EAR 119 Chordate Evolution

EAR 119 Chordate Evolution

1. Cladistic Tree

   • Cladogram
     • Branching diagram (Tree) resulting from a cladistic analysis
   • Clade
     • A complete branch on a cladogram

2. Cladistic terminology

   • Monophyletic
     • When it contains the last common ancestor (LCA) and all of its descendents
   • Paraphyletic
     • When it contains the LCA of all its members but not all the descendants of this LCA
   • Polyphyletic
     • When the descendants of its LCA include LCAs of other groups that are not included in the group in question

3. What makes a Chordate?

   • Radial Cleavage
   • Blastopore becomes anus
   • Enterocoelic coelom
   • Mesodermal skeleton
   • 5 synapomorphies (Ancestral characteristics)
     • Dorsal and tubular nerve chord
       • Non-chordates have ventral nerve chords
       • Non-chordates have a solid nerve chord whereas chordates have a hollow chord
       • Placed above the notochord in chordates
     • Notochord
       • Composed of connective tissue, fluid-filled vacuoles and collagen
       • Functions as a structural rod that prevents compression along the anterior-posterior (a-p) eaxis while being flexible sideways
       • Muscle contraction along the a-p axis thus results in a sweeping tail movement that serves as locomotion (motion) in some basal chordates
     • Pharyngeal slits
       • Pharynx = part of the digestive tract immediately behind the mouth
       • Likely original function to aid feeding
       • Appear in all chordates at some point during development
     • Postanal tail
     • Endostyle or Thyroid gland
       • Endostyle = groove with glands in the pharynx floor
       • Thyroid gland = same embryological origin as endostyle
       • Both involved in controlling metabolism
   • Haeckel, 1874, Arthropogenie

4. Tunicates - Most basal living Chordates

   • Sessile as adults (Cannot move)
   • Can be colonial
   • Lack mineralised hard parts
   • Marine filter feeders
   • No 'head' - anterior region specialised for feeding and containing a nerve centre
   • Notochord - only in larvae

5. Cephalochordates

   • With 'head' but lacking jaw
   • Marine or brackish
   • Vagrant
   • Lack mineralised hard parts
   • Filter feeders

6. Pikaia gracilens

   • musculature
   • notochord
   • possible pharyngial slits
   • Anterior antennae
   • paired anterior appendages

7. Metaspringgina walcotti

   • Myotomes
   • notochord
   • Pair of eyes
   • Cartilagous bars in pharynx associated with gills
   • post-anal tail

8. Agnathans (Jawless fish)

   • biting apparatus derived from pharyngeal bars
   • mouth commonly circular
   • cartilaginous skeleton
   • Muscular pharyngeal pump
   • Hagfish and Lampreys

9. Fossil Agnathans

   • Conodonts (Cambrian - Triassic)
     • Mainly known from their teeth
     • Teeth made of calcium phosphate (apatite)
     • Teeth compose a complex feeding apparatus
     • The conodont animal had no jaw
     • Important for biostratigraphy
   • Ostracoderms (Late Cambrian - Late Devonian)
     • Bone (however no mineralised endoskeleton)
     • Lateral line system
     • Inner ear with semi-circular canals

10. Ganthostomes (Jawed fish) - Mid Ordovician, Silurian - Recent

    • Evolution of jaws represents a major advantage
    • Jaws are derived from the anterior pharyngeal arches
    • Paired fins improved their manoeuvrability
    • 3 main groups
      • Placodermi (Mid Silurian - Late Devonian)
        • Bony dermal plates
        • Teeth lack dental tissue
        • Up to 10m long
        • No living descendants
        • Most important in the Devonian
      • Chondrichthyes (Mid Ordovician - Recent)
        • Cartilaginous skeleton
          • Absence of bone probably a secondary loss as ancestor seem to have possessed bone
        • Non-terminal mouth
        • Scales are pointed or cone-shaped and show no signs of growth
          • This type of scale first appears in the Mid-Ordovician
        • 2 main radiations
          • Early Devonian - End Permian
          • Jurassic - Recent
        • 2 main groups
          • Elasmobranchia (Sharks/Rays)
          • Chimaera (Holocephalans)
            • No scales; gills don't open to the surface
      • Teleostomi (Late Ordovician - Recent)
        • Paraphyletic group as it includes the ancestors to tetrapods
        • Osteichthyes (bony fish) = biggest group of Teleostomi
          • 2 main groups
            • Actinopterygii (Majority of modern fish)
              • Muscles are located within the main body wall (do not extend to fins)
                • Ray-finned fish
              • Palaeonisciformes = primitive actinopterygians with an assymetrical tail, thicker scales, and a different skull configuration
                • Modern Palaeonisciformes = sturgeons
              • Neopterygians - most dominant in Early Mesozoic
                • Different skull for greater jaw mobility
                • More efficient swimmers due to thinner scales and increasingly ossified vertebrae
            • Sarcopterygii (Today a small group, but ancestral to tetrapods)
              • Fins are supported by short internal bony elements
                • Fleshy-finned fish
                • Muscles extended into the fins
              • Never a diverse group
              • Gave rise to the first terrestrial vertebrates
                • Fins not suitable to support their weight on land
        • Most diverse group of vertebrates
        • Possess swim bladders (Chondrichthyes do not)
        • Body usually covered by overlapping scales
        • Terminal mouth
        • Fins are commonly strengthened by fin rays (lepidotrichia)

11. Rhipidistians (Early Devonian - Recent)

    • Paraphyletic group that includes stem group tetrapods
      • Tetrapods
        • Chridium (muscular limb) with well defined joints and digits
        • Labryrinthodonts - paraphyletic group that sits between Sarcopterygian fish and amphibians
          • Therefore basal tetrapods
          • 3-4 main groups
            • Ichthyostegalia
            • Anthracosaurs
            • Temnospondyls
            • Lepospondyls
            • Relationship to modern amphibians
              • Temno - only group to survive into Mesozoic
                • But might be a clade that did not leave any descendants
              • Not totally clear
          • Possessed bony scales (often confined by abdomen)
          • Many had a lateral line system when young
        • Acanthostega: most basal tetrapod known
          • Still possesses fin rays in tail
          • Polydactyle (>5 digits)
          • Weight-bearing shoulder and pelvic girdle
        • Lissamphibia (modern amphibians)
          • Frogs, Salamanders, Caecilians
          • Possess mucous glands in skin (often poisonous)
          • Modern Lissamphibians are very specialised and might not reflect much about their deep ancestry
          • No convincing fossil intermediates connect them to known fossil groups
        • Amniotes
          • Ambryos are enveloped in extra-embyronic membranes
          • Both embyro + membrane commonly in calcified or leathery shelled egg
          • Skull fenestration of Amniotes
            • Diapsid skull
              • One hole
            • Synapsid skull
              • Two holes
            • Anapsid
              • No holes
          • Parareptilia
            • Testudines (turtles) + some fossil groups
            • No temporal fenestrae (anaspid)
            • Might be the most primitive reptilians that live today
          • Eureptilia
            • Most basal = captorhinomorphs
              • Anapsid
              • Share the following features with other eureptilians
                • Long + slender limbs
                • Similar limb + ankle articulation
                • Similar arrangement of skull bones
              • Were small (<20cm) + had many teeth (in jaw + roof of mouth)
              • More independent of water than ancestors
    • Most dominant freshwater predators during the Late Palaeozoic

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