Lecture Outline: The Rise Of Animal Diversity

Open PDF Version

The Kingdom Animalia (Metazoa)
1. Definition and Ecological Impact
  1. Animals are highly portable eating machines, making the world a more dangerous place
  2. Timeline and Origin
    1. Animals are approximately 770 million years old, older than plants
    2. Animals originated in the ocean, later moving onto land after plants had established there
2. Classification and Phylogeny
  1. Animalia is a true clade (monophyletic taxon)
  2. The entire group of animals is known as Metazoa
  3. The sister taxon to animals is the Choanoflagellates
    1. These unicellular organisms closely resemble the choanocytes found in sponges
Basal Animal Groups
1. Sponges (Phylum Porifera)
  1. Porifera means "full of holes" or "full of pores"
  2. Sponges represent the basal group of animals
  3. They are multicellular, but lack true tissues (not considered complex multicellularity)
  4. They lack symmetry (unlike nearly all other animals)
  5. Body Organization and Feeding
    1. Water enters through pores into a hollow central cavity
    2. Contain specialized cells, including choanocytes (collar cells) and amoebocytes
    3. Choanocytes use flagella to create water currents, sweeping microscopic food inward
    4. Food is ingested by choanocytes via phagocytosis
    5. Amoebocytes deliver nourishment to other cells within the sponge
2. Cnidarians (and related groups like Ctenophores)
  1. They are members of Eumetazoa, meaning they possess true tissues
  2. Lack organization into complex organs
  3. Exhibit radial symmetry (symmetry based on a central point)
  4. Have a simple body plan featuring a gastrovascular cavity
    1. This cavity has only one opening, serving as both the mouth and the anus
  5. Often use stinging cells to paralyze prey
  6. Examples include jellyfish and anemones
3. Evolutionary Response of Protists
  1. Ancient protests that survived the appearance of animals became larger and more armored due to increased selection pressure
Defining Characteristics of Bilateria
1. Symmetry Types in Eumetazoa
  1. Radial symmetry: Symmetry through a central point (e.g., Cnidarians)
  2. Bilateral symmetry: Symmetry based on a central line, resulting in left/right mirror images (found in most animals, called Bilateria)
2. Germ Layers (Layers of development)
  1. Diploblasts: Animals developing from two germ layers
    1. Ectoderm (outer covering)
    2. Endoderm (inner lining/digestive tract)
  2. Triploblasts: Animals developing from three germ layers (includes most familiar animals, such as humans)
    1. Ectoderm, Endoderm
    2. Mesoderm (intermediate layer, develops into other organs)
3. Body Cavity (Coelom)
  1. Coelomates possess a body cavity (coelom), while Acoelomates do not
  2. The coelom significantly increases body mobility by allowing different internal layers to move without friction
  3. Example: Serous membranes (like the pericardium around the heart) create fluid-filled spaces to prevent damage during movement
4. Embryonic Development (Bilaterian Distinction)
  1. Bilaterians possess a complete digestive tract (mouth and anus)
  2. Development is classified by the formation sequence of the digestive openings
  3. Protostomes ("mouth first"): The first opening develops into the mouth (e.g., Arthropods)
  4. Deuterostomes ("mouth second"): The first opening develops into the anus (includes Chordata/humans)
Phylum Arthropoda: Adaptive Success
1. Arthropods are the most successful animal phylum in terms of adaptive radiation and number of species
2. The vast majority of the 1.5 million identified animal species are arthropods (approximately 1.3 million)
3. General Characteristics
  1. Possess a segmented body
  2. Protected by a hard exoskeleton made of chitin
  3. Often feature numerous legs; the number varies between groups
4. Class Insecta (Hexapods)
  1. Insects alone account for about 1 million species
  2. All insects have exactly six legs (hence the alternative name Hexapods)
  3. Success in flight
    1. Unlike birds and bats (who sacrifice two limbs for wings), insect wings are totally new structures
    2. Insects retain all six legs for walking while adding wings for flight, providing a competitive advantage
Phylum Chordata and the Vertebrate Lineage
1. Chordates are a small minority of animals; Vertebrates are a major sub-group within Chordata
2. Defining Characteristics (present at some point in development)
  1. Notochord: A slightly stiff, flexible rod important for muscle attachment and body undulation (swimming)
  2. Dorsal Hollow Nerve Cord: Develops into the central nervous system (brain and spinal cord)
  3. Post-anal Tail: A tail that extends past the anus
  4. Pharyngeal Slits: Openings in the pharynx used for gas exchange (gills in fish)
3. Non-Vertebrate Chordates
  1. Lancelets: Retain all four characteristics throughout their entire life
  2. Tunicates (Sea Squirts): Change shape as they develop
4. The Vertebrate Tree
  1. Vertebrates are defined by the presence of a vertebral column (spine), which first appeared in early fish
  2. Jawless Fishes (Agnathans)
    1. Primitive vertebrates lacking a hinged jaw
    2. Use a circular mouth for suction (e.g., hagfish, lampreys)
  3. Jawed Vertebrates (Gnathostomes)
    1. The innovation of the jaw greatly increased their success as eating machines
5. Gnathostome Subdivisions (Based on Skeleton)
  1. Cartilaginous Fishes (Chondrichthyans): Skeletons made of cartilage throughout life (e.g., sharks, rays)
  2. Bony Fishes (Osteichthyans): Skeletons made of truly mineralized, hard bone
6. Osteichthyan Subdivisions (Based on Fins)
  1. Ray-finned Fishes (Actinopterygii): Have sharp fins with minimal flesh
  2. Lobe-finned Fishes (Sarcopterygii): Have meaty, club-like fins
The Tetrapods and Land Invasion
1. Tetrapods
  1. Tetrapods ("four feet/limbs") evolved from lobe-finned fish ancestors
  2. Defined by four limbs with digits (fingers and toes)
  3. The invasion of land occurred multiple times in animal history (unlike in plants, which happened only once)
2. Fossil Evidence: The Missing Link
  1. Fossils like Tiktaalik represent a transitional form between ocean life and land-dwelling tetrapods
  2. It displayed both fish characteristics (scales, fins, gills) and tetrapod characteristics (eyes on top of the head, movable neck)
3. Amphibians
  1. Name means "dual life," referring to their lifestyle partly in water and partly on land
  2. Major groups include salamanders, frogs/toads, and Caecilians
  3. Caecilians live entirely underground and have evolved to lose their eyes
4. Amniotes (The lineage including mammals and reptiles/birds)
  1. Amniotes include all tetrapods except amphibians; they are more closely related to mammals than amphibians are
  2. Defined by four extraembryonic membranes present during development (whether or not the egg is hard-shelled)
  3. The Four Membranes
    1. Amnion: Sac surrounding the embryo for cushioning and shock absorption
    2. Allantois: Vascularized membrane that functions as a waste dump
    3. Chorion: Membrane that assists in the exchange of respiratory gases
    4. Yolk Sac: Food store filled with lipids (high energy density)
Mammals and Classification Toward Humans
1. Reptiles
  1. Birds are phylogenetically considered reptiles (specifically, the only surviving dinosaurs)
  2. Turtles (Chelonia) have an uncertain phylogenetic placement within reptiles
2. Mammals
  1. Defining traits: Production of milk and possession of true hair
  2. Three major groups of Mammals:
    1. Monotremes (Basal/Most Primitive)
      1. Lay hard-shelled eggs
      2. Produce milk that oozes directly from the skin (no nipples)
      3. Examples: Echidnas, duck-billed platypus
    2. Marsupials
      1. Give birth to very premature young
      2. Young complete development attached to a nipple inside an external pouch (marsupium)
    3. Eutherians (Placental Mammals, includes humans)
      1. Development occurs entirely within the uterus until birth
      2. Species development spectrum: Precocial (well-developed at birth) versus Altricial (requires extended parental care, like humans)
3. The Primate and Ape Lineage
  1. Humans are Primates, characterized by forward-facing eyes and manipulative hands and feet
  2. A subset of Primates are the Apes (including gibbons, gorillas, chimpanzees, and humans)
  3. Humans are phylogenetically most closely related to chimpanzees
Ecological Interactions and Conservation
1. Co-evolution with Plants
  1. Flowering plants (Angiosperms) and animals have co-evolved a close relationship, especially regarding pollination
  2. Example: A flower with a long tube co-evolved with a moth possessing an equally long proboscis to access nectar
2. Conservation Issues
  1. Overfishing of commercial fish targets older individuals, favoring the evolutionary success of fish that reach sexual maturity younger
  2. The continuous trend toward earlier maturity puts species at risk of extinction
  3. Mollusks (snails and slugs) are the most endangered major animal group
    1. Historically threatened by overharvesting (e.g., for buttons)
    2. Currently threatened primarily by global change, including global warming