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The Microvertebrate Fauna of the Upper Triassic (Revueltian) Snyder Quarry, North-Central New Mexico Andrew B

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The Microvertebrate Fauna of the Upper Triassic (Revueltian) Snyder Quarry, North-Central New Mexico Andrew B New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/56 The microvertebrate fauna of the Upper Triassic (Revueltian) Snyder Quarry, north-central New Mexico Andrew B. Heckert and Hillary S. Jenkins, 2005, pp. 319-334 in: Geology of the Chama Basin, Lucas, Spencer G.; Zeigler, Kate E.; Lueth, Virgil W.; Owen, Donald E.; [eds.], New Mexico Geological Society 56th Annual Fall Field Conference Guidebook, 456 p. This is one of many related papers that were included in the 2005 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. New Mexico Geological Society, 56th Field Conference Guidebook, Geology of the Chama Basin, 2005. p. 319-334. 319 THE MICROVERTEBRATE FAUNA OF THE UPPER TRIASSIC (REVUELTIAN) SNYDER QUARRY, NORTH-CENTRAL NEW MEXICO ANDREW B. HECKERT1 AND HILLARY S. JENKINS2 1New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 87104-1375 2Department of Earth and Ocean Sciences, Duke University, 103 Old Chemistry Box 90229, Durham, NC 27708 ABSTRACT.—The Snyder quarry is a well-documented assemblage of Late Triassic invertebrates and vertebrates from the Painted Desert Member of the Upper Triassic Petrified Forest Formation in the Chama Basin, north-central New Mexico. The presence of Revueltian index taxa, including the aetosaurs Typothorax coccinarum and Desmatosuchus chamaensis and the phytosaur Pseudopalatus buceros, demonstrate that the Snyder quarry is of Revueltian (early-mid Norian) age. Screenwashing matrix from the primary bonebed at the Snyder quarry yielded a moderately diverse assemblage of microvertebrates, some of which were not represented in the macrovertebrate fauna. Microvertebrate fossils from the Snyder quarry are mostly scales and bone fragments—complete teeth are unusually rare. New records include a tooth of the hybodontoid shark Lonchidion and numerous scales of a palaeoniscid fish tentatively assigned to aff. Turseodus. The microvertebrate assemblage differs some- what from the known macrovertebrate assemblage, and includes many more osteichthyan fossils. Osteichthyans dominate the microvertebrate fauna, and include semionotids, redfieldiids, palaeoniscoids, and indeterminate sarcopterygians. Osteichthyans are largely represented by scales, with the exception of the indeterminate sarcopterygians and actinopterygians, represented by fragments of dentigerous toothplates, fossils previously assigned to “colobodontids.” The microvertebrate tetrapod fauna represented by teeth includes metoposaurid amphibians, juvenile (?) phytosaurs (?), probable dinosaurs, aetosaurs and other diverse, unidentified archosauromorphs. Many of the vertebrae appear to pertain to small archosauromorphs. The microverte- brate assemblage is unusual in that recovered vertebrae and other non-cranial elements greatly outnumber intact teeth, which normally dominate Chinle microvertebrate assemblages. We interpret this as additional support for the hypothesis of a cata- strophic origin for the Snyder quarry vertebrate assemblage, as more typical Chinle Group microvertebrate assemblages are attritional deposits in which teeth greatly outnumber vertebrae. Entrada Sandstone INTRODUCTION Key 16 Reptile bones The Snyder quarry (NMMNH locality 3845) is an excep- 15 tionally rich Upper Triassic fossil assemblage from the Painted Conglomerate Desert Member of the Petrified Forest Formation in north cen- n Mudstone o 14 i tral New Mexico that has been the focus of much recent study t a Soft sediment deformation (Heckert et al., 2000, 2004; Zeigler et al., 2002a,b,c; Heckert m r o 13 and Zeigler, 2003; Heckert et al., 2003a,b; Hurlburt et al., 2003; F Siltstone t 12 n Lucas et al., 2003a,b; Rinehart et al., 2003; Tanner et al., 2003; i o Zeigler, 2003; Zeigler et al., 2003a,b,c,d,e) (Fig. 1). Macrofossil P Sandstone, crossbedded k vertebrates, principally the bones and teeth of the archosaurian c o 11 Sandstone, massive to blocky phytosaurs, aetosaurs, dinosaurs, and rauisuchians, dominate the R assemblage. However, in 1999 one of us (ABH), with the help Sandstone-pebbly/coarse of the New Mexico Friends of Paleontology, collected ~3000 lbs (~1400 kg) of screenwash matrix from the principal bone-bearing meters 10 10 interval of the site. Furthermore, ever since the initial prepara- 8 tion of Snyder quarry jackets in 1998, matrix from those jackets n 9 6 o i t 4 has been kept and screenwashed as well, and the junior author a 8 2 m conducted an undergraduate thesis and related studies based on r 0 o this material (Heckert et al., 2004; Jenkins, 2004; Jenkins and F 7 t s Heckert, 2004a,b). Here, we describe the microvertebrate fauna e r 6 from the Snyder quarry, some of which was briefly mentioned in o F 5 the annotated faunal list presented by Zeigler et al. (2003b). The d e i 4 f i nomenclature and taxonomy we use for this paper follows that r 3 t L-3845 developed by Heckert (2001, 2004) for Chinle microvertebrates e 2 P 1 100 km from older strata in Texas, New Mexico, and Arizona, and com- plete references for the taxa listed in the systematic paleontology FIGURE 1. Index map and stratigraphic section showing the geographic section are provided there. Unless otherwise noted, all specimens location and stratigraphic position of the Snyder quarry (after Heckert were recovered from screenwash concentrate, although a few et al., 2000a). Heckert & Jenkins, Fig. 1 320 HECKERT AND JENKINS of the more diagnostic fossils were actually recovered by crew The Snyder quarry has thus far yielded a single incomplete members during excavation for macrovertebrates. xenacanth tooth (Zeigler et al., 2003b). The specimen (NMMNH P-33112) is readily identifiable, possessing the unique bifid STRATIGRAPHY AND AGE geometry and extended lateral cusps characteristic of xenacanth teeth (Fig. 2A-B). Only one lateral cusp remains on this speci- The Snyder quarry is in the Painted Desert Member of the men, however, the orientation of the lateral and central cusps with Petrified Forest Formation, 28.5 m below the contact with the the base of the tooth is also diagnostic. The anterior lateral cusp overlying Rock Point Formation (Fig. 1). This stratigraphic posi- and lateral cutting edge are present together with a small central tion is approximately equivalent to the Canjilon quarry 4 km to cusp (Fig. 2A-B). The central and lateral cusps both align along the east and, probably, the Hayden quarry approximately 6 km to the labial edge of the tooth. The specimen has a sub-ovoid base the south and east. The Snyder quarry is also at nearly the same with a low root, and is referred to as “Xenacanthus” moorei in stratigraphic horizon as the type locality of Eucoelophysis bald- this study. wini Sullivan and Lucas near Orphan Mesa and several other The morphology of this specimen is representative of the xen- localities in the general vicinity (Lucas and Hunt, 1992; Hunt and acanth teeth found in other parts of the Chinle Group (Murry, Lucas, 1993a; Sullivan et al., 1996; Sullivan and Lucas, 1999; 1982, 1986; Heckert, 2001, 2004). However, much debate has Lucas et al., 2002, 2003). All of these localities are much lower occurred over the taxonomy of these Upper Triassic xenacan- stratigraphically than the famous Whitaker (Coelophysis) quarry thiformes (Heckert, 2001, 2004). Johnson (1980) first identified at Ghost Ranch, which is at least 50 m above the aforementioned Xenacanthus moorei from the lower Kalgary locality in Texas. theropod localities and in an entirely different stratigraphic unit, However, Schneider (1996) argued that the name “Xenacanthus” the Rock Point Formation (Sullivan et al., 1996; Lucas et al., should not be applied to Triassic xenacanths because
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