[D] Abydosaurus mcintoshi [sG] [T]
Describer
Chure, Britt, Whitlock & Wilson, J.A. 2010
Time
Cretaceous Early Cretaceous Late Albian Cenomanian
Classification
Saurischia Sauropodomorpha Sauropoda Brachiosauridae
Diet
Herbivore
Fossilsite
Cedar Mountain Formation, Carnegie Quarry at Dinosaur National Monument, Utah, US
Info
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 highwear 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.
Etymology
The generic name refers to Abydos, the Greek name for the city along the Nile River (now El Araba el Madfuna) that was the burial place of the head and neck of Osiris, Egyptian god of life, death, and fertility—an allusion to the type specimen, which is a skull and neck found in a quarry overlooking the Green River; sauros is the Greek word for lizard. The specific name honors Jack McIntosh for his contributions to Dinosaur National Monument and to the study of sauropod dinosaurs.
Holotype
Dinosaur National Monument (DINO) 16488, a nearly complete, articulated skull and lower jaws preserved in articulation with the first four cervical vertebrae.
Referred specimens
Many cranial and postcranial bones were found in association with the holotypic specimen in locality Dinosaur National Monument (DNM) 16, where excavations are ongoing. Cranial bones pertain to three additional individuals (DINO 17848, 17849, 39727), each of which can be referred to A. mcintoshi on the basis of autapomorphies and the absence of substantive differences with the holotype. DINO 17848 is the anterior portion of an articulated skull and lower jaws that was sawn through during excavation just anterior to the external nares. It preserves prenarial portions of the right and left premaxillae and maxillae; right and left dentaries; anterior portions of the right and left surangular and angular, right quadrate, pterygoid, and prearticular; and an associated right quadratojugal, ectopterygoid, dentary, splenial, and intercoronoid. DINO 17849 is a nearly complete, disarticulated skull that includes right premaxilla, jugal, lacrimal, postorbital, and palatine; right and left maxillae; nasals; pterygoids, ectopterygoids, and quadratojugals; left quadrate, a braincase with skull roof; and a complete set of upper teeth preserved as ‘dentures’— articulated teeth preserved with little or no surrounding dentigerous bones (see Britt et al. 2008). DINO 39727 is a braincase with a partial skull roof. The postcranial bones, which include a partial pelvis and sacrum with an articulated tail, a scapula, a humerus, and metacarpus, do not overlap with the holotype, but they were found in close association with DINO 17848 and DINO 17849. Although we cannot associate them directly with any one individual, their close association with the cranial bones and lack of other sauropod taxa in the quarry indicate that the postcranial bones almost certainly pertain to A. mcintoshi
Locality, horizon, and age
A. mcintoshi was collected from locality DNM 16, which is located 375 m WSW of the Carnegie Quarry at Dinosaur National Monument, in Utah, USA. The bonebearing horizon is 27 m above the base of the Cedar Mountain Formation, near the base of the Mussentuchit Member. The bones were preserved in the lower one half of a 3-m thick stack of low angle, trough cross-bedded to laminar-bedded, medium-grained, fluvial sandstones with thin interbeds of mudstone. These sandstones are incised into a 0–2-m thick smectitic mudstone. Together, these mudstones and sandstones mark the base of 12-m thick paleovalley incised into the Ruby Ranch Member of the Cedar Mountain Formation. Single crystal zircon U–Pb dating via LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) of three crystals comprising the youngest peak of a total of 63 dated crystals yields a mean age of 104.46±0.95 Ma. Thus, the Abydosaurus bone bed can be no older than the mid-Albian Stage of the Early Cretaceous. The minimum age of Abydosaurus is bounded by the Cenomanian-age Dakota Formation, which overlies the Cedar Mountain Formation and represents the transition to a marine facies during the first incursion of the western interior seaway (Eberth et al. 2006).
Diagnosis
A. mcintoshi is a titanosauriform sauropod based on the presence of camellate pneumaticity in its cervical vertebrae. It is diagnosed by a nasal lateral process directed anteriorly and lacking a posterior hook; maxilla narial process with dorsal articulation for the nasal; nasals overlap asymmetrically on the midline; external nares smaller than the orbit; upper crown apices shifted distally; upper tooth shafts twist through an arc of nearly 45°; and tooth wear present only mesially. Abydosaurus can be differentiated from other Early Cretaceous North American sauropods on the basis of its postcranial anatomy. The holotypic cervical centra of Abydosaurus are less elongate than those of Paluxysaurus (Rose 2007), the Cloverly sauropod, and probably Sauroposeidon (Wedel et al. 2000), with whose cervical series they do not quite overlap. The humerus referred to Abydosaurus is approximately the same size as that of Cedarosaurus (1.6 m; Tidwell et al. 1999), but is much broader at midshaft (26 vs. 18 cm). The humerus of Sonorasaurus is not complete (Ratkevich 1998), but it and other forelimb elements appear to be as gracile as of those Cedarosaurus (M. D’Emic, personal communication). Anterior caudal centra referred to Abydosaurus lack the deep lateral fossae and gentle anterior convexity present in Venenosaurus, and limb elements are more robust (Tidwell et al. 2001). Among cranial remains, the syntypic tooth of Astrodon johnstoni (Leidy 1865) superficially resembles lower teeth of Abydosaurus, but these similarities are not diagnostic among titanosauriforms. Teeth that have been assigned to Pleurocoelus nanus (Lull et al. 1911:pl. 14) more closely resemble upper teeth of Abydosaurus in the position of the crown apex and localized wear (see above). Despite these similarities, however, we refrain from drawing any taxonomic conclusions at this point due to uncertainty about the type series and constituency of Pleurocoelus nanus, its possible synonymy with Astrodon johnstoni (e.g., Carpenter and Tidwell 2005), and the extremely limited overlap with materials of Abydosaurus
Chure, Britt, Whitlock & Wilson, J.A. 2010
Time
Cretaceous Early Cretaceous Late Albian Cenomanian
Classification
Saurischia Sauropodomorpha Sauropoda Brachiosauridae
Diet
Herbivore
Fossilsite
Cedar Mountain Formation, Carnegie Quarry at Dinosaur National Monument, Utah, US
Info
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 highwear 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.
Etymology
The generic name refers to Abydos, the Greek name for the city along the Nile River (now El Araba el Madfuna) that was the burial place of the head and neck of Osiris, Egyptian god of life, death, and fertility—an allusion to the type specimen, which is a skull and neck found in a quarry overlooking the Green River; sauros is the Greek word for lizard. The specific name honors Jack McIntosh for his contributions to Dinosaur National Monument and to the study of sauropod dinosaurs.
Holotype
Dinosaur National Monument (DINO) 16488, a nearly complete, articulated skull and lower jaws preserved in articulation with the first four cervical vertebrae.
Referred specimens
Many cranial and postcranial bones were found in association with the holotypic specimen in locality Dinosaur National Monument (DNM) 16, where excavations are ongoing. Cranial bones pertain to three additional individuals (DINO 17848, 17849, 39727), each of which can be referred to A. mcintoshi on the basis of autapomorphies and the absence of substantive differences with the holotype. DINO 17848 is the anterior portion of an articulated skull and lower jaws that was sawn through during excavation just anterior to the external nares. It preserves prenarial portions of the right and left premaxillae and maxillae; right and left dentaries; anterior portions of the right and left surangular and angular, right quadrate, pterygoid, and prearticular; and an associated right quadratojugal, ectopterygoid, dentary, splenial, and intercoronoid. DINO 17849 is a nearly complete, disarticulated skull that includes right premaxilla, jugal, lacrimal, postorbital, and palatine; right and left maxillae; nasals; pterygoids, ectopterygoids, and quadratojugals; left quadrate, a braincase with skull roof; and a complete set of upper teeth preserved as ‘dentures’— articulated teeth preserved with little or no surrounding dentigerous bones (see Britt et al. 2008). DINO 39727 is a braincase with a partial skull roof. The postcranial bones, which include a partial pelvis and sacrum with an articulated tail, a scapula, a humerus, and metacarpus, do not overlap with the holotype, but they were found in close association with DINO 17848 and DINO 17849. Although we cannot associate them directly with any one individual, their close association with the cranial bones and lack of other sauropod taxa in the quarry indicate that the postcranial bones almost certainly pertain to A. mcintoshi
Locality, horizon, and age
A. mcintoshi was collected from locality DNM 16, which is located 375 m WSW of the Carnegie Quarry at Dinosaur National Monument, in Utah, USA. The bonebearing horizon is 27 m above the base of the Cedar Mountain Formation, near the base of the Mussentuchit Member. The bones were preserved in the lower one half of a 3-m thick stack of low angle, trough cross-bedded to laminar-bedded, medium-grained, fluvial sandstones with thin interbeds of mudstone. These sandstones are incised into a 0–2-m thick smectitic mudstone. Together, these mudstones and sandstones mark the base of 12-m thick paleovalley incised into the Ruby Ranch Member of the Cedar Mountain Formation. Single crystal zircon U–Pb dating via LA-ICPMS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) of three crystals comprising the youngest peak of a total of 63 dated crystals yields a mean age of 104.46±0.95 Ma. Thus, the Abydosaurus bone bed can be no older than the mid-Albian Stage of the Early Cretaceous. The minimum age of Abydosaurus is bounded by the Cenomanian-age Dakota Formation, which overlies the Cedar Mountain Formation and represents the transition to a marine facies during the first incursion of the western interior seaway (Eberth et al. 2006).
Diagnosis
A. mcintoshi is a titanosauriform sauropod based on the presence of camellate pneumaticity in its cervical vertebrae. It is diagnosed by a nasal lateral process directed anteriorly and lacking a posterior hook; maxilla narial process with dorsal articulation for the nasal; nasals overlap asymmetrically on the midline; external nares smaller than the orbit; upper crown apices shifted distally; upper tooth shafts twist through an arc of nearly 45°; and tooth wear present only mesially. Abydosaurus can be differentiated from other Early Cretaceous North American sauropods on the basis of its postcranial anatomy. The holotypic cervical centra of Abydosaurus are less elongate than those of Paluxysaurus (Rose 2007), the Cloverly sauropod, and probably Sauroposeidon (Wedel et al. 2000), with whose cervical series they do not quite overlap. The humerus referred to Abydosaurus is approximately the same size as that of Cedarosaurus (1.6 m; Tidwell et al. 1999), but is much broader at midshaft (26 vs. 18 cm). The humerus of Sonorasaurus is not complete (Ratkevich 1998), but it and other forelimb elements appear to be as gracile as of those Cedarosaurus (M. D’Emic, personal communication). Anterior caudal centra referred to Abydosaurus lack the deep lateral fossae and gentle anterior convexity present in Venenosaurus, and limb elements are more robust (Tidwell et al. 2001). Among cranial remains, the syntypic tooth of Astrodon johnstoni (Leidy 1865) superficially resembles lower teeth of Abydosaurus, but these similarities are not diagnostic among titanosauriforms. Teeth that have been assigned to Pleurocoelus nanus (Lull et al. 1911:pl. 14) more closely resemble upper teeth of Abydosaurus in the position of the crown apex and localized wear (see above). Despite these similarities, however, we refrain from drawing any taxonomic conclusions at this point due to uncertainty about the type series and constituency of Pleurocoelus nanus, its possible synonymy with Astrodon johnstoni (e.g., Carpenter and Tidwell 2005), and the extremely limited overlap with materials of Abydosaurus