Over 50 species of Saurichthys have been described (see list below). The species differ in size and show variability in their skeletal features. The latter can potentially be ascribed to changes in major developmental genes.[11] The use of subgenera (Eosaurichthys, Costasaurichthys, Lepidosaurichthys, Saurorhynchus, Sinosaurichthys) in the literature reflects differences in morphology between species groups.[12] Several species that were described predominantly in the 19th century are based on fragmentary fossils (often isolated teeth). These are mostly considered invalid species by modern taxonomic standards.
Louis Agassiz, who described the type species of Saurichthys in 1834, named it the "lizard fish" because of skeletal features that he thought were intermediate to reptiles and fishes.
Their exact phylogenetic position is uncertain, though it is agreed that they are not members of Neopterygii. Historically, they have been seen as close relatives of the Acipenseriformes (which includes living sturgeon and paddlefish) as part of the Chondrostei, though this has been strongly questioned by modern studies, which suggests that it may lie outside the Actinopterygii crown group.[5]
Saurichthys was an elongate, streamlined fish, commonly about 0.6 metres (2.0 ft) to 1 metre (3.3 ft) long. Some species were only a few decimetres long (e.g. Saurichthys minimahleri), while others could grow up to about 1.5 metres (4.9 ft) in length (specimen from the Middle Triassic of Turkey).[13][8]
Species of Saurichthys had an elongate bodyform superficially similar to the modern garfish or needlefish. Its dorsal and anal fins were placed opposite each other well back on the body, and the tail was symmetrical (abbreviate diphycercal[14]). These features would have made it a powerful swimmer.[15] Its jaws were extremely long, making up a third of the total body length, and ended in a sharp, beak-like tip. Two to six longitudinal scale rows are developed, with small scales in between in some species.
The axial skeleton consists of ossified neural and haemal arches. Haemal arches may develop spines. The neural arches often show spines as well as other projections interpreted as prezygapophyses and postzygapophyses. Ossified centra are missing. The axial skeleton shows regionalization, meaning that there are differences in bone morphology between segments of the axial skeleton. Some species show dedifferentiation of the axial skeleton.[16]
Saurichthys is classically interpreted as an ambush predator, similar modern gars and needlefish. It is suggested that it first approached its prey and then suddenly accelerated towards it.[15] Some species may have lived as subsurface cruisers (Sinosaurichthys).[17]
Specimens showing half-swallowed conspecific individuals suggest that cannibalism was relatively common in Saurichthys.[18] Fossil evidence, in the form of a bolus (ball-shaped mass) of bones in the same strata, indicates that Saurichthys attacked marine reptiles such as the tanystropheidLangobardisaurus, or possibly scavenged their corpses.[citation needed]
A study on the gastrointestinal tract of Saurichthys found similarities with present-day sharks and rays, in particular the many windings in the spiral valve. The many windings increased the surface area for digestion, which is sure to have provided the fish with more energy. It indicates that Saurichthys had an energy-laden lifestyle.[19]
Early Triassic species of Saurichthys[18] differ from later species most prominently in their more elongate postorbital portion of the skull (part of the skull behind the eyes) and their generally denser scale cover. Middle Triassic and Late Triassic species, on the other hand, typically have a short postorbital portion of the skull and their scale cover is reduced. This reduction includes both the number of the longitudinal scale rows and the size of individual scales.[8] These evolutionary trends are, however, not an indication for anagenesis, but rather the result of parallel evolution in different lineages of Saurichthys. The aforementioned trends are observed only in marine species. Late Triassic freshwater species of Saurichthys (e.g., S. orientalis, S. sui) retain an elongate postorbital skull portion and a denser scale cover, suggesting that freshwater environments served as refugia for species with a more primitive appearance.[8][20]
Fossils of gravid females provide evidence for (ovo-)viviparity in Saurichthys[21] and the oldest known example for viviparity in ray-finned fishes.
Internal fertilisation is also evidenced by specialized pelvic fin rays (Saurichthys calcaratus) or ventral scales (gonopodium; Saurichthys curionii, Saurichthys macrocephalus) that are interpreted as copulatory organs of males.[22]
This list includes species of Saurichthys that are generally considered valid (based on Romano et al.[8] and references cited below). The validity of species that are based on fragmentary material (e.g., isolated scales or teeth) is questionable (see below).
^ abcStensiö, E. (1925). "Triassic Fishes from Spitzbergen, Part II". Kungliga Svenska Vetenskapsakademiens Handlingar. 3: 1–126.
^ abCartanyà, J. (1999). "An overview of the Middle Triassic actinopterygians from Alcover, Mont-ral and El Pinetell (Catalonia, Spain)". In G. Arratia; H. P. Schultze (eds.). Mesozoic Fishes 2—Systematics and Fossil Record. München: Friedrich Pfeil. pp. 535–551.
^Kogan, Ilja (2016). "Acidorhynchus Stensiö, 1925 or Saurorhynchus Reis, 1892: how to call the Jurassic saurichthyid?". Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 279 (1): 123–126. doi:10.1127/njgpa/2016/0545.
^Mutter, R. J.; Cartanyà, J.; Basaraba, S. A. U. (2008). "New evidence of Saurichthys from the Lower Triassic with an evaluation of early saurichthyid diversity". In G. Arratia; H.-P. Schultze; M. V. H. Wilson (eds.). Mesozoic Fishes 4—Homology and phylogeny. München: Friedrich Pfeil. pp. 103–127.
^Kligman, Ben T.; Parker, William G.; Marsh, Adam D. (2017). "First record of Saurichthys (Actinopterygii) from the Upper Triassic (Chinle Formation, Norian) of western North America". Journal of Vertebrate Paleontology. 37 (5): e1367304. Bibcode:2017JVPal..37E7304K. doi:10.1080/02724634.2017.1367304.
^Schmid, Leonhard; Sánchez-Villagra, Marcelo R. (2010). "Potential genetic bases of morphological evolution in the triassic fish Saurichthys". Journal of Experimental Zoology Part B: Molecular and Developmental Evolution. 314B (7): 519–526. Bibcode:2010JEZB..314..519S. doi:10.1002/jez.b.21372. PMID20853419.
^ abcKogan, Ilja; Romano, Carlo (2016). "Redescription of Saurichthys madagascariensis Piveteau, 1945 (Actinopterygii, Early Triassic), with implications for the early saurichthyid morphotype". Journal of Vertebrate Paleontology. 36 (4): e1151886. Bibcode:2016JVPal..36E1886K. doi:10.1080/02724634.2016.1151886. S2CID87234436.
^ abFang, G.; Wu, F.X. (2022). "The predatory fish Saurichthys reflects a complex underwater ecosystem of the Late Triassic Junggar Basin, Xinjiang, China". Historical Biology. 35 (8): 1–11. doi:10.1080/08912963.2022.2098023. S2CID250567176.
^Maxwell, Erin E.; Argyriou, Thodoris; Stockar, Rudolf; Furrer, Heinz (2018). "Re-evaluation of the ontogeny and reproductive biology of the Triassic fish Saurichthys (Actinopterygii, Saurichthyidae)". Palaeontology. 61 (4): 559–574. Bibcode:2018Palgy..61..559M. doi:10.1111/pala.12355. S2CID135337591.
^Bürgin, Toni (1990). "Reproduction in Middle Triassic actinopterygians; complex fin structures and evidence of viviparity in fossil fishes". Zoological Journal of the Linnean Society. 100 (4): 379–391. doi:10.1111/j.1096-3642.1990.tb01866.x.