No surprises here.
Tricleidus seeleyi (Fig. 1; Andrews 1909; Middle Jurassic, Callovian, BMNH R3539) ) enters the large reptile tree (LRT, 1435 taxa) alongside Plesiosaurus.
Figure 1. GIF animation of Tricleidus skull demonstrating both the enormous gape and the filter-feeding teeth that permit water to be pushed out by a constricting throat sac.
Why such long, slender and closely intermeshing teeth?
Not only do those long teeth provide excellent spiky fish traps, but upon closing they act like whale baleen in trapping food inside the mouth cavity while permitting the expulsion of excess water without losing the fish trapped inside.
As in other plesiosaurs
the large jaw muscles filling the posterior skull ensure a strong bite. A small ridge posterior to the jaw joint blocks over-extension of the mandible.
References
Andrews CW 1909, 1910. A Descriptive Catalogue of the Marine Reptiles of the Oxford Clay, Part I. British Museum (Natural History), London, England: 205 pp.
The Pterosaur Heresies 