ORIGIN MAMMALS.pptx

origin of mammals

1. ORIGIN OF MAMMALS

2. Origin and evolution of Mammals The mammals evolved from the reptiles , during the triassic period. Mammals are an offshoot of the reptilian sub-class: Synapsida and more particularly of some type of triassic- Therapsida (the true mammal like reptiles). Pelycosaurs Therapsids Cynodonts Mammals By end of Triassic Marsupials Monotremes Eutherians Gradual acquisition of mammalian characteristics Reptilian- Synapsids

3. Establishment of mammalian characters Following characters were established during evolution of mammals : Hairy skin Double occipital condyle Secondary hard palate Squamosal-dentary jaw joint Three ear ossicles Large brain case Differentiated dentition Double headed ribs Fused bones of pelvic girdle Constant body temperature Development of diaphragm Limited growth

4. Ancestors of mammals: Synapsids Pelycosaur : (a) Dimetrodon ( b) Cotylorhyncus. Ref. : Book of vertebrates Life by Harvey Pough, Janis & Heiser Reptiles that gave rise to mammals were Synapsids that diverged from the main reptilian stock during the Permian Period. Synapsids include mammals and their extinct predecessors, they are commonly called mammal like reptiles. Synapsids are distinguished from other amniotes by the synapsid type of temporal fenestra plus a few other skull features. Synapsid- Basic skull structure Source: Book of zoology by Parker and Haswell

5. Therapsids Dinocephalian Therapsid- Moschops Gorgonopsid Therapsid- Lycaenops Cynodont - Probelesodon A reconstruction of the early Triassic Dicynodont Therapsid- Lystrosaurus. Therapsids descended from Pelycosaurs in the middle Permian and took over their position as dominant land vertebrates. The Therapsida was varied and successful group of reptiles which had been claimed almost to bridge the ‘entire evolutionary gap between a primitive reptile and a mammal’. More derived Therapsids include the Dicynodonts (Anomodontia) and the Theriodonts (Gorgonopsida, Therocephaliana and Cynodontia). The Upper Permian and Triassic Cynodontia were perhaps the most mammal-like of all. Ref. : Book of Vertebrate Life by Harvey Pough, Janis & Heiser.

6. PRIMITIVE MAMMALS OF THE MESOZOIC PERIOD Five orders of mammal are known from sediments of late Triassic & Jurassic age, these orders are: Docodonta: They appeared in the Upper Jurassic & they are known only from teeth. In these primitive mammals the upper molars were expanded. The molars do not have the form of interlocking triangles, though the upper jaw have three roots. Tricodonta: These are known from partial skull, teeth, palate & jaws. They were mainly carnivores. These animals ranged in size from small to large. There were three or four incisors in each jaw, well developed canines and up to nine cheek-teeth which cannot be separated into premolars and molars. The molar pattern was distinctive, in these three cones were arranged in a linear series. Ref . : textbook of Zoology Vertebrates by Parker and Haswell. Tricodonta- diagrammatic view showing different types of teeth.

7. Symmetrodonta: they were Jurassic and Cretaceous animals with three molars arranged in a symmetrical triangle but the cusp arrangement was entirely different from that of Tricodonta. They were carnivores and moderate in size. Pantotheria: all the Pantotherians were small in size, in the largest the jaw was only 4cm long. Their teeth were differentiated into incisors, canines, premolars and molars, but the number of molars is often as many as eight. Multituberculata: these mammals were very numerous and survived for over 100 million years. They were herbivorous and longer than any other mammalian order. They have rodent like incisors ,the cheek teeth carried longitudinally arranged rows of cusps probably used for grinding the food. Ref. : textbook of Zoology Vertebrates by Parker and Haswell. Pantotheria - lower jaw Multituberculata- the lower and upper molars, palatal view of skull .

8. EVOLUTION OF JAWS AND EAR Ref. : Science.education.nih.gov The original Synapsid condition of the lower jaw was a tooth-bearing dentary occupying the anterior half jaw, with a variety of bones forming the posterior half. The jaw articulation was between the articular in the lower jaw and the quadrate in the skull. In mammals and some cynodonts, the contact between the dentary and the squamosal formed a new jaw joint, the dentary-squamosal jaw joint. Over the course of evolution of mammals the articular and quadrate lost their purpose in the jaw joint and were put to new use in middle ear, connecting to stapes. In mammals these three bones or ossicles are known as malleus, incus and stapes.

9. CONCLUSION Simplified cladogram of Synapsids . Ref . : Book of Vertebrate life by Harvey Pough, Janis and Heiser

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