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A Latest Cretaceous (Earliest Puercan?) Local Fauna, Hell Creek Formation, Southeastern Montana

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A Latest Cretaceous (Earliest Puercan?) Local Fauna, Hell Creek Formation, Southeastern Montana Paludicola 10(1):50-91 September 2014 © by the Rochester Institute of Vertebrate Paleontology PRELIMINARY REPORT ON THE MAMMALS FROM LANE'S LITTLE JAW SITE QUARRY: A LATEST CRETACEOUS (EARLIEST PUERCAN?) LOCAL FAUNA, HELL CREEK FORMATION, SOUTHEASTERN MONTANA Thomas S. Kelly Research Associate, Department of Vertebrate Paleontology, Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, California 90007, USA ABSTRACT New records of mammalian fossils that are associated with non-avian dinosaurs from a quarry in the upper Hell Creek Formation of Powder River County, Montana, are described and include the following taxa: Meniscoessus robustus; Mesodma spp.; ?Cimolodon sp.; Turgidodon rhaister; Glasbius twitchelli; Protolambda mcgilli new species; Pediomys elegans; ?Leptalestes cooki; ?Leptalestes sp.; Didelphodon vorax; cf. Eodelphis sp.; Procerberus sp. (large); ?Procerberus sp. (small); Cimolestes sp.; Baioconodon sp., cf. B. nordicus; and Paleoungulatum hooleyi new genus and species. Stratigraphic evidence suggests that the mammalian and lower vertebrate taxa from the quarry are contemporaneous and late Cretaceous (Maastrichtian) in age. INTRODUCTION METHODS The upper Hell Creek Formation and the Measurements of teeth were made with an overlying Tullock Member of the Fort Union optical micrometer to the nearest 0.01 mm, whereas Formation of eastern Montana have long been known dentary measurements were made with vernier for their latest Cretaceous (Lancian North American caliper to the nearest 0.1 mm. Dental terminology for Land Mammal Age) and early Paleocene (Puercan Metatheria and Eutheria follows Nessov et al. (1998), North American Land Mammal Age) mammalian for Multituberculata follows Clemens (1963), and for assemblages, respectively (e.g., Sloan and Van isolated teeth of non-avian dinosaurs follows Larson Valen, 1965; Archibald, 1982; Lofgren, 1995; Cifelli and Currie (2013). Upper and lower teeth are et al. 2004). Lane's Little Jaw Site (LLJS) occurs in designated by capital and lowercase letters, the upper Hell Creek Formation on deeded, private respectively. All specimens are deposited in the land owned by Lewis and Tawny McGill of the collection of the Department of Vertebrate McGill Land and Livestock Corporation, Powder Paleontology at the Natural History Museum of Los River County, Montana. The site was named after Angeles County and detailed locality data are the landowner's son, Lane McGill, because he found available at this institution. the first small mammal jaw at the site. Subsequently, Cladistic analyses using parsimony were Kurtis W. Hooley, a local collector, and Lewis performed using Hennig86, V1.5, with explicit McGill opened a commercial quarry at the site. After enumeration and all character states unordered. realizing the scientific significance of the their finds Abbreviations are as follows: ap, greatest at the quarry, Hooley and McGill graciously donated anteroposterior length; CI, consistency index; L, left; the specimens described in this study. The LLJS LACM, Natural History Museum of Los Angeles Quarry has yielded well preserved mammal County; ln, natural log; OR, observed range; R, right; specimens along with representatives of RI, retention index; tal, talonid; tr, greatest transverse Chrondrichthyes, Actinoptergyii, Allocaudata, width; tra, anterior transverse width; tri, trigonid; trp, Testudines, Crocodylia, Choristodera, and non-avian posterior transverse width; UCMP, Museum of dinosaurs. The entire fauna from the LLJS Quarry is Paleontology, University of California, Berkeley. referred to the McGill Ranch Local Fauna in honor of Lewis and Tawny McGill. The purpose of this report GEOLOGIC SETTING is to describe the mammal specimens recovered from the LLJS Quarry and discuss their probable The LLJS Quarry (= locality LACM 7942) chronologic relationship to the specimens of non- occurs at the base of a filled channel cut in the upper mammalian taxa in the fauna. Hell Creek Formation near the southeastern end of Sixmile Ridge (Figure 1). A measured section was 50 KELLY—MAMMALS FROM HELL CREEK, MONTANA 51 taken from the top of a mesa about 0.6 km east of the higher ridges throughout the area east of the Powder LLJS Quarry down section to where the local base of River in the Powderville 30' x 60' Quadrangle (Vuke the Hell Creek Formation is covered by Quaternary et al, 2001). Vuke et al (2001) referred to this unit as alluvium (Figure 1, Appendix 1). The fossiliferous the boundary sandstone of the Hell Creek Formation horizon at the quarry occurs in a distinctive layer because it has yielded non-avian dinosaurs within the along the base of the channel cut. This layer is Powderville 30' x 60' Quadrangle (E.S. Belt, oral characterized by yellowish-buff, fine-grained communication in Vuke et al. [2001]). Vuke et al. sandstone matrix with numerous rounded claystone (2001) stated that "it is unlikely that the Cretaceous clasts and occasional pebble clasts. Iron oxide dinosaur bones and teeth were reworked into a incrusted small root casts are common in this layer, Paleocene basal Fort Union sandstone because a which were derived from smectitic claystone that the similar sandstone bed at the same stratigraphic channel is cut into. The fossils are distributed position within the south-adjacent Broadus 30' x 60' throughout this layer, unsorted by size, and consist Quadrangle contains Cretaceous palynomorphs (D. primarily of isolated elements less than 10 cm in size. Nichols, written communication, 1992) as well as Within the channel cut and overlying the basal dinosaur fossils (U.S. Bureau of Land Management, fossiliferous horizon the following layers are present Miles City)." Results of a field examination during in ascending stratigraphic order (Appendix 1): 1) a 2013 of most of the higher ridges surrounding and light gray, fine-grained sandstone; 2) a layer including Sixmile Ridge (e.g., Rattlesnake Ridge, consisting of yellowish-gray sandstone matrix with Cedar Ridge, Oliphant Butte, Pine Hill, and Soldier's rounded claystone clasts and occasional pebble clasts, Mount) appear to confirm Vuke et al.'s (2001) similar lithologically to the fossiliferous horizon; and working hypothesis of a relatively consistent 3) a layer of light gray, fine-grained sandstone. stratigraphic position for their boundary sandstone. Fossils are lacking in all of the layers above the Vuke et al. (2001) mapped the sediments fossiliferous horizon. The occurrence of two exposed on the mesa above the boundary sandstone distinctive sandstone layers containing rounded clay and east of the LLJS Quarry as the Tullock Member clasts that are each overlain by fine-grained of the Fort Union Formation (Figure 1). No distinct sandstone, indicates that a least two major flood outcrops typical of the Tullock Member were found events occurred during the cutting and filling of the on field examination in the 8-12 m of sediments on channel. this mesa above the boundary sandstone. Instead, the Above the channel cut and the smectitic mesa surface consists of a yellowish-brown, sandy claystone that the channel is cut into is a 3.3 m thick, soil that is covered with grass and sagebrush. structureless, yellowish-buff, fine grained sandstone. However, assuming that the top soil was derived Overlying this sandstone is a distinctive 1.5 m from the Tullock Member, which is probably present resistant, ledge-forming, well-cemented, fine-grained below the surface soil, this unit is shown hesitantly as sheet sandstone. This resistant, capping sandstone the Tullock Member on the stratigraphic column in can be traced laterally for at least 1.5 km northeast Figure 2. and 0.6 km southeast of the quarry, where it varies A second locality, McGill 1 (= locality LACM from 0.9 m to 2 m thick. There is no evidence that 7943), occurs on the McGill Ranch in a sandy this capping sandstone was breached when the fossil claystone just below a resistant, ledge-forming flaggy bearing channel was cut into the underlying smectitic sandstone within the Hell Creek Formation that is claystone, strongly suggesting that the channel cut lower in the section and about 0.7 km north of the occurred prior to the deposition of the capping LLJS Quarry (Figure 2). It has yielded a partial sandstone. This scenario appears to be further Triceratops skull, an ungual of Pachycelphalo- supported because the sandstone matrix of the saurus, a partial dentary of a crocodilian, and a fossiliferous horizon appears to have been derived vertebral centrum of Champsosaurus. from the yellowish-buff sandstone overlying the top of the channel cut and the clay clasts appear to be AGE OF FAUNA ripped up from smectitic claystone that the channel cuts into. If this scenario is correct, then the The McGill Ranch Local Fauna from the LLJS fossiliferous horizon predates the deposition of the Quarry is composed of fish, amphibians, reptiles, and resistant, capping sheet sandstone. mammals (Table 1). The most common taxa Overlying the sheet sandstone is a thick, buff to represented in the fossiliferous layer are turtle shell brown sandstone that weathers to form moderately fragments, gar scales and crocodilians followed by gentle, sagebrush covered slopes. Capping this non-avian dinosaurs, with ceratopsian teeth sandstone is a thick, resistant, cross-bedded, ledge- particularly common. Teeth of chondrichtyians and forming, yellowish brown sandstone. This capping actinopterygians are relatively common. Mammals, sandstone is extensive and exposed along most of the 52 PALUDICOLA,
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