Rhomaleosaurus

''Rhomaleosaurus'' (meaning "strong lizard") is an extinct genus of Early Jurassic (Toarcian age, about 183 to 175.6 million years ago) rhomaleosaurid pliosauroid known from Northamptonshire and from Yorkshire of the United Kingdom. It was first named by Harry Seeley in 1874 and the type species is ''Rhomaleosaurus cramptoni''. It was one of the earliest large marine reptile predators which hunted in the seas of Mesozoic era, with the type species ''R. cramptoni'' measuring long and weighing . Like other pliosaurs, ''Rhomaleosaurus'' fed on ichthyosaurs, ammonites and other plesiosaurs.

Species

''R. cramptoni''

In July 1848, a fossil of a large plesiosaur was unearthed in an Alum quarry at Kettleness, near Whitby, in Yorkshire, England. It was collected from the ''A. bifrons'' ammonite zone of the Whitby Mudstone Formation age, about 183 to 180 million years ago. The complete skeleton which preserved the skull, NMING F8785, was kept for five years at Mulgrave Castle, which was then owned by the Marquess of Normanby. In 1853, the Marquess introduced the interesting finding to the eminent Irish surgeon and anatomist, Sir Philip Crampton. The same year, Crampton transferred the fossil to Dublin to be displayed as a centrepiece at the 1853 British Association annual meeting. The Zoological Society of Ireland built a specially constructed building to house the large reptile. After a decade, still remaining undescribed, the specimen moved in the Royal Dublin Society museum and officially described by Alexander Carte and W. H. Bailey as a new species of ''Plesiosaurus''. Carte and Bailey named the species ''Plesiosaurus cramptoni'' after the Irish scientist, Sir Philip Crampton. In 1874, the British geologist Harry G. Seeley, based on this finding, which is now known as the holotype of the family Rhomaleosauridae, recognized and erected a new genus establishing ''Rhomaleosaurus''. Only in 2006 the skull of this specimen was finally prepared and enabled a restudy of this genus.

''R. propinquus''

''R. propinquus'' is known from the holotype WM 852.S, articulated almost complete skeleton which preserved the skull, exposed in dorsal view. It was collected from the ''A. serpentines'' ammonoid zone, of the Whitby Mudstone Formation. ''R. propinquus'' was first named by Tate and Blake in 1876 as a new species of ''Plesiosaurus''. Watson (1910) redescribed it as a species of ''Rhomaleosaurus''. Adam S. Smith (2007), in his thesis on the anatomy and classification of the family Rhomaleosauridae, suggested that ''R. propinquus'', is a junior synonym of ''Rhomaleosaurus zetlandicus''. Smith and Gareth J. Dyke (2008) considered this species to be valid.

''R. thorntoni''

''R. thorntoni'' is known from the holotype BMNH R4853, three-dimensional, partially complete skeleton which preserved most of the skull and mandibles. It was collected from Kingsthorp of Northamptonshire, dating to the Toarcian stage. It is the only well-known British Toarcian rhomaleosaurid discovered away from the Yorkshire coast, to date. ''R. thorntoni'' was first named by Andrews in 1922 and later revised by Cruickshank (1996) as a junior synonym of ''R. cramptoni'', alongside ''R. zetlandicus''. Adam S. Smith (2007) and Smith and Gareth J. Dyke (2008) considered this species to be valid.

''R. zetlandicus''

''R. zetlandicus'' is known from the holotype YORYM G503 (pictured), a nearly complete skull and vertebral column in association with parts of the limbs. It was collected from the Alum Shale of Whitby Mudstone Formation, Yorkshire, dating to the Toarcian stage. ''R. thorntoni'' was first named by Phillips in 1854 and its skull was described in detail by Taylor (1992). Later it was revised by Cruickshank (1996) as a junior synonym of ''R. cramptoni'', alongside ''R. thorntoni''. Adam S. Smith (2007) and Smith and Gareth J. Dyke (2008) considered this species to be valid.

Reassigned species

Through the years, various species have been referred to as ''Rhomaleosaurus''. However, according to Smith (2007), in his thesis on the anatomy and classification of the family Rhomaleosauridae, the genus ''Rhomaleosaurus'' has only three valid species: ''R. cramptoni'', ''R. thorntoni'' and ''R. zetlandicus''. Smith and Dyke (2008) also recognized ''R. propinquus'' as valid. Other species that previously fell under this genus were: ''R. megacephalus'' and ''R. victor''. Smith (2007) and Smith and Dyke (2008) pointed out that these species do not belong to ''Rhomaleosaurus'', as ''R. megacephalus'' is referred to ''Eurycleidus'' (or to a new genus by Smith and Dyke (2008)) and ''R. victor'' represents a new genus named ''Meyerasaurus'' by Smith and Vincent in 2010. Cladistic analyses by Ketchum & Benson, 2010, Benson ''et al.'', 2011 and Ketchum & Benson, 2011 found ''R. megacephalus'' to be basal to the clade containing ''Rhomaleosaurus'' and ''Eurycleidus'', thus it should be in its own genus as suggested by Smith and Dyke (2008). Following this, it has been placed in its own genus, ''Atychodracon''. (Smith, 2015).http://palaeo-electronica.org/content/pdfs/504.pdf

''Thaumatosaurus''

The name ''Thaumatosaurus'', which means 'wonder reptile', belonged to a genus of plesiosaur that was described by palaeontologist Christian Erich Hermann von Meyer, back in 1841. Meyer described the species ''Thaumatosaurus oolithicus'' based on partial skull, vertebral and limb remains, that were found in the Posidonia Shale of Holzmaden, Baden-Württemberg, Germany. In 1856, Meyer published a full description of ''Thaumatosaurus'' and later provided figures of the specimen. Richard Lydekker (1889) regarded ''Rhomaleosaurus'' as a synonym of ''Thaumatosaurus'' because Richard Lydekker
and Harry G. Seeley "refused steadfastly to recognize the generic and specific names proposed by one another". Lydekker referred continuously to the name ''Thaumatosaurus'', instead of ''Rhomaleosaurus''. Fraas (1910) recognized both generic names in his original description of ''R. victor'' (now ''Meyerasaurus''), but referring to the new species "Thaumatosaurus" victor. Many other researchers adopted the name "Thaumatosaurus". Today this taxon is regarded as a nomen dubium because the holotype can be referred to Pliosauroidea indet at best. The diagnostic specimens which previously were regarded as ''Thaumatosauruss specimens, now represent the holotypes of ''Eurycleidus'', ''Meyerasaurus'' and ''Rhomaleosaurus''.

Phylogeny

Smith & Dyke, 2008 redescribed the skull of ''R. cramptoni'' after its final preparation. Both Rhomaleosauridae and Pliosauridae were found to be monophyletic, and the relations between ''Rhomaleosauruss species were tested. The cladogram below follows Smith & Dyke (2008), with the asterisk noting species removed from ''Rhomaleosaurus'' to their own genera since their study.

Paleobiology

''Rhomaleosaurus'' may have been able to pick up scents while submerged by forcing water through passages in its skull containing sensory organs. This adaptation would have enabled it to hunt its prey in a similar manner to some modern shark species.

See also

* List of plesiosaur genera
* Timeline of plesiosaur research

References

{{Portal bar, Paleontology, United Kingdom

Sauropterygian genera
Early Jurassic plesiosaurs of Europe
Fossil taxa described in 1874
Rhomaleosaurids
Taxa named by Harry Seeley