5. Synapsid Lineage
• Amniota: Monophyletic group
A key morphological change adaptive for reproduction
on land
Arose from amphibian-like tetrapods in the early
Carboniferous period
The common ancestor of all reptiles and mammals
Were the dominant land animals for 70 million years
Passed their evolutionary peak by the time of the
emergence of the dinosaurs
6. • By the late Carboniferous period, the amniotes
had diverged into three lineages:
1. Synapsids
2. Anapsids
3. Diapsids
These groups were distinguished by no., size, and
position of lateral temporal openings (fossa) in
the skull
Used to facilitate attachment of jaw muscles
7. Divergence of Synapsids
• Early synapsids diverged into diverse 164 herbivorous
and carnivorous forms:
Pelycosauria: The most primitive of the two groups
Known from fossil remains in North America and South
Africa
Therapsida: The more advanced group, were the top
carnivores in the food web
Known from fossil remains in Russia, South Africa and
China
Have traditionally been referred to as ‘’mammal-like
reptiles ‘’
8. • Reptiles are not a monophyletic group but
rather paraphyletic
• As a result, the Class Reptilia is no longer
recognized as a valid taxon by cladists
• Reptiles are referred to as “amniotes”—
neither birds nor mammals
• Mammals and our transitional “mammal-like
reptiles” did not evolve from reptiles
• Reptiles and mammals shared a common
ancestor (the Amniota) from which each
group evolved in divergent ways
9. Pelycosaurs
• They were common by the end of the Pennsylvanian epoch
• By that time, they had radiated into three suborders.
1. Ophiacodontia: the most primitive, were semiaquatic and
ate fish
2. Edaphosauria: terrestrial herbivores and probably were
preyed on by the third group
3. Sphenacodontia: Genus Dimetrodon were the dominant
carnivores throughout the early Permian period
A unique feature—a reflected lamina of the angular bone in
the lower jaw
This feature was to become part of the development of the
middle ear in later synapsids and mammals
The sphenacodonts eventually gave rise to the Therapsida
13. Therapsids
• By the middle to late Permian period, all pelycosaurs were
replaced by the more advanced therapsids
• Therapsids may be divided into the following suborders:
Biarmosuchia, Dinocephalia, Anomodontia, and
Theriodontia
Dicynodontia: The largest and most successful group of
anomodonts
They enjoyed a worldwide distribution and were the
dominant terrestrial herbivores for 60 MY from the mid-
Permian until the late Triassic period
Theriodontia: Primarily carnivorous and much more diverse
and successful than the herbivorous anomodonts
14. Theriodontia
• The gorgonopsians were the prevalent theriodonts throughout the
late Permian period
They did not survive into the Triassic period
• Therocephalians were a much more advanced and diverse group
were extinct by the early Triassic period
• Advanced theriodont group, the Cynodontia: Some mammal-like
characteristics, including:
1. A secondary palate
2. Complex cheek teeth
Therocephalians did not show changes in position of the jaw
muscles, however, as did cynodonts
Only the cynodonts possessed the specialized cranial and skeletal
features that eventually led to the evolution of the mammals
17. CYNODONTIA
• Cynodonts existed for 70 million years, throughout the Triassic to
the middle Jurassic period
• During this time, the several recognized families of cynodonts
developed many of the transitional anatomical features leading
from synapsids to the earliest mammals
• Later cynodonts included diverse herbivores (gomphodonts and
tritylodonts) as well as carnivores (cynognathids and tritheledonts)
• Several cynodont characteristics approached mammalian grade—
that is, a level of organization similar to mammals
• These characteristics included:
1. Changes in dentition to tricuspid and double-rooted cheek teeth
2. Jaw structure and masseter muscles (increased dentary size with
reduction in postdentary bones and development of a glenoid
fossa on the squamosal bone)
18.
19. 1. Hearing
2. Their postcranial skeleton (differentiated
vertebrae, including modification of the first
two vertebrae—the atlas/axis complex,
modified pectoral and pelvic girdles, and
thoracic ribs)
Unlike all other vertebrae, the atlas and axis of
most mammals are not separated by an
intervertebral disk but rather by a synovial
joint
3. Their phalangeal formula
20. The left manus of (A) Sphenodon, showing the primitive amniote phalangeal formula (but a modified,
shortened digit IV); (B) Didelphis, showing the modified phalangeal formula and digital proportions of
mammals; (C) the Early Permian sphenacodontid ''pelycosaur'' Dimetrodon, showing the primitive synapsid
condition of the manus; and (D) the Late Permian gorgonopsian Lycaenops, showing an early therapsid
condition of the manus. The phalangeal formula for each is given below the manus. The ''extra'' phalanges
lost in mammals are indicated by stipple in A, C, and D. Abbreviations: Mc I, first metacarpal; 4, 5, separate
fourth and fifth distal carpals; 4+5, fused fourth and fifth distal carpals; I-V, first through fifth digits
21.
22. Cynodonts
• Changes in cynodonts and early Mesozoic mammals also
included:
1. More erect posture
2. Efficient movement
3. Adaptability in feeding
4. Development of the zygomatic arch and temporal openings of
the skull
5. Eventual development of the lateral wall of the braincase
• These developments reduced stress on the jaw joint, increased
the force of the bite, and protected the brain
• Competition with early mammals may have led to their
extinction
• They went extinct sometime during the Jurassic or Cretaceous
Period
23.
24.
25. • Eucynodontia: A clade of cynodont therapsids including
mammals and most non-mammalian cynodonts
• Cynognathians: Included the large carnivorous
genus Cynognathus and the herbivorous traversodontids.
Can be identified by several synapomorphies including a
very deep zygomatic arch that extends above the middle
of the orbit.
• Probainognathus: meaning “progressive jaw”
(Carnivorous)
• Prozostrodontia: Reduced prefrontal and postorbital
bones, with the disappearance of a strut of bone called
the postorbital bar separating the eye socket from the
temporal region
26. Mammaliaformes
• Characteristics of Mammaliaformes were as follows:-
1. Unfused symphysis between the dentary bones in the
lower jaw
2. The presence of a small hole within the eye socket
called the sphenopalatine foramen
3. A long sagittal crest extending to the rearmost part of
the lambdoidal crest at the back of the skull
4. A convex-shaped iliac crest and a reduced posterior
iliac spine on the hip
5. An acetabular notch on the ischium (a groove in the hip
socket)
6. Lactation and fur
7. Early mammaliaformes were probably nocturnal
27.
28.
29.
30. EARLY PROTOTHERIANS
• Members of the Family Morganucodontidae are among the most
primitive known mammals, with the Genus Morganucodon
abundant in the fossil history
• Morganucodontids may have been the ancestors of later major
groups, including the unknown ancestors of monotremes
• They may be considered “prototherian” only in that they did not
form the ancestry of the marsupials and placental mammals
• Several distinct mammalian structural features are present in
morganucodontidae and evolved among several different
lineages:
1. Dentary-squamosal articulation, although involvement of the
quadrate and articular bones remained
31. 2. Incisors and canines, the cheek teeth were differentiated into
premolars and molars
3. The occlusal surfaces of the upper and lower molars were clearly
mammalian
4. The cochlear region was large relative to skull size
5. The first two vertebrae were similar to those seen in later mammals,
and two occipital condyles were present
6. Thoracic and lumbar vertebrae and the pelvic region were distinct
from the reptilian pattern
7. Morganucodontids had a mammalian posture, with the legs beneath
the body, not splayed out as in reptiles
8. Vertebrae allowed flexion and extension of the spine during
locomotion
• These features continued to be refined in later groups of late
Jurassic and early Cretaceous lineages of mammals
• The most prominent of these lines were the triconodonts,
amphilestids, docodonts, and multituberculates groups defined on
the basis of tooth structure and associated adaptive feeding types
32. EARLY THERIANS
• Remains of the earliest therians (formerly known as “pantotheres”) occur in rock strata
that also contain the prototherian morganucodontids
• Two orders are known only from teeth and jaw fragments:
• Symmetrodonta
• Eupantotheria
• The earliest known symmetrodonts, within the Family Kuehnoetheriidae, are the
Genera Kuehneotherium and Kuhneon— very small carnivores or insectivores from the
late Triassic period
• Later Jurassic pantotheres radiated into numerous different lines and adaptive feeding
niches during the Cretaceous period
• A significant feature of pantotheres was their molars, which had three principal cusps in
triangular arrangement
• This tribosphenic tooth pattern allowed for both shearing and grinding food
• The most diverse family of eupantotheres, the Dryolestidae, may have been
omnivorous and survived into the early Cretaceous period
• Based on derived dental characteristics, advanced therians, probably originated within
the eupantothere Family Peramuridae by the middle to late Cretaceous period, if not
before
33. • Peramurids are known only from the late Jurassic Genus Peramus
• Genetic evidence indicates that the marsupial lineage split from a
therian ancestor about 173 mya, much earlier than estimated by
fossil evidence
• The earliest known marsupial, Kokopellia, is known from the middle
Cretaceous of North America
• The earliest known eutherian mammal, Eomaia scansoria, was
recently discovered from the Lower Cretaceous Yixian Formation of
northeastern China
• We estimate Eomaia to be about 125 myr
• This extends the oldest eutherian records with skull and skeleton by
about 40–50 myr
• In contrast to terrestrial locomotor features of other Cretaceous
eutherians, E. scansoria possessed fore- and hindfeet morphology
adaptive for scansorial locomotion
Such locomotor morphology may offer clues to the evolution of
scansorial abilities of early eutherians
