TWO DISTINCT FROG DEATH ASSEMBLAGES FROM THE SHEEP PASS FORMATION (MAASTRICHTIAN-EOCENE), EAST-CENTRAL NEVADA

Two separate horizons containing five complete and multiple disarticulated frogs have been discovered in the southern Egan Range of east-central Nevada. The horizons occur within Members B and C of the Sheep Pass Formation type section. Member B was previously dated as Maastrichtian-Paleocene and represents a shallow lacustrine, microbial laminated, dolomitic limestone. The oldest date for Member B is 66.1 ± 5.4 Ma based upon an U-Pb carbonate age from the base of the member. Member C is Late Paleocene-Early Eocene(?), determined by previous molluscan biostratigraphy, and consists of interfingering braided fluvial to lacustrine facies composed of sandstone, carbonaceous siltstone, and limestone.

Complete frogs vary in size from 93mm to over 200mm. Preliminary identifications designate two specimens extracted from Member B as Palaeobatrachus occidentalis based on the dorsal prominence and acetabular features on the ilia. Three articulated frogs from Member C have been identified as Eorubeta nevadensis based on the distally expanded sacral diapophyses and preserved equilength transverse processes. Unfortunately, bone is poorly preserved, in some cases only molds remain, due to modern erosional processes. Major identifying features are degraded, and further collection may substantiate these identifications. Associate taxa include ostracods, bivalves, high-spired and planispiraled gastropods, and agglutinated caddis fly larval cases. Taphonomic data from the frogs suggests that little to no transport occurred as evident by the well-articulated, life-like posture of the frogs and lack of abrasion. Several blocks of sandstone and carbonaceous siltstone containing “frog-hash” have been recovered from Member C, however the disarticulated specimens show no signs of abrasion or winnowing indicating minimal transport.

Possible hypotheses for the death assemblages include fungal disease as suggested in previous studies involving similar circumstances. A stronger explanation may exist in periodic dessication and/or salinity changes caused by climatic fluctuations, supported by mudcracked intervals, a lack of associated fish and aquatic reptile fossils, and an upsection shift to a more playa-like setting.