Habroichthys is an extinct genus of marine peltopleuriform ray-finned fish from the Middle Triassic, and is the only genus of the monotypic family Habroichthyidae. Fossils of the fish have been found in a number of countries being Slovenia, Italy, and China. Species within the genus were small with the largest species having a standard length of only 50 millimetres (2.0 in). Due to their small size, the preservation of the skull bones of the fish are usually poorly preserved, resulting in species being identified by the number of flank scales along with the placement of certain fins on the body. Within their environment, Habroichthys would have been a very common fish and would have contributed a large amount of biomass in their ecosystems. Overall, these fish would have most likely behaved like a number of modern schooling fish that swim near the water's surface. There are currently 12 valid species attributed to the genus.
History The type species of Habroichthys, H. minimus, was first descripted in 1939 by Brough based on an incomplete specimen (BMNH P. 19359) that lacked a number of skull elements along with the paired fins of the fish. The species would later be redescribed by Bürgin (1992) based on material that has been attributed to the Besano Formation and the Meride Limestone. Along with this, he would also describe a second species of the genus, H. griffithi, along with some specimens of indeterminate species. Between the description of the second species and 2010, a number of specimens had been found at various sites in Italy and Slovenia. The third species, H. broughti, would be described by Lin et al. (2011) based on material excavated from the Guanling Formation and mark the first specimen found in what would have been the Eastern Tethys Ocean. In 2016, Tintori et al. would reassigned a species of Peltopleurus , P. orientalis, to Habroichthys along with the description of another new species, H. dolomiticus, from the Dont Formartion. The most recent paper focusing on Habroichthys was published in 2025 by Conedera et al. in which seven new species were described along with a general overview of the paleoecology, paleoenvironment, and palaeobiogeography for the genus.
Description Habroichthys was a small, marine fish with most known species within the genus being under 30 millimetres (1.2 in), with the largest having a standard length of up to 50 millimetres (2.0 in). Due to this small size, preservation of skull elements is usually poor. In turn, descriptions of various species are usually identified from one another due to squamation and the position of the fins. Flanks of this fish are covered in a single row of tall scales that ends in a smaller, caudal semicircular scale. Species of the genus differ in the amount of scales in this row with the number ranging between 24 and 42 scales. The position of the dorsal, pelvic, and anal fins also differ between species with this being insertion point being labeled with the scale count. The position of these fins differ between the species along with slightly between individuals. The dorsal fin's position correlating between the 14th-26th scale, pelvic fin ranges between the 8th-16th scale, and anal fins range between the 15th-25th scale. Outside of these features of the postcranium, species of Habroichthys are overall very similar to one another. The caudal fin of is largely symmetrical though the attachment style of the fin rays identify it as a semiheterocercal tail. The skull of Habroichthys is short and almost box-shaped though with a more rounded transversal outline. All skulls of fish placed within the genus are toothless with a mandible that is longer than the maxilla. Along with this, the maxilla is free from the preopercle but does make contact with it. This is in contrast to other Peltopleuriformes that possess a maxilla that is sutured to the preopercle. The frontals, dermopterotics, and parietals of the fish are fused in most specimens. This fusion creating a single, shield-like structure on the skull roof. Within the operculum, the opercle is larger than the subopercle though the exact ratio between the two bones differ based on the species. Unlike a majority of other stem neopterygians along with a number of modern genera, Habroichthys lacked supraorbitals.
Ontogeny In comparison to adults, the smallest specimens of Habroichthys are more dorsoventrally flattened with a lack of scales outside of the large ones found on the flank. The flank scales are also more narrow with them directly extending to the fin insertions unlike in larger individuals. Bases on this, the shape and presence of scales in certain areas of the body can allow one to determine if a specimen is of an adult or a juvenile.
Species
Classification Due to the poor preservation of the skull bones of Habroichthys, the exact phylogenic placement of the genus has not been studied in depth. A number of features such as the flank scales, small size, and the lack of teeth do place it as a member of the order Peltopleuriformes but its placement within it isn't well understood. Even with this being the case, Habroichthys has been included in a few more recent phylogenies. However, it has been stated in papers focusing on the genus that these results are questionable. In these phylogenies, the genus either places near two other Triassic genera Luganoia and Venusichthys, neither of which are considered Peltopleuriformes.
Below is a phylogeny from Near and Thacker, 2024 showing one of the few phylogenies that include Habroichthys:
Paleobiology
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