Chapter 31

Deuterostomes Characteristics

Blastopore becomes the anus, and the mouth forms from a secondary opening during development on the opposite side

• Radial cleavage

• Coelomates

• Coelom develops from mesodermal pockets that bud off during gastrulation

  • Gastrulation: a process that forms the three germ layers

• Triploblastic

• Bilateral symmetry (and pentaradial in adult sea stars)

• Have internal skeletons

• Includes 3 clades:

  • Echinoderms: sea stars, sea urchins, and their relatives

  • Hemichordates: acorn worms and pterobranchs

  • Chordates: sea squirts, lancelets, and vertebrates

Echinoderms

• Pentaradial symmetry (in adult sea stars) and Bilateral symmetry (in sea star larvae and other echinoderms)

• Internal skeleton made of fused calcified plates

• Water vascular system: network of water-filled canals leading to extensions called tube feet

  • Functions: locomotion, feeding, and gas exchange

  • Water enters through the madreporite and enters the ring canal around the esophagus

    • the madreporite is located on the outside aboral side and it filters water into the water vascular system

• They have no head and move equally well in all directions

• The mouth is on the oral side (facing the sea floor) and the anus is on the aboral side

• Crinoids (sea lilies and feather stars) were more abundant and species-rich 300-500 mya

  • Sea lilies attach to the substrate by a stalk

    • substrate: where they attach themselves to

    • they use the tube feet on their arms for filter feeding

  • Feather stars grasp the substratum with flexible appendages that allow limited movement

• Echinozoans (sea urchins, sand dollars, and sea cucumbers)

  • Sea urchins lack arms, have moveable spines that attach to the underlying skeleton by ball-and-socket joints, and some produce toxins

    • sea urchins catch plankton with their tube feet or scrape algae from rocks with rasping structure

  • Sand dollars are flattened relatives of sea urchins

  • Sea cucumbers lack arms, have a body with an anterior mouth and posterior anus

  • They all have tube feet that are used to attach to substrates, rather than using it for locomotion

• Asterozoans (sea stars and brittle stars)

  • Sea stars have gonads and digestive organs located in the arms

    • tube feet are used in locomotion, to capture prey, gas exchange, and attachment

  • Brittle stars have flexible arms composed of jointed plates

• The extinct yunnanozoans may be ancestral deuterostome

• Only 6 groups (of 23, known fossils) survive today; many clades were lost during mass extinctions

Hemichordates

• bilateral symmetry

• three-part body plan: proboscis, collar, and tru

  

  Acorn Worms

  • up to 2m long

  • burrow in soft marine sediment

  • prey is captured with large proboscis, which is covered in sticky mucus

• Some pterobranch species form colonies

• main difference between hemichordates and echinoderms: the way they feed (water entering madreporite vs proboscis for feeding)

Chordates

All chordates possess:

• Dorsal hollow nerve cord: develops into brain, spinal cord, and the CNS

• Notochord: dorsal supporting rod

  • In tunicates, it is lost during metamorphosis to the adult stage

  • In vertebrates, it is replaced by the vertebrae (vertebral column)

• Pharyngeal gill slits: are ancestral but often lost or modified in adults; will develop into gills; helps in the formation of jaws

  • In tunicates and lancelets, the pharynx is used to filter feed

  • In fish and larval amphibians, pharyngeal arches become gill arches that support the gills

  • Pharyngeal arches also develop into elements of the

    vertebrate jaws, parts of the tongue, larynx,

    trachea, eustachian tube, and middle ear

• Post-anal tail: extends beyond the anus

Tunicates (urochordates)

• also known as sea squirts (ascidians)

• Form colonies by budding a single founder. Colonies may be meters across

• The adult body is enclosed in a “tunic” (a protective mucus

  covering) of proteins and polysaccharides secreted by the

  epidermis

• Pharyngeal basket filters prey from the water

• Larvae have pharyngeal slits, a hollow nerve

  cord, and notochord in the tail region

• Adults are sessile

• The swimming, tadpole-like larvae suggest a relationship between tunicates and vertebrates

Lancelets (cephalochordates)

• Very small, less than 5cm

• All chordate synapomorphies are retained throughout life

• Burrow in sand with head protruding; also swim

• Use their pharyngeal gill slits for suspension feeding (aka filter feeding)

  • Enlarged pharynx forms a pharyngeal basket for filtering prey from the water

• Fertilization takes place in the water

Vertebrates

• Vertebral column replaces the notochord during early development

• Anterior skull encloses a large brain

• Rigid internal skeleton supports an extensive muscular system that gets oxygen from the circulatory system and is controlled by the nervous system

• Internal organs suspended in a coelom

• Well-developed circulatory system driven by a ventral heart

• These structural features can support large, active animals, and they allowed vertebrates to diversify widely

• Jawless fish

  • Hagfish

    • weak circulatory system with 3 small hearts, a partial cranium, no stomach, and no jaws

    • skeleton is cartilage - no vertebrae

    • blind → produce large amounts of slime as a defense

    • they have a specialized structure to capture prey and tear up dead organisms

    • development is direct

    • adult can change sex from year to year

  • Lampreys (more advanced than hagfish)

    • have a complete skull and vertebrae (cartilaginous)

    • adults of many species are parasitic; the round mouth is used to attach to fish and rasp at the flesh. some adults are non-feeding (have stored energy from larval stage)

    • live in freshwater or are anadromus (live in salt water but migrate to freshwater to breed)

    • some species are endangered due to loss of critical habitat

    • ectoparasites

• Gnathostomes (jawed fish)

  • jaws evolved from gill arches late in the Ordovician

  • jaws and teeth improved feeding efficiency and prey capture

  • diversified rapidly and became dominant

  • most have paired fins for stabilization and swimming

  • one lineage gave rise to the bony vertebrates with calcified internal skeletons of rigid bone, which split into 2 main lineages: ray-finned fish and lobe-limbed vertebrates

  • Chondrichthyes (sharks, rays, skates, chimaeras)

    • skeletons of cartilage

    • no operculum or swim bladder (oil-rich liver instead)

    • flexible, leathery skin

    • sharks swim using lateral undulations of the body

    • most sharks are predatory; some strain plankton from the water

    • skates and rays swim by flapping enlarged pectoral fins and live on the ocean floor and feed on animals in the sediments

    • chimaeras live in deep-sea cold waters and possess modified dorsal fins that contain toxins

  • Osteichthyes (bony fish)

    • have a swim bladder: organs of buoyancy that allow the fish to maintain its position at specific depths

    • Ray-finned fish:

      • most are covered by scales

      • their gills open to a chamber covered by the operculum. movement of the operculum enhances water flow over the gills

      • very diverse in what they eat: filtering plankton, rasping algae from rocks, eating corals, digging into sediments for smaller animals (predation), eating terrestrial fruits and organisms that fall in the water

      • complex behaviors: maintain schools, build nests, choose mates, and care for young

      • most marine fish move into shallow water to lay eggs – coastal water and estuaries are extremely important

      • some are anadromous (ex: salmon)

    • Lobe-limbed vertebrates

      • have paired pelvis and pectoral fines developed into muscular fins jointed to the body by an enlarged bone. these vertebrates could support themselves in shallow water, and later move on land

        • they are intermediate appendages between fish fins and terrestrial tetrapod limbs

      • the evolution of lung-like sacs in fish, that supplemented their gills, was a crucial step in the transition of vertebrates to land

      • some aquatic lobe-limbed vertebrates began to use terrestrial food sources, became adapted to life on land, and evolved to become ancestral tetrapods

      • Tiktaalik: aquatic

      • Acanthostega: semi-terrestrial

      • tetrapods split into amphibians and amniotes

    • Coelacanths: ancient, lobe-finned fish that were once thought to be extinct, but living ones were found in South Africa

      • have fleshy, limb-like fins and are more closely related to lungfish and tetrapods

      • have a cartilaginous skeleton that is a

        derived feature

    • Lungfish: important in the Devonian; 6 species survive in tropical swamps

      • have lungs and gills: they can burrow in mud when ponds dry up and survive many months in an inactive state while breathing

  Amphibians

  • Most remain tied to moist habitats; they lose water easily through the skin; eggs dry out if exposed to air

  • Some species are entirely aquatic. In others, adults live on land but must return to water to lay eggs; larvae develop in the water (but some species have direct development)

  • There are a variety of reproductive modes and

    parental care

  • Three groups of amphibians:

    • Caecilians – wormlike, limbless, tropical burrowing animals

    • Tailless frogs and toads (anurans)

      • greatest number of species

      • some have tough skins and other adaptations that allow them to live in dry habitats

      • many are arboreal (live in trees); some completely aquatic

      • all have a short vertebral column and pelvic region modified for hopping or kicking in the water

      • males call to attract females and defend territories

      • some species lay a few eggs and guard the nest or carry the eggs on the body

    • Tailed salamanders

      • many live in moist soil and rotting logs

      • one group has lost the lungs and relies on gas exchange through skin and mouth lining

• Amniotes (reptiles, birds, mammals)

  • have features that enable them to conserve water and exploit terrestrial habitats

  • Amniotic egg: relatively impermeable to water, providing a contained aqueous environment for the embryo, while its leathery or brittle shell reduces water evaporation but allows gas exchange

    • stores food in the form of yolk → embryos develop using the energy from the yolk

    • extraembryonic membranes protect the embryo from drying and assist gas exchange and excretion of nitrogen

  • The 4 internal membranes:

    1. amnion: fluid-filled sac surrounding the embryo

    2. yolk sac: contains a rich store of nutrients for the developing embryo

    3. chorion (and allantois): enable the embryo to obtain oxygen from the air and dispose of carbon dioxide

    4. allantois: helps dispose of metabolic waste

    • In mammals, the egg lost its shell entirely and the functions of the extraembryonic membranes were retained and expanded. the yolk sac initially provides nutrients, the allantois contributes to the formation of the umbilical cord, and the chorion forms the majority of the placenta

  • Adult amniotes have tough skin with scales, feathers, or hair to prevent drying

  • Kidneys allow the excretion of concentrated urine and allow the excretion of nitrogen wastes without losing a lot of water

  • During the Carboniferous period, amniotes split into two major groups: the reptiles and mammals

  • Reptiles: half of the living species are birds, the only living descendants of the dinosaurs

    • Lepidosaurs (lizards and snakes)

      • Squamates: lizards and snakes

      • Tuataras: resemble lizards; only one species survives

      • Lepidosaurs have skin covered with horny scales and their gas exchange is only through the lungs

      • the 3-chambered heart partially separates oxygenated from deoxygenated blood. This generates high blood pressure and can sustain a relatively high metabolism

      • most lizards are insectivores, but some are herbivores and predators

      • the largest lizard is the Komodo dragon (East Indies)

      • Snakes are limbless squamates, and all of them are carnivores. many evolved venom glands

    • Turtles: have changed very little since the early Mesozoic

      • dorsal and ventral bony plates form a shell (dorsal shell is an expansion of the ribs)

      • sea turtles come ashore only to lay eggs - human exploitation has resulted in declining populations - all sea turtles are now endangered

    • The two surviving archosaur groups: crocodilians and birds

    • Crocodilians (crocodiles, caimans, gharials, alligators)

      • carnivores

      • mostly stay in water in tropical/warm regions

      • build nests on land or floating piles of vegetation - heat from decaying matter warms the eggs

    • Birds

      • a specialized group of theropods- predatory dinosaurs that were bipedal, had hollow bones, a furcula (wishbone), three-fingered feet and hands, and a pelvis that pointed backward

      • modern birds are endothermic: regulate body temperature by producing and retaining metabolic heat

      • 2 groups of living bird species that diverged in the late Cretaceous period:

        • Palaeognaths: secondarily flightless or weak flyers: tinamou, rhea, emu, kiwi, cassowary, ostrich

        • Neognaths: most retained ability to fly - many more species (the normal birds we see)

    • Archaeopteryx: transitional fossil (extinct) between birds and dinosaurs

      • scales modified into feathers (for insulation)

      • clawed fingers on forelimbs assisted in clambering over tree branches

  • Mammals

    • coexisted with dinosaurs throughout the Mesozoic and diversified rapidly after the mass extinction at the end of the Cretaceous

    • range in size from tiny shrews to blue whales (largest animal on Earth)

    • highly differentiated teeth reflect their varied diets

    • have sweat glands, mammary glands, hair (thickness and amount varies), and a four-chambered heart that completely separates oxygenated from deoxygenated blood

      • in cetaceans (whales and dolphins), layers of fat replace hair for insulation

      • humans learned to use clothing for insulation

    • eggs are fertilized internally and the embryos develop in the female uterus

    • the embryo is contained in an amniotic sac that is homologous to one of the membranes of the amniotic egg

      • a placenta connects the embryo and uterus wall for nutrient and gas exchange and waste elimination via the female’s circulatory system

    • 2 groups of living mammals:

      • Prototherians: duck-billed platypus and echnidas/monotremes - lay shelled eggs and have sprawling legs

      • Therians: all other mammals: Marsupials and Eutherians

    • Marsupials:

      • carry and feed young in a ventral pouch. young are born early, and crawl into pouch for further development

      • most are in Australia and south america

      • the virginia opossum is the only marsupial in north america

      • herbivores, insectivores, and carnivores

      • none live in the oceans and none can fly (but some arboreal species are gliders)

      • kangaroos are the largest marsupials (larger species were exterminated by humans)

      • note: marsupials are considered non-placental mammals but they still have a temporary placenta, which is different than the normal placenta that eutherians have

    • Eutherians (placental mammals: rodents, dolphins, primates, etc.)

      • have placentas; young are more developed at birth than marsupials

      • extremely varied in form and ecology

      • some species grew large and became the dominant terrestrial predators

Viviparous, Oviparous, Ovoviviparous

• Viviparous: give birth to live, well-developed young (ex: humans/mammals)

• Oviparous: lay eggs that develop outside the parent’s body (ex: birds, lizards, sea turtles)

• Ovoviviparous: fertilized eggs are retained within the female's body, but the developing embryos are nourished by the yolk within the egg, rather than directly from the mother (ex: garter snake guppies)

  
  

  note: most anurans are oviparous but some are viviparous

Primates

• One lineage of small, arboreal, insectivorous eutherians underwent extensive adaptive radiation to become the primates

• Grasping limbs with opposable digits was a major adaptation to arboreal life and distinguishing the primates

• Primates are categorized into two main clades:

  • Wet-nosed primates: lemurs, lorises, galagos

  • Dry-nosed primates: tarsiers, new world monkeys, old world monkeys, and apes

    • all new world monkeys are arboreal and many have a prehensile tail (ex: spider monkeys)

    • some old world monkeys are arboreal, while others are terrestrial. none have prehensile tails (ex: mandrills)

    • a prehensile tail is a tail that an animal has adapted to grasp or hold objects

  • Asian apes (gibbons and orangutans) descended from two of the ape lineages

    • Orangutans are the closest living sister groups of modern African apes (gorillas, chimpanzees, and humans)

  • One lineage split to form chimpanzees and the hominin clade (modern humans and extinct relatives)

    • Chimpanzees are the most closely related to humans

  • Earliest protohominins (ardipithecines) had bipedal locomotion:

    • more energetically economical than quadrupled locomotion

    • frees forelimbs to manipulate and carry objects

    • elevates the eyes to look for prey

    • Australopithecines descended from ardipithecines.

      • The most complete skeleton found to date is “Lucy” (Australopithecus afarensis), in Ethiopia, about 3.5 million years ago.

      • A. Afarensis: her discovery changed our evolutionary understanding of human evolution

  • Homo erectus spread as far as eastern Asia - the first hominin to leave Africa

    • they were nearly as large as modern people

    • their brains were smaller with comparatively thick skulls

    • they used fire and stone tools

  • In the lineage leading to Homo sapiens, brain size increased rapidly, while jaw muscles decreased in size

    • neoteny (evolutionary adaptation): adult human skill stays about the same shape as the baby skull and is large relative to other features. in chimpanzees, skull shape changes dramatically with maturation

    • increasing brain size was probably favored by an

      increasingly complex social life. features that increased communication between individuals would have been favored