Anaspids are characterized by a large, tri-radiate spine (red) posteriorly to the series of branchial openings. It is assumed that the most primitive anaspids, such as Pharyngolepis (top), possessed a long, ribbon-shaped, ventrolateral fin-fold (green). More advanced forms, such as Rhyncholepis (bottom), possessed a shorter paired fin-fold (green) and enlarged, spine-shaped, median dorsal scutes. –Philippe Janvier
Anaspida ("shieldless ones") is an extinct group of jawless fish that existed from the early Silurian period to the late Devonian period.[2] They were classically regarded as the ancestors of lampreys,[3] but it is denied in recent phylogenetic analysis,[4] although some analysis show these group would be at least related.[5] 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.[6] Anaspids all had prominent, laterally placed eyes with no sclerotic ring, with the gills opened as a row of holes along either side of the animal, typically numbering anywhere from 6-15 pairs. The major synapomorphy for the anaspids is the large, tri-radiate spine behind the series of the gill openings.[1]
Taxonomy
Now that Jamoytius and its close cohorts, i.e., Euphanerops, have been moved to Jamoytiiformes,[7]Class Anaspida now consists of two orders, the monogeneric Lasaniida, which contains the genus Lasanius and represents a basal anaspid group,[8] and Birkeniida, which contains all other recognized anaspid taxa.[9] 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.[9] Two recently described genera, Kerreralepis[8] and Cowielepis,[10] are considered to be Birkeniida incertae sedis.
↑ Janvier, Philippe (2003). Early Vertebrates. Oxford University Press. ISBN978-0-19-852646-9.
↑ Sansom, Robert S., et al. "Taphonomy and affinity of an enigmatic Silurian vertebrate, Jamoytius kerwoodi White." Palaeontology 53.6 (2010): 1393-1409.
1 2 Blom, Henning. "New birkeniid anaspid from the Lower Devonian of Scotland and its phylogenetic implications." Palaeontology 55.3 (2012): 641-652.
1 2 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.
↑ 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.
↑ Haaramo, Mikko (2003). "†Anaspida – anaspids". In Mikko's Phylogeny Archive. Retrieved November 25, 2018.
Agnatha is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both living (cyclostomes) and extinct species. Among recent animals, cyclostomes are sister to all vertebrates with jaws, known as gnathostomes.
Cephalaspidomorphs are a group of jawless fishes named for Cephalaspis of the osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day. They are the closest relatives of jawed fishes, who emerged from within them and they would survive if the jawed fish are included.
Placoderms are members of the class Placodermi of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches.
Ostracoderms are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic and thus does not correspond to one evolutionary lineage. However, the term is still used as an informal way of loosely grouping together the armored jawless fishes.
Heterostraci is an extinct subclass of pteraspidomorph, Ostracoderm, jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci existed from the mid-Ordovician to the conclusion of the Devonian.
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.
Lasanius is a genus of basal jawless fish from the Early Silurian, around 443.8 million years ago, known from fossils found near Lesmahagow, Scotland. Specimens range from 13.3 to 74.5 mm in length.
Rhyncholepis was an extinct genus of anaspida from the Late Silurian. Fossils of species R. butriangula and R. parvula have been found in Ringerike, Norway, and Oesel, Estonia. The genus has two species, described in 1911 and 2002.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
Traquairaspidiformes is an order of extinct heterostracan agnathan fish known from the Silurian and Early Devonian periods. Fossils are predominantly known from Late Silurian fluvial deposits from Wales and England: some species were also found in strata representing shallow water marine environment in Canada and North America.
Thyestiida is an order of bony-armored jawless fish in the extinct vertebrate class Osteostraci.
Cowielepis is an extinct genus of jawless fish in the class Anaspida. It is from the Cowie Harbour fish bed of Stonehaven, Scotland, which age was originally considered as the Silurian, one study reassigned to Early Devonian (Lochkovian), and 2023 study suggested Wenlock age again.
Birkeniiformes is an extinct order of jawless fish belonging to the class Anaspida.
Septentrioniidae is an extinct family of jawless fish belonging to the order Birkeniiformes.
Septentrionia is an extinct genus of jawless fish belonging to the family Septentrioniidae. It is the type genus of its family.
Liivilepis is an extinct genus of jawless fish belonging to the family Septentrioniidae.
Anaspidomorphi (anaspidomorphs) is an extinct superclass of jawless fish.
Jamoytiiformes is an extinct order of prehistoric jawless fish in the superclass Anaspidomorphi.
Ramsaasalepis is an extinct genus of jawless fish. Its the type and only genus of the family Ramsaasalepididae, and contains the single species Ramsaasalepis porosa.
Tahulalepididae is an extinct family of jawless fish belonging to the order Birkeniiformes.
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