Middle Jurassic - Early Cretaceous, 160–136 Ma
|Mounted skeleton of Stegosaurus stenops, Natural History Museum, London|
Stegosauridae is a group of armored dinosaurs comprising all species more closely related to Stegosaurus than to Huayangosaurus. Their fossil range extends into the early Cretaceous period, exceeding that of other stegosaurians, such as the huayangosaurids, which had died out by the end of the Jurassic. Stegosaurids are characterized by rows of plate-like osteoderms arranged vertically along the neck, body, and tail, which graded with varying abruptness into a series of spikes. These may have had a number of uses: display and/or thermoregulation are likely functions for the plates, while evidence suggests that the spikes were used at least partly for defense.
Stegosaurids were large and powerful dinosaurs. Their front legs were shorter than their rear, making them slow-moving. When Othniel Charles Marsh first found Stegosaurus, he portrayed the animal with very short front legs and neck, though new evidence shows that some species had relatively long front limbs. Several species show sexual dimorphism in the sacrum with the putative female having an extra pair of sacral ribs. There are also two sizes of parascapular spines in Lexovisaurus which can be ascribed to such variation.
In 2010 Míriam Reichel, using the 3D-modelling software ZBrush, created two digital models of Stegosaurus teeth differing in the presence or not of serrations. She found that Stegosaurus likely had beaks (rhamphotheca). She also found that stegosaurs were capable of shearing small branches, and suggested that the same data could be applied to other stegosaurids.
Evolution and classification
In contrast with early stegosaurians, like Huayangosaurus, stegosaurid skulls are shallower and the difference between the long hindlimb and short forelimb larger. The osteoderms of stegosaurids can attain a large size either as the broad plates of Stegosaurus or the long spikes of Kentrosaurus.
The earliest stegosaurian is thought to be Lexovisaurus from the Bathonian of England. There was found a massive femur of the juvenile Lexovisaurus. The youngest is Dravidosaurus from the Coniacian of India. Possible Maastrichtian stegosaurids may also exist in Indian deposits, but these fossils have not been described.
Stegosauridae was first defined by Paul Sereno in 2005 as the most inclusive clade containing Stegosaurus stenops but not Huayangosaurus taibaii. It is divided into two subfamilies: Dacentrurinae and Stegosaurinae. The subfamily Stegosaurinae was first recognized by Nopcsa in 1915. It comprises, according to the definition of Sereno (1998), the eponymous Stegosaurus and all stegosaurids more closely related to it than to Dacentrurus. A second subfamily, the Dacentrurinae, was proposed by Mateus et al. (2009) to include Dacentrurus and all stegosaurids closer in relation to it than to Stegosaurus.
More primitive stegosaurids, like Lexovisaurus, Kentrosaurus, and Tuojiangosaurus, are characterized by the comparatively large osteoderms running along their backs and reduced lateral osteoderms. Contrasting with the abrupt transition from plate to spike in stegosaurines, primitive stegosaurids have their plates grade into spines in a zone with osteoderms showing an intermediate shape.
The cladogram below follows an analysis by Mateus et al. (2009).
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