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CHAPTER PACE I INTRODUCTORY
PRIMITIVE JAWLESS VERTEBRATES
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abbreviations see Figure amphibians ancestors appear aquatic arch archosaurian armor arthrodires artiodactyls birds body bones bony fishes braincase canines carnivores cheek teeth contrast creodonts Cret Cretaceous crossopterygians cusps dentition dermal developed Devonian dinosaurs dorsal early elements elongated Eocene extinct Family feet fins forms fossil front fused gill girdle herbivorous hypocone inches incisors Jurassic known late later lemurs limbs living lower jaw mammalian mammals marsupials Mesozoic Mioc Mioc.-R Miocene molars muscles nasal North America nostrils Old World Olig Oligocene orbit original paired palate Paleocene pelycosaur perissodactyls Perm Permian placental plates Pleist Pleist.-R Pleistocene Plioc Plioc.-R Pliocene posterior premaxilla premolars present presumably probably reduced region reptiles reptilian ribs rodents sauropods sharks side similar skeleton skull length South structure SUBORDER tail tend Tertiary tetrapods thecodont therapsids toes tooth Trias Triassic typical ungulates upper molars usually ventral vertebrates