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Major subgroups

Tuojiangosaurus, Field Museum, ChicagoStegosaurus, Yale Peabody Museum
Two familiar stegosaurs: Tuojiangosaurus (Field Museum traveling exhibit, Chicago) and Stegosaurus (Yale Peabody Museum, New Haven, CT.)

Compared to other dinosaur subgroups, Stegosauria is a relatively small clade, with at most 13 valid genera spanning only a limited time interval from the Middle Jurassic until the Early Cretaceous.  Scientists largely agree that Stegosauria is the sister taxon (most closely related group) to Ankylosauria, the group of tank-like and heavily armored herbivores that includes Ankylosaurus, Edmontonia, Euoplocephalus, and Pinacosaurus.  Together, Stegosauria and Ankylosauria, along with a handful of poorly-known primitive forms (Scelidosaurus, Scutellosaurus, Emausaurus), comprise the Thyreophora, a major group of ornithischian (bird-hipped) dinosaurs.  However, while the position of stegosaurs within the broader family tree of dinosaurs is well-constrained, the genealogical relationships within Stegosauria are poorly known and controversial.  Very few studies have attempted to untangle these relationships, and the few published analyses are largely based on literature descriptions and not personal observation of many important stegosaur fossils.  Susannah Maidment is undertaking a careful analysis of stegosaur phylogeny and evolutionary as part of her doctoral dissertation at Cambridge University, and until her study is complete it is premature to speculate on stegosaur interrelationships.  Thus, I will briefly describe several stegosaur genera individually below.

Stegosaurus, Carnegie MuseumStegosaurus

Certainly the best-known stegosaur, Stegosaurus is known from the Upper Jurassic Morrison Formation of the western United States.  This genus was first described by O.C. Marsh in 1877, and today is known from several specimens that may represent up to four different species.  The plates of Stegosaurus are large, broad, and plate-like, and the tail contained either two or four pairs of robust spikes.  The throat region is covered by a series of small osteoderms, but parascapular spines in the shoulder region are absent.  Mounted specimens of Stegosaurus can be seen in several American museums, including the Carnegie Museum in Pittsburgh (photo), the Peabody Museum at Yale University, and the Denver Museum of Natural History and Science.


Tuojiangosaurus, Field Museum, ChicagoTuojiangosaurus

One of several Chinese stegosaurs, Tuojiangosaurus is known from two partial skeletons discovered in Late Jurassic rocks in Sichuan Province.  A total of 17 pairs of plates and spines ornament the back and tail.  Some plates are spherical in the neck region, and overall the plates are smaller and more robust than those in Stegosaurus.  Vertebrae located near the base of the tail possess one unique character: thin plates on the neural spines.  This stegosaur likely reached lengths of approximately seven meters.  Photo to the left was taken by the author at a traveling exhibit at the Field Museum of Natural History in Chicago.





Kentrosaurus, BerlinKentrosaurus

The best-known African stegosaur, Kentrosaurus is represented by an array of isolated cranial, vertebral, and limb elements from the Upper Jurassic Tendaguru Formation in Tanzania.  This material includes multiple examples of the same bones, which has allowed scientists to assess growth patterns and sexual dimorphism (morphological differences between males and females) in this genus.  Kentrosaurus is characterized by a handful of unique features, including simple cheek teeth, a prominent foramen in the quadrate bone of the skull, and tail vertebrae with kinked neural arches.  Photo at left was taken by S.C.R. Maidment at Museum fur Naturkunde, Berlin.






Huayangosaurus, MelbourneHuayangosaurus

One of the oldest stegosaurs, Huayangosaurus is known from the Middle Jurassic of Sichuan Province, China.  It is represented by a single well-preserved skeleton, along with other fragmentary postcranial elements.  Although stegosaur phylogeny is still largely unresolved, published analyses consider Huayangosaurus to be the most primitive known member of the group.  Unique features are numerous, and include a oval-shaped depression between the premaxilla and maxilla bones of the skull, a high tooth count in the maxilla (25-30 teeth), and a single fused carpal block in the hand.  Photo above was taken by S.C.R. Maidment at a traveling exhibit in Melbourne, Australia.

Paranthodon, BMNHParanthodon

Paranthodon is a poorly-known yet intriguing stegosaur genus recovered from the Upper Jurassic Kirkwood Formation of Cape Province, South Africa.  It is only known from a partial skull, shown at left (photo courtesy of S.C.R. Maidment, British Museum of Natural History, London).  Its teeth are similar to those in other stegosaurs, but the snout region is characterized by a long and broad process on the premaxilla that extends caudally (backwards).




Hesperosaurus, SauriermuseumHesperosaurus

As with Stegosaurus, Hesperosaurus is known from the Upper Jurassic Morrison Formation of the United States.  It was originally described based on a single skeleton (approximately 5-6 meters long), but a second specimen has recently been discovered.  This specimen is shown at left (photo courtesy of S.C.R. Maidment, Sauriermuseum, Switzerland).  Unique features of Hesperosaurus include expanded ribs in the neck region and ossified tendons in the back region.  Ossified tendons are known in many other herbivorous dinosaurs, but absent in all other stegosaurs.  As Hesperosaurus is known from the same deposits as Stegosaurus and the two genera are similar in many features, it is possible they are synonymous.



Wuerhosaurus, IVPP, BeijingWuerhosaurus

One of the most fragmentary stegosaurs ever described, Wuerhosaurus is known from various postcranial elements discovered in Lower Cretaceous rocks in two different locations in China. Fossils from each locality have been described as separate species.  Little can be said about the anatomy of this animal, but several unique features have been proposed, including: elongate neural spines in vertebrae at the base of the tail, an expanded end of the ischium bone of the pelvis, and long and large plates.  The image to the left is a reconstruction of the pelvic girdle and sacral vertebrae in dorsal view, courtesy of S.C.R. Maidment.  The original specimen is located in the Institute of Palaeontology and Palaeoanthropology in Beijing.







Other Stegosaurs

Other stegosaur genera not discussed here include: Chungkingosaurus, Craterosaurus, Dacentrurus, Gigantspinosaurus, and Lexovisaurus.  For more information on these genera, see P.M. Galton and P. Upchurch (2004).

Author: Stephen Brusatte
Last updated: 01/11/2006
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