|The Vertebrates||Plesiosauroidea (2)|
Diapsida ├─Archosauromorpha └─Lepidosauromorpha ├─Sauropterygia │ ├─Placodontia │ ├─Pachypleurosauridae │ └─┬─Nothosauridae │ └─Plesiosauria │ ├─Pliosauroidea │ └─Plesiosauroidea │ ├─Attenborosaurus │ └─┬─Plesiosaurus │ └─┬─Eretmosaurus │ ├─Sthenarosaurus │ └─┬─Microcleidus │ └─┬─Elasmosauridae │ └─Cryptocleidoidea └─Lepidosauriformes ├─Sphenodontia └─Squamata
Since Plesiosaurus was for so long a garbage taxon, most references to Plesiosaurus, other than P. dolichodeirus, actually refer to something else. However there is another probably valid species, which goes by the awkward name of P. guilelmiimperatoris. In addition, P. brachypterygius is a probable junior synonym of P. guilelmiimperatoris.
P. guilelmiimperatoris Dames 1895 (originally called Plesiosaurus guilelmi imperatoris) is known from the later Toarcian or Epsilon Lias of Holzmaden, Würtemberg, southern Germany. According to Storrs (1997), it is distinguished from P. dolichodeirus by a more delicate skull and a more robust, straighter humerus. Length 3.40 meters.
P. brachypterygius von Heune 1923 was also recovered from the epsilon Lias (Toarcian) of Holzmaden. Length 2.5 to 2.7 meters. Storrs 1997) considers this species a synonym of P. guilelmiimperatoris, while Robert Bakker asserts that it is an early elasmosaur, and transfers it to the genus Microcleidus (as M. brachypterygius). The skull, shown at left, does not appear to conform to the diagnosis of Plesiosaurus. Note the following features:
a) the cheek is strongly emarginated;
b) the upper temporal fenestra is not rounded;
c) the squamosal is not strongly triradiate -- at least the anterior process is weak; and
d) the temporal bar is robust.
On the other hand, the overall shape and proportions of the skull are similar to Plesiosaurus dolichodeirus. The dentition and postcranial skeleton are very similar.
References: Storrs (1997). ATW020706.
Eretmosaurus: Seeley 1874b. E. rugosus (= Plesiosaurus rugosus) Owen 1840b
Range: Late Triassic (Rhaetian) to Early Jurassic (Sinemurian) of Europe (UK).
Phylogeny: Plesiosauroidea ::: Sthenarosaurus + (Microcleidus + (Elasmosauridae + Cryptocleidoidea)) + *.
Characters: Length 4-5 m; 35+ cervical vertebrae; coracoids truncated anteriorly, not extending anterior to glenoid; scapulae extend ventromedially, fused to each other and coracoids on midline; "clavicular arch degenerate".
Note: Eretmosaurus is often placed among the Pliosauroidea, as was one of the earliest representatives of the group. It is known from isolated remains in the latest Triassic (Rhaetian) of England and possibly Scotland, and the earliest Jurassic (lower Lias) of England. A headless skeleton described by the famous paleontologist Sir Richard Owen, from the Sinemurian of England, forms the basis for this species. The head, although unknown, was clearly quite small, to fit on the end of the long neck. The general appearance very Plesiosaurus-like. Indeed for a long time this genus was included under the Plesiosauridae. It has recently been suggested, on the basis of the long neck, that Eretmosaurus is an elasmosaurid, the earliest known member of the family. However the individual neck vertebrae are not elongated as in later elasmosaurids, and it is just as, or more, likely, that the tendency towards increased neck length evolved simultaneously among a number of lineages of early Plesiosauria.
There is of course no guarantee that the poorly known chronologically earlier by some eight or ten million years) specimens belong to the same species as the later, Sinemurian holotype. However, they may (even this is uncertain) belong to the same genus.
Links: The Plesiosaur Site - Species; Plesiosauria Translation and Pronunciation Guide;
References: Taylor & Cruickshank (1993)
Sthenarosaurus: Watson 1911? 1909?. S. dawkins Watson 1911? 1909?.
Range: Early Jurassic (Toarcian) of Europe (UK)
Phylogeny: Plesiosauroidea ::: Eretmosaurus + (Microcleidus + Elasmosauridae + Cryptocleidoidea)) + *.
Characters: robust build; 2.8-3.0 m; 18 cervical vertebrae known, up to 28-32 possible; cervical vertebrae very strongly tapered (i.e. vertebrae at base of neck are much larger); coracoids short & thick; pelvis very broad & heavy.
Note:  This is another headless genus that has been placed almost everywhere except the Elasmosauridae. The cervical vertebrae show no trend to elongation. As a result, it has been estimated that there were 28 to 32 neck vertebrae. However, the vertebral column is also strongly tapered, to the extent that the smallest cervical, of the 18 known cervicals, is 1/8th the size of the largest. Extrapolating this trend over a further 12 vertebrae would likely result in a head the size of a cough drop.  On the basis of features of the pectoral (shoulder) girdle it resembles the Plesiosaurus. White (1940) although it has also been placed in the Pliosauroidea or related to Attenborosaurus. This genus is now considered incertae sedis (of uncertain relationships), but may perhaps possibly represent a transitional stage between the Plesiosaurus and the Cryptocleididae.
Links: The Plesiosaur Site - Species; Plesiosauria Translation and Pronunciation Guide. MAK990830, ATW020706.
Microcleidus: Watson 1909. M. ("Plesiosaurus") homalospondylus Owen 1865, M. ("Plesiosaurus") macropterus Seeley 1865b.
Range: Early Jurassic (Toarcian) of Europe (UK)
Phylogeny: Plesiosauroidea :::: Elasmosauridae + Cryptocleidoidea) + *.
Characters: jugal excluded orbital margin by extended maxilla-postorbital contact; neck very long, with 40 cervical vertebrae; cervical ribs double-headed; cervical centra elongated; 22 dorsals; 3 sacrals; 28+ caudals; clavicles very small (hence the name); interclavicle absent.
Note: Note that the image lacks the heavy gastralia and ventral shield typical of plesiosauroids, has very short ribs, and very tall, robust neural spines. Whether this is an error, a taphonomic artifact, or the actual condition, we cannot say. It is certainly a strange animal. As with many other plesiosaurians, Microcleidus has led a full and varied taxonomic life, having bee variously referred to the Cryptocleididae (White, 1940), Elasmosauridae (Brown, 1981) and Plesiosauroidea i.s. The current tendency seems to be to place it among the cryptocleidid elasmosaurs, although we have not yet made this move.
Links: NEW SVP MARINE REPTILES; The Plesiosaur Site - Species; IV EUROPEAN WORKSHOP ON VERTEBRATE PALAEONTOLOGY (abstract on p. 29); Plesiosauria Translation and Pronunciation Guide always a good source). MAK000218, ATW040712.
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