Lab # 6 -- INTRODUCTION TO DINOSAURS

Section 1. Tetrapods

In this section we will begin to apply what we have learned in previous labs about systematics to the question of amniote and especially reptile phylogeny. But first we will do an exercise on the basic features of the vertebrate skull.

You have, no doubt, committed to memory all of the bones of the basic reptilian skeleton. These are very important to today's work. In case your dog ate your lecture handout with all this information on it, it has been included in today's lab handout. Before you begin the questions, take a moment to familiarize yourself with the terminology again.

Take a look at the "Tree of Life" website again. (http://phylogeny.arizona.edu). By clicking on the following links in order you can trace the history of life, essentially the cladogram, from the chordates to the dinosaurs. Make some sidetrips on the way! The links from the above webpage are: Chordata, Craniata, Vertebrata, Gnathostomata, Sarcopterygii, terrestrial vertebrates, Amniota, Diapsida, Archosauromorpha, Archosauria, Dinosauria.

Part 1. The Skull

1. Examine a series of skulls belonging to different groups of the Tetrapoda. Make outline sketches of the given skulls and label the cranial openings of each (where present): a) nares

b) orbits

c) suborbital fenestra

d) upper and lower temporal fenestrae

e) antorbital fenestra

f) mandibular fenestra

2. a) Which arrangements of fenestrae are characteristic of specific groups? b) Construct a cladogram for these animals based on the skull openings; show synapomorphies and autapomorphies on the cladogram. 3. Identify the frontals, nasals and maxillae on each specimen, and label on your drawings. Is there any systematic change of relative size of these bones among the archosaurs?

Part 2: Postcranial Skeleton

1. Compare and make sketches (labeling the bones) of the following features of the amphibian, dinosaur, pigeon, and monkey skeletons: a) pelvic girdle

b) pectoral girdle

c) forelimb & hand

d) hindlimb & foot

2. How does the posture differ among the various groups?

Part 3: Relationships

Examine the cladograms below. On the basis of soft-part anatomy, which cladogram makes the most sense? On the basis of cranial openings which cladogram makes most sense? Label the cladograms (with characters) and justify your choices. What ways can we find to test our choice of which is best? What does examining an outgroup do?

Section 2. Introduction to Dinosaurs

In this section we will examine specifically the relationship between the two major groups within the Dinosauria - the Saurischia and the Ornithischia - by comparing skeletal characters. Remember: Crocs + dinosaurs (including birds) = Archosauria.

The following is a list of shared derived characters for Dinosauria, Ornithischia, and Saurischia. Keep in mind that the ornithischians and saurischians are equivalently-ranked sister taxa within the dinosaurs.

Dinosauria:

perforate acetabulum
expanded ilium
minimum of 3 sacral vertebrae
long, s-shaped neck
offset digits 1 and 5 on hand

Saurischia: Ornithischia

manus digit 2 longest backward, splint-like pubis
thumb very robust predentary bone
metacarpal 1 short and twisted leaf-shaped teeth
neck at least 1/3 presacral length ilium elongated forward
reduced antorbital opening
nutcracker-like jaws
cheeks

Questions: 1. Compare and sketch the skulls of Thescelosaurus (an ornithischian dinosaur), Coelophysis (a non-avian saurischian dinosaur), and the modern bird. For each skull, label the synapomorphies that you can see.

2. Compare the skeletons of Allosaurus and the pigeon. Sketch the following features, labeling the bones, and describe the morphological changes needed to get from the primitive saurischian to avian condition.

3. Construct a cladogram including the pigeon, Allosaurus, Archaeopteryx (see diagram), and Thescelosaurus. Choose a 5th taxon from last weekís lab to use as your outgroup. Label the synapomorphies and autapomorphies for each taxon. There will be more than 1 for each taxon.