JAWLESS FISHES (NOT "AGNATHA") 4 general groups, 2 fossil, 2 recent 2 fossil (with bony exoskeleton) Ord-Dev) Heterostraci diplorhina--no nasohypophyseal duct Astraspida, Pteraspida, etc., Thelodonta Osteoscraci monorhina--nasohypophyseal duct present Cephalaspida and Anaspida (not known where Cambrian Anatolepis fits) 2 recent (no bone, aspidin, or dentin) Myxiniformes 1 semicircular canal, 1 nasal opening dorsal Petromyzontiformes 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 Osteostraci-- Cephalaspida-- 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 benthic 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 active continuous growth (but small size) Jaymoitius 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) Heterostraci 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 Pteraspids Cyathaspids Amphiaspids Coelolepids, unique scales, fish shapes New material from northern Canada primitive heads and caudal fins