osteostracan การใช้

• *" A New Osteostracan Fauna from the Devonian of the Welsh Borderlands and Observations on the Taxonomy and Growth of Osteostraci"
• Most osteostracans had a massive cephalothorac shield, but all Middle and Late Devonian species appear to have had a reduced, thinner, and often micromeric dermal skeleton.
• Most of the ostracoderms, such as thelodonts, osteostracans, and galeaspids, were more closely related to the gnathostomes than to the surviving agnathans, known as cyclostomes.
• Almost nothing is known of the rest of the body, save that it had a pair of well-developed pectoral fins, similar to osteostracans and gnathostome fish.
• Classes of vertebrates listed in chronological order from oldest to most recent include heterostracans, osteostracans, coelolepid agnathans, acanthodians, osteichthyan fishes, chondrichthyan fishes, amphibians, reptiles, mammals, and birds.
• Placoderms also share certain anatomical features only with the jawless osteostracans; because of this, the theory that placoderms are the sister group of chondrichthyians has been replaced by the theory that placoderms are a group of stem gnathostomes.
• French paleontologist Phillipe Janvier placed " B . stensioi " in its own order, "'Brindabellaspida "', because of how the internal anatomy of the braincase resembled more like those of osteostracans or galeaspids than those of other placoderms.
• The dorsal portion of its head armor differs from osteostracans in that the orbits of the eyes are set apart from each other, and that the shield has no pineal foramen ( the " hole " between the eyes of " Cephalaspis " and its relatives ), and that an opening at the base of the rostrum gives very little hints about the nature of the nasal openings.
• It is characterised by a number of features of its armoured headshield, including the presence of raised rims around its eyes, the shape of its lateral and median fields ( depressed areas of the headshield covered by small loose plates, which are common to most Osteostraci ), its prominent dorsal spine, fairly long cornual processes ( backwards pointing spines on each side of the headshield ) and ornamentation . " Janaspis " was fairly small compared with other osteostracans, with a headshield measuring less than 60mm.