Mammals evolved from reptiles during the Triassic period, descending from reptilian synapsids. Key mammalian characteristics developed over time, including hairy skin, specialized dentition, and a high metabolic rate allowing for a constant body temperature. Early mammals originated from cynodont therapsids in the Late Triassic and included primitive groups like docodonts, triconodonts, symmetrodonts, pantotheres, and multituberculates which dominated until the rise of modern mammal groups in the Cenozoic. Key skull and jaw bones were adapted over generations to form the mammalian middle ear from ancestral jaw structures.

1. ORIGIN OF
MAMMALS

2. • 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

10. THANK YOU