''Andreolepis'' 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 ...
of
prehistoric
Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use of ...
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
, which lived around 420 million years ago.
[ It was described by Walter Gross in 1968 based on scales found in the Hemse Formation in ]Gotland
Gotland (, ; ''Gutland'' in Gutnish), also historically spelled Gottland or Gothland (), is Sweden's largest island. It is also a province, county, municipality, and diocese. The province includes the islands of Fårö and Gotska Sandön to the ...
, Sweden
Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic country located on ...
. It is placed in the monogeneric
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
Andreolepididae and is generally regarded as a primitive member of the class
Class 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 differentl ...
Actinopterygii
Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.
The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...
[The Paleobiology Database](_blank)
/ref> based on its ganoid
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 ...
scale structure; however some new research regards it as a stem group of osteichthyans
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...
.
Researchers have used microremains of an ''Andreolepsis'' to determine its origins and found that it dated back to the late Silurian.
''Andreolepis'' was capable of shedding its teeth by basal resorption
Resorption is the absorption of cells or tissue into the circulatory system, usually by osteoclasts.
Types of resorption include:
* Bone resorption
* Herniated Disc Resorption
* Tooth resorption
* Fetal resorption
* Blood resorption
See also
...
, which is considered a rather primitive mode of tooth replacement. This makes it informative about the evolution of teeth.
Fossil remains are mostly limited to scales, platelets and fragmented bones. At first only the species ''A. hedei'' was described. Scales, platelets and a spine from the Central Urals in Russia have thereafter been assigned to a new species, ''A. petri'', due to differences in fossil morphology. Remains have been found in Russia
Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
, and ''A. hedei'' fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s have also been uncovered in the Hemse Formation of Sweden
Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic country located on ...
, the Himmiste Beds Formation
Himmiste is a village in Saaremaa Parish, Saare County, Estonia. (retrieved 28 July 2021)
Before the administrative reform in 2017, the village was in Lääne-Saare Parish
Lääne-Saare Parish ( et, Lääne-Saare vald) was a rural municipali ...
of Estonia
Estonia, formally the Republic of Estonia, is a country by the Baltic Sea in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the sea across from Sweden, to the south by Latvia, a ...
, Latvia
Latvia ( or ; lv, Latvija ; ltg, Latveja; liv, Leţmō), officially the Republic of Latvia ( lv, Latvijas Republika, links=no, ltg, Latvejas Republika, links=no, liv, Leţmō Vabāmō, links=no), is a country in the Baltic region of ...
, and the West Khatanzeya Formation
West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sun sets on the Earth.
Etymology
The word "west" is a Germanic word passed into some R ...
of Nova Zemlya
Novaya Zemlya (, also , ; rus, Но́вая Земля́, p=ˈnovəjə zʲɪmˈlʲa, ) is an archipelago in northern Russia. It is situated in the Arctic Ocean, in the extreme northeast of Europe, with Cape Flissingsky, on the northern island, ...
, Russia. Other fossils were found in Great Britain
Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It is ...
, the former of which it was originally described from. ''Andreolepis'' fossils have also recently been recovered from Western Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
and were identified as ''Andreolepis'' sp. aff. ''A.'' ''petri'' due to the resemblance to ''A. petri'' scales.
Environment
Fossils of ''Andreolepis'' have been found in marine sediment, indicating that this fish lived in a marine environment, in both shallow and deeper waters. Remains of acanthodians
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, which ...
, anaspids, heterostracans
Heterostraci (Ancient Greek, ἕτερος+ὄστρακον "those itha different shell" i is pl. of -us is an extinct subclass of pteraspidomorph jawless vertebrate that lived primarily in marine and estuary environments. Heterostraci exis ...
, osteostracans, thelodonts
Thelodonti (from Greek: "feeble teeth")Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996. is a class of extinct jawless fishes with distinctive scales instead of large plates of armor.
There is much ...
and bivalves
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
have also been found in the same sediment layers. Examples of encountered vertebrate genera are '' Gomphonchus, Nostolepis, Archegonaspis, Thyestes
In Greek mythology, Thyestes (pronounced , gr, Θυέστης, ) was a king of Olympia. Thyestes and his brother, Atreus, were exiled by their father for having murdered their half-brother, Chrysippus, in their desire for the throne of Olym ...
, Paralogania, Phlebolepis
''Phlebolepis'' is an extinct thelodont agnathan genus belonging to the family Phlebolepididae. Whole fossils are found in Late Silurian (Ludlow epoch) aged strata from Saaremaa, Estonia
Estonia, formally the Republic of Estonia, is a co ...
'' and ''Thelodus
''Thelodus'' (from el, θηλή , 'nipple' and el, ὀδούς , 'tooth') is an extinct genus of thelodont agnathan that lived during the Silurian period. Fossils have been found in Europe, Asia and North America. ''.
Phylogeny
The exact position in the phylogenetic tree is debated. ''Andreolepis'' has been considered a primitive actinopterygian, partly based on scale characteristics and the presence 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 ...
, a homologue to true enamel, which was thought to be limited to actinopterygians whereas true enamel is limited to sarcopterygians. It has also been suggested to be a basal osteichthyan. For example, the teeth lack enamel and have a broad tooth field, well-developed dental organization is absent and tooth production is extraosseous, which are indications that ''Andreolepis'' is located at the base of the osteichthyes.
Gross has formerly placed ''Andreolepis'' in the family Lophosteidae, but considering the distinct differences between the genera ''Lophosteus'' and ''Andreolepis'', the latter was placed in the new family Andreolepididae.[ These two genera form the oldest and most basal osteichthyans that are known thus far.]
Evolutionary significance
Squamation and scale characteristics
The scales of ''Andreolepis'' have a rhombic shape and contain a thin monolayer of ganoine. The squamation pattern has been divided into ten morphotypes, each presumably covering a distinct section of the body. However, some variation in scale morphology might be due to different developmental stages and different species, as no distinction was made between ''A. hedei'' and ''A. petri'' when reconstructing the squamation. Peg-and-socket articulation of the scales is already present in this basal fish genus and its scales are reminiscent of those of actinopterygian scales in early developmental stages, which indicates that developmental heterochrony might have been a mechanism by which differences in scale morphology evolved.
Tooth replacement and evolution of enamel
Previously it was thought that the dentary of ''Andreolepis'' did not contain true teeth, but instead harbored denticles. The lack of teeth and the recognition of initial denticle organisation suggested a basal phylogenetic position within the osteichthyes. It was even argued that the presumed dentary fossil of ''A. hedei'' is uninformative of dental evolution, as the fossil did not represent dental development, but rather development of the dermal skeleton. This would mean the tooth-like structures of ''Andreolepis'' neither match with teeth of chondrichthyans nor with those of osteichthyans and are more similar to the development of structures in dermal scales. Recently it was shown that ''A. hedei'' did have functional teeth that were shed by basal-resorption, something that was overlooked during previous research efforts due to methodological limitations.
The location of the resorption cups, places where resorption of the tooth base took place, and newly formed teeth are not perfectly aligned, which suggests a flexible form of tooth replacement. Nonetheless, some form of patterning can be recognised in the teeth. Multiple layers of resorption cups have been found, which means shedding by resorption could take place multiple times. The presence of a primitive form of tooth development in the most basal osteichthyans sheds light on the manner by which this has evolved.
Fossils including those of ''Andreolepis'' together with genetic inferences also helped to elucidate the evolution of enamel. The scales of ''Andreolepis'' contain the enamel homologue ganoine, but the dermal bones and teeth don't. Moving up in the phylogenetic tree, more derived extinct and extant species show a shift of enamel-containing structures from the scales, to the dermal plate and eventually the teeth, with enamel lost in dermal teeth-like structures and in some cases even in the teeth of the most derived groups of tetrapods and teleosts. This might indicate that enamel first evolved on dermal tissues like scales and only later in teeth.
References
{{Taxonbar, from1=Q4756098, from2=Q1523624
Palaeonisciformes
Prehistoric ray-finned fish genera
Transitional fossils
Pridoli life
Silurian genus first appearances
Silurian genus extinctions
Silurian bony fish
Silurian fish of Europe
Silurian Estonia
Fossils of Estonia
Silurian Russia
Fossils of Russia
Silurian Sweden
Fossils of Sweden
Fossil taxa of Gotland
Fossil taxa described in 1968