Megalichthyidae is an extinct

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

tetrapodomorph The Tetrapodomorpha (also known as Choanata) are 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. Advance ...

s which lived from the Middle– Late Devonian to the Early Permian. They are known primarily from freshwater deposits, mostly in the Northern Hemisphere (Europe, the Middle East, North Africa and North America), but one genus ('' Cladarosymblema'') is known from Australia, and the possible megalichthyid '' Mahalalepis'' is from Antarctica.

Description

Megalichthyids were fairly primitive tetrapodomorphs, retaining a largely fish-like appearance. Like some other primitive

sarcopterygian Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includ ...

s, their bodies were covered in rhomboid

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 ...

that possessed a layer of

cosmine Cosmine is a spongy, bony material that makes up the dentine-like layers in the scales of the lobe-finned fishes of the class Sarcopterygii. Fish scales that include layers of cosmine are known as cosmoid scales. Description As traditionally d ...

(a porous, mineralised tissue). The scales however lacked the peg-and-socket articulations found in some other groups. The skulls of megalichthyids had a rather low and broad shape. Like the scales, the

dermal bone A dermal bone or investing bone or membrane bone is a bony structure derived from intramembranous ossification forming components of the vertebrate skeleton including much of the skull, jaws, gill covers, shoulder girdle and fin spines rays ( le ...

s of the skull bore a layer of cosmine. The

nostril A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbi ...

s were somewhat elongated, slit-like holes that were bordered in the front by the lateral rostral bone and in the back by the posterior tectal bone. The

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

(one of the bones of the upper jaw) had a well-developed posterior process, and bore a large

tusk Tusks are elongated, continuously growing front teeth that protrude well beyond the mouth of certain mammal species. They are most commonly canine teeth, as with pigs and walruses, or, in the case of elephants, elongated incisors. Tusks share ...

that either interrupted the main tooth row or was located further inside the mouth, depending on the species. The

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...

(another bone of the upper jaw, located behind the premaxilla) was rather tall, especially towards the rear end. The

vomer The vomer (; lat, vomer, lit=ploughshare) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right max ...

s (a pair of bones in the

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly separ ...

) were short and broad, with a well-developed mesial process in some cases. The

lacrimal bone The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks of ...

had a prominent notch. The front end of the

parietal bone The parietal bones () are two bones in the skull which, when joined at a fibrous joint, form the sides and roof of the cranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is nam ...

had a concave shape where it contacted the posterior nasal bone. The

pineal foramen A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...

(a hole located at the top of the skull in some vertebrates, containing the parietal eye) was absent. The

supratemporal bone The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) relative to the squamosal and latera ...

had a forwards-pointing process that lacked a covering of cosmine. The

squamosal bone The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral c ...

was nearly equal in height and width. The preopercular was a fairly short, vertically aligned bone. The opercular series (a set of bones protecting the

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s) consisted of three bones, the opercular and two suboperculars. The opercular was large and almost square-shaped. The suboperculars were in contact with the last or second-last of the submandibular bones (a group of bones lying beneath the

dentary bone In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

of the lower jaw). The

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

s were large and had a fan-like shape.

Classification

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 ...

'', the

type genus In biological taxonomy, the type genus is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearing type of a nominal ...

of Megalichthyidae, was historically grouped together with various other primitive tetrapodomorphs into the family Osteolepididae, starting with an 1891 article by the English palaeontologist

Arthur Smith Woodward Sir Arthur Smith Woodward, FRS (23 May 1864 – 2 September 1944) was an English palaeontologist, known as a world expert in fossil fish. He also described the Piltdown Man fossils, which were later determined to be fraudulent. He is not relate ...

. Osteolepididae is nowadays thought to be paraphyletic, being diagnosed mainly by features that are widely distributed among tetrapodomorphs and other osteichthyans. The family Megalichthyidae was erected in 1902 by the American palaeontologist

Oliver Perry Hay Oliver Perry Hay (May 22, 1846 – November 2, 1930) was an American herpetologist, ichthyologist, and paleontologist. Hay was born in Jefferson County, Indiana, to Robert and Margaret Hay. In 1870, Hay graduated with a bachelor of arts from Eu ...

. Within the family, he included the genera '' Coelosteus'', ''

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 ...

'', ''Megalichthys'', '' Rabdiolepis'', '' Rhizodopsis'', '' Sauripteris'' and '' Strepsodus'', most of which are no longer regarded as megalichthyids. The family thereafter went into disuse for a long time, but was treated as a subfamily of Osteolepididae (as Megalichthyinae) by Vorobyeva (1977). Megalichthyidae was resurrected as a separate family in a 1992 article by Young ''et al.'', who assigned ''Megalichthys'', '' Ectosteorhachis'', '' Megistolepis'' and the newly described '' Mahalalepis'' to the group. Later studies have disagreed about which genera should be included, but it is widely agreed that at least ''Megalichthys'', ''Ectosteorhachis'' and '' Cladarosymblema'' belong to Megalichthyidae. A few genera of "osteolepidids" (e.g. ''

Osteolepis ''Osteolepis'' (from el, ὀστέον 'bone' and el, λεπίς 'scale') is an extinct genus of lobe-finned fish from the Devonian period. It lived in the Lake Orcadie of northern Scotland. ''Osteolepis'' was about long, and covered with ...

'' 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 ...

'') have often been recovered in a clade with the megalichthyids. This larger clade was given the name

Megalichthyiformes Megalichthyiformes is an extinct clade of basal tetrapodomorphs which first appeared during the Devonian period. It was named in 2010 by Michael I. Coates and Matt Friedman, who defined it as a stem-based taxon containing all tetrapodomorphs clo ...

in a 2010 article, and is defined as "including all tetrapodomorph sarcopterygians more closely related to ''Megalichthys'' than ''Eusthenopteron''". Below is a

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

adapted from a phylogenetic analysis by Clement ''et al.'' (2021). Here, the family Canowindridae was found to be the sister group of Megalichthyidae, while four "osteolepidid" genera form a basal grade leading to both groups. Megalichthys NT

Distribution

Fossils attributed to megalichthyids have been found mainly in freshwater deposits, dating to between the Middle– Late Devonian and the Early Permian. North American representatives of the family include ''Ectosteorhachis'', ''Megalichthys'', and the possible megalichthyids '' Lohsania'', ''Rhizodopsis'' and '' Sterropterygion''. European megalichthyids include '' Askerichthys'', ''Megalichthys'', '' Palatinichthys'', and the possible members '' Cryptolepis'', '' Megistolepis'', '' Megapomus'' and ''Rhizodopsis''. Megalichthyids are also known from Morocco (''Megalichthys''), Turkey ('' Sengoerichthys''), Australia (''Cladarosymblema'') and Antarctica ('' Mahalalepis'').

References

{{Taxonbar, from=Q1917037 Megalichthyiforms Carboniferous bony fish Prehistoric lobe-finned fish families Late Devonian first appearances Late Devonian animals Cisuralian extinctions Taxa named by Oliver Perry Hay