[D] Sarahsaurus aurifontanalis [sG] [T]
Describer
Rowe, Sues & Reisz 2010
Time
Jurassic Early Sinemurian Pliensbachian
Classification
Saurischia Sauropodomorpha
Diet
Herbivore
Fossilsite
Kayenta Formation (Glen Canyon Group, Northern edge of Gold Spring Wash drainage basin, Arizona, US
Info
Abstract
Sauropodomorph dinosaurs originated in the Southern Hemisphere in the Middle or Late Triassic and are commonly portrayed as spreading rapidly to all corners of Pangaea as part of a uniform Late Triassic Early Jurassic cosmopolitan dinosaur fauna. Under this model, dispersal allegedly inhibited dinosaurian diversification, while vicariance and local extinction enhanced it. However, apomorphy-based analyses of the known fossil record indicate that sauropodomorphs were absent in North America until the Early Jurassic, reframing the temporal context of their arrival.
We describe a new taxon from the Kayenta Formation of Arizona that comprises the third diagnosable sauropodomorph from the Early Jurassic of North America. We analysed its relationships to test whether sauropodomorphs reached North America in a single sweepstakes event or in separate dispersals. Our finding of separate arrivals by all three taxa suggests dispersal as a chief factor in dinosaurian diversification during at least the early Mesozoic. It questions whether a ‘cosmopolitan’ dinosaur fauna ever existed, and corroborates that vicariance, extinction and dispersal did not operate uniformly in time or under uniform conditions during the Mesozoic. Their relative importance is best measured in narrow time slices and circumscribed geographical regions.
Etymology
Named in honour of Sarah (Mrs Ernest) Butler, whose broad interests in the arts, the sciences and medicine have enriched Texas in so many marvellous ways, and sauros (Gr., lizard); second part of binomen from aurum (L., gold) and fontanalis (L., of the spring), in reference to Gold Spring, Arizona, where the holotype was discovered.
Holotype
TMM 43646-2, a partial skull (premaxilla, frontal, quadrate and braincase) and a nearly complete, largely articulated postcranial skeleton (figures 2 and 3). The holotype is ontogenetically mature based on fusion of the opisthotic-exoccipitals to the basioccipital, fusion of the neural arches to their centra along the entire vertebral column, and fusion of the sacral and caudal ribs to their respective vertebrae.
Locality and horizon
Northern edge of Gold Spring Wash drainage basin, in northeastern Arizona, USA; middle third of the ‘Silty Facies’ of the Kayenta Formation (Glen Canyon Group); Early Jurassic (Sinemurian–Pliensbachian).
Referred specimens
TMM 43646-3, a partial postcranial skeleton from the holotype quarry; and Harvard University, Museum of Comparative Zoology, MCZ 8893 (figure 4), a crushed but nearly complete skull and mandible, with cervical and caudal vertebral fragments, the distal end of a humerus and a femoral shaft. Referral of the MCZ specimen is based on skeletal elements shared with the holotype, including the braincase, quadrate, frontal, premaxilla, cervical vertebrae and humerus, in which the two specimens are identical in all character scores and in all other respects. The only notable difference is that MCZ 8893 represents a less mature individual, with open sutures between the exoccipital-opisthotics and basioccipital, whereas the holotype was fully mature and has closure of these sutures.
Diagnosis
Among sauropodomorphs, Sarahsaurus is unique in the configuration of a low wall between the basicranial tubera with a central anterior fossa; presence of spine tables on its dorsal vertebrae; a manual phalangeal formula of 2-3-4-2-2; and presence of a pubic foramen. Sarahsaurus is also diagnosed by the largest unique suite of character states of any taxon analysed in either matrix. Our diagnoses are based on the pruned analysis, because unequivocal local apomorphic states cannot be taken from consensus trees. In the pruned Yates matrix, Sarahsaurus had a branch length (BL) of 61 steps and in the Upchurch et al. matrix 35 steps. Most characters are homoplastic with taxa elsewhere on the tree, but the combinations were unique in all tests.
Rowe, Sues & Reisz 2010
Time
Jurassic Early Sinemurian Pliensbachian
Classification
Saurischia Sauropodomorpha
Diet
Herbivore
Fossilsite
Kayenta Formation (Glen Canyon Group, Northern edge of Gold Spring Wash drainage basin, Arizona, US
Info
Abstract
Sauropodomorph dinosaurs originated in the Southern Hemisphere in the Middle or Late Triassic and are commonly portrayed as spreading rapidly to all corners of Pangaea as part of a uniform Late Triassic Early Jurassic cosmopolitan dinosaur fauna. Under this model, dispersal allegedly inhibited dinosaurian diversification, while vicariance and local extinction enhanced it. However, apomorphy-based analyses of the known fossil record indicate that sauropodomorphs were absent in North America until the Early Jurassic, reframing the temporal context of their arrival.
We describe a new taxon from the Kayenta Formation of Arizona that comprises the third diagnosable sauropodomorph from the Early Jurassic of North America. We analysed its relationships to test whether sauropodomorphs reached North America in a single sweepstakes event or in separate dispersals. Our finding of separate arrivals by all three taxa suggests dispersal as a chief factor in dinosaurian diversification during at least the early Mesozoic. It questions whether a ‘cosmopolitan’ dinosaur fauna ever existed, and corroborates that vicariance, extinction and dispersal did not operate uniformly in time or under uniform conditions during the Mesozoic. Their relative importance is best measured in narrow time slices and circumscribed geographical regions.
Etymology
Named in honour of Sarah (Mrs Ernest) Butler, whose broad interests in the arts, the sciences and medicine have enriched Texas in so many marvellous ways, and sauros (Gr., lizard); second part of binomen from aurum (L., gold) and fontanalis (L., of the spring), in reference to Gold Spring, Arizona, where the holotype was discovered.
Holotype
TMM 43646-2, a partial skull (premaxilla, frontal, quadrate and braincase) and a nearly complete, largely articulated postcranial skeleton (figures 2 and 3). The holotype is ontogenetically mature based on fusion of the opisthotic-exoccipitals to the basioccipital, fusion of the neural arches to their centra along the entire vertebral column, and fusion of the sacral and caudal ribs to their respective vertebrae.
Locality and horizon
Northern edge of Gold Spring Wash drainage basin, in northeastern Arizona, USA; middle third of the ‘Silty Facies’ of the Kayenta Formation (Glen Canyon Group); Early Jurassic (Sinemurian–Pliensbachian).
Referred specimens
TMM 43646-3, a partial postcranial skeleton from the holotype quarry; and Harvard University, Museum of Comparative Zoology, MCZ 8893 (figure 4), a crushed but nearly complete skull and mandible, with cervical and caudal vertebral fragments, the distal end of a humerus and a femoral shaft. Referral of the MCZ specimen is based on skeletal elements shared with the holotype, including the braincase, quadrate, frontal, premaxilla, cervical vertebrae and humerus, in which the two specimens are identical in all character scores and in all other respects. The only notable difference is that MCZ 8893 represents a less mature individual, with open sutures between the exoccipital-opisthotics and basioccipital, whereas the holotype was fully mature and has closure of these sutures.
Diagnosis
Among sauropodomorphs, Sarahsaurus is unique in the configuration of a low wall between the basicranial tubera with a central anterior fossa; presence of spine tables on its dorsal vertebrae; a manual phalangeal formula of 2-3-4-2-2; and presence of a pubic foramen. Sarahsaurus is also diagnosed by the largest unique suite of character states of any taxon analysed in either matrix. Our diagnoses are based on the pruned analysis, because unequivocal local apomorphic states cannot be taken from consensus trees. In the pruned Yates matrix, Sarahsaurus had a branch length (BL) of 61 steps and in the Upchurch et al. matrix 35 steps. Most characters are homoplastic with taxa elsewhere on the tree, but the combinations were unique in all tests.