A sharp-billed stem ibis from the Fossil Butte Member of the Green River Formation

Abstract We report a new species of stem ibis (Threskiornithidae) from the early Eocene Fossil Butte Member of the Green River Formation. Rhynchaeites mcfaddeni n. sp. is represented by an articulated partial skeleton and was a diminutive species, with an estimated body mass of under 300 g. The holotype specimen is preserved in an unusual manner, leaving the skull, presacral vertebral column, wings, and feet in approximate life position despite the loss of the trunk, pelvis, tail, and upper legs. This peculiar mode of preservation may represent a case of the “stick ‘n’ peel” model of preservation, by which a portion of a carcass becomes firmly adhered to the sediment by substances released during decay. Like other members of the stem ibis genus Rhynchaeites , Rhynchaeites mcfaddeni n. sp. lacks the dense pitting of the bill tip associated with abundant Herbst corpuscles, which is developed in extant ibises. However, the new species differs from all stem and crown ibises in possessing a sharply tapering beak tip, which suggests a unique mode of foraging, possibly including prying mollusks from their shells. Given that Rhynchaeites messelensis Wittich, 1898 is one of the most common birds in the prolific Messel deposits but Rhynchaeites mcfaddeni n. sp. is represented by a single specimen after more than 50 years of intense collecting in the Green River Formation, it is likely the two species had divergent foraging strategies, with the former perhaps feeding closer to the lake margin and the latter in streams. UUID: http://zoobank.org/60d25b3b-9c76-4553-99c4-a390d3a5548d