The ‘Last Hurrah of the Reigning Darwinulocopines’? Ostracoda (Arthropoda, Crustacea) from the Lower Jurassic Moenave Formation, Arizona and Utah, USA

Lucas S Antonietto,James I Kirkland,Celina A Suarez,Lisa E Park Boush,Andrew R.C Milner

doi:10.1017/jpa.2017.150

Abstract

AbstractAn ostracode fauna is described from lacustrine sediments of the Hettangian, Lower Jurassic, Whitmore Point Member of the Moenave Formation. The Moenave is well known for its rich, Late Triassic?–Early Jurassic fossil record, which includes fossil fishes, stromatolites, ostracodes, spinicaudatans, and a diverse ichnofauna of invertebrates and vertebrates. Four ostracode species, all belonging to the suborder Darwinulocopina, were recovered from these sediments:Suchonellina globosa,S. stricta,Whipplella? sp. 1, andW.? sp. 2. The diversity and composition of the Whitmore Point Member ostracode fauna agree with previous interpretations about Lake Dixie and nearby paleoenvironments as shallow lakes inhabited by darwinulocopine species that survived the effects of the Central Atlantic Magmatic Province and the subsequent end-Triassic extinction and quickly recolonized these areas, thanks to asexual reproduction by parthenogenesis. The Lake Dixie region, in its geographical isolation, could represent the last episode of darwinulocopine dominance in nonmarine environments before the Late Jurassic diversification of the cypridocopine/cytherocopine modern ostracodes.

Highlights

The present work identifies and redescribes some of the species of an ostracode fauna that was recovered from sediments of the Hettangian, Lower Jurassic, Whitmore Point Member, Moenave Formation, Glen Canyon Group
In the present taxonomic review, four ostracode species were identified from sediments of the Whitmore Point Member, deposited in and along the margins of a lacustrine environment, referred throughout the region as Lake Dixie, and associated/ interbedded mudflat environments during the earliest Jurassic
The composition and stratigraphic position of this ostracode fauna makes it an interesting case to understand the demise of darwinulocopines as the main nonmarine ostracodes, both in terms of abundance and diversity, during early to middle Mesozoic times

Summary

Introduction

The present work identifies and redescribes some of the species of an ostracode fauna that was recovered from sediments of the Hettangian, Lower Jurassic, Whitmore Point Member, Moenave Formation, Glen Canyon Group. We extend the scientific knowledge of ostracode diversity from this formation by addressing their taxonomy, along with major diversification trends of the ostracode suborder Darwinulocopina Sohn, 1988 (darwinulocopines) during the early Mesozoic. According to Schudack (2006), the ostracode fauna of the Moenave Formation comprises a restricted number of poorly known species of darwinulocopines (genus Darwinula Brady and Robertson in Jones, 1885) and an indeterminate genus of cypridocopines (suborder Cypridocopina Jones in Chapman, 1901). The Late Jurassic Morrison Formation, which is widespread through the midwestern USA, presents a diverse fauna of early nonmarine cytherocopines (suborder Cytherocopina Gründel, 1967, e.g., Timiriasevia Mandelstam, 1947, and Theriosynoecum Branson, 1936). The faunal composition of the Moenave and Morrison formations is in accordance with major evolutionary trends of these three taxa during the late Paleozoic–early Mesozoic (Whatley, 1988; Sames and Horne, 2012)

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