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.
The family tree of the Enaliosauria is here. The complete reptile family tree is here.