Taxonomy and Carnivorus Dinosaurs
I have come across a good example of how paleontologist use character traits to classify dinosaurs. The example is short and simple and could be used - with a few modifications in a biology class.
The example comes from Thomas Holtzs’ article Chasing Tyrannosaurus and Deinonychus Around the Tree of Life: Classifying Dinosaurs
If you look at the list below you will see four dinosaurs and nine traits listed. With these nine traits we can create a simple cladogram for the four dinosaurs. Traits or characters fall into four different catagories: primitive, unique, shared derived and convergent.
If you look at the chart below, you will notice that all four dinosaurs have three traits in common: hinge in lower jaw, wishbone and bipedality. These are considered the primitive traits that are found in the common ancestor of all four dinosaurs (if you assume the common ancestor did not have the three traits then each would have had to evolve independently in all four dinosaurs - which is a much more complicated scenario). Unfortunately, the three primitive traits do not tell us much, so lets look at unique traits. There are two unique traits in the list (retractable sickle claw and backward pointing pubis) found only in Deinonychus. While these distinguish Deinonychus they don’t help us learn which of the other three dinosaurs was the closest relative of Deinonychus. So we seem to be at an impass.
Allosaurus Deinonychus Albertosaurus Tyrannosaurus
Hinge in lower jaw Yes Yes Yes Yes
Wishbone Yes Yes Yes Yes
Bipedal Yes Yes Yes Yes
Retractable sickle claw No Yes No No
Backawards-pointing pubis No Yes No No
Number of fingers 3 3 2 2
Third metatarsal in foot Unpinched Unpinched Pinched Pinched
Astragalus (ankle bone) Short Tall Tall Tall
Tip of ischium Expanded Pointed Pointed Pointed
Let’s look at the chart again. There are several traits on the list that are shared between two or more - but not all - of the dinosaurs. These are two fingers (rather than three) on the hand, pinched third metatarsal (rather than unpinched), tall astragalus (rather than short) and pointed ischium (rather than expanded). None of these traits occur in Allosaurus (or meat eaters previous to Allosaurus). At this point we can say that the condition seen in Allosaurus is the ancestral condition (although this does not mean Allosaurus was the actual ancester of the other three). At this point then, we can say we have two clade, one composed of Allosaurus and the other composed of the other three dinosaurus:
This leaves the relationships between Deinonychus, Albertosaurus and Tyrannosaurus to work out. There are three posibilities. First, Deinonychus could be more closely related to Albertosaurus. Second, Deinonychus could be more closely related to Tyrannosaurus. Third, Tyrannosaurus is more closely related to Albertosaurus. Lets go back and look at our trait list. We have determined that the hinge in the lower jaw, the wishbone and bipedality are primitive traits shared by all four dinosaurs. We have also determined the tall astragalus and pointed tip of the ischium evolved after the lineages for Deinonychus, Albertosaurus and Tyrannosaurus split from the Allosaurus lineage. We have also determined that the retractable claw and the backward pointing pubis separate Deinonychus from the others. Let’s look at the second option above (that Deinonychus is more closely related to Tyrannosaurus).
In order for this to be the case 11 evolutionary changes would have had to take place: one change for the three primitive features shared by all, one for each of the unique traits in Deinonychus and 2 for the remaining two traits in Albertosaurus and Tyrannosaurus (which would have evolved independently in each lineage). There is a simpler explanation - one involving fewer evolutionary changes. This is the third option listed above, that Albetosaurus and Tyrannosaurus are more closely related to each other which requires only nine evolutionary changes, which I will leave to the reader to work out (consult the article by Holtz, linked to above, for a more detailed explanation and better cladograms).
