DIPLODOCIDAE

Whitlock, John A., 2011, A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda), Zoological Journal of the Linnean Society 161 (4), pp. 872-915 : 887-888

DIPLODOCIDAE

Diplodocidae is a well-supported node (decay index = 7, 20 synapomorphies) inclusive of six taxa: Apatosaurus , Barosaurus , Dinheirosaurus , Diplodocus , Supersaurus , and Tornieria . Apatosaurus is recovered as the basal-most member of the clade. Supersaurus is recovered one node higher. Dinheirosaurus and Tornieria are recovered in a polytomy with the clade ( Barosaurus , Diplodocus ) in both strict and 50% majority-rule consensus trees. Similarly, Rauhut et al. (2005) recovered Barosaurus , Dinheirosaurus , Diplodocus , and Tornieria in a four-way polytomy; a 50% majority-rule tree obtained using a rescored matrix (see ‘ Amazonsaurus ’ and ‘ Amphicoelias ’ below) recovers a fully-resolved Diplodocidae , with Apatosaurus basal to all other diplodocids and Tornieria basal to the clade ( Dinheirosaurus ( Barosaurus , Diplodocus )). Dinheirosaurus and Tornieria are difficult to differentiate confidently, as no elements overlap between them: Dinheirosaurus is known primarily from two posterior cervical vertebrae and a series of articulated dorsal vertebrae ( Bonaparte & Mateus, 1999); Tornieria is known primarily from cranial, caudal, and appendicular elements ( Remes, 2006). Any hypothesis of their relative relationships is therefore predicated entirely on synapomorphies and symplesiomorphies with the surrounding members of Diplodocidae and not with each other, and must be treated with caution.

The arrangement of the three best-known taxa in both trees – ( Apatosaurus ( Barosaurus , Diplodocus )) – is consistent with the relationships recovered by traditional studies and all cladistic analyses ( Calvo & Salgado, 1995; Casanovas et al., 2001; Wilson, 2002; Harris & Dodson, 2004; Upchurch et al., 2004; Gallina & Apesteguía, 2005; Harris, 2006c; Remes, 2006; Salgado et al., 2006; Lovelace et al., 2008). The relationship ( Apatosaurus ( Tornieria ( Barosaurus , Diplodocus ))) is also consistent with Remes (2006). As in all previous analyses, Barosaurus and Diplodocus are well-supported sister taxa. They share seven synapomorphies (Appendix 4), three of which are ambiguous and may characterize a group inclusive of Dinheirosaurus , Tornieria , or both ( Table 6).

The recovery of Supersaurus as a basal diplodocine is at odds with the result of Lovelace et al. (2008), who recovered Supersaurus as the sister taxon to Apatosaurus . Examination of their matrix reveals that the Apatosaurus + Supersaurus sister relationship in their analysis is supported by a single synapomorphy: scapular glenoid strongly bevelled medially. The two taxa also share five plesiomorphic characters: anterior caudal centra not doubling in length over the first 20 vertebrae; cylindrical anterior caudal centra; ulnar proximal condylar processes subequal; short neural spines on first caudal (reversal); and short posterior dorsal neural arches. Three of these characters (anterior caudal centra not doubling in length over the first 20 vertebrae, cylindrical anterior caudal centra, and short posterior dorsal neural arches) are ambiguously resolved, and do not differentiate the preferred tree of Lovelace et al. (2008) from this analysis. Supersaurus and the diplodocines, however, share two derived states in their analysis: mid-cervical centra with an elongation index (EI; centrum length: posterior centrum height; Upchurch, 1998) greater than or equal to four; and anterior caudal centra with pneumatopores.

Supersaurus cannot be scored for Lovelace et al.’s (2008) character 166, ulnar proximal condylar processes subequal, as the referred ulna (BYU 13744) was removed from the taxon by those authors earlier in the same paper. After rescoring that single cell, a PAUP* search returns twice as many equally parsimonious trees (48). A strict consensus of those trees drops Apatosaurus , Suuwassea , and Supersaurus into a polytomy at the base of Flagellicaudata with Dicraeosauridae and what remains of Diplodocidae . The 50% majority-rule tree agrees with the conventional topology, with Apatosaurus and Supersaurus in a polytomy at the base of Diplodocidae and Suuwassea in a polytomy with Dicraeosauridae and Diplodocidae . In the current analysis, the clade Supersaurus + MDD is supported by three unambiguous synapomorphies (Appendix 4), suggesting that this clade is more probably correct than a sister relationship between Apatosaurus and Supersaurus .