DICRAEOSAURIDAE

Whitlock, John A., 2011, A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda), Zoological Journal of the Linnean Society 161 (4), pp. 872-915 : 888-890

DICRAEOSAURIDAE

Dicraeosauridae is well supported (decay index = 11; 13 unambiguous synapomorphies) and contains Amargasaurus , Brachytrachelopan , Dicraeosaurus , and Suuwassea . The recovered relationship ( Amargasaurus ( Brachytrachelopan , Dicraeosaurus )) is consistent with the two prior hypotheses of their relationships ( Rauhut et al., 2005; Sereno et al., 2007) and is well supported (decay index = 7; eight unambiguous synapomorphies).

Salgado et al.’s (2006) analysis is the only other to recover Suuwassea as a basal dicraeosaurid. Harris & Dodson (2004), Harris (2006c), and Sereno et al. (2007) all recovered Suuwassea in a polytomy with Dicraeosauridae and Diplodocidae . Harris (2006c), after pruning Alamosaurus , Euhelopus , Haplocanthosaurus , Losillasaurus , Jobaria , Malawisaurus , Nigersaurus , and Rebbachisaurus from the matrix, recovered Suuwassea as a basal dicraeosaurid supported by three synapomorphies: subtriangular lateral temporal fenestra; tall cervical neural spines; and elongate distal-most caudal vertebrae.

Gallina & Apesteguía (2005), Rauhut et al. (2005), Remes (2006), and Lovelace et al. (2008) recovered Suuwassea as a basal diplodocid. This was supported in the analysis of Rauhut et al. (2005) by only a single derived character, presence of a divided centroprezygapophyseal lamina (cprl) in middle and posterior cervical vertebrae. Suuwassea’s position in the analyses of Gallina & Apesteguía (2005), Remes (2006), and Lovelace et al. (2008) was based on that same synapomorphy and one symplesiomorphy, either presence of dorsal pleurocoels ( Gallina & Apesteguía, 2005) or lack of a basisphenoid/ basipterygoid recess ( Remes, 2006; Lovelace et al., 2008). In the present analysis, the cprl of Suuwassea is not considered to be divided, although there is a superficially similar morphology present. In many sauropods, the roof of the neural canal forms an anteriorly projecting ‘lip’ that grades gently on its lateral margins into the cprl; in some taxa (i.e. Suuwassea ) the lateral narrowness of the pedicel and the robustness of this neural canal lip can create the appearance of a divided cprl. A true divided cprl can be distinguished by the lack of an anteroposterior ‘offset’ between the anterior margin of the cprl and its medial branch. In Suuwassea and similar taxa, the lip of the neural canal is set back from the cprl and the two are easily distinguished. Rescoring this character from ‘1’ to ‘0’ in all three analyses ( Table 7) results in equally parsimonious topologies with Suuwassea as either a basal dicraeosaurid or a basal diplodocid. The position of Suuwassea in the present analysis is supported by a Templeton test, which rejects the placement of Suuwassea either as a basal diplodocid (P <0.01) or as a basal diplodocine (P <0.01).

Suuwassea represents the only recorded instance of a Laurasian member of Dicraeosauridae , and the second recorded instance of sympatric dicraeosaurids and diplodocids ( Tornieria and Dicraeosaurus are known from the same layers in Tendaguru). Suuwassea lacks a number of unambiguous synapomorphies with Amargasaurus and more derived dicraeosaurids – fused frontals (character 19), decreased pneumaticity in vertebrae (characters 81, 111), bifid anterior cervical vertebrae (character 85), and tall neural spines (characters 87, 93, 118) – suggesting that it is a basal dicraeosaurid. Harris (2006a, b, c, 2007) and Lovelace et al. (2008) have noted the coarse similarities between Suuwassea and Apatosaurus , although as both are basal members of their respective clades they may be expected to retain many plesiomorphic characters. The presence of basal members of both clades in North America may indicate a Laurasian origin for Flagellicaudata (see ‘Palaeobiogeographical implications’, below).

Age abbreviations: EK, Early Cretaceous; LK, Late Cretaceous. Area abbreviations: EU, Europe; NA, North America; SA, South America. Clade abbreviation: MDT = more derived taxa

Rebbachisauridae is recovered as a monophyletic group containing Histriasaurus , Rebbachisaurus , Cathartesaura , Limaysaurus , Zapalasaurus , Nigersaurus , and the unnamed Spanish rebbachisaurid. Other sauropods included in this group are Nopcsaspondylus , Rayososaurus , and three unnamed elements, CCC 017, MACN PVN35, and MIWG 6544 ( Table 8). Histriasaurus is recovered as the basalmost member of the group, just outside the clade Rebbachisaurus + MDD, primarily because of the retention of the hyposphene-hypantrum articulation. As in the analysis of Sereno et al. (2007), the remaining taxa are divided into two distinct lineages, here named Limaysaurinae and Nigersaurinae ( Table 5). Cathartesaura and Limaysaurus are recovered as sister taxa, as are Nigersaurus and the Spanish rebbachisaurid, similar to Sereno et al. (2007). Here, however, Zapalasaurus is recovered as the basal-most member of Nigersaurinae, unlike the position at the base of Limaysaurinae recovered by Sereno et al. (2007). This change in position is the result of a combination of updated character scorings based on first-hand observations ( Table 9) and the addition of novel characters not present in the analysis of Sereno et al. (2007). The phylogenetic position of Zapalasaurus is supported here by two unambiguous synapomorphies: presence of an epipophysealprezygapophyseal lamina (eprl) on cervical vertebrae (character 78) and caudal neural spines with triangular lateral processes derived from the lateral lamina (character 123). The recovery of Zapalasaurus at the base of Nigersaurinae breaks up the (South America (Europe, Africa)) distribution of derived rebbachisaurids recovered by Sereno et al. (2007) and requires a more complicated explanation (see ‘Palaeobiogeographical implications’, below). Limaysaurinae is supported by two unambiguous synapomorphies: accessory lateral lamina connecting the postzygodiapophyseal lamina (podl) and the sprl on posterior cervical vertebrae (character 96); and sprl- Character state abbreviations: ‘0’, primitive; ‘1’, derived; ‘?’, unknown.

spinopostzygapophyseal lamina (spol) contact on anterior caudal vertebrae (character 137).

Zapalasaurus was recovered outside the clade Rebbachisauridae + Flagellicaudata by Salgado et al. (2006), on the basis of anterior caudal neural spines without spinoprezygapophyseal laminae present and extending onto the lateral surface of the neural spine. Here, Zapalasaurus is scored ‘?’ for this character (138); spinoprezygapophyseal laminae are definitely present on the anterior-most preserved caudal vertebrae, although they do not appear to extend onto the lateral surfaces. The preservation of the anteriormost caudal is somewhat ambiguous. It is possible that in more anterior caudal vertebrae, these laminae would take on the form seen in other diplodocoids. For this reason, it scored here as ‘?’. Constraining Zapalasaurus (as presently scored) outside the clade Rebbachisauridae + Flagellicaudata requires an additional step (274 total). Rescoring Zapalasaurus as primitive for this state (0), without changing its position in the topology, adds an additional step to the tree (274); constraining the rescored Zapalasaurus outside the clade Rebbachisauridae + Flagellicaudata in this rescored analysis requires still one more step (275), making the placement of Zapalasaurus within Rebbachisauridae a more parsimonious hypothesis than alternative positions. Differences amongst these topologies are too small to calculate meaningful test statistics for comparison, such as with a Templeton test.

The clade Rebbachisauridae is not as strongly supported as either Diplodocidae or Dicraeosauridae (decay indices = 2, 7, 11, respectively), largely as a consequence of fragmentary taxa at the base of the clade. There are, however, three nonhomoplastic synapomorphies uniting Rebbachisauridae (Appendix 4). The clade Limaysaurinae + Nigersaurinae is supported by 22 synapomorphies, but all of these are ambiguous as a result of missing data. Some, all, or none may in fact be recovered as synapomorphies of Rebbachisauridae once scorings for basal forms are known ( Table 6). Support for internal nodes is less robust, and the fragmentary nature of all but Limaysaurus and Nigersaurus pushes a number of potential synapomorphies of more inclusive groups further up the tree.

Removing the isolated crown MNHN MRS 1524a from the scoring of Rebbachisaurus does not affect the results of the analysis. The presence of a second, lingual planar wear facet (character 67) is suggested by this crown, but cannot be definitively identified and so was scored as unknown here. This is potentially a synapomorphy of a larger group than was recovered here (Limaysaurinae + Nigersaurinae; Appendix 4).