Vertebrate Classification and Characteristics

Ch. 18- The Fishes: Vertebrate Success in Water

• Vertebrates are ectotherms.
• Chapter focuses on fishes and their adaptations for aquatic life.

Ch. 19- Amphibians: The First Terrestrial Vertebrates

• Covers the transition of vertebrates from water to land with amphibians as the initial group.

Ch. 20- Nonavian Reptiles: Diapsid Amniotes

• Discusses reptiles, specifically non-avian reptiles, as diapsid amniotes.

Chordate Basics

• Key characteristics of Chordates (the "Big Four"):
  • Notochord: A flexible rod providing structural support.
  • Pharyngeal slits: Grooves in the pharynx.
  • Dorsal tubular nerve cord: A hollow nerve cord located on the back.
  • Post-anal tail: A tail extending beyond the anus.
• Subphyla within Chordata:
  • Urochordata
  • Cephalochordata
  • Craniata

Phylogeny

• Phylogenetic tree showing the relationships among different groups:
  • Common ancestor of chordates gave rise to:
    • Chordates
    • Vertebrates
    • Gnathostomes
    • Osteichthyans
    • Lobe-fins
    • Tetrapods
    • Amniotes
• Key evolutionary milestones:
  • Jaws, mineralized skeleton
  • Vertebrae
  • Lungs or lung derivatives
  • Lobed fins
  • Limbs with digits
  • Amniotic egg
  • Milk

Subphylum Craniata

• Characterized by a skull surrounding:
  • Brain
  • Olfactory organs
  • Eyes
  • Inner ear
• Infraphyla within Craniata:
  • Hyperotreti (Hagfish)
• Vertebrata
• Superclass Petromyzontimorpha (Lamprey)
• Superclass Gnathostomata (Jawed vertebrates)

Phylogeny of Craniata

• One interpretation of the phylogeny of Craniata, emphasizing fishes

  • Includes:
    • Subphylum Cephalochordata
    • Class Myxini
    • Class Petromyzontida
    • Various Ostracoderm taxa
    • Class Placodermi
    • Infraphylum Vertebrata

• Superclass Gnathostomata

  • Class Sarcopterygii
  • Class Chondrichthyes
  • Class Actinopterygii

• Evolutionary innovations:

  • Enamel on teeth
  • Jaws
  • Paired appendages
  • Vertebrae present
  • Three-part brain with cranial nerves
  • Lateral line system
  • Skeleton of endochondral bone
  • Bony operculum

Fishes

• Aquatic craniates (vertebrates) spanning several phylogenetic groups
• Jawless fishes (“Agnatha”)
  • Hagfish - Infraphylum Hyperotreti; Class Myxini
    • Lack vertebrae, have fibrous “skull”
    • 4 pairs of tentacles, copious slime
    • Benthic, scavengers, ecology little known

Lamprey

• Infraphylum Vertebrata
• Class Petromyzontida (=Hyperoartia)
  • Larvae filter feed, spawn in freshwater
  • Adults in Great Lakes or Marine
  • Some parasites, others never feed

Gnathostomes

• Derived Characters of Gnathostomes
• Gnathostomes (“jaw mouth”) are named for their jaws, hinged structures used to grip and slice food
• Jaws are hypothesized to have evolved by modification of skeletal rods that supported the pharyngeal (gill) slits

Fossil Gnathostomes

• The earliest gnathostomes in the fossil record are an extinct lineage of armored vertebrates called placoderms (~440 mya)
• Another group of jawed vertebrates called acanthodians radiated during the Silurian and Devonian periods (444 - 359 mya)
• Three lineages of jawed vertebrates survive today: chondrichthyans, ray-finned fishes, and lobe-fins

Cartilaginous Fishes

• Chondrichthyes
  • Cartilaginous skeleton
  • Placoid scales
  • Male claspers
• Sharks
  • 5-7 gill slits, rostrum
  • Varied lifestyles
  • Conveyer-belt teeth
• Skates & Rays
  • Laterally flattened, expanded fins
  • Many with crushing teeth
• Ratfish
  • Scaleless, buck-teeth, head-claspers, light organs

Bony Fishes

• “Osteichthyes”
  • Bone in skeleton and/or scales
  • Bony operculum over gills
  • Not monophyletic (tetrapods are descendents of Sarcopterygiians)
• Class Actinopterygii “ray-finned fishes”
  • Fins supported by bony rays
  • Highly diverse (>25K species)
• Class Sarcopterygii
  • Lobe-finned fishes
    • Muscular, bony support to fins- homologous with tetrapod legs
• Coelacanth
  • Deep sea, “living fossil”
• Lungfish
  • Specialized lungs allow air breathing
  • Some aestivate during dry spells

Tetrapods

• Vertebrates with four muscular limbs supported by bony girdles
  • Adapted for terrestrial life (at least part of life)
  • Lungs
  • Pectoral & Pelvic girdles
  • First emerge in Devonian Period
• Taxonomic note: All further “class” (etc.) designations are in flux
• Tiktaalik
• Extinct Amphibians
• Carboniferous Tetrapods

Tetrapoda

• Digits in limbs, skull modifications
• Four limbs-terrestrial locomotion
• Movable head and snap-and-grab jaw mechanism
• Numerous extinct taxa
• "Amphibia"
  • Unique skull modifications
• Caudata
• Lissamphibia
• Anura
• Gymnophiona

Class Amphibia

• (= Lissamphibia)
  • Successful group pre-dating Amniotes, most kinds long extinct.
  • 3 extant groups affinities unclear (2 highly specialized)
• Living amphibians have moist, glandular skin devoid of keratinized structures; some produce potent toxins
• Larvae usually aquatic with metamorphosis
• Tw0-chambered heart

Order Anura

• Frogs & Toads
  • “toad” = terrestrial
  • Highly diverse lifestyles
  • Highly modified skeletal structures: leaping legs
• Aquatic larvae: tadpoles
• Reproduction- Diverse too!
• Courtship calls & strange parenting
• Predatory Adults
  • Projectile tongue

Order Caudata

• (=Urodela)
  • Salamanders & Newts
    • Newts= toxic
    • Mimicry
  • Long tail, most with 2 pair limbs
  • Sirens with only 1 pair
  • Feeding- most “grab”, some specialized projectile tongues
• Most with internal fertilization via spermatophores
• Paedomorphosis
  • Axolotls & others
• Most common in Northern hemisphere, mountain regions
• Largest in China

Order Gymnophiona

• (=Apoda)
  • Caecilians
    • Tropical, aquatic or burrowing
    • Legless, nearly blind
    • Modified skulls & musculature
    • Feeding behavior
• Complex parental care
  • Egg-layers and viviparous
  • Maternal feeding

Amniotes

• Tetrapods with amniotic eggs
• Phylogenetic classification named for skull structures
  • Synapsida: single temporal opening- mammals
  • Anapsida: no temporal opening- extinct (formerly contained turtles)
  • Diapsida: two temporal openings, some secondarily lost (ie., turtles)

Developmental Adaptations of Amniotes

• The colonization of land by vertebrates was made possible only after the evolution of the shelled egg
• Embryos are surrounded by fluid in a sac called the amnion
  • This protects the embryo from desiccation and allows reproduction on dry land
  • Mammals and reptiles including birds are called amniotes for this reason

Extraembryonic Membranes

• The four extraembryonic membranes that form around the embryo:
  • The chorion functions in gas exchange
  • The amnion encloses the amniotic fluid
  • The yolk sac encloses the yolk
  • The allantois disposes of waste products and contributes to gas exchange

Amniote Phylogeny

• Extraembryonic membranes: amnion, chorion and allantois; anapsid skull
• Synapsida
  • Mammalia
• Diapsida
  • Lepidosauria
    • Squamata
      • Geckos
      • Lizard lineage
      • Amphisbaenia
      • Snakes
    • Sphenodontia
  • Testudines
    • Turtles
      • Loss of skull openings, plastron and carapace formed by dermal bones fused with axial skeleton
  • Archosauria
    • Crocodylia
    • Dinosuaria
      • Pterosauria+
      • Ornithishia+
      • Saurishia +
      • Aves
        • Unique skull characteristics, muscular gizzard.

The Origin and Evolutionary Radiation of Reptiles

• Fossil evidence indicates that the earliest reptiles lived about 310 mya (Carboniferous)
• The first major group to emerge were parareptiles, which were mostly large, stocky quadrupedal herbivores
• As parareptiles were dwindling, the diapsids were diversifying
• The diapsids consisted of two main lineages: the lepidosaurs and the archosaurs

Lepidosaurs and Archosaurs

• The lepidosaurs include tuataras, lizards, snakes, and extinct mososaurs
• The archosaur lineage produced the crocodilians, pterosaurs, and dinosaurs
  • Pterosaurs were the first tetrapods to exhibit flight
• The dinosaurs diversified into a vast range of shapes and sizes
  • They included bipedal carnivores called theropods, the group from which birds are descended

Testudines

• the Turtles
  • Anapsid condition- but thought secondary
  • Fossil record few clues
  • Long lives, slow reproduction- many with conservation issues
• Modified spine & ribs support shell
• Long necks!
• Toothless beaks
  • All lay eggs (on land)
  • Diverse ecology

Lepidosauria

• Tuatara & Squamates
  • Tuatara: lizard-like Sphenodontia; “living fossils” in New Zealand
  • Squamates: “scaly ones” include the lizards, snakes & amphisbaenians
    • Keratinized scales
    • Paired hemipenes
    • Varied lifestyles, both oviparous and live-bearing

Squamate Diversity

• Varied lifestyles and structures

Snake Skulls & Feeding

• Eating things bigger than their heads

Archosaurs

• Ruling Reptiles
  • Famous extinct members include Pterosaurs and Dinosaurs.
• Living representatives:
  • Crocodylians
  • Birds
    • Hip structures allow bipedality
    • Highly diverse
• Both living groups lay eggs and demonstrate complex parental behavior

Crocodylia

• Few remaining species, all semi-aquatic predators
  • Crocodiles
  • Alligators & Caimans
  • Gavials
• Complicated behavior-
  • Courtship
  • Parental care
  • Predation