Citation:

Tschopp, E., & Mateus O. (2012).  A sternal plate of a large-sized sauropod dinosaur from the Late Jurassic of Portugal. 10th Annual Meeting of the European Association of Vertebrate Paleontologists ¡Fundamental! . 20, 263-266.: European Association of Vertebrate Paleontologist

Notes:

ISBN: 978–84–938173–4–3

Sternal plate of a large-sized sauropod dinosaur from the Late Jurassic of Portugal

Emanuel Tschopp & Octávio Mateus
CICEGe - FCT, Universidade Nova de Lisboa, 2829-516 Caparica, PT & Museu da Lourinhã, Rua João Luis de Moura, 2530-158 Lourinhã, PT; tschopp.e@campus.fct.unl.pt; omateus@fct.unl.pt

Introduction
Sauropods of the Late Jurassic of Portugal are Lusotitan atalaiensis, Dinheirosaurus lourinhanensis, Turiasaurus riodevensis, Lourinhasaurus alenquerensis, and possibly Apatosaurus (Antunes and Mateus, 2003; Mateus, 2009; Mannion et al., 2011). The skeletal remains consist of incomplete specimens. Only one sternal plate was ever reported (Lapparent and Zbyszewski, 1957: p. 41), but was neither figured nor can it be located in any collection at present. An isolated, second, right sternal plate (ML 684) from Lage Fria (Atalaia, Lourinhã, Western Portugal), is herein described. It is partly damaged anteromedially and was recovered from the Sobral Unit of the Lourinhã Formation (Tithonian, Late Jurassic).

Description
The sternal plate ML 684 is 100 cm long, 43.5 cm wide, and about 2 cm thick medially. It is reniform in outline, with a weakly concave lateral and a convex medial margin (Fig. 1). It is flat dorsally, and bears a weak, broad ridge ventrally. The ridge extends close to the lateral edge, following its curvature. The lateral side of the ridge is steeper than the medial one. The sternal plate tapers towards both the lateral and the medial edges, medially more so than laterally. The margins are rugose, only the central-most portion of the lateral edge is smooth. No lateroanterior process (sensu (Sanz et al., 1999) is present. The posteromedial corner of the sternal plate bears a thin and rugose, subtriangular extension, as in the sternal plates of Haplocanthosaurus delfsi CMNH 10380 (McIntosh and Williams, 1988) or Camarasaurus lentus WDC BS-104 (Wilhite, 2005). Whereas the dorsal surface of this extension is smooth, the ventral one is highly irregular and pitted. Without this extension, the posterior edge of the sternal plate would be gently rounded. The ventral surface of the plate bears several circular pits. These pits are 12 to 14 mm wide and about 4 mm deep at their deepest point.

Comparison and discussion
Due to its posteromedial extension, the sternal plate ML 684 resembles the one of Haplocanthosaurus delfsi CMNH 10380, or Camarasaurus lentus WDC BS-104 (McIntosh and Williams, 1988; Wilhite, 2005). However, the rugose surface and a faint suture between the extension and the plate body indicates that this does not belong to the main plate. It is located where the sternal ribs are interpreted to attach (Filla and Redman, 1994). Its rugose surface implies a cartilage covering (Ikejiri et al., 2005). As sternal ribs are usually cartilaginous, the posteromedial extension most probably represents a partly ossified sternal rib, fused to the sternal plate. The extensions in H. delfsi CMNH 10380, and C. lentus WDC BS-104 might thus be sternal ribs as well. Ignoring the extension, ML 684 best resembles the sternal plates of Camarasaurus supremus AMNH 5761, Apatosaurus ajax NSMT-PV 20375, or Turiasaurus riodevensis (Osborn and Mook, 1921; Upchurch et al., 2004a; Royo-Torres et al., 2006). Although incomplete, also the element of Losillasaurus giganteus shows some affinities with ML 684 (Casanovas et al., 2001).
Both Turiasaurus riodevensis and Losillasaurus giganteus are Late Jurassic Iberian sauropods. The Portuguese sauropod taxa mentioned above and the Spanish form Galvesaurus herreroi complete the reported Jurassic Iberian sauropod record. G. herreroi is distinct from ML 684 in having a dorsal instead of a ventral ridge, and a straight posterior border (Barco, 2009). The large size of ML 684 (rivaling even the Giraffatitan brancai elements, which reach 110 cm; Janensch, 1961) makes an identification as Lourinhasaurus alenquerensis and Losillasaurus  giganteus improbable as well: while the latter has a sternal only 60% the length of ML 684 (Casanovas et al., 2001), L. alenquerensis does not preserve sternal elements. However, being in the same size class as the closely related Camarasaurus (Barco, 2009), ML 684 can be compared to the largest, morphologically similar Camarasaurus sternal (AMNH 5761, length 67,5 cm; Osborn and Mook, 1921). For both L. giganteus and L. alenquerensis, an approximate size increase of more than 50% would thus be needed to have an individual large enough to carry ML 684.
As a diplodocid sauropod (Mannion et al., 2011), one would expect a typical triangular plate for Dinheirosaurus lourinhanensis, markedly different from ML 684. On the other hand, given the similar element of Apatosaurus ajax NSMT-PV 20375, an assignment of ML 684 to D. lourinhanensis seems possible. However, comparing the sizes of the sternals of NSMT-PV 20375 and ML 684 an individual almost 20% bigger than the only known specimen ML 414 would be needed to carry a sternal plate as ML 684. An assignment of ML 684 to D. lourinhanensis is thus unlikely.
Lusotitan atalaiensis is a brachiosaurid about the size of Giraffatitan brancai (Antunes and Mateus, 2003). In addition to G. brancai, also the brachiosaurs Paluxysaurus jonesi and Cedarosaurus weiskopfae preserve sternal plates. All these elements are similar to ML 684 in outline. However, G. brancai has a thicker anterior corner, and no ventral ridge (Janensch, 1961), P. jonesi has an acute anterior end (Rose, 2007), and C. weiskopfae has more elongate plates than ML 684 (V. Tidwell, pers. comm.). Like ML 684, L. atalaiensis was found in the Sobral Member of Lourinhã Fm. in Atalaia (Antunes and Mateus, 2003). As the morphological differences concern widely varying features (see below), an assignment of ML 684 to L. atalaiensis is possible.
The sternal plate of the eusauropod Turiasaurus riodevensis is reniform, but more slender than ML 684 (Royo-Torres et al., 2006). With a length of 950 mm, it almost reaches the size of ML 684. Finally, also the stratigraphic level of T. riodevensis coincides with the level of ML 684 (Royo-Torres et al., 2006). The most probable identification of ML 684 is thus T. riodevensis or Lusotitan atalaiensis - awaiting the find of a sternal plate undoubtedly belonging to the latter taxon.

The sternal plate showed some boring holes, up to 11 mm in diameter, and 8 mm deep, possibly caused by dermestid or silphid beetles, similar to the coeval occurrences in the bones from Morrison Formation (Britt et al., 2008). This borings are, however, larger than the previous reported.

Evolution of sauropod sternal plates
Isolated sauropod sternal plates are difficult to identify (Wilhite, 2005). However, some trends can be observed in their morphology (Fig. 2): basal Eusauropoda like Shunosaurus lii usually show rounded sternal plates, some with expanded anterior corners (Zhang, 1988). More derived Eusauropoda have reniform plates, often with shallow dorsal or ventral ridges. Within Diplodocoidea, Rebbachisauridae show crescentic (Calvo and Salgado, 1995), and Diplodocidae subtriangular to triangular plates with dorsoventrally thick anterior ends (Hatcher, 1901). Somphospondylia mirror the evolution of rebbachisaurs, developing crescentic sternal plates. Some taxa as Alamosaurus or Malawisaurus have straight posterior edges (Gilmore, 1946; Gomani, 2005). Advanced titanosaurs like Alamosaurus or Opisthocoelicaudia evolve very long sternal plates compared to humerus length (Gilmore, 1946; Borsuk-Bialynicka, 1977). Whereas shallow ridges or expanded anteroventral corners appear convergently in various groups, a prominent anteroventral ridge only occurs in derived titanosaurs like Lirainosaurus or Neuquensaurus (Huene, 1929; Sanz et al., 1999).
Based on this review, four additional characters are suggested that were not included in the phylogenetic analyses of Salgado et al., (1997); Upchurch, (1998); Sanz et al., (1999); González Riga, (2002); and Upchurch et al., (2004b): 1) sternal plate, general shape: short or elongate; 2) sternal plate, anteromedial corner: gently curved or forming a distinct corner; 3) sternal plate, dorsoventrally expanded anterior end, not followed by ridge: absent or present; 4) sternal plate, anteroventral ridge: absent/shallow or prominent.

Acknowledgements
We thank N. Knötschke, V. Tidwell, R. Kosma, S. Suteethorn, J. Le Loeuff, M. Brett-Surman, C. Mehling, and J. Carballido, who provided pictures of previously undescribed sternal plates. E. Tschopp is supported by the doctoral fellowship SFRH / BD / 66209 / 2009 of FCT-MCTES, and the project PTDC/BIA-EVF/113222/2009.

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