Evolution of mammamls

EVOLUTION OF MAMMAMLS

Mammals Mystery

By Muhammad Tahir Waseem Rol. 28 7th Semester

SUMMAMRY

“How Mammals evolved? Late paleozoic or early mesozoic Evolutionary hierarchy Amniotes Synapsids Therapsids

THE DARK AGE OF MAMMALS

Mesozoic era Dinosaurs Mammals like reptiles Nocturnal

DEFINITION OF MAMMALS

Mammals use two bones for hearing that all other amniotes use for eating.

 In the mammal configuration, the quadrate and articular bones are much smaller and form part of the middle ear. Note that in mammals the lower jaw consists only of the dentary bone.

 Mammals also have a double Occipital condyle; they have two knobs at the base of the skull that fit into the topmost neck vertebra, while other tetrapods have a single occipital condyle.

THE ANCESTRY OF MAMMALS

AMNIOTES

The first fully terrestrial vertebrates were amniotes.

 Eggs had internal membranes that allowed the developing embryo to breathe but kept water in.

Middle Carboniferous

SYNAPSIDS

Identification Holes behind each eye

Purpose Made the skull lighter without sacrificing

strength. Provided attachment points for jaw muscles. Saved energy by using less bone

THERAPSIDS

Therapsids descended from pelycosaurs in the middle Permian

They differ from pelycosaurs in several features of the skull and jaws, including larger temporal fenestrae and incisors that are equal in size.

Progress towards an erect limb posture. Gradual development of a bony

secondary palate.

THERAPSIDS FAMILY

Only the dicynodonts, therocephalians, and cynodonts survived into the Triassic.

CYNODONTS

Arose in the late Permian. Cynodonts' mammal-like features include

further reduction in the number of bones in the lower jaw, a secondary bony palate, cheek teeth with a complex pattern in the crowns, and a brain which filled the endocranial cavity.

 Trirachodon

FROM CYNODONTS TO CROWN MAMMALS

Mesozoic synapsids that had evolved to the point of having a jaw joint composed of the dentary and squamosal bones are preserved in few good fossils, mainly because they were mostly smaller than rats.

Docodonta

Crown group Mam

mals

Mammaliaformes

Prozostrodon

Cynodonts

DOCODONTS

Among the most common Jurassic mammaliaforms.

 Noted for the sophistication of their molars. Castorocauda- A fish eating docodont.

AUSTRALOSPHENIDA  12OMYA

Appear to have tribosphenic molars. Monotremes are thought to be evolved from

them.

MONOTREMES

Teinolophos, from Australia, is the earliest known monotreme.

 It was also a basal monotreme and predated the radiation of modern monotremes.

Monotremes have some features that may be inherited from the cynodont ancestors:Like lizards and birds, they use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole").

They lay eggs that are leathery and uncalcified, like those of lizards, turtles and crocodilians.

MULTITUBERCULATES

Rodents of the Mesozoic.  They existed for approximately 120 million

years but were eventually outcompeted by rodents,

becoming extinct during the early Oligocene. Their "molars" have two parallel rows of

tubercles.

THERIA

Theria ("beasts"), is the clade originating with the last common ancestor of the Eutheria (including placentals) and Metatheria(including marsupials).

No interclavicle. Tribosphenic molars.

METATHERIA

The living Metatheria are all marsupials. Didelphimorphia first appeared in the late

Cretaceous and still have living representatives. 

The best-known feature of marsupials is their method of reproduction.

The mother develops a kind of yolk sack in her womb that delivers nutrients to the embryo.

Embryosof  bandicoots, koalas and wombats additionally form placenta-like organs that connect them to the uterine wall.

Pregnancy is very short, typically four to five weeks.

EUTHERIA

Early Cretaceous have also been classified as eutherian birth.

study also reported that eutherians did not significantly diversify until after the catastrophic extinction at the Cretaceous–Paleogene boundary, about 66 million years ago.

From a paleontologist's point of view, eutherians are mainly distinguished by various features of their teeth, ankles and feet.