Cranial anatomy of Shunosaurus and Camarasaurus (Dinosauria: Sauropoda) and the phylogeny of the Sauropoda



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Texas Tech University


The sauropods were the most spectacular of all dinosaurs, and the largest terrestrial animals that have ever lived. Yet, their skull anatomy and phylogeny are poorly known. New material of two sauropod dinosaurs, Shunosaurus from the Middle Jurassic of China and Camarasaurus from the Upper Jurassic of North America, provides critical information about their skull anatomy, brain morphology, tooth replacement pattern, kinesis, and feeding habits.

Shunosaurus is a primitive sauropod but exhibits the following cranial autapomorphies: tooth morphology is a combination of cylindrical and spatulate form; pterygoid is extremely slender and small; quadrate ramus of the pterygoid is forked, quadratojugal participates in the jaw articulation; occlusal level of the maxillary tooth row is convex downward, whereas that of the dentary is concave upward, acting like a garden shear; lingual wall of the dentary is considerably lower than the labial wall; and the replacing teeth invade the labial side of the functional teeth.

Camarasaurus is a relatively advanced sauropod, but retained spatulate teeth. The derived skull characters include: crowns of teeth are expanded transversely; there is a prominent longitudinal groove along the interdental plates of the maxilla; subnarial opening is located in the rostral comer of the extemal nostril opening; three external occipital vein foramina are present in the occiput; the basipterygoidal shelf is deep and complex; the crista prootic is large and conceals the basiocciphal tubera and the middle ear region.

The sauropods were entirely herbivorous, but their tooth morphology and skull mechanics indicate two different styles of feeding habit. Basal sauropods with lateral narial openings and spatulate teeth were probably high browsers; they developed a shearing jaw action, and probably relied on hard and resistant food, such as cones and twigs of conifers In contrast, in advanced sauropods, high, median nares and peglike teeth may indicate their low browsing habh; their peglike teeth were designed for cropping action by tooth to tooth contact. They probably fed on soft, aquatic vegetation.

Shunosaurus and Camarasaurus are compared with other Jurassic and Cretaceous sauropods of the world. Numerical cladistic analysis of 76 cranial characters among 14 sauropod taxa generates a corroborated hypothesis of phylogenetic relationships of sauropods. The following successive clades are established: Sauropoda, Eusauropoda, Neosauropoda, Homalosauropoda, Diplodocoidea, Dicraeosauridae and Nemegtosaurinae. Seven sauropod families are recognized: Cetiosauridae {Shunosaurus), Mamenchisauridae {Omeisaurus, Mamenehisaurus), Camarasauridae {Euhelopus, Camarasaurus, Brachiosaurus), Titanosauridae {Antaretosaurus), Diplodocidae {Diplodoeus, Barosaurus, Apatosaurus), Dicraeosauridae {Dicraeosaurtds, Amargasaurus) and Nemegtosauridae {Nemegtosaurus, Quaesitosaurus). Shunosaurus is the most primitive taxon and is the sister group of all other sauropods, Eusauropoda. Camarasaurus, on the other hand, is more advanced and is the closest relative of a newly defined taxon, Homalosauropoda.



Shunosaurus -- Anatomy, Saurischia -- Phylogeny, Camarasaurus -- Anatomy, Craniology