Dinosaurs: Their Lives, Their Deaths and Their Evolution!

by Dr. Bob Gardner
Department of Mathematics
Department of Physics and Astronomy
Institute of Mathematical and Physical Sciences
East Tennessee State University

The Timeline

The Earth and solar system condensed out of a giant cloud of gas and dust (called the solar nebula) about 4.6 billion years ago.

The young Earth was very hot and was hit by many smaller objects which were orbiting around in the early solar system.
The Earth continued cooling and after about a billion years, the first forms of life arose. These first living things were bacteria and primitive algae. In fact, these first organisms are the ancestors of all living things on our planet today, including you and me!

Since the history of the Earth is so long (4.6 billion years), geologists break the time period up into smaller pieces. The largest of these slices of time is called an eon. The Archaean Eon lasted from 4.6 to 2.5 billion years ago. The Proterozoic Eon lasted from 2.5 billion years ago until 543 million years ago. The final eon is the Phanerozoic Eon which started 543 million years ago and continues today.

The oldest rocks found on Earth date from the Archaean and are 4 billion years old. The evolution of the first life forms is thought to have occurred at around 3.5 billion years ago. The oldest known fossils are calcareous structures called {\it stromatolites} and were produced by cyanobacteria.

They were found in Australia and date from 3.4 billion years ago. The Archaean Eon is marked mostly by the absence of fossil life, however.

The Proterozoic Eon includes the presence of several life forms, but only of the simplest kinds (mostly single celled). Sometimes the Archean Eon and the Proterozoic Eon are together called the Precambrian.

But if we are interested in dinosaurs, then we have to look much, much farther ahead in time.

The Phanerozoic Eon is divided into three blocks of time called {\it eras}.

The Paleozoic Era (meaning "ancient life") lasted from 543 million years ago until 248 million years ago. This era is further divided into several periods:
the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. It is during the early Paleozoic Era that there was a tremendous increase in the evolution of complex multicellular organisms (sometimes called the Cambrian explosion). Many of the types of plants and animals we are familiar with first appeared during the Paleozoic Era, such as: jellyfish, coral, crabs, insects, spiders, fish, amphibians, reptiles, ferns, and conifers. However, there were still no people, no mammals, no birds, no flowering plants, and NO DINOSAURS! So let's move on to the next era.

The second era in the Phanerozoic Eon is the Mesozoic Era

(meaning "middle life") which lasted from 248 million years ago until 66 million years ago. The Mesozoic Era is often called the "age of reptiles." It is during the Mesozoic Era that mammals, birds, and flowering plants first evolved. It is also the era in which the ichthyosaurs, plesiosaurs, pterosaurs, and most importantly for us, the dinosaurs lived! Even though mammals first appeared early in the Mesozoic, they didn't evolve much beyond small rat-like creatures. In particular, there were no bears, elephants, monkeys, and especially NO PEOPLE during the Mesozoic Era. Before we explore the Mesozoic in more detail, let's look at the third and final era in the Phanerozoic Eon.

We live in the Cenozoic Era which started 66 million years ago.

The Cenozoic Era is divided into two periods: the Tertiary which lasted from 66 million years ago to 2 million years ago, and the Quaternary.
These periods are further divided into the epochs seen here. During these epochs, most of the mammals with which we are familiar evolved including whales, bats, monkeys, and humans. Some other strange mammals evolved and became extinct during these times, including the saber toothed tiger Smilodon, the rhinoceros-like Teleoceras,
the "dawn horse" Hyracotherium (sometimes called"eophippus") an ancestor of modern horses, and mammoths.
In fact, some of these animals lived here in east Tennessee!

A particularly interesting group of animals evolved in the late Cenozoic: the apes. Today, there are six groups of apes:

gibbons (of which there are nine species), orangutans,
chimpanzees, bonobos (also called pygmy chimpanzees), and humans. The evolutionary relationships between these animals is as seen here.
Gibbons and orangutans branched off long before gorillas, chimpanzees, and humans. Humans branched off from the other apes about 7 or 8 million years ago. Chimpanzees and bonobos became distinct only about 1.5 million years ago. These dates are based on studies of both anatomical and genetic differences. During the last 5 million years, several very interesting species lived. They include
Australopithecus afarensis which lived from about 4 million years ago to about 3 million years ago,
Australopithecus robustus which lived from 2 million to 1 million years ago,
Homo habilis which lived from 2.5 million years ago until 1.6 million years ago,
Homo erectus which lived from 1.8 million years ago to 100,000 years ago, and
Neandertals which lived from 230,000 years ago until 30,000 years ago. What makes these animals so interesting is that they are our genetic ancestors and our evolutionary cousins. These species represent a very small part of the unbroken evolutionary tree from those first living organisms that appeared 3.5 billion years ago to every living thing on planet Earth today, including us!

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