The fossils of mammals offer a glimpse into the evolutionary process. Other vertebrate animals, such as reptiles and fish, have existed for millions of years prior to the evolution of mammals. Mammals appear to have branched off from reptiles more than 100 million years ago. They rose to sudden dominance 65 million years ago, after the Cretaceous meteor impact causing the dinosaurs to become extinct. Indeed, reptiles lost their position as the most numerous vertebrates on the planet. The study of mammal fossils since that time shows the development of evolutionary dead ends as well as the first steps taken by the ancestors of mammals still prevalent today. Humans are among the descendants of these earliest mammals.
What is a Mammal?
Mammals are distinguished from other vertebrate animals by a set of common physical characteristics and abilities. They are distinguished from fish by their ability to breathe air. In contrast to reptiles, mammals grow hair, which assists in keeping their bodies warm. They are endotherms, creatures that control their own body temperature through internal mechanisms. This capacity allows them to maintain activity for longer periods, even during inclement weather.
There are a number of other features, which set mammals apart from other animals. However, the most significant feature is that from which this animal class derives its name. Female mammals give birth to live young and possess mammary glands. With these latter organs, they provide nourishing milk for their young until the latter are able to take care of themselves. This physical characteristic has resulted in a behavioral and social characteristic that is unusual among other animals. Mammals take care of their young for extended periods after birth, in contrast to other animals, which abandon their fertilized eggs to the whims of chance and environment.
History of Mammals
Early research suggested that mammals sprang up almost at around the same time that the dinosaurs became extinct, some 65 million years ago. Prior to that, it was obvious that small, furry creatures had existed in ecological niches. However, continuing studies of dinosaur fossils suggest that the trend toward a mammalian body structure began hundreds of millions of years ago.
As far as present day researchers can tell, true mammals did not appear until late in the Triassic period. This was roughly 200 million years ago. The earliest mammal fossils show small, rat-like creatures, which bear some resemblance to modern mammals. However, they still laid eggs and exhibited certain other behaviors, which they had in common with reptiles.
Earliest Mammals Fossils
Monotremes are among the earliest fossils of mammals. They predate the dinosaur extinction by some 60 million years. They were still egg-layers, but their eggs were leathery like those of modern turtles rather than calcified. These creatures did possess a unique method of providing nourishment for their young. They fed their young by sweating milk through their abdomens. Scientists believe that the modern spiny anteater and the duckbilled platypus are the surviving descendants of these proto-mammals.
Marsupials appear in great numbers around the same time. A fossil of a 125-million year old Sinodelphys was discovered in northern China. The fossil was in such good condition that paleontologists could detect tufts of fur among the remains. Other fossils in the same area show the simultaneous existence of placental mammals, which are the ancestors of most mammals in existence today.
However, all of these mammals display features, which clearly show that they occupy a transitional stage between reptiles of those eras and the mammals of today. It is not until the end of the Cretaceous and the sudden disappearance of the dinosaurs that the fossil record begins to show the existence of creatures, which are unmistakably mammals, according to modern classifications. The earliest example of a bat appears in the fossil record just 15 million years after this extinction event.
Post-Cretaceous Fossils
The current geological era is known as the Cenozoic. It began 65 million years ago. During the earliest part of this era, mammals rapidly grew in size and distributed themselves across the face of the earth in an ever-increasing variety of species. Many early mammals were much bigger than any that survive today. However, none ever reached the size of the largest dinosaurs.
Mammals reached a global dominance in diverse forms about 15 million years ago. It was not until two million years ago, however, that mammals achieved the massive forms whose fossils have excited the imaginations of museum-goers around the world. At this time, the entire planet experienced severe ecological and climactic changes. This more recent segment of the Cenozoic is known as the Pleistocene.
The Earth has experienced severe ice ages in these last two million years. Large land mammals developed and left behind some exquisitely preserved fossils. Among them are fossils of creatures, which obviously bear direct relationships with mammals still walking the Earth today. The ancestors of humans also appear during this time.
Examples of such creatures are the woolly mammoth, the mastodon, megatherium, aurochs and the smilodon. The first two were massive elephant-like creatures with long tusks. They were covered in long fur that, no doubt, kept them warm in the cold weather of the time. Megatheriums were gigantic, slow moving mammals that fed on fruits and nuts. They looked like giant sloths. The auroch was the ancestor of today’s cows. They were much larger, standing nearly six feet tall at the shoulders. The last auroch died only a few hundred years ago. The smilodon was a fierce, cat-like creature that may have weighed as much as 1,000 lbs.
These mammals are extinct now but you can find smaller mammals, which obviously are descended from these fossilized ancestors. They appear to have been the most dominant life forms of their time period. Not far from their fossils, researchers find those of bipedal primates, which are the ancestors of modern humans. Hominid fossils include ramapithecus, which was an ape-like hominid and neanderthalensis, a species of homo sapiens that bore many resemblances to humans of today. Fossils of cro-magnon humans are some of the most interesting because these fossils show anatomically modern human characteristics. Their remains are often accompanied by their tools, which makes them unique and unlike any other finds of mammal fossils.
