Peloneustes philarchus (originally Plesiosaurus philarchus Seeley 1869; Linder 1913; Ketchum and Benson 2011; 3m length; NHMUK R4058; Middle Jurassic) is traditionally considered a small pliosaurid. Here it nests as a polycotylid plesiosaur with Trinacormerum. Several specimens are known.

Trinacromerum osborni (Cragin 1888, Williston 1908, T. bentonianum) late Late Cretaceous, ~3 m in length, has been classified as a polycotylid plesiosaur. Derived from a sister to Pistosaurus, Trinacromerum was a sister to Plesiosaurus and phylogenetically preceded Brachauchenius and Kronosaurus.

Distinct from Pistosaurus, the skull of Trinacromerum had a longer rostrum created by an elongated maxilla. The teeth were relatively shorter. The lower rim of the upper temporal bar was lower than the orbit and inline with the jawline. The naris was closer to the orbit, which was larger and laterally oriented.

The cervical neural spines were narrower. The cervical ribs were shorter and more robust. Far fewer gastralia were present.

The scapula and coracoid were relatively larger, serving as anchors for larger flipper/paddle muscles. The humerus was larger. The ulna and radius were tranformed into discs incorporated into the mosaic of short metacarpals and phalanges, which were more numerous.

The pubis was much larger. The ischium was larger. The femur was almost identical to the humerus and similarly, the tibia and fibula were reduced to discs incorporated into the mosaic of short metatarsals and phalanges, which were more numerous.

Dolychorhynchops osborni (Williston 1902; Late Cretaceous, 80mya; FHSM VP-404) is a sister to Trinacromerum with a narrow, flat skull. Above the skull was inappropriately made wider and taller and several bones were overlooked. Note the palatal extension of the premaxilla to the internal nares, as in other eusauropterygians, overlooked by prior workers. Here hypothetical hemals were added. Gastralia were likely present.

Ketchum HF and Benson RJ 2011. The cranial anatomy and taxonomy of Peloneustes philarchus (Sauropterygia, Pliosauridae) from the Peterborough Member (Callovian, Middle Jurassic) of the United Kingdom. Paleontology 54(3):639–665.

Linder H 1913. Beiträge zur Kenntnis der Plesiosauria-Gattungen Peloneustes und Pliosaurus. Geologische und Palaeontologische Abhandlungen, 11, 339–409.

O'Keefe FR 2001. A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Acta Zoologica Fennica 213:1-63.

O'Keefe FR 2004. On the cranial anatomy of the polycotylid plesiosaurs, including new material of Polycotylus latipinnis Cope, from Alabama. Journal of Vertebrate Paleontology. 24 (2): 326–340.

Seeley HG 1869. Index to the fossil remains of Aves, Ornithosauria, and Reptilia, from the Secondary System of Strata arranged in the Woodwardian Museum of the University of Cambridge. Deighton, Bell and Co., London, xxiii + 143 pp.

Williston SW 1902. Restoration of Dolichorhynchops osborni, a new Cretaceous plesiosaur. Kansas University Science Bulletin 1 (9): 241–244.

Williston SW 1908. North American Plesiosaurs: Trinacromerum. Journal of Geology 16(8): http://www.jstor.org/stable/30068152

wiki/Trinacromerum
wiki/Dolichorhynchops
wiki/Peloneustes