1 Submitted on September 14, 2006. Accepted on November 16, 2007. 2 University of Wyoming, School of Arts and Sciences, Laramie, Wyoming, 82071, U.S.A. E-mail: geodave@uwyo.edu. 3 Big Horn Basin Foundation, 110 Carter Ranch Road, Thermopolis, Wyoming, 82443, U.S.A. 4 The Wyoming Dinosaur Center, 110 Carter Ranch Road, Thermopolis, Wyoming, 82443, U.S.A. Arquivos do Museu Nacional, Rio de Janeiro, v.65, n.4, p.527-544, out./dez.2007 ISSN 0365-4508 MORPHOLOGY OF A SPECIMEN OF SUPERSAURUS (DINOSAURIA, SAUROPODA) FROM THE MORRISON FORMATION OF WYOMING, AND A RE-EVALUATION OF DIPLODOCID PHYLOGENY 1 (With 15 figures) DAVID M. LOVELACE 2, 3 SCOTT A. HARTMAN 4 WILLIAM R. WAHL 3, 4 ABSTRACT: A new specimen of Supersaurus vivianae is described, providing additional information about the osteology of Supersaurus. The single Supersaurus individual that the WDC quarry produced allows a re- examination of elements referred to Supersaurus from the Dry Mesa quarry. The osteology supports maintaining the generic distinction of Supersaurus. Phylogenetic evaluation finds a monophyletic Apatosaurinae containing [Apatosaurus + Supersaurus] + Suuwassea, and a monophyletic Diplodocinae containing [Diplodocus + Seismosaurus] + Barosaurus, although the generic distinction of Seismosaurus is not supported in the current analysis. Key words: Dinosauria. Sauropoda. Supersaurus. Phylogeny. Morrison Formation. RESUMO: Morfologia de um espécime de Supersaurus (Dinosauria, Sauropoda) da Formação Morrison de Wyoming e uma reavaliação da filogenia de diplodocídeos. Um novo espécime de Supersaurus vivianae é descrito, acrescentando informações sobre a osteologia de Supersaurus. O único indivíduo de Supersaurus coletado no afloramento WDC permite o re-exame dos elementos referidos a Supersaurus do afloramento de Dry Mesa. A osteologia suporta a manutenção da distinção genérica de Supersaurus. Uma avaliação filogenética resultou em um grupo monofilético Apatosaurinae contendo [Apatosaurus + Supersaurus] + Suuwassea, e um grupo monofilético Diplodocinae contendo [Diplodocus + Seismosaurus] + Barosaurus, embora a distinção genérica de Seismosaurus não esteja suportada na presente análise. Palavras-chave: Dinosauria. Sauropoda. Supersaurus. Filogenia. Formação Morrison. INTRODUCTION Diplodocoid taxa rank among the earliest described and best-known sauropods (MARSH, 1896; HATCHER, 1901; HOLLAND, 1906; LULL, 1919; GILMORE, 1936), with new taxa continuing to be described, such as Suuwassea (HARRIS & DODSON , 2004) and Dinheirosaurus (BONAPARTE & MATEUS, 1999). Recent studies have provided needed attention to diplodocoid phylogenetic systematics (UPCHURCH et al., 2004; TAYLOR & NAISH, 2005; MCINTOSH, 2005; HARRIS, 2006), yet several diplodocid taxa have remained problematic due to their fragmentary nature, notably Seismosaurus and Supersaurus. In 1985, J.A. Jensen erected three sauropod genera based on material collected from Dry Mesa Quarry: Ultrasauros macintoshi; Dystylosaurus edwini; and Supersaurus vivianae. All three have had complex nomenclatural histories (e.g., JENSEN, 1987; CURTICE, 1995; CURTICE et al., 1996; CURTICE & STADTMAN, 2001), with the types of both Ultrasauros and Dystylosaurus eventually sunk into Supersaurus vivianae (CURTICE, 1995; CURTICE & STADTMAN, 2001). In addition, some of the specimen numbers have changed in the last two decades. The name Supersaurus was erected for a single scapulocoracoid, BYU 12962 (JENSEN, 1985). Dozens of elements have been referred to this taxon since. Some referrals, such as the matching right scapulocoracoid, are unambiguous. Other elements have been referred based on quarry location, relative size, and hypotheses of phylogenetic