Available at Primitive Mammals: in an Egg or Pouch Dane McDonald 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 mammals

 It was the synapsid reptiles that gave rise to mammals. Mammal 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 ancestors

 Kuehnneotheriidae which is ancestral to the latter therian mammals (marsupials and placentals)  Morganucodontidae which is ancestral to the monotremes. Mammal 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 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.” (  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 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 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 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 Monotremes s_animals/odd_echidna.html Tachyglossus aculeatus Ornithorhynchus 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 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. Reproduction 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. Marsupials 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 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. Reproduction in Marsupials

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.

Web References 1] 2] 3] http//palaeo.gly.bris.ac.uk/communicatio 4] 5] 6] http//animaldiversity.ummz.edu/site/accounts 7] http// 8] 9] 10] Not available

Literature  Dawson, TJ (1983) Monotremes and Marsupials: the other mammals. Edward Arnold Ltd, London. ISBN  Kemp, TS (1982) Mammal-like Reptiles and the Origin of Mammals. Academic Press, London. ISBN