Taxonomic Affinities of the Putative Titanosaurs from the Late Jurassic
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Taxonomic affinities of the putative titanosaurs from the Late Jurassic Tendaguru Formation of Tanzania: phylogenetic and biogeographic implications for eusauropod dinosaur evolution Philip D. Mannion1*, Paul Upchurch2, Daniela Schwarz3 and Oliver Wings4 1 Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK 2 Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, UK 3 Museum für Naturkunde Berlin, Invalidenstrasse 43, 10115 Berlin, Germany 4 Niedersächsisches Landesmuseum Hannover, Willy-Brandt-Allee 5, 530169 Hannover, Germany * Author for correspondence (Email: [email protected]) RRH: TENDAGURU SAUROPOD DINOSAURS LRH: P. D. MANNION ET AL. Abstract The Late Jurassic Tendaguru Formation of Tanzania, southeastern Africa, records a rich sauropod fauna, including the diplodocoids Dicraeosaurus and Tornieria, and the brachiosaurid titanosauriform Giraffatitan. However, the taxonomic affinities of other sympatric sauropod taxa are poorly understood. Here, we critically reassess and redescribe these problematic taxa, and present the largest phylogenetic analysis for sauropods (117 taxa scored for 542 characters) to explore their placement within Eusauropoda. Janenschia robusta has played a prominent role in discussions of titanosaur origins, with various authors referring at least some remains to Titanosauria, a clade otherwise known only from the Cretaceous. Re-description of the holotype of Janenschia, and all referable remains, supports its validity and placement as a non-neosauropod eusauropod. It forms a clade with Haestasaurus, from the earliest Cretaceous of the UK, and the Middle/Late Jurassic Chinese sauropod Bellusaurus. Phylogenetic analysis and CT scans of the internal pneumatic tissue structure of Australodocus bohetii tentatively support a non-titanosaurian somphospondylan identification, making it the only known pre-Cretaceous representative of that clade. New information on the internal pneumatic tissue structure of the dorsal vertebrae of the enigmatic Tendaguria tanzaniensis, coupled with a full re-description, results in its novel placement as a turiasaur. Tendaguria is the sister taxon of Moabosaurus, from the Early Cretaceous of North America, and is the first turiasaur recognised from Gondwana. A previously referred caudal sequence cannot be assigned to Janenschia and displays several features that indicate a close relationship with Middle–Late Jurassic East Asian mamenchisaurids. It can be diagnosed by six autapomorphies, and we erect the new taxon Wamweracaudia keranjei gen. et sp. nov. The presence of a mamenchisaurid in the Late Jurassic of southern Gondwana indicates an earlier and more widespread diversification of this clade than previously realised, prior to the geographic isolation of East Asia. Our revised phylogenetic dataset sheds light on the evolutionary history of Eusauropoda, including supporting a basal diplodocoid placement for Haplocanthosaurus, and elucidating the interrelationships of rebbachisaurids. The Tendaguru Formation shares representatives of nearly all sauropod lineages with Middle Jurassic–earliest Cretaceous global faunas, but displays a greater range of diversity than any of those faunas considered individually. Biogeographic analysis indicates that the Tendaguru sauropod fauna was assembled as a result of three main phenomena during the late Early and/or Middle Jurassic: (1) invasions from Euramerica (brachiosaurids, turiasaurs); (2) endemism in west Gondwana (dicraeosaurids, diplodocids); and (3) regional extinctions that restricted the ranges of once widespread groups (mamenchisaurids, the Janenschia lineage). Multiple dispersals across the Central Gondwanan Desert are required to explain the distributions of Jurassic sauropods, suggesting that this geographic feature was at most a filter barrier that became easier to cross during the late Middle Jurassic. Keywords: BioGeoBEARS, Biogeography, Central Gondwanan Desert, Dispersal, Extended Implied Weights, Gondwana, Mesozoic, Rebbachisauridae, Regional Extinction, Titanosauria INTRODUCTION The Late Jurassic section of the Tendaguru Formation of southeastern Tanzania (Fig. 1), East Africa, preserves a rich fossil fauna, including plants (Bussert, Heinrich & Aberhan, 2009, and references therein), invertebrates (Aberhan et al., 2002), fish (e.g. Arratia, Kriwet & Heinrich, 2002), lissamphibians (Aberhan et al., 2002), mammals (Heinrich, 1998, 1999a, 2001), sphenodontids (Aberhan et al., 2002), squamates (Broschinski, 1999), crocodylomorphs (Heinrich et al., 2001), pterosaurs (Unwin & Heinrich, 1999; Costa & Kellner, 2009) and, most famously, dinosaurs (Maier, 2003). Within Dinosauria, ornithischians are represented by abundant material of the basal iguanodontian Dysalotosaurus lettowvorbecki (Virchow, 1919; Hübner & Rauhut, 2010) and the stegosaur Kentrosaurus aethiopicus (Hennig, 1915; Galton, 1982). Theropod remains are less common: with the exception of a partial skeleton of the ceratosaur Elaphrosaurus bambergi (Janensch, 1920, 1925a; Rauhut & Carrano, 2016), only isolated and fragmentary remains are known (Janensch, 1925a; Rauhut, 2005, 2011), although some of these are recognised as distinct taxa, i.e. Veterupristisaurus milneri (Rauhut, 2011) and Ostafrikasaurus crassiserratus (Buffetaut, 2012). Sauropod dinosaurs are both abundant and diverse in the Tendaguru Formation (Janensch, 1929a; Heinrich, 1999b), represented by multiple skeletons of diplodocoids, i.e. the diplodocid Tornieria africana (Fraas, 1908; Remes, 2006) and the dicraeosaurids Dicraeosaurus hansemanni and D. sattleri (Janensch, 1914a; Schwarz-Wings & Böhm, 2014), and titanosauriforms, i.e. the brachiosaurid Giraffatitan (‘Brachiosaurus’) brancai (Janensch, 1914a; Taylor, 2009). A second titanosauriform, Australodocus bohetii, is known only from two associated cervical vertebrae (Remes, 2007). Originally described as a second diplodocid taxon (Remes, 2007), Whitlock (2011a, b) demonstrated its titanosauriform affinities (see also D’Emic, 2012; Mannion et al., 2012) and suggested a brachiosaurid identification, whereas the analyses of Mannion et al. (2013) recovered it within the somphospondylan clade, as a titanosaur. The remaining two Tendaguru sauropod taxa have even less certain taxonomic affinities: Janenschia robusta (Fraas, 1908; Sternfeld, 1911; Wild, 1991; Bonaparte, Heinrich & Wild, 2000) and Tendaguria tanzaniensis (Bonaparte et al., 2000). Janenschia robusta has a complex and convoluted taxonomic history. Two sauropod skeletons (‘A’ and ‘B’) were collected from near Tendaguru Hill (Fig. 1) by a reconnaissance expedition led by Eberhard Fraas in 1907 (Fraas, 1908; Bonaparte et al., 2000; Remes, 2006). Fraas (1908) described them both as species of the new genus Gigantosaurus: G. africanus (skeleton ‘A’) and G. robustus (skeleton ‘B’). However, this genus name was preoccupied by ‘Gigantosaurus megalonyx’ from the Late Jurassic of the UK (Seeley, 1869) and so Sternfeld (1911) provided Tornieria as a replacement name, with the two Tendaguru species becoming T. africana and T. robusta. The German Tendaguru Expedition of 1909–1913 collected approximately 230 tonnes of additional dinosaur material (Maier, 2003), including elements from the type locality of skeleton B (Janensch, 1914b; Bonaparte et al., 2000). In his study of this material, Janensch (1922) considered the two Tornieria species to be distinct and referred T. africana (G. africanus) to the North American genus Barosaurus, creating the new combination B. africanus. Janensch (1922) also considered that the original genus name, Gigantosaurus, was now available because ‘Gigantosaurus megalonyx’ had been shown to be a nomen dubium by Lydekker (1888) and thus ‘removed’ from the literature; consequently, he revalidated G. robustus. However, once used, a genus name does not become available again and thus Janensch’s (1922) revalidation was erroneous (Wild, 1991). As Tornieria was now considered a junior synonym of Barosaurus, and the two Tendaguru species are clearly distinct at higher taxonomic levels (Janensch, 1922), Wild (1991) proposed a new generic name for T. robusta (G. robustus): Janenschia robusta, with skeleton ‘B’ (a near complete hindlimb) designated as the holotype. Janensch (1914b, 1922, 1929a, 1961) referred additional material, although Bonaparte et al. (2000) and Mannion et al. (2013) concluded that only some of it could be unequivocally assigned to Janenschia. Many authors have considered Janenschia (or at least elements at some point referred to Janenschia) to represent a titanosaur. Thus, along with potentially Australodocus (Mannion et al., 2013), it would represent the earliest and only pre-Cretaceous body fossil occurrence of this radiation of derived sauropods (Janensch, 1929a; McIntosh, 1990; Wild, 1991; Jacobs et al., 1993; Upchurch, 1995; Salgado & Calvo, 1997; Wilson & Sereno, 1998; Upchurch, Barrett & Dodson, 2004; Curry Rogers, 2005; Mannion & Calvo, 2011). However, Bonaparte et al. (2000) disputed the titanosaurian affinities of Janenschia and instead suggested close affinities with the basal macronarian Camarasaurus from the Late Jurassic of North America. Royo-Torres & Cobos (2009) proposed that some of the material included in Janenschia might be referable to the non-neosauropod eusauropod clade Turiasauria (see also Britt et al., 2017), otherwise known only from western Europe (Royo-Torres, Cobos & Alcalá, 2006) and North America (Royo-Torres et al., 2017a).