Anaspida ("shieldless ones") is an extinct group of

jawless fish Agnatha (; ) or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both extant taxon, living (Cyclostomi, cyclostomes such as hagfish ...

that existed from the early

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

period to the late

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

period. They were classically regarded as the ancestors of lampreys, but it is denied in recent

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analysis, although some analysis show these group would be at least related. Anaspids were small marine fish that lacked a heavy bony shield and paired fins, but were distinctively hypocercal.

Anatomy

Compared to many other ostracoderms, such as the Heterostraci and Osteostraci, anaspids did not possess a bony shield or armor, hence their name. The anaspid head and body are instead covered in an array of small, weakly mineralized scales, with a row of massive scutes running down the back, and, at least confirmed among the birkeniids, the body was covered in rows of tile-like scales made of aspidine, an acellular bony tissue. Anaspids all had prominent, laterally placed eyes with no sclerotic ring, with the

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s opened as a row of holes along either side of the animal, typically numbering anywhere from 6-15 pairs. The major

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...

for the anaspids is the large, tri-radiate spine behind the series of the gill openings.

Taxonomy

Now that '' Jamoytius'' and its close cohorts, i.e., '' Euphanerops'', have been moved to

Jamoytiiformes ''Jamoytius kerwoodi'' is an extinct species of primitive, eel-like jawless fish known from the Patrick Burn Formation in Scotland, dating to the Llandovery epoch of the Early Silurian period. Long thought of as a "basal anaspid," ''J. kerwoodi' ...

,Sansom, Robert S., et al. "Taphonomy and affinity of an enigmatic Silurian vertebrate, Jamoytius kerwoodi White." Palaeontology 53.6 (2010): 1393-1409.

Class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

Anaspida now consists of two orders, the monogeneric Lasaniida, which contains the genus '' Lasanius'' and represents a basal anaspid group,Blom, Henning. "New birkeniid anaspid from the Lower Devonian of Scotland and its phylogenetic implications." Palaeontology 55.3 (2012): 641-652. and Birkeniida, which contains all other recognized anaspid taxa.Blom, Henning, T. Märss, and C. G. Miller. "Silurian and earliest Devonian birkeniid anaspids from the Northern Hemisphere." Earth and Environmental Science Transactions of the Royal Society of Edinburgh 92.03 (2001): 263-323. Birkeniida is further divided into several families, including Birkeniidae, Pterygolepididae, Rhyncholepididae and Pharyngolepididae, which contain those taxa known from whole body fossils (in addition to several taxa known only from scales) and the family Septentrioniidae, whose subtaxa are known exclusively from scales. Two recently described genera, '' Kerreralepis'' and '' Cowielepis'',Blom, Henning. "A new anaspid fish from the Middle Silurian Cowie Harbour fish bed of Stonehaven, Scotland." Journal of Vertebrate Paleontology 28.3 (2008): 594-600. are considered to be Birkeniida ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

''. Some recent studies have suggested that anaspids are stem- cyclostomes, more closely related to

hagfish Hagfish, of the Class (biology), class Myxini (also known as Hyperotreti) and Order (biology), order Myxiniformes , are eel-shaped Agnatha, jawless fish (occasionally called slime eels). Hagfish are the only known living Animal, animals that h ...

and lampreys than to

jawed fish Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant taxon, extant vertebrates, including all living bony fishes (bot ...

. A newer taxonomy based on the work of Mikko's Phylogeny Archive, Nelson, Grande and Wilson 2016 and van der Laan 2018. Lasanius NT small

* Class †Anaspida Janvier 1996 non Williston 1917 ** Order † Endeiolepidiformes Berg 1940 *** Family † Endeiolepididae Stensiö 1939 corrig. **** Genus †'' Endeiolepis'' Stensiö 1939 ** Order † Birkeniiformes Berg 1940 *** Genus †'' Cowielepis'' Blom 2008 *** Genus †'' Hoburgilepis'' Blom, Märss & Miller 2002 *** Genus †'' Kerreralepis'' Blom 2012 *** Genus †'' Maurylepis'' Blom, Märss & Miller 2002 *** Genus †'' Rytidolepis'' Pander 1856 *** Genus †'' Schidiosteus'' Pander 1856 *** Genus †'' Silmalepis'' Blom, Märss & Miller 2002 *** Genus †'' Vesikulepis'' Blom, Märss & Miller 2002 *** Family † Pharyngolepididae Kiær 1924 corrig. **** Genus †'' Pharyngolepis'' Kiaer 1911 *** Family † Pterygolepididae Obručhev 1964 corrig. **** Genus †'' Pterygolepis'' Cossmann 1920 'Pterolepis'' Kiaer 1911 non Rambur 1838 non De Candolle ex Miquel 1840; ''Pterolepidops'' Fowler 1947 *** Family † Rhyncholepididae Kiær 1924 corrig. **** Genus †'' Rhyncholepis'' Kiær 1911 non Miquel 1843 non Nuttall 1841 *** Family † Tahulalepididae Blom, Märss & Miller 2002 **** Genus †'' Tahulalepis'' Blom, Märss & Miller 2002 **** Genus †'' Trimpleylepis'' Miller, Märss & Blom 2004 *** Family † Lasaniidae Goodrich 1909 **** Genus †'' Lasanius'' Traquair 1898 *** Family † Ramsaasalepididae Blom, Märss & Miller 2003 **** Genus †'' Ramsaasalepis'' Blom, Märss & Miller 2003 *** Family † Birkeniidae Traquair 1899 **** Genus ?†'' Vilkitskilepis'' Märss 2002 **** Genus †'' Ctenopleuron'' Matthew 1907 **** Genus †'' Saarolepis'' Robertson 1945 'Anaspis'' Robertson 1941 non Geoffroy 1762 non Thomson 1893**** Genus †'' Birkenia'' Traquair 1898 *** Family † Septentrioniidae Blom, Märss & Miller 2002 **** Genus †'' Liivilepis'' Blom, Märss & Miller 2002 **** Genus †'' Manbrookia'' Blom, Märss & Miller 2002 **** Genus †'' Ruhnulepis'' Blom, Märss & Miller 2002 **** Genus †'' Spokoinolepis'' Blom, Märss & Miller 2002 **** Genus †'' Septentrionia'' Blom, Märss & Miller 2002

Notes

External links

* * {{Taxonbar, from=Q132837 Prehistoric fish classes Paleozoic jawless fish Llandovery first appearances Early Devonian extinctions