Whether to escape from the many large carnivorous fishes and sharks that were roaming the late Paleozoic seas, or to take advantage of the plethora of arthropods that were quickly colonizing the land, lobe-fins extended their forays further out on terra firma. Some lobe-fins developed stronger limb bones, more flexible joints, and a most remarkable new feature: toes! Moving around on land meant dealing with a problem never before encountered by vertebrates, gravity. In water, vertebrates weigh almost nothing. On land, compact muscular bodies needed to work just to keep their bellies from scraping on the land. Trim, svelte bodies were the goal for land lubbers!
In attempting to truly conquer land, early tetrapods ran into a serious snag. Sure, they could roam freely to find food, shelter, and even an attractive member of the opposite sex, but they still had to return to the water to start off the next generation. Tetrapod eggs were leaky bags that shriveled up and died if not kept in water. So, when cupid's arrow struck, it was off to find the nearest pond. This made colonization of much of the earth's dry land impossible! The solution to this problem was not so simple, however. A surface coating that resisted drying had evolved long before. The skin of some of the new tetrapods was composed of many layers of dead cells works quite well to keep moisture inside the body. But, eggs needed more.
The embryo developing inside also needed a constant source of oxygen to breathe. Skin came in one of two varieties, gas permeable or water resistant, but not both. In what has been called one of the more remarkable steps in vertebrate evolution, a new, multilayered coating surrounded the egg giving it protection from the elements and, at the same time, independence from water. This new "amniotic" or "cleidoic" egg gave one group of tetrapods, the amniotes, permanent rights to land.
With all of land now available to amniotes, new evolutionary experiments were going on rapidly. Only two of these survived to today. One of these lead to the great reptile and bird lineage that exploded onto the scene with the great dinosaur age. The second line was slower off the mark, but persisted to give rise to the mammals. Near the beginning of this split, it was difficult to tell members of the two apart. There were reptiles that looked a bit like mammals and mammal ancestors that looked an awful lot like reptiles. Subtle differences in the shape and position of the limbs make it clear who was who, however. Early in the evolution of the group that gave rise to the mammals, the synapsid amniotes held their legs beneath their body, instead of at their sides. The side to side wiggle that we find in early chordates, lobe-fins and reptiles is all but gone in the line that leads to mammals. With the addition of longer limbs, more upright, and potentially faster land dwellers weren't too far off.
Although it is easy to think of the characteristics that distinguish mammals now: hair covering the body, mammary glands to feed newborns, and internal development of embryos, and so on, none of these apply to the earliest mammals. Instead, early mammals, like all of their amniote ancestors, laid eggs and may have given little or no parental care to their young.
What the first mammals did have that was otherwise not known was highly differentiated teeth that functioned in different ways. Different cutting, biting, and chewing teeth that we know all too well were found in the earliest of mammals. Chewing teeth, our molars, were especially significant. Not only do the surfaces of the upper and lower teeth have to match perfectly for chewing teeth to work properly, but these teeth had to be strongly anchored to the jaws to keep them from wiggling out of place. However, once jaw teeth were firmly established, a wealth of new foods became available to mammals. Imagine, for instance, trying to eat grass or leaves with teeth like those of an alligator!

All Text © Dr. Carl Ferraris

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