Friday, February 17, 2012

Lab #4: Dinosauria. Alex Valigosky and Jeff Walker

Coelophysis - a dog-sized Saurischian predator
This past week we were privileged enough to observe the fine specimens of Dinosuria on display at the Cleveland Museum of Natural History

Dinosauria were the ruling reptiles of the Mesozoic. Their diversity included an estimated 1000 species and 500 genera. Some of the largest were massive marine reptiles but the group also included many smaller, even lizard-like members. All of the dinosaurs (save the avian clades) we observed at the CMNH fell into one of two groups based on pelvic girdle anatomy. This is the main reason for the separation of the groups, but as you will see, there are many other diagnostic characteristics.

The Ornithischia are those dinosaurs with a bird-like pelvic girdle. The pubis extends posteriorly and contacts the ischium. The primary Ornithischian we observed was Triceratops horridus, who was standing opposite T. rex in the display. 
Triceratops horridus, a typical Ornithischian. 
Triceratops was unique in its anatomy, but also exhibited characteristic Ornithischian traits. The quadrate bone is elongate and massive, almost completely armoring the base of the skull and neck. From this protrude its namesake three horns. The pelvic girdle of Triceratops is bird-like, but also massive. There were many sacral vertebra articulated to the ilium, and the ischium and pubis were very posteriorly extended.

Tyrannosaurus rex 
The Saurischia are those dinosaurs that have a lizard-like pelvic girdle. These types of girdles are three-pronged. The pubis extends ventrally and anteriorly.
The digits and manus of Saurischians are defining characteristics, as well. The manus is more than 45% percent the length of the humerus and radius combined. At the distal end of this lies an asymmetrical manus with digit II being the largest. The manus and arms bones pictured belong to Tyrannosaurus rex. These are very small in relation to the rest of the body and scientific debate continues as to what the purpose of these reduced limbs was. Two hypotheses are that they were used for holding down prey while feeding, or that they were used by males to hold down females while copulating.

Photos showing articulation of limbs to body in Tyrannosaurus rex (left) and Haplocanthosuarus delfsi (right), and above the girlles of various Saurishian and Ornithischian dinosaurs.

Nannotyrannus or juvenile Tyrannosaurus?? 
On display at the entrance to the museum is a fossil specimen of questionable origin. We identified it as a Saurischian based on pelvic girdle anatomy. Reading the placard associated the specimen, we learned that the identity of this Saurischian is debated as either a juvenile Tyrannosaurus or as a Nannotyrannus.

One whole side of the hall of dinosaurs was devoted to mammals, a somewhat distant relative. It is important to note that the mammal lineage is that of Synapsida and that they therefore exhibit the synapsid skull condition. Pictured below is a mammoth skull that shows its synapsid condition followed by a picture of the pelvic girdle. A large predatory bird, Diatryma, was also viewed, which exhibited the highly derived skull and pelvic girdles of the birds (descendants of Saurischians)

Mammoth skull and pelvic girdle. 
Cranial anatomy and pelvic girdles of Diatryma - a large predatory bird
One of the lab exercises was to identify the skull condition of several fossil specimens. Included here were the Phytosaurus, Ichthyosaurus, and Stenopterygius. We determined that the latter two were exhibiting the euryapsid condition. We also observed the skulls of several Dinosaurs to determine their fenestral condition. We found Corythosaurus, Parasurolophus, and Edmontosaurus to be diapsid Ornithischians. The reduced or lacking antorbital fenestra led us to this conclusion.

(Top pictures) Diapsid condition exhibited by Corythosaurus, Parasurolophus, and Edmontosaurus, all Ornithisichians.  (Bottom pictures) Euryapsids, Stenopterygius on top, and Ichthyosaurus on the bottom
Dimetrodon limbatus. Note the synapsid skull condition. 

A unique specimen on display was Dimetrodon limbatus. This specimen is a reptile demonstrating the synapsid condition. It is believed this is a basal member of the group that led to modern Synapsida.

Pictured below is Sphenodon punctatus, this is a Leposauromorph that also lived through the Mezozoic; there are still, surprisingly, two extant species still living in New Zealand today.

Sphenodon puncatus

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