Polacanthus (from the Ancient Greek polys-/πολύς- "many" and akantha/ἄκανθα "thorn" or "prickle") is an extinct genus of ankylosaurian dinosaurs from the early Cretaceous (130–125 million years ago) of England.Holtz, Thomas R. Jr. (2012) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,https://www.geol.umd.edu/~tholtz/dinoappendix/appendix.html Several species have been named in the genus Polacanthus, but only the type species, Polacanthus foxii, is currently seen as valid. There are not many fossil remains of this dinosaur, and some important anatomical features, such as its skull, are poorly known. It grew to about long. Its body was covered with armour plates and spikes. It may be a basal member of the Nodosauridae or part of a separate family, the Polacanthidae.
History of study Discovery
Polacanthus foxii was discovered by the Reverend William Fox on the Isle of Wight in early 1865, at Barnes High at the southwest coast. Fox at first planned to have his friend Alfred Tennyson name the new dinosaur during a meeting on 23 July 1865, when the remains were shown to paleontologist Richard Owen. Tennyson proposed Euacanthus Vectianus but this name was ultimately rejected.Tennyson, H., 1897, "Note about Polacanthus", In: Alfred Lord Tennyson, A memoir by his son, The MacMillan Company, p. 23-24 In September 1865, Fox in a lecture to the British Association reported on the find and let it be named Polacanthus foxii by Owen, hereby perhaps circumventing the convention that an author does not name a taxon after himself.Fox W. (1865). "On a new Wealden saurian named Polacanthus". Report of the British Association for the Advancement of Science, 1865 for 1864, p. 56 The text of the lecture, only published in 1866, was more or less reproduced by him in anonymous articles in the Geological Magazine and the Illustrated London News of 16 September 1865.Anonymous. 1865. "Fossil reptiles from the Wealden of the Isle of Wight". Geological Magazine 2: 432Anonymous. 1865. "A new Wealden dragon. Order, Sauria; Family, Dinosaurian; Genus, Polacanthus; Species, foxii". Illustrated London News: 270 This procedure caused some confusion as no corresponding 1865 publication by Owen exists. Some have therefore contended that Thomas Huxley in 1867 became the author of the name,Huxley, T.H., 1867, "On Acanthopholis horridus, a new reptile from the Chalk Marl", Geological Magazine, 4: 65-67 while others give Fox, Owen or "Anonymous" as the author. The generic name is derived from Greek πολύς, polys, "many" and ἄκανθα, akantha, "thorn", in reference to the many spikes of the armour. The specific name honours Fox.Martill, D.M. and Naish, D., 2001, "Armoured dinosaurs: thyreophorans." Pages 147–184 in Martill, D.M. and Naish, D. (eds.). Dinosaurs of the Isle of Wight, The Palaeontological Association, London.
The holotype, NHMUK PV R175, was found in a layer of the Upper Wessex Formation dating from the Barremian. It is an incomplete skeleton with the head, neck, anterior armour and forelimbs missing but including dorsal vertebrae, a sacral rod of five dorsosacrals, the sacrum, most of the pelvis, most of the left hindleg, the right thighbone, twenty-two tail vertebrae, ribs, chevrons, ossified tendons, a pelvic shield, twenty-two spikes and numerous ossicles. The skeleton was in 1881 studied by John Whitaker Hulke, while it was still in the possession of Fox. Hulke published the first detailed description of the find, noting that the specimen had badly deteriorated over the years, the dermal armour having almost fully fallen apart.Hulke, J.W., 1881, "Polacanthus foxii, a large undescribed dinosaur from the Wealden Formation in the Isle of Wight", Philosophical Transactions of the Royal Society of London, 172: 653–662 The same year Fox died, his collection was acquired by the British Museum of Natural History, including the Polacanthus fossil. This was after arrival in the museum in 1882, reassembled by preparator Caleb Barlow, painstakingly putting all the pieces together with Canada balsam, much to the wonder of Hulke who in 1881 had called this a hopeless undertaking. This allowed Hulke to redescribe the specimen in 1887, with a special attention to the armour arrangement.Hulke, J.W., 1887, "Supplemental Note on Polacanthus Foxii, Describing the Dorsal Shield and Some Parts of the Endoskeleton, Imperfectly Known in 1881", Philosophical Transactions of the Royal Society of London. B 178: 169–172 In 1905, when it was mounted by the museum, the specimen was again described by Franz Nopcsa who for the first time provided an illustration of the possible spike configuration.Nopcsa, F., 1905, "Notes on British dinosaurs. Part II. Polacanthus", Geological Magazine 2: 241-250 Later, the specimen was stored in the museum basement. Additional specimens Numerous other specimens from Wight and Great Britain have since been referred to Polacanthus. These mostly consist of single bones or armour elements. Several specimens that were discovered prior to the holotype were at various points considered to belong to Polacanthus. In 1843 John Edward Lee reported the discovery on Wight of three such specimens, consisting only of armour pieces. They were already lost before the description was published.Lee, J.E., 1843, "Notice of Saurian Dermal Plates from the Wealden of the Isle of Wight", Annals and Magazine of Natural History 11: 5-7 In 1859, geologist Ernest P. Wilkins mentioned the presence in his collection of numerous scutes, spikes and vertebrae from Wight, referred by him to Hylaeosaurus.Wilkins, E.P. and Brion, John & Sons. 1859. A concise exposition of the geology and antiquities of the Isle of Wight. [E W] Topography of the island [J B] Newport, T. Standford, 98 pp After his death, his collection was moved several times and the pieces were lost.
A second partial skeleton, from which parts had been removed since 1876, was identified and fully excavated by Dr. William T. Blows in 1979;Blows, W.T., 1982, "A preliminary account of a new specimen Polacanthus foxi (Ankylosauria, Reptilia) from the Wealden of the Isle of Wight", Proceedings of the Isle of Wight Natural History and Archaeological Society for 1980 7(5): 303-306 it is also in the London Natural History Museum as specimen NHMUK R9293. It is the first specimen to show skull elements, neck vertebrae and unequivocal anterior armour.Blows W.T. (1987). The armoured dinosaur Polacanthus foxi, from the Lower Cretaceous of the Isle of Wight, Palaeontology. 30, 557–580 More contentious are finds from mainland England. In 2014 a partial skeleton was reported from Bexhill in Sussex, specimen BEXHM 1999.34.1-2011.23.1 discovered in the early summer of 1998 by David Brockhurst in the Ashdown Pevensey Quarry. This dates from the Valanginian.William T. Blows & Kerri Honeysett, 2014, "First Valanginian Polacanthus foxii (Dinosauria, Ankylosauria) from England, from the Lower Cretaceous of Bexhill, Sussex", Proceedings of the Geologists' Association 125: 233–251 In 1999, 2007 and 2011, remains from Spain were referred to Polacanthus.Pereda-Suberbiola, X.; Meijide, M.; Torcida, F.; Welle, J.; Fuentes, C.; Izquierdo, L.A.; Montero, D.; Pérez, G. & Urién, V. 1999. "Espinas dermicas del dinosaurio anquilosaurio Polacanthus en las facies Weald de Salas de Los Infantes (Burgos, España). Estudios Geológicos 55: 267-272Pereda-Suberbiola, X., C. Fuentes, M. Meijide, F. Meijide-Fuentes, and M.J. Meijide-Fuentes. 2007. "New remains of the ankylosaurian dinosaur Polacanthus from the Lower Cretaceous of Soria, Spain". Cretaceous Research 28: 583–596Gasulla, J.M.; Ortega, F.; Pereda-Suberbiola, X.; Escaso, F. & Sanz, J.L. 2011. "Elementos de la armadura dermica del dinosaurio anquillosaurio Polacanthus Owen, 1865, en al Cretácico inferior de Morella (Castellón, España)". Ameghiniana 48(4): 508-519
A 2020 review of British ankylosaurian fossils concluded that none of these additional specimens could be confidently referred to Polacanthus, which would therefore be represented only by the holotype.Thomas J. Raven, Paul M. Barrett, Stuart B. Pond & Susannah C. R. Maidment (2020) Osteology and Taxonomy of British Wealden Supergroup (Berriasian–Aptian) Ankylosaurs (Ornithischia, Ankylosauria), Journal of Vertebrate Paleontology, DOI: 10.1080/02724634.2020.1826956
Description
Polacanthus was a medium-sized ankylosaur. In 2001, Darren Naish and David Martill estimated its length at . In 2010, Gregory S. Paul supported the initial length estimate, while providing a body mass estimate of .Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 229 Thomas Holtz, in 2012, gave a lower estimation of and . A taxon once assigned to Polacanthus, P. rudgwickensis (now Horshamosaurus), was approximately 30% larger.
Its hindlimbs are relatively long for an ankylosaur, with the holotype right femur measuring about in length.
In 2011 Barrett e.a. indicated two possible unique traits, autapomorphies: the floor of the neural canal is deeply cut by a groove with a V-shaped transverse profile; the caudal spikes have triangular bases in side view and narrow points. In 2020, a study concluded to a single autapomorphy: the ischia at half length curve towards each other, their rear ends touching at their inner sides.
The subsequent describers have always dedicated much effort at restoring the armour configuration. Hulke understood that Polacanthus had a large "pelvic shield" or "sacral shield", a single fused sheet of dermal bone over its hips (sacral area) which perhaps was not attached to the underlying bone and decorated with tubercles. This feature is shared with other "polacanthine" (basal nodosaurids) dinosaurs such as Gastonia and Mymoorapelta. With the holotype, this shield is 108 centimetres wide and 90 centimetres long. It features four horizontal rows of larger keeled osteoderms per side, surrounded by smaller ossicles.
These latter are sometimes completely fused to form flat armour plates. Hulke thought that on the tail there were two rows of keeled osteoderms per side. Of a set of spikes found with the fossil, he assumed they had adorned the sides of the rump. A different arrangement was hypothesised by Nopcsa. He thought that both the tail and the front of the body including the neck featured two parallel rows of spikes, one per side. On the front body each row would have consisted of five spikes and he claimed that seven of these had been conserved with the fossil, five of the right side and two of the left. The tail rows would have consisted of twenty-two shorter pairs, fifteen spikes being still extant, eight of the left side and seven of the right. As the spikes are asymmetrical their position can more or less be deduced. Blows in 1987 basically agreed with Nopcsa but also distinguished three spike types, a Type A, B and C, allowing him to classify additional fossil finds, which often differed from the holotype spikes in several details. In 2013 a footprint was found by Henley Hobbs and his father on the Isle of Wight. Now a picture of the footprint is inside the dinosaur farm.
Classification Species Polacanthus is known definitively only from its holotype specimen, representing the species P. foxii. However, numerous other species have been erroneously assigned to the genus Polacanthus in the past.
In 1924, Edwin Hennig named a Polacanthus becklesi, the specific name honouring collector Samuel Beckles, based on specimen BMNH R1926, a piece of an ilium associated with armour plates, found on Wight in the nineteenth century.Hennig, E., 1924, "Kentrurosaurus aethiopicus. Die Stegosaurier-Funde vom Tendaguru, Deutsch-Ostafrika", Palaeontographica Supplement 7: 101-254 Today this is often considered a junior synonym of P. foxii. It was assumed to be a different species because the armour is smoother on top, but this was likely caused by water erosion of the fossil.