The Tet Zoo field guide to ostrich dinosaurs (part II)

So, on to more ornithomimosaurs, aka ostrich dinosaurs (part I here). This time, the ornithomimids: this is the ornithomimosaur clade that includes only the edentulous arctometatarsalian taxa. Yes, I said arctometatarsalian*. However, note that some authors have incorrectly regarded Ornithomimidae as synonymous with Ornithomimosauria... and one author has even included therizinosauroids and alvarezsaurids within Ornithomimosauria. For those who don't keep up to date with the phylogeny of non-avian theropods, character evidence indicates that ornithomimosaurs are stem-group coelurosaurs, probably closer to maniraptorans than are tyrannosaurs and compsognathids (if you need to see this portrayed graphically, go to the theropod cladogram here: ornithomimosaurs are number 11; maniraptorans are number 12).

* We have the excellent Thomas R. Holtz to thank for this most pulchritudinous of terms. In theropods that exhibit an arctometatarsalian pes, the three largest metatarsals are not all cylindrical in shape (as is normal for theropods): the middle metatarsal (mt III) is laterally compressed at its proximal end, so much so that it's obscured from view by the proximal ends of mt II and IV. It was initially suggested that the arctometatarsalian pes characterized a coelurosaurian clade called - what else - Arctometatarsalia (Holtz 1994).

Enough of the preamble...

Archaeornithomimus asiaticus is a basal member of Ornithomimidae and was discovered in 1923 by an American Museum of Natural History expedition to Iren Dabasu, Mongolia. Described in 1933 as the first Asian species of Ornithomimus, Dale Russell argued in 1972 that it should be regarded as a distinct genus because, in contrast to Ornithomimus, its three metacarpals differed in length, and its foot bones were robust. The new generic name Archaeornithomimus (meaning 'ancient bird mimic') was therefore coined. Recent studies have found Archaeornithomimus to be the most basal member of Ornithomimidae (Kobayashi & Lü 2003). Unlike other ornithomimids, Archaeornithomimus still possessed a first metatarsal. It was also smaller than other ornithomimids, probably only reaching 2 m. Unfortunately however, doubt remains as to whether the many elements described as belonging to Archaeornithomimus all belong to the same species and it has even been suggested that some of the more 'primitive looking' Archaeornithomimus bones actually belong to Garudimimus. A partial Archaeornithomimus skull reportedly discovered in 1959 is presently lost. A North American species from the Arundel Clay of Maryland, named Ornithomimus affinis in 1920, has been regarded as a member of Archaeornithomimus. However, this is almost certainly incorrect.

Known from 14 juvenile and adult specimens discovered in 1997 in the Aptian-Albian Ulansuhai Formation of Inner Mongolia (China), Sinornithomimus dongi was named in 2003 by Yoshitsugu Kobayashi and Jun-Chang Lü. The preservation of 14 specimens together strongly suggests that these animals were gregarious and that a single event was responsible for their death and rapid burial. Abundant gastroliths were preserved in all the Sinornithomimus specimens, supporting the idea that these theropods were herbivorous. As in other ornithomimids, the anatomy of the jaw edges in Sinornithomimus show that the bones here were sheathed by beak tissue, and part of the upper jaw seems to have formed a sharp cutting edge. Sinornithomimus is like Gallimimus, but unlike North American ornithomimids, in that its snout tip was U-shaped when seen from above. Its three fingers were nearly equal in length and its finger claws were only weakly curved but, in contrast to more derived ornithomimids, its first metacarpal was tipped with a roller-like joint and it would therefore have been better at grasping than these other forms. Adult individuals of Sinornithomimus were around 3 m long [adjacent skull pic from sinofossa].

Anserimimus planinychus, named by Rinchen Barsbold in 1988, is a poorly known ornithomimid from the Maastrichtian Nemegt Formation of Mongolia. As indicated by the specific name (which means 'flat claws'), the hand claws of Anserimimus were straighter and broader than those of other ornithomimids, and had flatter undersides. The upper arm of Anserimimus was unusual compared to that of other ornithomimids, having large muscle attachments and a wide distal end. Its hands were odd: the first metacarpal was longer than the others, and the metacarpals were fused together. Furthermore, it's been suggested that the arm sockets of Anserimimus faced sideways more than they did in other ornithomimids (except Gallimimus). Presumably, these unusual forelimb features show that Anserimimus was doing something with its arms and hands that other ornithomimids weren't, but just what we don't know. At present only a single Anserimimus specimen is known, and this is lacking its skull. Recent studies suggest that Anserimimus is particularly closely related to Gallimimus, so it is conceivable that these two represent a uniquely Asian clade. Anserimimus means 'goose mimic' but Anserimimus was no more like a goose than was any other ornithomimosaur.

One of the biggest and best known members of Ornithomimosauria, Gallimimus bullatus is a Mongolian ornithomimid from the Maastrichtian Nemegt Formation. Discovered in 1967 by the Mongolian Palaeontological Expedition, Gallimimus was named by Halszka Osmólska and colleagues in 1972. Gallimimus was a giant among ornithomimids, reaching 6 m, and was originally illustrated with a strange snout that curved upwards at its tip (Osmólska et al. 1972). This no longer seems correct: Gallimimus had a blunt snout tip that did not curve upwards. Its snout was relatively broad, being U-shaped from above, and its lower jaw was deeper and proportionally shorter than that of other ornithomimids and with a shovel-like tip. Gallimimus had proportionally shorter arms, smaller hands and shorter claws than other ornithomimids. These features may have meant that Gallimimus did not use its forelimbs to grasp vegetation as, maybe, other ornithomimids did. As Osmólska and colleagues suggested, perhaps Gallimimus 'raked' the ground in order to uncover food. The anatomy of the neck in Gallimimus reminded its describers of the neck of Gallus, the chicken. Consequently, Gallimimus means 'chicken mimic'. Its specific name refers to the bony bulla, or capsule, that formed the base of its braincase.

Struthiomimus altus is one of several very similar, closely related North American ornithomimids. Originally named Ornithomimus altus by Lawrence Lambe in 1902, Struthiomimus - the name means 'ostrich mimic' - was given its own genus by Henry Osborn in 1917. The type specimen, discovered near Steveville, Alberta, in the Campanian Dinosaur Park Formation, consisted only of pelvic and hindlimb material but a far superior specimen [shown in adjacent image], missing only the end of the tail and the skull roof, was discovered by Barnum Brown near Steveville in 1914. Struthiomimus is also known from the Maastrichtian Horseshoe Canyon Formation. While Struthiomimus would have looked strikingly similar to the other North American ornithomimids, it had a proportionally longer body and tail and shorter hindlimbs than these species. Its hands, and its hand claws, were particularly long. Because its second and third fingers were the same length and probably worked together in unison, they may have been bound together in the same sheath of skin. This hand probably best functioned as a hooking device, probably for bringing branches or fern fronds toward the mouth (Nicholls & Russell 1985). Struthiomimus grew to a length of 4.5 m.

Named by Othniel Marsh in 1890 for a foot and hand from the Maastrichtian Denver Formation of Colorado, Ornithomimus velox remains poorly known. However, a second Ornithomimus species is well represented by many good specimens: O. edmontonicus, described from the Campanian Dinosaur Park Formation of Alberta in 1933 [adjacent image shows Greg Paul's restoration of this species]. This species is also known from the Maastrichtian Horseshoe Canyon Formation. Beak tissue is preserved in some Ornithomimus specimens. In 2001 it was noted that vertical ridges on the beak's inner surface looked similar to structures used by ducks to filter food particles from water, and it was proposed that ornithomimids were filter-feeders (Norell et al. 2001). However, these structures are seen in turtles and other beaked animals and actually have nothing to do with filter feeding (Barrett 2005). Ornithomimus was probably mostly herbivorous, grabbing at branches with its slim hands and snipping off leaves with its pointed beak. In Ornithomimus the fingers were even more similar in length than those of other ornithomimids, so its hand made an even more effective 'hook'. In 1972 two ornithomimids were described as the new genus Dromiceiomimus (meaning 'emu mimic') but in 2004 it was shown that these specimens actually belonged to O. edmontonicus (Russell 1972, Makovicky et al. 2004).

And, no, I haven't forgotten Deinocheirus: in the text it wasn't included in the ornithomimosaur section [adjacent Luis Rey image of said beast from here]. More soon, maybe I'll end up putting the entire field guide here on the blog. But not yet: coming next - the terrifying sex organs of male turtles! And you also have amazing iguanas, more Triassic crurotarsans, and a ton of other stuff to look forward to. And on Monday I begin a totally new chapter in my life; more on that when I'm allowed to talk about it.

Refs - -

Barrett, P. M. 2005. The diet of ostrich dinosaurs (Theropoda: Ornithomimosauria). Palaeontology 48, 347-358.

Holtz, T. R. 1994. The phylogenetic position of the Tyrannosauridae: implications for theropod systematics. Journal of Palaeontology 68, 1100-1117.

Kobayashi, Y. & Lü, J.-C. 2003. A new ornithomimid dinosaur with gregarious habits from the Late Cretaceous of China. Acta Palaeontologica Polonica 48, 235-259.

Makovicky, P. J., Kobayashi, Y. & Currie, P. J. 2004. Ornithomimosauria. In Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 137-150.

Nicholls, E. L. & Russell, A. P. 1985. Structure and function of the pectoral girdle and forelimb of Struthiomimus altus (Theropoda: Ornithomimidae). Palaeontology 28, 643-677.

Norell, M. A., Makovicky, P. & Currie, P. J. 2001. The beaks of ostrich dinosaurs. Nature 412, 873-874.

Osmólska, H., Roniewicz, E. & Barsbold, P. 1972. A new dinosaur, Gallimimus bullatus n. gen., n. sp. (Ornithomimidae) from the Upper Cretaceous of Mongolia. Palaeontologica Polonica 27, 103-143

Russell, D. A. 1972. Ostrich dinosaurs from the Late Cretaceous of western Canada. Canadian Journal of Earth Sciences 9, 375-402.