19 mammal origins

8/10/2019 19 Mammal Origins

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THE ORIGIN OF MAMMALS

What traits have mammals got that make them unique?

A. Glands

Of all the vertebrates, mammals have the greatest variety of integument (=skin) glands

All are derivatives of the epidermis

1.Sweat Glands

Tubular coiled glands that occur over much of the body

Secrete a watery sweat that when evaporated on the skin's surface draws heat from the

skin and cools itMuch reduced or absent in rodents and whales

2. Scent Glands

Present in nearly all mammals

Used in communication with members of the same species

3.Sebaceous glands

These are oil-secreting glands

Function to soften and lubricate the skin and hair and to kill some kinds of bacteriaIn humans, they are highly concentrated on the face

4. Mammary Glands

Probably modified apocrine glands

Occur on all female mammals and in a rudimentary form on all male mammals

In females these glands produce milk; all mammalian females nourish their young with

milk, a balanced diet rich in fats, sugars, proteins, minerals and vitamins

B. Hair

All mammals possess hair, which helps to insulate the animal and maintain a constant

body temperature.In some mammals hair has been substantially reduced, e.g. in whales it is reduced to a

few hair bristles around the snout and eyelids

Mammals characteristically have 2 kinds of hair:

Underhair - dense, soft hair; important as insulation Guard Hair - course and longer: functions in protection and provides coloration


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C. Diaphragm

Mammals have a diaphragm, a sheet of muscle and tendon that separates the body cavity

into two sections.No other animal has a diaphragm.

D. High metabolic rate/high body temperature

Though this trait is not unique to mammals, they are one of just two important livinggroups of vertebrates that possess the trait

Note:

All of these traits involve "soft stuff"; it's hard to judge their appearance in the fossilrecord.

Instead, we look at novel hard parts in mammals that correlate with this soft stuff.

E. Big brains with modified skulls

F. Three ear bones

G. Complex teeth

H. Reduced number of bones in the lower jaw

These hard parts arose in a mosaic fashion over millions of years of synapsid evolution,

from pelycosaurs to therapsids, and from therapsids to mammals.These transformations were in no sense a straight line "march" from pelycosaur to

mammal.Rather, "important" mammalian traits arose on some lines, whereas other traits that

weren't ultimately passed on to long-lasting descendents arose on other lines.

Mammal teeth are different from teeth of other vertebrates

Mammal teeth have more complex shapesthat the teeth of other vertebrates.

Many more cusps, cones, and ridges.

They have teeth that differ from front-to-backin the jaw: Incisors, canines, premolars,

molars.

Complex occlusion between teeth.

When chewing, mammal teeth lock together in a very complex and rigid fashion.


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When did this happen?

Pelycosaurs have simple, cone-shaped teeth that don't lock together tightly during biting.

Some do have big "canine" teeth, but little other front back differentiation.

Some therapsids have much more complex shapes and occlusion and substantial front-back differentiation.

Extremely complex teeth and occlusion, and full front-back differentiation developed

after the first appearance of true mammals.

Why innovations in teeth?

As therapsids and early mammals began to eat a greater diversity of foods, particularlyplants, they needed to develop specialized chewing systems.

Consequences?

Mammals have only one set of replacement teeth (most vertebrates have many).

They can't continuously replace their teeth and maintain complex occlusion.

Mammal teeth have a thick coating of enamel to make sure they last a lifetime.

Mammal skulls are different from skulls of other vertebrates

They greatly increase their brain size.

They have elaborate musculature on their head to control chewing.Many have a big crest on the top of their heads for muscle attachment.

They enlarge the synapsid opening greatly to accommodate added muscles and swellingbrain.

Mammals have new solutions to breathing/walking problems.

Secondary palate:allows simultaneous chewing and breathing.

This structure first began to develop in therapsids.

Diaphragm:pumps air in and out without ribs

Upright posture and a spinethat flexes in the vertical, not horizontal, plane.Pumps air in-and-out, rather than from side-to-side, when the animal moves


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Mammals have bizarre ears and jaws

Mammals have three bones in their ears that transmit sound from the eardrum to the inner

ear.

Eardrum - malleus - incus - stapes - inner ear

A Brief Overview on Ear-Jaw Evolution among Mammals

The oldest reptiles having mammal-like features, the synapsids, occur in rocks of

Pennsylvanian age formed about 305 mya.

However, the first mammals do not appear in the fossil record until Late Triassic time,

about 210 mya.

Hopson (1994) noted, "Of all the great transitions between major structural grades within

vertebrates, the transition from basal amniotes [egg-laying tetrapods except amphibians]

to basal mammals is represented by the most complete and continuous fossil record....Structural evolution of particular functional systems has been well investigated, notably

the feeding mechanism... and middle ear, and these studies have demonstrated the gradual

nature of these major adaptive modifications."

A widely used definition of mammals is based on the articulation or joining of the lower

and upper jaws.

In mammals, each half of the lower jaw is a single bone called the dentary; whereas inreptiles, each half of the lower jaw is made up of three bones.

The dentary of mammals is joined with the squamosal bone of the skull.

This condition evolved between Pennsylvanian and Late Triassic times.

Evolution of this jaw articulation can be traced from primitive synapsids (pelycosaurs), toadvanced synapsids (therapsids), to cynodonts, to mammals.

In mammals, two of the extra lower jawbones of synapsid reptiles (the quadrate and

articular bones) became two of the middle-ear bones, the incus (anvil) and malleus(hammer).

Thus, mammals acquired a hearing function as part of the small chain of bones that

transmit air vibrations from the eardrum to the inner ear.

Where did these extra ear bones come from?

Pelycousaurs(like lizards and amphibians) transmitted sound from the eardrum to theinner ear with just the staples, which had evolved earlier via transformation of an old gill

support bone.

The joint between the head and lower jaw was between the quadrate (on the skull) andthe articular (on the lower jaw).


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In many therapsids,the quadrate and articular reduce in size and shift towards the back

of the head.Sound waves were transmitted via the eardrum (located on the lower jaw), through the

articular-quadrate jaw joint, and on to the stapes and inner ear.

Some sound waves also moved up from the feet, into the shoulder, through the jaw joint,

and on into the head.

In true mammals, the articular-quadrate joint lost all function in jaw opening and closing.

Instead, the articular became the malleus, the quadrate transformed in the incus, and both

bones were transferred to the lower jaw and used solely for hearing.As a consequence,the jaw joint had to move forward, and was now between the major bone in the lower jaw

(the dentary) and a different bone on the skull.

Summary

This shift to hearing with three shaking bones, and moving the jaw joint forward, is theshared novelty that unites all true mammals.Ultimately, mammals would keep just one bone in their lower jaw, and the ear bones

would shift to the skull, but this was not the case early in mammal evolution.


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Who are the early mammals?

Morganucodon: late Triassic:

The first mammal

Small, shrew-like insectivore, agile climber and jumperGood mammalian ear, but it is attached to lower jaw, not the skull.

Upright posture for hind limb.

Primitive in many other respects (more than one bone in lower jaw, sprawling front

limbs).

Monotremes: Cretaceous-Recent

Egg-laying, aquatic predators on arthropods and wormsMilk oozes from the skin (no breasts).

Hair

Ear bones shift from lower jaw to skull during embryonic development.Electroreception (receptors on front edge of rostrum)

Multituberculates: Jurassic-Eocene

Small rodent-like animals (rat- to rabbit-sized)

Important small herbivore in Cretaceous and early Cenozoic.

Incisor (grasp/puncture), splicing blade-shaped premolar, grinding molars.Single bone in lower jaw.Fossil record shows that they had hair.

Some may have had live birth.

Marsupials: Cretaceous-Recent

Pouched mammals.

Born as gross little embryos.

Crawl into pouch, attach to nipple, and develop.

Cretaceous ones were fairly opposum-like in their ecology.Later ones are more diverse.Today, they are most diverse on Australia and South America.

They share complex type of molar tooth shape with Placental mammals.


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Placentals: Cretaceous-Recent

Nourish their young internally with a placenta, a complex intergrowth of maternal tissue

with the embryonic membranes of the amniotic eggs.neonate tissue

Placentals give birth to offspring that are more "adult" like and independent.

Cretaceous ones were shrew-like in their ecology.

Later placentals are spectacularly diverse.

How do Placentals and Marsupials differ?

Marsupials Placentals

Slower growth and maturation. Faster growth and maturation.

Smaller brains.Biggers brains.

Slower metabolic rates. Higher metabolic rates.

The first two differences probably relate to the more efficient feeding of developingplacental mammals relative to marsupials.

The reason for the latter difference is not clear.

What is one advantage of having young develop in a pouch?

Flexibility in inconstant environment.

19 mammal origins

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