Discovery of a short-necked sauropod dinosaur from the Late Jurassic period of Patagonia

Abstract

Sauropod dinosaurs are one of the most conspicuous groups of Mesozoic terrestrial vertebrates. They show general trends towards an overall increase in size and elongation of the neck, by means of considerable elongation of the length of individual vertebrae and a cervical vertebra count that, in some cases, increases to 19 (ref. 1). The long neck is a particular hallmark of sauropod dinosaurs and is usually regarded as a key feeding adaptation. Here we describe a new dicraeosaurid sauropod, from the latest Jurassic period of Patagonia, that has a particularly short neck. With a neck that is about 40% shorter than in other known dicraeosaurs, this taxon demonstrates a trend opposite to that seen in most sauropods and indicates that the ecology of dicraeosaurids might have differed considerably from that of other sauropods. The new taxon indicates that there was a rapid radiation and dispersal of dicraeosaurids in the Late Jurassic of the Southern Hemisphere, after the separation of Gondwana from the northern continents by the late Middle Jurassic.

Taxonomy of Late Jurassic diplodocid sauropods from Tendaguru (Tanzania)

“…The hemapophyses are only rudimentary developed, but show the forked shape with elongate cranial and caudal processes that is synapomorphic for Flagellicaudata (Upchurch 1998;Upchurch et al 2004a;Harris 2006c). The threesided, ventrally concave form of the centra is typical for diplodocines (Marsh 1895;Upchurch 1998;Wilson 2002;Upchurch et al 2004a;Rauhut et al 2005;Harris 2006c;Salgado et al 2006), but the grade of elongation is unusually low for this group. Since both vertebrae measure only 15 cm in combined length, and because the hemapophyses are only weakly developed, it may well be that specimen Ki 75-77 represents a juvenile individual.…”

“…12A). The characters of the vertebrae identify specimens Z and C as Diplodocinae (Upchurch 1998;Wilson 2002;Upchurch et al 2004a;Rauhut et al 2005;Harris 2006c;Salgado et al 2006), but these elements are not diagnostic on genus level. The differences in the shape of the tibia between skeleton k (Tornieria) and skeleton C also speaks against an assignment of the material from site C to Tornieria, although the grade of individual variation in sauropods is not known (compare Bonnan et al 2008).…”

“…In the maxillae, replacement teeth of identical shape are preserved. All these characters are diagnostic for Diplodocoidea (Calvo & Salgado 1995;Upchurch 1995Upchurch , 1998Wilson 2002;Upchurch et al 2004a;Rauhut et al 2005;Harris 2006c;Salgado et al 2006). Resembling Apatosaurus Marsh, 1877 and Diplodocus (Holland 1924;Berman & McIntosh 1978), but unlike Suuwassea Harris, 2006 and Dicraeosaurus (Janensch 1935-36), the nasal process of the premaxilla is continuous with, not medially offset from the lateral side of the tooth-bearing portion of the bone.…”

“…Specimens MB.R.2736 (E 6), MB.R.2722 (XI a 10), H 9, and MB.R.2735 (Ki 13) have a prominent, hook-shaped ambiens process caudodorsally. An enlarged ambiens process is a derived trait of Flagellicaudata (McIntosh 1990a;Wilson 2002;Rauhut et al 2005;Salgado et al 2006), but its hook-like shape is synapomorphic for Diplodocinae (Upchurch 1998;Upchurch et al 2004a;Harris 2006c). Therefore, these four specimens may be referred to Diplodocinae indet.…”

“…This is important because the original descriptions of "Barosaurus africanus" by Janensch (1935Janensch ( -36, 1961 still serve as the primary source of anatomical data for the Tendaguru sauropods, and false taxonomic assignments may lead to substantial misinterpretations in further analyses (e.g., of phylogeny or histology). Recent descriptions of newly discovered diplodocoids (Carvalho et al 2003;Harris & Dodson 2004;Salgado et al 2006;Harris 2006aHarris , 2006bHarris , 2007 and phylogenetic studies (Upchurch 1998;Wilson 2002;Salgado et al 2004;Upchurch et al 2004a;Rauhut et al 2005;Harris 2006c; Salgado et al 2006) provide a better database for assessing the taxonomic status of individual fossils. The material can be divided into two groups: a) diplodocoid material with phylogenetically diagnostic characters, and b) material that is not diagnostic beyond Sauropoda indet.…”