Tristichopterus is an extinct genus of Devonian eotetrapodiform fish in the family Tristichopteridae. The type species, T. elatus, was first described by Philip Grey Egerton in 1861 from remains of Givetian age found in Scotland. In 2013, a second species, Eusthenopteron kurshi was attributed to the genus based on remains from Latvia, originally assigned to Eusthenopteron, but this has not been universally recognised, With a maximum length of sixty centimetres, is the smallest genus in the family. Tristichopterus was thought by Egerton to be unique for its time period as a fish with ossified vertebral centers, breaking the persistent notochord rule of most Devonian fish. but this was later reinspected and shown to be only partial ossification by Dr. R. H. Traquair. Tristichopterus alatus closely resembles Eusthenopteron and this sparked some debate after its discovery as to whether it was a separate taxon.
History of discovery The two first specimens of Tristichopterus were dug up from sediments of the Old Red Sandstone in Caithness by Charles William Peach and described by Sir Philip Egerton in 1861. A lot of confusion has surrounded this taxon as the first specimens lacked head, fin, and dentition osteology. The original classification by Egerton was to put it in the same family as Dipterus with Coelacanthi. In 1864 and 1865 Peach obtained further specimens of the genus with clear paired fin, head, and dentition osteology that prevented its placement within the Coelacanthi clade with Dipterus. Ramsay Traquair in 1875 instead included Tristichopterus in the Cyclopteridae family. Later Tristichopterus was assigned its own family. Eusthenopteron foordi and Tristichopterus alatus are fairly similar in a number of regards but have a couple features between them that prevented the dissolution of either genus upon E. foordi’s discovery by Whiteaves in 1883. Eusthenopteron can be distinguished from Tristichopterus by the possession of two fang pairs on the ectopterygoid and posterior coronoid, a very long posterior coronoid (twice as long as anterior and middle), and ethmosphenoid longer than the oticooccipital and a more symmetric caudal fin. Eusthenopteron is also distinctly larger than Tristichopterus. The original defining features that were used to initially separate the two were the greater symmetricity of E. foordi’s caudal fin and the presence of two cutting edges in the laniary teeth of E. foordi. In 2013, "Eusthenopteron" kurshi, originally described in 2008 from remains found in Latvia, was reassigned to the Tristichopterus in one study, but this reassignment was not recognised by a later 2014 study.
Description
The general form of Tristichopterus is slender with fins biasing towards the posterior parts of its body. The caudal fin is partially between the heterocercal condition of Dipterus and the diphycercal shape of Sirenidae. The rays that compose the fins of Tristichopterus are fine and innumerable and overlapping as in Dipnoi. The typical specimen is 10.5 inches in length, its head taking up a fifth of that measurement. The greater depth of its body being just behind its subacutely lobate pectorals, measuring at two inches. The first dorsal originates six inches back and the second and 1 and ¼ inches further back. The sets of ventral and anal tails are placed opposite each other. The start of the lower lobe of the caudal fin occurs 8 ¼ inches from the front, its upper lobe beginning a little further back. The pectorals are attached 2 ¼ inches from the front. Most common length of an adult Tristichopterus was thought to be fifteen inches. The head of Tristichopterus sports a cranial buckler for protection, similar to Saurodipterini, and is divided into an anterior, longer, and a posterior, wider, parts. External nares on the original specimen were not observed and thought to be closer to the lip, like in Osteolepis and Diplopterus. On the labial margin of the snout Tristichopterus has small conical teeth. The aforementioned cranial shield on its posterior margin is composed of three parts: one mesial and polygonal in shape and two lateral, triangular in shape. These dermal bones are equivalent to transverse supral temporal chain elements in Polypterus and Lepidosteus. A large oblong plate covers a significant portion of the cheek in front of the opercular bones contacting the hind part of the maxilla. The plate articulates just in front of the orbit with a hyomandibular mechanism similar to Gyroptychius, possibly a pre operculum. The orbit itself is placed very far forward on the skull.
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