''Marsdenichthys'' 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 ...

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 ...

tetrapodomorph. Fossils have been found from Mount Howitt in Victoria,

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 ...

from strata that are Givetian-

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 ...

in age. Mount Howitt is an important site that has been the source of many tetrapodomorph fossils, including ''

Beelarongia ''Beelarongia'' is a genus of prehistoric lobe-finned fish which lived during the Late Devonian period (Frasnian stage, about 375 to 385 million years ago). Fossils have been found in Victoria, Australia Australia, officially the Co ...

'' and '' Howittichthys'', both of which were first described from the locality. Mount-howitt-summit

''Marsdenichthys'' was first named in 1985 on the basis of material collected from Mount Howitt. The genus was initially described as a member of the

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 ...

Eusthenopteridae (now known as the

Tristichopteridae Tristichopterids (Tristichopteridae) were a diverse and successful group of tetrapodomorph fishes living throughout the Middle and Late Devonian. They first appeared in the Eifelian stage of the Middle Devonian. Within the group sizes ranged from ...

), the first known from the southern hemisphere. However, the classification of ''Marsdenichthys'' within Tetrapodomorpha has often been debated and there is not yet a consensus on the exact

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

relationships of the genus.

Classification

When it was originally described as a tristichopterid by Long (1985), ''Marsdenichthys'' was considered to be the

basalmost In phylogenetics, basal is the direction of the ''base'' (or root) of a rooted phylogenetic tree or cladogram. The term may be more strictly applied only to nodes adjacent to the root, or more loosely applied to nodes regarded as being close to th ...

member of the family, based on such characters as the presence of extratemporal bones, the size ratio between the parietals and

postparietal Postparietals are cranial bones present in fish and many tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were dermal bones situated along the midline of the ...

s, and the presence of rounded

scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...

with an internal median boss. At the time, this feature was thought to be unique to tristichopterids, but Long suggested that ''Marsdenichthys'' could be considered a specialized "osteolepidid" if the scale morphology was later found to be present in members of the family. Osteolepididae is now known to be

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

along with the superorder

Osteolepiformes Osteolepiformes, also known as Osteolepidida, is a group of prehistoric lobe-finned fishes which first appeared during the Devonian period. The order contains the families Canowindridae, Megalichthyidae, Osteolepididae and Tristichopterida ...

, but several "non-tristichopterid osteolepiformes" have since been found with the scale morphology previously seen only in ''Marsdenichthys''. These include the genera '' Medoevia'', '' Canowindra'', and '' Rhizodopsis''. Based on scale ornamentation, ''Marsdenichthys'' was assigned to the family Rhizodopsidae by Long (1999). However, the only phylogenetic analyses that included ''Marsdenichthys'' either placed it with ''

Eusthenopteron ''Eusthenopteron'' (from el, εὖ , 'good', el, σθένος , 'strength', and el, πτερόν 'wing' or 'fin') is a genus of prehistoric sarcopterygian (often called lobe-finned fishes) which has attained an iconic status from its close ...

'' in a clade within a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

Osteolepiformes, a polytomy with ''Eusthenopteron'' basal to ''

Megalichthys ''Megalichthys'' is a genus of prehistoric lobe-finned fish which lived during the Devonian and Carboniferous periods. It is the type genus of the family Megalichthyidae. The type species is ''M. hibberti''. The species ''M. mullisoni,'' named f ...

'', or a polytomy basal to ''Medoevia'' and '' Gogonasus''. It is possible that the classification of ''Marsdenichthys'' remains unresolved because there is a lack of derived characters in the incomplete Mount Howitt specimens. However, Holland ''et al.'' (2010) recently described new specimens from Mount Howitt that preserve previously unknown derived characters, mainly in the cheek and palate. However, because Holland ''et al.'' (2010) did not provide a phylogenetic analysis using the new specimens, the classification of ''Marsdenichthys'' remains unresolved.

References

{{Taxonbar, from=Q6773256 Prehistoric fish of Australia Prehistoric lobe-finned fish genera Devonian bony fish Canowindrids