METHODS AND MATERIALS
Quarried Blocks with Microvertebrate Fossils
MNA locality (loc.) 318 (Figure 3.1) occurs in a 2 m high exposure (8 m in lateral extent) of calcareous grayish-green mudrocks interbedded between white biogenic limestones (micrites) containing occasional external molds of freshwater gastropods. Screenwashing of the mudrocks from the locality and inspection in the field had previously indicated that abundant microvertebrate fossils and rare gastropod fragments were exposed on edge in a single bedding plane for about 1 m along the face of the outcrop (Figure 3.2). Screened samples from above this bedding plane had produced no small vertebrate remains, and samples screened below the bedding plane produced very few. Therefore, the fossiliferous horizon in the cliff face was quarried by blocks.
A relatively large block of matrix, including the fossiliferous level, was exhumed with hammers and chisels, plaster jacketed, and prepared in the laboratory. Several small blocks were collected and prepared in the same manner. The blocks were numbered, labeled with north arrows, and their position relative to other blocks was sketched. Sedimentary matrix was painstakingly removed to the level of the fossils with carbide needles and insect pins under a dissecting microscope over an eight-month period. Once prepared, blocks were individually mapped while maintaining their relative positions, and a master map of those portions of the quarried deposit was drawn. A grid pattern of 10 cm x 10 cm squares was superimposed on the map. The general aspect of the microvertebrate deposit and the quality of bones preserved in it are shown in
Taphonomic data collection followed
Munthe and McLeod (1975), and data analyses followed approaches much like those of
Maas (1985) and
Dodson and Wexlar (1979), along with the application of biological as well as taphonomic criteria (e.g., Korth and Evander 1986).
The trend (compass orientation) of the proximal end of 82 identifiable, elongate bones was measured, and their frequencies among 24 sectors of 15° each was plotted on a circular histogram. If proximal and distal ends were broken off or indiscernible, the azimuth of the larger end was measured. If there was no difference in size between ends, the bone was not used in the analysis. Plunge was not measured because all the bones were lying essentially horizontal on a relatively flat bedding plane. Nevertheless, the circular distribution data were treated as a diametrically bimodal distribution (Zar 1984) in case no significant direction of orientation occurred and a mean angle of orientation could still be calculated. The nonparametric Rayleigh test (Zar 1984) was used to test the null hypothesis that the sampled population of bones is uniformly (randomly) distributed.
An attempt was made to identify all skeletal elements, and where possible, to identify the species to which certain skeletal elements belonged. Some teeth, dentaries, and skull fragments were carefully removed from the prepared quarried blocks after mapping. This procedure was necessary to identify small mammals whose tooth crowns were either buried in the sediment or lying obliquely, thereby preventing detailed in situ evaluation with a microscope. The removed elements were assigned numbers that were recorded on individual block maps. These fossils were then completely prepared, usually with the aid of water-soluble Carbo-Wax (General Electric Co.). All fossils were examined for signs of abrasion and transport by water.
In quantifying the vertebrate taxa at the House Mountain fossil locality, the relative abundance was based on crania, dentaries, and isolated premolars and molars (Table 1). No attempt was made to count symmetrical pairs of paired elements or greatest number of ipsilateral elements. Nor was any attempt made to match or to separate pairs of contralateral elements based on similar degrees of tooth wear. A simple total number of halves for each paired element was counted and divided by two in order to determine the MNI represented. Thus,
Table 1 may slightly underestimate the actual numbers present in the deposit.
Modern Owl Pellets for Comparison
Recent owl pellets were collected from several localities in Arizona in order to make taphonomic comparisons with the fossil assemblage from House Mountain. Owl pellets analyzed in the greatest detail came from three localities. The first two localities, occupied by the barn owl (Tyto alba), occurred in agriculturalized mesquite grassland between Portal, Arizona, and Rodeo, New Mexico. These materials were analyzed by generally following the methods of
and Wexlar (1979) on pellets gathered from captive owls. Counts were made of each skeletal element except ribs, and the degree of completeness and breakage patterns of specific elements were noted. Mammal, bird, reptile, and amphibian skeletal elements were tallied separately. No species identifications were made for postcranial bones of mammals from these sites, but cranial elements were identified at lower taxonomic levels. A count of the minimum number of individuals (MNI) was based on major elements (e.g., dentary, femur). Data from the modern owl pellets, including prey taxa represented, proportional representation of skeletal elements, and patterns of bone breakage were compared with published studies and with the fossils from House Mountain.
Expected number of individual bones (NIb) was based on the number of the elements found in a typical small mammal skeleton multiplied by the minimum number of individual small mammals. This result was then compared to the number of individual specimens (NISP); NISP often exceeds the "expected" NIb because of multiple fragmentations of a given element (see Results and Discussion).
The third modern pellet locality was a habitual roost for barn owls at a short cliff south of Sugarloaf Mountain, Yavapai County, Arizona. Along the base of this cliff was an accumulation of small vertebrate bones from regurgitation pellets (Figure 5.1-5.4) representing prey remains from many seasons. The exact duration of accumulation is unknown, but probably represents at least one season of occupation of the roost.
Small vertebrate bones at this site were spread across an area of ground estimated at 12 m2. The density of bones varied greatly in different portions of the area. Compass orientation of an arbitrary sample of long bones was determined as in the fossil sample. I measured the trend of 95 bones in a sparsely vegetated area that sloped slightly downhill away from the cliff. Because of the slope, there was reason to expect that a specific mean direction might result. Therefore, a modification of Rayleigh's test was used (Durand and Greenwood 1958;
Zar 1984) to consider the null hypothesis that the population angles are randomly distributed. Two 0.25 m2 segments (approximately equal to the quarried area of the Pliocene deposit) within the densest concentration of bones were sampled in 25 increments each 10 cm x 10 cm (Figure 6.1-6.4). Data on prey species present, prey sizes, and proportional representation of all skeletal elements (including ribs) were collected also.
A few modern pellets were collected opportunistically near Camp Verde, Arizona, from a great horned owl (Bubo virginianus) roost. These pellets were examined in less detail than were the barn owl pellets from the other localities, but some incidental results are mentioned below.
Age-frequency profiles were produced for Prodipodomys idahoensis, the most abundant rodent in the House Mountain fossil assemblage. In order to determine the ontogenetic age of individual kangaroo rats, arbitrary tooth-wear categories similar to those used by neontologists were established (see
Figure 7). In addition, tooth crown-height measurements of certain cheek teeth were made (Table 2). Crown heights were measured from the apices of the dentine tracts as follows (terminology is from
Barnosky 1986; see also
Czaplewski 1990): P4, from labial dentine tract to metacone and from lingual tract to entostyle; M1, from labial tract to paracone and from lingual tract to protostyle; p4, from labial tract to hypostylid and from lingual tract to entoconid; m1, from labial tract to protostylid and from lingual tract to metaconid.