Plesiosauria

Thililua

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Skull and neck of Thililua in right lateral view. From Bardet et. al. (2003).

Thililua has 30 cervical vertebrae. Each cervical has a longitudinal ridge on its lateral surfaces, this is a convergent feature with elasmosaurids. The skull of Thililua is relatively large with a short postorbital region and elongate rostrum. The type specimen (MHNGr.PA.11710) consists of a complete skull, entire cervical and pectoral series, and the first four dorsal vertebrae. Skull and neck...

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Thalassomedon

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The holotype of Thalassomedon was discovered by R. L. Landberg in 1939 in Baca County, Colorado. Thalassomedon has a short and deep atlas-axis and 62 cervical vertebrae, and there is no pectoral or pelvic bar in the adult condition (Carpenter 1999). The species name haningtoni is sometimes wrongly spelled ‘hanningtoni’ (as in Carpenter 1999) or ‘haringtoni’ (as in the AMNH...

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Thalassiodracon

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Thalassiodracon lectotype specimen (NHMUK 2018*).

The species Plesiosaurus hawkinsii was introduced in 1838 for a small plesiosaurian from Street, Somerset. The new genus name Thalassiodracon was erected decades later following an examination of a referred skull in Cambridge (CAMSM J.46986). Thalassiodracon is from the Rhaetian-Hettangian boundary, so it is latest Triassic or earliest Jurassic in age. Thalassiodracon means ‘Sea Dragon’, which “alludes to the colloquial...

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Simolestes

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Simolestes has variously been allied with the Pliosauridae and the Rhomaleosauridae. The most noticeable difference between Simolestes and the other pliosaur taxa from the Oxford Clay (Liopleurodon, Peloneustes, Pachycostasaurus), is its much shorter snout and mandibular symphysis, a character is shares with the Rhomaleosauridae. However, this is probably a convergent character. Older descriptions of this taxon note the spatulate symphysis...

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Rhomaleosaurus

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Reconstruction of Rhomaleosaurus thorntoni. From Smith and Benson (2014).

Rhomaleosaurus is the largest known Lower Jurassic pliosaur and was the top predator in early Jurassic marine ecosystems. It has a reinforced skull to help resist torsion and a ferocious set of teeth, a combination of characters perfect for snatching and killing cephalopods, fish, and other marine reptiles. Historically, the genus Rhomaleosaurus has been interchangeable with now invalid ‘Thaumatosaurus’. See...

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Plesiosaurus

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Reconstruction of Plesiosaurus dolichodeirus in lateral view. From Conybeare (1824).

Plesiosaurus was the first plesiosaur discovered and named (De la Beche and Conybeare 1821). The first 1821 description was based on partial remains so the anatomy of Plesiosaurus – particularly the identity of the isolated bones and how they articulated together – involved some guesswork. An attempted reconstruction of the forelimb of Plesiosaurus by De la Beche and Conybeare (1821)....

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Peloneustes

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Peloneustes is one of several pliosaur genera from the Oxford Clay Formation in the UK. It is one of the better known pliosaurs from this horizon, represented by 12 nearly complete skulls and several skeletons. It is also the most abundant pliosaur from the Peterborough Member of the Oxford Clay Formation (Ketchum and Benson 2011b). Peloneustes is characterised by an...

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Pachycostasaurus

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Mounted skeleton of Pachycostasaurus in the Peterborough Museum. Photo by Adam Smith.

Pachycostasaurus is approximately 3.1 meters long. Its rib cage and vertebrae exhibit thickened bone (Cruickshank et al. 1996) a condition termed pachyostosis. This heavy ossification is unusual in plesiosaurs (another exception may include Kronosaurus), although it is common in basal sauropterygians, especially pachypleurosaurs. Pachycostasaurus probably used the extra ballast provided by its heavy bones to traverse the sea bed searching...

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Occitanosaurus

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Mounted skeleton of 'Occitanosaurus'. From a postcard.

‘Occitanosaurus’ was described by Sciau et al. (1990) and Bardet et al. (1999). The genus name was erected for ‘Plesiosaurus’ tournemirensis by Bardet et al. (1999). The taxon was originally regarded as an early elasmosaurid but is now considered a member of the Microcleididae. ‘Occitanosaurus’ was approximately 3.9 meters long. The type material (MMM J. T. 86-100), in the Musée...

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Morturneria

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Skull of Morturneria in lateral view. From Chaterjee and Small (1989).

The taxon was originally named ‘Turneria seymourensis‘ by Chatterjee and Small (1989) but this genus was preoccupied and so Chatterjee and Creisler (1994) later revised the name to Morturneria seymourensis. The holotype specimen is TTU P 9219, an incomplete skull and mandible plus associated cervical vertebrae, from the Lopez de Bertodano Formation of Seymour Island, Antarctica. Morturneria was regarded by...

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Meyerasaurus

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The holotype specimen of Meyerasaurus (SMNS 12478) from the Posidonia-Schiefer, Holzmaden, Germany.

M. victor was originally described and figured by Fraas (1910). Historically, M. victor has been regarded as a species of Rhomaleosaurus and is often associated in the literature under the defunct name ‘Thaumatosaurus’. See my article about this history of ‘Thaumatosaurus‘ here. Smith and Vincent (2010) identified M. victor as generically distinct from Rhomaleosaurus and erected the new name...

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Macroplata

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Macroplata is a rhomaleosaurid plesiosaur from the Lower Jurassic of Warwickshire, UK. Macroplata was named and briefly described by Swinton (1930a) based on a single skeleton from Harbury, Warwickshire, and the skeleton was redescribed in detail by Ketchum and Smith (2010). Photograph of Macroplata tenuiceps in situ in Harbury, Warwickshire. White (1940) referred the species Plesiosaurus longirostris to Macroplata, and...

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Liopleurodon

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Mounted skeleton of Liopleurodon ferox in the Tubingen Museum, Germany (from Martill and Naish, 2000).

Liopleurodon is a pliosaur that hardly needs introduction since appearing as the villain in the BBC’s ‘Walking with Dinosaurs’ TV series. This led to popular misconceptions about the size of Liopleurodon, which is known to have reached adult sizes in the region of seven metres long, nowhere near the gargantuan 25m estimate proposed by the series and later perpetuated elsewhere....

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Libonectes

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Libonectes was erected for ‘Elasmosaurus morgani’. The pectoral girdle and a forelimb of the holotype are figured (most recently by Welles 1962, fig. 12) but now lost (Carpenter 1999). Libonectes morgani (Welles 1949) is the type and only species. Type material of L. morgani (SMUSMP 69120) consists of a skull, cervical vertebrae and gastralia, from the Britton Formation, Eagle...

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Leptocleidus

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Skull of Leptocleidus capensis in lateral view (from Cruickshank, 1997)

Lower Cretaceous plesiosaurs are rare, so Leptocleidus is important because it fills a gap in the fossil record of plesiosaurians. Leptocleidus was once considered to be a late surviving member of the family Rhomaleosauridae but it has recently been reidentified as a close relative of polycotylids. The fossils of all known species of Leptocleidus were preserved in inshore brackish or...

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