Origin of Mammals | Biology

In this article we will discuss about:- 1. Introduction to Mammals 2. Definition of Mammal 3. Biological Organisation 4. Archaic Mam­mals 5. Ancestries 6. Evidences 7. Evidences 8. Molar Hypotheses 9. Probable Ancestry 10. Biological Trends.

Contents:

1. Introduction to Mammals
2. Definition of Mammal
3. Biological Organisation of Mam­mals
4. Archaic Mam­mals
5. Ancestries of Mammals
6. Evidences from Living Mammals
7. Evidences from Serology Support Reptilian Ancestry of Mammals
8. Molar Hypotheses Support Reptilian Ancestry of Mammals
9. Probable Ancestry of Mammals
10. Biological Trends in Mammalian Evo­lution

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1. Introduction to Mammals:

Mammals stand at the pinnacle of evo­lutionary success amongst the living orga­nisms. The question of mammalian ancestry is of special interest to man, because their history includes ours in a broader sense, as man is included under this great class, Mammalia.

Mammals constitute a very unique group and separate themselves from others by having many morphological and physiological characteristics which help them to overcome in hospital environmental hurdles.

A survey of the past geological records reveals that in Mesozoic era, there was a storm of evolution. This era is re­garded as the age of reptiles. The majestic and dominant forms were the Dinosaurs, the then giant rulers of the horizon.

When the Dinosaurs ruled the earth, a group of small and insignificant creatures evolved under their shadows. Though lowly in origin, but unique in their endowments, they constituted the mammals.

The cause of extinction of Dinosaurs is quite obvious. Of other factors, hugeness in size, lowly developed nervous system, inability to re­tain constant body temperature, can be regarded as the prime causes of extinction.

Mammals, with efficient mechanism of re­gulating body temperature, well-developed nervous system, efficient power of swift movement confronted the challenges of evolution. From such a lowly start arose the mammals—which became diversified and ruled the earth.

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2. Definition of Mammal:

Mammals are warm-blooded vertebrates. The body is covered more or less with hairs. Young mammals are born alive (excepting the egg-laying monotremes) and are nourished by mothers with milk in infancy secreted by the mammary glands. Separate opening of the anus and the urino-genital system (excepting monotremes) is characteristic.

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3. Biological Organisation of Mam­mals:

For a better understanding of the mammalian ancestry, a generalised survey of the mammalian organisation of existing as well as of the extinct forms is desirable.

Morphological Peculiarities:

Mammals possess:

(a) Heterodont dentition, with differentiation into incisors, canines, pre­molars and molars.

(b) Thecodont teeth, i.e., each tooth is embedded in an alveo­lar pocket on the jaw bone.

(c) Diphyodont teeth, i.e., milk set is replaced by per­manent set of teeth.

(d) Normal dental battery is:

(e) Presence of two Aoccipital condyles.

(f) Lower jaw is made up of one pair of bones—the dentaries.

(g) Articular and quadrate bones are transformed into mall­eus and incus respectively.

(h) Secondary palate is complete and functional.

(i) Cer­vical vertebrae are seven in number.

(j) Ribs are double headed with capitulum and tuberculum.

(k) Vertebrae are gastrocentrous.

(l) Coracoid of the pec­toral girdle is absent as a separate bone and is represented as the coracoid process of the scapula.

(m) Heart is fully four- chambered with only left aortic arch.

(n) Mature erythrocytes are enucleated, bi­concave and circular.

(o) A muscular diaphragm separates the thoracic and ab­dominal cavities.

(p) Presence of sweat and sebaceous glands.

(q) Highest development of brain, great development of neopallium and presence of corpus callosum (absent in monotremes, rudimentary in marsupials).

(r) Pinna is present ex­cepting some aquatic mammals.

Physiological Peculiarities:

(a) Complete separation of oxygenated and deoxygenated blood with a free scheme of circulation.

(b) Homoiothermous condition is a new attribute which keeps the body temperature constant and is independent of environmental fluctuation.

(c) Viviparous excepting monotremes and young’s are nourished in the uterine cavity by placenta in embryonic condition.

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4. Archaic Mam­mals:

A short survey of the archaic mammals will be informative to gain a clear idea regarding the ancestry of mam­mals. Mammals became well flourished at the beginning of Cenozoic era but have started their true career in the Mesozoic era. So the description of some Mesozoic and Cenozoic mammals will serve the purpose.

Mesozoic Mammals:

The mammal-l­ike reptiles were quite abundant in Permian period. But at the early Triassic time, those forms became gradually extinct and ultimately they became almost entirely absent by the end of Triassic. But again in the Jurassic and sub­sequent periods fossil mammals became more abundant.

Mesozoic mammals possessed some basic characteristics:

(a) small size,

(b) sharp-pointed teeth,

(c) teeth were generally heterodont and

(d) habits were varied.

Pantodonta:

The fossil forms were dis­covered in upper Triassicperiod. Dentition was more like that of reptile than mam­malian. But the simple jaw consisting of a single bone established their mammalian affinity. The fossil records of this group arc rather incomplete.

Allotheria:

They were very important group amongst the extinct mammals and their fossils were recorded in the upper Triassic rocks of Germany. The most important mammalian charac­teristic was the nature of the molar teeth which had two to three longitudinal rows of tubercles. The presence of these tubercles gave the name Aluitituberculata to this group.

Triconodonta:

These fossils were re­corded in Jurassic period. The molars had three cusps arranged in a single longi­tudinal row.

Symmetrodonta:

This group was confined to the upper Jurassic period of England and North America. The molars had three main cusps arranged in a triangular fashion.

Pantotheria:

The symmetrodonta was distantly related to this group. The molars had four or more cusps. The three principal cusps assumed the form of a trigon. Their fossils were recorded throughout the Jurassic period.

Cenozoic Mammals:

Besides the archaic mammals, a few important Cenozoic mammals need mentioning in connection with the ancestry of mammals. They belong to three groups, Creodonta, Condylarthra and Amblypoda. Creodonta had small reptile-like brain, carnivore-like teeth and terminal phalanges with claws.

Condylarthra represented the primitive ungulates and structurally formed a sort of connect­ing link between the hoofed and clawed mammals. The cranial portion was located behind the orbit. Teeth were suited for herbivorous habit. Molars were low crown­ed. Amblypoda was short footed and hoofed forms of almost elephantine size.

Fate of Archaic Mammals:

With the close of reptilian age, the Mesozoic mam­mals became well flourished in the Meso­zoic era and exhibited wide adaptive radiation. But very quickly they became extinct and were replaced by modernised mammals which were fully equipped to face all eventualities of nature. The cause of extinction of archaic mammals is pri­marily due to their incapabilities to res­pond to new environmental conditions.

The archaic mammals had a brief evolu­tionary career and they were subjected to racial extinction. The challenges of life were received by modernised mammals who flourished well in time and space. Some of the archaic forms transmutated into higher types while some became emigrant. Marsupials passed beyond the limits of South America and crossed the Antarctic land-bridge to come to Australia where they still persist.

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5. Ancestries of Mammals:

In tracing out the folds of life it is often said that the higher forms have originated from lower forms. So the ancestor of mammals must be some lower vertebrate forms. The chances for the fishes and the birds are out of question, because one is too low and the other is highly specialised.

Ancestry from them is in no case possible. Now remains the possibilities for the amphibia and the reptiles to hold the respected position of mammalian ancestor.

a. Amphibian Ancestry:

The amphibian ancestry of mammals was emphasised by T. H. Huxley in the year 1880. He advo­cated that mammals arose from amphibia. He assumed that Hypotheria is a sub- mammalian evolutionary stage, which arose from amphibia and holds the key of mammalian evolution. In support of his contention, Huxley put forward some argu­ments. First, amphibia as well as mammals possess double occipital condyles.

Another argument in this context is that in mammals only the left aortic arch is present but in amphibians and reptiles both the arches are present. Of the two arches, the left one is weakly developed in reptiles which indirectly supports amphi­bian ancestry of mammals. But this theory was subjected to vigorous criticism.

The points of resemblance are not rigid enough to support Huxley’s assumption excepting the occurrence of double occipital con­dyles. The occipital condyles in amphibians and mammals are’ not strictly homologous structures but differ in development.

In amphibians the occipital condyles are the derivatives of the exoccipitals, but in mammals the source is the basioccipital. This assumption is further weakened by the lack of any intermediate forms and palaeontological data.

Huxley’s view of mammalian origin through Hypotheria is untenable today and does not offer any tangible evolutionary significance. In spite of that this view is still kept as a piece of historical document in the discussion of mammalian ancestry. The very concept of mammalian ancestry directly from amphibia without the inter­vention of reptiles is most unscientific and absurd.

b. Reptilian Ancestry:

The reptilian an­cestry of mammals is full of evolutionary significance and gives a vivid indication of biological transformation towards the mammalian organisation. The reptilian ancestry of mammal can be established from three angles,

(i) Evidences furnished by some living mammalian groups,

(ii) From serological tests and

(iii) From palae­ontology.

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6. Evidences from Living Mammals:

Some of the primitive living mammals give the strongest support of reptilian affinity of mammals. The monotremes and the marsupials occupy intermediate status bet­ween reptiles and mammals.

Monotremes Offer Strongest Support:

Mono­tremes constitute a sort of connecting link between the mammals and their reptilian ancestor and serve to bridge the gap bet­ween the two. Monotremes are undou­btedly mammalian, but possess many rep­tilian features too.

The reptilian features are:

(a) Pectoral girdle is typically reptilian with a large coracoid, an interclavicle and without scapular spine.

(b) Pelvic girdle possesses an epipubic bone.

(c) Ribs ex­cepting the cervical ones are single headed.

(d) Presence of cervical ribs.

(e) Bones of the skull are typically reptilian. LacrymaL bone is absent, jugal is reduced or absent, auditory bulla is absent, presence of rep­tilian provomer.

(f) Corpus callosum is absent.

(g) Reproductive system is based on reptilian plan.

(h) Oviparous.

(i) Presence of shallow cloaca.

(j) Meroblastic segmentation.

(k) They are not fully homiothermous.

It appears that the monotremes are roughly fifty per cent, mammal and fifty per cent, reptile in their organisation. They are very primitive and represent the end product of an independent evolutionary line of mammal-like reptilian stock.

Marsupials—also Offer Support:

The mar­supials also show many reptilian features.

These features are:

(i) Brain lacks corpus callosum and the olfactory part is comparatively large.

(ii) Presence of epipubic bone.

(iii) Palate is deficient in ossification.

(iv) Epicondylar foramina occur frequently.

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7. Evidences from Serology Support Reptilian Ancestry of Mammals:

Serological tests have revealed that the reptilian blood shows close affinity with that of mammals which are warm blooded.

Palaeontological Evidences in Support of Reptilian Ancestry:

The fossil remains of the mammal-like reptiles offer a sound evolutionary basis of mammalian origin through reptiles. The mammal-like reptiles are:

Synapsida:

Synapsida comprises of a very interesting group of reptiles that appeared in the rocks of late Carboniferous time. They form a bridge between the primitive- reptiles and the primitive mammals. The latest genera approached so close towards mammalian stage that it is really question­able whether they should be included amongst mammals or reptiles.

They show­ed the tendency of the loss of bones in the skull. Teeth showed differentiation. The lateral temporal opening was situated be­hind the eye and it became enlarged gradually. The synapsida had a large number of genera. The early members were very close to the ancestral Cotylosaurs, while some of the latest genera approached very close to the mammals.

Pelycosaurs—the first synapsid:

They ini­tiated a line of evolutionary development which is most completely recorded in geological history. The first Pelycosaurs are the Ophiacodon of which Varanosaurus was a primitive member. In Varanosaurus, the skull was narrow, marginal teeth were socketed and showed little regional specialisation.

Bifurcation of Pelycosaurs. The Pely­cosaurs evolved along two lines—the large plant eating Edaphosaurs and the large aggressive carnivorous Sphenacodonts, Eda­phosaurs.

Springing from an Ophiacodon ancestry in Permian age, the Edaphosaurs diverged into a different line. The skull was small, stout and shallow. The teeth were not differentiated. Sphenacodonts. The second evolutionary line from the Pelycosaurs was the Sphenacodonts which led the channel of evolution. Teeth were strongly differen­tiated which made the refinement of the skull.

Therapsids:

The fossils of these forms were recorded in the middle and late Permian and in Triassic age. They were the forms that took the road leading towards mam­malian organisation. Possession of the following characters certainly supports the contention.

These are:

(a) There was a strong trend of lateral temporal fossa to enlarge.

(b) Presence of a secondary palate.

(c) Dentary was enlarged at the expense of other bones.

(d) Double occipital condyles were pre­sent.

(e) Teeth were differentiated.

(f) Quadrate and quadratojugal were reduced and loosely attached with the skull.

The therapsids followed two lines of adaptive radiations.

(A) Anomodonts:

They were archaic therapsids who retained many primitive features of the Pelycosaurs. They lack secondary palate and the pineal opening was very large. The dentary was moderately developed. The typical repre­sentative of this group is exemplified by Tapinocephalid (Moschops).

(B) Dinocephalians:

They were the most successful therapsids of phylogenetic longevity. They appeared in the middle of Permian to lower Triassic. Bones of the temporal region of the skull were thin due to en­largement of temporal opening. Acromion process was present on the scapula. The phalanges were reduced to mammalian count (2-3-3-3-3).

Thecodonts. When the Dinocephalians were evolving in their own way, the theriodonts developed rapidly in a direc­tion that was led directly to the mammals. The theriodonts were varied in shape. Cynognathus was a typical genus of the theriodonts. Cynodontia had many mantmalian features.

They possessed the following mammalian characters:

(a) Double occi­pital condyles.

(b) Lower jaw was almost made up of a single bone—dentary.

(c) Heterodont dentition.

(d) Presence of hard palate.

(e) Double headed ribs.

(f) Sca­pular spine was present.

Ictidosauria formed a bridge between reptiles and mammals. They had almost crossed the threshold line that separates the reptiles from mammals.

They possess:

(i) Skull was highly organised and mam­mal-like. The cranium was spacious.

(ii) Quadrate was greatly reduced.

(iii) Lower jaw was made up of dentary.

(iv) Presence of double occipital condyles.

(v) Hetero­dont dentition.

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8. Molar Hypotheses Support Reptilian Ancestry of Mammals:

Mammals possess characteristic molar teeth. The origin of molar teeth in the phylogenetic history of mammal has given rise to a number of theories. All the theories indirectly support reptilian an­cestry of mammals.

Tritubercular Theory:

Cope and Osborn hold that mammalian molar teeth passed through a trituberculate stage found in Cynognathus. Multitubercular theory of Woodward and Goodrich advocates that mammalian molar teeth arose from a multituberculate stage found in Cynodontia (Diademodon) and the modern mammals are the evolutionary derivatives of the Diademodon.

Compromise theory of diphylectic origin of Mivart and Beddard advances the logical idea of mammalian evolution from Cynodontia having both tritubercular and multitubercular molar teeth.

Judging all the theories the inevitable conclusion that can be drawn is that rep­tilian ancestry is full of evolutionary signi­ficance. Cumulative evidences from palae­ontology, comparative morphology and serology thus prove that the mammals sprang from reptiles. Very likely the source is best represented by either Cynodonts or Ictidosauria.

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9. Probable Ancestry of Mammals:

According to most evolutionists of to­day, the Theriodont reptiles hold the key of mammalian ancestry. From Cynodontia, the Ictidosauria and the mammalia arose and underwent parallel evolution. The probable line of ancestry is shown in Fig. 1.18.

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10. Biological Trends in Mammalian Evo­lution:

A review of the fossil mammal like reptiles reveals that the evolution of mam­mals is preceded through the following biological trends.

(a) The gradual growth of brain which is related to the increment of the cranial cavity.

(b) Gradual acquisition of hard palate.

(c) Loss and condensation of some bones of the skull and lower jaw.

(d) Gradual reduction of elements of the pectoral girdle.

(e) Gradual condensation, differentiation and compactness of the general body system to increase the efficiency.

Time of Origin:

The mammal like reptiles were abundant in Permian and in early Triassic periods. The probable time is assigned either to be late Permian or early Triassic.

Place of Origin:

There are two views regarding the place of origin of mammals. First view is that the mammals arose in South Africa as evidenced from the abun­dance of Therapsid fossils. Based from the history of reptilian disappearance the se­cond view holds that the place, of origin was Central Asia.

Cause of Origin:

From Permian to Jurassic age, there was a great climatic fluctuation. There had been periodic ari­dity alternating with glacial periods. So nature has to provide a new mecha­nism of temperature control. The achievement of temperature control probably saved the insignificant mammals from the one stroke of geological changes.

At the present state of knowledge it will be better not to go too far regarding the origin of mammals. Further researches on palaeontology, comparative anatomy are necessary t6 picture out the complete­ness of mammalian ancestry.

However, with all certainties and probabilities, the insignificant Cynodonts whose existence was at the mercy of the mighty Dinosaurs, survived the perilous existence and gave origin to mammalian form of life in time and space.