Mammalian trends Endothermy Sensory specializations Heterodont dentition Skeletal simplification Dentary/squamosal jaw joint Lactation and increased parental.

Mammalian trends• Endothermy

• Sensory specializations

• Heterodont dentition

• Skeletal simplification

• Dentary/squamosal jaw joint

• Lactation and increased parental care

See Table 2.2 for a longer list of diagnostic characteristics.

Mammalian Skin

Adapted from Romer, A. S., and Parsons, S. T. The Vertebrate Body. Saunders, 1977.

Skin Glands

• Sweat glands• Sebaceous glands• Scent and musk glands

Mammary Glands

Adapted from Hildebrand, M. Analysis of Vertebrate Structure. John Wiley & Sons, 1974.

• Lactation and suckling promote social bonds

• Milk composition varies

– Seal milk has 12 times the fat and 5 times the protein of cow milk

• Glandular ducts

• Teat, nipple, or hair tufts

• Under endocrine control

Mammary Glands

Hair

• Dead epidermal cells with keratin• Outer layer of cuticular scales• Deeper cortex and medulla layers• Color determined by pigments (melanin)• Pelage—coat of hair

– Provides insulation– May undergo seasonal molting

FIGURE 03: Structure of a guard hair and cuticular scale patterns of the guard hairs of some mammals

Adapted from Teerink, B. J. Hair of West-European Mammals: Atlas and Identification Key. Cambridge University Press, 1991.

Pelage coloration

• Countershading• Camouflage• Disruptive coloration

– Zebra stripes

• Warning coloration– Skunks

• Intraspecific communicationFIGURE 04: The pattern of postjuvenile molt in the

golden mouse (Ochrotomys nuttalli)

Adapted from Linzey, D. W. and Linzey, A. V., J., Mammalogy. 48 (1967): 326-241.

Fat and Energy

• Adipose tissue– Energy storage– Source of heat and water– Thermal insulation

• Desert or temperate zone mammals– Store fat in tail or abdomen

• Boreal or arctic mammals– Subcutaneous layer of blubber

Circulatory System

• Endothermy requires highly efficient circulation– Systemic and pulmonary circuits– 4-chambered heart– Biconcave red blood cells

• Heart rate varies with:– body size– activity state (e.g. hibernation)

Circulatory SystemFIGURE T03: Heart Rates of Selected Mammals

Data are from Altman and Dittmer (1964: 235); names updated.

Respiratory System

• Trachea—bronchi—bronchioles—aveolar ducts—alveoli

• Human lungs contain ~300 million alveoli or 70 m2 of respiratory surface area

• Muscular diaphragm aids ventilation

– Limb and body movements also assist

Reproductive System• Females

– Both ovaries functional– Ova fertilized in uterine tubes

Adapted from Smith, H. M. Evolution of Chordate Structure. Holt, Rinehart, and Winston, 1960.

• Males– Erectile copulatory organ—penis– Os penis or baculum variable– Testes usually held in scrotum

FIGURE 08: Ventral view of penises of New Guinean murid

rodents

Adapted from Lidicker, W. Z., Jr., J. Mammalogy 49 (1968): 609-643.

Brain• Neopallium highly developed• Corpus callosum present

(Hedgehog) Adapted from Romer, A. S., and Parsons, S. T. The Vertebrate Body. Saunders, 1977; (dolphin) adapted by Norris, K. S. Whales, Dolphins and Porpoises. University of California Press, 1966.

FIGURE 09: Left sides of the brains of a hedgehog and a dolphin

Sense Organs• Olfaction

– Olfactory receptors distributed across the mucosal surfaces of the mesethmoid and vomeronasal organ areas

FIGURE 10: Cutaway view of

the nasal chamber of an Abert’s squirrel

• Hearing– High acuity (especially in nocturnal mammals)– Sound use in:

• Communication• Orientation to environment• Locating food• Avoiding enemies

– Infrasound to ultrasound– External pinna leads to external auditory meatus– Middle ear has three ossicles encased in bony bulla

FIGURE 11: Lateral view of the right middle ear chamber (anterior is to the right) of Abert’s

squirrel, with the auditory bulla largely

removed

• Middle ear

Other Senses

• Vision– Similar to other amniotes– Tapetum lucidum in nocturnal mammals– Retina with photoreceptors (rods and cones)

• Rods enable vision in low light (grayscale)• Cones enable color vision in brighter light

– Eyes reduced in some fossorial mammals• Tactile

– Vibrissae on muzzle (mystacial pad)

Digestive System

• Salivary glands• Simple esophagus• Stomach simple or complex

– Multi-chambered in many herbivores– Site of microbial fermentation in ruminants

• Caecum – May be site of microbial digestion

Digestive System

FIGURE 13: The four-chambered “stomach” of a

ruminant artiodactylAdapted from Storer, T. L., and Usinger, R. L. General Zoology. McGraw-Hill, 1965.

Muscular System

• Limb muscles highly adapted to style of locomotion

• Trunk muscles aid postural control and breathing

• Superficial muscles allow skin to move – Assist in suckling– Facial expression– Ear movements

The Skeleton• Simplification of

skeletal elements– Metabolic savings– Lighter skeleton for

quicker movements• Greater ossification

– Well-braced muscle attachments

• Determinate growth– Epiphysis fuses to

diaphysis

Adapted Stock, C. Rancho. La Brea: A record of Pleistocene life in California, Science Series, no. 13. Los Angeles County Museum, 1949.

The Skull• Akinetic skulls• Single craniomandibular jaw joint• Braincase large• Sagittal and lambdoidal crest may be present• Zygomatic arch usually present• Secondary palate present• Turbinal bones within nasal cavities• Foramina pass cranial nerves and vessels• Three middle ear bones transmit sound to cochlea• Tympanic bulla• Hyoid bones support tongue

FIGURE 17A: Side view of the skull of the African wild dog (Lycaon pictus), showing the bones, foramina, and teeth

Teeth

• Heterodont dentition– Incisors, canines, premolars, molars

• Deciduous dentition– Incisors, canines, and premolar “milk teeth”

replaced by permanent dentition

Dental formula• Specifies number and position of teeth on one side

Incisors 3/3, Canines 1/1, Premolars 4/4, Molars 2/3OR

(3/3, 1/1, 4/4, 2/3) × 2 = 42

Individual teeth designated with upper case letters for upper teeth and lower case letters for lower teeth

P3 is upper premolar 3M2 is lower molar 2

Evolution of teeth

Adapted from Spencer, R. S. Major Features in Vertebrate Evolution: Short Courses in Paleontology. No. 7. Paleontological Society, University of Tennessee, 1994.

• Tribubercular pattern – 3 cusps in

triangle in upper molars

– Talonid added to lower molars (triboshenic)

Adapted from Crompton, A. W., and Hiiemae, K., Discovery 5 (1969): 23.

Mastication• Complex chewing movements

– Initially crush and puncture food– Later sliced by shearing surfaces of molars

FIGURE 21: Molars of the Virginia opossum

(B) Adapted from Crompton, A. W., and Hiiemae, K., Discovery 5 (1969): 23.

Carnivory vs. Herbivory

• Carnivores – Cheek teeth become blade-like (carnassials)– Adapted for slicing flesh– Jaw action is scissor-like

• Herbivores– Cheek teeth become quadrate with hypocone– Jaw action is horizontal and transverse

FIGURE 22: Comparisons of the occlusal surfaces of the right upper cheek teeth of a carnivore

Adapted from Crompton, A. W., and Hiiemae, K., Discovery 5 (1969): 23.

Tooth structure• Cementum binds tooth to jaw• Inner dentine• Outer enamel

FIGURE 23: Generalized sections of mammalian

teeth, showing the internal structure

Terminology

• Brachydont—short-crowned teeth• Hypsodont—high-crowned teeth• Ever-growing—grow continuously• Diastema—space between incisors/canines

and cheek teeth• Bunodont—rounded cusps on molars• Lophodont—cusps form ridges• Selenodont—cusps form crescents

Cusp terminology

Modified from Romer, A.S. Vertebrate Paleontology. University of Chicago Press, 1966.

FIGURE 25: Basic cusp pattern of mammalian molars

Axial Skeleton• Rib cage and sternum• Vertebral column

– Greater head movement– Dorsoventral flexion of spine– Five vertebrae types

• Cervical, Thoracic, Lumbar, Sacral, Caudal

FIGURE 26: Vertebrae of the gray fox

Limbs and Girdles

• Main form of propulsion in most mammals• Fore and aft movement of limbs

• Pelvis– Ilium, ischium, and pubis

• Shoulder girdle– Scapula and clavicle (clavicle may be reduced or

absent)

Manus and Pes

• Manus (hand or forefoot)• Pes (foot or hindfoot)• Five digits is ancestral—highly modified in

several lingeages• Pollex (thumb) and Hallux (big toe) have two

phalanges—remaining digits have three phalanges

Wrist and Ankle