New Mexico Geological Society Annual Spring Meeting — Abstracts

Recently, one of us (MS) discovered a prolific new locality (NMMNH locality 3845) in the badlands of the Petrified Forest Formation near Ghost Ranch, New Mexico. This locality yields fossils that represent a diverse vertebrate and invertebrate fauna. The invertebrate fauna consists primarily of conchostracans, although a possible decapod crustacean was also discovered at the site. The vertebrate fauna consists of both fish and abundant tetrapods. A partial, articulated fish skeleton represents a semionotid, and numerous scales have also been recovered from the site. The tetrapod fauna is dominated by archosaurs, including indeterminate phytosaurs, the aetosaur Desmatosuchus haplocerus, a coelophysoid theropod dinosaur, and another, larger theropod; a distal humerus of a possible cynodont? was also recovered. Plants are represented by non-diagnostic woody debris.

Stratigraphically, the site is high in the Petrified Forest Formation, approximately 60 m below the Entrada Sandstone. Tetrapod biochronology, based on the presence of the aetosaur Typothorax; and the phytosaur Pseudopalatus, indicates a Revueltian (early-mid Norian, approximately 210-218 Ma) age for the Petrified Forest Formation in this area.

Tetrapod bones from locality 3845 are extremely well-preserved and occur primarily in a bonebed of greenish gray intraformational conglomerate that fines upward into sandy mudstone of similar colors. Conchostracans are preserved in close association with tetrapod bones. The fish skeleton was found approximately 1.5 m above this horizon in laminated mudstone above a similar, fining-upward sequence. The decapod? was found in a spoil pile at the site, but was probably originally close (<1 m) to the bone-bearing horizon.

Coelophysoid theropod material recovered thus far includes elements of the pelvic girdle and the hind limbs as well as a partial tooth. The presence of two proximal left tibiae and fibulae indicates the presence of at least two individuals. An aberrant tetrapod specimen consists of a fused tibia-fibula-astragalus-calcaneum that is also fused proximally. This probably represents a large (>3 m body length) theropod with an as-yet-undetermined pathology.

Particularly important aspects of this site are: (1) the abundant and highly diverse faunal elements, including aquatic invertebrates, fish, and both semi-aquatic and terrestrial tetrapods; (2) the exceptional preservation that characterizes all taxa found at the site; (3) the 10+ m extent of the fossiliferous horizons; and (4) the relatively rare taxa, including decapods?, dinosaurs, and cynodonts? found at the site.