A dyrosaurid crocodilian from the cretaceous (Maastrichtian) Escondido formation of Coahuila, Mexico
Shiller, Thomas A.
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Two fragmentary marine crocodylian specimens were discovered in Upper Cretaceous marine strata near the town of Sabinas in Coahuila, Mexico. Although exposures are poor in the vicinity of the collection sites, associated ammonites indicate that the specimens were recovered from either the Sauz Creek Member or lower part of the Cuevas Creek Member of the Escondido Formation, and are Maastrichtian in age. The two specimens share several unique features, including a reduced seventh dentary alveolus and flat mandibular rostrum that indicate they pertain to the extinct crocodylian family Dyrosauridae. The two are assigned to the same species, referred to here informally as the "Sabinas dyrosaur." The proportions of the mandible in the Sabinas dyrosaur are unusual. Its rostrum is narrow, but relatively short, and intermediate in form between typical longirostrine and brevirostrine dyrosaurs. The distal end of the mandibular rostrum is uniquely shaped with laterally expanded margins and enlarged, closely spaced, first and second alveoli. Dentary alveoli thirteen through fifteen are grouped together, with the fourteenth alveolus laterally offset and confluent with the thirteenth. The splenials participate in only a small part of the mandibular symphysis, and do not diverge laterally with the dentaries in the mandibular rami. The mandible lacks internal or external mandibular fenestra. These unique features, and unusual combination of character states, indicate that the Sabinas dyrosaur represents a previously unknown species. Based on criteria used to recognize marine reptile feeding guilds, the skull and tooth morphology indicate that the Sabinas dyrosaur had a durophagus diet, and may have preyed on the thin-shelled ammonites that are abundantly preserved in the same deposits. Restoration of the fragmentary skull suggests that the Sabinas dyrosaur had a total body length of about 5 to 6 meters, and was among the largest dyrosaurs known. Although dyrosaurid crocodylians were abundant and diverse in Africa during Late Cretaceous and Paleogene time, the Sabinas dyrosaur represents only the second species thus far known from North America. Its occurrence in Mexico is compatible with an hypothesized trans-Atlantic dispersal event of dyrosaurs from Africa to North America during Late Cretaceous time.