Sauropod haemal arches: morphotypes, new classification and phylogenetic aspects

Abstract

Sauropod haemal arches are caudal bony structures that have been traditionally incorporated into two different types observed in different anatomical views: Y-shaped (anterior view) and forked (lateral view). This research proposes a classification combining information observed in anterior and lateral views. Four types of ‘Y-shaped’ and six types of ‘V-shaped’ chevrons were recognised. Complete chevron series in some eusauropods allows the comparison of topological equivalent structures along the tail and also among taxa. A basal titanosaur from Argentina exhibits mid-caudal chevron morphology in which more than one cranial and caudal process is present, arising from the distal blades as well as from the proximal rami, a condition not seen before in a sauropod dinosaur. The morphological variability seen in sauropod chevrons along the tail is in close relationship with the development and distribution of muscle. caudofemoralis longus, as seen in extant crocodiles and as previously proposed for non-avian theropods and for caudal centra and transverse processes of sauropod dinosaurs. Two new characters related to middle chevrons are proposed here, in which the transitional morphology is described.

Keywords:

• Sauropoda
• Y-shaped
• V-shaped
• forked
• caudofemoralis longus

Acknowledgements

We thank all the people who allowed us to see the specimens under their care: Marcelo Reguero, Alejandro Kramarz, Jorge Calvo, Carlos Muñoz, Leonardo Filippi, Jaime Powell and Zoltán Csiki. Leonardo Salgado, Sebastián Apesteguía and Jeff Wilson made useful comments about the interpretation of the chevrons of MMCH-Pv 47. Juliana Sterli and José Carballido are deeply thanked for their constructive comments on the manuscript. Paul Sereno provided unpublished photos of Nigersaurus taqueti, Mike Taylor and Takehito Ikejiri provided information of the chevrons of Giraffatitan brancai and Camarasaurus, respectively. Yanina Herrera helped us with the C. latirostris dissections and also supplied us information about the chevrons of metriorhynchiid crocodyliform. Jorge González is thanked for helping on dissections. Phil Mannion revised the English spelling of the manuscript. We thank the authorities of the CICYTTP-CONICET (Diamante, Entre Ríos) where dissections were made and C. Piña for providing the specimens of C. latirostris. Yacaré Porá S.A. provided several specimens of C. latirostris for study at Fundación Azara. Mike D'Emic and an anonymous reviewer made fruitful comments on the manuscript. This research was partially funded through a Jurassic Foundation Grant awarded to AO. This contribution used TNT ver 1.1, a program made freely available thanks to a subsidy by the Willi Hennig Society.

Notes

1. Email: [email protected]

2. Email: [email protected] [email protected]

Additional information

Notes on contributors

Pablo Ariel Gallina

1

Juan Ignacio Canale

2