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, 97 (4), 379-91

First Complete Sauropod Dinosaur Skull From the Cretaceous of the Americas and the Evolution of Sauropod Dentition

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First Complete Sauropod Dinosaur Skull From the Cretaceous of the Americas and the Evolution of Sauropod Dentition

Daniel Chure et al. Naturwissenschaften.

Abstract

Sauropod dinosaur bones are common in Mesozoic terrestrial sediments, but sauropod skulls are exceedingly rare--cranial materials are known for less than one third of sauropod genera and even fewer are known from complete skulls. Here we describe the first complete sauropod skull from the Cretaceous of the Americas, Abydosaurus mcintoshi, n. gen., n. sp., known from 104.46 +/- 0.95 Ma (megannum) sediments from Dinosaur National Monument, USA. Abydosaurus shares close ancestry with Brachiosaurus, which appeared in the fossil record ca. 45 million years earlier and had substantially broader teeth. A survey of tooth shape in sauropodomorphs demonstrates that sauropods evolved broad crowns during the Early Jurassic but did not evolve narrow crowns until the Late Jurassic, when they occupied their greatest range of crown breadths. During the Cretaceous, brachiosaurids and other lineages independently underwent a marked diminution in tooth breadth, and before the latest Cretaceous broad-crowned sauropods were extinct on all continental landmasses. Differential survival and diversification of narrow-crowned sauropods in the Late Cretaceous appears to be a directed trend that was not correlated with changes in plant diversity or abundance, but may signal a shift towards elevated tooth replacement rates and high-wear dentition. Sauropods lacked many of the complex herbivorous adaptations present within contemporaneous ornithischian herbivores, such as beaks, cheeks, kinesis, and heterodonty. The spartan design of sauropod skulls may be related to their remarkably small size--sauropod skulls account for only 1/200th of total body volume compared to 1/30th body volume in ornithopod dinosaurs.

Figures

Fig. 1
Fig. 1
Palaeogeography and exposures of the Cedar Mountain Formation in the area of the Abydosaurus mcintoshi quarry at Dinosaur National Monument, Utah. a Early Cretaceous (120 Ma) paleocoastline map (Mollweide projection) with latitude and longitude lines spaced at 30° intervals (modified from Blakey 2006). Star identifies position of Dinosaur National Monument. b Photograph of Dinosaur National Monument showing the location of locality DNM 16 (star), which is approximately 375 m WSW of the Carnegie Quarry visitor center (40°26′24″ N, 109°18′18″ W)
Fig. 2
Fig. 2
Stratigraphy and geochronology of DNM 16. a Stratigraphic section indicating position of skulls and zircon samples DMMZ-16 and PV-1. b Histogram of number of zircon crystals (blue) and relative age probability curve (red) of single crystal detrital zircon U/Pb dating of sample PV-1, the mudstone underlying the bone-bearing sandstone; 104.46 ± 0.95 Ma = Youngest Peak Mean Age, 2 sigma (3 crystals, mean square weighted deviation = 0.0112, probability = 0.099). See Online Resources 1 and 2 for geochronologic data
Fig. 3
Fig. 3
Photographs and interpretive line drawings of the holotypic skull of Abydosaurus mcintoshi gen et sp. nov (DINO 16488) in left lateral (a) and right lateral (b) views. Gray tone indicates matrix, hatching indicates broken bone. Abbreviations: a angular, aof antorbital fenestra, asaf anterior surangular foramen, d dentary, en external naris, eo exoccipital-opisthotic, fr frontal, h hyoid, j jugal, la lacrimal, ls laterosphenoid, ltf lateral temporal fenestra, m maxilla, n nasal, oc occipital condyle, or orbit, os orbitosphenoid, p parietal, paof preantorbital fenestra, pm premaxilla, po postorbital, pop paroccipital process, pr prearticular, prf prefrontal, psaf posterior surangular foramen, ptf postemporal fenestra, q quadrate, qj quadratojugal, sa surangular, sc scleral plates, snf subnarial foramen, spl splenial, splf splenial foramen, sq squamosal
Fig. 4
Fig. 4
Reconstruction of the skull of A. mcintoshi based on holotypic and referred specimens (DINO 16488, 17848, 17849, 39727) in anterior (a) and left lateral (b) views. Computed tomography cross-section through the third cervical vertebra just posterior to the diapophysis (c) reveals camellate pneumaticity. Photographs of left premaxillary tooth 1 (d) and right dentary tooth 5 (e) in lingual, mesial, and cross-sectional views show differences in tooth shape. Note twisting of carina in the premaxillary tooth, which has an apical wear facet. Cross-sections were taken at 5 mm intervals along the tooth axis. Abbreviations: nc neural canal, pcdl posterior centrodiapophyseal lamina, r2 cervical rib 2, r3 cervical rib 3
Fig. 5
Fig. 5
Temporal patterns in sauropodomorph tooth shape. The plot shows tooth slenderness index (crown height/crown width) for sauropodomorph genera throughout the Mesozoic (see Online Resource 5 to match sauropod species and SI). SI has been logged to show proportional differences between taxa. The orange field indicates non-sauropod sauropodomorphs (‘prosauropods’), the yellow field indicates basal sauropods, the red field indicates diplodocoids, and the blue field indicates macronarians. Half-tone drawings show representative members of each group (not to scale). The dashed gray vertical lines indicate the range of tooth breadths present in ‘prosauropods’, which may be used as a proxy for the primitive condition in sauropods. Phylogenetic uncertainty regarding the affinities of Jobaria is indicated by cross-hatching; the transparent blue and yellow fields indicate the shape of the tooth space when Jobaria is included within macronarians and basal sauropods, respectively. Time scale based on Gradstein et al. (2004)

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