Stegosaurus

Stegosaurus is a dinosaur genus from the Late Jurassic (from 155 to 150 million years ago), having the spectacular distinctive characteristics of kite-shaped upright plates along their backs and two pairs of spikes in the posterior part of their tails. Due to these features, Stegosaurus belongs to the most recognizable dinosaurs. Fossil records of Stegosaurus are known from the western United States and from Portugal.

Smiley face

Known occurrences of Stegosaurus fossils.

Discovery

Stegosaurus was described and named by Otheniel Charles Marsh in 1877, during the Bone Wars, from fossil remains found in Colorado[1] . These first found remains are now the holotype of Stegosaurus armatus. Marsh originally believed that the bones belonged to an aquatic, turtle-like reptile, and its plates layed flat, covering the dorsal side of the animal.

Smiley face

Stegosaurus stenops skeleton. By EvaK - Self-photographed, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=2686788

Valid and formerly assigned species

There are three valid species of the genus Stegosaurus: S. ungulatus, S. stenops, and S. sulcatus. All the other, initially described species are considered to be invalid[2].

From these, S. ungulatus was described by Marsh in 1879, based on fossils from Como Bluff, Colorado, and the meaning of its name is "hoofed roof lizard"[3]. It was about 9 meters long, which means that this was the longest between the < i>Stegosaurus species. Outside of North-America, its remains were also discovered in Portugal[4]. This species can be easily delimitated from S. stenops on the basis of its longer hind limbs, and proportionately smaller, more pointed plates which have wide bases and narrow tips, furthermore, it had several small and flat, spine-like plates just before its spikes on the tail. The type specimen of S. ungulatus was found with eight spikes, but they were moved away from their otriginal position. It is not clear yet, that it indicates the presence of four pairs of spikes in the same specimen, or they came from a different individual. However, this latter option is more likely[5].

Stegosaurus stenops is the best known species of the genus. It was described in 1887 by Marsh[6], and the meaning of its name is "narrow-faced roof lizard". Its fossils were found in Garden Park, north of Canon City, and its fossil material includes at least one complete skeleton. The plates of this species were broad and proportionally larger, and it had rounded tail plates. The plates had an altering arrangement, in a staggering double row. Stegosaurus stenops was about 7 meters long.

Stegosaurus sulcatus was described in the same work of Marsh in 1887[6], but its type specimen had only a partial skeleton. It can be differentiated from other Stegosaurus species chiefly on the basis of its large and furrowed spikes, which had large bases. Probably one of the large spikes found with the type specimen belongs to the shoulder hip, which is quite unusual in the other species of the genus. Some authors regard it possibly blonging to a separate genus[7].

Stegosaurus armatus, which was the first species of the genus to be described by Marsh in 1877[1], due to the quite fragmentary state of the holotype, is considered now as a nomen dubium, and it was replaced by Stegosaurus stenops as the type species of the genus[8]. Stegosaurus armatus, described by Marsh in 1881, is considered to be a nomen nudum, due to its lost material and unadequate description[9]. Stegosaurus duplex is now considered as a junior synonym of S. ungulatus[5]. Some further species, like S. marshi, S. priscus, S. longispinus, and S. madagascariensis were reassigned to different genera[7][10][11].

Smiley face

Stegosaurus stenops. By DataBase Center for Life Science (DBCLS) - https://doi.org/10.7875/togopic.2020.186, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=93725152

Description and anatomy

Stegosaurus species were large, quadrupedal herbivorous dinosaurs, with massive body-structure, having rounded backs and shorter forelegs than hindlegs. Their most spectacular distinctive feature was a double-row of kite-shaped plates, in a vertical arrangement along the back, and the two pairs of spikes in the posterior part of the their tail. The largest specimens could grow up to 9.1 meters long, and could weight even 5.3-8.1 tons [12][13]. Stegosaurus had a long and narrow skull, which was very small compared to its body. It had small, triangular, and flat teeth, which helped them to grind the food. Compared to the overall body-size of this dinosaur, its braincase was quite small compared to other dinosaurs. Stegosaurus had between 17-22 dermal back-plates and spines [14], and these were not directly jointed to the skeleton. These so-called osteorerms had a bony core, in a similar way like the those present in todays crocodiles.The largest of these plates could reach even 60cm in hight and 60cm in breadth[14]. It is likely that these plates were covered by keratinous sheaths, possibly providing to the plate as a whole sharp cutting edges[15]. Probably the plate arrangement on the back varied between species. Based on fossil evidence, it seems that S. stenops had it plates in two staggered rows, while S. ungulatus might have at least the ones on it tails arranged in pairs[5].

Paleobiology

Stegosaurus was first believed by Marsh to be bipedal due to its shorter forelegs[16], but later he changed his mind considering the estimated body mass of the dinosaur[17]. The capability of Stegosaurus for rearing up on its hindlegsand and using its tail for support is a matter of debate[18][19]. Track fossils suggest that this dinosaur lived in herds, composed of different aged animals[20]. The plates of Stegosaurus probably played a role in thermoregulation, as their outer surface was highly vascularised, thus the air around them could cool down the blood[21]. However, these also could be helpful in absorbing the heat from the sun[22]. They also might play a role in intimidating predators[9] or in sexual dimorphism[23]. When being attacked, due to its longer and strong hindlegs and its mass centre near to the hip, Stegosaurus could its rear maneuver easily, allowing to keep its tail pointed at the predator[24]. The use of thagomizers (tail spikes) in the animal's defence was supported by a punctured tail vertebra of Allosaurus, into which a thagomizer fits perfectly[25]. Due to a large canal in the hip region of the spinal cord, Stegosaurus was believed for long time to have a second brain in its hip, which might have beeen responsible for controlling reflexes in the caudal part of the body[26]. However, other dinosaurs, and even birds also possess such sacro-lumbar expansion, which contains the so called glycogen body, which serves stored energy supply for the nervous system[27]. Stegosaurus was a herbivorous dinosaur. Stegosaurus had small, peg-shaped teeth with horizontal wear facets, and the movement of its jaws was probably limited to up-down movements[28]. The low position of the skull suggests that this dinosaur browsed the low-growing vegetation. Probably their diet consisted of fruits, moss, ferns, and low-growing horsetails, cycads, and conifers.

Scientific references

[1] Marsh, O. C. (1877): A new order of extinct Reptilia (Stegosauria) from the Jurassic of the Rocky Mountains. American Journal of Science, 3 (14): 513-514. doi: 10.2475/ajs.s3-14.84.513

[2] Carpenter, K.; Galton, P.M. (2001): Othniel Charles Marsh and the Myth of the Eight-Spiked Stegosaurus. In Carpenter K (ed.). The Armored Dinosaurs. Indiana University Press. pp. 76-102. ISBN 978-0-253-33964-5. [3] Marsh, O. C. (1879): Notice of new Jurassic reptiles. American Journal of Science, 3 (18): 501-505. doi10.2475/ajs.s3-18.108.501

[4] Escaso, F.; Ortega, F.; Dantas, P.; Malafaia, E.; Pimentel, N.L.; Pereda-Suberbiola, X.; Sanz, J.L.; Kullberg, J.C.; Kullberg, M.C.; Barriga, F. (2007): New Evidence of Shared Dinosaur Across Upper Jurassic Proto-North Atlantic: Stegosaurus From Portugal. Naturwissenschaften, 94 (5): 367-74. doi: 10.1007/s00114-006-0209-8.

[5] Galton, P.M. (2010): Species of plated dinosaur Stegosaurus (Morrison Formation, Late Jurassic) of western USA: new type species designation needed. Swiss Journal of Geosciences. 103 (2): 187-198. doi: 10.1007/s00015-010-0022-4.

[6] Marsh, O. C. (1887): Principal characters of American Jurassic dinosaurs, part IX. The skull and dermal armour of Stegosaurus. American Journal of Science. 3 (34): 413-17. doi: 10.2475/ajs.s3-34.203.413.

[7] Maidment, S.C.R.; Norman, D.B.; Barrett, Paul M.; Upchurch, P.(2008): Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia). Journal of Systematic Palaeontology. 6 (4): 1. doi: 10.1017/S1477201908002459.

[8] Maidment, S.C.R.; Brassey, C.; Barrett, P.M.(2015): 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. PLOS ONE. 10 (10): e0138352. doi: 10.1371/journal.pone.0138352.

[9] Gilmore, C.W. (1914): Osteology of the armored Dinosauria in the United States National Museum, with special reference to the genus Stegosaurus. Series: Smithsonian Institution. United States National Museum. Bulletin 89 (89).

[10] Galton, P.M.; Carpenter, K.(2016): The plated dinosaur Stegosaurus longispinus Gilmore, 1914 (Dinosauria: Ornithischia; Upper Jurassic, western USA), type species of Alcovasaurus n. gen. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. 279 (2): 185-208. doi: 10.1127/njgpa/2016/0551.

[11] Galton, P.M. (1981): Craterosaurus pottonensis Seeley, a stegosaurian dinosaur from the Lower Cretaceous of England, and a review of Cretaceous stegosaurs. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 161 (1): 28-46.

[12] Holtz, Thomas R. Jr. (2012): Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2011 Appendix.

[13] Benson, R.B.J.; Campione, N.E.; Carrano, M.T.; Mannion, P.D.; Sullivan, C.; Evans, David C.; et al. (2014): Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage. PLOS Biol. 12 (5): e1001853. doi: 10.1371/journal.pbio.1001853.

[14] Revan, A. (2011): Reconstructing an Icon: Historical Significance of the Peabody's Mounted Skeleton of Stegosaurus and the Changes Necessary to Make It Correct Anatomically. Doctoral dissertation, faculty of the Department of Geology and Geophysics, Yale University.

[15] Christiansen, N. A.; Tschopp, E. (2010): Exceptional stegosaur integument impressions from the Upper Jurassic Morrison Formation of Wyoming. Journal of Geosciences, 103 (2): 163-171. doi: 10.1007/s00015-010-0026-0.

[16] Marsh, O.C. (1880): Principal characters of American Jurassic dinosaurs, part III. American Journal of Science. 3 (19): 253-59. doi: 10.2475/ajs.s3-19.111.253.

[17] Marsh, O.C. (1891): Restoration of Stegosaurus. American Journal of Science, 3 (42): 179-81. doi: 10.2475/ajs.s3-42.248.179.

[18] Bakker, R.T. (1978): Dinosaur feeding behavior and the origin of flowering plants. Nature, 274 (5672): 661-63. doi: 10.1038/274661a0.

[19] Carpenter, K. (1998): Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park, Colorado. Modern Geology, 23: 127-44.

[20] Rajewski, G. (2008): Where Dinosaurs Roamed. Smithsonian: 20-24.

[21] de Buffrénil, V. (1986): Growth and Function of Stegosaurus Plates". Paleobiology. 12 (4): 459-73. doi: 10.1017/S0094837300003171.

[22] Main, R.P.; Padian, K.; Horner, J. (2000): Comparative histology, growth and evolution of archosaurian osteoderms: why did Stegosaurus have such large dorsal plates?. Journal of Vertebrate Paleontology, 20: 56A.

[23] Davitashvili L (1961). The Theory of sexual selection. Izdatel'stvo Akademia nauk SSSR. p. 538.

[24] Bakker, R.T. (1986): The Dinosaur Heresies. William Morrow, New York. ISBN 978-0-8217-2859-8.

[25] Carpenter, K.; Sanders, F.; McWhinney, L.; Wood, L. (2005): Evidence for predator-prey relationships: Examples for Allosaurus and Stegosaurus. In Carpenter K (ed.). The Carnivorous Dinosaurs. Indiana University Press. pp. 325-50. ISBN 978-0-253-34539-4.

[26] Fastovsky, D.E.; Weishampel, D.B. (2005): Stegosauria: Hot Plates. In Fastovsky DE, Weishampel DB (eds.). The Evolution and Extinction of the Dinosaurs (2nd ed.). Cambridge University Press. pp. 107-30. ISBN 978-0-521-81172-9.

[27] Buchholz (née Giffin) EB (1990). "Gross Spinal Anatomy and Limb Use in Living and Fossil Reptiles". Paleobiology. 16 (4): 448-58. doi: 10.1017/S0094837300010186.

[28] Galton, P.M.; Upchurch, P. (2004): Stegosauria. In: Weishampel DB, Dodson P, Osmólska H (eds.). The Dinosauria (2nd ed.). University of California Press. ISBN 978-0-520-24209-8.