33 Did they have gills 34 Did they have scales 35 Give the age period name and dates of the earliest fish from these two references 36 Where did Astraspis live List four states where they have been found THE EVOLUTION OF FISH WITH JAWS Jawed fish are gnathostomes as opposed to the ostracoderms or jawless fish The classification of the gnathostomes is shown in Box 12 2 A simplified version of some of the traditional nomenclature is included in blue Box 12 2 Classification of the Gnathostomes GNATHOSTOMES Osteostraci fossil armored jawless vertebrates called ostracoderms Gnathostomata vertebrates with jaws Placodermi placoderm fish armored jawed vertebrates Chondrichthyes sharks and rays Teleostomi teleost fish Acanthodii spiny fish Osteichthyes bony fish Actinopterygii ray finned fish most modern fish Sarcopterygii lobe finned fish and terrestrial vertebrates Class Sarcopterygii Coelacanthimorpha coelacanths Order Coelacanthiformes Dipnoi lung fish Subclass Dipnoi Onychodontiformes extinct fish Porolepimorpha extinct fish Osteolepimorpha extinct lobe finned fishes such Eusthenopteron Rhizodontimorpha extinct lobe finned fishes Terrestrial vertebrates Subclass Tetrapodomorph Janvier P 1997 Gnathostomata Jawed Vertebrates Tree of Life Web Project http tolweb org tree group Gnathostomata contgroup Vertebrata Scroll down to see the diagram relating jaws in red to gill arches in green in this reference Zimmer C 1996 Death From the Pleistocene Sky Discover 17 3 http discovermagazine com 1996 mar deathfromtheplei720 37 What animal is shown in the diagram with jaws and gill arches in the Gnathostomata reference Hint See the caption 38 What does the Zimmer article propose as a reason for the development of jaws