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Mexican Mesozoic Reptiles: new unnamed ornithomimid + chubby-tailed polycotylid plesiosaur

Ben Creisler

The new book Dinosaurs and Other Reptiles from the Mesozoic of Mexico
recently came out.


Browsing the articles, a couple of items jump out for me.

Chapter 9. Mexican Saurischian Dinosaurs
Héctor E. Rivera-Sylva and Kenneth Carpenter

mentions a new still unnamed ornithomimid first described in a thesis:

"Ornithomimid n.g., n.sp.
Aguillon-Martinez (2010) described material from the Cerro del Pueblo
Formation of Coahuila and noted differences from other ornithomimids
that suggested a new taxon. The material included a partial adult
skeleton (CPC 16/237) found at La Majada, near Saltillo. The specimen
anterior and posterior caudal vertebrae, conjoined right and left
pubes, right femur, right tibia missing distal end, proximal end of
right tibia, distal end of left fibula, proximal end of metararsal II,
proximal and distalends of metatarsal III, complete metatarsal IV,
right digit IV phalanx 1, right digit IV phalanx 3, digit III phalanx
1, left digit II-1, and a partial pedal ungual. Three additional
specimens from La Majada and La Parrita were referred to this taxon,
of which one was a juvenile."


Chapter 5. Plesiosaurs, Reptiles between Grace and Awe
Eberhard Frey and Wolfgang Stinnesbeck

This chapter mentions the Vallecillo polycotylid, a spectacular,
virtually complete specimen with soft-tissue body outline preserved,
about 2 m long. The wide shape of the tail outline is a bit of a
surprise and gives the post-neck part of the body an amphora like

"The soft tissue lining has a reddish-brown ground color. It is
preserved along the left side of the postorbital part of the skull and
continues along the left side of the neck (Figs. 5.11A–B). On the
right side of the neck, the soft tissue stain is restricted to the
cervicothoracic transition. On both sides of the trunk, the soft
tissue seam is preserved along both sides of the shoulder girdle and
flanks with a width between 50 and 70 mm. The seam continues onto the
tail base without any constriction. In the tail, soft tissue stain
reaches a maximum width of 170 mm immediately terminalto the ischiadic
plates. From there it gradually converges toward the end of the tail.
Patches of a gray soft tissue matter are preserved along the left side
of the body and both sides of the tail. Wrinkled dark tissue is
visible between the ribs and the central elements of the gastralia."

Another recent paper in Paludicola argued that the flattened terminal
caudal vertebrae on the some plesiosaurs indicated a narrower tail
with a vertical fin at the end for steering or propulsion.


The chapter also gives more details on the “Monster of Aramberri," a
giant Late Jurassic pliosaur that has a healed bite from an even
bigger animal and a fatal bite from another. The remains have yet to
be described but appear to be distinct from European species.