Palaeorhynchidae

Members of Palaeorhynchidae were generally medium-sized fish with taxa such as Homorhynchus colei having a standard length of 375 millimetres (14.8 in). The main exception to this is the genus Pseudotetrapturus from the Oligocene of the Caucasus that has been estimated to have a total body length of about 4 metres (13 ft).^[5][6]

The main diagnostic trait in the skulls of Palaeorhynchidae when compared to other groups of billfish is the downturned, spade-like flange on the maxilla. Similar to some modern billfish like marlins, palaeorhynchids possessed rows of teeth on both the upper and lower jaws with the teeth of juveniles being most likely larger than those seen in adults proportionally. The locations of the mouth where these teeth are present on the upper jaw does differ between genera with them being absent on the premaxilla of Palaeorhynchus and only present on the maxilla in Pseudotetrapturus.^[2][1] While the upper and lower jaws of palaeorhynchids are usually around the same length, unlike in modern billfish, the genus Homorhynchus differs with the upper jaw being much longer than the lower jaw.^[3] When fully preserved, the jaw or jaws (depending on the species) take up about 60%-70% of the skull length. The jaws of the fish are usually straight though they curve inwards anteriorly in Pseudotetrapturus.^[4][2]

Unlike the skull, the slender and elongate postcrania of palaeorhynchids is overall very similar to modern billfish. The main difference between them and extant members is the single soft dorsal fin that runs from the skull to around the caudal fin of the fish. This fin, however, is split into two sections with the anterior having much longer fin rays that connect to a fan-shaped pterygiophore that has three ridges. This is in contrast to the much shorter fin rays on the posterior section that each are attached to wedge-shaped pterygiophores that are wider as one gets further from the vertebrae. Almost all of vertebrae, excluding ones that are the most anterior and posterior, possess neural and haemal spines that have a plate-shaped projection at the posterior edge.^[2] The anal fin of palaeorhynchids is similar in morphology to the anterior section of the dorsal fin though it is much shorter. The height of the anal and dorsal fins differ between genera with them being longer than the maximum body depth in Palaeorhynchus and much shorter in Homorhynchus.^[2]

The earliest member of Palaeorhynchidae is Palaeorhynchus zorzini from the Ypresian of Italy, specifically from the famous localities within Monte Bolca. It is generally considered to be the basal most member of the genus, suggesting that the family originally diversified in warm, shallow water within the Tethys Sea. Along with Palaeorhynchus, a number of other genera such as Homorhynchus and Aglyptorhynchus, if a true member of the family, would also appear during the middle to late Eocene with all of these genera being specifically found in the western Paratethys. The family would not reach the eastern Paratethys until the Oligocene with earlier genera along with the genus Pseudotetrapturus being found in sediments in the Caucasus. All generally agreed upon members of the family would go extinct in the Oligocene with the youngest potential member of the family, Aglyptorhynchus, going extinct in the Miocene.^[5]