Suborder Stegosauria

- image by  Brenner Fishman

The Stegosaurs are a distinctive component of the Jurassic megafauna. These striking dinosaurs are characterised by pairs of bony plates along the back (presumably for the purpose of thermoregulation) and varying numbers of pairs of spikes along the tail and, in some species the back, hips, and even shoulders.  Despite their protective spikes, they were never a very diverse group.  Reasonably common during the middle and late Jurassic, the stegosaurs were almost wiped out by the Tithonian extinction event.  A few stragglers continued on to the early or middle Cretaceous.

The following taxonomic arrangement is a tentative presentation of Stegosaurid systematics, based on Linnean/Evolutionary Systematic classification of the Stegosaurs. (note - the following is unofficial and should not be taken as authoritative!) Some of these groupings are not formally named and in fact future discoveries may reveal a completely different picture. As most biologists and paleontologists now advocate the Cladistic stance, the following is therefore suggested to complement (but not replace) the current paradigm.

1. cheeks 2. armor 3. armor as plates or spines on back 4. armor as plates 5. armor as spines 6. armor as covering or small plates over back 7. uneven enamel on teeth 8. jaw joint below tooth rows 9. bony shelf on back of head

Note that this cladogram only includes the  Genasauria and not some more primitive forms like the Fabrosauria.

family Huayangosauridae

The Huayangosaurs were early ancestral forms that lived during the early to the middle Jurassic. They are mostly a lot smaller than the later stegosaurs. The skull is short and high relative to later stegosaurs, and retains the archosaurian antorbital fenestra (opening in front of the eyes). Although Huayangosaurus is the only certain form, it is likely that other earlier types like Tatisaurus and Emausaurus also belong here. An early Cretaceous jaw of uncertain affinites called Regnosaurus northamptoni has been placed in this group, on the basis of its strong resemblance to the lower jaw of Huayangosaurus, but in view of the incomplete nature of the specimen and its much later date (contemporary with the latest advanced stegosaurines) I remain sceptical.

Huayangosaurus taibaii Dong, Tang, and Zhou, 1982

Comments: Huayangosaurus is one of the best-known stegosauroids, with remains belonging to over a half dozen individuals. The skull (left) is deep with a relatively short snout, and has what may be little "horns," which may be a gender-related character. It is also unique for having armor scutes, something like those of scelidosaurs and ankylosaurs, in a row down its sides. This is one of the things that indicate that these three groups may be combined in a single clade, the Thyreophora. (I have not given the Thyreophora a linnean ranking here). Huayangosaurus is protected by two pairs of tail spikes and a pair of spines on the shoulders

Tatisaurus oehleri Simmons, 1965

Comments: It is still controversial whether or not Tatisaurus is a proper stegosaurian, but it is the earliest known (but not the most primitive) member of the dinosaurian clade Thyreophora, a group that included various types of armoured dinosaurs.  It is still not very far removed from its Fabrosaur ancestors, but within a few million years Tatisaurus, or a form like it, evolved into large armoured quadrepeds. This poorly-known ornithischian has been related to Scelidosaurus, but seems to be more closely related to the stegosauria. Possibly it represents a form transitional between or ancestral to the two groups.

Emausaurus ernsti Haubold, 1991

Comments: This animal has been compared to a miniature version of Huayangosaurus, the skull of which is very similiar.

family Stegosauridae

The most diverse and advanced of the Stegosauria, the family Stegosauridae included two subfamilies and it would seem several tribes.  The more advanced genera, such as Stegosaurus, had large bony plates aranged in alternating pairs along the back.  These would have been very efficent heat regulation devices.  Other more primitive types had smaller plates and pairs of spikes along the tail and, in some species the back, hips, and even shoulders.   The unusually large hind limbs and high hips served a double purpose: for rearing up to browse on trees and to swing the spiked tail more efficently

Paul E. Olsen divides the stegosauria into two families, Kentrosauridae and Stegosauridae.  More usually however Kentrosaurus is included within the Stegosauridae.  Perhaps a compromise would be to have the Kentrosaurs and Stegosaurs as two subfamilies of the family Stegosauridae.  The more primitive Kentrosaurines would then be transitional betweneen the very primitive Dacentrurinae and the more advanced Stegosaurinae.

subfamily Dacentrurinae

These were late persisting primitive types, that seem to have characteristics indicating a type ancestral to other stegosaurids (even if they themselves are too late in time to be an actual ancestor). The vertebrae are more primitive and the forelimbs longer in comparison to the hind-limbs. Although Dacentrurus armatus is the only certain species, an early femur has been compared top that species.

Dacentrurus armatus Lucas, 1902b

Comments: This is the first stegosaur described by science, although the original name, Omosaurus, had to be changed to Dacentrurus, (as it was already applied to another animal). It is the most primitive known stegosaurid, with primitive vertebrae and long forelimbs, and some researchers have suggested it deserves to be put in a separate family. It is also among the largest of the stegosaurs. There are also a number of specilised features in the vertebrae and hip that prevent from being an actual ancestor. As with many early-named European dinosaurs, the species seems to be something of a garbage taxon for fragmentary remains of a generally similiar type, as it is unlikely that one species would persist for a period of some 10 or 12 million years or so. Some remains suggest individuals of 10 meters in length, larger than the biggest Stegosaurus.

?Dacentrurus phillipsi (Seeley, 1893)

Comments: "O." phillipsi is based, like "O." vetustus, on the femur from a juvenile stegosaurid. In may be a very primitive stegosaurid, possibly a species of Dacentrurus.

Subfamily Kentrosaurinae

These are primitive stegosaurids, small to large in size, with generally small spikey plates and numerous spikes along the tail, and sometimes the hips and shoulders as well.  A paraphyletic group

Kentrosaurus aethiopicus Hennig, 1915

Comments: Kentrosaurus is the best known of the primitive spiny stegosaurs. The six pairs of plates are small and located on the neck to the mid back, with the sixth pair transitional between plate and spine. Beyond this are five large pairs of spines along the back and tail, as well as a pair of prominent shoulder spines. Most specimens known are juveniles. The skull contains a number of primitive features. This animal lived at the same time as the larger and more advanced Stegosaurus.

Chialingosaurus kuani Young, 1959

Comments: High narrow skull, slender limb-bones, small plate-like spines

Chungkingosaurus laminaplacodus Dong, Zhou, & Zhang,1983

Comments: The skull is high and narrow skull, and the sacrum and humerous primitive, all indicating a less-specialised form then Stegosaurus or Tuojungosaurus. There are large thick spine-like bony plates (intermediate between spines and plates) and apparently four or five pairs of tail spines. May be transitional between Kentrosaurines and Stegosaurines

Craterosaurus pottonensis Seeley, 1874

Comments: fragmentary remains - relationships uncertain; tentatively assigned to this group

subfamily Stegosaurinae

These are advanced stegosaurids, with generally larger plates and spikes that tend to be limited to the end of the tail, although primitive forms still retain additional spines. One could distinguish two tribes, the paraphyletic or ancestral Tuojiangosaurini, and the specialised Stegosaurini

tribe "Tuojiangosaurini"

A paraphyletic taxon; intermediate between the Kentrosaurs (which they resemble) and the Stegosaurini, Generally retain the Kentrosaurine pattern of back and hip spines

Lexovisaurus durobrivensis (Hulke, 1887)

Comments: Superficially quite similar to Kentrosaurus, this early stegosaurine is a more closely related to Stegosaurus. It has large thin plates, and spines on the hip. In view of the time difference i is unlikely that the fragmentary Kimmeridgian material belongs to the same species as the Callovian material

Lexovisaurus? vetustus (Huene, 1910)

Comments: tentatively assigned to the genus Lexovisaurus, although the material is too incomplete to be sure. The oldest known stegosaurid

Monkonosaurus lawulacus Zhao, 1983

Comments: Dermal plates much like Stegosaurus. Sacral neural spines broad and lower than other stegosaurs. However the large thin bony plates and closed sacral fenestrae are progressive features, this poorly known animal is therefore very tentatively assigned to this group

Paranthodon africanus (Broom, 1910)

Comments: tentatively assigned to this group

Yingshanosaurus jichuanensis Zhou, 1984

Comments: broad spikes on shoulders; seems to be closely related to Tuojiangosaurus.

Tuojiangosaurus multispinus Dong, Li, Zhou, & Zhang,1973

Comments: Tthe largest known Jurassic Asian stegosaur. The skull (left) is similiar to that of Stegosaurus. There 17 pairs of small spiny plates, as well as spines on the shoulders, hips, and tail

tribe Stegosaurini

Stegosaurines are distinguished by large size, no spines on hips or shoulders, only two (or rarely 4) pairs of tail spines, relatively short forelimbs, small elongate head, large plates in life covered with skin and blood vessels. This tribe includes only two genera, Stegosaurus and Wuerhosaurus

The skull of Stegosaurus. Key to abbreviations

Note on Stegosaurus species: The controversial dinosaurologist Bob Bakker distinguishes between two types of Stegosaurs - those with long legs and relatively smaller plates (Stegosaurus proper) and a more primitive form with shorter legs and larger plates (Diracodon).  Dinogeorge however is critical of proposing a new stegosaur genus on this basis.  (seeDinosaur Genera List corrections #63).  See alsocomments by Stanley Friesen (Dinosaur Reference Center).  Also, different sources (both in print and on the Web) give different lists of species; e.g. some are synonymised with others.   It may be that spme of these species are only subspecies or local variants.  However, for the sake of completeness (and with the risk of further muddling the issue.... ) I have listed all the types here.

Stegosaurus armatus armatus Marsh,1887

Comments: The type species of the genus. Has longer legs and relatively smaller plates than S. stenops; 2 pairs of tail spines

Stegosaurus armatus ungulatus Marsh, 1879

Comments: Along with Dacentrurus armatus, this is the largest known stegosaurid. The limbs are long and slender. It has four pairs of tail spines, and the middle part of the tail bears paired spine-plates. The sacral centra are broad and rounded and lack a ventral keel. This may be variant of S. armatus, rather than a distinct species.

Stegosaurus stenops stenops Marsh, 1877

Comments: S. stenops has very large plates and relatively shorter legs. The sacral centra have a ventral keel. There are two pairs of tail spines. Bob Bakker sugests this species should be relocated under its original genus Diracodon

Stegosaurus stenops laticeps Marsh, 1881

Comments: This form may be same as S. stenops

Stegosaurus longispinus Gilmore,1914

Comments: This species is distinguished by two pairs of very elongate (a meter in length) tail spines

Wuerhosaurus homheni Dong,1973

Comments: Among the last of the stegosaurs, Wuerhosaurus is closely related to Stegosaurus, with a similiar arrangement of plates and spines. It differs in that Wuerhosaurus's plates are long and low, unlike the tall lozenge-shaped plates of Stegosaurus.

Wuerhosaurus ordosensis Dong,19p3

Comments: A small species of this genus

Stegosaurid brains

Stegosaurids had small brains by Dinosaurian standards, although not so small as to fall below the average range of reptilian brain to body weight ratio (the dim-witted dinosaur - all brawn and no brain is in fact a myth).  Stegosaurus is famous for its "second brain" - actually an enlarged pelvic nerve plexus situated in the hip region.  In fact all dinosaurs and birds and many other Sauropsids have such organs (so do mammals, but not developed to such an extent).

This figure shows the Kentrosaurus brain (a) next to the much larger and pelvic nerve ganglion (b)

Stegosaurid thermoregulation

The photograph shows the side and top-bottom view of the large plates on the back of the Stegosaurus.  Originally thought to have been a form of armour-plating, the plates of advanced stegosaurs were, as can be seen here, actually quite thin and full of holes for the accomodation of blood vessels.  For all large animals heat disipation is a big problem (that is why African elephants have such large ears for example), and the Stegosaur plates clearly served this purpose.  The plates tend to be small and poorly developed (or even absent) in primitive Stegosaurids, but more developed in more advanced forms, indicating that they conferred an adaptive advantage.

Links

Stegosauroidea - Thescelosaurus!

Stegosauria cladogram, short essay, and list of all known genera.

The Real Jurassic Park - Morrison and Tendaguru Formations - part of Professor Paul Eric Olsen's superb coverage of dinosaurs and their predecessors in the DINOSAURS AND THE HISTORY OF LIFE - GEOLOGY V1001x site

Stegosauria Mikko's phylogeny site

Jurassic Gallery - fantastic artwork by M. Shiraishi. In Japanese and English

Galton, P. M., 1990: Stegosauria. 435-455, in Weishampel, D. B., Dodson, P., & Osmólska, H. (eds.), The Dinosauria University of California Press, Berkley, Los Angeles, Oxford, 1990. A number of images from this chapter have been used to illustrate various species listed in the present page

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