AN EXCEPTIONAL NEW THALATTOSAUR (REPTILIA) FROM THE LATE TRIASSIC (NORIAN) HOUND ISLAND VOLCANICS OF SOUTHEASTERN ALASKA

In May 2011, the skeleton of a small (1.5 m) vertebrate was discovered in Triassic marine rocks exposed in an intertidal zone near Kake, Alaska. Following preparation at the University of Alaska Museum in Fairbanks the specimen, UAMES 23258, was identified as being a thalattosaur, a group of small-bodied (1-4 m) secondarily aquatic Triassic reptiles. UAMES 23258 was recovered from the Hound Island Volcanics in a calcareous shale unit interbedded with volcaniclastic-rich biogenic limestone. The Hound Island Volcanics are part of the Alexander Terrane and were deposited at low paleolatitudes (10-20 degrees North) within a volcanic island arc complex. Based on conodont and bivalve biostratigraphy, the specimen is middle Norian in age.

Compared to all other known thalattosaurs, UAMES 23258 exhibits a unique rostral morphology. Both the premaxilla and dentary are straight and very tapered and pointed anteriorly, in contrast to the ventrally deflected rostrum of Thalattosaurus. The presence of teeth in the upper toothrow is unknown, but the dentary teeth are gracile and recurved and the anterior third of the dentary is edentulous. This suite of characters also distinguishes UAMES 23258 from Nectosaurus, of which fragmentary remains had previously been reported from the formation. UAMES 23258 is the only articulated thalattosaur material known from the Hound Island Volcanics and Alaska. Significantly, it is the most complete and well-articulated thalattosaur known from North America and being Norian in age, it is also one the youngest thalattosaurs known.