Lab 4: Dinosauria (Kelly and Maggie)

During the Mesozoic time period, dinosaurs dominated the Earth. Some were incredibly tiny, while others were some of the largest animals ever on the planet. Over 1,000 species of dinosaurs (non-avian) have been described, and many of these fall into two major lineages: Saurischia and Ornithischia.

Saurischia

Saurischian hip (Tyrannosaurus rex): (A) Ischium (B) Ilium (C) Pubis

The Saurischian dinosaurs are know as the "lizard-hipped" dinosaurs. This refers to the positioning of the pelvis bones, particularly in regards to the pubis, which extends anteriorly and ventrally. In addition to the positioning of the pelvis, there are many other characteristics that set this group apart. For example, the length of the manus is more than 45% of the length of the humerus and radius combined. In addition, Digit II on the manus is the dominant digit, which gives the manus a distinctly asymmetrical look. While there are many other characteristics, these were two things that we noticed most often.

Examples of Saurischians

Tyrannosaurus rex: Note that the manus is more than 45% the length of the humerus and radius combined, as well as the distinctly asymmetrical manus, with Digit II being dominant.

Haplocanthus delfsi: While not a strictly Saurischian characteristic, note how, in the left photo, the head of the femur is facing medially and contacts the laterally-facing acetabulum in the hip. This allowed for the dinosaurs to hold themselves high above the substrate. In the left photo, note the three-pronged pelvic girdle and how the pubis projecting anteriorly and ventrally.

Allosaurus fragilis: Like in T. rex, note that the manus is more than 45% the length of the humerus and radius combined, as well as the distinctly asymmetrical manus, with Digit II being the dominant digit. In the right photo, note the three-pronged pelvic girdle and how the pubis projecting anteriorly and ventrally.

Ornithischia
 

Ornithischian hip (Triceratops horridis): (A) Pubis (B) Ilium (C) Ischium

The Ornithischian dinosaurs are known as the "bird-hipped" dinosaurs. Like the Saurischian dinosaurs, this name refers to the positioning of the pubis in the pelvic bones. In these dinosaurs, the pubis typically extends posteriorly. Other characteristics separate the Ornithschians from the Saurischians, including a reduced antorbital fenestra and the presence of at least five sacral vertebrae. Also, Digit V has no phalanges; it has been reduced to a small metatarsal. Again, there are many other characteristics that we noticed, but these are some examples of the ones that stood out the most.

Examples of Ornithischians

Triceratops horridis: Note the ventral margin of the antorbital fenestra that parallels the maxillary tooth row, as well as the lack of gastralia. In the bottom left-hand corner of the photo on the right, note how Digit V of the foot has been reduced to a small metatarsal with no phalanges.

L to R: Edmontosaurus, Parasaurolophus, Corythosaurus. All display the Diapsid condition. The reason these dinosaurs are all placed in to Ornithischia is because they either have very reduced or no antorbital fenestra.

Temporal Fenestration
While at the museum, we were able to put our knowledge concerning temporal fenestration to the test. The synapsid condition was apparent in the mammal skeletons, and the diapsid or modified diapsid condition was present in the dinosaur skulls.  

L to R: Irish elk, saber-toothed cat, wooly mammoth

Dimetrodon limbatus

Dimetrodon limbatus is a basal synapsid, not a dinosaur. This creature is on the lineage that lead to modern mammals. Therefore, D. limbatus was more closely related to mammals than to dinosaurs.

Dimetrodon limbatus. Note the synapsid condition of the skull.

Sphenodon punctatus

Sphenodon punctatus

The tuatara, Sphenodon punctatus, today displays the best example of the diapsid condition. Endemic to New Zealand, there are only two species still around today.