Megalichthyidae is an extinctfamily oftetrapodomorphs which lived from theMiddle–Late Devonian to theEarly Permian. They are known primarily fromfreshwater 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 megalichthyidMahalalepis is from Antarctica.[5]
Megalichthyids were fairly primitive tetrapodomorphs, retaining a largely fish-like appearance. Like some other primitivesarcopterygians, their bodies were covered in rhomboidscales that possessed a layer ofcosmine (a porous, mineralised tissue). The scales however lacked the peg-and-socket articulations found in some other groups.[3][5]
The skulls of megalichthyids had a rather low and broad shape. Like the scales, thedermal bones of the skull bore a layer of cosmine. Thenostrils were somewhat elongated, slit-like holes that were bordered in the front by thelateral rostral bone and in the back by theposterior tectal bone. Thepremaxilla (one of the bones of the upper jaw) had a well-developed posterior process, and bore a largetusk that either interrupted the main tooth row or was located further inside the mouth, depending on the species. Themaxilla (another bone of the upper jaw, located behind the premaxilla) was rather tall, especially towards the rear end. Thevomers (a pair of bones in thepalate) were short and broad, with a well-developed mesial process in some cases. Thelacrimal bone had a prominent notch. The front end of theparietal bone had a concave shape where it contacted theposterior nasal bone. Thepineal foramen (a hole located at the top of the skull in some vertebrates, containing the parietal eye) was absent. Thesupratemporal bone had a forwards-pointing process that lacked a covering of cosmine. Thesquamosal bone was nearly equal in height and width. Thepreopercular was a fairly short, vertically aligned bone. Theopercular series (a set of bones protecting thegills) 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 thesubmandibular bones (a group of bones lying beneath thedentary bone of the lower jaw). Thepectoral fins were large and had a fan-like shape.[3][5]
Megalichthys, thetype genus of Megalichthyidae, was historically grouped together with various other primitive tetrapodomorphs into the familyOsteolepididae, starting with an 1891 article by the English palaeontologistArthur Smith Woodward. Osteolepididae is nowadays thought to beparaphyletic, being diagnosed mainly by features that are widely distributed among tetrapodomorphs and otherosteichthyans.[5][6]
The family Megalichthyidae was erected in 1902 by the American palaeontologistOliver Perry Hay. Within the family, he included the generaCoelosteus,Eusthenopteron,Megalichthys,Rabdiolepis,Rhizodopsis,Sauripteris andStrepsodus,[7] 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).[8] Megalichthyidae was resurrected as a separate family in a 1992 article by Younget al., who assignedMegalichthys,Ectosteorhachis,Megistolepis and the newly describedMahalalepis to the group.[8] Later studies have disagreed about which genera should be included, but it is widely agreed that at leastMegalichthys,Ectosteorhachis andCladarosymblema belong to Megalichthyidae.[5]
A few genera of "osteolepidids" (e.g.Osteolepis andGogonasus) have often been recovered in a clade with the megalichthyids. This larger clade was given the nameMegalichthyiformes in a 2010 article, and is defined as "including all tetrapodomorph sarcopterygians moreclosely related toMegalichthys thanEusthenopteron".[9]
Below is acladogram adapted from a phylogenetic analysis by Clementet al. (2021). Here, the familyCanowindridae was found to be the sister group of Megalichthyidae, while four "osteolepidid" genera form a basal grade leading to both groups.[5]
"Osteolepididae" | ||
Fossils attributed to megalichthyids have been found mainly infreshwater deposits, dating to between theMiddle–Late Devonian and theEarly Permian. North American representatives of the family includeEctosteorhachis,Megalichthys, and the possible megalichthyidsLohsania,Rhizodopsis[10] andSterropterygion.[11] European megalichthyids includeAskerichthys,Megalichthys,Palatinichthys, and the possible membersCryptolepis,Megistolepis,Megapomus andRhizodopsis.[12] Megalichthyids are also known from Morocco (Megalichthys), Turkey (Sengoerichthys), Australia (Cladarosymblema) and Antarctica (Mahalalepis).[5]
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