Clavicles, interclavicles, gastralia, and sternal ribs in sauropod dinosaurs: new reports from diplodocidae and their morphological, functional and evolutionary implications

Abstract

Ossified gastralia, clavicles and sternal ribs are known in a variety of reptilians, including dinosaurs. In sauropods, however, the identity of these bones is controversial. The peculiar shapes of these bones complicate their identification, which led to various differing interpretations in the past. Here we describe different elements from the chest region of diplodocids, found near Shell, Wyoming, USA. Five morphotypes are easily distinguishable: (A) elongated, relatively stout, curved elements with a spatulate and a bifurcate end resemble much the previously reported sauropod clavicles, but might actually represent interclavicles; (B) short, L-shaped elements, mostly preserved as a symmetrical pair, probably are the real clavicles, as indicated by new findings in diplodocids; (C) slender, rod-like bones with rugose ends are highly similar to elements identified as sauropod sternal ribs; (D) curved bones with wide, probably medial ends constitute the fourth morphotype, herein interpreted as gastralia; and (E) irregularly shaped elements, often with extended rugosities, are included into the fifth morphotype, tentatively identified as sternal ribs and/or intercostal elements. To our knowledge, the bones previously interpreted as sauropod clavicles were always found as single bones, which sheds doubt on the validity of their identification. Various lines of evidence presented herein suggest they might actually be interclavicles - which are single elements. This would be the first definitive evidence of interclavicles in dinosauromorphs. Previously supposed interclavicles in the early sauropodomorph Massospondylus or the theropods Oviraptor and Velociraptor were later reinterpreted as clavicles or furculae. Independent from their identification, the existence of the reported bones has both phylogenetic and functional significance. Their presence in non-neosauropod Eusauropoda and Flagellicaudata and probable absence in rebbachisaurs and Titanosauriformes shows a clear character polarity. This implicates that the ossification of these bones can be considered plesiomorphic for Sauropoda. The proposed presence of interclavicles in sauropods may give further support to a recent study, which finds a homology of the avian furcula with the interclavicle to be equally parsimonious to the traditional theory that furcula were formed by the fusion of the clavicles. Functional implications are the stabilizing of the chest region, which coincides with the development of elongated cervical and caudal vertebral columns or the use of the tail as defensive weapon. The loss of ossified chest bones coincides with more widely spaced limbs, and the evolution of a wide-gauge locomotor style.

Fig. 1

Compiled quarry map of the two excavation periods at the Howe Quarry (AMNH map below; SMA map above). Arrows indicate supposed clavicles at SMA, arrowheads possible locations of the supposed clavicle at AMNH. Circles indicate gastral or sternal baskets (full circles: SMA; dashed circles: AMNH), rectangle marks the SMA pair of symmetrical bones. AMNH map modified from Bird (1985); SMA map drawn by Esther Premru.

Fig. 2

Detail of the 1991 quarry map, with sections producing associated morphotype C–E elements enlarged (from left to right: clusters M 21, F 27 and D 28). The morphotype C–E elements are highlighted in grey in the enlarged sections.

Fig. 3

Morphotype A elements, to scale. (a) AMNH 30900; (b) SMA I 24-4; (c) SMA M 25-3; (d) SMA L 22-3; (e) SMA L 27-7. Scale bar: 10 cm. Gray areas in (a) indicate broken surfaces. Note the bifurcate end on top and the spatulate end at the bottom.

Fig. 4

Pair of Morphotype B elements SMA K 24-3 (outer bone) and SMA K 24-6 (inner bone) in internal (a) and external (b) view. Short leg of L-shaped bones shown in perpendicular view below. Note the considerable bend of this portion in respect to the main axis of the bone. Scale bar: 10 cm.

Fig. 5

Pair of Morphotype B elements AMNH 30789 in internal (a) and external (b) view. Scale bar: 10 cm.

Fig. 6

Morphotype B elements of the diplodocid DQ-SB, articulated with the acromia (arrowheads) of the scapulae, as they were found. Co, coracoid; MB, morphotype B element; Sc, scapula. Picture courtesy of H. Galiano.

Fig. 7

Coracoid with taphonomically attached morphotype B element (MB) of the non-somphospondylian macronarian SMA 0009 in posteroventral (a) and lateral (b) view. Coracoid made semitransparent in order to visualize better the morphotype B element. Arrows indicate brightly coloured matrix present between the MB and the coracoid. CF, coracoid foramen; GL, glenoid surface. Scale bar: 2 cm.

Fig. 8

Morphotype C elements SMA H 20-7 (a) and L 21-5 (b). Both elements are incomplete, fracture surface at the top is indicated by the grey area. Scale bar: 10 cm.

Fig. 9

Morphotype D elements SMA D 28-5 (a), M 21-2 (b) and M 21-8 (c). The bottom end of M 21-8 is broken. Scale bar: 10 cm.

Fig. 10

Proposed articulation between two morphotype D elements (left, SMA D 28-5; right, SMA D 28-14) in three views (internal/dorsal view in the centre, grey lines indicate the same morphological landmarks on the respective elements). Note the similarity to the central portion of the fused morphotype D element (Fig. 9c). Scale bar: 5 cm.

Fig. 11

Morphotype E elements SMA H 21-3 (a), N 22-12 (b) and M 21-15 (c). Note the irregular shapes that do not allow an assignation to any other morphotype. Dotted lines in (a) indicate direction of the broken hook-like projection. Scale bar: 10 cm.

Fig. 12

Reconstruction of the pectoral girdle and the chest region of an indeterminate diplodocid sauropod, based on the finds reported. Light grey elements represent pectoral girdle elements not discussed in the paper, dark grey elements mark the bones identified as chest bone morphotypes in this paper. Anterior (a) and ventral (b) view. Abbreviations: aDR, anterior dorsal ribs; Cl, clavicle (morphotype B); Co, coracoid; DR, dorsal rib; Ga, gastralia (morphotype D); In, interclavicle (morphotype A); pDR, posterior dorsal ribs; Sc, scapula; SP, sternal plates; SR, sternal ribs (morphotypes C and E); VC, vertebral column. Modified from Schwarz et al. (; a) and Filla & Redman (; b).

Fig. 13

Evolution of the furcula, comparison between the two hypotheses. Note the gap within Dinosauriformes in the furcula-interclavicle hypothesis. Line drawings scaled to same size. Eaton & Stewart (: Hesperoherpeton); Chatterjee (: Parasuchus); Klima (: Ornithorhynchus); Rieppel (: Lacerta); Steyer et al. (: Aphanerama); Benton & Walker (: Erpetosuchus); Martz (: Typothorax); Vickaryous & Hall (: Dimetrodon; 2010: Alligator, Basilicus, Gallus, Leptoceratops); Remes (: Euparkeria); Dilkes & Sues (: Doswellia).