Friday, February 17, 2012

Lab 4: Dinosauria: Matt Knestrick & Megan Thornhill


In this lab, we explored the Cleveland Museum of Natural History to calssify extinct members of the reptilian group Dinosauria. Some were classified as Saurischian (reptile-like) or ornithischian (bird-like). While each group has many characteristics that separated them, the major features include their hip bones.

Nannotyrannus versus Baby Tyrannosaur rex

The first specimen we observed was a fossilized skeleton of what is either Nannotyrannus or juvenile Tyrannosaur rex. Because Tyrannosaur rex is similar in size to Nannotyrannus when it’s a juvenile, it is usually necessary to determine its identity based on whether it is ornithischian or saurischian. Tyrannus rex is saurischian, while Nannotyrannus is ornithischian. The specimen had the saurischian qualities of having a three-pronged pelvic girdle with an anterior pointing pubis. However, it also displayed the ornithischian quality of having a reduced antorbital fenestra (hole in front of it’s eye socket), with it’s lower edge running parallel to its teeth line. Because of this, it was impossible to determine which of the two species it was, a similar problem faced by scientists who discovered the specimen.

Saurischian vs. Ornithischian

We also classified specimen of Tyrannus rex, Triceratops horridis, Haplocanthosaurus delfsi, Allosaurus fragilis, Coelophysis and Diatryma as being saurischian or ornithischian. These specimen were much easier to classify, as they displayed many prominent characteristics of their respective group.

Tyrannus rex was classified as being
saurischian, since it had a three-pronged pelvic
girdle, a distinctly asymmetrical manus with a prominent 2nd digit, and a manus more than 45% the length of the humerus and radius combined.

Triceratops horridis was classified as ornithischian. The pubis bone of its pelvic girdle was facing posteriorly, making it look like the same structure as the ischium. It also had the reduced antorbital fenestra with the bottom margin running parallel to the teeth, and at least five sacral vertebrae connecting the pelvic girdle to the vertebral column.

Haplocanthosaurus delfsi was saurischian. It displayed the three-pronged pelvic girdle typical of this class. Allosaurus fragilis was ornithischian though, as it had the posteriorly facing pubis bone in its pelvic girdle.

Diatryma was determined to be ornithischian, as it had the pubis bone facing posteriorly. This condition of a bird pelvis is why that posterior facing pubis bones are classified as ornithischian pelvic girdles.

Coelophysis was a ornithischian, since it had the posteriorly facing pubis bone.

Temporal fenestration

We also examined the fenestration of various prehistoric specimen to show the typical Diapsid conditions of birds and dinosaurs, the anapsid condition of very basal reptiles, and the synapsid condition of prehistoric mammels.

The mammels on displayed, in particularly the mammoths, had girdles typical of terrestrial mammels, and presented the synapsid condition.

Corythosaurus, Parasaurolophus, and Edmontosaurus skulls were also examined. They all showed the diapsid condition of more derived reptiles. They also lack, or have very reduced antorbital fenestra, indicating that they are all ornithischian.

We also observed various basal aquatic reptiles, such as Phytosaurus, Ichtyosaurus, and Stenopterygius. These specimen showed a range of temporal fenestration, like the Diapsid condition shown.

Dimetrodon limbatus was another basal dinosaur on display. It showed the synapsid condition, which is unique for reptiles, which are typically diapsid. This indicates that this species is more

The final specimen we observed was a model of a
Sphenodon punctatus. While there are only a few extant species of tuatara, it is from an extremely ancient family that flourished during the closely related to mammels, and is one of many species that bridges the gap between mammels and reptiles.

same periods as the other specimen on display. Because of this, it is of great importance in studying the evolution and adaptations of reptiles.

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