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]
Appearance
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]
Reproduction
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]
Species
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.
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 (Molecular and Developmental Evolution). 314B (7): 519–526.
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.