Zenaspidida

	Zenaspidida Zenaspidida is an extinct order of osteostracans, a group of jawless stem-gnathostomes. They possessed a distinct headshield, which varied in width to length ratio by species. Description The head shield is dome-shaped and extremely large in comparison to the main body. The abdominal section of this shield has a less developed median dorsal crest. As a rule for this order, the nasohypophysial opening is larger than the nasal division. The pineal plate seen in other osteostracans is barely developed or completely absent. The median dorsal field is notably broad, and the lateral fields are widened in the posterior, but reach back no further than the proximal section of the dorsal surface of the cornual processes. The ornamentation on the head shield can have singular, large tubercles, or groups of tubercles which range in size. This is often used to speciate. Classification The cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram u ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
	Zenaspididae Zenaspididae is an extinct family of jawless fish in the order Zenaspida Zenaspidida is an extinct order of osteostracans, a group of jawless stem-gnathostomes. They possessed a distinct headshield, which varied in width to length ratio by species. Description The head shield is dome-shaped and extremely large in ....A New Osteostracan Fauna From The Devonian Of The Welsh Borderlands And Observations On The Taxonomy And Growth Of Osteostraci. Joseph N. Keating, Robert S. Sansom and Mark A. Purnell, Journal of Vertebrate Paleontology 32(5):1002–1017, September 2012 References External links * Osteostraci Prehistoric jawless fish families {{Paleo-jawless-fish-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
picture info	Zenaspis Powriei ''Zenaspis'' is an extinct genus of jawless fish which existed during the early Devonian period. Due to it being jawless, ''Zenaspis'' was probably a bottom feeder. The fish was around 10 inches (25 cm) in length. It had a horseshoe shaped head that probably sheltered a small brain. References Osteostraci genera Devonian jawless fish Early Devonian fish of Europe {{Devonian-jawless-fish-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
	Zenaspis ''Zenaspis'' is an extinct genus of jawless fish which existed during the early Devonian period. Due to it being jawless, ''Zenaspis'' was probably a bottom feeder. The fish was around 10 inches (25 cm) in length. It had a horseshoe shaped head that probably sheltered a small brain. References Osteostraci genera Devonian jawless fish Early Devonian fish of Europe {{Devonian-jawless-fish-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
picture info	Osteostracan The class Osteostraci (meaning "bony shells") is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian. Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all known agnathans. This is due to the development of paired fins, and their complicated cranial anatomy. The osteostracans were more similar to lampreys than to jawed vertebrates in possessing two pairs of semicircular canals in the inner ear, as opposed to the three pairs found in the inner ears of jawed vertebrates. They are thought to be the sister-group to pituriaspids, and together, these two taxa of jawless vertebrates are the sister-group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]
	Osteostraci The class Osteostraci (meaning "bony shells") is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian. Anatomically speaking, the osteostracans, especially the Devonian species, were among the most advanced of all known agnathans. This is due to the development of paired fins, and their complicated cranial anatomy. The osteostracans were more similar to lampreys than to jawed vertebrates in possessing two pairs of semicircular canals in the inner ear, as opposed to the three pairs found in the inner ears of jawed vertebrates. They are thought to be the sister-group to pituriaspids, and together, these two taxa of jawless vertebrates are the sister-group of gnathostomes. Several synapomorphies support this hypothesis, such as the presence of: sclerotic ossicles, paired pectoral fins, a dermal skeleton with three layers (a basal layer of isopedin, a middle layer of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu]