Opisthocoelia: first effort towards recognition of sauropods as a distinct reptile group

The history of the classification of the eusauropod Cetiosaurus is well-known because it was initially seen by its describer, Richard Owen, as representing either an unknown race of aquatic reptiles or a gigantic crocodyliform, until it was recognized as a dinosaur in the 1870s. However, buried in the very superfluous process of the classification of Cetiosaurus within Reptila during the 1840-1880 interval is the fact that Owen himself reinforced his perception of Cetiosaurus as a gigantic marine crocodile when he unintentionally planted the seeds for long-term recognition of sauropods as a distinct reptile group by including Cetiosaurus in a new group, christened Opisthocoelia (Greek for "posterior hollows"), in a paper published by him in 1859.

Since the publication of his 1842 paper whereby the name Dinosauria was coined, Richard Owen knew that the proportions of the caudal vertebrae that he named Cetiosaurus medius and C. brevis (the latter now Pelorosaurus) discriminated Cetiosaurus from other Mesozoic crocodyliform taxa known to him, but he needed to buy time to determine whether including Cetiosaurus in a new clade within Crocodilia was warranted. Mantell (1850) had coined the new genus Pelorosaurus for Cetiosaurus conybeari (a junior objective synonym of brevis) after recognizing that the humerus NHMUK 28626 found at the type locality of the syntype caudals of C. brevis in Cuckfield, West Sussex, belonged to the same animal as the caudals, and while Pelorosaurus was the first-named sauropod to be recognized as terrestrial by its describer, Owen (1859a) classified Pelorosaurus as a gigantic crocodile on the grounds that Mantell (1850) had NHMUK 28626 incorrectly oriented. In a paper working out the classification of both extant and fossil reptiles, Owen (1859b) erected a new suborder within Crocodilia to include Cetiosaurus and the tetanuran theropod dinosaur Streptospondylus (which is now recognized as a theropod), which he named Opisthocoelia. According to Owen (1859b), Opisthocoelia could be discriminated from his other crocodilian suborders, Procoelia and Amphicoelia, in having opisthocoelous dorsal vertebrae with concave articulations at the rear end of the dorsal centra. Although the material known for Cetiosaurus was still extremely limited and not enough to illuminate the true appearance of this genus relative to extant and extinct crocodyliforms, this did not deter Owen (1859b) from taking the first crucial step to formally place Cetiosaurus in a systematic grouping of its own.

Following recognition of Cetiosaurus as a member of Dinosauria by Huxley (1870) and Phillips (1871), Owen (1875) admitted that his initial classification of Cetiosaurus within Crocodilia was untenable and realized that Cetiosaurus was a dinosaur after noting numerous similarities between the dorsal vertebrae of Cetiosaurus oxoniensis (referred by him to the Late Jurassic nominal species 'C. longus') and those of Hylaeosaurus, Iguanodon, and Scelidosaurus in their opisthocoelous nature. However, he did not touch upon whether or not he still regarded Streptospondylus as a close relative of Cetiosaurus even though Hulke (1872) came to regard Streptospondylus as also a dinosaur and possibly related to not just Cetiosaurus but also Ornithopsis and Eucamerotus. Seeley (1874), agreeing with the re-assessment of Cetiosaurus as a dinosaur by Phillips (1871), included Cetiosaurus and his new genus Craterosaurus in a distinct grouping within Dinosauria, utilizing the name Cetiosauria (first mentioned in Owen 1859c*) for it. When Edward Drinker Cope described the genera Amphicoelias and Camarasaurus from the Morrison Formation, he recognized them as forming a grouping with Ornithopsis, but did not mention Opisthocoelia in his papers naming Amphicoelias and Camarasaurus. His arch-rival during the Bone Wars, Othniel Charles Marsh, got the chance to apply a name to a grouping of exclusively long-necked sauropodomorphs when he coined the higher-level taxonomic name Sauropoda for the dinosaur group formed by long-necked sauropomorphs in a paper published by him in 1878, unaware of the names Opisthocoelia and Cetiosauria. Cope (1883) treated Opisthocoelia as a senior synonym of Sauropoda, basing his diagnosis for Opisthocoelia on some of the characters used by Marsh (1878) to define Sauropoda. 

* Although Cetiosauria was first used by Owen (1859c) and not Seeley (1874), Owen did specify whether his Cetiosauria was a provisional name for the group he named Opisthocoelia should future study judge Cetiosaurus was too morphologically distinct from crocodyliforms to be classified within Crocodilia. Osborn (1898) used Cetiosauria as a senior synonym of Sauropoda but incorrectly attributed Cetiosauria to Seeley (1874) rather than Owen (1859c). Hatcher's (1903) comments about Cetiosauria having subordinal rank only within Sauropoda leave room open for using Cetiosauria to phylogenetically define a eusauropod clade including Neosauropoda, Mamenchisauridae, Turiasauria, Cetiosaurus, and any other eusauropods more closely related to Neosauropoda than to Shunosaurus.  

Given that Sauropoda has had universal currency in the literature, it is important to note that although Riggs (1903) and Matthews (1915) both used Opisthocoelia as a senior synonym for Sauropoda, the taxonomic content of Opisthocoelia listed by Owen (1859b), while paving the way for Cetiosaurus and Pelorosaurus to be one day recognized as constituting previously unknown clade of dinosaurs, would seemingly make the name Opisthocoelia equivalent to Saurischia (as stated by Mickey Mortimer) given that Streptospondylus is now classified as a theropod and has opisthocoelous dorsal vertebrae, as does Eustreptospondylus. Also, the dorsal vertebrae of Camarasaurus and Amphicoelias are opisthocoelous and amphicoelous respectively as noted by Cope (1877), and the presence of opisthocoely in both the cervical and dorsal vertebrae has an uneven distribution among sauropods (Fronimos 2016). Thus, the name Opisthocoelia can be considered not as a synonym of Sauropoda but as a prelude to eventual recognition of sauropods as distinct reptile ground in their own right by taking into account dorsal vertebral features distinguishing sauropods from all extinct and extant crocodyliforms.

References:

Cope, E.D., 1877. On Amphicoelias, a genus of saurians from the Dakota Epoch of Colorado. Paleontology Bulletin 27: 1–5.

Cope, E.D., 1883. On the characters of the skull in the Hadrosauridae. Proceedings of the Philadelphia Academy of Natural Sciences 35: 97-107.

Fronimos, J.A., 2016. Functional Mechanics of Concavo-convex Articulations and Neurocentral Sutures in the Vertebral Column of Sauropod Dinosaurs. Unpublished dissertation, University of Michigan.

Hatcher, J. B., 1903. Osteology of Haplocanthosaurus with description of a new species, and remarks on the probable habits of the Sauropoda and the age and origin of the Atlantosaurus beds. Memoirs of the Carnegie Museum 2: 1–72.

Hulke, J. W., 1872. Appendix to a "Note on a new and undescribed Wealden Vertebra," read 9th February 1870, and published in the Quarterly Journal for August in the same year. Quarterly Journal of the Geological Society 28 (1–2): 36–38.

Huxley, T.H., 1870. On the Classification of the Dinosauria, with observations on the Dinosauria of the Trias. Quarterly Journal of the Geological Society of London 26: 32–51.

Mantell, G. A. 1850. On the Pelorosaurus: an undescribed gigantic terrestrial reptile, whose remains are associated with those of the Iguanodon and other saurians in the strata of Tilgate Forest, in Sussex. Philosophical Transactions of the Royal Society of London 140: 379–390.

Marsh, O.C., 1878. Principal characters of American Jurassic dinosaurs. Part I. American Journal of Science, Series 3 16: 411–416.

Matthew, W. D. 1915. Dinosaurs, with Special Reference to the American Museum Collections. American Museum of Natural History, New York.

Osborn, H. F. 1898. Additional characters of the great herbivorous dinosaur Camarasaurus. Bulletin of the American Museum of Natural History 10: 219–233.

Owen, R., 1859a. Monograph on the fossil Reptilia of the Wealden and Purbeck Formations. Supplement no. II. Crocodilia (Streptospondylus, &c.). Palaeontographical Society Monograph 11: 20–44.

Owen. R., 1859b. On the orders of fossil and recent Reptilia, and their distribution in time. Report of the British Association for the Advancement of Science 29:153–166. 

Owen, R., 1859c. On the Classification and Geographical Distribution of the Mammalia. London, UK: John W. Parker and Son.

Owen, R., 1875. Monographs of the fossil Reptilia of the Mesozoic formations, part II (genera Bothriospondylus, Cetiosaurus, Omosaurus). Palaeontographical Society Monographs 29:15–93.

Phillips, J. 1871. Geology of Oxford and the Valley of the Thames. Oxford, UK: Clarendon Press.

Riggs, E. S. 1903. Structure and relationships of opisthocoelian dinosaurs. Part I, Apatosaurus Marsh. Field Columbian Museum, Geological Series 2: 165–196.

Seeley, H. G. 1874. On the base of a large lacertian cranium from the Potton Sands, presumably dinosaurian. Quarterly Journal of the Geological Society, London 30: 690–692.