Transcript Dane_macdonald

Primitive Mammals: in an Egg
or Pouch
Dane McDonald
2413521
Dr. Knight
Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/
The Origin of mammals
 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.
Mammal ancestors
 It was the synapsid reptiles that gave rise to mammals.
http//palaeo.gly.bris.ac.uk/communication
Mammal ancestors
 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 ancestors
 Kuehnneotheriidae which is ancestral to the latter therian
mammals (marsupials and placentals)
 Morganucodontidae which is ancestral to the monotremes.
taken from Dawson (1983)
Evolution of present-day mammals
 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 mammals
taken from Dawson (1983)
Evolution of present-day mammals
 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
Theria and Marsupionta hypothesis
 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 hypothesis
taken from Grutzer and Graves (2004)
Monotremes vs Marsupials
 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
 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
http://www.naturebase.net/plant
s_animals/odd_echidna.html
Ornithorhynchus anatinus
Tachyglossus aculeatus
Reproduction in Monotremes
 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 Monotremes
http://www.naturebase.net/plants_animals/odd_echidna.html
Reproduction in Monotremes
 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.
Marsupials
 The marsupials are represented by 3 orders, 16 families and
249 species.
 Animals such as kangaroos and opposums are represented
in this group.
http://www.enchantedlearning.com/coloring/marsupial.shtml
Reproduction in Marsupials
 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 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.
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.
www.nhc.ed.ac.uk
 Thus the primitive mammals have
survived and are extensively found in
Australia.
Web 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
Literature

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