''Andreolepis'' 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 ...

prehistoric Prehistory, also called pre-literary history, is the period of human history between the first known use of stone tools by hominins million years ago and the beginning of recorded history with the invention of writing systems. The use o ...

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

, 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 Provinces of Sweden, province/Counties of Sweden, county (Swedish län), Municipalities of Sweden, municipality, a ...

Sweden Sweden, formally the Kingdom of Sweden, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. It borders Norway to the west and north, and Finland to the east. At , Sweden is the largest Nordic count ...

. It is placed in the monogeneric

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Andreolepididae and is generally regarded as a primitive member of the

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 ...

Actinopterygii Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class (biology), class of Osteichthyes, bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built ...

The Paleobiology Database
/ref> based on its ganoid scale structure; however some new research regards it as a stem group of osteichthyans. 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 Bone resorption is resorption of bone tissue, that is, the process by which osteoclasts break ...

, 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 country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

, 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 preserve ...

s have also been uncovered in the Hemse Formation of

, the Himmiste Beds Formation of

Estonia Estonia, officially the Republic of Estonia, is a country in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the Baltic Sea across from Sweden, to the south by Latvia, and to the east by Ru ...

Latvia Latvia, officially the Republic of Latvia, is a country in the Baltic region of Northern Europe. It is one of the three Baltic states, along with Estonia to the north and Lithuania to the south. It borders Russia to the east and Belarus to t ...

, and the West Khatanzeya Formation of Nova Zemlya, Russia. Other fossils were found in

Great Britain Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...

, 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 country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

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, anaspids, heterostracans, osteostracans, thelodonts and

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

have also been found in the same sediment layers. Examples of encountered vertebrate genera are '' Gomphonchus, Nostolepis, Archegonaspis,

Thyestes In Greek mythology, Thyestes (pronounced , , ) 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 Olympia. They took refuge ...

, Paralogania, Phlebolepis'' and '' Thelodus''.

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, 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 Prehistoric bony 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