New Mexico Geological Society Annual Spring Meeting — Abstracts

The Krzyzanowski bonebed (NMMNH locality 3764) is an extremely rich vertebrate locality in Upper Triassic Chinle Group strata in the Blue Hills near St. Johns in east-central Arizona. The fauna of the Blue Hills includes the aetosaur Stagonolepis and the phytosaur Rutiodon, both index fossils of the Adamanian (latest Carnian) land-vertebrate faunachron. The bonebearing horizon is low in the Blue Mesa Member of the Petrified Forest Formation and consists of an intraformational conglomerate that rapidly (<10 cm) fines upward into a bentonitic mudstone. The entire bonebed appears to be pedogenically modified, as the strata are color-mottled, and the bones are frequently encrusted with an iron-rich concretionary coating. Fossils from the Krzyzanowski bonebed consist of disarticulated, but associated (often jumbled) bones of small tetrapods and fish, and the largest elements recovered thus far are less than 20 cm maximum dimension. To date, the macrovertebrate fauna consists of actinopterygian and coelacanth fish, extremely fragmentary metoposaurid amphibians (confined to the base of the bonebed), phytosaurs, at least one sphenosuchian, several fish, probable theropods, and a possible ornithischian. Among the most important of these are a fish with an elongate, edentulous rostrum, and a tiny dentary bearing teeth that closely resemble those of Triassic ornithischians. Microvertebrates previously reported from the quarry include actinopterygian fish, at least two archosauriforms, probable theropods, an ornithischian dinosaur, and another probable herbivorous dinosaur. We have also recovered several new records, including possible prosauropods, the ornithischian Tecovasaurus or a closely allied form, and additional theropods, sphenosuchians, and archosauromorphs.

There are two particularly important aspects of the Krzyzanowski bonebed: (1) the extreme richness of the deposit; and (2) its preservation of small vertebrates, some of which were previously known only from microvertebrate remains. The richness of this deposit cannot be overstated—one small (<5 kg) jacket has yielded over 100 identifiable fossil elements, including parts of at least five skulls. This particular jacket is probably abnormally rich, but there are still hundreds, if not thousands, of bones in the eight other jackets awaiting preparation.

The macrovertebrate fauna of the Krzyzanowski bonebed includes diverse taxa that possess teeth and other elements previously only known from screen washing. The bonebed therefore is potentially a “Rosetta stone” where isolated teeth from microvertebrate faunas can be matched to more readily identifiable skulls and lower jaws. Thus, the bonebed not only provides a glimpse into a Late Triassic ecosystem, but could lend insight into ecosystems represented by more fragmentary fossils elsewhere.