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Dravidosaurus ("South India lizard") is a dubious genus of Late Cretaceous reptiles, variously interpreted as either a Ornithischia (possibly a Stegosauria) dinosaur or a plesiosaur. The genus contains a single species, D. blanfordi, known from mostly poorly preserved fossils from the Coniacian (Late Cretaceous) of southern India.
Dravidosaurus was originally described as a late-surviving stegosaur in 1979, younger in age than other known stegosaurs by tens of millions of years. This classification was questioned by Sankar Chatterjee in 1991, who suggested that the fossils were actually plesiosaurian. Chatterjee did however not formally reclassify any of the fossil specimens and did not examine all of them. Since 1991, researchers have variously followed Chatterjee's assessment, maintained Dravidosaurus as a stegosaur, or considered it an indeterminate ornithischian dinosaur.
Researchers in favor of a stegosaurian identity point to the presence of plates and spikes among the fossils, as well as certain morphological features. In 2017, Peter Galton and Krishnan Ayyasami reaffirmed that Dravidosaurus was a stegosaur and announced that further likely stegosaurian fossils from the same original site were currently being studied.
The fossils attributed to Dravidosaurus included the holotype GSI SR Pal 1, a partial skull, as well as fossils identified as an isolated tooth, a sacrum, an ilium, an ischium, ten armor plates, and a Thagomizer, designated (in order) as GSI SR Pal 2–7. Yadigiri and Ayyasami identified several of the skull bones in GSI SR Pal 1, of which the most well-preserved were the Parietal bone, Frontal bone, supraorbitals, Squamosal bone, and Quadrate bone. In addition to the armor plate GSI SR Pal 6, nine other fossils identified as armor plates were found associated with the referred specimens. These fossils have since their discovery been housed in the Palaeontological Laboratory of the Geological Survey of India. The fossils attributed to Dravidosaurus were at the time of its description determined to not be worn and to indicate that there had not been much transportation before burial. The hard limestone matrix around the fossil made extraction and preparation, done using a dental drill and chiselling. As of 2024, several bones originally attributed to Dravidosaurus is not available in the collections of the Geological Survey of India.
The generic name Dravidosaurus comes from south india, a term often used for the southern part of India in which the Trichinopoly Group is situated. Dravidosaurus thus literally means "Dravidanadu lizard", though the name is sometimes interpreted as "lizard from south India". The specific name blanfordi honours H.F. Blanford, responsible for the pioneering research on the Cretaceous in southern India.
Since they also identified what they considered to be diagnostic traits among the fossils, differentiating them from other stegosaurs known at the time, Yadagiri and Ayyasami erected the new genus Dravidosaurus. In terms of the proportions of the skull itself, Dravidosaurus was determined to be similar to Stegosaurus. Among the features that distinguished GSI SR Pal 1 were the postfrontal being absent, the beak being slightly different from that of Stegosaurus, the Postorbital bone being thin and straight, and the Pterygoid bone being thick and rectangular. GSI SR Pal 2 was distinguishable from the similar teeth of Kentrosaurus through possessing three rather than six crenulations. In addition to these features, Yadagiri and Ayyasami also distinguished Dravidosaurus by features of its sacrum, which indicated that it possessed ribs that were more slender than those of Kentrosaurus.
If Dravidosaurus blanfordi was a stegosaur, it would have been one of small size. In fact, at an estimated length of just three metres (10 ft), Dravidosaurus would be the smallest known stegosaur. Its narrow skull was reconstructed by Yadagiri and Ayyasami to have measured 20 centimetres (7.9 in) long and 7 centimetres (2.8 in) wide, making it proportionally smaller relative to the rest of the body when compared to stegosaurs known at the time. One part of the skull, identified as the anterior portion of the premaxilla, preserved parts of a stout, curved up, and pointed beak, 3.5 centimetres (1.4 in) long. Among the fossils of Dravidosaurus, Yadagiri and Ayyasami identified ten armor plates and a spike. The plates were largely triangular in shape, with stout bases. They were otherwise relatively thin, about 1 centimetre (0.4 in) in thickness. The referred plates ranged in height from 5 to 25 centimetres (2–9.8 in) and in length from 3 to 15 centimetres (1.2–5.9 in). The spike, identified as a tail spike, measured 15 centimetres (5.9 in) in length and was slightly curved. Assuming a stegosaurian identity, this spike possessed a notable unique trait in that it had an expanded middle region; it bulged at the center with a diameter of 3 centimetres (1.2 in) before tapering towards the base, where it had a diameter of 2.2 centimetres (0.9 in). If Dravidosaurus was a stegosaur, it would like other stegosaurs have been herbivorous.
Opinions on Dravidosaurus have varied within the palaeontological community following Chatterjee's reclassification. Dravidosaurus was still considered a stegosaur, without comment, by Carpenter & Currie (1992) and Loyal, Khosla & Sahni (1998). Several more recent works have either supported Chatterjee's opinion that the fossils are plesiosaurian, such as Verma (2015), Verma et al. (2016), and Rozadilla et al. (2021), or maintained that independent redescription and assessment of it is needed, such as Maidment (2010). Wilson, Barrett & Carrano (2011) listed Dravidosaurus as an ornithischian, though did not view this as "demonstrable". Tidwell & Carpenter (2005) considered Dravidosaurus to be a "questionably identified ornithischian dinosaur". Khosla & Lucas (2020) likewise referred to Dravidosaurus as an ornithischian dinosaur, though noted that its taxonomic validity was "under discussion".
Chatterjee's suggestion that Dravidosaurus was a plesiosaur was first explicitly questioned by Peredo Superbiola et al. (2003). This study pointed out that the skull and armor plates figured in the original description, specimens Chatterjee had admittedly not examined, were "certainly not plesiosaurian" but also stated that the fossils were in need of redescription. Similar criticism was offered by Peter Galton & Upchurch (2004), who also noted that the skull and armor plate described in 1979 could not be from a plesiosaur and consequently maintained Dravidosaurus as a stegosaur. Fastovsky & Weishampel (2005) followed Galton & Upchurch's opinion, noting that features of the skull as well as the presence of plates and spikes suggested that Dravidosaurus was a stegosaur. In 2012, Galton again affirmed his belief that Dravidosaurus was a stegosaur due to the presence of plates and a stegosaur-like tooth among the material. Galton also encouraged new examinations of the specimens. The 'Dino Directory' of the London Natural History Museum, written by Paul Barrett, considers Dravidosaurus to be a stegosaurian dinosaur, noting that its fossils were "once thought" to have been plesiosaurian but also that its taxonomical classification is not yet agreed.
In 2017, Galton and Ayyasami together reaffirmed the stegosaurian classification of Dravidosaurus, stating that they saw no similarities between the photographs of the fossils of Dravidosaurus in its original description and the pelvic and hindlimb elements of plesiosaurs. They noted that the small tooth referred to Dravidosaurus was especially unlikely to be plesiosaurian. Furthermore, Ayyasami announced that he was in the process of working on new undescribed and likely stegosaurian bones from the original site of the Dravidosaurus fossils.Peter M. Galton; Krishnan Ayyasami (2017). " Purported latest bone of a plated dinosaur (Ornithischia: Stegosauria), a "dermal plate" from the Maastrichtian (Upper Cretaceous) of southern India". Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. 285 (1): 91–96. doi:10.1127/njgpa/2017/0671.
In addition to the marine life found in the upper Anaipadi Formation, terrestrial matter was in the area evidently prone to being carried out to sea. Among other finds recovered in the unit are for instance a large amount of petrified wood. The presence of large quantities of wood indicates that land with dense vegetation was located relatively close to the marine environment in which the Dravidosaurus fossils were buried, meaning that it is not impossible that it (if a terrestrial animal) could have been carried out to sea.
Other than Dravidosaurus, no prospective dinosaur fossils have been reported from the Anaipadi Formation or the Trichinopoly Group as a whole. The overlying Ariyalur Group, which dates to the Campanian and Maastrichtian, has however preserved scant theropod and sauropod fossil material and, according to Yadagiri and Ayyasami in 1979, possibly further stegosaurian fossils. These supposed even later stegosaurian fossils have however never been figured or formally described. In 2017, Galton and Ayyasami reinterpreted some previously assigned Maastrichtian "stegosaur" fossils as sauropod bones but noted that stegosaurs may still have survived to the Maastrichtian in India due to the presence of the ichnogenus Deltapodus, commonly identified as stegosaurian footprints, in the Maastrichtian-age Lameta Formation.
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