Mammalian Diversity and Evolution

Chordate Classification

• Subphylum Chordata - Classes and Types of Chordates:

  • Cephalochordata (e.g., lancelets)

  • Urochordata (e.g., tunicates)

  • Agnatha (jawless fishes)

  • Chondrichthyes (cartilaginous fishes)

  • Osteichthyes (bony fishes)

  • Amphibia (amphibians)

  • Reptilia (reptiles)

  • Aves (birds)

  • Mammalia (mammals)

• Major Features of Related Classes:

  • Lancelets and Tunicates: Primitive chordate features including a notochord and pharyngeal slits.

  • Agnatha: Jawless structure with cartilage skeletons and absence of paired fins.

  • Chondrichthyes: Cartilage-based skeleton, strong jaw structures, and powerful swimming capabilities.

  • Osteichthyes: Ossified (bony) skeleton and backbone, streamlined for survival and buoyancy control via swim bladders.

  • Amphibia: Development of lungs and limbs; unique dual life stages involving aquatic larvae and terrestrial adults.

  • Reptilia: Extensive adaptation to land, featuring thick skin and amniotic eggs that resist drying.

  • Aves: Unique feathers for insulation and flight; specialized skeletal adaptations to reduce weight.

  • Mammalia: Distinctive hair; mammary glands for feeding youth; highly adapted to specialized environments globally.

Amniota and its Clades

• Clade Amniota includes:

  • Clade Synapsida:

  • Includes mammals and their extinct ancestors.

  • Order Therapsida:

    • Suborder Cynodontia

    • Class Mammalia:

    • Subclass Yinotheria:

      • Infraorder Australosthenida

      • Order Monotremata (Prototheria)

    • Subclass Theriiformes:

      • Infraclass Holotheria

      • Cohort Marsupalia (Metatheria)

      • Cohort Placentalia (Eutheria)

  • Clade Sauropsida:

  • Includes all modern reptiles, extinct dinosaurs, and birds.

  • Subclass Diapsida: Most reptiles and birds.

  • Subclass Anapsida: Extinct turtles (traditionally defined; paraphyletic group).

Mammalian Classification & Diversity

• Mammal Diversity:

  • Species Count: Approximately $4,450$ species categorized into $1,000$ genera and $21$ orders.

  • Habitat: Highly adaptable, mammals are found globally in terrestrial, aquatic, and aerial habitats.

• Origin of Mammals:

  • Developed from synapsid reptiles in the Triassic period ($\sim 300$ million years ago).

  • Survived the mass extinction event that eliminated the dinosaurs ($\sim 65$ million years ago).

  • Earliest mammals were small, nocturnal insectivores and scavengers, likely avoiding competition with larger reptiles.

Characteristics of Mammals

Mammals are distinguished by a suite of physical and physiological traits essential for their success:

1. Hair: Composed of keratin and produced by specialized follicles. Functions include insulation, protection, camouflage, and sensory touch through vibrissae.

2. Integumentary Skin Glands: Mammals possess a diverse array of glands:

   • Sweat Glands: Used primarily for thermoregulation and cooling.

   • Scent Glands: Utilized for marking territory, communication, and defense.

   • Sebaceous Glands: Secretes oils to lubricate and waterproof skin and hair.

   • Mammary Glands: Specialized glands providing nutritious milk to offspring.

3. Skeletal Specializations:

   • Occipital Condyles: Two condyles at the base of the skull for specialized neck movement.

   • Secondary Bony Palate: An essential structure that allows mammals to eat and breathe simultaneously.

   • Cervical Vertebrae: Most mammals consistently possess $7$ cervical vertebrae.

4. Heterodont Dentition: Teeth are differentiated into specific types based on function:

   • Incisors: For biting and cutting.

   • Canines: For piercing and tearing.

   • Premolars and Molars: For grinding and crushing.

   • Dental Formulae: The primitive (ancestral) formula is $3:1:4:3$, while the modern human formula is $2:1:2:3$.

5. Advanced Physiological Systems:

   • Movable eyelids and fleshy external ears (pinnae).

   • Endothermy: Maintaining a constant internal body temperature.

   • Excretory System: Advanced kidneys that process nitrogenous waste into urea.

   • Reproduction: Internal fertilization and specialized embryonic development within the uterus (in most clades).

Systems of Mammals

Circulatory System

• $4$-Chambered Heart: Completely separates oxygenated and deoxygenated blood, ensuring efficient delivery of oxygen to active tissues.

Respiratory System

• Advanced Lungs: Extensive surface area provided by numerous alveoli for efficient gas exchange.

• Diaphragm: A muscular partition that contracts to create a vacuum for lung inflation, separating the thoracic and abdominal cavities.

Digestive System

• Dietary Adaptations: Digestive tracts vary significantly based on diet.

  • Herbivores: Feature long intestines and the rumen (in ruminants) for breaking down cellulose.

  • Oral Processing: Secondary palate ensures high metabolic rates can be maintained by breathing while chewing food.

• Behavioral Adaptations: Includes migration (e.g., baleen whales) or hibernation to cope with seasonal food scarcity.

Nervous System

• Encephalization: Mammalian brains are approximately $15\%$ larger than those of other vertebrates of similar size.

• Neocortex (Cerebrum): Highly developed, allowing for complex motor control, sensory processing, and higher-order social behaviors.

Mammalian Diet Types

• Carnivores: Possess sharp canines and carnassial teeth for meat processing.

• Herbivores: Rely on specialized molars for grinding fibrous plant material; some have specialized gut bacteria for fermentation.

• Omnivores: Maintain flexible, generalized dentition to handle a mixed diet of plants and animals.

Reproduction in Mammals

• Developmental Patterns:

  • Monotremes: The most primitive mammalian reproduction; they lay leathery eggs (e.g., platypus, echidna).

  • Marsupials: Give birth to very underdeveloped young that complete growth inside a pouch, characterized by short gestation and long lactation.

  • Placentals (Eutherians): Utilize a complex placenta for nutrient and waste exchange, allowing for extended intrauterine development.

• Lifecycle Metrics: Species differ in gestational lengths and weaning times, often correlated with body size and environmental stability.

Important Orders of Mammals

Order Monotremata

• egg laying mammals

• nurse young with milk

• 3 species- duck billed platypus, and 2 ant-

• eaters (echidnas)

• only found in Australia and New Guinea

• most primitive of all mammals

Infraclass/ cohort-Marsupalia

• marsupials

• 280 species mostly in Australia and New Guinea

• out-competed by placentals in the Americas

• live young with immature birth

-live in a pouch

-baby kangaroos must climb climb to pouch with no parental aid

-short gestation, long lactation

• only one marsupial in US- opossum

Order Insectivora: Shrews and moles; mostly small and burrowing.

• the hedgehogs and moles

• long pointed noses they use to search for worms and insects

• most primitive placental mammal

Order Rodentia: The largest order ($\sim 40\%$ of all placental species); characterized by specialized continuously growing incisors.

• chisel-like incisors that continue to grow, a pair per jaw

• very high reproductive capacity

• (Latin: rodere – to gnaw)

  - Beavers, mice, rats, squirrels, woodchucks

Order Lagomorpha: Rabbits and hares; similar to rodents but with an extra pair of upper incisors.

- Four front incisors (doubled up, Another pair behind the first.)

- rabbits, hares, pikas

Order Chiroptera: Bats; the only mammals capable of true powered flight, with forelimbs modified into wings.

SuperOrder -Xenarthra (Edentata)

- No teeth

- Anteaters and armadillos

Order Carnivora: Diverse group including cats, dogs, and seals; primarily meat-eaters.

• meat eating predators-cats, dogs, weasels, bears, raccoons…

• small incisors, large canines

• sharp claws

• keen senses of sight and smell

• Latin: carni vorare –

  flesh eating)

• teeth adapted for

  tearing flesh

Clade Pinnipedia

- Walruses, seals, sea lions

- Fin-footed

- Formerly an Order, now within Order Carnivora

- migration

- young have more fur than adults

- modified limbs for swimming

Order Perissodactyla: Odd-toed ungulates (e.g., horses, Equines (Zebras, donkeys), rhinoceros, tapirs).

Order Artiodactyla: Even-toed ungulates (e.g., deer, cattle, pigs, hippopotamuses,

camels, llamas, peccaries,antelopes, sheep,goats, giraffes, cattle)

InfraOrder - Cetacea(Latin: cetus – whale)

- Within Order Artiodactyla

- Front limbs – flippers

- no hind limbs

- whales, dolphins, porpoises

• exclusively aquatic

• live birth at sea

• no rear appendages

• horizontal tail called a fluke

• groups are called pods

• 2 families

Order Proboscidea: Elephants; defined by a muscular trunk and ivory tusks.

Order Sirenia: Manatees and dugongs; fully aquatic herbivores.

Order Primates

• the primates

• eyes forward

• nails instead of claws

• complex behavior

• variety of sizes and habitats

• grasping hands (and in some cases feet) opposable thumbs

• - Large cerebral hemispheres

  - Eyes face forward

  - Nails on fingers

  - Monkeys, humans, and

  other apes

Apes

• Old World distribution

• Loss of tail; no chin

• Typically, a 5-Y cusp pattern on molars

• Complex society with dominance hierarchies

(also seen in some Old World monkeys, such

as baboons)

• Two families: Hylobatidae (gibbons) and Homonidae (great apes)

Tarsiers (Tarsius spp.)

a.k..a. “those things with the great big spooky eyes”

• Five living species, native to Indonesia, Philippines, Malaysia

• Eyes larger than brain (!) and fixed; head turns 180°(convergent with owls)

• Capable of leaping up to two meters

• Taxonomy still controversial. Once grouped with “prosimians” (lemurs, etc.), but today

majority opinion places them with haplorrhines (monkeys and apes)

Platyrrhini or Ceboidea

(Monkeys of the Americas; “New Word Monkeys”)

• 2133/2133 or 2132/2132 dentition, like Strepsirrhines

• Often have prehensile tail

• Still commonly used scent marking, like lemurs

• Nostrils apart and pointing sideways

Catarrhini (African and Eurasian “Old World” Primates)

• Tail not prehensile or absent

• Nostrils close and point down

• 2123/2123 dental formula (i.e., one fewer

premolar than other primates)

• No scent marking

• Includes Old World monkeys (Cercopithecoidea) and apes (Hominoidea)

Evolution of Primates

General Characteristics

• Development of forward-facing eyes (binocular vision) for depth perception.

• Grasping hands and feet with opposable digits for arboreal navigation.

• Increased reliance on social structures and maternal care.

Major Groups

Strepsirhines: Including lemurs and lorises; often nocturnal with a moist rhinarium.

• Most species are native to Madagascar, and are extremely endangered. A few are native to Africa (galagos, pottos) or India and Southeast Asia (lorises)

• Note the rhinarium (cold, wet nose), related to scent marking and greater dependence of

smell

• Often one claw and/or the lower incisors are specialized for grooming

• Haplorhines: Including monkeys, apes, and humans; generally diurnal with larger brains.

Fossil Record and Human Origins

Molecular Clock Estimates of Divergence Times

Common ancestor of humans and chimpanzees: 5 million years ago

Common ancestor of great apes: 8 million years ago

Common Ancestor of Hominoidea: 10 million years ago

• Hominid Evolution: Milestones include bipedalism, tool use, and cranial expansion.

• Early History

• 70 million years ago oldest known fossil primate, †Purgatorius from Montana, consisting of a single tooth, and some ankle bones

• 60 million years ago: oldest semi-complete fossil primate, †Plesiadapis from Wyoming

and France

• 45 million years ago: diverse adapids and omomyids: Eosimias from China is close to

common ancestry of haplorrhines(tarsiers + monkeys +apes)

Key Species:

†Ardipithecus ramidus(extinct)

- 4.4 million yeas old; found at Aramis Ethiopia in 1993-4

- Teeth appear more ape-like than any other hominid.

- More recent than the common ancestor between humans and chimpanzees .

Australopithecus: Early bipedal ancestors.(extinct)

• First fossils discovered in Southern Africa in 1925, by Dr. Raymond Dart

• Ten species named so far, between 4.2 million and 1.4 million years old

• Two general categories of species: “gracile” (slight build) and “robust” (heavier build)

• With once known exception (†A. bahrelghazali, discovered in Chad in 1995), all are confined to eastern Africa.

• How to we know that Australopithecacines walked upright?

- Bowl-shaped pelvis with broad ilium

- Angled hip and knee joints

- Foramen magnum below skull

- S-shaped vertical column

- Enlarged big toe in line with others

- Footprints(at Laetoli, Kenya; 3.7 million years old)

†Homo naledi(extinct)

• Discovered in a cave in South Africa

• 15 individuals in the cave – Burial site

• Brain the size of an orange, 1/3 size of modern

• Modern looking jaw

• Between 2.5 and 2.8 MYA

Homo habilis: Known for the earliest sophisticated stone tools.(extinct)

• Some anthropologists divide this species into at least two: true †H. habilis and the

somewhat earlier †H. rudolfensis

• Simple tools. Evidence of stone piles used as hunting blinds or hut foundations

• Brain size: 650 cc average; range is 500-800 cc(overlaps with †Australopithecus range)

• Age: 2.4 -1.6 million years ago. Coexisted with robust †Australopithecusines

†Homo erectus(extinct)

• First hominid found outside of Africa

– Found in 1891 in Java, Indonesia (“Java man”)

– Later found in Zhoukoudian (Chou-Kou-Tien), Near

Peking, China

– European sites such as Terra Amata in France, Dmanisi in Georgia (Country)

– African erectus-like fossils are oldest; often assigned a separate species, †Homo ergaster

– Brain size: average 850 cc

• Age: 1.8 million years – 500,000 years ago

“Archaic Homo sapiens”

• Informal name for hominids with larger brains

than H. erectus/ ergaster

• Divided into several species, such as Homo rhodesiensis and H. heidelbergensis.

Taxonomy is problematic: some fossils could either be †H. erectus or archaic H. sapiens

• First appeared 800,000 years ago; apparently coexisted with †H. erectus in places

• Brain size: 1000 -1200 cc

Denisovans

• First found in Denisova Cave, Altai mountains.

• More robust than Ancient Homo sapiens.

• This population split from Neanderthal lineage about 600,000 years ago, and from Archaic H.

sapiens, 800,000 years ago.

• Show extensive and complicated tool us.

• Show evidence of crossing the Wallace line.

Neanderthals: Closely related species with complex cultures and evidence of interbreeding with modern humans.

• Age: 180,000 - 30,000 years ago

• Brain size: ~1400 cc (same as modern

humans)

• Controversy: Are these are subspecies of Homo sapiens, or a separate species? (and

how can you tell from fossils?)

• First found in Neander River valley, Germany: now known from Europe and Middle East

• “Classic Neandertals” From Europe are physically distinctive (Probably adaptation for

cold climate, and food):

– Long low skull vault

– Weak chin

– Large brow ridges

– Broad projecting mid-facial region

– Short stocky ,powerful , barrel-chested build

• Neandertals from elsewhere (Iraq, Israel) don’t always show these features

Homo Sapiens

• Homo sapiens

  • Age: 100,000 years ago to present

  • Brain size: Avg. 1400 cc

  • Oldest known fossils are in Africa (Omo,

  Ethiopia: ~100,000 years old ) and Israel (Jebel

  Qafzeh, 92,000 years old)

  • Oldest in South Asia 70,000 to 40,000 years.

  • Earliest H. sapeiens found in Europe are

  referred to as “Cro-Magnons” named after a

  site in France

  – Oldest 35,000-40,000 years old

  – Some interbreeding with Neandertal

• Cultural Milestones: Emergence of cave art, musical instruments, and complex burial rituals