| Tristichopteridae | |
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| Teeth ofPlatycephalichthys, inCosmoCaixa Barcelona | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sarcopterygii |
| Clade: | Tetrapodomorpha |
| Clade: | Eotetrapodiformes |
| Family: | †Tristichopteridae Cope,1889 |
| Genera | |
| Synonyms | |
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Tristichopterids (Tristichopteridae) were a diverse and successful group of fish-liketetrapodomorphs living throughout the Middle and LateDevonian. They first appeared in theEifelian stage of the Middle Devonian. Within the group sizes ranged from a few tens of centimeters (Tristichopterus) to several meters (Hyneria andEusthenodon).
Some tristichopterids share some of the features of theelpistostegalians, a diverse clade of tetrapodomorphs close to the origin of (and including) tetrapods.[1][better source needed] This mainly concerns the shape of the skull and a reduction in size of the posterior fins.
An old and persistent notion is thatEusthenopteron was able to crawl onto land using its fins. However, there is no evidence actually supporting this idea.[2] All tristichopterids had become extinct by theend of the Late Devonian.
The Tristichopteridae were medium-sized (Tristichopterus, 30 cm) to very large animals. The smallest forms (such asTristichopterus) attained lengths of 30 cm, and the largest forms (Hyneria) could grow several meters long, possessed teeth up to 5 cm long, and were the largest predators in their faunal communities.[3] Tristichopterids had a strongly developed fin skeleton. In the skeleton of the front fins, which were deeply attached, the elements of the forelimb of the later tetrapodomorphs, such as the humerus, ulna and radius, can already be recognized.
The parts of the bones directed outward from the body trunk were flattened. The pelvis had long, tapering pubic branches (pubic rami) that possibly met in a cartilaginous symphysis. The ventral fins also contained the major bones of the hind legs, femur, tibia, and fibula. Ankle bones (tarsal bones) and toes cannot yet be identified. The limbs were oriented to the side and could only be moved back and forth about 20 to 25°. The anal fin sat on a basal, fleshy peduncle. The caudal fin possessed three lobes, initially asymmetrical in basal forms and becoming more symmetrical externally in later forms. Thescales lacked thecosmine layer found in more primitive tetrapodomorphs. Most of the body was covered in round, overlappingelasmoid scales with a median projection on the inner side. Thicker, enlarged scales occurred on the bases of the fins.[4] The earliest-known fossilised evidence of bone marrow has been found in Eusthenopteron, which may be the origin of bone marrow in tetrapods.[5]
The ontogeny of the Tristichopteridae is relatively well-known, and can be somewhat reliably inferred throughEusthenopteron.[6]Eusthenopteron differs significantly from some later Carboniferous tetrapods in the apparent absence of a recognized larval stage and a definitive metamorphosis. In even the smallest known specimen ofEusthenopteron foordi (at 29 mm), thelepidotrichia cover all of the fins, which does not happen until after metamorphosis in genera likePolyodon. This might indicate thatEusthenopteron developed directly, with the hatchling already attaining the general body form of the adult.[7]
The family Tristichopteridae was named byEdward Drinker Cope in 1889, who established it as the sole family of the orderRhipidistia. In turn, Cope deemed Rhipidistia andActinistia as the two orders forming a superorder he named Rhipidopterygia.[8] In 1955,Lev Berg erected a family named Eusthenopteridae to include the generaTristichopterus andEusthenopteron, and placed this family within the orderRhizodontiformes.[9] More genera would later be placed in Eusthenopteridae, with Vorobyeva and Obruchev (1964) statingEusthenodon and possiblyPlatycephalichthys are members of this family, while Carroll (1988) addedHyneria and tentatively assignedDevonosteus andLitoptychus to the group.[10][11] Currently, Eusthenopteridae is deemed ajunior synonym of Tristichopteridae.[12]
In the past, Tristichopteridae was assigned to the orderOsteolepiformes, a group of tetrapodomorphs that is now consideredparaphyletic.
The following phylogeny is modified from Schwartz (2012). Here,Platycephalichthys is not considered a member of the Tristichopteridae:[1]
| Tristichopteridae | |
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