An owl-pellet accumulation of small Pliocene vertebrates from the Verde Formation, Arizona, USA
A small quarry in Pliocene lacustrine deposits at House Mountain in the Verde Formation of central Arizona produced a high concentration of microvertebrate fossils of early Blancan (early Pliocene) age. Within a single bedding plane on excavated blocks, a miniature (0.25 m2) bone bed was exposed piecemeal under a microscope and mapped. The deposit contained about 300 recognizable skeletal elements representing a small finch and several kinds of rodents. Concomitant screenwashing of the site produced 1085 more rodent and bird bone fragments plus shrew, bat, and salamander remains. The House Mountain assemblage clearly exhibits the characteristics of nocturnal raptor (owl) pellet accumulations. Comparison with modern owl pellet accumulations in Arizona, including a quantitative consideration of bones from an area equal to that quarried for fossils showed similar partial association of the elements of individual skeletons. Weathering of bones prior to burial probably was limited to brief subaerial exposure to rainfall and insect activity or deposition in calm, shallow water sufficient to disintegrate owl pellets but not sufficient to completely dissociate some skeletal elements of individual prey animals. A medium-sized owl (100 to 400 g) probably accumulated the House Mountain microvertebrates. Contrary to other evidence, the age-frequency profile of extinct kangaroo rats, Prodipodomys, does not indicate attritional mortality. Had screenwashing been used as the only method of recovery of fossils, much taphonomic information would have been lost. However, unless the taphonomic information for a given site is crucial or equivocal, the additional time spent on quarrying and mapping may be unwarranted.
KEY WORDS: microvertebrates; owl pellets; Pliocene; quarry map; rodents; taphonomy
PE Article Number: 14.3.30A
Copyright: Society of
Vertebrate Paleontology November 2011
Submission: 15 June 2007. Acceptance: 16 July 2011