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Abstract
A nearly complete skeleton of a juvenile sauropod from the Lower Morrison Formation (Late Jurassic, Kimmeridgian) of the Howe Ranch in Bighorn County, Wyoming is described. The specimen consists of articulated mid-cervical to mid-caudal vertebrae and most appendicular bones, but cranial and mandibular elements are missing. The shoulder height is approximately 67 cm, and the total body length is estimated to be less than 200 cm. Besides the body size, the following morphological features indicate that this specimen is an early juvenile; (1) unfused centra and neural arches in presacral, sacral and first to ninth caudal vertebrae, (2) unfused coracoid and scapula, (3) open coracoid foramen, and (4) relatively smooth articular surfaces on the limb, wrist, and ankle bones. A large scapula, short neck and tail and elongate forelimb bones relative to overall body size demonstrate relative growth. A thin-section of the mid-shaft of a femur shows a lack of annual growth lines, indicating an early juvenile individual possibly younger than a few years old. Pneumatic structures in the vertebral column of the specimen SMA 0009 show that pneumatisation of the postcranial skeleton had already started in this individual, giving new insights in the early ontogenetic development of vertebral pneumaticity in sauropods.
The specimen exhibits a number of diplodocid features (e.g., very elongate slender scapular blade with a gradually dorsoventrally expanded distal end, a total of nine dorsal vertebrae, presence of the posterior centroparapophyseal lamina in the posterior dorsal vertebrae). Although a few diplodocid taxa, Diplodocus, cf. Apatosaurus, and cf. Barosaurus, are known from several fossil sites near the Howe Ranch, identification of this specimen, even at a generic level, is difficult due to a large degree of ontogenetic variation.
Acknowledgements
We cordially thank Hans-Jakob ‘Kirby’ Siber for giving us this great opportunity to study the specimen SMA 0009 and further material at the Sauriermuseum Aathal. Urs Moeckli also supported our visits to SMA. We thank Kenneth Carpenter, John McIntosh, Virginia Tidwell, and Jeffrey Wilson for valuable discussions. Jacques Ayer shared unpublished data on the geology of the Howe Stephens Quarry with us. Peter Larson allowed us to access to the SMA 0009 during a preparation at the Black Hills Institute. For their interest and helpful discussions on the matter we are indebted to David Lovelace, Ray Wilhite, Oliver Wings and the members of the German DFG Research Group (DFG No. 533) ‘Biology of the Sauropod Dinosaurs: The Evolution of Gigantism’. Jeffrey Wilson critically reviewed an earlier draft of this manuscript and we wish to thank him for his helpful comments and suggestions for improvement. We also wish to acknowledge the editor Gareth Dyke for his efforts. We thank furthermore the following persons for access to collections under their care: Bernd Herkner (Senckenbergmuseum Frankfurt a.M.), Wolf-Dieter Heinrich, David Unwin (Museum für Naturkunde der HU Berlin), Daniel Brinkman, Walter Joyce (Yale Peabody Museum), Carl Mehling, Mark Norell, Ivy Rutzky (American Museum of Natural History), David Berman, Amy Henrici, Matt Lamanna, Norm Wuerthele (Carnegie Museum of Natural History), Robert Prudy, Michael Brett-Surman (Smithsonian Institution, Washington DC), José F. Bonaparte, Alejandro Kramarz (Museo Argentino de Ciencias Naturales, Buenos Aires), and Logan Ivy (Denver Museum of Natural History). For this study, DS was financed by the Swiss National Science Foundation (SNF No. 200021-101494/1 and 200020-109131/1). The Wyoming Dinosaur Center research grant partially supported TI.
Notes
† Current address: Museum of Paleontology and Department of Geological Sciences, The University of Michigan, 1109 Geddes Avenue, Ann Arbor, MI 48109-1079, USA. [email protected]
† Suuwassea was decribed (Harris and Dodson Citation2004) as belonging to Flagellicaudata with no closer assignment to Diplodocidae or Dicraeosauridae possible. According to a description of the new dicraeosaurid taxon Brachtrachelopan, the latest phylogenetic framework on sauropods (Rauhut et al. Citation2005) and more detailed descriptions of Suuwassea by Harris (Citation2006b, Citation2006a), the taxon might also belong to Diplodocidae, being closely related to Apatosaurus.
‡ The remains of CM 566 consists of a neural arch of a mid-(?) dorsal vertebra and a number of appendicular bones. This juvenile sauropod was first described as ‘Elosaurus parvus’ (Peterson and Gilmore Citation1902), but revised to be synonymous to Apatosaurus excelsus (McIntosh Citation1981). In a revision of Apatosaurus, Upchurch et al. (Citation2004b) again replaced CM 566 and additional material into the new combination Apatosaurus parvus, owing to some synapomorphies of the postcranial skeleton. Because the taxonomic assigment of CM566 is still under controversial view, we refer to the remains just as Apatosaurus with no species assignment.
