Tuesday, January 9, 2018

Is "Apatosaurus" minimus a camarasaurid?

With the recent erection of Galeamopus for the diplodocid "Diplodocus" hayi by Tschopp et al. (2015), but also growing understanding of basal somphospondylan evolution (e.g. Averianov et al. in press; D'Emic et al. 2013; Saegusa and Ikeda 2014) and a preliminary assessment of the systematic affinities of "Apatosaurus" minimus Mook, 1917 by Taylor and Wedel (2012), I have had the chance to compare this problematic sauropod from the Morrison Formation with members of Macronaria, especially non-titanosaurian taxa, to better interpret placement within Neosauropoda. Indeed, McIntosh (1990a.b) and Upchurch et al. (2004) rejected the placement of minimus in Apatosaurus by Mook (1917) because of the height of the neural spines, low ilia with preacetabular processes being directed strongly laterally, and an ischial articular surface of the pubis nearly 50 percent of the pubic length. They considered it likely that "Apatosaurus" minimus was a derived member of Macronaria. Taylor and Wedel (2012) elaborated further, noting that the taxon has a mosaic of basal diplodocoid and macronarian characters, including tall neural spines and flaring ilia, but unpublished cladistics results were inconclusive. 

In their cladistic analyses of Diplodocoidea, Tschopp et al. (2015) noted that "Apatosaurus" minimus shares with Camarasaurus and most somphospondyls six sacral vertebrae and widely splayed preacetabular lobes of the ilium, even as they noted that the pubic morphology of AMNH 675 resembles Camarasaurus. For example, six sacral vertebrae are present in non-titanosaur somphospondyls like Euhelopus (Wilson and Upchurch 2009), Huabeisaurus (D'Emic et al. 2013), and Tambatitanis (Saegusa and Ikeda 2014), and some specimens referred to Camarasaurus (AMNH 690, BYU 17465, GMNH-PV 101) also have six sacral vertebrae (Tidwell et al. 2005). However, Upchurch et al. (2004) noted that AMNH 675 differs from titanosauriforms in that the cranial part of the ilium has a subtriangular outline in lateral view. Moreover, the somphospondylan Sibirotitan has five sacral vertebrae rather than six, in contrast to all other basal Somphospondyli (Averianov et al., in press) and "Apatosaurus" minimus.   

Although Taylor and Wedel (2012) listed tall neural spines on the sacral vertebrae as a diplodocoid synapomorphy for "Apatosaurus" minimus, the non-neosauropod eusauropod Cetiosauriscus, previously classified as a diplodocoid following McIntosh (1990a) and Upchurch et al. (2004) but now placed outside Diplodocoidea following Heathcote & Upchurch (2003) and Rauhut et al. (2005) also possesses tall sacral neural spines (Upchurch et al. 2004). Moreover, Tschopp et al. (2015, Supplementary Table 108) report that tall sacral neural spines occur among non-diplodocoid neosauropods. Given that tall neural spines of the sacral vertebrae are present in non-neosauropod eusauropods, it could be parsiminous to interpret this character as having evolved in more than one eusauropod clade, suggesting that tall sacral neural spines are a reversal in "Apatosaurus" minimus within Macronaria.  

Judging from comparisons of AMNH 675 with non-titanosaurian macronarians and non-neosauropod eusauropods, and evaluation of the characters cited by Taylor and Wedel (2012), the best parsimonious conclusion to be drawn is that "Apatosaurus" minimus may be a derived macronarian closely related to Somphospondyli, but possibly phylogenetically intermediate between Camarasauridae and Titanosauriformes. The number of sacral vertebrae is distinct from Camarasaurus (except in AMNH 690, BYU 17465, GMNH-PV 101) and more closely matching members of Somphospondyli except Sibirotitan, but the subtriangular outline of the cranial portion of the ilium in lateral view excludes "A." minimus from Titanosauriformes, while the tall neural spines distinguish the species not just from camarasaurids but also from titanosauriforms.              


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