VERTEBRATE FOSSILS FROM THE UPPER(?) JURASSIC TRAIL FORMATION, KETTLE ROCK SEQUENCE: FIRST RECORDED EVIDENCE OF DINOSAUR REMAINS IN THE SIERRA NEVADA, CALIFORNIA

The Kettle Rock Sequence (KRS), exposed in the vicinity of Taylorsville, California, represents the northernmost exposure of Jurassic volcanic arc rock within the Sierra Nevada. The KRS consists of Lower to Middle Jurassic, proximal-facies, volcanic arc rock of the Kettle Formation (KF), conformably overlain by 2,600-m of felsic to silicic, Middle to Upper(?) Jurassic flow rock, ash-flow tuff, tuff breccia and sparsely fossiliferous shallow marine to subaerial clastic rock of the Cooks Canyon (CCF), Lucky S (LSF) and Trail (TF) formations. Two fossil locations in the upper KF have yielded Callovian ammonites. A felsic marine tuff from the lower CCF has yielded molds of possible Oxfordian bivalves. Prior investigations in the LSF and TF had uncovered only poorly preserved plant fossils (fern fragments).

In 1995 and 1997, dinosaur bone fragments were discovered in coarse-grained fluvial rock within the lower third of the TF in the vicinity of Evans Peak. Most often, the recovered vertebrate material weathers to a light blue-grey color but can appear black on fresh surfaces. The bone fragments are commonly rectangular in shape and 2 to 4 cm in length. Quality of preservation is good, but because the material appears as small, angular, broken fragments in volcanic cobble conglomerate, the prospect of recovering larger unbroken material is poor.

The two largest specimens recovered include the proximal end of an anterior dorsal rib (120 mm long and 95 mm wide at the proximal end) and a possible mid-section of a neural vertebra spine (7 cm long). The rib fragment is broken just shy of the head and tubercle, but does show most of the bifurcation. The size and robustness of the rib fragment suggest a rib cage similar in size to that of a modern American bison. The vertebrate fragments recovered from the Upper (?) Jurassic TF represent the first recorded evidence of dinosaur remains in the Sierra Nevada and are among only a handful of Jurassic dinosaur finds noted from California.