1 / 21

Primitive Mammals: in an Egg or Pouch

Primitive Mammals: in an Egg or Pouch. Dane McDonald 2413521 Dr. Knight. 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.

maude
Download Presentation

Primitive Mammals: in an Egg or Pouch

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Primitive Mammals: in an Egg or Pouch Dane McDonald 2413521 Dr. Knight

  2. 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.

  3. http//palaeo.gly.bris.ac.uk/communication Mammal ancestors • It was the synapsid reptiles that gave rise to mammals.

  4. 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:

  5. taken from Dawson (1983) Mammal ancestors • Kuehnneotheriidae which is ancestral to the latter therian mammals (marsupials and placentals) • Morganucodontidae which is ancestral to the monotremes.

  6. 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.

  7. taken from Dawson (1983) Evolution of present-day mammals

  8. 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

  9. 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.

  10. taken from Grutzer and Graves (2004) Theria and Marsupionta hypothesis

  11. 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.)

  12. http://www.naturebase.net/plants_animals/odd_echidna.html Ornithorhynchusanatinus Tachyglossus aculeatus 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

  13. 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.

  14. http://www.naturebase.net/plants_animals/odd_echidna.html Reproduction in Monotremes

  15. 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.

  16. http://www.enchantedlearning.com/coloring/marsupial.shtml Marsupials • The marsupials are represented by 3 orders, 16 families and 249 species. • Animals such as kangaroos and opposums are represented in this group.

  17. 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.

  18. 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.

  19. www.nhc.ed.ac.uk 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.

  20. 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

  21. 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

More Related