Early Cretaceous pterosaur guano deposit from central Oregon, USA

Abstract

Excavation of a green breccia lens in the otherwise shaley Hudspeth Formation of central Oregon recovered a surprising variety of fossils of different kinds: plant impressions, mollusc shells, and vertebrate bones and teeth. Some ammonites, such as Mortoniceras inflatum, which date the deposit as late Albian (103 Ma), were preserved with shell intact below, but dissolved above back to sutures. Intact ammonites were outnumbered by angular ammonite fragments in the size range 1–2 cm. Marine clams, snails and ammonites were found in the same bed as a variety of fossil plants, including horsetails, ferns, cycads, and conifers. Also recovered were bones of fish, ichthyosaurs, dinosaurs, and pterosaurs. Orientation of large fossil logs and branches reveal derivation from the east, where a shoreline of shingle beaches and alluvial fans created the interfingering Gable Creek Conglomerate. The matrix-supported green breccia is massive and ungraded, and interpreted as a mass flow deposit. Enrichment in phosphorus compared with shale above and below is evidence that its fine-grained component includes guano. This guano and unusual fragmentation of ammonites are interpreted as evidence that the Oregon pterosaur, Bennettazhia oregonensis, was a mollusc-eater and formed large colonies on nearby cliffs, like modern gull rookeries.