mammalian characteristics chapter 20. lactation presence of mammary glands for which the class is...
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Lactation Presence of Mammary Glands for which the class is
named. This is a unique feature of the class, Function is lactation – provide milk to the developing
young, All mammalian females lactate Mammary glands absent from males of marsupials but
present in male monotremes and eutherians Only Eutherians have nipples, others have mammary hairs,
so young ones suckle from the mother’s fur
Determinate growth Epiphyses feature of determinate
growth in mammals Growth zones of cartilage between
diaphysis and epiphysis until cartilage is used up (limited growth span)
At maturity ossify with diaphysis (shaft)
Skeletal System: Cranial Features Heterodentition
Incisors, canines, premolars & molars 2 sets of dentition 1st set = milk teeth: incisors, canines and
premolars 2nd set = second set of original teeth + later-
erupting teeth the molars Mammals are the only animals that masticate
food and swallow it as a bolus of food Therians have special molars called
tribosphenic molars
Cranial & Postcranial features Dermal bones completely enclose the brain Zygomatic arch : bowed out bones of that
formed the lower boarder of temporal opening
3-boned middle ear Dentary; single bone that forms the
mammary lower jaw Upright posture with limbs underneath the
body –now : dogs, cats, horses In opossums, semi-sprawling stance –
primitive condition
Postcranial features (fig 20.4) Unique ankle joint called
crurotarsal joint Between tibia and ankle bone called
astragalus Proximal ankle bone the calceneum
projects to form the calcaneal heel The heel is the point of insertion of
calf muscles : Gastrocnemius
Postcranial features Ilium is rod shaped, & directed
forward Pubis and ischium are short
Contrast with plate-like pubis of & ischium of reptiles
Femur has a trocanter for the attachment of gluteal muscles which are the major retractors of the hind limbs
Postcranial features All have 7 cervical vertebrae
exceptions giraffe (8); matees (7) Unique atlas-axis complex
Formed by the 1st two cervical vertebrae
Joint between head & atlas allows up & down head movement
Joint between atlas & axis allow the side to side movement
Postcranial features Ribs restricted to the thoracic
vertebrae Jaw adductor muscles
Temporalis: from head to dentary’s coronoid process
Masseter Extend from the zygomatic arch to outside
of the lower jaw Unique in mammals Enables sideways movement of jaws
Postcranial features Diagastric muscles
Jaw opening muscles in Therian mammals
Innervated by Cranial Nerves 7 (back ½) and 5 (front ½)
Facial expression muscles-unique Innervated by cranial nerve 7.
Integument Skin has epidermal, dermal and
hypodermal (with fat cells) layers Epidermis varies in thickness
Humans thickest in palms and soles Rodents – very delicate epidermis Elephants, rhinoceroses,
hippopotamuses once classified as pachyderms because, epidermis is several hundred cells thick
Integument: Hair Camouflage Communication Sensation via (vibrissae or whiskers)
Grow on muzzle, eyes, lower legs, & have touch receptors
Insulation Composed of keratin Grows from hair follicles
Integument: Glands Secretory glands that develop from the
epidermis 3 major types of skin glands Eccrine gland:
produce a watery secretion with little organic content
Function as sweat glands in humans In most animals, restricted to soles of the
feet, tail and areas that contact surfaces to improve adhesion
Integument: Glands Sebaceous glands
Over entire body Produce oily lubricant sebum that lubricates
and waterproofs the hair Apocrine
Type of gland in which the apical part of the secretory cell breaks down in the process
Secretions used in chemical communication In humans found in armpits and pubic region
Integument: Claws, Nails, Hooves, Horns
Part of the integument Claws, nails and hooves are
accumulation of keratin that protect the terminal phalanx of the digits
Horns can be keratin (e.g. rhino) or bone covered by keratin as in cows
Cardiovascular system Heart is four chambered Complete ventricular septum Single systemic arch (aorta)
Birds retained right systemic arch Mammals retained the left systemic arch as
aorta Do not have the double systemic arch seen
in reptiles Mature Red blood cells lack a nucleus
Respiratory system Large lobed lungs Finely branching bronchioles that
terminate as alveoli Diaphragm that aids in breathing
Divides the pleurocavity into the Peritoneal cavity: surround viscera Pleural cavity surround the lungs
Urogenital system Retain bladder, secretes dilute urine Cloaca replaced by separate openings for
urogenital and alimentary canal systems Testes placed in a scrotum outside of the
body Urethra common passage for urine and
sperms, hence is the penis Glans= tip of penis; single in eutherians
and forked in monotremes and marsupials os penis (baccalum) contain bone
Urogenital system Renal portal system does not
appear in mammals Have the loop of Henle which is
ideal for salt secretion Marine reptiles and birds have salt
glands in their heads that secrete excess salt
Sex determination Genetic X & Y chromosomes Gene on Y chromosome initiates
male gonadal development and female gonadal development occurs in its absence
The brain Highly developed neopallium (roof of the
forebrain): extended portion of the forebrain
tectum (visual center in lower vertebrates) reduced to corpora quadrigemina: functions mainly as a relay center for auditory information and to control visual reflexes
corpus callosum in eutherians provides additional communication
Major Lineages of Mammals Allotheria (Multituberculates)
Extinct Prototheria
Monotremes Theria
Metatheria or Marsupials or Eutheria: Placentals
Multituberculates Rodent or squirrel- like semi-arboreal
omnivores Could rotate foot backward to descend trees
headfirst. Prehensile tail (figure 20.20
Narrow pelvis: maybe gave birth to poorly developed young
Broad molars that are multicusped and specialized for grinding rather than shearing
Monotremes vs Therians Toothless bird-like beak which
contains receptors for sensing prey Reptile-like cervical ribs Reptile like pectoral girdle with
clavicle, interclavicle, precoracoid, coracoid, scapula
Large epipubic bone on pelvic girdle Lay 1 or 2 eggs, incubate 7-10 days No nipples
Monotremes vs TheriansMonotremes Lay eggs No teeth in adults No nipples No cochlea No external ear( pina) Retain sclerotic ring
around eyes Septomaxilla bone Retain ribs on cervical
vertebrae
Therians Give birth to young Tribosphenic molars Nipples Cochlea in inner ear External ear (pina) No sclerotic rings
around the eyes No septomaxilla bone No cervical ribs
Monotremes vs Therians No crurotarsal joint Reptile like pectoral
girdle with clavicle, interclavicle, coracoid, scapula (not as big)
Large epipubic bone on pelvic girdle
Crurotarsal ankle joint with complete superposition of astragalus on the calceneum (fig 20.12)
Lost interclavicle & coracoid, expanded the scapula. Clavicle lost in dogs and horses
Small epipubic bones
Eutherian vs MarsupialsEutherian Non inflected angle to
the dentary bone Rectangular shape of
nasal bones Auditory bulla (bone
elaboration around the ear to increase hearing
Post-orbital bar bone (bar of bone behind orbit in herbivores
Marsupials Inflected angle
(distinct) Flared or diamond
shaped nasal bones Lack an auditory
bulla or a very small one from a different bone
No postorbital bar
Eutherians vs Marsupials Replaces all teeth
except molars Dental formula I3/3; C1/1; P4/4; M3/3
( 44 teeth, e.g whales)
Many have fewer teeth; primitive
Humans have I2/2; C1/1; P2/2; M3/3
Lack epipubic bone
Replace only last molars
Dental formula I3/4 C1/1 P3/3 M4/4
55 teeth More incisors and
molars Epipubic bone that
project forward from the pubis
Monotremes Toothless as adults, but teeth in juveniles Egg laying mammals Have primitive anatomical features Reptile-like pectoral girdle with clavicle,
coracoid and scapula Large epipubic bone on pelvic girdle Lay 1 or 2 eggs, incubate 7-10 days
Platypus : Australia Echidna: Australia ( short nosed) Guinea (long
nosed)
Therians: Marsupials Diverse (275 spp): See Table 20.1 See also figure 20.16 Smaller brain than eutherians Short gestation length New borne crawl into pouch & attach to
the nipple Separated from Eutherian since late
mesozoic Central and South America, Australia and
New Guinea
Therians: Eutherians Placental mammals 93% of all mammals Approximately 4100 spp Chorio-allantoic placenta Longer gestation period Young are more developed Relatively short lactation length Rapid growth & development- facilitated
by high energy milk
Major orders of placental mammals
Insectivora Shrews, moles, hedgehogs, Madagascan tenrecs Chiroptera (bats) Carnivora (bears, canids, felids, cats, seals, sea
lions, walruses Rodentia: mice & squirrels Lagomorpha: pikas, rabbits, hares Perissodactyla:odd toed ungulates) Artiodactyla:even toed ungulates) Cetacea:whales, dolphins Primates: monkeys, apes, gorillas, humans
Anatomical differences between Eutherians and Marsupials Eutherian Non-inflected angle to
the dentary bone Rectangular shape of
the nasal bones Auditory bulla- bone
elaboration around the ear region
Post-orbital bar- bar of bone behind the orbit
Replace all teeth except molars
Marsupial Inflected angle to the
dentary bone Flare or diamond
shaped nasal bones Lack a bulla or it is a
very small formed from a different bone
Lacks post-orbital bar Replaces only the
last premolar
Anatomical differences between Eutherians and Marsupials
Eutherian Dental formula
I3/3 C1/1 P4/4 M 3/3
Many have fewer teeth than this e,g humans have 32, dolphins ~ 44
Lack epipubic bones
Marsupial Dental formula
I3/4 C1/1 P3/3 M4/4
Have epipubic bones that project forward from the pubis
Anatomical differences between Therians and Monotremes Therians Give birth to
young Have nipples No sclerotic rings
around the eyes Lack septomaxilla
bone in skull No ribs on cervical
vertebrae
Lay eggs No nipples Sclerotic cartilage
rings. Do not ossify to form bone
Have septomaxilla bone in skull
Ribs retained in monotremes
Anatomical differences between Therians and Monotremes
Therians More derived hinge joint
between tibia and the astralagus in the ankle
There is complete superposition of astragalus on the calceneum
Shoulder girdle more derived Coracoid & interclavicle
bones do not appear in the shoulder girdle
Clavicle lost in running eutherians (dogs & horses)
Scapula expanded by appearance of scapula spine
Monotremes Not seen in
monotremes Reptile-like
shoulder girdle Coracoid &
interclavicle present
Clavicle present