Many of the most important and diagnostic mammalian characteristics serve to further intelligence and sensibility, promote endothermy, or to increase the efficiency of reproduction or the securing and processing of food.

Basic structural body plan is inherited from Therapsid mammal-like reptiles.

Survival through mammalian evolution was perhaps due to their ability to move and to think more quickly than their Archosaurian counterparts.

Morphological trends were toward structural simplification:
    • skull and jaw bones lost or reduced in size
    • limbs and limb girdles simplified, reduced, and less laterally splayed

Fossil record provides little evidence on when endothermy actually developed.

Diagnostic or Distinguishable Characteristics of Mammals:

Soft Tissues
Skin glands:  Mammalian skin contains several kinds of glands not found in other vertebrates.
    Mammary Glands: Provide nourishment for the young during their postnatal period of rapid growth.
    Milk Composition: Milk composition varies with species:
    Cow’s milk 85% H2O
    Dry weight 20% Fat
                     20% Proteins
                     60% Sugars - largely lactose
    also have sweat, sebaceous, scent, and musk glands

Hair: bodies of mammals typically covered with hair, which has no structural homology in other vertebrates.
    - perhaps developed before a scaly covering lost in Therapsid reptiles
    - consist of dead epidermal cells that are strengthened by keratin

Fat and energy storage:
Fat and adipose tissue but are of vital importance as:
    • energy storage
    • a source of heat and water
    • thermal insulation
    Lives of many mammals punctuated by times of crisis when food is in short supply or energy demands are usually high

Circulatory system:
    Highly efficient system with four-chambered heart acting as a double pump
        The RIGHT side receives venous blood from the body and pumps it to the lungs for oxygenation.
        The LEFT side receives oxygenated blood from the lungs and pumps it to the body.
    Erythrocytes biconcave, enucleated disk as possible mechanisms for increased oxygen-carrying capacity.

Respiratory system:
    Lungs are large and, together with the heart, virtually fill the entire thoracic cavity.
    Movements of air into and out of the lungs and volume of exchange due primarily to muscular diaphragm.

Reproductive system:
    Both ovaries are functional and the ova is fertilized in the oviducts.
    Embryo develops within the uterus and lies within a fluid-filled amniotic sac
    Nourishment for embryo comes from the maternal blood stream via placenta
    Male testes typically contained within the scrotum outside the body cavity

    Enlargement of the brain’s cerebral hemisphere
   Neopallium - functions as center for sensory stimulus and initiation of motor activity

Sense organs:
    Sense of smell acute as a result of development of the turbinate bones
    Olfactory lobes enlarged in carnivorous and insectivores but lost in porpoises and dolphins
    Hearing highly developed due to three middle ear bones: malius, incus, stapes, and external pinnae.
   Tapetum lucidum - reflective structure within choroid that improves night vision by reflecting light
   Vibrissae - tactile hairs/whiskers in the muzzles and lower legs of some mammals.

Digestive system:
    salivary glands are present -specialized in anteaters: mucilaginous material makes the tongue sticky

Musculature system:
    Limb and trunk musculature highly plastic
    Variations for high speed locomotion

The Skeleton
Basic changes from Reptiles to Mammals
    • simplification of skeletal elements
    • reduction in the size and number of bones
    • limbs and girdle systems simplified
    • axial skeleton becomes more rigid
    • ossification of large parts of the skeleton
    • development of epiphysis and diaphyses
    • increased brain case size
    • sagittal and lambdoidal crest increased
    • temporal muscle origin
    • zygomatic arch protects eyes and provides an origin for the masseter
    • turbinal bones within the nasal cavity (improved smell/saturation of air)
    • foramina allow passage of cranial nerves
    • 3 middle ear ossicles
    • dentary bone articulates directly with the squamosal
    • hyoid apparatus supports trachea, larynx, and base of the tongue
    • Heterodonty - specialized for feeding/diet
    • Originate in the premaxilla, maxilla, and dentary
    • Dentine covered by enamel
    Axial skeleton - limbs and girdles:
    • five well-differentiated vertebrae: cervical, thoracic, lumbar, sacral, caudal
    • sternum well developed to form a rigid rib cage
    • limb motion generally restricted to fore-aft directions in distal joints; more solid hip and shoulder attachments
    • pelvic girdle has characteristic shape: illium projecting forward and ischium and pubis back - all solidly fused
    • standard pattern of bones in manus and pes (hand and foot) 2-3-3-3-3

Diagnostic Mammalian Traits:

Mammal - a hairy, endothermic, homeotherm which, in most cases, bears live young which are nursed from mammary glands

Studying the evolution of Mammals:
    • helps us understand where and why they are distributed
    • understand evolution
    • understand our past

Geologic Time

Era  =»  Period  =»  Epoch


   Precambrian or Protozoic- to 570mya
        First life - algae, bacteria, worms
   Paleozoic (old) - 570 - 225mya
        Age of fishes
   Mesozoic (middle) - 225 - 65mya
        Age of reptiles  ~20 reptilian orders
   Cenozoic (recent) - 65mya to present
        Age of mammals  ~30 mammalian orders

Mesozoic mammals tended to be somewhat insignificant - limited fossil evidence indicated holding  to conservative mouse-like form and quadrupedal locomotion

Dramatic adaptive burst following the extinction of the dinosaurs

Why so many mammals in such a short time?
   Plate tectonic and continental drift
Mesozoic - Pangea 230 mya
               Laurasia - Europe and Asia - northern
               Gondwanaland - India, S. America, Africa, Antarctica, Australia, southern

When mammals arose, continents were fairly close together
    - pieces breaking off with groups of mammals
    - different conditions evolved different mammals
    - geographic isolation

Mammals evolved from Synapsid reptiles

Evolution of the Skull:
Synapsid (one window) skulls:
Order Pelycosauria
    1. primitive order that gave rise to the Therapsids
    2. first group to depart radically from the basic reptilian design = 300mya
    3. “bowl lizards” pelvic structure allowed organs to be carried off of the ground- increased agility
    4. many changes in size, teeth, skulls, jaw musculature
    5. many forms evolved
    6. may have had increased body temperature, appetites, and feeding effectiveness
    1 1/2 - 3m long
    Predaceous - jaws deep, bigger muscles, canines developing and lots of sharp teeth - no true heterodonty
    Unique sail structure - display and/or thermoregulation.
Order Therapsida
    1. term refers to characteristic mammal-like arch of the cheekbone- may be for harder bite
    2. size variable from a rat to a bear
    3. limbs thin, and body raised off ground- faster, more active?
    4. secondary palate developing
    5. dentary becoming larger
    6. phalangeal formula develops 2-3-3-3-3
    7. chemical communication develops- sites for well developed vomeronasal organs
    8. fossils suggest smooth instead of scaly skin- no indications of hair
    9. thought to have laid eggs, but no remains have been located
    egg guarding =» egg brooding =» egg retention =» live birth
    10. milk evolved from secretions to keep humidity high in egg brooders?
    11. middle ear bones developed- allowed for higher frequencies to be heard: hearing sounds of insects - insectivory- (maybe) - tooth differentiation
Disappearance??  mid-Jurassic during the rise of dinosaurs
    - forced to become nocturnal??
    - forced toward small size??

Order Therapsida; Infraorder Cynodontia (dog tooth)

Increased locomoter efficiency and increased oxygen uptake allowed and aided in chasing and securing prey = differentiation in teeth

Greater mastication prior to swallowing - increased surface area for better enzyme action

Dentary-Squamosal jaw articulation developing with reduction of the quadrate and articular - become free to aid in transmission of sounds and vibrations.

Secondary palate increased surface area of the nasal cavity
    - allowed for simultaneous mastication and respiration
    - warm, moisturize, and clean air
    - increase sense of smell - chemical smells/pheromones (vomeronasal organ)

Masseter muscle has essentially the same attachment as in modern mammals - insertion on lateral surface of dentary and originates on the zygomatic arch; enhanced control of transverse jaw movement.

First Mammals - after Cynodonts
    ~ 10cm long
    long snouts
    rows of complex teeth - probably insectivorous
    partially arboreal
    probably nocturnal - favored by endothermy

Early mammals were small - at least an order of magnitude smaller than Cynodonts
     - arose in Triassic
     - stayed small with Cretaceous (140mya)

Why stay small?
    - competition from moderate-sized non-dinosaur reptiles (turtles, crocodiles) - probably not, lived in different habitats
    - competition from small immature dinosaurs - carnivorous when young
    - lacked sophisticated evaporative cooling mechanism
    - more important features were evolving than size:  maternal care, soft anatomy, and physiology each demonstrated by similarities between therians and non-therians

Mesozoic Mammalian Radiation
Current evidence indicates that mammals probably evolved monophyletically from cynodont reptiles

Early mammals displayed structural features that distinguish them from even the most advanced cynodonts:

Early radiation best described as a dichotomy between two early groups, the Kuehneotheriidae and Morganucodontidae.
Morganucodontidae - basic triconodont molar - may have evolved triconodons, docodonts, and monotremes.

Kuehneotheriidae- triangular molars - may have given rise to the therians (symmetrodonts, pantotheres, marsupials, and eutherians)

 Increasing evidence indicates more complex relationships among early mammals - only the monotremes, marsupials, and eutherians survive today.
Order Triconodonta - late Triassic early cretaceous
    - one of the oldest primitive prototherians
    - predaceous
    - largest = house cat size
    - heterodont
    - 14 teeth in dentary
    - canines large
    - molar cusps 3, arrangement in front-to-back row
Order Docodonta - late Jurassic
    - roughly quadrate teeth
    -cusp not aligned anterioposteriorly
Order Symmetrodonta - late Triassic to late Cretaceous
    - probably predaceous
    - 3 fairly symmetrical cusp

Order Multituberculata - first appeared in the late Jurassic period to Tertiary
    - first mammalian herbivores
    - wide spread in both the old and new worlds
    - Ecologically equivalent to rodents
    - strongly built lower jaw with attachment for powerful jaw muscles
    - 2 or 3 incisors
    - diastema in front of premolars, 3 parallel cuspules
    - olfactory lobes enlarged
It has been generally accepted that eutherians and metatherian mammals evolved from the Order Pantotheria
    - profile of the ventral border of the dentary bone is interrupted by an angular process
    - the lower molar has a posterior “heel” which is separated by the talonid
    - the trigonid section of the pantotheric lower molar and the triangular upper molar resemble the corresponding teeth of some primitive eutherians and metatherians

During the Cretaceous, land dwellers were banned from intercontinental movement by oceans and seaways
    - populations of mammals on different continents evolved in isolation under different environmental conditions
    - earliest known marsupials are from late Cretaceous Canada, Westerns N.A. and Peru.
    - mammalian radiation coincided with a burst of flowering plants (Angiosperms), Lepidoptera (moths and butterflies), Isoptera (termites), and Coleoptera (beetles).


Partial Classification of Mammals                    ? = Extinct

Kingdom Animalia
    Phylum Chordata
    Subphylum Vertebrata
        Class Mammalia
            Subclass Prototheria
                Infraclass Eotheria ?
                    Order Tricodonta ?
                        Family Morganneodontidae ?
                    Order Docodonta ?
                        Family Docodontidae ?
                Infraclass Ornithodelphia
                    Order Monotremata
                        Family Tachyglossidae
                        Family Ornithorhynchidae
                Infraclass Allotheria ?
                    Order Multituberculata ?
                        (?) Family Haramyidae ?
            Subclass Theria
                Infraclass Trituberculata ?
                    Order Symmetrodonta ?
                        Family Kuchneotheriidae ?
                    Order Pantotheria ?
                Infraclass Metatheria - all marsupial mammals
                Infraclass Eutheria - all placental mammals



Monotremes and Marsupials can easily be considered apart from other mammals

• both are relatively primitive

• have different reproductive patterns

Monotremes - egg-layers

Marsupials - bear tiny, poorly developed young

Monotremes diverged from other mammals ~190mya

Marsupials diverged from Eutherians ~100mya



Represented by three genera, each with a single species. Monotreme translates into "one-hole" - refers to the presence of a cloaca, a single reproductive/excretory outlet

Monotremes retain many reptilian traits:


Refers to the diet of many, but . . . taxonomic uncertainty and disagreements on classification are causing many problems - the order serves as a convenient "catch-all"

Most primitive eutherian order

Third largest order with ~77genera and 400spp.

Rodents ~1700spp.

Chiroptera ~850spp.

Distributed through most of both hemispheres except Australian region, northern part of South America, and polar regions

Originally thought to have evolved in Old World (Europe and Asia) and moved into the New World

- earliest fossil Insectivores (Batodon) from mid-Cretaceous North America ~100mya
- oldest members of clearly recognized families - soricids and talpids - from Eocene ~50mya
"Grab-bag" of forms making it difficult to form subclassifications - many generalized forms
- some could be lumped
- some may be considered as separate orders
- Butler (1972) ". . . any fossil eutherian not closely related to one of the other orders is classified in the order Insectivora."

Order Macroscelidea - Elephant shrews

Order Scandentia - Tree shrews

General characteristics:
- usually small
- long narrow snout
- 5 clawed digits
- usually short, close-set fur
- anterior vena cavae paired
- pinnae small to absent
- minute eyes - some covered with skin
- scrotum when present anterior to penis
- most insectivorous
- terrestrial, fossorial, semiaquatic
- plantigrade - heels touch ground when walking
General cranial traits: - small brain case with smooth cerebral hemisphere
- no auditory bullae - ring-shaped tympanic bone
- jugal reduced or absent
- zygomatic arch absent in some
- usually enlarged and specialized incisors with sharp shearing cusps
- canines usually reduced
- some genera retain tribosphenic molars

Second largest order of mammals with ~170 genera and 850 species

Characterized as the only mammal to have evolved true flight

Represents the most poorly understood/misunderstood groups of mammals

Relatively recent biological research has revealed:

Bats have nearly cosmopolitan distribution, being absent only from the arctic and polar regions and from isolated oceanic islands

Frequently abundant members of temperate faunas but reach their highest densities and greatest diversities in tropical and subtropical areas

Bats occupy a number of terrestrial environments, including:


Because of their small size, ability to fly, delicate structure, and greatest abundance in tropical areas where fossilization rarely occurs, but fossils are rare
  • Icaronycteris index - from Eocene beds in Wyoming the oldest known undoubtable bat material
  • first described by Jepsen in 1966
  • claws on the first 2 digits of the hand
  • fairly short, broad wings
  • Other fossils prove insectivory during Eocene - moth scales in gut
  • Late Eocene and Oligocene deposits in France have shown evidence for Microchiroptera families Emballonuridae, Megadermatidae, Rhinolophidae, and Vespertilionidae
  • Megachiroptera appeared in Oligocene in Italy
  • Fossil record shows little change in some families since Eocene, indicating good adaptations to their particular environments - a sharp contrast to Oligocene horses which were sheep-sized and 3-toed

    Oligocene Tadarida were nearly identical to present-day members of the Molossidae family

    Paleocene origins of bats seems probable, followed by a late Cretaceous divergence from insectivorous stock


    The most important feature about bats is their adaptation for flight
    • bones of the arm and hand (except thumb) are elongated and slender

    • flight membranes extend from the body and hind limbs to the arm and the 5th digit, between the fingers, from the hind limbs to the tail, and from the arm to the occipitopollicalis muscle - patagia

    • the hind foot is free from flight membranes, which may extend to the ankle
    • Uropatagium (tail membrane) may not always be present as in the tail - multiple combinations

    • ears usually have some form of a tragus - flap in the front part of the ear - different shapes in different species

    Calcar may or may not be present - bony element extending off the foot which helps to support the uropatagium - may also be keeled (cartilagenous extension)

    Shoulder Modifications:
    • enlarged tuberosity of the humerus locks against the scapula at the top of the upstroke

    The hind limbs: Reduction in Wing Weight:



    Primates represent the 7th largest order with 51 genera and 168sp

    Order Scandentia - tree shrews as primates

    5 genera, ~16sp
    Suborder Streposhirhini
    Suborder Haplorhini
    upper lip divided
    upper lip not divided
    orbit and temporal fossa continuous
    postorbital plate
    lacrimal foramina inside orbit
    outside orbit
    smaller braincase
    larger braincase
    foramen magnum posterior
    olfactory oriented
    vision oriented
    more hairy face
    less hairy face

    Rhinarium - fleshy pad with moisture glands on end of nose - cool and wet

    Most nonhuman primates occur in tropical areas - because of cultured and biological plasticity man adapts to most biomes

    Primate Pattern


    Centers around adaptation to tree life and arboreal existence, secondarily evolved for life on the ground

    Qualities to exploit forest canopies:

    Xenarthra (Edentata), Philodota, and Tubulidentata all share a major structural trend, the loss or simplification of dentition Xenarthrans (armadillos, sloths, anteaters): underwent tertiary radiation in South America

    Pholidota (pangolins) and Tubulidentata (aardvark): Old World groups, each of which has conservatively maintained a single structural plan.

    Order Xenarthra = Edentata

    "strange articulation" = new                     "no teeth" = old

    Not of great importance but are remarkably interesting because of their unique structure and unusual ecological roles, large fossil types, and remarkable Tertiary radiation in South America

    All share a series of distinctive morphological features:

    Major Xenarthrous structural trends are toward a reduction and simplification of the dentition, specialization in the limbs for climbing or digging, and rigidity of the axial skeleton


    An ancient, profitable (and honorable) occupation

    Appeared in the early Paleocene (Creodonts) before most of the recent mammalian orders

    Probably evolved in response to the food resources offered by expanding array of herbivores

    Most recent carnivores are predaceous and have a remarkable sense of smell

  • many carnivorous
  • may be omnivorous - bears
  • may be specialized - cats
  • Cursorial abilities may be limited - Ursidae, Procyonidae
                              well developed - cheetah, canids

    General Characters:

    ORDER CETACEA (Greek = whale)

    Mysticeti - baleen whales 11spp.

    Odontoceti - toothed whales 67spp.

    Noted as being mammals that are most fully adapted to aquatic life

    Fossil information indicates cetaceans are an old and successful group:

    ORDER RODENTIA  (Greek "to gnaw")

    About half of all mammals currently alive are rodents

    Plants are the most abundant food source - rodents adapted as herbivores to take advantage of this food supply

    30 families
    418 genera
    ~1750 species

    Why so successful?

    Taxonomic relationships are difficult to understand Convergence - distantly related but look alike Heteromyidae and Dipodidae - morphologically similar in utilizing dry habitats Divergence - closely related but look nothing alike Geomyidae related to Heteromyidae - gophers fossorially adapted/kangaroo rats long legged and saltatorial Parallelism - closely related and pursue similar modes of life Muridae and Cricetidae - Old and New World rodents frequently lumped together into the same family - look very much alike and are clearly related through the fossil record Evolutionary radiation has allowed Rodentia to occupy niches filled by other orders South America species resemble: rabbits     =  Hydrochoerus sp. (Capybara)
    antelope   =  Cavia sp. (Cavy)
    Size extremely variable: Largest:    South America Hydrochoerus sp. - Capybara ~100 lbs North America Castor canadensis - beaver ~50 lbs Smallest: Baiomys sp. Perognatus flavus
    Reithrodontomys humilis ~5g
    Fossil record for rodents is not very good due to small and fragile bones

    Early divergence of suborders based on mandibles - Split by Tullburg (1899) and Wood (1985)

    Sciurognathi - the angular process of the dentary bone originates in the plane that passes through the alveolus of the incisor and is ventral to the alveolus

    Hystricognathi - the angular process originates lateral to the vertical plane of the alveolus

    Most distinguishing trait for the order are based on dental characters: Incisors No Canines - long diastema

    Cheek Teeth

    Dental formula reduced to a maximum of 22 teeth
    1/1 0/0 2/1 3/3 = 22

    Dentition best suited for herbivory but varies

    Onychomys - carnivorous
    Dipodomys - omnivorous
    Rodents have been broken out into four taxonomic groups based on jaw musculature and mandible/skull articulation Protrogomorphs - represents the primitive condition
    Sciuromorphs - squirrel-like
    Hystricomorphs - porcupine-like
    Myomorphs - mouse-like



  • The origin of the medial masseter from the zygomatic arch to an extensive area on the side of the rostrum
  • passes through the often greatly enlarged infraorbital foramen
  • occurs in Dipodidae and most Hystricognaths
  • Myomorphs

    Complicated jaw action allows the lower cheek teeth to move transversely or anterioposteriorly against the upper teeth, producing a crushing and grinding action.

    Cheek teeth and incisors perform distinctively different functions - musculature is required to move the lower jaws into power positions for teeth to function - "division of labor"


    Rabbits, hares, and pikas are not a very diverse group but are important members of many terrestrial communities, and are nearly cosmopolitan in distribution and were only absent from Antarctica, Australia, and southern South America

    Two families with 13 genera and 80 species

    Taxonomic origins are from the Paleocene of China - appear to share a common origin with rodents within the Paleocene order Anagalida Conservatism in evolutionary design may be related to the limitations of their functional position as "miniature ungulates" - direct competition with Artiodactyla may have limited lagomorphs to a single limited adaptive zone


    Not a taxonomic term but is a general term for a large group of cursorial forms

    The name implies that a hoof is present

    Two Orders:

    Perissodactyla - horses, rhinos, tapirs - "odd toed"
    Artiodactyla - pigs, peccaries, hippos, camels, deer, antelope, cattle, sheep, goats - "even toed"
    Came from the Condylarthra - a diverse group of ungulate-like mammals that existed from Cretaceous to the Oligocene

    Exceptional cursorial ability - Why?

    Perissodactyls Artiodactyla
    Cursorial Specializations

    Two factors determine speed:

    Length of stride:
    1) Lengthen the limb

    2) Loss or reduction of clavicle

    3) Flexion of the spine Increased rate of stride:

    1) Extra joints

    How done? 2) Specialization in musculature 3) Ungulate "ankle/wrist" specializations


    Mammals with no reduction in limb elements

    tarsals and metatarsals


    tiba and fibula
    (often fused)
    (becomes weight bearing)
    other tarsals
    (fused in canon bone)

    4) Other specializations

    Feeding Specializations for Herivorous Diets:
    1) Cheek teeth

    2) Digestion of plant material Artiodactyls more efficient digesters - may live longer in areas with short food supply

    Microflora and Fauna:

    Ruminants = Cud chewers

    General pattern of stomach activity: