Sarahsaurus

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Sarahsaurus
Temporal range:
Ma
[1]
Illustration of the skull in multiple views
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: †Sauropodomorpha
Clade: †Massopoda
Genus: †Sarahsaurus
Rowe, Sues & Reisz, 2011
Species:
†S. aurifontanalis
Binomial name
†Sarahsaurus aurifontanalis
Rowe, Sues & Reisz, 2011

Sarahsaurus is a

sauropodomorph dinosaur which lived during the Early Jurassic period in what is now northeastern Arizona, United States.[2]

Discovery and naming

Illustration of the bones in the holotype

All specimens of Sarahsaurus were collected from the

MCZ 8893) was assigned to the genus.[2] The latter specimen was originally referred to as Massospondylus sp.[3] The complete skull is crushed and split horizontally, separating the skull roof from the palate; this split was caused by periodic swelling and shrinkage of the surrounding clay after burial. While the holotype individual was mature, the second skull indicates a less mature individual as indicated by open suture lines that separate skull bones of the braincase (the bones enclosing the brain).[2][1]

Sarahsaurus was first described by Timothy B. Rowe,

Connecticut River Valley, and Seitaad of the later Navajo Sandstone of Early Jurassic Utah; a genus of basal sauropodmorph from Early Jurassic deposits around Nova Scotia called Fendusaurus remains undescribed.[5] It is thought to have appeared through a dispersal event that originated in South America and was separate from those of the other two sauropodomorphs.[2] The animal is notable for possessing very large, powerful hands, possibly suggesting that it was an omnivore.[6]

Description

Sarahsaurus was a basal (or "primitive") member of the sauropodomorph lineage, which also included the more derived (or "advanced")

Gregory Paul, in a 2016 popular book, estimated its length at 4 metres (13 ft) and its body weight at 200 kilograms (440 lb).[8]

Skull

The external naris (skull opening for the nostril) was less than half the size of the orbit (eye socket). It was bordered by the nasal, premaxilla, and, to a small extent, the maxilla, the latter two forming the upper jaw. Between the premaxilla and maxilla there was a large opening, the subnarial foramen. The antorbital fossa, an additional skull opening seen in most dinosaurs that was situated between the external naris and orbit, was less than half the length of the orbit. Much of the upper margin of the orbit was formed by the frontal bone of the skull roof; this bone, however, did not contribute to the front margin of the upper temporal fenestra at the rear end of the skull roof. Instead, this front margin was formed by the postorbital and parietal bones, with the front part of the parietal being forked to connect with the latter. Between the parietal and the supraoccipital, which follows behind the parietal at the back of the skull, was another opening, the postparietal fenestra, which was large in Sarahsaurus. A unique feature of the genus can be found on the underside of the braincase, where a shallow ridge spanned between the basipterygoid processes, bony downwards projections of the braincase that brace the latter against the palate. The lower jaw curved downwards towards its tip, and the jaw joint was located well below the level of the tooth row. There is no evidence for a beak.[2][1]

Each side of the upper jaw was lined with 20 teeth – four in the premaxilla, the front bone of the upper jaw, and 16 in the maxilla which followed behind. The dentary bone of the lower jaw likewise had 20 teeth. The teeth were moderately heterodont, i.e., tooth shape varied within the jaw, and sat slightly obliquely in the jaw, slightly overlapping each other. Their cutting edges were serrated with up to 20 denticles per tooth.[2]

Postcranium

The spine comprised 10 cervical (neck) vertebrae; 14 dorsal (trunk) vertebrae; three sacral (hip) vertebrae; and ca. 50 caudal (tail) vertebrae. In contrast to the later

air sacs invading the bone (postcranial skeletal pneumaticity). The scapula (shoulder blade) was hourglass-shaped. In the holotype specimen, the scapula and coracoid were unfused, probably connected only via cartilage; this might indicate that the individual was not yet fully mature. The holotype specimen includes the sternum (breastbone) and the clavicle (collarbone), which are rarely found with dinosaur skeletons. The clavicula is a rod-like bone that articulates with the lower end of the scapula in an immobile joint. It has been controversial if the left and right clavicles of sauropodomorphs were attached to each other at their ends, thus connecting the left and right shoulder girdles, or if they were pressed against their scapulae along their lengths. The orientation of the clavicle in Sarahsaurus confirms that the former was the case; the ends of both clavicles would probably have been connected to each other by a ligament, though they were not fused. In side view, the scapula was tilted between 45° and 65° relative to the horizontal. This would have allowed the left and right coracoids to almost touch each other at the body midline, while the left and right sterna, which were attached to each other at the midline, would have been located directly behind the coracoids.[2][1]

The humerus (upper arm bone) was more than half the length of the femur (upper thigh bone), and had a large crest on its front side for muscle attachment, the deltopectoral crest, which extended for half the length of the bone and had an S-shaped edge. In the hand, the ungual (claw bone) of the first finger was the largest.[2]

In the hip, the

archosaurs. The femur was straight and elliptical in cross-section. The tibia (shin bone) reached 84% of the length of the femur.[2]

Classification

Holotype braincase in multiple views

In a

derived than the former and more primitive than the latter.[9] A 2018 cladistic analysis by Chapelle and Choiniere recovered Sarahsaurus and Ignavusaurus as massospondylids, mirroring the original assignment of Sarahsaurus to Massospondylus.[10]

Plateosauria

Paleoecology

Restoration

Age and habitat

A definitive radiometric dating of the Kayenta Formation has not yet been made, and the available stratigraphic correlation has been based on a combination of radiometric dates from vertebrate fossils, magnetostratigraphy and pollen evidence.[11] It has been surmised that the Kayenta Formation was deposited during the Sinemurian and Pliensbachian stages of the Early Jurassic period, approximately 199 to 182 million years ago.[12] The Kayenta Formation is part of the Glen Canyon Group that includes formations not only in northern Arizona but also parts of southeastern Utah, western Colorado, and northwestern New Mexico. The formation was primarily deposited by rivers. During the Early Jurassic, the land that is now the Kayenta Formation experienced rainy summers and dry winters. By the Middle Jurassic period it was being encroached upon from the north by a sandy dune field that would become the Navajo Sandstone.[13] The animals here were adapted to a seasonal climate and abundant water could be found in streams, ponds and lakes.

Paleofauna

Sarahsaurus shared its

therapsids, lizard-like animals, and dinosaurs, which provided evidence that these animals were also present.[20] Non-vertebrates in this ecosystem included microbial or "algal" limestone,[19] freshwater bivalves, freshwater mussels and snails,[13] and ostracods.[21] The plant life known from this area included trees that became preserved as petrified wood.[17]

References

  1. ^ a b c d e f Adam D. Marsh, Timothy B. Rowe (2018):Anatomy and systematics of the sauropodomorph Sarahsaurus aurifontanalis from the Early Jurassic Kayenta Formation:https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0204007
  2. ^
    PMID 20926438
    .
  3. doi:10.1080/02724634.1985.10011850
    .
  4. ^ "New Fossil Suggests Dinosaurs Not So Fierce After All".
  5. ^ Fedak TJ. Description and evolutionary significance of the sauropodomorph dinosaurs from the Early Jurassic (Hettangian) McCoy Brook Formation. Ph.D. Dissertation, Dalhousie University. 2006.
  6. ^ [1]
  7. ISBN 978-0-253-35701-4
    .
  8. ISBN 9780691167664
    .
  9. PMID 22096511.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  10. ^ Chapelle KEJ, Choiniere JN. (2018) A revised cranial description of Massospondylus carinatus Owen (Dinosauria: Sauropodomorpha) based on computed tomographic scans and a review of cranial characters for basal Sauropodomorpha. PeerJ 6:e4224 https://doi.org/10.7717/peerj.4224
  11. ^ J. M. Clark and D. E. Fastovsky. 1986. Vertebrate biostratigraphy of the Glen Canyon Group in northern Arizona. The Beginning of the Age of the Dinosaurs: Faunal change across the Triassic-Jurassic boundary, N. C. Fraser and H.-D. Sues (eds.), Cambridge University Press 285–301
  12. ^ Padian, K (1997) Glen Canyon Group In: Encyclopedia of Dinosaurs, edited by Currie, P. J., and Padian, K., Academic Press.
  13. ^
    S2CID 32010827. {{cite book}}: Missing or empty |title= (help
    )
  14. ^ Gay, R. 2010. Kayentavenator elysiae, a new tetanuran from the early Jurassic of Arizona. Pages 27–43 in Gay, R. Notes on early Mesozoic theropods. Lulu Press (on-demand online press).
  15. ^ a b Lucas, S. G.; Heckert, A. B.; Tanner, L. H. (2005). "Arizona's Jurassic fossil vertebrates and the age of the Glen Canyon Group". In Heckert, A. B.; Lucas, S. G. (eds.). Vertebrate paleontology in Arizona. Bulletin. Vol. 29. Albuquerque, NM: New Mexico Museum of Natural History and Science. pp. 95–104.
  16. ^ a b Jenkins, F. A., Jr., Crompton, A. W., and Downs, W. R. 1983. Mesozoic mammals from Arizona: new evidence in mammalian evolution. Science 222(4629):1233–1235.
  17. ^ a b Jenkins, F. A., Jr. and Shubin, N. H. 1998. Prosalirus bitis and the anuran caudopelvic mechanism. Journal of Vertebrate Paleontology 18(3):495–510.
  18. ^ Curtis, K., and Padian, K. 1999. An Early Jurassic microvertebrate fauna from the Kayenta Formation of northeastern Arizona: microfaunal change across the Triassic-Jurassic boundary. PaleoBios 19(2):19–37.
  19. ^ a b Luttrell, P. R., and Morales, M. 1993. Bridging the gap across Moenkopi Wash: a lithostratigraphic correlation. Aspects of Mesozoic geology and paleontology of the Colorado Plateau. Pages 111–127 in Morales, M., editor. Museum of Northern Arizona, Flagstaff, AZ. Bulletin 59.
  20. ^ Hamblin, A. H., and Foster, J. R. 2000. Ancient animal footprints and traces in the Grand Staircase-Escalante National Monument, south-central Utah. Pages 557–568 in Sprinkel, D. A., Chidsey, T. C., Jr., and Anderson, P. B. editors. Geology of Utah's parks and monuments. Utah Geological Association, Salt Lake City, UT. Publication 28.
  21. ^ Lucas, S. G., and Tanner L. H. 2007. Tetrapod biostratigraphy and biochronology of the Triassic-Jurassic transition on the southern Colorado Plateau, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 244(1–4):242–256.
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