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THE ORIGIN, PHYLOGENY AND EVOLUTION OF
MAMMALS

ORIGIN OF MAMMALS

Definition

The animal kingdom has the following recognized groups

1 The Protozoa divided into 13 phyla namely Phylum Euglenozoa, Alveolata, Dinoflagellata, Apicomplexa, Ciliophora, CAxostylata, Retortamonada, Opalinata, Microspora, Rhizopoda, Granuloreticulosa and Actinopoda.

2. The Sponges namely Phylum Porifera

3. The Radiate animals namely Phylum Cbidaria and Phylum Ctenophora

4. The flatworms namely Phylum Paltyhelminthes,

5. The round worms namely Phylum Rotifera, Phylum Nemata formerly called Phylum Nematoda

6. The earthworms - Phylum Annelida

7. The Arthropods –

8. The Mollusca – Phylum Mollusca

9. The Echinoderms – Phylum Echodermata

10. The Deuterostomes – Phylum Chaetognata, Phylum Hemichordata, Phylum Chordata

The Phylum Chordata has three subphyla namely

i.                    Subphylum Urochordata

ii.                  Subphylum Cephalochordata

iii.                Chordata.

The subphylum chordate consists of several classes  of animals with notochord and there are five major groups namely

i.                    Fishes (Pisces)

ii.                  Amphibians (Amphibia)

iii.                Reptiles (Reptilia)

iv.                Birds (Aves)

v.                  Mammals (Mammalia).

Introduction to Mammals

Mammals are animals belonging to the Class Mammalia.

They are chordates, possessing a notochord manifested by the vertebral column in adults. Characteristically, they are animals with well developed milk producing mammary glands and they exhibit elaborate care of their young ones which are born alive, except in the extremely primitive ones which are egg layers. Although there are other features possessed by mammals, the ones mentioned here distinguish them from other animals.

Other features of mammals would be discussed later.

Evolution and Phylogeny of Mammals

Phylogeny

Mammals evolved from reptiles, but it is difficult to trace their their phylogenetic relationship with the extant reptiles. Why is this so? This so because the reptilian subclass (Subclass Synapsida), which gave rise to mammals diverged early on the evolutionary tree. The relationship of mammals to the synapsids can therefore only be traced using fossil records.

The catastrophe which befell the dinosaurs some 65 million years ago and led to their extinction gave a chance for the rapid evolution and success of mammals. While the dinosaurs existed they dominated the earth such that the mammals existing at that time were mainly insectivorous, unnoticeable shrew-like animals that foraged for food at night while the dinosaur had become inactive since they were diurnal. The large mammals were therefore non-existent.

That was the status quo until the extinction of the dinosaurs following which the surviving mammals suddenly became grazers, browsers and predators, and they evolved along different lines.

The first Mammals

Going by this phylogenetic history, it is established that the first mammals appeared some 230 million years ago. This coincided with the time the dinosaur started dominating the earth.

Features of the first set of mammals

These mammals were distinct from their reptilian ancestors and co-inhabitants of the earth in that their body was covered with a coat of hairs called fur. The fur, being an insulator, provided warmth for the mammal, an essential condition for its warn-blooded nature

The Direct Antecedents of Mammals

Among the reptiles, the direct antecedent to mammals were the Pelicosaurs, a group which lived in the carboniferous and early Permian periods (that is 340 – 260 million years ago).

These were extremely primitive reptiles. Notable among them, however, is ophiacodon which shows striking features that indicates its ancestry with mammals. Such characters are in the skull structure, including the presence of a single opening in the temple region of the skull. These pelicosaus were succeeded in the late Permian and the early Triassic (that is some 225 million years ago) by the therapsids.

The therapsids were carnivores and active four-footed runners. They had their elbow and knee swung in towards the body as in their contemporary descendants. The advantage of this arrangement is that it makes for better support and speed. In the advanced Triassic species features such as skull, jaw, dentition and limbs closely resemble the mammalian pattern, one of these mammal like reptiles called Lycaenops was discovered in the later Permian of South Africa.

Intelligent activity alertness and the aforementioned characteristics of mammals were developed during the period of domicile under the dinosaurs.

EVOLUTION AMONG MAMMALS

From the ancestral therapsids, evolution of mammals have taken place giving rise to animals of different forms and habits. In this regard there are three main limes of evolution. They are presented here in order to evolution and advancement.

The Prototheria: these are the first to evolve from the therapsids. They are also called monotremes. They appeared in the Jurassic period, some 180 million years ago. They are the most primitive among mammals. These animals are restricted to Australia and have only two surviving members namely the Duck-bill platypus and the Spiny ant–eaters.

The monotremes bear many characters which are diagnostic of mammals. They however retain certain features of their primitive reptilian stock, notably oviposition (egg-laying).

The duck-bill is somewhat “amphibious” being semi-aquatic. Its feet are webbed, its body covered with for and it frequent strews where it hunts for snails and mussels. The ant-eater on the other hand is has its body covered with protective spiny hairs. It is endowed with powerful clawed feet which endears its remarkable digging ability and making it effective hunter of subterranean termites. One attribute common to the monotremes is that thy both make nests in borrows where the shelled eggs are hatched and the hatchlings are nursed. Also they are toothless as adults. In the duck-bill, the teeth are replaced by flat and duck-like horny bills while slender horny bills take place of teeth in the adult ant-eater.

The Metatheria: These are also called the marsupials or pouched mammals. They are more advanced than the monetremes. They evolved in the early cretaceous (that is about 130 million years ago) coinciding with the extinction of the dinosaurs. The marsupials give birth to live offspring, the young are born tiny and immature. Typically therefore, the females posses a pouch on their bellies where the newborn are nursed for some time after birth. The marsupials lack placentas needed to nourish the foetus during gestation (pregnancy). They are therefore primitive mammals. Examples of marsupial include the Oppossum the Tasmanian wolf, the Kangaroos and so on. The marsupials are found in Australia, although the opossum is also found in the Americas. Some marsupials such as the kangaroos are herbivores while others like the Tasmanian wolf are carnivorous.

The Eutheria: these are the placental mammals simply called the placentals. They are regarded as the true mammals. They contrast sharply with marsupials in that they possess the placenta – an nutritive connection between the mother and the foetus during gestation. This enables the foetus to develop to a much more advanced stage before parturition (birth). The placentals are the most advanced and they include all higher mammals. They evolved in the Paleocene epoch during the tertiary period (some 65 million years ago) after the extinction of the dinosaurs. Therefore has been rapid evolution of diverse forms of these mammals. For this and their wide distribution and abundance reference to mammals are usually made to the placentals.

Abundance and Diversity of mammals

There are about 4,000 species of mammals and they occupy almost every environment that supports life on earth. Consequently, mammals are regarded as the most biologically successful group in the Animal Kingdom. They are highly diverse in size, shape, form and function. The pigmy shrew, for instance is a terrestrial mammal measuring less than 4cm in length and just a few grams in weight. In contrast, the blue whale is an aquatic mammal whose weight is about 190 tonnes. Also, Kitti’s hog-nosed bat (1.5 g) is only a little bigger than the bumble bee. The mammals also range from the ardent tunnellers like the moles to the specialized fliers like the bats. Bats are peculiar in that they are the only mammal group capable of true flight. In difference to the wings of pterosaurs and birds, the wing of bats is a web drawn tout by four of the five “fingers”. Ranges of mammals also include the monkeys hanging on tress to dolphins that live entirely in the sea.