Late Pleistocene (Rancholabrean) mammals from fissure deposits in the Jurassic Todilto Formation, White Mesa mine, Sandoval County, north-central New Mexico Gary S. Morgan, [email protected], and Larry F. Rinehart, [email protected], New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, New Mexico 87104 Abstract A late Pleistocene (Rancholabrean) fossil site containing partial to nearly complete articu- lated skeletons of large mammals preserved in fissure deposits was discovered in 2005 at the White Mesa mine near San Ysidro in San- doval County, north-central New Mexico. The fissures are in gypsum of the Jurassic Todilto Formation. Structural analysis suggests the fissures opened to the surface during the Pleistocene in response to extension associ- ated with the Rio Grande rift. Bones were found approximately 9–12 m (30–40 ft) below the modern land surface in three different fis- sures within approximately a 15-m (~50-ft) radius. Four species of large mammals are preserved in the fissures, each known from at least one partial to nearly complete skeleton: stilt-legged horse (Equus cf. E. francisci), camel (Camelops hesternus), extinct bison (Bison anti- quus), and mule deer (Odocoileus hemionus). The most fossiliferous fissure, containing one skeleton each of a camel and a bison, was 18 cm (7 in) wide and contained bones over a depth of approximately 40–50 cm (15–20 in) and a length of approximately 3.5–4 m (11–13 ft). The camel consists of a partial skeleton of a very old individual of Camelops hester- nus with heavily worn teeth and evidence of osteoarthritis on the articular surfaces of several limb bones and toes. The bison is a FIGURE 1—Map showing the location of the White Mesa mine, Sandoval County, north-central nearly complete articulated skeleton of a New Mexico. juvenile Bison antiquus with deciduous pre- molars, unerupted third molars, and unfused The fissures were wider near the surface and of 2005. A study of the structural geology epiphyses on all limb bones. A second nearby narrower at depth. The animals either died of these fissure deposits was recently pub- fissure contained an articulated front limb of from falling into the fissures or were unable lished (Rinehart et al. 2006). This paper an advanced juvenile or young adult Equus to climb out and starved to death. The artic- summarizes the vertebrate paleontology cf. E. francisci and an articulated hind limb ulated condition of the skeletons and the lack of bite marks on the bones suggest the and taphonomy of the late Pleistocene fis- of a second adult Camelops hesternus. A third sure deposits from the White Mesa mine. fissure had a nearly complete skeleton of a fissures were inaccessible to predators or young adult female Odocoileus hemionus. The scavengers that would have damaged and To our knowledge, the White Mesa mine White Mesa mine differs from most New disassociated the carcasses. The bodies of the represents the first record of Pleistocene Mexico Pleistocene cave faunas in the pre- ungulates began to decompose in the wider fissure deposits in New Mexico, although dominance of articulated skeletons and the portion of the fissures nearer the surface. several Pleistocene cave sites with vertical rarity of small vertebrates, although a par- Finally, the mostly decomposed skeletons openings functioned as natural traps. tial skeleton of the pocket gopher Thomomys still held together by connective tissues fell Abbreviations used are: LF (Local talpoides was found in the same fissure that and became lodged in the lower and nar- Fauna); NMMNH (New Mexico Museum produced the deer skeleton. An AMS radio- rower portion of the fissures where they were eventually discovered. of Natural History). The abbreviations for carbon date on a Camelops metacarpal from tooth positions in mammals are standard, the White Mesa mine yielded an age of 12,910 ± 60 14C yrs B.P. with upper case letters for upper teeth The predominance of grazing ungulates Introduction and lower case letters for lower teeth: I/i in the fissures, including horse, camel, and (upper/lower incisors), C/c (upper/lower bison, suggests that the habitat in the vicin- Workers at the White Mesa mine in Sando- canines), P/p (upper/lower premolars), ity during the late Pleistocene consisted of val County, New Mexico, discovered several and M/m (upper/lower molars). All mea- a grassland. The following is a hypothetical skeletons of large late Pleistocene mammals surements are in millimeters (mm). sequence of events that led to the preserva- in 2005 in fissure-fill deposits developed tion of partial to nearly complete skeletons in gypsum of the Middle Jurassic Todilto of large Pleistocene mammals in very narrow Locality fissures that were not wide enough to accom- Formation. Field parties from the New modate the living (or recently dead) animals. Mexico Museum of Natural History, aided The White Mesa Mine Local Fauna The fissures were open to the surface in the by the miners, in particular David Pino and (NMMNH locality L-6112; Fig. 1) was col- late Pleistocene and acted as natural traps Lambert Pino, collected these fossils over lected from the White Mesa mine, oper- into which the animals inadvertently fell. a period of several months in the summer ated by the American Gypsum Company May 2007, Volume 29, Number 2 NEW MEXICO GEOLOGY 39 on land belonging to Zia Pueblo, approxi- 3 was located 10 m (~33 ft) south of fissure LF (NMMNH site L-6112). The specimens mately 6 km (~4 mi) southwest of San 1 and contained a complete deer skeleton, from the White Mesa mine are curated Ysidro in Sandoval County, north-central a partial skeleton of a pocket gopher, and in the vertebrate paleontology collection New Mexico (35°32’N, 106°48’W). Gypsum several isolated bones of a small mouse- of the New Mexico Museum of Natural from the White Mesa mine is mined from sized rodent. Besides removing all exposed History (NMMNH); however, the fossils the ~30-m (~100-ft) thick Tonque Arroyo bones from this fissure, we also collected a remain the property of Zia Pueblo. Most Member of the Middle Jurassic Todilto For- small sample of the sediment (several kilo- of the accounts are divided into three sec- mation and is used to manufacture sheet- grams) for screenwashing. Fissure 3 was tions, referred specimens, description, and rock or wallboard. During the removal of 25 cm (10 in) in width, and the fossils were remarks. Three of the species are members gypsum with heavy machinery, Pleistocene found over a depth of approximately 20–30 of the extinct Pleistocene megafauna, Equus fossils were discovered in three fissure-fill cm (8–10 in) and a length of less than 1 m cf. E. francisci, Camelops hesternus, and Bison deposits within a radius of about 15 m (~50 (~3 ft). antiquus, whereas Odocoileus hemionus and ft) approximately 9–12 m (~30–40 ft) below The lithology of the fissure fills consists of Thomomys talpoides are living species. A the present land surface (Fig. 2). These fis- brecciated fragments of gypsum in a matrix small mouse-sized rodent represents a sures were open to the surface during the of poorly consolidated, silty, fine- to medi- sixth species in the fauna. late Pleistocene and trapped large mam- um-grained gypsum sandstone with sparse mals. The site has produced articulated or quartz grains. The gypsum fragments are Order PERISSODACTYLA associated skeletal material of five species mostly 1–3 cm (0.4–1.2 in) in diameter, but Family EQUIDAE of mammals, including a horse (Equus cf. the largest are as much as 10 cm (4 in) or Equus cf. E. francisci Hay 1915 E. francisci), a camel (Camelops hesternus), more. The proportion of gypsum breccia to Stilt-legged horse a bison (Bison antiquus), a deer (Odocoileus sand varies considerably among the vari- Referred specimen hemionus), and a pocket gopher (Thomomys ous fissure deposits. The brecciated frag- —NMMNH 54152, par- talpoides), all of late Pleistocene (Ranchola- ments make up ~25–75% of the fill. Some tial front limb from fissure 2, including the brean) age. stratification is evident, with beds that are distal two-thirds of the humerus, radius- more or less sandy showing lateral conti- ulna, carpals (scaphoid, lunar, pisiform, Fissures and fissure-fill deposits nuity. Fissure 3 had a higher percentage of magnum, pyramidal, trapezoid, trape- fine-grained sand than the other two fossil- zium), metacarpal 3 (including associated There were at least 44 fissures within 100 m bearing fissures and was the only fissure sesamoids), lateral metacarpals 2 and 4, (~325 ft) of the fossil site (locality NMMNH that contained small mammals. and proximal, medial, and ungual (hoof) L-6112), only three of which contained The trends and widths of 44 fissures phalanges. bones, here designated fissures 1, 2, and in the White Mesa mine floor at the level Description—A nearly complete articu- 3. Fissure 1 contained the largest number of locality L-6112 were measured (Rine- lated front limb of an advanced juvenile or of bones, including an articulated skeleton hart et al. 2006). All were within a 100-m possibly a young adult stilt-legged horse of a bison and an articulated skull, lower (~325-ft) radius of the fossil site and had a Equus cf. E. francisci was found in fissure jaws, and cervical vertebrae of a camel nearly vertical orientation. Fissure widths 2 (Fig. 4D). No other postcranial elements, (Fig. 3). The camel was situated above the range from 1 to 34 cm (0.4 in to 14 in). A teeth, or cranial material of a horse were bison skeleton in the fissure and was heav- few of the narrower fissures are open and found in this fissure or disassociated in the ily damaged by the milling machine that unfilled in spots.