''Santanichthys diasii'' is a species of extinct fish that existed around 115 million years ago during the

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

age. ''S. diasii'' is regarded as the basal-most

characiform Characiformes is an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, more than 2000 different species are described, including the well-known piranha and tetras.; Buckup P.A.: "Relationshi ...

, and is the earliest known member of Otophysi. It appears as a small fish, similar in appearance to a modern-day herring little more than 30 millimeters in length. Its most striking characteristic is the presence of a

Weberian apparatus The Weberian apparatus is an anatomical structure that connects the swim bladder to the auditory system in fishes belonging to the superorder Ostariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes c ...

, which makes it the most primitive known member of the order

Characiformes Characiformes is an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, more than 2000 different species are described, including the well-known piranha and tetras.; Buckup P.A.: "Relationsh ...

, the order in which modern-day

tetra Terrestrial Trunked Radio (TETRA; formerly known as Trans-European Trunked Radio), a European standard for a trunked radio system, is a professional mobile radio and two-way transceiver specification. TETRA was specifically designed for use by ...

s (including

piranha A piranha or piraña (, , or ; or , ) is one of a number of freshwater fish in the family Serrasalmidae, or the subfamily Serrasalminae within the tetra family, Characidae in order Characiformes. These fish inhabit South American rivers, fl ...

s) are classified. ''Santanichthys'' has been unearthed from numerous locations throughout

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

, in rocks dating to the

Cretaceous Period The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of t ...

. Its presence in these strata is seen as an indicator for the biogeography and evolution of its order.

Anatomy and morphology

''Santanichthys'' is a small,

fish that would have superficially resembled more advanced tetras or the unrelated herrings. Maximum length appears to be around 3-4 centimeters standard length. The species possesses a primitive but complete

Weberian Apparatus The Weberian apparatus is an anatomical structure that connects the swim bladder to the auditory system in fishes belonging to the superorder Ostariophysi. When it is fully developed in adult fish, the elements of the apparatus are sometimes c ...

, formed from the first four vertebrae of the spinal column. This is the earliest occurrence of the structure found in any known taxon of fish. Anatomically, the fish is similar to many others within its order. The body is semi-laterally compressed and ovate. The fins are more or less complete for a characiform - ''Santanichthys'' possesses a single

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal c ...

anal Anal may refer to: Related to the anus *Related to the anus of animals: ** Anal fin, in fish anatomy ** Anal vein, in insect anatomy ** Anal scale, in reptile anatomy *Related to the human anus: ** Anal sex, a type of sexual activity involving s ...

and

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

and paired

pectoral Pectoral may refer to: * The chest region and anything relating to it. * Pectoral cross, a cross worn on the chest * a decorative, usually jeweled version of a gorget * Pectoral (Ancient Egypt), a type of jewelry worn in ancient Egypt * Pectorali ...

and pelvic fins. Skeletal-wise, the fish has from 30-40 elongated

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

. A complex suspensorium can be found in the taxon. Jaw teeth however, are noticeably absent.

Cycloid scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as w ...

s are wholly present throughout the entire body.

Range and distribution

Fossils of ''Santanichthys'' were first unearthed from the Romualdo Member of the

Santana Formation The Santana Group is a geologic group, formerly included as the middle part of the Araripe Group, in the Araripe Basin of northeastern Brazil. The group comprises the Crato, Ipubi and Romualdo Formations and is dated to the Aptian to Albian sta ...

in northeastern

. The formation, situated in the hills of the

Araripe Basin The Araripe Basin () is a rift basin covering about ,Neto et al., 2013, p.1 in Ceará, Piauí and Pernambuco states of northeastern Brazil. It is bounded by the Patos and Pernambuco lineaments, and is situated east of the Parnaíba Basin, southwe ...

, is highly renowned for its rich Early Cretaceous fauna. The specific rock strata where fossils of ''Santanichthys'' were gathered from date back to the Aptian stage of the Cretaceous period. A large part of this faunal assemblage are masses of fossilized fish found at semi-regular intervals, including shoals of ''Santanichthys'' in various states of preservation. Aside from the Santana, ''Santanichthys'' fossils have also been found in other

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

deposits in Brazil. The Cretaceous deposits of the Brazilian Riachuelo Formation, specifically the Taquari Member have yielded specimens of ''Santanichthys''. Fossils of the taxon have also been unearthed from the Codo Formation in the Parnaiba Basin of Northeastern Brazil. The

type specimen In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes th ...

, DGM-DNPM 647P was taken from

deposits of the Romualdo Member in the Santana Formation. It is a fossil of a complete fish, around 4 centimeters long from snout to the base of the tail. At least two dozen specimens are kept by the American Museum of Natural History as AMNH 20050-20075. Some specimens of ''Santanichthys'' were taken from the fossilized stomach contents of other, larger fish such as AMNH 19439 and AMNH 12826.

Evolutionary history

''Santanichthys diasii'' is the only species within the genus ''Santanichthys''. It is classified in the order

, an order that includes the

s. Analysis of its morphological characters, including the presence of enlarged lagenar capsules has placed it as one of the most basal characiforms. This, combined with the Early Cretaceous origin of these specimens, makes ''Santanichthys'' the earliest known otophysan; The next characiforms up the

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

date from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

period already. This has several implications on the evolution of the Characiformes. All modern characiforms are strictly freshwater species. The Santana Formation is a marine

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

formation and thus ''Santanichthys'' was most likely an inhabitant of brackish or marine waters. This supports a marine-to-freshwater model of the evolution and dispersal of characiform species from the breakup of the continent Gondwana (

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

and

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...

) to the present day.

Taxonomic history

''Santanichthys diasii'' was first described within the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

'' Leptolepis'' as ''Leptolepis diasii'' in 1958. The specimens, fossilized fish from the

no more than thirty centimeters long apiece, were simply described as "prehistoric

teleosts Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Teleo ...

" and not much anatomical comparison was done between the specimens and other previously-known prehistoric fish. The second ''Leptolepis'' species described at the time of discovery, ''L. diasii'' found itself orphaned when the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

for the genus, ''L. bahiaensis'' was re-examined and reclassified in a different genus. ''L. diasii'' was deemed to be too different and anatomically advanced than others in the genus, and was rotated out of the genus in a 1991 classification by Maisey. The species' current genus, ''Santanichthys'' was coined in the same year when the species was redescribed by its original describer. Because of several perceived anatomical structures, it was reclassified within the Clupeomorpha. Subsequent analyses of available ''Santanichthys'' fossil material have determined some structures to be akin to a primitive

, prompting reclassification of the taxon as a basal otophysan and within the

References

{{Taxonbar, from=Q7420069 Prehistoric ray-finned fish genera Characiformes genera Early Cretaceous fish Prehistoric fish of South America Early Cretaceous animals of South America Cretaceous Brazil Fossils of Brazil Fossil taxa described in 1958