Fearfully Great Lizards

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HONR 259C: “Fearfully Great Lizards”: Topics in Dinosaur Research

Patterns and Processes of Evolution

February 13, 2003

Group member names:

Part I: Homology & Adaptation

Questions 1 - 5 refer to the following diagram of the skeleton of a human arm:

1. How many bones are in the upper arm?

2. How many bones are in the forearm?

3. Roughly how many bones are in the wrist? What is their shape?

4. Roughly how many bones are in the palm? What is their shape?

5. Roughly how many bones are in the fingers? What is their shape? Are they larger or smaller than the bones of the palm?
Using the information in your answers to questions 1-5, identify the upper arm, forearm, wrist, palm, and fingers in each of the following arm skeletons. They are to different scales, but all show the arm in the same general orientation.

6. Opossum

7. Mosasaur (an extinct marine lizard)

8. Bat

Sometimes, bony elements are lost or fused. The following arm skeletons display such changes. See if you can still identify the regions like you did in 6-8.

9. Peccary (a fast-running pig relative from the New World)

10. Pigeon

The general patterns that we see reflected in this wide variety of vertebrate forelimbs reflect an underlying similarity based on common ancestry: a pattern called homology. At the same time, these examples show a different kind of similarity based on the animals’ adaptation to their life styles. The following page shows a variety of forelimb skeletons. Among them are limbs adapted for the life styles listed below. For each life style:

  • Identify the animal that is adapted to it

  • Briefly describe what it is about the skeleton that fits the adaptation.

11. Swimming:

12. Flying:

13. Running

14. Grasping and climbing.

Part II. Intermediate Forms

Although the fossil record, by its nature, is spotty, we identify trends toward different anatomical adaptations in creatures living at successive time periods. The following illustrations represent a group of fossil organisms that make up such a series. In this case, the series shows the progressive development of a group of dinosaurs called the ceratopsians. Their ancestor was a medium-sized (c. 1 m long) bipedal plant-eater, while the most derived ceratopsians (the horned ceratopsids) were rhinoceros-to-elephant sized quadrupeds. Below is a series of ceratopsians, with oldest/most primitive on the bottom and the youngest/most specialized at the top.

Anchiceratops (late Late Cretaceous, ~4.5 m long)

Protoceratops (late Late Cretaceous, ~1.8 m long)

Archaeoceratops (early Late Cretaceous, ~1.5 m long)

Psittacosaurus (late Early Cretaceous, ~1.5 m long)

(Middle Jurassic, ~1 m long)

15. Based on the figures in the previous page, indicate the evolutionary changes you observe in:

Size of the dinosaurs:

Relative size of the skull of the dinosaurs:

Relative size of the tail of the dinosaurs:

Development of the frill on the back of the skull:

Development of horns on the face:

Other changes?

Part III: Convergence

Below are a series of skulls different animals, modern and extinct. Each pair represents a specialized species and one that closely approximates the form of the ancestor of the specialized form. These pairs are:

  • Gavialis, a modern fish-eating crocodilian; and Pseudohesperosuchus, a primitive meat-eating land relative of the crocodilians

  • Suchomimus, a spinosaurid dinosaur; and Eustreptospondylus, a more primitive torvosaurid dinosaur

  • Tursiops, the bottlenose dolphin; and Harpagolestes, a land dwelling meat-eating relative of the cetaceans (whales and dolphins)

Gavialis Pseudohesperosuchus

Suchomimus Eustreptospondylus

Tursiops Harpagolestes
17. How does the number of teeth in Gavialis compare to the number in Pseudohesperosuchus?
In Suchomimus compared to Eustreptospondylus?
In Tursiops compared to Harpagolestes?
18. Are the snout shapes of the three specialized forms more similar to their closest relatives or more similar to each other?

19. The teeth of Gavialis, Suchomimus, and Tursiops are all simple cones; those of Pseudohesperosuchus and Eustreptospondylus are blade-like while those of Harpagolestes are a combination of nipping incisors, biting canines, and grinding premolars and molars. What might be the reason for the shared similarity of the teeth of the three specialized forms?

20. How might you explain the dissimilarity of the ancestors but the great similarities between the advanced forms? What factors might result in the common features shared by the advanced forms?

Part IV. Divergence

Dinosauria (the dinosaurs) is a group of reptiles that dominated Mesozoic terrestrial ecosystems. During this time they demonstrated great ecological diversity. Where did this great diversity come from?

On the next page is a diagram showing a small selected set of dinosaurs that might help explain this. These do not always represent direct ancestor-descendant sequences, but do give you a general approximation of some of the trends in various dinosaur groups. The masses of the dinosaurs are indicated. 1 kg (1 kilogram) = 2.2 lbs; 1 t (1 metric tonne) = 2200 lbs.
21. How similar are the dinosaurs of the Late Cretaceous to each other? Are any obvious anatomical similarities (other than giant body size) apparent between these different forms? Do you expect that these animals all had the same (or similar) ecological habits?

22. Trace each group back through time. Do you see any particular trends in shape and/or size through the various groups? Start with Lagosuchus and head up to one of the four Late Cretaceous forms. Describe the changes you see.

23. Are the dinosaurs of the Early Jurassic more similar or less similar to each other than those of the Late Cretaceous?

24. Time, of course, ran from the Middle Triassic to the Late Cretaceous. Briefly describe the pattern of change overall from Lagosuchus (representing the common ancestor of dinosaurs) to the diversity seen later on, using the concepts of descent with modification and natural selection.

Part V. Heterochrony

Recall the concept of heterochrony: evolution by the change of the rate of development of parts of the body from the ancestral rate of development. Heterochrony can be divided into two general types:

  • Pedomorphosis: The adult form of the descendant is more similar to the juvenile state of the ancestor than to the adult state of the ancestor

  • Peramorphosis: The adult form of the descendant goes further in its specialization than the adult form of the ancestor

We’ll look at heterochrony in the evolution of the long-necked herbivorous sauropodomorph dinosaurs. Thecodontosaurus from the Early Jurassic represents the primitive form. Below we see the skull and skeleton of the juvenile on the left, and the skull (based on the more completely known Plateosaurus) and skeleton of the adult on the right. The juvenile shown was about 1 m long; the adult a bit over 2 m.


Camarasaurus from the Late Jurassic will represent the giant Sauropoda. Below is a restoration of the skull and skeleton of an adult Camarasaurus:

25. In terms of the length of the snout, is the skull of Camarasaurus pedomorphic or peramorphic?

26. In terms of the size of the skull relative to the rest of the body, is Camarasaurus pedomorphic or peramorphic?

27. In terms of the heaviness of the front limbs compared to the hindlimbs, is Camarasaurus pedomorphic or peramorphic?

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