''Asterolepis'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

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 coi ...

placoderms from the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

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 relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

and

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

. 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 ancient Greek, βένθος (bénthos), meaning "t ...

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 distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

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 and they were first named Chelonichthys. Later very fine specimens were found in the

Old Red Sandstone The Old Red Sandstone 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 northeastern seaboard of North America. It also exte ...

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 abandoned 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 this the incus and malleus are developed. Meckel's cartilage arises from the first ...

are present in the ''Asterolepis'' that are important features shared between antiarchs and other

Gnathostomata Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...

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

thorax The thorax or chest is a part of the anatomy of humans, 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 divisions of the cre ...

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 projection ...

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 post ...

. ''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 Sclerotic rings are rings of bone found in the eyes of many animals in several groups of vertebrates, except for mammals and crocodilians. They can be made up of single bones or multiple segments and take their name from the sclera. They are bel ...

. 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 we ...

s like the armour which are modified

cosmoid scales 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 scale (zoology), scales, which can also provide effective Underwater camouflage, camouflage through the us ...

consisting of a bony basal layer, a layer of

dentin Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...

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, bichirs, and coelacanths. It is composed of rod-like, pseudoprismat ...

. 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 located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...

, 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 the Am ...

. 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 th ...

. The ''Asterolepis'' of

Stromness Stromness (, non, Straumnes; nrn, Stromnes) 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. E ...

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 ''Pterichthyodes'' is a genus of antiarch placoderm fishes from the Devonian period. Its fossils have been discovered in Scotland. They were one of the first species recognized for what they were, as their fossils are common in the Old Red Sand ...

''. 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 The Blue Fiord Formation is a geologic formation in Nunavut. It preserves fossils dating back to the Devonian period. See also * List of fossiliferous stratigraphic units in Nunavut This is a list of fossiliferous stratigraphic units in Nunavu ...

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 sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

, 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 be ...

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, slate, and shale. Most of the particles of which the stone is composed are less than and are too sm ...

and black argillaceous

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

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

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

Cuche Formation The Cuche Formation ( es, Formación Cuche, 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 Devo ...

, 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