Geranosaurus atavus

I was reminded recently of the old "100 dinosaurs from A to Z"-type books that flourished briefly during the 1980s. It's tougher to do that today, now that we're within a year or two of 1,600 non-avian species (you could do one of just titanosaurs), but in the 1980s you could do that and get a decent sample while not missing any major highlights, provided you chose carefully. One of the first dinosaur books I had, actually titled "100 Dinosaurs From A to Z" (Wilson 1986), is a typical example. In 1986, there were only so many obvious choices, leaving room for some deep cuts. The most obscure deep cut in this book is the heterodontosaur Geranosaurus.

I brought this up before, but Wilson opted to include both Lycorhinus and Geranosaurus while omitting Heterodontosaurus, which is honestly an odd allocation of heterodontosaurs. Not that I'm complaining, because otherwise where would a five-year-old in 1986 have learned about either Geranosaurus or Lycorhinus, but still: How did these obscure heterodontosaurs get into the book? In hindsight, certain similarities with Michael Benton's "The Dinosaur Encyclopedia"/"Kingfisher Pocket Book of Dinosaurs", published in 1984, make me suspect that someone associated with Wilson's book paid close attention to Benton's book. (For one thing, the 100 dinosaurs in Wilson's book are almost entirely a subset of the 130 or so in Benton's book. There are also textual similarities, such as the content in the Parksosaurus entry.) Benton was clearly in a heterodontosaur mood, including not only Geranosaurus and Lycorhinus, but Heterodontosaurus as well. What we have thus seems to be some kind of heterodontosaur kismet: Benton made an obscure selection, and it survived into Wilson's book. And the reason I'm writing about something that's hardly a speck of dust in either edition of "The Dinosauria"? Because I have a soft spot for an obscure genus that was in one of my first dinosaur books. (See, authors? Your choices do have meaning. Wilson could have decided to include Echinodon instead, and then where would we be?)

Enough philosophizing and reminiscing. What is Geranosaurus?

Geranosaurus atavus was named in 1911 by Robert Broom along with several other dinosaur species from the Early Jurassic of South Africa. Broom had a collection of bones obtained by Mr. G. S. T. Mandy from a roadcut near the top of Barkly Pass in Eastern Cape, South Africa. These consisted of fragmentary skull material including a decent portion of the lower jaw, "slender birdlike hind-limb bones", and some poorly preserved vertebrae that he thought were too large to belong to the skull. Broom therefore limited the holotype to the jaw bones, represented by most of the predentary and left dentary, about half of the right dentary, and a fragment of maxilla. He did not specify a collection number, but this material is now cataloged as SAM-PK-1871 (Iziko South African Museum, Cape Town, South Africa), with the limb bones cataloged as SAM-PK-1857 and the vertebrae lost (Norman et al. 2011).

The presence of a predentary made it obvious that the owner was an ornithischian, or predentate of Broom's usage. The animal was not large; the predentary was only 12 mm long (0.47 in), and the more complete dentary measured 73 mm long (2.9 in). There were nine teeth in the dentary, with the first dentary tooth enlarged into what Broom referred to as a canine. A fragment of maxilla included teeth with flat-topped, chisel-shaped crowns showing weakly developed ridging on the buccal surfaces. This is basically the extent of the anatomical information; nothing is related about the vertebrae or the limb bones except for the "birdlike" form of the latter.

A crop of Figure 18 in Broom (1911), showing the holotype partial lower jaw.

Broom reported that the fossils came from the Cave Sandstone (today the Clarens Formation), which he interpreted as Early Jurassic in age but noted that others had interpreted as Triassic. More importantly, he also observed that in either case the new species would be the oldest known "predentate". This actually gives Geranosaurus more gravitas than you might realize, because it would hold this position as oldest ornithischian (or approximate co-oldest with Scelidosaurus) for fifty years (Lycorhinus was described in 1924, but it was thought to be a cynodont until the description of Heterodontosaurus in 1962). (One might think that the world's oldest ornithischian would merit more than a page of text at the end of a batch of descriptions of "prosauropods" and one line drawing, but apparently not.) Broom did not explain why he had named this new genus and species Geranosaurus atavus, but "atavus" means "ancestor" in Latin, which presumably refers to its position as oldest ornithischian, and "geranos" is Greek for "crane". "Of course!" you say. "Cranes are famous for their deadly canines!" (Actually, this is seen as a reference to the limb bones.)

Heterodontosauridae was not described until 1962 (Crompton and Charig 1962), leaving G. atavus little to do for many years except sit in a metaphorical box labeled "world's oldest ornithischian". With the description of Heterodontosaurus, it was much more evident what Broom's species was up to, but its very fragmentary remains prevented it from going on to fame and fortune. Damage over the years, such as to the maxillary teeth, also hinders recognition of some of the original features cited by Broom (Norman et al. 2011). Recent reviews of heterodontosaurids (Norman et al. 2011; Porro et al. 2011; Sereno 2012) have agreed that G. atavus is a heterodontosaurid. Sereno (2012) held out some hope for the species, noting several differences from other heterodontosaurs. There are only eight dentary teeth after the fang, arranged in a bowed line, and they are all about the same size, unlike other heterodontosaurs which have more teeth, arranged in a straight line, and differing more strongly in size. G. atavus also has no significant gap between the fang and the next dentary tooth, unlike Heterodontosaurus and Lycorhinus. Finally, Crompton and Charig (1962) had reported that there was no arch between the premaxilla and maxilla for the dentary fang, although Sereno (2012) noted that this part of the maxilla did not seem to be preserved. Sereno did not consider these differences sufficient for taxonomic purposes, but did consider them evidence for another heterodontosaur species in the Clarens Formation besides Heterodontosaurus tucki.

References

Benton, M. 1984. The dinosaur encyclopedia. Simon & Schuster, Inc., New York, New York.

Broom, R. 1911. On the dinosaurs of the Stormberg, South Africa. Annals of the South African Museum 7(4):291–308.

Crompton, A. W., and A. Charig. 1962. A new ornithischian from the Upper Triassic of South Africa. Nature 196(4859):1074–1077.

Norman, D. B.,A. W. Crompton, R. J. Butler, L. B. Porro, and A. J. Charig. 2011. The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962: Cranial anatomy, functional morphology, taxonomy, and relationships. Zoological Journal of the Linnean Society 163(1):182–276.

Porro, L. B., R. J. Butler, P. M. Barrett, S. Moore-Fay, and R. L. Abel. 2011. New heterodontosaurid specimens from the Lower Jurassic of southern Africa and the early ornithischian dinosaur radiation. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101(Special Issue 3–4):351–366.

Sereno, P. C. 2012. Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs. Zookeys 226:1-225.

Wilson, R. 1986. 100 dinosaurs from A to Z. Grosset & Dunlap, New York, New York.