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Lecture 6

BIOC50H3 Lecture Notes - Lecture 6: Heterostraci, Invertebrate, Myllokunmingia

Biological Sciences
Course Code
Jason Weir

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Lec 6 – Early Vertebrates
Vertebrate relatives
-Notochord – stiff, rod-shaped body found in embryos of all chordates – primitive axis of
embryo; persists in some as main axis
-Closest relative – two invertebrate phylum (have a notochord but no spine, skeleton or
-1. Urochordates – larva uses the notochord and muscle fibers in tail-like structure to
swim actively – as adults become most sedentary, attached to the sea floor
-Cephalochordates – notochord important in the adult stages – primitive brain of sensory
-Several adaptations over time (eyes, head -2 , skull -3, spine -4, first bone-like elements
-5, paired fins, cellular bone -6, jaws, pelvic fins -7, bony exoskeleton disappears -8,
endoskeleton of bone -9, lungs (homologous) -10)
Precursors to early Craniates - Haikouichthys and Myllokunmingia (distinct head with small
eyes), craniates are defined by having a skull (either bone or cartilage) (Hagfish), lamprey,
Heterostracans (lacked endoskeleton), Osteostracans (paired fins), Gnathostomes (jawed
-Evolution of the Jaw
-water was taken in at the mouth and passed out through the gill slits
-between each group (gill arch = a thin strip of bone or cartilage that supported the gill)
-probably connected originally with respiration and only at a latter date became important
in feeding
-joint in a forward gill arch may have allowed to mouth to open wider, increasing water
flow over the gills and allowing greater oxygen intake, forming a primitive jaw which later
evolved teeth
Placoderms – carnivorous; extensive armor plating on head and neck
Sharks and Rays
-Early Devonian to Present – lacked bony exoskeleton, endoskeleton of cartilage
Boney Fish
-Late Silurian to present
-Endoskeleton of bone rather than cartilage
-Diversified so rapidly in late Silurian and early Devonian that we have very few fossils of
the earliest forms
-Rayfins – around 24,000 species described – very thin fins composed of webs of skin
supported by numerous thin radiating bones called rays (dominated marine and
freshwater environments)
-Lobefins – fins are stronger than any found in rayfins; three major clades = all extant
-Coelacanth – living found in 1938, second found in 1999; “living fossil”
-Lungfish – closest living sister group to land vertebrates – can breathe air with the aid of
one or two lungs homologous to lungs in land vertebrates; other fish groups breath air
through a vascularized swim bladder
-Osteolepiformes – extinct transitional forms between fish and land vertebrates
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