Diapsida ├─Archosauromorpha └─Lepidosauromorpha ├─Sauropterygia │ ├─Placodontia │ ├─Pachypleurosauridae │ └─┬─Nothosauridae │ └─Plesiosauria │ ├─Pliosauroidea │ └─Plesiosauroidea │ ├─Elasmosauridae │ └─Cryptocleidoidea │ ├─Cryptocleididae │ └─Tricleidia │ ├─Tricleidus │ └─┬─Cimoliasauridae │ │ ├─Kimmerosaurus │ │ └─Cimoliasaurus │ └─Polycotylidae └─Lepidosauriformes ├─Sphenodontia └─Squamata
Range: Late Jurassic
Phylogeny: Cryptocleidoidea: Cryptocleididae + * : Tricleidus + (Cimoliasauridae + Polycotylidae). ATW020709.
Tricleidus: T. seeleyi Andrews 1909.
Range: Late Jurassic (Callovian) of Europe (Peterborough, UK).
Phylogeny: Tricleidia : (Cimoliasauridae + Polycotylidae) + *.
Characters: 2.5-3.0 m; skull 22 cm; rostrum shorter & skull table taller than in Cryptocleidus; parietals form sagittal crest; quadrate overlaps quadrate ramus of pterygoid posteromedially; paroccipital process relatively long and slender; occipital condyle ringed by a groove; occipital condyle formed from basioccipital only; pterygoid with articular process for basisphenoid; teeth with dense longitudinal ridges; dentaries with 17 tooth pairs; premaxillae with 5 teeth each, 1st and 5th small & 2nd to 4th large; anterior most maxillary tooth (6th upper) small and 8th & 9th uppers large; 26+ cervical vertebrae with relatively amphicoelous centra; cervical centra length only slightly exceeds height (but never the width) in most anterior vertebrae; pectoral girdle with "large interclavicle and a pair of well developed elongated clavicles"; clavicles are triangular, well-developed, and visceral to interclavicle which separates them in the midline; the interclavicle is well-developed and plate-like; the coracoids meet the scapulae in midline in 'adults'; width of posterior cornua of the coracoids exceeds interglenoid width in 'adults'; the humerus is not greatly expanded distally, and articulates with four epipodials; probable trap-feeder, but with fewer teeth than Cryptocleidus.
Note: supposed Tricleidus remains from Wyoming are now believed to be a distinct genus. See Tate Geological Times.
Links: Plesiosauria Translation and Pronunciation Guide; The Plesiosaur Site - Species ATW020709
Cimoliasauridae: originally an unfashionable family based on scrappy materials and a vague definition, to wit: "A family in the Plesiosauroidea with neck relatively shorter than in the Plesiosauridae and the Elasmosauridae. Teeth of the general Plesiosaurian type (crowns high and slender). The height of at least the middle and posterior cervical centra approximately equal to the length, but the breadth of these centra considerably greater than the length; the end faces of the centra flat or slightly concave. Ribs single-headed." See Plesiosauria Translation and Pronunciation Guide. It has been resurrected as a clade, but remains an unfashionable family based on scrappy materials and a vague definition.
Range: Late Jurassic to Late Cretaceous of Europe, North America, Australia, & new Zealand
Phylogeny: Tricleidia :: Polycotylidae + * : Kimmerosaurus + Cimoliasaurus.
Links: Plesiosauria Translation and Pronunciation Guide; plesiosarus Swedish). ATW020709.
Kimmerosaurus: K. langhami Brown 1981.
Range: Late Jurassic (Kimmeridgian) of Europe (Kimmeridge Clay, UK). Known from 3 partial skulls & several cervicals.
Phylogeny: Cimoliasauridae : Cimoliasaurus + *.
Characters: skull broad, but very lightly built and more elongated than in Cryptocleidus; no parietal sagittal crest; quadrate overlaps quadrate ramus of the pterygoid anterolaterally; paroccipital process relatively short & massive; occipital condyle is not ringed by a groove, and extends onto pedicles of exoccipitals; tooth ornament absent; teeth highly recurved, thin (laterally compressed), sharply pointed, with 36 dentary pairs & 6 premaxillary pairs.
Note: Postcranial skeleton is not known. Possible junior synonym of Colymbosaurus.
Links: Plesiosauria Translation and Pronunciation Guide; The Plesiosaur Site - Species; FILTER FEEDING REPTILES. ATW030114.
Cimoliasaurus: C. maccoyi Etheridge 1904, C. planus, numerous others, all nomina dubia.
Range: Early Cretaceous II (Aptian? Albian?) to Late Cretaceous of Australia, North America, Europe & New Zealand.
Phylogeny: Cimoliasauridae : Kimmerosaurus + *.
Characters: 3-8 m; and long, slender, recurved teeth; teeth conical, with oval cross section at base, becoming more flattened toward tip, somewhat flattened & without carinae; teeth coated with enamel, smooth, with some growth cracks, usually 2-5 cm long; relatively short neck, for holding fish ; centra dorsoventrally compressed and constricted in the middle, with oval articular surfaces and two large nutritive foramina; vertebrae morphologically similar, but decreasing in size from mid-dorsal toward sacrum;
Note:  This is odd, because we are also informed that no skull is known.  The Australian species, C. maccoyi, is said to differ from the others in lacking lateral ridges on the cervical centra. It is likely that this is a garbage taxon without diagnosable characters.  The following portion of a description of the vertebra from Leidy sounds interesting, but I can't quite make out what he means:
The sides of the body of the vertebræ form, together with the sides of the vertebral arch and the upper part of the transverse processes, a nearly uniform slope, broken only by a slight elevation formed by the apparent sutural coossification of the transverse process with the body. The under part of the body between the transverse processes nearly forms a level surface, more or less elevated into a ridge between the venous foramina, and depressed along a line with the position of the latter.
Image: Cimoliasaurus vertebra from the Late Cretaceous of New Jersey. From D&D Fossils & Meteorites.
Links: Marine Reptiles (NOT Dinosaurs); Cope - Elasmosaurus2; Cope Additional Note; (these are where Cope explains how he managed to put the head on the wrong end of an elasmosaur!); Plesiosauria Translation and Pronunciation Guide; The Plesiosaur Site - Species; Cimoliasaurus vertebrae $45; Cretaceous Fossils: Cimoliasaurus magnus Page; Big Brook Plesiosaur Page (are these vertebrae from a plesiosaur?); Exponat mesice - Regionalni Muzeum v Teplicich (Czech); Leidy, 1865 OOK: portions of Leidy's description. The OOK treatment is, of course Best on the Web); Plesiosaur History OOK: the early papers); Big Brook Plesiosaur Page very nice page with summary of anatomy, other information, and a rough life reconstruction).
References: Molnar (1991). ATW030708. MAK990821.
Using this material. All material by ATW is public domain and may be freely used in any way (also any material jointly written by ATW and MAK). All material by MAK is licensed Creative Commons Attribution License Version 3.0, and may be freely used provided acknowedgement is given. All Wikipedia material is either Gnu Open Source or Creative Commons (see original Wikipedia page for details). Other graphics are copyright their respective owners