 1899, or 1900, or 1902; it depends on who you read, but Nopcsa himself was of the opinion it was 1899), several Maastrichtian formations of western Romania (see also Brusatte et al. 2017)
• Zalmoxes shqiperorum Weishampel et al. 2003, several Maastrichtian formations of western Romania (see also Godefroit et al. 2009 and Brusatte et al. 2017)

This omits a couple of dubious and/or synonymous taxa we don't need to concern ourselves with, as well as some unnamed forms. Due to historical usage, what we currently call Rhabdodon priscus may be concealing additional species, and there is a fragmentary rhabdodont from Lower Cretaceous rocks in north-central Spain (the Vegagete ornithopod; Dieudonné et al. 2016). Now if we could just get one from Germany or Italy or Switzerland, between France and Austria, we'd have a (very, very niche) tour across Europe: "Walk in the footsteps of the rhabdodonts!" Admittedly, it doesn't have quite the cachet of other tourist options, but you could do worse.

Figure 1 in Ősi et al. 2012. Book your next vacation now!

Historically speaking, there is a certain lack of precision in the name "Rhabdodon". In part, this is an unintended consequence of Nopcsa's work: he came to the conclusion that the Austrian, French, and Romanian fossils represented the same genus, things kind of slid down to include just one species as well, and nobody else challenged the idea until R. septimanicus came along in 1991. (A brief stretch in the 1980s when Mochlodon came back was the result of a taxonomic issue, namely that the name "Rhabdodon" had been given to a snake about 40 years before Matheron's dinosaur. The whole thing had to be hashed out by the ICZN. Obviously, the snake lost.) With only one name in use for small to medium ornithopods of the Upper Cretaceous of Europe, it was inevitable that a lot of fossils falling into that category were assigned to Rhabdodon or Rhabdodon priscus. R. priscus has since been pruned of Central and Eastern European fossils, but there is still work to be done on French and Spanish fossils.

With Rhabdodon being the only rhabdodontid for about 80-90 years, there was little call for any special classification. It tended to be classified as an iguanodontid, in the old sense of the term ("it's in the Cretaceous, it's an ornithopod, it's too little and not specialized enough to be a hadrosaurid, and it's too heavy for a hypsilophodontid"), although occasionally it snuck into Hypsilophodontidae. Weishampel et al. (2003) introduced Rhabdodontidae when they named Zalmoxes, and this clade has generally been found at about the "seam" where Iguanodontia begins (the exact placement depends on author and clade definitions). Dieudonné et al. (2016) coined Rhabdodontomorpha for a slightly larger clade, incorporating Rhabdodontidae and the Australian Muttaburrasaurus. (I'd have been tempted to make Rhabdodontomorpha stem-based rather than node-based, but maybe that's just me.) [2017/11/06: As Daniel Madzia pointed out in the comments, Madzia et al. (2017) redefined Rhabdodontomorpha as branch-based (see their Appendix 1). Incidentally, the subject of their paper, an ornithopod from the Czech Republic they name Burianosaurus augustai, plots within or near the rhabdo-clades depending on the data set.]

The basic body plan of a rhabdodontid seems to be that of a fairly tubby or robust basal iguanodont. They are traditionally restored as bipeds, although the arms are well-developed. The limb bones are more robust than those of a typical "hypsilophodont" or dryosaurid, with stocky proportions (relatively short shin compared to thigh, like old friend Thescelosaurus, although unsurprisingly this is not quite as true for smaller rhabdodontids). However, it should be noted that although we have a good-sized sample of rhabdodontid fossils, articulation and association have been harder to come by. Rhabdodontid size varies more than you might suspect. The Eastern and Central European species have been noted for their small size (estimated at around 1.5 to 2.5 m long as adults), and because these parts of Europe are interpreted as having been islands in the Late Cretaceous, the species now referred to Mochlodon and Zalmoxes have sometimes been interpreted as island dwarf species. Rhabdodon itself, though, was not a dwarf anything; at around 5 to 6 m long, it was more like a smallish Camptosaurus. In fact, when Ősi et al. (2012) had a look at the growth of various rhabdodontid species and the body sizes of other ornithopods, they found no unequivocal evidence that the island species had been dwarfed, and instead suggested that Rhabdodon was a giant which evolved from an otherwise small-bodied lineage.

Figure 15 in Ősi et al. 2012. As the song says, one of these things is not like the others.

The rhabdodontid skull is the piece that makes rhabdodontids stand out among all the various critters near the base of Iguanodontia. It's boxy, heavily built, and proportionally large. Rhabdodontids bucked the general trend among ornithopods that went from a small number of separate smallish teeth to lots and lots of interlocked small teeth (the famous "dental battery" of hadrosaurids). Instead, they went for fewer, larger teeth. The apex of this to date is Matheronodon, which had a grand total of eight active teeth in the upper jaws (four per side). In a further bit of dental engineering, Matheronodon's tooth positions were running a sort of time share: instead of replacement teeth forming behind the old tooth in the same socket, it had paired sockets, one with the active tooth and the other with the growing replacement. When the active tooth fell out, the replacement tooth was ready in the other socket, either anterior or posterior to the old one (Godefroit et al. 2017). (This would have also given it a permanently gap-toothed appearance.)

The large teeth of rhabdodontids appear to have been strongly adapted for shearing, indicating that they were tackling tough foods. A couple of interesting things present themselves here. First of all, you may be familiar with another group of herbivorous dinosaurs with jaws and teeth interpreted as adapted for cutting: the ceratopsians. As noted by Godefroit et al., Europe is not noted for its ceratopsians. Perhaps there is a connection? Second, there is one other ornithopod that has been suggested as a shearer: Muttaburrasaurus. Could we be seeing evidence of a larger and more diverse radiation of shearing ornithopods than a handful of species in the Late Cretaceous of Europe?

Figure 2 in Godefroit et al. (2017): breaking out the shears. For orientation, this is a lateral view of the right maxilla (cheekbone), with the anterior end on the right. The part sticking up articulates with another skull bone. The grey blocks are superimposed teeth; dark gray is the active tooth, light gray is the replacement tooth. An active tooth and a replacement tooth are paired, so that when the active tooth wears out, the replacement tooth in the other socket will take over.

References

Brusatte, S. L., M. Dumbravă, M. Vremir, C.-S. Zoltán, R. Totoianu, and M. A. Norell. 2017. A catalog of Zalmoxes (Dinosauria: Ornithopoda) specimens from the Upper Cretaceous Nălaţ-Vad Locality, Haţeg Basin, Romania. American Museum Novitates 3884.

Buffetaut, E., and J. Le Loeuff. 1991. Une nouvelle espèce de Rhabdodon (Dinosauria, Ornithischia) du Crétacé supérieur de l’Hérault (Sud de la France). Comptes Rendus de l’Académie des Sciences, série II, 312:934–948.

Bunzel, E. 1871. Die Reptilfauna der Gosau-Formation in der Neuen Welt bei Wiener-Neustadt. Abhandlungen der Kaiserlich-Königlichen Geologischen Reichsanstalt 5:1-18.

Dieudonné, P-E, T. Tortosa, F. Torcida Fernández-Baldor, J. I. Canudo, and I. Díaz-Martínez. 2016. An unexpected early rhabdodontid from Europe (Lower Cretaceous of Salas de los Infantes,Burgos Province, Spain) and a re-examination of basal iguanodontian relationships. PLoS ONE 11(6):e0156251. doi:10.1371/journal.pone.0156251.

Godefroit, P., V. Codrea, and D. B. Weishampel. 2009. Osteology of Zalmoxes shqiperorum (Dinosauria, Ornithopoda), based on new specimens from the Upper Cretaceous of Nălaţ-Vad (Romania). Geodiversitas 31(3):525–553.

Godefroit, P., G. Garcia, B. Gomez, K. Stein, A. Cincotta, U. Lefèvre, and X. Valentin. 2017. Extreme tooth enlargement in a new Late Cretaceous rhabdodontid dinosaur from southern France. Scientific Reports 7:13098.

Madzia, D., C. A. Boyd, and M. Mazuch. 2017. A basal ornithopod dinosaur from the Cenomanian of the Czech Republic. Journal of Systematic Palaeontology. doi:10.1080/14772019.2017.1371258.

Matheron, P. 1869. Notice sur les reptiles fossiles des dépots fluvio-lacustres crétacés du bassin à lignite de Fuveau. Mémoire de l'Académie Impériale des Sciences, Belles-Lettres et Arts de Marseille 1869:345–379.

Nopcsa, F. 1899. Dinosaurierreste aus Siebenbürgen. Schädel von Limnosaurus transsylvanicus nov. gen. et spec. Denkschriften der kaiserlichen Akademie der Wissenschaften Wien, Mathematisch-Naturwissenschaftliche Classe 68:555–591.

Ősi, A., E. Prondvai, R. Butler, and D. B. Weishampel. 2012. A. R. Evans, editor. Phylogeny, histology and inferred body size evolution in a new rhabdodontid dinosaur from the Late Cretaceous of Hungary. PLoS ONE 7(9):e44318. doi:10.1371/journal.pone.0044318.

Seeley, H. G. 1881. The reptile fauna of the Gosau Formation preserved in the Geological Museum of the University of Vienna. With a note on the geological horizon of the fossils at Neue Welt, east of Wiener Neustadt. Quarterly Journal of the Geological Society of London 37:620–707.

Weishampel, D. B., C.-M. Jianu, Z. Csiki, and D. B. Norman 2003. Osteology and phylogeny of Zalmoxes (n. g.), an unusual euornithopod dinosaur from the Latest Cretaceous of Romania. Journal of Systematic Palaeontology 1(2):65–123.