Dinosauria: This week in lab we went to the Cleveland Museum of Natural History to observe the skeletal anatomy of dinosaurs, as well as that of some other extinct reptiles and synapsids. During the Mesozoic, non-dinosaurs were the dominant tetrapods, with roughly 1,000 species described from fossil remains. Non-avian dinosaurs are part of the larger clade Ornithodira, which also includes birds, meaning that they possess antorbital fenestrae, supra-acetabular crests, enlarged fourth trohanters, and a digitigrade stance. Additionally, the monophyly of Dinosauria is supported by numerous other osteological characters. The two major clades of dinosaurs are the Ornithischia (bird-hipped dinosaurs) and Saurischia (lizard-hipped dinosaurs). These clades are most easily recognized by their hip morphology, but there are also many other characters that unite these groups.
Tyrannosaurus rex or Nanotyranus: The first specimen that we observed was a theropod dinosaur that was either a specimen of a juvenile Tyrannosaurus rex or Nanotyranus. The debate about the actual identity of the specimen is ongoing. In either case, the skeleton clearly shows the saurischian hip condition, in which the boot-shaped pubis projects ventrally. It was also clear that the manus was longer than 45% of the rest of forelimb, and that the manus was asymmetrical, with digit II being dominant, which are both characteristic of saurischian dinosaurs. The antorbital fenestra was also very obvious on this specimen. All dinosaurs exhibit the diapsid condition of temporal fenestration, and both the subtemporal fenestra and supratemporal fenestra were seen in this specimen and all other dinosaurs.
Tyrannosaurus rex and Triceratops horridus: Next we looked at specimens of an adult Tyrannosaurus rex and Triceratops horridus. The Tyrannosaurus was clearly a saurischian dinosaur and appeared very similar to the Tyrannosaurus/Nanotyrannus specimen. Triceratops, on the other hand, was an ornithischian dinosaur, exhibiting the characteristic hip with the pubis directed posterior. Other ornithischian characteristics visible on Triceratops included: a toothless and roughened rostral tip of the premaxilla, exclusion of the maxilla from the margin of the external naris by the lateral process of the premaxilla, buccal emarginations of the upper and lower jaws, a mandibular condyle that was set below the tooth row, and the absence of gastralia.
Extinct Mammals: We examined the skeletons of several extinct mammals. These mammals possessed a hip morphology that was very different from that of any of the dinosaurs observed. They also all exhibited the synapsid condition of a single subtemporal fenestra demonstrated by the presence of a zygomatic arch.
Diatryma: We also examined the skeleton of the extinct bird Diatryma. The hip of this bird looked similar to that of the ornithischian dinosaurs, in that the pubis was directed posteriorly and closely associated with the ischium. Despite this similarity, it is now believed that birds evolved from saurischian theropods, rather than from ornithischian dinosaurs.
Corythosaurus, Parasauralophus, and Edmontosaurus: We next observed the skulls of Corythosaurus, Parasauralophus, and Edmontosaurus. Though all of these dinosaurs possessed supratemporal and subtemporal fenestrae, none possessed prominent antorbital fenestrae, indicating reduction of the antorbital fenestrae and placing these dinosaurs within Ornithischia. This was confirmed by the observing the posteriorly directed pubis of Edmontosaurus, the only specimen of the three which was a complete skeleton.
Haplocanthosaurus delfsi and Tyrannosaurus rex Pelvic Girdle: We observed the limbs and girdles of the large saurischian sauropod Haplocanthosaurus delfsi. Rather than extending laterally from the body (like many non-dinosaur amniote contemporaries), the limbs extend ventrally from the body to give Haplocanthosaurus an upright stature. This is achieved with a laterally-facing acetabulum and a femur head that is inturned and directed medially to articulate with the acetabulum. The Tyrannosaurus rex pelvic girdle has the boot-shaped pubis that projects ventrally, the saurischian hip condition.
Allosaurus fragilis: We examined the pelvic girdle and skeleton of the saurischian Allosaurus fragilis. This species has the saurischian hip condition, as well as a conspicuous antorbital fenestra.
Coelophysis: We examined the specimen of Coelophysis on display. This dinosaur has characteristics of both Saurischia and Ornithischia, suggesting that it is a very basal member of Dinosauria before the lineage split in two. Coelophysis has a conspicuous antorbital fenestra, as well as elongate posterior cervicals, two characteristics of Saurischia. However, the manus was not longer than 45% of the rest of forelimb.The pelvic girdle was unique in appearance, not resembling that of Ornithischia or Saurischia. Coelophysis has a classic diapsid skull condition.
Phytosaurus, Ichtyosaurus, and Stenopterygius: Next, we observed Phytosaurus, Ichtyosaurus, and Stenopterygius. Phytosaurus superficially resembled a crocdilian skull, although it does not actually belong to Crocodylotarsi. This reptile had a classic diapsid skull. Ichtyosaurus and Stenopterygius are both ichthyosaurs. These aquatic reptiles both have the euryapsid condition of possessing supratemporal fenestration, but lacking subtemporal fenestration.
Dimetrodon limbatus: In the final display, we looked at a specimen of Dimetrodon limbatus, an early synapsid. The aspect of the skeleton that demonstrates that Dimetrodon is a synapsid, and not a diapsid reptile, is that it possesses only subtemporal fenestration, and lacks supratemporal fenestration.
Sphenodon punctatus: Sphenodon punctatus, the tuatara, is one of two extant species of the Order Spenodontida. This reptile is endemic to New Zealand, and has the classic diapsid condition. Two characteristics of Sphenodon punctatus are spines on the nape and a parietal eye on the dorsal view of the head.