''Marsdenichthys'' 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 ...

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

tetrapodomorph Tetrapodomorpha (also known as Choanata) is a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advanced for ...

. Fossils have been found from

Mount Howitt Mount Howitt, also known as ''Toot-buck-nulluck'' in the Gunai language, is a mountain in Victoria, Australia, named for Alfred William Howitt. Located in the Wonangatta Moroka Unit of the Alpine National Park approximately 170 km north-e ...

Victoria Victoria most commonly refers to: * Queen Victoria (1819–1901), Queen of the United Kingdom and Empress of India * Victoria (state), a state of Australia * Victoria, British Columbia, Canada, a provincial capital * Victoria, Seychelles, the capi ...

, Australia from strata that are

Givetian The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian Stage. It is named after the town of Givet in ...

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 ...

in age. Mount Howitt is an important site that has been the source of many tetrapodomorph fossils, including '' Beelarongia'' and '' Howittichthys'', both of which were first described from the locality. Mount-howitt-summit

The genus ''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 ) 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 ...

Eusthenopteridae (now known as the

Tristichopteridae Tristichopterids (Tristichopteridae) were a diverse and successful group of fish-like tetrapodomorphs living throughout the Middle and Late Devonian. They first appeared in the Eifelian stage of the Middle Devonian. Within the group sizes ranged ...

), 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 () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

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 Tetrapod, tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were Dermal bone, dermal bones situated alo ...

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 ...

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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

along with the superorder

Osteolepiformes Osteolepiformes, also known as Osteolepidida, is a group of prehistoric lobe-finned fishes which first appeared during the Devonian Geological period, period. The order (biology), order contains the families Canowindridae, Megalichthyidae, Osteol ...

, but several "non-tristichopterid osteolepiformes" have since been found with the scale morphology previously seen only in ''Marsdenichthys''. These include the genera '' Medoevia'', ''

Canowindra Situated on the Belubula River, Canowindra (pronounced ) is a historic township and the largest population centre in Cabonne Shire, New South Wales, Cabonne Shire. The town is located between Orange, New South Wales, Orange and Cowra, New South ...

'', and '' Rhizodopsis''. Based on scale ornamentation, ''Marsdenichthys'' was assigned to the family Rhizodopsidae by Long (1999). However, the only

phylogenetic analyses In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as Computational phylogenetics, phylogenetic inference. It infers the relationship among organ ...

that included ''Marsdenichthys'' either placed it with ''

Eusthenopteron ''Eusthenopteron'' (from 'stout', and 'wing' or 'fin') is an extinct genus of prehistoric marine lobe-finned fish known from several species that lived during the Late Devonian period, about 385 million years ago. It has attained an iconic ...

'' in a clade within a

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

Osteolepiformes, a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tree ...

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 ''Gogonasus'' (meaning "snout from Gogo") was a lobe-finned fish known from three-dimensionally preserved 380-million-year-old fossils found from the Gogo Formation in Western Australia. It lived in the Late Devonian period, on what was once a ...

''. 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 Tetrapodomorpha Prehistoric lobe-finned fish genera Late Devonian sarcopterygians Devonian sarcopterygians of Australia