The Evolving View of Stegosaurus

Mounted skeleton of Stegosaurus ungulatus at the Carnegie Museum. The tail, plate, and spike arrangement have been updated in this mount to reflect current thinking following the study by Carpenter (1998). Photo by Perry Quan, CC-By-SA 2.0.

Seeing as how I've been working on restorations of two different stegosaur species this month, I thought I'd write up a quick review of the most famous aspect of these iconic dinosaurs: Their big, triangular plates. For a complete overview of the history and interpretation of Stegosaurus, be sure to see Ken Carpenter's 1998 paper.

Interpretation of the life appearance of stegosaurs has changed several times since they were discovered by O.C. Marsh in 1877. The first stegosaur fossil (belonging to the species S. armatus) were found near the town of Morrison, Colorado, but the specimen was disarticulated and only a few of the plates were preserved. Initially, Marsh thought that these plates played flat along the animal's back, forming a sort of turtle-like shell, or like the tiles on a roof (hence the name Stegosaurus, which means "roofed lizard"). Marsh also initially believed that stegosaurs were aquatic, due to this turtle-like appearance, but also that they would have walked on two legs on land.

By 1880, Marsh had studied the specimen further and changed some of his initial conclusions. He reversed his stance on the lifestyle of stegosaurs, recognizing them as land-living animals that were too heavy to have walked mostly on two legs (though he thought they could probably rear up on two legs when necessary). Marsh's recognition that the bases of the plates were asymmetrical also led him to propose for the first time that they stuck upright out of the back. He suggested that there were multiple rows of vertical plates covering the back.

Marsh's original 1890s restoration of Stegosaurus.

However, the discovery of a more complete specimen prompted Marsh to change this view yet again, describing S. stenops as having paired plates on the back and a single row on the tail. In a skeletal diagram published in 1891, he portrayed a generic Stegosaurus species as an arch-backed quadruped with only a single row of plates protruding vertically along both the back and tail.

Marsh died in 1899, but the job of preparing and interpreting his stegosaur specimens continued. In 1910, R.S. Lull examined the known specimens of Stegosaurus with the aim of solving the question of plate arrangement in order to produce the first-ever museum mount for the Yale Peabody Museum (which is still there unaltered, Marsh's eight spikes and all). Lull concluded that the double row of paired plates was correct, and that the plates were relatively widely spaced, above the top of the rib cage. This interpretation of Stegosaurus is probably the most famous, and the one that found its way into mid-20th century pop culture.

Charles W. Gilmore revised this interpretation only a few years later, when preparation of Marsh's S. stenops specimen was finally completed. Gilmore used the specimen to argue for an alternating, rather than paired, double row of plates, situated close together on the spine. This interpretation was used in several mid-20th century mounted skeletons, including the Carnegie Museum's and the American Museum of Natural History's. The latter remained unchanged during the dinosaur hall renovation of the early 1990s and is still on display in its original form.

Mounted skeleton and model of Stegosaurus stenops at the AMNH. This mount retains the posture and armor arrangement suggested by Gilmore in 1914. Photo by InSaphoWeTrust, CC-By-SA 2.0.

For most of the 20th century, depictions of Stegosaurus followed either the older Lull interpretation or the more scientifically correct Gilmore version. That was, of course, until the Dinosaur Renaissance came along to challenge the status quo. I first remember being shocked to see the next iteration of Stegosaurus in the 1987 book The Rise and Fall of the Dinosaur by Joseph Wallace. This book was my personal introduction to the dinosaur Renaissance, and was probably the first time I'd encountered Dino Renaissance-era artwork and ideas by Greg Paul, Doug Henderson, and the late Steven Czerkas, who passed away this month.

Czerkas' 1987 Stegosaurus model showing raised neck and tail, with single row of plates. From Dinosaurs: A Global View, originally posted at Love in the Time of Chasmosaurs by Marc Vincent.  

The book was full of photos of Clerks' incredible dinosaur sculptures, including his own brand new interpretation of Stegosaurus. Czerkas portrayed Stegosaurus with huge plates (extended significantly by keratin, which was unusual in art at that time) arranged in a single row along the very peak of the back, but deflected slightly from side to side in an alternating fashion, and with a high, flat back, horizontal tail, and a slender neck held in a high, upright position. The new look of Stegosaurus was so revolutionary it was the subject of a fairly lengthy piece in The New York Times - "A Revisionist's View of Stegosaurus" by John Noble Wilford, August 1st, 1986 (several months before the publication of Bakker's The Dinosaur Heresies). Czerkas' radical new view of Stegosaurus was the one repeated in any forward-thinking dinosaur books during the 1990s, and rubbery plastic version based on Czerkas' sculpture was the first Jurassic Park toy I bought when it was released ahead of the movie in 1992 or '93.

The original Jurassic Park stegosaur toy by Kenner, clearly inspired by Czerkas' revised reconstruction. Photo from JPtoys.com.

Like some other Dinosaur Renaissance ideas, though, this new Stegosaurus went a little too far to the opposite extreme of older interpretations. Several new specimens discovered during the 1990s, some more complete than any previous fossil, helped clarify the plate problem, and reversed Czerkas' conclusions. As Carpenter showed in his 1998 study, Gilmore turned out to have been right: the plates were situated in double, alternating rows high on the back. Careful examination of the shape of the first few tail vertebrae confirmed Czerkas' hypothesis that the tail was held high and upright at least at the base, and was held aloft by passive tendons and ligaments rather than muscles. Recent analyses by Mike Taylor, Matt Wedel, & Darren Naish (2009), Taylor & Wedel (2013), and Taylor (2014) have shown that the necks of all tetrapods, stegosaurs included, are normally held in an extremely retraced/upright position, so the overall posture suggested by Czerkas, at least, seems to have been correct.

About those spikes...

Spike arrangement and possible ranges of motion for the tail in Stegosaurus, from Carpenter 1998.  Carpenter argued for the more inflexible version B due to locking of the tail by the plates, though perhaps the amount of soft tissue involved would have mitigated this effect? 

Richard Owen suggested as early as 1875 that the spikes often associated with dinosaurs were outgrowths of the wrist and used for defense. Marsh followed this interpretation in his initial description of Stegosaurus, but since there were more than two spikes, he guessed that the remainder must have been arranged along the forelimb. He discarded this idea when he found the first articulated stegosaur arm. Marsh later found enough specimens with the spikes near the tail that he concluded that must have been their natural position, and even began drawing distinctions between species and sexual morphs based on number or presence of tail spikes. The assumption based on his 1891 skeletal was that the spikes pointed upright. This was followed by later scientists, though no explanation or detailed rationale was ever given.

Carpenter (1998) re-examined the spikes, again because of the need to re-mount a museum specimen, and concluded that the spikes actually pointed posteroleterally (out and to the rear) based on several lines of evidence, including the alignment of grooves on their bases with the vertebrae in articulated skeletons having to be re-arranged to achieve an upright orientation.

How did they move?

Carpenter (1998) notes several times that the staggered rows of plates would have severely lowered lateral mobility. The body and tail simply would not have been able to move much without the plates coming apart from the vertebrae (see diagram of the tail above). So how could Stegosaurus move at all, let alone swing that tail far enough to the side to hit anything? Carpenter himself offers a partial solution: The skin covering the back must have been incredibly thick, and the plates anchored and buried pretty deep inside fibrous soft tissue. (This is contrary to some Renaissance-era suggestions by Bakker that the plates were somehow attached to the muscles, allowing them to be flexed up and down). The plates were anchored in tough, fibrous skin up to at least the bottom third of their height (Galton, 2012). As Carpenter notes, the fact that the plates were anchored in a thick layer of skin would have allowed the skin and plates to slide relative to the underlying bone and musculature, which should have increased overall mobility.

Below are two in-progress rough drafts showing possible extremes in range of motion (my own hypothesis based on the above info) for the neck and tail of Stegosaurus. Note that these depict two different species: S. ungulatus (top) and S. stenops (middle and bottom). Also note the lateral spike arrangement, and the degree to which the plates are buried in skin.

Quick addendum: I noticed after writing this article that it looks like the new "Sophie" mount at the Natural History Museum in London appears to have been mounted with Czerkas-style plate and spike arrangement. This is supposed to be the most complete stegosaur specimen yet. Does anyone know if this plate arrange,went is supported by the articulation of the specimen, or were they just following an older model?

Sophie the (Czerkas-style?) stegosaur. 

References:

• Carpenter, K. (1998). "Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park Colorado". The Upper Jurassic Morrison Formation: An Interdisciplinary Study. Part 1. Modern Geol. 22. pp. 127–44.
• Galton, P.M. (2012). "Stegosaurs". Pp. 483-504 in Brett-Surman, M.K., Holtz, T.R. Jr., and Farlow, J.O. The Complete Dinosaur - Second Edition. Bloomington: Indiana University Press.
• Taylor, M.P. (2014). "Quantifying the effect of intervertebral cartilage on neutral posture in the necks of sauropod dinosaurs." PeerJ, 2:e712.
• Taylor, M.P. and Wedel, M.J. (2013). "The effect of intervertebral cartilage on neutral posture and range of motion in the necks of sauropod dinosaurs." PLoS ONE, 8(10):e78214.
• Taylor, M.P., Wedel, M.J., and Naish, D. (2009). "Head and neck posture in sauropod dinosaurs inferred from extant animals." Acta Palaeontologica Polonica, 54(2):213–220.