A new zygodactylid species indicates the persistence of stem passerines into the early Oligocene in North America

Tobin L Hieronymus,David A Waugh,Julia A Clarke

doi:10.1186/s12862-018-1319-6

Tobin L Hieronymus, David A Waugh + Show 1 more

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https://doi.org/10.1186/s12862-018-1319-6

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Abstract

BackgroundThe lake deposits of the informal Ruby Paper Shale unit, part of the Renova Formation of Montana, have yielded abundant plant fossils that document Late Eocene – Early Oligocene global cooling in western North America. A nearly complete small bird with feather impressions was recovered from this unit in in 1959, but has only been informally mentioned.ResultsHere we describe this fossil and identify it as a new species of Zygodactylus, a stem lineage passerine with a zygodactyl foot. The new taxon shows morphological traits that are convergent on crown Passeriformes, including an elongate hallux, reduced body size, and a comparative shortening of proximal limb elements. The fossil documents the persistence of this lineage into the earliest Oligocene (~ 33 Ma) in North America. It is the latest occurring North American species of a group that persists in Europe until the Miocene.ConclusionsEocene-Oligocene global cooling is known to have significantly remodeled both Palearctic and Nearctic mammal faunas but its impact on related avifaunas has remained poorly understood. The geographic and temporal range expansion provided by the new taxon together with avian other taxa with limited fossil records suggests a similar pattern of retraction in North America followed by Europe.

Highlights

The lake deposits of the informal Ruby Paper Shale unit, part of the Renova Formation of Montana, have yielded abundant plant fossils that document Late Eocene – Early Oligocene global cooling in western North America
Parts of this clade were first described from the Miocene of Germany and France [7, 11], partial skeletons are only known from the early Oligocene and early Eocene of these same regions and the Eocene of North America
The limb proportions of the new taxon are convergent on the ranges seen in crown Passeriformes, most notable in the length of proximal phalanx of the hallux

Summary

Introduction

The lake deposits of the informal Ruby Paper Shale unit, part of the Renova Formation of Montana, have yielded abundant plant fossils that document Late Eocene – Early Oligocene global cooling in western North America. Zygodactylidae [1] is an extinct clade of small birds that possess a zygodactyl foot, with two pedal digits that are caudally deflected [2,3,4,5,6]. This clade includes the genera Zygodactylus [7], Primozygodactylus [8], Eozygodactylus [9], and Primoscens [10]. Described species are most abundant in Eocene deposits of North America and Europe, they are known from the Oligocene and early Miocene of Europe [2, 3, 7, 11]. Zygodactylids have been identified as allied with extant songbirds, or stem passerine taxa [3]

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