{{Short description|Extinct genus of bony fishes}} {{Automatic taxobox | fossil_range = Pridoli<br>~{{fossil range|418.7|416}} | parent_authority = Märss, 2001 | display_parents = 2 | taxon = Andreolepis | authority = Gross, 1968 | subdivision_ranks = Species | subdivision = * ''A. hedei'' {{small|Gross, 1968}} * ''A. petri'' {{small|Märss, 2001}} | type_species = '''''Andreolepis hedei''''' | type_species_authority = Gross, 1968 }}
'''''Andreolepis''''' is an extinct genus of prehistoric fish, which lived around 420 million years ago.<ref name=kirj/> It was described by Walter Gross in 1968 based on scales found in the Hemse Formation in Gotland, Sweden.<ref name=":0">{{Cite journal|last=Gross|first=Walter|date=1968|title=Fraglich Actinopterygier-Schuppen aus den Silur Gotlands|url=http://dx.doi.org/10.1111/j.1502-3931.1968.tb01736.x|journal=Lethaia|volume=1|issue=2|pages=184–218|doi=10.1111/j.1502-3931.1968.tb01736.x|bibcode=1968Letha...1..184G |issn=0024-1164|url-access=subscription}}</ref> It is placed in the monogeneric family Andreolepididae and is generally regarded as a primitive member of the class Actinopterygii<ref name="The Paleobiology Database">[https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=34968 The Paleobiology Database]</ref> based on its ganoid scale structure; however some new research regards it as a stem group of osteichthyans.<ref name=":1">{{Cite journal|last1=Botella|first1=Hector|last2=Blom|first2=Henning|last3=Dorka|first3=Markus|last4=Ahlberg|first4=Per Erik|last5=Janvier|first5=Philippe|date=August 2007|title=Jaws and teeth of the earliest bony fishes|journal=Nature|language=En|volume=448|issue=7153|pages=583–586|doi=10.1038/nature05989|pmid=17671501|bibcode=2007Natur.448..583B|s2cid=4337868|issn=1476-4687}}</ref>
Researchers have used microremains of an ''Andreolepsis'' to determine its origins and found that it dated back to the late Silurian.<ref name=":2">{{Cite journal|last1=Chen|first1=Donglei|last2=Janvier|first2=Philippe|last3=Ahlberg|first3=Per E.|last4=Blom|first4=Henning|date=August 2012|title=Scale morphology and squamation of the Late Silurian osteichthyanAndreolepisfrom Gotland, Sweden|journal=Historical Biology|volume=24|issue=4|pages=411–423|doi=10.1080/08912963.2012.668187|bibcode=2012HBio...24..411C |s2cid=84869644|issn=0891-2963}}</ref>
''Andreolepis'' was capable of shedding its teeth by basal resorption, which is considered a rather primitive mode of tooth replacement.<ref name=":3">{{Cite journal|last1=Chen|first1=Donglei|last2=Blom|first2=Henning|last3=Sanchez|first3=Sophie|last4=Tafforeau|first4=Paul|last5=Ahlberg|first5=Per E.|date=2016-10-17|title=The stem osteichthyan Andreolepis and the origin of tooth replacement|journal=Nature|volume=539|issue=7628|pages=237–241|doi=10.1038/nature19812|pmid=27750278|bibcode=2016Natur.539..237C|s2cid=205251184|issn=0028-0836}}</ref> This makes it informative about the evolution of teeth.<ref name=":3" />
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.<ref name="kirj" /> Remains have been found in Russia, and ''A. hedei'' fossils have also been uncovered in the Hemse Formation of Sweden, the Himmiste Beds Formation of Estonia, Latvia,<ref name="kirj">{{Cite journal|last=Märss|first=T.|date=2001|title=Andreolepis (Actinopterygii) in the upper Silurian of northern Eurasia|journal=Proceedings of the Estonian Academy of Sciences, Geology|volume=50|issue=3|pages=174–189|doi=10.3176/geol.2001.3.03|s2cid=67798212|doi-access=free}}</ref> and the West Khatanzeya Formation of Nova Zemlya, Russia.<ref name="The Paleobiology Database" /> Other fossils were found in Great Britain, the former of which it was originally described from.<ref name="kirj" /> ''Andreolepis'' fossils have also recently been recovered from Western Australia and were identified as ''Andreolepis'' sp. aff. ''A.'' ''petri'' due to the resemblance to ''A. petri'' scales.<ref>{{Cite journal|last1=Burrow|first1=Carole J.|last2=Turner|first2=Susan|last3=Trinajstic|first3=Kate|last4=Young|first4=Gavin C.|date=2019-02-27|title=Late Silurian vertebrate microfossils from the Carnarvon Basin, Western Australia|url=http://dx.doi.org/10.1080/03115518.2019.1566496|journal=Alcheringa: An Australasian Journal of Palaeontology|volume=43|issue=2|pages=204–219|doi=10.1080/03115518.2019.1566496|bibcode=2019Alch...43..204B |s2cid=134883695|issn=0311-5518|url-access=subscription}}</ref>
== 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.<ref name="kirj" /> Remains of acanthodians, anaspids, heterostracans, osteostracans, thelodonts and bivalves have also been found in the same sediment layers. Examples of encountered vertebrate genera are ''Gomphonchus, Nostolepis, Archegonaspis, Thyestes, Paralogania, Phlebolepis'' and ''Thelodus''.<ref name="kirj" />
== 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,<ref name=":4">{{Cite journal|last=Schultze|first=Hans-Peter|date=2016|title=Scales, Enamel, Cosmine, Ganoine, and Early Osteichthyans|journal=Comptes Rendus Palevol|volume=15|issue=1–2|pages=83–102|doi=10.1016/j.crpv.2015.04.001|bibcode=2016CRPal..15...83S |issn=1631-0683|doi-access=free}}</ref> which was thought to be limited to actinopterygians whereas true enamel is limited to sarcopterygians.<ref name="kirj" /><ref name=":0" /><ref>{{Cite journal|last=Janvier|first=Philippe|date=1978|title=On the oldest known teleostome fish Andreolepis hedei Gross (Ludlow of Gotland), and the systematic position of the lophosteids|journal=ENSV TA Toim. Geol.|volume=27|pages=86–95}}</ref><ref name=":4" /> It has also been suggested to be a basal osteichthyan.<ref name=":1" /><ref name=":2" /><ref name=":3" /><ref>{{Cite journal|last1=Zhu|first1=Min|last2=Zhao|first2=Wenjin|last3=Jia|first3=Liantao|last4=Lu|first4=Jing|last5=Qiao|first5=Tuo|last6=Qu|first6=Qingming|date=2009|title=The oldest articulated osteichthyan reveals mosaic gnathostome characters|url=http://dx.doi.org/10.1038/nature07855|journal=Nature|volume=458|issue=7237|pages=469–474|doi=10.1038/nature07855|pmid=19325627|bibcode=2009Natur.458..469Z|s2cid=669711|issn=0028-0836|url-access=subscription}}</ref><ref>{{Cite journal|last1=Friedman †|first1=Matt|last2=Brazeau ‡|first2=Martin D.|date=2010-01-29|title=A reappraisal of the origin and basal radiation of the Osteichthyes|url=http://dx.doi.org/10.1080/02724630903409071|journal=Journal of Vertebrate Paleontology|volume=30|issue=1|pages=36–56|doi=10.1080/02724630903409071|bibcode=2010JVPal..30...36F |s2cid=86402320|issn=0272-4634|url-access=subscription}}</ref> 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.<ref name=":3" />
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.<ref name=kirj/> These two genera form the oldest and most basal osteichthyans that are known thus far.<ref name=":1" />
{{Clade|{{cladogram|title=Phylogeny of Gnathostomes |caption=Simplified Phylogeny based on Chen, D., Blom, H., Sanchez, S. et al. The stem osteichthyan Andreolepis and the origin of tooth replacement. Nature 539, 237–241 (2016). |clades= {{Clade | {{Clade |1=Placoderms |2={{Clade |1=Placoderms |2={{Clade |1=''Entelognathus'' |2={{Clade|1={{Clade |1=Acanthodians |2= Crown Chondrichthyes}} |2={{Clade |1=''Lophosteus'' |2={{Clade |1=''Andreolepis'' |2= Crown Osteichthyes}} }} }} }} }} }} }} }}|style=font-size:100%;line-height:100%}}
== Evolutionary significance ==
=== Squamation and scale characteristics === The scales of ''Andreolepis'' have a rhombic shape and contain a thin monolayer of ganoine.<ref name="kirj" /> The squamation pattern has been divided into ten morphotypes, each presumably covering a distinct section of the body.<ref name=":2" /> 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.<ref name=":2" />
=== 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.<ref name=":1" /> 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.<ref name=":5">{{Cite journal|last1=Cunningham|first1=John A.|last2=Rücklin|first2=Martin|last3=Blom|first3=Henning|last4=Botella|first4=Hector|last5=Donoghue|first5=Philip C. J.|date=2012-05-23|title=Testing models of dental development in the earliest bony vertebrates, Andreolepis and Lophosteus|journal=Biology Letters|volume=8|issue=5|pages=833–837|doi=10.1098/rsbl.2012.0357|pmid=22628098|pmc=3440983|issn=1744-9561|doi-access=free}}</ref> 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.<ref name=":5" /> 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.<ref name=":3" />
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.<ref name=":3" /> 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.<ref name=":3" />
Fossils including those of ''Andreolepis'' together with genetic inferences also helped to elucidate the evolution of enamel.<ref name=":6">{{Cite journal|last1=Qu|first1=Qingming|last2=Haitina|first2=Tatjana|last3=Zhu|first3=Min|last4=Ahlberg|first4=Per Erik|date=2015-09-23|title=New genomic and fossil data illuminate the origin of enamel|url=http://dx.doi.org/10.1038/nature15259|journal=Nature|volume=526|issue=7571|pages=108–111|doi=10.1038/nature15259|pmid=26416752|bibcode=2015Natur.526..108Q|s2cid=4457535|issn=0028-0836|url-access=subscription}}</ref> 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.<ref name=":6" />
== References == {{Reflist}}
{{Osteichthyes}} {{Taxonbar|from1=Q4756098|from2=Q1523624}}
Category:Prehistoric bony fish genera Category:Transitional fossils Category:Pridoli life Category:Silurian genus first appearances Category:Silurian genus extinctions Category:Silurian bony fish Category:Silurian fish of Europe Category:Silurian Estonia Category:Fossils of Estonia Category:Silurian Russia Category:Fossils of Russia Category:Silurian Sweden Category:Fossils of Sweden Category:Fossil taxa of Gotland Category:Fossil taxa described in 1968