June 30 , 2016

by Justin Tweet

After my years of puzzlement versus Leonardo, I have the greatest sympathy for the early generations of dinosaur paleontologists, being confronted with the often-scanty remains of beasts that had never been seen by anyone. We've already touched on Marsh dealing with Atlantosaurus and Cope dealing with Monoclonius recurvicornis. Ankylosaurians were no exception to the confusion. If a dinosaur skeleton is like a puzzle, a partial ankylosaur skeleton is like getting 300 pieces of a 2,000 piece puzzle, with the only guide being a hand-drawn copy of the box cover drawn by a right-handed person using their left hand, who is also in denial about the state of their vision. The puzzle, of course, is a stand of birch trees in a Great Lakes winter, in black and white. Small wonder that ankylosaurs spent most of the 20th century in the guise of magnified grouchy horned toads, covered in a pavement of armor laid out with enough regularity to satisfy the most tyrannical of 1950s suburb planners.

By 1905, the world's sample of described ankylosaurs included the following:
the front half of Hylaeosaurus armatus (partially prepared);
the back half of Polacanthus foxii;
part of Nodosaurus textilis (minimally described);
part of Hoplitosaurus marshi;
teeth of Palaeoscincus;
the partial skull and cervical armor of Stereocephalus tutus, not to be given its present name Euoplocephalus until 1910;
the lower jaw of Sarcolestes leedsi (only recognized as coming from an armored dinosaur in 1901);
the partial upper jaw of Priodontognathus phillipsi (ditto, except swap 1902 for 1901);
the soul-destroying morass that is Acanthopholis and friends;
an assortment of bones from Austria divided into several taxa, the only one with much modern recognition being Struthiosaurus austriacus;
and various bits and pieces.
In other words, enough bones to get a decent grasp of the skeleton if you work hard and don't mind some travel, but minimal information on where all those blasted plates, spikes, and scutes are supposed to go. At that point, the main frame of reference consisted of a nice skeleton (partially prepared) of Scelidosaurus, the European complex of stegosaurs then known as Omosaurus, and Marsh's beloved Stegosaurus. It is into this world that Samuel Wendell Williston brought a new genus and species, Stegopelta landerensis (Williston 1905).

Given that the original description is in Science and the articles then weren't any longer than they are today, there are approximately two columns of text divided into four paragraphs, of which one describes the specimen, two describe the stratigraphy, and one comments on the relationships of the dinosaur and gives it its name. Williston immediately compares his new beast to Stegosaurus, regarding Stegopelta as half the size and featuring a "heavy bony carapace" [sic; part of the ilium] two inches (five cm) thick which is fused to an overlying pavement mosaic of pentagonal plates [sic; photographs show plates with five, six, or seven sides, with a preference for hexagonal] resembling those of Glyptodon. These plates are "about four inches [10 cm] in diameter, scrobiculate and somewhat elevated in the middle." (Our new vocabulary word "scrobiculate" means "pitted", more or less, and I can't wait for an occasion to use it.) Williston interprets the elevations as supporting nonbony spines. He does not think that the carapace covers the entire body, because he also has a number of flattened and keeled scutes (although no spines), and draws a comparison to the sacral shield of Polacanthus. From the description, it sounds like his keeled scutes belong to the partial cervical ring later illustrated (Carpenter and Kirkland 1998). The rest of the anatomical description is concerned with matters we would now recognize as generally distributed among ankylosaurs. The extensive stratigraphic description presents a picture of marine beds populated by three or four species of plesiosaurs, a marine croc, and abundant snails and bivalves (although these are described as of freshwater to brackish affinities). Like Nodosaurus, the conclusion in hindsight is that Stegopelta is a bloat-and-float. He attributes the fauna to a Niobrara-age unit, and names it the Hailey Shale, a name which never caught on. Today the source rock is identified as the Belle Fourche Member of the Frontier Formation, which is somewhat older than the Niobrara. Williston does not dwell on the locality itself, but it is thought to have been near Conant Creek in Fremont County, Wyoming (Carpenter and Kirkland 1998). "Landerensis" is a reference to Lander, Wyoming.

But, I digress. We were about to get to the main show, Moodie (1910). Moodie relates that the skeleton had once been encased in a brown sandy clay concretion, but then eroded out and been reduced to fragments which were further trampled by cattle. The first unpromising scraps found were thought to belong to an indeterminate plesiosaur, but Williston identified "stegosaurian" teeth and thus a more exhaustive search yielded about 300 pounds (140 kg) of fragments. We pause here to allow the sounds of preparators wailing and begging for mercy to die down.

Anyway, per Moodie, that pile translated to fragments of the upper and lower jaws with fragmentary teeth, seven fragmentary back vertebrae, part of the sacrum, two tail vertebrae, rib fragments, the left and partial right ulna, fragments of the radii, the left and partial right ilium, the right and partial left pubis, the distal end of the right tibia, the ends of the right fibula, metatarsal fragments, scutes, and a generous helping of additional fragments. (The catalog number is left unstated, but today the material is reposited at the Field Museum of Natural History in Chicago as FMNH UR88.) He makes a few mistakes, most notably identifying the left scapula as the right pubis as noted by Carpenter and Kirkland (1998), and in hindsight is somewhat more optimistic about the completeness of certain elements than warranted. Overall, though, this is not bad, considering what he started with. By this time, he has correctly identified Williston's "carapace" as part of the ilium, and again draws attention to the scrobiculate plates. The eminences on the shield scutes often feature larger pits, again interpreted as the bases of nonbony spines. Later, Tanke and Rothschild (2002) would suggest that the pits on the bosses are pathological. The sacral shield is compared to that of Polacanthus, which differs in being composed of many small unscuplted plates which form a continuous surface over the pelvis; in contrast, the shield of Stegopelta is composed of much larger and distinctly sculpted plates, and is described as discontinuous. Interestingly, by this time Stegopelta also features "huge bony spines", not previously reported. What we would now recognize as a cervical ring is interpreted as a girdle probably found around the tail, continuing the glyptodont analogy. Moodie's conception of the armor includes elongate single plates with horn covering over the tail vertebrae, although it is not explained what this is based on. Another piece is identified as a bifid (split) spine, which he interprets as coming from the base of the tail; based on more complete ankylosaurs, a shoulder position is more likely, but this is apparently the first time such a spine had been identified.

The teeth are similar to those of the late unlamented Palaeoscincus, and he briefly entertains but then dismisses the possibility that Stegopelta is a synonym, based on stratigraphy, a high rate of evolution in dinosaurs, and a comparison to "Stereocephalus", then thought to possibly be the same as Palaeoscincus. He refers to twelve back vertebrae (after previously referring to seven), and the suspicion crosses my mind that at least some of them are the neck vertebrae reported by Carpenter and Kirkland (1998). The suspicion grows with Moodie's comment about how much less robust they are compared to the tail vertebrae. The pubis error is one where the advantage of time is easily appreciated; from the illustration, its identity as a scapula is immediately obvious even to someone like me, without much firsthand experience of ankylosaurians but a decent amount of book work. At the time, though, the relevant bones of the ankylosaur skeleton were little known. Moodie interprets the animal as two-thirds the size of Stegosaurus, 7 feet (2 m) tall at the hips and 16 to 18 ft (about 5 m) long (proportions suggesting Stegosaurus was used as the model, especially with the poor representation of the limb bones). Moodie closes by discussing two new taxa, Brown's Ankylosaurus magniventris (which for unstated reasons he assumes has a sacral shield like that of Stegopelta) and Wieland's Hierosaurus sternbergii, which Moodie suspects is synonymous with Stegopelta. Neither Nodosaurus or Hoplitosaurus make an appearance, which is unfortunate (especially in the case of the former). Lull (1921) remedies that situation, at least for Nodosaurus, which he notes is larger and has much different pelvic armor.

Like most ankylosaurians, little was heard from Stegopelta for the next few decades. It made a cameo in Coombs (1978) to be synonymized with Nodosaurus. Carpenter and Kirkland (1998) returned to the genus and separated it from Nodosaurus again, re-diagnosing the genus with the aid of nearly eighty years of research. One interesting tweak is that the sacral shield is not in fact fused directly to the ilia. Ford (2000) put it into its own subfamily within Ankylosauridae, to be called Stegopeltinae (also to include Glyptodontopelta mimus and what would be named Aletopelta coombsi), but this has not caught on. The most recent review, Arbour and Currie (2016, available 2015), did not find them to make a distinct clade, and placed Stegopelta and Glyptodontopelta in the nodosaurids. They kept Stegopelta and Nodosaurus separate, but noted that they had not examined the type of Nodosaurus textilis. See also Burns (2008) and Arbour et al. (2011) for some additional recent commentary on Stegopelta.

References:

Arbour, V. M., and P. J. Currie. 2016. Systematics, phylogeny, and palaeobiogeography of the ankylosaurid dinosaurs. Journal of Systematic Palaeontology 14:385–444.

Arbour, V. M., M. E. Burns, and P. J. Currie. 2011. A review of pelvic shield morphology in ankylosaurs (Dinosauria: Ornithischia). Journal of Paleontology 85:298–302.

Burns, M. E. 2008. Taxonomic utility of ankylosaur (Dinosauria, Ornithischia) osteoderms: Glyptodontopelta mimus Ford, 2000: a test case. Journal of Vertebrate Paleontology 28:1102–1109.

Carpenter, K., and J. I. Kirkland. 1998. Review of Lower and middle Cretaceous ankylosaurs from North America. Pages 249–270 in S. G. Lucas, J. I. Kirkland, and J. W. Estep, editors. Lower and Middle Cretaceous terrestrial ecosystems. New Mexico Museum of Natural History and Science, Albuquerque, New Mexico. Bulletin 14.

Coombs, W. P. 1978. The families of the ornithischian dinosaur order Ankylosauria. Palaeontology 21(1):143–170.

Ford, T. L. 2000. A review of ankylosaur osteoderms from New Mexico and a preliminary review of ankylosaur armor. Pages 157–176 in S. G. Lucas, and A. B. Heckert, editors. Dinosaurs of New Mexico. New Mexico Museum of Natural History and Science, Albuquerque, New Mexico. Bulletin 17.

Lull, R. S. 1921. The Cretaceous armored dinosaur, Nodosaurus textilis Marsh. American Journal of Science (5th series) 1(2):97–126.

Moodie, R. L. 1910. An armored dinosaur from the Cretaceous of Wyoming. Kansas University Science Bulletin 5:257–273. (text) (plates)

Tanke, D., and B. Rothschild. 2002. DINOSORES: An annotated bibliography of dinosaur paleopathology and related topics—1838–2001. New Mexico Museum of Natural History and Science, Albuquerque, New Mexico. Bulletin 20.

Williston, S. W. 1905. A new armored dinosaur from the Upper Cretaceous of Wyoming. Science 22(564):503–504.

Williston, S. W. 1908. Review: The Ankylosauridae. The American Naturalist 42(501):629–630