CHAPTER 11. MESOZOIC ERA (225 TO 65 MYA):

AGE OF THE REPTILES

Triassic Period (225 - 190 MYA)

Following the period of mass extinction, the reptiles dominated the landscape. The huge reptiles, known collectively as archosaurs, "ruling lizards," and the dinosaurs in particular, came to predominate. Also during this period, mammals evolved and conifers became abundant. This period also saw the development of those insects known as the true flies (order Diptera), along with the earliest bugs (order Hemiptera). Some 200 million years ago, the members of that highly socially-organized order Hymenoptera developed. The wasps developed first and from these animals came the ants and bees. The development of advanced insects would, in turn, lead to the development of flowering plants.

The discovery of a small paramammal called Thrinaxodon has provided fascinating evidence of the transition in this period to true mammals. But by the end of the Triassic, they and other paramammals vanished. Small hairy mammals, the first true mammals, replaced them. One of the most significant evolutionary advances of the mammal was in the maternal care of offspring.

Mammals gradually developed three important subclasses. Subclass Prototheria contains the Order Monotremata, of which that odd Australian animal, the platypus, is a member. This creature is an egg-laying mammal with a duck-like bill. Subclass Metatheria contains the marsupials, pouched animals, such as the Kangaroo. The higher mammals in the subclass Eutheria developed the placenta, which is a blood-rich lining supplying nutrients to the developing fetus. There are many placental mammals. A few of the orders are Insectivora (shrews and moles); Chiroptera (bats); Carnivora (cats, dogs, bears, and seals); Rodentia (rats, mice, squirrels, porcupines, and beavers); Lagomorpha (hares and rabbits); Artiodactyla (even-toed ungulates, such as cattle and deer); Perissodactyla (odd-toed ungulates, such as horses and rhinoceroses); Proboscidea (elephants); Cetacea (whales and porpoises); and Sirenia (sea cows).

The important thing to realize about the early mammals is how vulnerable they were, since they developed in an age dominated by dinosaurs. The earliest mammals learned to survive by inhabiting all possible environments, even the ones so cold that they inhibited reptiles. They survived by hiding in the smallest cracks and crevices, and also by burrowing in the ground. The early mammals survived by being afraid. Their brain made this possible, but would also spell trouble for future mammals, namely humans. The mammal introduced a new bioprogramming language called emotion. (Nevertheless, the mammals still retained the older, reptilian parts of the brain.) Doctor Paul MacLean (1970) called the new addition, the limbic brain, because, like a limb, it surrounds and embraces the oldest and much smaller, reptilian parts of the brain. The limbic system deals with the emotional feelings that guide behavior. For instance, affection became superimposed over the reptile's sexual drive, and fear superimposed over those programs that instructed the reptile to stay alive by avoiding larger creatures. (In laboratory experiments, after destruction of parts of the limbic system, young mammals cease to play and the females are deficient in maternal behavior. It is as though these animals regress toward a reptilian condition.)

The limbic brain includes the deep clusters of nerve cells that form the basal ganglia, hippocampus, cingulate gyrus, and the hypothalamus. The basal ganglia are located close to the base of the brain. They vary in size and shape from an almond to a plum, and each has a twin in the opposite side of the brain. Their names are caudate (tail-like), putamen (shell-like), globus pallidus (pale globe), and amygdala (almond). The basal ganglia play an important role in initiating and controlling muscle movements and in modulating and relaying sensations. Some of them also share in the control of emotional reactions.

Parts of the limbic brain itself are responsible for specific emotions. The amygdala influences emotional reactions to odors. Mild electrical stimulation of this nerve-cell cluster causes rats to become suddenly ferocious and violent. Likewise, stimulation of a cluster associated with pleasure causes a cat to purr contentedly.

The hippocampus is from the Greek words hippo (horse) and campus (sea creature). This is because the two structures, one on the right and the other on the left, are the size and shape of a little sea horse. A major function of the hippocampus is its role in memories concerned with emotions such as scents and things past.

Over and around the corpus callosum, that unites the two halves of the cerebrum, are the cingulate gyrus. Each cingulate gyrus plays a prominent role in regulating the autonomic nervous system, which adjusts blood pressure, heart rate, breathing, the size of the pupils, and other bodily, or psychosomatic, responses that accompany emotional feelings and reactions. The cingulate gyri are also responsible for primitive feelings and expressions of such emotions as despair and the isolation cry of an infant animal deserted by its parents.

Jurassic Period (190 - 136 MYA)

During the Jurassic period, dinosaurs roamed amid vast forests of conifers and ferns. A little after the time the dinosaurs appeared, a creature called a proto-avis appeared. Proto-avis may had a common ancestor with the crocodile where the reptile shoulder evolved into a wing. The alternative theory is that the birds were dinosaurs of the Archaeopteryx type that developed wings and flew. The Archaeopteryx look much like the present day Hoatzin bird of Amazonian South America. This bird has habits possibly similar to those of Archaeopteryx. The babies are the only birds with free toes located in the wings that they use to hang onto tree branches. The young climb trees from which they often dive into the water.

One of the most significant advances for birds was the development of warm bloodedness, a characteristic shared with the mammals. This characteristic made these animals more independent of the vagaries of climatic changes.

Cretaceous Period (136 - 65 MYA)

Toward the end of the Cretaceous period, the dinosaurs started to become extinct. One reason for this was that a huge meteor struck the Mexican coast in the vicinity of the Yucatan peninsula. As the dinosaurs declined, the mammals began to change from their nocturnal habits to diurnal ones. Mammals similar to the marsupial opossum came into existence.

Around 112 million years ago, the flowering plants evolved. These angiosperms had what is popularly recognized as a flower (rather than a cone) along with seeds covered with fleshy tissue (i.e., fruits). Botanists divide the angiosperms into two large groupings: dicots and monocots. The dicots developed first, followed by the monocots. There are many differences between the two groupings. In the embryo, the dicotyledons have two cotyledons (i.e., the first leaves sent up by the germinating seed). The monocots have one cotyledon. The dicots are either herbaceous or woody. (An herb is an annual that dies completely or a perennial that dies back to the ground at the end of the growing season because they lack the firmness resulting from secondary growth.) Since the dicots dominated the land, many of the monocots became small and took to the water. They often dominate the water's edge around lakes and ponds. Dicots usually have annual growth rings that can be used to determine the age of dicot trees. Monocot trees, such as palms, are fibrous, not woody, and have a soft inner part, so there are no annual growth rings. Monocot leaves have veins that are usually parallel to each other, and their flower parts, such as the sepals and petals, usually occur in threes or multiples of threes.

The most primitive flowers are those belonging to the magnolia subclass. Some of the orders in the Magnolia subclass are magnolia, laurel, water lily, butter cup, and poppy. These flowers developed in association with the long-established beetles. Beetles were plentiful when the proangiosperms appeared and may have been the first animal pollinators. The accidental beetle pollination provided a more efficient method of pollination than chance pollination by wind. Many insects which we usually think of as pollinators, had not yet appeared when the first angiosperms evolved. For instance, butterflies and moths did not develop until 70 million years ago.

 

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