Anaspida

Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived during the Silurian and Devonian periods.[1] They are classically regarded as the ancestors of lampreys.[2] Anaspids were small marine agnathans that lacked heavy bony shield and paired fins, but have a striking highly hypocercal tail. They first appeared in the early Silurian, and flourished until the Late Devonian extinction,[3] where most species, save for lampreys, went extinct due to the environmental upheaval during that time. Anatomy Unusually for an agnathan, anaspids did not possess a bony shield or armor. The head is instead covered in an array of smaller, weakly mineralized scales.[4] They have large, laterally placed eyes with no sclerotic ring, with the gills opened as a row of holes along either side of the animal, typically numbering anything from 6-15 pairs. The major synapomorphy for the anaspids is the large, tri-radiate spine behind the series of the gill openings An anapsid is an amniote whose skull does not have openings near the temples.[1] Anapsid skull of Caretta caretta (Loggerhead sea turtle), a Testudine While "anapsid reptiles" or "anapsida" were traditionally spoken of as if they were a monophyletic group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related. Scientists still debate the exact relationship between the basal (original) reptiles which first appeared in the late Carboniferous, the various Permian reptiles which had anapsid skulls, and the Testudines (turtles, tortoises, and terrapins). However, it was later suggested the anapsid-like turtle skull may be due to reversion rather than to anapsid descent. The majority of modern paleontologists believe the Testudines are descended from diapsid reptiles which lost their temporal fenestrae. More recent morphological phylogenetic studies with this in mind placed turtles firmly within diapsids,[2][3][4] some p

ace turtles as a sister group to extant archosaurs[5][6] or, more commonly within Lepidosauromorpha.[7][8][9][10][11] Some modern paleontologists still believe that Testudines are the only surviving branch of this ancient evolutionary grade, which includes groups such as procolophonids, millerettids, and pareiasaurs,[12] although that view is not generally accepted. All molecular studies have strongly upheld the placement of turtles within diapsids; some place turtles within Archosauria,[13] or, more commonly, as a sister group to extant archosaurs.[14][15][16][17][18] However, one of the most recent molecular studies, published in 23 February 2012, suggests that turtles are lepidosauromorph diapsids, most closely related to the lepidosaurs (lizards, snakes, and tuataras).[19] Reanalysis of prior phylogenies suggests they classified turtles as anapsids both because they assumed this classification (most of them studying what sort of anapsid turtles are) and because they did not sample fossil and extant taxa broadly enough for constructing the cladogram. Testudines were suggested to have diverged from other diapsids between 200 and 279 million years ago, though the debate is far from settled.[7][14][20] Most of the other reptiles with anapsid skulls, including the millerettids, nycteroleterids, and pareiasaurs, became extinct in the late Permian period by the Permian-Triassic extinction event. But the procolophonids managed to survive into the Triassic. Anaspididae is a family of freshwater crustacean that is endemic to Tasmania, Australia.[1] The family contains 3 genera and 5 species. This group of crustaceans are considered living fossils.[1] They are commonly and collectively known as the Tasmanian anaspid crustaceans. Anaspidids have stalked eyes, long antennae and antennules, and a slender body with no carapace. The two species of Allanaspides[2][3] and the single species of Paranaspides[4] are all listed as vulnerable on the IUCN Red List.