Frontiers: The Stegosaurian Dinosaurs
As deadines approach and exams draw close, the countdown to the winter holidays grudgingly advances at a slow pace. Fortunately, a regular Thursday’s seminar in mid November brought with it a hearty and amusing sense of relief to the palpable tension that lingers in the Grant Institude at this time of the year.
Of the many wondrous exhibits of the Natural History Museum, Earth Hall features the most complete skeleton of a Stegosaurian dinosaur. Its acquisition was made possible by the donations of 70 wealthy donors to the museum1, one of whom had the skeleton named after his daughter, Sophie. Before its debut in 2014, Dr Susannah Maidment and her team were instrumental in the anatomical and paleoecological study of the affectionately dubbed specimen.
Without a doubt, dinosaurs have become one of the most publicised topics in Earth sciences. The gravity of their reputation is often preceded by child-like excitement and enthusiasm in households worldwide, thanks to film classics like Jurassic Park. In the seminar, Dr Maidment begins with an introduction to basic anatomy and the current state of academic knowledge on Stegosaurian dinosaurs. As part of the wider group of armoured dinosaurs, the stegosaurs are a sister taxon to the plated ankylosaurs and have been found in a multitude of places across the globe, two genera of which were excavated in the British Isles: Lexovisaurus and Scelidosaurus2.
However, most fossils are found as crushed and fragmentary segments rather than complete specimens. These so-called “roadkill fossils” often make dinosaur paleontology a challenging field when identifying the anatomy of long-dead species such as Stegosaurus stenops. Because the last definitive description of Stegosaurus was given in 1914, the recent fame of Sophie is owed to its rare case as a virtually complete skeleton as part of a relatively understudied taxon of dinosaurs that were most active around the Late Jurassic epoch about 163 million years ago3.
After its accidental discovery during a severe windstorm, Sophie was excavated from the 147 to 156 million-year-old4 Morrison Formation of the Red Canyon Ranch quarry near Shell, Wyoming, USA in 2004. Found with only a few missing toe bones and some broken skeletal parts, Sophie—known then as specimen SMA RCR0603—was prepared and kept at the Sauriermuseum in Aathal, Switzerland before its relocation to the Natural History Museum.
Of its many distinguishing features, stegosaurs are most recognised by their double row of kite-shaped plates extending from the nape of their necks to the two pairs of spikes protruding at the end of their long tails. Using a variety of empirical techniques, Dr Maidment and her team were able to discern several characteristics of Sophie’s life that would not have been preserved after its death, such as body mass. One such technique, known as the linear bivariate method, associates the log sum of the circumferences of a fully-grown animal’s femur and humerus with the log of its estimated body mass in a positive linear relationship; the higher the sum of the circumferences, the larger and heavier the animal.
On the other hand, the study also utilised a volumetric approach, in which Sophie’s body mass is estimated by projecting a convex hull around its near-complete skeleton. This convex hull was then expanded to a size that would be proportional to the typical dimensions of soft tissue supported within the confines of an animal’s skeleton. This method produced an estimated size indicative of 1,560 kg5. However, the linear bivariate method estimates Sophie’s body mass to have been 3,752 kg5 instead, the body size equivalent of which was fondly described by Dr Maidment as an obese Stegosaurus—or as internet meme culture would call Oh Lawd He Comin’6.
However, because of notable details such as a disarticulated skull and visible vertebrae sutures in the specimen’s skeleton, Sophie is indicated to have been a young-adult Stegosaurus at the time of its death, which was likely to have happened due to starvation or disease3. When the linear bivariate method was applied with the dimensions of a known adult Stegosaurus, it was confirmed to have vastly overestimated Sophie’s body mass.
The estimation of a dinosaur’s body mass is a crucial feature, as it allows for the determination of a myriad of its other characteristics, such as movement and feeding habits. Comparable to many bipedal dinosaurs such as Tyrannosaurus rex, stegosaurs were estimated to have supported up to 85% of their body weight on its hind limbs7, which places their centre of mass just behind their hips. However, its size and weight indicate that stegosaurs are known to be quadrupedal animals that would have been unable to raise their front legs past their shoulders.
Combined with shared features in their cranial skeleton with herbivorous dinosaurs like Plateosaurus and Erlikosaurus8, it is likely that the stegosaurs may have adopted a raised posture for feeding off tall trees—with a bite force equivalent to that of a sheep. Although its walking style is yet to be determined, a recent study that employed a self-learning model to estimate styles of stable movement for the bipedal Tyrannosaurus rex9 is now being utilised in attempts to model the gaits of quadrupedal animals such as Stegosaurus stenops. On a side note, the study also confirms that Tyrannosaurus rex, also known amusingly as ‘murder chickens’ in internet meme slang10, cannot run at the terrifying speeds we might have witnessed in Jurassic Park.
As for the iconic spinal plates, current literature assumes that they may have been used for reasons such as protection against predators or regulating internal body temperatures. According to Dr Maidment however, several observations refute these ideas. Firstly, many other stegosaurs are noted to not have had plates as large as those of Sophie’s, which means that their plates would have been ineffective thermoregulators due to inadequate surface area. Secondly, the spinal plates that line Sophie’s back were observed to have only been up to 3mm thick3. Therefore, it was unlikely that stegosaurs would have used their plates for protection against predators such as allosaurs, for whom biting on a spinal plate would have been akin to a human chomping down on Doritos. The Stegosaurians overall were also observed to have featured various morphologies of plates, with some being relatively thicker or flatter than Sophie’s. Thus, it is more likely that spinal plates may have been used as aesthetic displays for mating purposes. At present however, these deductions are difficult to test.
In a future study to be published in January 2020, Dr Maidment’s latest project is her involvement in the discovery and examination of a new, definitive species of stegosaur. Named after the Berber word for “mountain” (adrar) and “lizard” (tiklit)11, Adratiklit boulahfa has been classified not only as the oldest genus of stegosaur to have been found, but also the first of its kind to have been excavated in North Africa. Found in Boulahfa in the Middle Atlas Mountains of Morocco, this genus was observed to be more closely related to the European stegosaurs Dacentrurus and Miragaia than to the southern African taxa Kentrosaurus and Paranthodon; the excavation of this genus presents a possibility than more armoured dinosaurs may have resided in Gondwana than was originally assumed. Although the specimen was acquired by the Natural History Museum from local commercial fossil dealers, Dr Maidment expressed that current efforts are underway to collaborate with a local museum in Fez as well as local farmers and paleontologists in the search for more Gondwanan stegosaurs like Adratiklit boulahfa.
2 Stegosaurus (2019)
3 The Postcranial Skeleton of an Exceptionally Complete Individual of the Plated Dinosaur Stegosaurus stenops (Dinosauria: Thyreophora) from the Upper Jurassic Morrison Formation of Wyoming, U.S.A. (2015)
S.C.R. Maidment, C. Brassey, P.M. Barrett
4 Morrison Formation (2019)
5 Body mass estimates of an exceptionally complete Stegosaurus (Ornisthischia: Thyreophora): comparing volumetric and linear bivariate mass estimation methods (2015)
C.A. Brassey, S.C.R. Maidment, P.M. Barrett
6 Chonk / Oh Lawd He Comin’ (2019)
Know Your Memes
8 Decoupled form and function in disparate herbivorous dinosaur clades (2016)
S. Lautenschlager, C.A. Brassey, D.J. Button, P.M. Barrett
9 Investigating the running abilities of Tyrannosaurus rex using stress-constrained multibody dynamic analysis (2017)
W.I. Sellers, S.B. Pond, C.A. Brassey, P.L. Manning, K.T. Bates
10 Dinosaurs Explained (2019)
11 North Africa’s first stegosaur: Implications for Gondwanan thyreophoran dinosaur diversity (2020)
S.C.R. Maidment, T.J. Raven, D. Ouarhache, P.M. Barrett
12 Adratiklit (2019)
(Photo by Jon Butterworth on Unsplash)