''Asterolepis'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

antiarch Antiarchi ("opposite anus") is an order of heavily armored placoderms. The antiarchs form the second-most successful group of placoderms after the arthrodires in terms of numbers of species and range of environments. The order's name was coin ...

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

s from the

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 ...

North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating Direction (geometry), direction or geography. Etymology T ...

and

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

and

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

. They were heavily armored flat-headed

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

detritivore Detritivores (also known as detrivores, detritophages, detritus feeders or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...

s with distinctive jointed limb-like

pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

s and hollow spine. The armor plate gives the ''Asterolepis'' a box-like shape. Its pectoral fins are also armored but the caudal and dorsal fin are not. The first fossils were named by M. Eichwald in 1840 after noticing star-like markings on the fossils.

Etymology

"Aster-" means star while "-lepis" means scales. Confusion surrounded the first fossils discovered of this genus, as naturalists were unable to ascertain their place among fishes. In 1844, another author named this same genus ''Chelonichthys''. Later very fine specimens were found in the

Old Red Sandstone Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It ...

of Russia and Professor Asmus of Dorpat sent them to the British Museum, noticing fossils exhibited star-like markings. That is when the name ''Chelonichthys'' was recognized as a synonym for ''Asterolepis'' which Eichwald proposed.

Description

Primitive features such as jaw ossifications, a

palatoquadrate In some fishes, the palatoquadrate is the dorsal component of the mandibular arch, the ventral one being Meckel's cartilage. The palatoquadrate forms from splanchnocranium in various chordates including placoderms and acanthodians. See also * H ...

, and a

Meckel's cartilage In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel's cartilages (right and left; also known as Meckelian cartilages), above which the incus and malleus are located. Meckel's cartilage arises from the fir ...

are present in the ''Asterolepis'' that are important features shared between

antiarchs Antiarchi ("opposite anus") is an order of heavily armored placoderms. The antiarchs form the second-most successful group of placoderms after the arthrodires in terms of numbers of species and range of environments. The order's name was coin ...

and other

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

''.'' Additionally the ''Asterolepis'' has box-like dermal armour covering the head and

thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main di ...

with highly modified pectoral fins that are enclosed in interlocking dermal plates protecting a cartilaginous

endocranium The endocranium in comparative anatomy is a part of the skull base in vertebrates and it represents the basal, inner part of the cranium. The term is also applied to the outer layer of the dura mater in human anatomy. Structure Structurally, t ...

and gill region. Also, the vertebral column and the pectoral girdle are cartilaginous. The ''Asterolepis'' does not have an anal fin or pelvic fins. However it does have a caudal and dorsal fin. Both are covered in similar scales.

Armour plates

The ''Asterolepis'' has multiple interlocking dermal plates that form its body armour; anterior ventro-lateral (AVL) plate, posterior lateral plate, posterior ventro-lateral (PVL) plate, anterior median dorsal (AMD) plate, two anterior dorsolateral (ADL) plates, two mixilateral plates, and one posterior median dorsal (PMD) plate. The area considered to be the face is also armored with interlocking dermal plates; nuchal plate, paired paranuchal plate, paired postmarginal plate, postpineal plate, premedian plate, paired lateral plate, and semilunar plate. The cheek region of the head shield is formed by three bones: prelateral, opercular/ sub-marginal, and infraprelateral plates where the prelateral and submarginal plates form the lateral region and the infraprelateral plates form the ventral region The paired pectoral fins are also plated with plates of the dorsal central series (CD1-CD4), plates of the medial marginal series (MM1-MM4), and the plates of the lateral marginal series (ML1-ML4). The postpineal plate is broader than longer. The premedian plate is thickens as you move posterior. The posterior wall of the premedian plate concaves to form the anterior wall of the orbito-nasal cavity and shows prerostral process. The pineal plate is small, trapezoidal, thin bone that covers the most posterior part of the orbito-nasal cavity. It has a rough ornamented surface and a smooth visceral surface that has a round pineal pit. Also, rough postero-ventro-laterally facing processes in the antero-lateral projections of the plate are present. The medial marginal plate 4 of the pectoral fin displays short spaced spines that go lateral which are absent on the distal part of the plate. The posterior of this plate is narrow unlike the anterior part that is flat and large. The dorsal central plate 1 of the pectoral fin is long and the external dorsal articular area that is anterior to the unornamented area, is finely covered with tiny meshes The distal segment of the pectoral fin armour was shorter than the proximal segment and formed by 13 bones: the third, fourth, and fifth bones of the central dorsal row, central ventral row, middle marginal row, lateral marginal row, and the terminal. These bones were small, elongated, and hexagonal with the external surface usually ridged. The proximal region of the pectoral fins had articular surfaces covered with narrow grooves.

Surface sculpturing of bones

The surface ornamentation on the nuchal plate is tuberculated and on the AVL plate, the ornamentation is tuberculated as there are

tubercle In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projectio ...

s that are arranged in parallel rows. On the postpineal plate some tubercles fuse to form small ridges and some of these radiate from the center of the plate. The middle pit-line groove is well marked and connected to the supratemporal pit-line groove, in turn linked to an external openings for the

endolymphatic duct From the posterior wall of the saccule a canal, the endolymphatic duct, is given off; this duct is joined by the ductus utriculosaccularis, and then passes along the aquaeductus vestibuli and ends in a blind pouch ( endolymphatic sac) on the pos ...

. ''Asterolepis'' is part of the family Asterolepididae which was characterized by tuberculated surface sculpturing. The organization of the small tubercles were in random arrangement, arranged in radial rows that were sometimes positioned on low crests, or was absent from the majority of the bones. This was only seen in ''Asterolepis syasiensis'' where surface sculpturing was absent except on the anterior mediodarsal where relatively low tubercles were present at the anterior margin while the rest of the bone surface was smooth.

Orbito-nasal cavity

The internasal wall of the rostral plate connects the ventral, anterior, and dorsal walls in order to divide the nasal sacs which are relatively small. The nasal sacs are bound between the rostral plate and the rhinocapsular section of the cartilaginous endocranium and are placed some distance from the telencephalon and opened antero-dorsally. They are located posterior to the nares which were located in the anterior part of the nasal sacs. The structure of the orbito-nasal cavity in the ''Asterolepis ornata'' has been studied, providing a detailed description of the premedian, rostral, and pineal plates, and bones of the

sclerotic ring The scleral ring or sclerotic ring is a hardened ring of plates, often derived from bone, that is found in the eyes of many animals in several groups of vertebrates. Some species of mammals, amphibians, and crocodilians lack scleral rings. The rin ...

. The orbito-nasal fenestra is positioned in the middle of the head shield, leaning more towards the anterior end and is usually spectacle-shaped. Eyes are enclosed within the sclerotic capsule and the nasal cavities open directly above the head of the fish. The visceral surface of the sclerotic ring is smooth and bears small pores and pits. The sclerotic ring is open wide ventrally and the sclerotic capsule consists of three fine plates, sclerotic bones one through three (anterior, medial, and lateral). At the Lode Quarry, Latvia, two specimens of ''Asterolepis ornata'' were found to have a fossa deep in the orbital fenestra that is treated to be the hypophysial foramen. The ''Asterolepis'' is blind and eyes and nostrils are directed antero-latero-dorsally. Water would reach the nasal sacs directly through the nostrils and exit the sacs laterally along the anterior process of the sclerotic ring. The tail is covered

ganoid scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as w ...

s like the armour which are modified cosmoid scales consisting of a bony basal layer, a layer of

dentin Dentin ( ) (American English) or dentine ( or ) (British English) () is a calcified tissue (biology), tissue of the body and, along with tooth enamel, enamel, cementum, and pulp (tooth), pulp, is one of the four major components of teeth. It i ...

e, and an outer layer of

ganoine Ganoine or ''ganoin'' is a glassy, often multi-layered mineralized tissue that covers the scales, cranial bones and fin rays in some non-teleost ray-finned fishes, such as gars and bichirs, as well as lobe-finned coelacanths. It is composed of ...

. Since the tail doesn't have a high degree of mineralization in comparison to the scales of the internal skeleton making then less likely to be preserved in the fossil record. The tail is also has a

dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

, which is an identifying characteristic of Antiarchs.

Juvenile ''Asterolepis'' features

The medioventral bone, located at the center of the ventral wall of the armor, is absent form the trunk shield at early developmental stages of the ''Asterolepis ornata''. In juvenile ''Asterolepis'' found at the Lode Quarry, Latvia, the caudal region was covered with very small, rounded scales. The central row of the distal segment in the pectoral fins of juvenile ''Asterlepis ornata'' are fused together unlike their adult counterparts.

Taxa

''Asterolepis chadwiki'' remains were found in the lower part of the Upper Devonian continental Kataberg Formation in

Sullivan County, New York Sullivan County is a county in the U.S. state of New York. As of the 2020 census, the population was 78,624. The county seat is Monticello. The county's name honors Major General John Sullivan, who was labeled at the time as a hero in th ...

. These remains were the first record of the ''Asterolepis'' in North America. The discovery of ''A. chadwiki'' extended the stratigraphic and geographic range of this characteristically Middle Devonian fish by occurring in the middle of the Senecan Series or middle

Frasnian The Frasnian is one of two faunal stages in the Late Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Givetian Stage and followed by the Famennian Stage. Major reef-building was under way during ...

. The ''Asterolepis'' of

Stromness Stromness (, ; ) is the second-most populous town in Orkney, Scotland. It is in the southwestern part of Mainland, Orkney. It is a burgh with a parish around the outside with the town of Stromness as its capital. Etymology The name "Stromnes ...

is believed to be the oldest organism discovered in the most ancient geological system of Scotland. Several species within the genus have been described: ''Asterolepis concatenata'', ''Asterolepis maximus'', ''Asterolepis ornata'', ''Asterolepis verrucosa'', ''Asterolepis estonica'', ''Asterolepis chadwiki'', ''Asterolepis cornutus'', ''Asterolepis dellei'', ''Asterolepis essica'', ''Asterolepis radiata'', and ''Asterolepis sysasiensis''. Asterolepis has two sister taxa, '' Microbrachium'' and '' Pterichthys''. Both sister taxa are also carnivores and ''Pterichtyhs'' is similarly blind.

Stratigraphic formations

Taxa considered to be sisters to ''Asterolepis'' are ''Microbrachium'' and ''Pterichthys.'' In the Blue Fiord Formation of Canada (Nunavut), ''A. sp'' was discovered and dated to be from the Eifelian period. At the Mikhailovskii Mine, Zheleznogorsk, of the Russian Federation, fossils of ''A. radiata, A. syasiensis, A sp. and A ornata,'' were dated to be from the Lower Frasnian. They were found in beds of

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

, and clay strata with no other faunal remains, underlying Middle Jurassic formations. The

depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...

of the sediments from the Mikhailovskii Mine was lagoonal. The

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

comprised a combination of poorly lithified gray/blue

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...

and black argillaceous

. In Armagh, United Kingdom, ''A. verrucosa'' was discovered in marine

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

and dated to around the Mississippian. Fossils of ''Asterolepis'' have also been found in the

Cuche Formation The Cuche Formation (, Cc) is a geological formation of the Floresta Massif, Altiplano Cundiboyacense in the Eastern Ranges of the Colombian Andes. The sequence of siltstones, shales, and sandstone beds dates to the Late Devonian and Early Carbon ...

, Boyacá, Colombia.

References

{{Taxonbar, from=Q4810771 Antiarchi Placoderm genera Placoderms of Europe Placoderms of North America Paleozoic life of Nunavut Placoderms of South America Devonian animals of South America Devonian Colombia Middle Devonian animals Late Devonian animals Middle Devonian first appearances Late Devonian genus extinctions Fossil taxa described in 1840