A dinosaur defined

This exercise is a part of Educator Guide: What Makes a Dinosaur? / View Guide

These questions are based on the feature article “What makes a dinosaur?

Due to the length of the article, you may want to assign each section of the article to a specific group of students. For example, divide your class into five groups, and assign each group a section to become an expert on. The sections include: “Shared traits,” “Dino survivors,” “Uprooting the family tree,” “Catch-22” and “Finding Teleocrater.” Each group should also be assigned to read the introduction. Allow time for each expert group to discuss and summarize its segment. Next, assign new groups, each including at least one “section expert” from each of the original five groups. Each “section expert” should summarize his or her section of the article so that the new group has a summary of the whole article. Finally, each new group should work together to answer the questions below.  

1. What is the only characteristic currently known to be unique to all dinosaurs, and why is it important?

Possible student response: Dinosaurs had holes (as opposed to a dimple or small depression) in their hip sockets. That probably enabled the ancient reptiles to position their legs underneath them instead of on either side of them like crocodiles and turtles.

2. What are four other characteristics once believed to be unique to dinosaurs, but are now known to have been shared by some other early reptiles? What other reptile(s) have the characteristic?

Possible student response: (1) A deep depression at the top of the skull probably related to bite strength as an attachment site for jaw muscles — Teleocrater had that too. (2) An enlarged crest on the upper arm bone where some muscles attached — some dinosauromorphs, or prehistoric reptiles that are closely related to dinosaurs but do not have complete holes in their hip sockets, such as Silesaurus opolensis had that too. (3) Bony projections at the back of the neck vertebrae — dinosauromorphs such as Silesaurus opolensis and Asilisaurus kongwe had that too. (4) An extra, fourth muscle attachment site at the point on the femur that meets the hip — some dinosauromorphs such as Marasuchus lilloensis had that too.

3. How did dinosaurs get their name, and why did paleontology seem less complicated back then?

Possible student response: In 1842, the British paleontologist Richard Owen gave several giant reptile fossils the collective name of Dinosauria, which means “fearfully great lizards.” Back then, far fewer examples of dinosaur and contemporary non-dinosaur reptile fossils were known, and therefore it was much easier to find anatomical features that appeared to be shared by one group but not others.

4. What two extinctions helped shape the evolution of dinosaurs, and what key characteristics likely helped dinosaurs survive?

Possible student response: The Permian-Triassic extinction occurred 252 million years ago, just before the origin of the dinosaurs, and wiped out many categories of animals but not the ancestors of the dinosaurs. The Triassic-Jurassic extinction occurred 201 million years ago and wiped out many categories of animals other than the dinosaurs. Currently we do not have a good understanding of what key characteristics facilitated dinosaurs’ survival during the second extinction.

5. What are the three major branches of dinosaurs as discussed in the article?

Possible student response: (1) Ornithischians such as Triceratops with three horns on its head and Stegosaurus with plates on its back. (2) Sauropods such as Brachiosaurus. (3) Theropods such as Tyrannosaurus rex.

6. What is the traditional view of the relationships among these three branches of dinosaurs, and what evidence supports it?

Possible student response: Ornithischians are more distantly related to sauropods and theropods, and branched off earlier. Sauropods and theropods are more closely related to each other and branched apart later. Ornithischians had a pubis bone that pointed down toward the tail, and sauropods and theropods had a pubis bone that pointed down toward the front.

7. What is the new hypothesis of the relationships among these three branches of dinosaurs, as proposed by Matthew Baron, David Norman and Paul Barrett, and what evidence supports the hypothesis?

Possible student response: Sauropods are more distantly related to ornithischians and theropods and branched off earlier. Ornithischians and theropods are more closely related to each other and branched apart later. Ornithischians have more than a dozen different anatomical features in common with theropods but not sauropods.

8. Why did paleontologists decide to essentially repeat the new family tree study? What did the replication study find?

Possible student response: After the new hypothesis of the relationships among the branches of dinosaurs was proposed by Matthew Baron’s team, other paleontologists recognized the lack of detailed descriptions for the fossils used in the study. Given the inherent subjectivity of phylogenetic analyses, a group of paleontologists decided to travel around the world to carefully examine and properly assess 457 characters of different fossils from 74 species used in the study. The team found that the traditional view of dinosaur relationships was still the best fit. But the result was not statistically significant. The traditional dinosaur family tree wasn’t that much more likely to be correct than Baron and his team’s tree or even a third possible hypothesis — in which ornithischians are grouped closer to sauropods, and theropods are the outsiders.

9. How could you summarize two main points of the article?

Possible student response: (1) The relationship of dinosaurs to other early reptiles is less clear than it used to be, since almost all of the characters that were once believed to be unique to dinosaurs are now known to have been shared by various other early reptiles. (2) The relationship among the three major branches of dinosaurs is less clear than it used to be, since different characteristics of the fossils in each branch can be interpreted as indicating that any two branches are more closely related to each other and more distantly related to the third branch.

10. What other questions do you still have after reading the article?

Possible student response: Can comparisons of the DNA sequences of surviving related species (birds, for example) shed any light on the relationships between some of these extinct animals? Can enough residual DNA or amino acid sequences be extracted from fossilized bones that have been better preserved in cold regions, or from blood samples preserved in insects in amber, to clarify the relationships?

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