Dravidosaurus
| Dravidosaurus Temporal range: Late Cretaceous,
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| Holotype skull of Dravidosaurus (GSI SR Pal 1), with known parts in black and unknown parts in red (restored assuming a stegosaurian identity) | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia (?) |
| Genus: | †Dravidosaurus |
| Species: | †D. blanfordi
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| Binomial name | |
| †Dravidosaurus blanfordi Yadagiri & Ayyasami, 1979
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Dravidosaurus is a controversial taxon of Late Cretaceous reptiles, variously interpreted as either a ornithischian, possibly stegosaurian, dinosaur or a plesiosaur. The genus contains a single species, D. blanfordi, known from seven 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.
Discovery and naming
Dravidosaurus blanfordi was described in 1979 by P. M. Yadagiri and Krishnan Ayyasami,[1][2] based on fossils recovered from the Coniacian[3][4] Anaipadi Formation of the Trichinopoly Group in southern India.[5] The fossils were discovered at a site near the village of Siranattam.[5] The Dravidosaurus fossils were the first ornithischian dinosaur fossils reported from India.[6]
The fossils attributed to Dravidosaurus included the holotype GSI SR Pal 1, a partial skull, as well as a single tooth, a sacrum, an ilium, an ischium, an armor plate, and a tail spike, designated (in order) as GSI SR Pal 2–7.[7] These fossils have since their discovery been housed in the Palaeontological Laboratory of the Geological Survey of India.[7] The generic name Dravidosaurus comes from Dravidanadu, a term often used for the part of India in which the Trichinopoly Group is situated (see for instance Dravidian languages). The specific name honours William Thomas Blanford, responsible for the pioneering research on the Cretaceous in southern India.[7]
Classification and description
Classification debate
Examinations of the poorly preserved[8] fossils referred to Dravidosaurus have since their discovery caused some researchers to either doubt their identity as stegosaurian or consider the taxon a nomen dubium.[5][6][9] Most notably, the American palaeontologist Sankar Chatterjee visited the site in 1991 and expressed doubt that the fossils were dinosaurian at all. Chatterjee instead interpreted the Dravidosaurus fossil material he examined as the "highly weathered" pelvic and hindlimb elements of a plesiosaur, though presented no concrete morphological evidence.[3][10] Chatterjee and Dhiraj Kumar Rudra also described fossil plesiosaur material from the same site as the Dravidosaurus fossils in 1996.[3] Neither publication formally reidentified or reclassified any of the fossils.[4] In 1996, Chaterjee and Dhiraj K. Rudra still formally classified Dravidosaurus as "Stegosauria nomen dubium", though they once again stated that they during their 1991 visit "could not see anything related to the stegosaurian plates and skull claimed by these authors" and maintained that the bones they had seen might be plesiosaurian.[10]
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)[7] and Loyal, Khosla & Sahni (1998).[2] Several more recent works have either supported Chatterjee's opinion that the fossils are plesiosaurian, such as Verma (2015),[11] Verma et al. (2016),[12] and Rozadilla et al. (2021),[13] or maintained that independent redescription and assessment of it is needed, such as Maidment (2010).[3] Wilson, Barrett & Carrano (2011) listed Dravidosaurus as an ornithischian, though did not view this as "demonstrable".[14] Tidwell & Carpenter (2005) considered Dravidosaurus to be a "questionably identified ornithischian dinosaur".[15] Khosla & Lucas (2020) likewise referred to Dravidosaurus as an ornithischian dinosaur, though noted that its taxonomic validity was "under discussion".[16]
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.[4] Similar criticism was offered by 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.[8] 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.[17] In 2017, Galton & Ayyasami reaffirmed the stegosaurian classification of Dravidosaurus, stating that the photographs of the fossils of Dravidosaurus in its original description bore no resemblance to 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.[18]
Implications
No certain and undisputed stegosaurian fossil remains have been recovered in deposits from the Late Cretaceous. If Dravidosaurus was a stegosaur, it would consequently represent the last known member of the group by a timeframe of tens of millions of years. This would suggest either that the stegosaurian fossil record is poorly sampled throughout the world or that the stegosaurs persisted in what today is India for a long time after they had gone extinct elsewhere.[9]
If Dravidosaurus blanfordi was a stegosaur, it would have been one of small size;[7] at an estimated length of three metres (10 ft), it would be the smallest known stegosaurian species.[8] The preserved parts of the skull of Dravidosaurus are reportedly of similar proportions as the skull of Stegosaurus, though the skull was proportionally smaller than in other genera in regard to the rest of the body. Yadagiri and Ayyasami noted that the plates along the back of Dravidosaurus were large and triangular, with thick bases, and that the tail spikes had an expanded middle region, a unique trait.[8] Yadagiri and Ayyasami believed Dravidosaurus to be closely related to the European genus Dacentrurus.[19]
Palaeoenvironment
The Anaipadi Formation preserves fossils from a neritic environment (the relatively shallow part of the ocean above the drop-off of the continental shelf).[11] The Dravidosaurus fossils come from the upper portion of the unit, which is marked by the presence of the ammonite Kossmaticeras theobaldianum.[1] The Anaipadi Formation preserves a rich mollusc fauna,[11] including common fossils of ammonites and inoceramids,[20] as well as brachiopods. Fossils of marine reptiles have also been found, although they are rare.[11] It has been suggested that the abundant brachiopods and inoceramids in the upper Anaipadi Formation indicate a transgressive environment.[20]
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.[20]
See also
References
- ^ a b Yadagiri, P., and Ayyasami, K., (1979). "A new stegosaurian dinosaur from Upper Cretaceous sediments of south India." Journal of the Geological Society of India, 20(11): 521–530.
- ^ a b Loyal, Raminder S.; Khosla, Ashu; Sahni, Ashok (1998). "Gondwanan dinosaurs of India: Affinities and palaeobiogeography". Journal of African Earth Sciences. 26 (2): 630. doi:10.1016/S0899-5362(97)83516-6.
- ^ a b c d Maidment, Susannah C. R. (2010). "Stegosauria: a historical review of the body fossil record and phylogenetic relationships". Swiss Journal of Geosciences. 103 (2): 199–210. doi:10.1007/s00015-010-0023-3. ISSN 1661-8734.
- ^ a b c Pereda Superbiola, Xabier; Galton, Peter M.; Torcida, Fidel; Huerta, Pedro; Izquierdo, Luis Ángel; Montero, Diego; Pérez, Gustavo; Urién, Victor (2003). "First Stegosaurian Dinosaur remains from the Early Cretaceous of Burgos (Spain), with a review of Cretaceous stegosaurs". Estudios Geológicos. 55 (2): 148. doi:10.7203/sjp.18.2.21640.
- ^ a b c Parmar, Varun; Prasad, G. V. R. (2020). "Vertebrate evolution on the Indian raft - Biogeographic conundrums". Episodes Journal of International Geoscience. 43 (1): 461–475. doi:10.18814/epiiugs/2020/020029.
- ^ a b Prasad, Guntupalli V. R. (2020). "First Ornithischian and Theropod Dinosaur Teeth from the Middle Jurassic Kota Formation of India: Paleobiogeographic Relationships". Biological Consequences of Plate Tectonics. Springer. p. 22. doi:10.1007/978-3-030-49753-8_1.
- ^ a b c d e Carpenter, Kenneth; Currie, Philip J. (1992). Dinosaur Systematics: Approaches and Perspectives. Cambridge University Press. p. 264. ISBN 978-0-521-43810-0.
- ^ a b c d Peter M. Galton; Paul Upchurch (2004). "Stegosauria". In David B. Weishampel; Peter Dodson; Halszka Osmólska (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 343–362. ISBN 0-520-24209-2.
- ^ a b Raven, Thomas J. (2021). The Taxonomic, Phylogenetic, Biogeographic and Macroevolutionary History of the Armoured Dinosaurs (Ornithischia: Thyreophora) (PDF) (PhD thesis). University of Brighton.
- ^ a b Chatterjee, S., and Rudra, D. K. (1996). "KT events in India: impact, rifting, volcanism and dinosaur extinction," in Novas & Molnar, eds., Proceedings of the Gondwanan Dinosaur Symposium, Brisbane, Memoirs of the Queensland Museum, 39(3): iv + 489–731 : 489-532
- ^ a b c d Verma, Omkar (2015). "Cretaceous vertebrate fauna of the Cauvery Basin, southern India: Palaeodiversity and palaeobiogeographic implications". Palaeogeography, Palaeoclimatology, Palaeoecology. 431: 53–67. doi:10.1016/j.palaeo.2015.04.021. ISSN 0031-0182.
- ^ Verma, Omkar; Khosla, Ashu; Goin, Francisco J.; Kaur, Jasdeep (2016). "Historical biogeography of the Late Cretaceous vertebrates of India: Comparisons of geophysical and paleontological data". New Mexico Museum of Natural History and Science Bulletin. 71: 320.
- ^ Rozadilla, Sebastián; Agnolín, Federico; Manabe, Makoto; Tsuihiji, Takanobu; Novas, Fernando E. (2021). "Ornithischian remains from the Chorrillo Formation (Upper Cretaceous), southern Patagonia, Argentina, and their implications on ornithischian paleobiogeography in the Southern Hemisphere". Cretaceous Research. 125: 104881. doi:10.1016/j.cretres.2021.104881. ISSN 0195-6671.
- ^ Wilson, Jeffrey A.; Barrett, Paul M.; Carrano, Matthew T. (2011). "An associated partial skeleton of Jainosaurus cf. septentrionalis (Dinosauria: Sauropoda) from the Late Cretaceous of Chhota Simla, Central India". Palaeontology. 54 (5): 981–998. doi:10.1111/j.1475-4983.2011.01087.x.
- ^ Tidwell, Virginia; Carpenter, Kenneth (2005). Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press. p. 467. ISBN 978-0-253-34542-4.
- ^ Khosla, Ashu; Lucas, Spencer G. (2020), Khosla, Ashu; Lucas, Spencer G. (eds.), "Historical Background of Late Cretaceous Dinosaur Studies and Associated Biota in India", Late Cretaceous Dinosaur Eggs and Eggshells of Peninsular India: Oospecies Diversity and Taphonomical, Palaeoenvironmental, Biostratigraphical and Palaeobiogeographical Inferences, Cham: Springer International Publishing, pp. 31–56, doi:10.1007/978-3-030-56454-4_2, ISBN 978-3-030-56454-4
- ^ Fastovsky, David E.; Weishampel, David B. (2005). The Evolution and Extinction of the Dinosaurs. Cambridge University Press. pp. 129–130. ISBN 978-0-521-81172-9.
- ^ 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.
- ^ Sahni, Ashok; Rana, R. S.; Prasad, G. V. R. (1987). "New Evidence for Paleobiogeographic Intercontinental Gondwana Based on Late Cretaceous-Earliest Paleocene Coastal Faunas from Peninsular India". In McKenzie, Garry D. (ed.). Gondwana Six: Stratigraphy, Sedimentology, and Paleontology. American Geophysical Union. p. 214. ISBN 978-0-87590-067-4.
- ^ a b c Ayyasami, Krishnan (2006). "Role of oysters in biostratigraphy: A case study from the Cretaceous of the Ariyalur area, southern India". Geosciences Journal. 10 (3): 237–247. doi:10.1007/BF02910367. ISSN 1598-7477.