4 general groups, 2 fossil, 2 recent

2 fossil (with bony exoskeleton) Ord-Dev)
          diplorhina--no nasohypophyseal duct
          Astraspida, Pteraspida, etc., Thelodonta
          monorhina--nasohypophyseal duct present
          Cephalaspida and Anaspida
     (not known where Cambrian Anatolepis fits)
2 recent (no bone, aspidin, or dentin)
          1 semicircular canal, 1 nasal opening dorsal
          2 semicircular canals, 1 nasal opening terminal
          fossil Mayomyzon from Mazon Cr. (Bardach & Zangerl 68
                    65mm, larv, ad., no teeth Science 162:1265)
          (Miles: 2nasal sacs in embryos of MYX and PETR)

Ordovician first appearance
Silurian-Devonian abundance
Devonian extinction, 
     (unless Petromyzontiformes, lampreys, are derived from them)

2 groups of armored, jawless vertebrates:

Osteostraci and Heterostraci

     unified bony head shield 
     with 2 eyes, median nostril/hypophyseal op
     pineal eye opening
     electrosensory organs
     2 semicircular canals
     filter feeding gill chamber
     ventral of head with small plates, mouth, no jaws, gill Os
     Stensio polished sections showed gill chambers, sensory sys
          unified perichondral bone around brain case
          (1927 SK Ssvalbard Ishavet)
          (1932 Bull BMNH)
     propulsive heterocercal tail
     dorsal fin
     pectoral (?)fin


     size limited by bone formation--no growth after bone formed
     Denison 52 Fieldiana: bone histology, with cells

     e.g see Aceraspis and Cephalaspis

  Anaspida (and Petromyzontiformes?)

     about 5 families and 10 genera
     size mostly ca 5" to 10"
     fish shape
     share basic characters ith Cephalaspids
     but small plates on head and around mouth
     gill openings in line on sides
     anal fin
     lateral fin folds and hypocercal tail
     e.g. Birkenia

     surface feeder
     continuous growth (but small size)

     a primitive anaspid
     branchial basket like lampreys cartilaginous
     circular mouth matches holes in Dictyocaris (sympatric)
     see Ritchie 1968 Palaeontol 11:21-39

Galeaspida or Engalaspida
     Lower Devonian of China
     head shield with small ventral plates
     multiple gill openings
     no electrosensory fields
     single nasal opening for 2 nasal sacs
     2 semicircular canals (no horizontal canal)

     Aspidin--apatite, but acellular
     large dorsal and ventral plates on head and thorax
     plates thin, form after adult size was reached
     no internal skeleton
     Cambrian Anatolepis
     Ordovician forms in Harding Sandstone, and Black River MI


  Coelolepids, unique scales, fish shapes

  New material from northern Canada
     primitive heads and caudal fins