Vertebrate Diversity Review

Vertebrate Diversity - Part Two

Learning Objectives

  • Unit Objectives:
    • Explain how the vertebrate body plan evolved.
    • Identify the major groups of vertebrates.
    • Trace the evolutionary history of Homo sapiens.
  • Lecture Objectives:
    • Identify key derived characters of chordates.
    • Describe the evolutionary history of vertebrates.
    • Describe gnathostomes and explain the evolutionary advantage of having a jaw.
    • Explain how the first tetrapods may have arisen.
    • Identify key derived characteristics of amniotes and give examples of amniote groups.
    • Differentiate between monotremes, marsupials, and eutherian mammals.
    • Identify key hominin lineages and describe their characteristics.

Amphibians

  • Definition: Amphibians (class Amphibia) are represented by about 6,150 species in three clades:
    • Salamanders: (Urodela, meaning "tailed ones")
    • Frogs: (Anura, meaning "tail-less ones")
    • Caecilians: (Apoda, meaning "legless ones")
Salamanders
  • Salamanders belong to Urodela and have tails.
  • Habitat: Some are aquatic, while others live on land as adults or throughout their life.
Frogs
  • Frogs (Anura) lack tails.
  • Adaptation: They possess powerful hind legs for locomotion on land.
  • Toads are frogs with leathery skin.
Caecilians
  • Caecilians (Apoda) are characterized by their legless, nearly blind appearance, resembling earthworms.
  • The absence of legs is a secondary adaptation.

Lifestyle and Ecology of Amphibians

  • The term "amphibian" means "double life," referring to the metamorphosis from an aquatic larva to a terrestrial adult.
  • Example: A frog's larval stage (tadpole) is an aquatic herbivore with gills, a lateral line system, and a long finned tail.
    • During metamorphosis, the tadpole develops legs, lungs, a pair of external eardrums, and a carnivorous digestive system.
  • Habitat: Most amphibians are found in damp environments; some are strictly aquatic or terrestrial.
  • Physiology:
    • Moist skin complements lungs for efficient gas exchange.
    • Fertilization is external in most species requiring moist environments for eggs.
    • Parental care varies with some species caring for eggs on their backs, in mouths, or in their stomachs.
  • Conservation Status: Amphibian populations have been declining due to various factors:
    • Disease from chytrid fungus
    • Habitat loss
    • Climate change
    • Pollution
    • At least 9 amphibian species have become extinct in the last 40 years; over 100 may also be extinct.

Amniotes

  • Definition: Amniotes are a group of tetrapods whose living members include reptiles (including birds) and mammals.
  • Named for the key derived character, the amnion, which comprises membranes protecting the embryo.
  • Extraembryonic membranes:
    • Amnion: Encloses the embryo in a fluid-filled sac, decreasing the reliance on an aquatic environment for reproduction.
Amniotic Egg
  • The amniotic egg is a significant adaptation to terrestrial life.
  • Most reptiles and some mammals possess shelled amniotic eggs.
  • Other adaptations:
    • Relatively impermeable skin.
    • Use of rib cage for lung ventilation.

Evolution of Early Amniotes

  • Living amphibians and amniotes diverged from a common ancestor around 350 million years ago.
  • Early amniotes thrived in warm, moist environments and adapted to diverse conditions over time.
  • Description: Earliest forms were predatory, resembling small lizards with sharp teeth; later groups included herbivores.

Reptiles

  • Clade Definition: Includes tuataras, lizards, snakes, turtles, crocodilians, birds, and some extinct groups.
  • Characteristics:
    • Scales create a waterproof barrier.
    • Most reptiles lay shelled eggs on land with internal fertilization.
Thermoregulation
  • Most reptiles are ectothermic, absorbing external heat for body temperature regulation through behavioral adaptations.
  • Birds: Defined as endothermic, able to maintain body temperature metabolically.

Origin and Evolutionary Radiation of Reptiles

  • Fossil evidence shows earliest reptiles existed around 310 million years ago, resembling lizards.
  • Key derived character of diapsids: A pair of holes on each side of the skull behind the eye sockets for muscle attachment.
  • Diapsid Lineage: Comprised of two main lineages:
    • Lepidosaurs: Tuataras, lizards, snakes, and extinct mososaurs.
    • Archosaurs: Turtles, crocodilians, pterosaurs, and dinosaurs.
Archosaurs
  • Pterosaurs: The first tetrapods to exhibit flapping flight, went extinct about 66 million years ago.
  • Dinosaurs: Exhibited considerable diversity, ranging from herbivores with defense adaptations to bipedal carnivorous theropods, including ancestors of birds.
  • Evolutionary Insights:
    • Evidence shows many dinosaurs were agile and fast-moving.
    • Signs of parental care were observed in some dinosaur species.
    • Some anatomical evidence suggests that certain dinosaurs were endotherms.
    • Extinction: Majority of dinosaurs, except birds, became extinct by the end of the Cretaceous (~66 million years ago) possibly due to asteroid impact.

Turtles

  • Distinctive traits:
    • Lack holes in the skull behind the eye socket.
    • Fossil records indicate turtles once possessed these skull holes but lost them through evolution.
    • All turtles have a box-like shell made of fused upper and lower shields to vertebrae, clavicles, and ribs.
  • Adaptations:
    • Some species adapted to desert environments, while others are aquatic (living in ponds and rivers).
    • The largest turtles inhabit the sea and many sea turtle species are endangered.

Crocodilians

  • Crocodilians (alligators and crocodiles) belong to an archosaur lineage dating back to the late Triassic.
  • Presently, they are confined to warm regions.

Lepidosaurs

  • One lineage represented by the tuataras, a group of lizard-like reptiles found exclusively on small islands off New Zealand, facing threats from introduced rats.
  • Major living lineage:
    • Squamates which include lizards and snakes, varying in size from the tiny Jaragua lizard (16 mm) to the Komodo dragon (3 m).
  • Snakes: Legless lepidosaurs that evolved from lizards; characterized by adaptations for prey capture:
    • Chemical sensors
    • Heat-detecting organs
    • Venom
    • Loosely articulated jawbones
    • Elastic skin

Evolution of Birds

  • Birds are endothermic, yet many features of their reptilian anatomy have been modified for flight.
  • Major adaptations include:
    • Wings with keratin feathers.
    • Weight-saving features: absence of a urinary bladder, single ovary in females, small gonads, and loss of teeth.
  • Advantages of flight:
    • Enhanced hunting and scavenging, escape from predators, and migration.
    • Flight requires high energy expenditures, acute vision, and fine muscle control.
  • Complex behaviors: Includes elaborate courtship rituals.
  • Reproductive traits: Fertilization is internal, and eggs must be kept warm through parental brooding.

Mammals

  • Definition: Mammals are represented by more than 5,300 species.
  • Key derived characteristics include:
    • Fur or hair and a subcutaneous fat layer for insulation.
    • Kidneys that conserve water from wastes.
    • Endothermy and a high metabolic rate.
    • Efficient respiratory and circulatory systems.
    • Relatively large brain-to-body size ratio.
    • Extensive parental care.
    • Differentiated teeth.
Monotremes
  • Monotremes include egg-laying mammals like echidnas and the platypus.
  • Unique characteristics:
    • Females lack nipples and secrete milk from glands on their bellies; young suck milk from the female's fur.
Marsupials
  • Marsupials include opossums, kangaroos, and koalas.
  • Reproductive process:
    • The embryo develops in the uterus and is nourished by the placenta.
    • A marsupial is born early in development and completes its growth in a pouch called a marsupium.
    • Marsupium can open to the front or rear depending on species.
  • Australia exhibits convergent evolution leading to diverse marsupials resembling eutherians found in other regions.
Eutherians
  • Eutherians possess a more complex placenta compared to marsupials.
  • Young eutherians complete embryonic development in a uterus, connected to the mother via the placenta.

Primates

  • The order Primates includes lemurs, tarsiers, monkeys, and apes, with humans classified among the ape group.
  • Primates share these characteristics:
    • Adaptations for grasping: hands and feet structured for gripping, with flat nails instead of claws.
    • Forward-facing eyes providing depth perception (notably in monkeys and apes).

Primate Taxonomic Organization

  • Three main groups of living primates:
    • Lemurs, lorises, and bush babies.
    • Tarsiers.
    • Anthropoids (monkeys and apes).
Evolutionary History
  • The oldest known tarsier fossils date back to approximately 55 million years, indicating closer kinship with anthropoids than lemurs.
  • First monkeys evolved in the Old World (Africa and Asia).
  • The appearance of monkeys in the New World (South America) occurred roughly 25 million years ago.
  • Old World and New World monkeys underwent separate adaptive radiations due to long-term geographical separation.
Anthropoid Classification
  • The group of anthropoids includes gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans.
  • Apes diverged from Old World monkeys approximately 25–30 million years ago.

Hominins

  • Species Homo sapiens is about 200,000 years old, relatively young when considering life on Earth has existed for 3.5 billion years.
Common Misconceptions
  • Misconception: Early hominins were chimpanzees or evolved directly from them.
    • Correction: Hominins and chimpanzees share a common ancestor.
  • Misconception: Human evolution follows a linear pathway leading directly to Homo sapiens.
    • Correction: Hominin evolution included many branches and coexisting species, although only modern humans survive today.
Derived Characteristics of Hominins
  • Key traits include:
    • Upright posture and bipedal locomotion.
    • Enlarged brains capable of language, symbolic thought, artistic expression, and complex tool usage.
    • Reduced jawbones and jaw muscles for improved speech and tool use.
    • Shortened digestive tracts, reflecting dietary changes.
Closest Living Relative
  • Humans and chimpanzees share about 99% of their genomes.
  • Differences arise from the expression of 19 regulatory genes, where changes can lead to significant morphological and functional effects.

End of Lecture Review

  • Question: Vertebrates and tunicates share _.

    • A) jaws adapted for feeding
    • B) a notochord and a dorsal, hollow nerve cord
    • C) a high degree of cephalization
    • D) an endoskeleton that includes a skull

  • Question: Living vertebrates can be divided into two major clades. Select the appropriate pair.

    • A) the marsupials and the eutherians
    • B) the cyclostomes and the gnathostomes
    • C) the chordates and the tetrapods
    • D) the urochordates and the cephalochordates

  • Question: Unlike eutherians, both monotremes and marsupials _.

    • A) have some embryonic development outside the uterus
    • B) lack nipples
    • C) lay eggs
    • D) are found in Australia and Africa
    • E) include only insectivores and herbivores

  • Question: As hominins diverged from other primates, which characteristic appeared first?

    • A) reduced jawbones
    • B) language
    • C) enlarged brain
    • D) the making of stones
    • E) bipedal locomotion

  • Question: Which option could be considered the most recent common ancestor of living tetrapods?

    • A) a salamander with legs supported by a bony skeleton but moved with side-to-side bending typical of fishes
    • B) a sturdy-finned, shallow-water lobe-fin characterized by skeletal supports similar to terrestrial vertebrates
    • C) an armored, jawed placoderm with two pairs of appendages
    • D) an early terrestrial caecilian with a secondarily evolved legless condition
    • E) an early ray-finned fish that developed bony skeletal supports in paired fins