An Introduction to Dinosaurs

FAS Astronomers Blog, Volume 31, Number 12.

As you look around, you might notice that there are no dinosaurs, or at least what we think of as dinosaurs. The dinosaurs ruled the Earth for millions of years before disappearing some 66 million years ago.

What is a Dinosaur?

Dinosaurs are reptiles that lived in the distant past (for the most part). Okay, non-avian dinosaurs lived in the past. Avian (flying) dinosaurs are still with us today – we know them as birds. Dinosaurs are distinguished from other reptiles by their hip structure (more on this below). They also have legs underneath their body, unlike other reptiles (e.g., lizards and alligators) with legs off to the side.

Setting the stage for Dinosaurs

The Earth is thought to be around 4 ½ billion years old. Its history (Geologic Time) is broken into four eons (Hadean, Archean, Proterozoic, and Phanerozoic). The first three are often combined into the Precambrian supereon. The last, the Phanerozoic eon, started 539 million years ago, and is divided into the Paleozoic, Mesozoic, and Cenozoic eras. It was during the Paleozoic era that life in the form of complex plants and animals took hold. Then, during the Mesozoic, along came the dinosaurs.

The evolution of vertebrates during the Paleozoic era laid the foundation for dinosaurs. The Paleozoic extended for over 250 million years from the Cambrian period through to the Permian period. Fish became dominant during the Devonian period and evolved into the first four-limbed tetrapods that crawled onto the land. Tetrapods known as amniotes split into mammal ancestors (synapsids) and reptile ancestors (sauropsids/diapsids). Some of these mammal ancestors, such as therapsids, came to dominate the landscape during the Permian period. Then the Permian-Triassic (P-T) extinction hit. This is called the Great Dying, when life nearly came to an end. The larger therapsids eventually disappeared, leaving only small cynodonts to carry on.

Life slowly recovered from the Great Dying during the Triassic period. Archosaurs (“ruling reptiles”) took over and dominated much of the Triassic. They eventually led to the first dinosaurs, flying pterosaurs, and crocodylomorphs (ancestors of crocodiles).

The dinosaurs first appeared around 230 million years ago during the middle Triassic. Herrerasaurus and Stautikosaurus were once thought to be examples of these early dinosaurs. However, Eoraptor was discovered in 1991 and may have slightly predated the others. More recently, the fossilized remains of Nyasasaurus were found to be around 243 million years old. However, Nyasasaurus may be a dinosauriform, a group of reptiles that immediately preceded the dinosaurs, rather than a true dinosaur itself.

The Age of Dinosaurs

With the Triassic-Jurassic extinction, the true age of dinosaurs began. The dinosaurs ruled the Earth for around 150 million years, but really dominated the landscape during the Jurassic and Cretaceous periods. Many of the large plant eating sauropods were found during the Jurassic period. Most of the large meat eating therapods came along later during the Cretaceous period.

The reign of dinosaurs (and the dinosaurs themselves) ended with the Cretaceous-Paleogene (K-Pg) extinction around 66 million years ago. The K-Pg extinction was caused by a 6- to 9-mile-wide asteroid that hit the Earth near the present-day town of Chicxulub, Mexico in the Yucatan peninsula.

After the demise of the dinosaurs, mammals, which had been small and insignificant, began to flourish during the Cenozoic era eventually leading to humans and the world we know today.

Discovery of the Dinosaurs

No one knew anything about dinosaurs until the first half of the 19^th century when two “paleontologists” discovered fossils associated with three unidentified animals.

• William Buckland found evidence of Megalosaurus and published his work in 1824.
• Gideon Mantell discovered the remains of Iguanodon in 1822 (published in 1825).
• Mantell later found the remains of Hylaeosaurus in 1833.

In 1842, Richard Owen (Owen 1842) noticed similarities among the three. He concluded that they were large lizard like reptiles, which he classified as Dinosauria (Dinosaurs) meaning “fearfully great lizards” (or “terrible lizards” per many of today’s references). Soon it became clear that an entire group of extinct creatures roamed the Earth sometime in the distant past.

A few years later, Harry Seeley (Seeley 1887) divided dinosaurs into two distinct groups based on their hip structure (“lizard hipped” Saurischians and “bird hipped” Ornithischians).

• Saurischians were some of the more famous dinosaurs (Sauropods such as Brontosaurus/Apatosaurus and Theropods such as Tyrannosaurus rex). They had pubic bones pointed down and slightly forward and their pubis and ischium bones were split apart.
• Ornithischians were smaller plant-eating dinosaurs such as Stegosaurus and Triceratops. They had pubic bones pointed backward with their pubis and ischium bones close together.

The Bone Wars

The hunt for dinosaurs soon made its way to America. One of the first repositories of dinosaur fossils was found in New Jersey. Othniel Charles Marsh and Edward Drinker Cope decided to cooperate and share the findings from this site. Marsh then bribed someone at the site to bypass Cope and send all the fossils only to him. This began a competition that lasted for the rest of their lives and then some.

Cope was well off financially and had the means to support his fossil hunting. Marsh was not, but he had a rich uncle named George Peabody, who built the Yale Peabody Museum. Peabody appointed Marsh to run the museum and Marsh eventually inherited a great deal of wealth when Peabody passed away.

Fossil sites were soon discovered in the west during the building of the Transcontinental Railroad. So, around 1872, Cope headed out there in search of new sources for dinosaur bones. At the same time, Marsh, who stayed back east, sent students to prospect for fossils in his place. Well, they didn’t play nice. They spied on each other. They paid people off attempting to acquire all the fossils for themselves. They went so far as to steal fossils from each other.

Early on, Marsh made sure to criticize every published mistake Cope made, including one where Cope placed the skull of a dinosaur at the end of its tail. Later Cope, during a Congressional investigation into the U.S. Geological Survey (headed by Marsh), organized Marsh’s employees to testify against him. Their war found its way into the public arena when they began attacking each other with a series of articles in the New York Herald. Because of this, Marsh eventually had to resign his Geological Survey position. Later Cope became head of the National Association for the Advancement of Science but had to sell off much of his fossil collection due to financial problems.

It isn’t clear who won the Bone Wars. Marsh (80) discovered more species than Cope (56) including well know dinosaurs such as Stegosaurus and Allosaurus. Some say they wouldn’t have come close to these numbers if they had cooperated, although competition clearly didn’t bring out the best in either of them. They both ended up spending much of their financial resources. But they generated publicity, captured people’s attention, and made dinosaurs popular with the general public. So, maybe the two individuals lost, but the public and paleontology won.