Vertebrate Evolution and Diversity

• lobed finned fishes are ancestors of tetrapods
• Order of evolution of traits:

  neurocranium - vertebral elements - jaws - swim bladder (precursor of lungs) - bony appendages - four limbs - amniotic egg
• Key concepts:

  1. chordates have a notochord and a dorsal hollow nerve cord
  2. gnathostomes are vertebrates that have jaws
  3. tetrapods are gnathostomes that have limbs
  4. amniotes are tetrapods that have a terrestrial adapted egg
  5. mammals are amniotes that have hair and produce milk
  6. humans are mammals that have a large brain and bipedal locomotion
• chordates belong to the clade Deuterostomia (have bilateral symmetry) and include all vertebrates and two groups of invertebrates, the urochordates and cephalochordates
• 11 clades of living chordates: cum pca adarm

  1. cephalochordata
  2. urochordata
  3. myxini
  4. petromyzontida
  5. chondrichthyes
  6. actinopterygii
  7. actinistia
  8. dipnoi
  9. amphibia
  10. reptilia
  11. mammalia
• four derived characteristics of chordates : dorsal hollow nerve chord, notochord, pharyngeal slits, post-anal tail
• Subphylum Cephalochordata

  • lancelets/Branchiostoma/ Amphioxus
  • marine suspension feeders by burrowing through the sediment with the cirri upright used for filter feeding
• Subphylum Urochordata

  • tunicates or sea squirts
  • chordate characteristics are present only in larval stage
  • exist either as solitary individual structures or colonies
  • filter feeders: water flows from incurrent siphon to mouth to excurrent siphon
• • Ancestral chordates may have resembled lancelets
  • the same Hox genes that organize the vertebrate brain are expressed in lancelet's simple nerve cord tip
  • genome sequencing suggests that:
  • genes associated with thyroid and heart are common to all chordates
  • genes associated with transmission of nerve impulses are unique to vertebrates
• Hox genes

  • similar arrangement of chromosomes across species
  • homologous and orthologous genes
  • are transcription factor genes that specify bod region and give rise to organs
• Homologous genes: genes that descend from a common ancestral DNA sequencing

  
  

  Orthologous genes: genes evolved from a common ancestral gene by speciation and have retained similar functions in dif species

  
  

  Paralogous genes: genes evolved through duplication event results to difference in sequence composition and function
• 5 general vertebrate groups: fishes, amphibians, reptiles, birds, mammals
• There are 9 major vertebrate classes (2 extinct and 7 extant)
• Class Agnatha

  • Ostracoderms
  • Cyclostomes
• Ostracoderms are jawless fishes
• Ostracoderms are extinct by the end of Devonian period
• Ostracoderms


  Armored fishes without jaws, body is covered with bony dermal armor consisting of broad plates and smaller tile-like scales
• Osteotraci


  • Most likely ancestors of jawed fishes
• Cyclostomes


  • Extant eellike boneless lampreys and hagfishes
• Hagfishes

  Cartilaginous skull and axial rod of cartilage derived from notochord but lack jaws and vertebrae
• Hagfishes


  • Small brain, eyes, ears, and tooth-like formations, buccal funnel, rasping tongue
• Hagfishes are marine and scavenger animals
• Lampreys


  Jawless vertebrates that feed by clamping their mouth onto a live fish and suck blood (parasitic)
• Lampreys


  • Cartilaginous segments surround the notochord and arching partly over the nerve cord (vertebral element)
• Lampreys are marine and freshwater animals
• Conodonts were the 1st vertebrates with mineralized skeletal elements
• Mineralization - skeleton may have formed into either cartilaginous or bony fragments

  • acquires strength and structure
  • reservoir for Ca and P
• Gnathostomes include sharks and their relatives, ray-finned fishes, lobe-finned fishes, amphibians, reptiles (including birds), and mammals
• Class Acanthodii

  • extinct
  • spiny sharks, oldest
  • head and body were protected by a dermal armor of bony plates and scales
  • had stout spines supporting their fins, fixed in place and non-movable (like shark's dorsal fin)
• Class Placodermi

  • also extinct
  • head and neck armored with heavy bony plates
  • jaw has sharp bony blades
• Class Chondrichthyes

  • 2 subclasses: Subclass Elasmobranchii (sharks and rays) and Subclass Holocephali (chimeras and ratfish)
  • cartilaginous fishes skeleton
  • largest and most diverse group of Chondrichthyes includes the sharks, rays, and skates
  • the reproductive tract, excretory system, and digestive tract empty into a common cloaca
  • shark eggs are fertilized internally but embryos can develop in different ways
• Class Osteichthyes (Osteichthyans)

  • the vast majority of vertebrates belong to this clade
  • includes the bony fish and clade of tetrapods
  • aquatic osteichthyans are the vertebrates that we informally call fishes
  • 2 subclasses: Subclass Actinopterygii (ray-finned fishes) and Subclass Sarcpterygii (lobe-finned fishes)
• • Actinopterygii
  • include all familiar aquatic osteichthyans
  • originated during Silurian period
  • the fins, supported by long, flexible rays are modified for maneuvering and defense
• Sarcopterygii

  • muscular pelvic and pectoral fins
  • Silurian period
  • example is coelocanth
• fins of some lobe fins evolved into the limbs and feet of tetrapods
• Derived characters of tetrapods:

  • 4 limbs and feet with digits
  • neck (allows separate movement of head)
  • fusion of pelvic girdle to backbone
  • absence of gills (except for some aquatic species
  • ears for detecting airborne sounds
• Evolutionary adaptations to living on land:

  • improved respiration
  • protective and insulating body coverings
  • more efficient reproduction (on land)
  • paired muscular appendages
• Class Amphibia

  • represented by 6,150 species
  • 3 orders: Order Urodela (salamanders), Order Anura (frogs and toads), Order Apoda (caecilians)
  • external fertilization, eggs need moist environment
• amniotes are a group of tetrapods whose living members are the reptiles (birds, mammals)

  • diverged from synapsids (mammal'-like reptilians)
• Temporal fenestrae are post-orbital opening in the skull that allows muscles to expand and lengthen

  • anapsid - no temporal fenestrae
  • synapsid - one temporal fenestrae
  • diapsid - two temporal fenestrae