Journey to the age of the dinosaurs

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

Directions: After students have had a chance to review the article “What makes a dinosaur?” lead a classroom discussion based on the questions that follow.

Discussion questions:

1. What are the four major eras of Earth’s history, and what happened to life during each one?

During the Precambrian time period  (approximately 4.6 billion to 541 million years ago), Earth’s atmosphere lacked oxygen and the planet experienced periods of extreme heat and cold. Single-cell life formed in the oceans and ultimately evolved into photosynthetic unicellular life, such as algae, that produced oxygen. A variety of multicellular soft-bodied aquatic animals that consumed algae also evolved. During the Paleozoic Era (approximately 541 to 252 million years ago), some marine organisms evolved to have hard parts (shells, exoskeletons, teeth and bones), and then some of their descendants colonized dry land along with plants. The era ended with a mass extinction, possibly related to all the continents joining together to form Pangaea. The Mesozoic Era (252 to 66 million years ago) was when dinosaurs prevailed and birds and mammals evolved. The Mesozoic Era ended with another mass extinction, caused by the Chicxulub asteroid impact, the Deccan volcano eruptions or some combination of both events. The Cenozoic Era (66 million years ago to the present) has been most notable for an explosion of mammalian species.

2. What are the three periods of the Mesozoic Era, and what happened to dinosaurs and other reptiles during each period?

During the Triassic Period (approximately 252 to 201 million years ago), dinosaurs evolved from earlier reptiles and split into theropods such as Coelophysis, early sauropods such as Plateosaurus and early ornithischians such as, possibly, Pisanosaurus. There were also a wide range of other reptiles, but most died off during the Triassic-Jurassic extinction event. Dinosaurs, aquatic reptiles such as ichthyosaurs, crocodile ancestors, pterosaurs (flying reptiles) and other reptiles such as turtles and lizards survived. During the Jurassic Period (approximately 201 to 145 million years ago), those surviving groups expanded and rose to dominate Earth. During the Cretaceous Period (approximately 145 to 66 million years ago), dinosaurs continued to dominate the Earth until the K-T extinction event.

3. How are fossils formed and preserved?

To become a fossil, a dead organism is usually buried in the ground or in sediments at the bottom of a body of water. Soft parts of the organism decay away more quickly, while hard parts decay more slowly. Over time, the hard parts are replaced by minerals. Thus, most fossils have the same shape as the original object, but they are made of rock and not what the original organism was made of. Some fossils are simply impressions in mud (such as a footprint or shell print) that harden over time. Fossils can also be made when insects and other small organisms become trapped in tree resin, which then hardens over time into amber.

4. What general features did many theropods have in common? What are some examples of well-known dinosaurs that belonged to the theropods?

Theropods were carnivorous predators that walked on their hind legs, had smaller front legs and used their tails for balance. Theropods included the small Triassic Coelophysis, the larger Jurassic Dilophosaurus and Allosaurus, and the very large Cretaceous T. rex and Spinosaurus.

5. What general features did many sauropods have in common? What are some examples of well-known dinosaurs that belonged to the sauropods?

Sauropods were large, long-necked dinosaurs that walked on four legs and ate plants. They developed from Triassic dinosaurs, such as Plateosaurus, that were smaller, had shorter necks and shorter front legs with the ability to walk on either two or four legs. The most famous sauropods, such as Apatosaurus, Diplodicus and Brachiosaurus, lived during the Jurassic Period. Most sauropods during the Cretaceous were still very large but were less well-known.

6. What general features did many ornithischians have in common? What are some examples of well-known dinosaurs that belonged to the ornithischians?

Ornithischians were a diverse group of dinosaurs. About all that ornithischians have in common is that they were dinosaurs (complete hole in the hip socket), were not theropods or sauropods and were plant eaters. They tended to be four-legged with lots of armor to protect against predators, or two-legged (or four-legged with the option of standing on two legs) with the ability to run away from predators. Pisanosaurus may be an example of an early ornithischian. Later, much more famous ornithischians included Stegosaurus, which had plates on its back and spikes on its tail; Ankylosaurus, which had armor on its back and a club on its tail; Triceratops, which had three horns and a shield on its head; and hadrosaurs (sometimes called duck-billed dinosaurs).

Extension prompts:

7. What are some very common fossils of marine animals that were wiped out during the Permian-Triassic and Cretaceous-Tertiary extinctions?

Trilobites were wiped out during the Permian-Triassic extinction. Ammonites were wiped out during the Cretaceous-Tertiary extinction. Both are very widely available and inexpensive fossils.

8. What evidence in terms of the diversity of ornithischians, when compared with theropods and sauropods, could indicate that our current knowledge about ornithischians may be very incomplete or flawed?

Theropod species have a lot in common with each other, as do sauropod species. But ornithischians are incredibly diverse and do not have much in common with each other. Theropods have very clear ancestors, and sauropods have reasonably clear ancestors. But the origins of ornithischians are much murkier. As explained in this issue’s article, paleontologists argue whether ornithischians are more closely related to theropods, more closely related to sauropods, or are equally distant from both. Perhaps our understanding of ornithischians, and of their origin from Triassic ancestors such as Pisanosaurus, may be essentially correct and more details will be filled in as more fossils are discovered. However, the problems just listed suggest that we may have mistakenly grouped unrelated or only distantly related dinosaurs into a catch-all category. Perhaps ornithischians are not one branch of the dinosaur family tree, but rather two or more independent branches that should not be lumped together.

Discussion questions:

1. How could DNA analysis of living species potentially shed light on the relationships among extinct animals?

Comparisons of DNA sequences among surviving species could show how closely or distantly each is related to the others, and potentially how closely or distantly related extinct cousins may have been. One could compare all types of surviving reptiles (crocodiles, turtles, Komodo dragons), birds (descended from theropods), platypuses, marsupials and placental mammals (which branched off from more reptile-like ancestors).

2. Where might it be possible to find DNA or amino acid sequences of extinct species in order to analyze and compare them?

Though, even under ideal conditions, there seems to be a time limit for DNA preservation, it is possible to extract residual DNA or amino acid sequences from preserved marrow deep inside fossilized bones or preserved tissue deep inside fossilized eggs, from samples that have been preserved in cold regions and from mosquitoes or other insects trapped in amber.

Extension prompts:

3. How might you automate the process of looking for key similarities and differences among bones of different species?

Automating the process might reduce human judgment, opinions, biases and errors, or draw attention to previously overlooked features. Three-dimensional laser scanning, ultrasound, X-rays or other methods might be used to construct a 3-D image of the exteriors (or better yet, both the exteriors and interiors) of large numbers of bones from different species. Computer algorithms might be used to sort the bones into more closely and less closely related groups, and to identify those features that best distinguish which groups species should be assigned to.