Biomechanics of the extremely elongated neck of the Triassic archosauromorph Tanystropheus

Abstract

Extremely elongate necks have convergently evolved in several amniote lineages, including both aquatic and terrestrial forms (Fig. 1). The development of such a feature brings with it advantages in obtaining food items, but also biomechanical challenges, such as flexibility, stability, lift, and inertia. In Tanystropheus, a particularly long-necked Triassic archosauromorph, the neck is composed of only 13, mostly extraordinarily elongated and slender cervical vertebrae and accompanying rod-like, overlapping ribs, making it arguably the most extreme example of neck elongation in tetrapod evolution (Fig. 1;1-6). Understanding the function of this remarkable neck provides insights into the limits of neck elongation in amniotes and the evolution of morphological novelties in Triassic reptiles. Here we present the first quantitative biomechanical analysis of the Tanystropheus neck using a digital model based on three-dimensionally preserved bones. We assessed its range of motion (ROM) and performed finite element analysis (FEA) on the individual cervical ribs and the neck model in different configurations. Our results indicate that the neck of Tanystropheus was not extremely stiff, as previously postulated, and the ribs likely did not impair its movements. They transferred tensile forces towards the base of the neck, similar to what hypothesized for sauropods7. This study elucidates the bauplan of an extremely specialized animal and brings us closer to understanding the patterns of achieving neck elongation in vertebrates. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=186 SRC="FIGDIR/small/735087v1_fig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@6732a3org.highwire.dtl.DTLVardef@12d9dcdorg.highwire.dtl.DTLVardef@1d331a0org.highwire.dtl.DTLVardef@5f0a91_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFigure 1.C_FLOATNO Comparison of the cervical vertebral column anatomy of some long-necked tetrapods. Reconstructions not to scale. Based on: Tanystropheus conspicuus - Rytel6; Dinocephalosaurus orientalis - Spiekman et al.8; Mamenchisaurus youngi - Pi et al.9, Ouyang and Ye10; Struthio camelus - Mivart11; Quetzalcoatlus - Padian et al.12; Chelodina longicollis - Herrel et al.13; Albertonecte vanderveldei - Kubo et al.14, Sachs et al.15; Giraffa camelopardalis - Badlangana et al.16. The numbered nodes correspond to the following taxa: 1. Tetrapoda; 2. Diapsida; 3. Archosauromorpha; 4. Tanysauria; 5. Avemetatarsalia; and 6. Saurischia. C_FIG