Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/

Primitive Mammals: in an Egg or Pouch

Dane McDonald

2413521

Dr. Knight

In some way reptiles can be considered as the ancestors of mammals.

These reptiles, however, had little in common with modern reptiles.

Fossil evidence indicates that a split occurred between the early reptilian ancestors of mammals and the evolutionary line that gave rise to today’s living reptiles.

This occurred approximately 300mya in the Carbiniferous period, which was the time of the great swamp forests.

At this time many of the characteristics of modern reptiles had not yet evolved, and hence were not present in the reptilian ancestors of mammals.

In conclusion, today’s reptiles can therefore not be considered as the “evolutionary stage” preceding mammals.

The Origin of mammalsThe Origin of mammals

It was the synapsid reptiles that gave rise to mammals.

Mammal ancestorsMammal ancestors

http//palaeo.gly.bris.ac.uk/communication

The history of these animals can be divided into a series of evolutionary branches. Three major radiations occurred:

Carboniferous and Permian pelycosaurs

Permian and Triassic noncynodont therapsids

Cynodont therapsids

All mammals are descended from the latter group which shows distinct mammal- like structure.

At the end of the Triassic (190mya) there was a split in the cynodont lineage

The result was two lineages that represent present-day mammals:

Mammal ancestorsMammal ancestors

Kuehnneotheriidae which is ancestral to the latter therian mammals (marsupials and placentals)

Morganucodontidae which is ancestral to the monotremes.

Mammal ancestorsMammal ancestors

taken from Dawson (1983)

During the Jurassic (200mya) the dinosaurs dominated the earth. Hence early mammals were confined to a nocturnal lifestyle or protected habitats to survive competition or predation.

This period of “dormancy” resulted in major innovations that opened up a large new adaptive zone.

Being nocturnal resulted in the following developments:

Relatively large eyes

More acute hearing

Developed vocal cords

Pronounced whiskers

Another important feature that evolved in early mammals were new jaw attachments and modified teeth.

Evolution of present-day mammalsEvolution of present-day mammals

Evolution of present-day mammalsEvolution of present-day mammals

taken from Dawson (1983)

Food could now be chewed more effectively.

This rather small modification had a cascade of impacts on mammalian development:

“More efficient mastication of food→ faster digestion → rapid delivery of energy to the body, hence it was possible to accommodate greater metabolic requirements such as those associated with maintenance of a high body temperature and increased activity.”(www.bobpickett.org/evolution_of_mammals.htm)

All these factors combined to give mammals a greater awareness of the environment and survival advantages.

There was also a demand for constant fine-tuning of the integration of brain and bodily equipment.

Hence there was the opportunity for increased evolution of reproductive strategies, maternal behaviour, parental care, communication between individuals and learning 

Evolution of present-day mammalsEvolution of present-day mammals

Theses are the two hypothesis that attempt to explain the emergence and positions of the extant mammals ( i.e. monotremes, marsupials, placentals)

The Theria hypothesis states that monotremes are the oldest branch of the mammalian tree and are placed outside the Theria (marsupials and placentals).

There is increasing fossil evidence to support this hypothesis.

The Marsupionta hypothesis suggests that monotremes and marsupials should be aligned as sister groups within the mammalian subclass marsupionta

Most of the characters used in this hypothesis are considered ancestral or their interpretation is controversial.

Theria and Marsupionta hypothesisTheria and Marsupionta hypothesis

Theria and Marsupionta hypothesisTheria and Marsupionta hypothesis

taken from Grutzer and Graves (2004)

Monotremes are isolated from the other mammals by a key physiological difference: their defecatory, urinary and reproductive systems all open into a single duct called the cloaca.

In contrast to this characteristically reptilian feature, other mammal females have separate openings for reproduction, defecation, and urination: the anus, vagina and urethra.

Marsupials,on the other hand, differ from placental mammals in their reproductive traits

Hence reproductive traits and strategies can be used to compare and contrast the extant primitive mammals ( i.e. monotremes and marsupials.)

Monotremes vs MarsupialsMonotremes vs Marsupials

The monotremes are represented by two families, which represent three species:

Family: Ornithorhynchidae (platypus)

Species: Ornithorhynchus anatinus

Family: Tachyglossidae (echidnas/spiny anteater)

Species: Tachyglossus aculeatus, Zaglossus bruijni

MonotremesMonotremes

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Tachyglossus aculeatusOrnithorhynchus anatinus

Monotremes are the only extant group of egg-laying mammals

Do monotremes follow a DISTINCTLY reptilian pattern?

NO, reproduction in monotremes differ from the basic patterns in both reptiles and mammals.

Eggs are retained for some time in the mother who actively provides the eggs with nutrients

Literature suggests a combination between oviparity and ovoviviparity.

Oviparity: laying of eggs as in reptiles

Ovoviviparity: the embryo derives nutrients from the egg, but hatching takes place within the female tract and birth takes place thereafter.

Monotremes are seasonal breeders and the season typically lasts between 1-3 mths between July and October.

Reproduction in MonotremesReproduction in Monotremes

Reproduction in MonotremesReproduction in Monotremes

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During copulation, males use spurs on their feet to hold females (and hence execute ground-shattering positions!!)

The female usually lays 1-3 small eggs that are covered by a leathery shell.

The eggs are incubated for about 12 days and hatch outside the mother

“The newly hatched young are vulnerable, blind, and are fed by the mother’s milk.”

The young sucks up milk as it oozes out of a gland onto the abdominal surface.

Babies only leave the burrow after approximately 17 wks of age.

Reproduction in MonotremesReproduction in Monotremes

The marsupials are represented by 3 orders, 16 families and 249 species.

Animals such as kangaroos and opposums are represented in this group.

MarsupialsMarsupials

http://www.enchantedlearning.com/coloring/marsupial.shtml

Marsupials are viviparous ( i.e. they give birth to live young).

Females typically have a pouch or marsupium in which their young are reared during infancy.

Marsupials have a short gestation period and spend only 12-13 days in the reproductive tract.

The reproductive tract has two primary functions:

Reception and transport of spermatozoa to the egg for fertilization.

Production of egg coats and shells and provision of nourishment for the developing embryo.

The female has two vaginas that lead to different compartments within the uterus but open externally through a single orifice.

Males usually have a corresponding two-pronged penis, which only passes sperm.

Reproduction in MarsupialsReproduction in Marsupials

The pregnant female develops a yolk sack in her womb which delivers nutrients to the embryo.

The young are born in an underdeveloped state after about 4-5 wks.

Despite their incredibly small size, they are able to crawl into a pouch on the mother’s abdomen and attach to the teat after birth.

Marsupial young are nurtured in the pouch until they reach a level of development that is similar to that of young placentals.

Reproduction in MarsupialsReproduction in Marsupials

How did primitive mammals survive?How did primitive mammals survive?

It is generally expected that primitive mammals would be outcompeted when the more advanced mammals like the placentals evolved.

However, Australia split away from what is now Antarctica millions of years ago due to continental drift.

Hence it was this island isolation that acted as a barrier against invasion and competition from more advanced placental mammals.

Thus the primitive mammals have survived and are extensively found in Australia.

www.nhc.ed.ac.uk

Web ReferencesWeb References

1] www.wikipedia.org/wiki/cynodont

2] www.bobpickett.org/evolution_of_mammals.htm

3] http//palaeo.gly.bris.ac.uk/communicatio

4] http://en.wikipedia.org/wiki/Monotremes

5] http://en.wikipedia.org/wiki/Marsupials

6] http//animaldiversity.ummz.edu/site/accounts

7] http//www.nhc.edu.ac.uk/index.php?page

8] http://en.wikipedia.org/wiki/Platypus

9] http://www.naturebase.net/plants_animals/odd_echidna.html

10] Not available

LiteratureLiterature

Dawson, TJ (1983) Monotremes and Marsupials: the other mammals. Edward Arnold Ltd, London. ISBN 0-7131-2853-4

Kemp, TS (1982) Mammal-like Reptiles and the Origin of Mammals. Academic Press, London. ISBN 0-12-404120-5