Geological Magazine

Original Article

Onset of the Ordovician cephalopod radiation – evidence from the Rochdale Formation (middle Early Ordovician, Stairsian) in eastern New York

BJÖRN KRÖGERa1 c1 and ED LANDINGa2

a1 Université Lille 1, Laboratoire Géosystèmes (UMR 8157 CNRS), UFR des Sciences de la Terre – bâtiment SN5, 59655 Villeneuve d'Ascq cedex, France; formerly Museum für Naturkunde, Humboldt Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany

a2 New York State Museum, Albany, New York 12230, USA

Abstract

The Rochdale Formation of eastern New York (= Fort Ann and lower Bascom formations, designations abandoned) is now recognized to record the earliest stages of the Great Ordovician Radiation of cephalopods. The earliest Bassleroceratidae, Tarphyceratidae and endoceridans on the east Laurentian shallow carbonate platform occur in the upper, thrombolite-bearing member of the Rochdale. This fauna demonstrates that the earliest radiation of Ordovician nautiloids took place in the late Tremadocian and is best recorded in tropical platform facies. Revision of this cephalopod fauna based on approximately 190 specimens collected along a 200 km, N–S belt in easternmost New York has provided new information on inter- and intraspecific variation of earlier described species. The ellesmerocerid Vassaroceras and the endocerids Mcqueenoceras and Paraendoceras are emended. New taxa include Bassleroceras champlainense sp. nov. and B. triangulum sp. nov., Mccluskiceras comstockense gen. et sp. nov., Exoclitendoceras rochdalense gen. et sp. nov. and Paraendoceras depressum sp. nov. A rank abundance plot of 146 specimens from a locality in the Lake Champlain lowlands provides information on the community structure of a nautiloid fauna in which the longiconic cyrtoconic Bassleroceras is shown to dominate strongly. The nautiloid community structure of the Rochdale Formation is similar to that of the underlying Tribes Hill Formation (late early Tremadocian) with respect to the depositional setting, diversity and evenness but displays a remarkably increased taxonomic distinctness.

(Received February 19 2007)

(Accepted September 19 2007)

(Online publication May 09 2008)

Keywords:

  • Early Ordovician;
  • cephalopods;
  • evolution;
  • Laurentia;
  • New York

Correspondence:

c1 Author for correspondence: bjoekroe@gmx.de

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