Santanichthys
Encyclopedia
Santanichthys diasii is a species of extinct fish that existed around 115 million years ago. It is the earliest known fish that can be classified in the Otophysi. It appears as a small fish, similar in appearance to a modern-day herring
no more than 30 centimeters in length. Its most striking characteristic is the presence of a Weberian apparatus
, which makes it the most primitive known member of the Order Characiformes
, the order in which modern-day tetra
s (including piranhas
) are classified in. Santanichthys has been unearthed from numerous locations throughout Brazil
, in rocks dating to the Cretaceous Period. Its presence in this strata is seen as an indicator for the biogeography and evolution of its order.
, 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
, anal and caudal fin and paired pectoral and pelvic fins. Skeletal-wise, the fish has from 30-40 elongated vertebrae. A complex suspensorium can be found in the taxon. Jaw teeth however, are noticeably absent. Cycloid scales are wholly present throughout the entire body.
in northeastern Brazil
. The formation, situated in the hills of the Araripe Basin, 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
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, DGM-DNPM 647P was taken from Albian
deposits of the Romualdo Member in the Santana Formation. It is a fossil of a complete fish, around 29 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.
Characiformes
, an order that includes the tetra
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 date from the Late Cretaceous
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 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
and South America
) to the present day.
Leptolepis
as Leptolepis diasii in 1958. The specimens, fossilized fish from the Santana Formation
no more than thirty centimeters long apiece, were simply described as "prehistoric teleosts" 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
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 Weberian Apparatus
, prompting reclassification of the taxon as a basal otophysan and within the Characiformes
.
Herring
Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...
no more than 30 centimeters in length. Its most striking characteristic is the presence of a Weberian apparatus
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 collectively referred to as the Weberian ossicles...
, which makes it the most primitive known member of the Order Characiformes
Characiformes
The Characiformes are an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, there are a few thousand different species, including the well-known piranha and tetras.-Taxonomy:...
, the order in which modern-day tetra
Tetra
thumb|right|250px|Pristella tetra — [[Pristella maxillaris]].thumb|right|250px|Golden Pristella tetra, a [[morph |morph]] of [[Pristella maxillaris]].thumb|right|250px|[[Silvertip tetra]] — Hasemania nana....
s (including piranhas
Piranhas
Piranhas is a historic city and municipality in the western of the State of Alagoas, in the Northeast Region of Brazil. Located on the bank of the São Francisco River, just at the border with the State of Sergipe, Piranhas was founded in 1891 and originally named Floriano Peixoto...
) are classified in. Santanichthys has been unearthed from numerous locations throughout Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
, in rocks dating to the Cretaceous Period. Its presence in this strata is seen as an indicator for the biogeography and evolution of its order.
Anatomy and morphology
Santanichthys is a small, characiform fish with some external anatomical features that resemble clupeiforms. Maximum length appears to be around 30 centimeters standard length. The species possesses a primitive but complete Weberian ApparatusWeberian 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 collectively referred to as the Weberian ossicles...
, 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 fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
, anal and caudal fin and paired pectoral and pelvic fins. Skeletal-wise, the fish has from 30-40 elongated vertebrae. A complex suspensorium can be found in the taxon. Jaw teeth however, are noticeably absent. Cycloid scales are wholly present throughout the entire body.
Range and distribution
Fossils of Santanichthys were first unearthed from the Romualdo Member of the Santana FormationSantana Formation
The Santana Formation is a geologic Lagerstätte in northeastern Brazil's Araripe Basin where the states of Pernambuco, Piauí and Ceará come together. The geological formation, named after the village of Santana do Cariri, lies at the base of the Araripe Plateau...
in northeastern Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. The formation, situated in the hills of the Araripe Basin, is highly renowned for its rich Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
fauna. The specific rock strata where fossils of Santanichthys were gathered from date back to the Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
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
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
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, DGM-DNPM 647P was taken from Albian
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 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...
deposits of the Romualdo Member in the Santana Formation. It is a fossil of a complete fish, around 29 centimeters long from snout to the base of the tail. At least two dozen specimens are kept by the American Museum of Natural History
American Museum of Natural History
The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...
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 orderOrder (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Characiformes
Characiformes
The Characiformes are an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, there are a few thousand different species, including the well-known piranha and tetras.-Taxonomy:...
, an order that includes the tetra
Tetra
thumb|right|250px|Pristella tetra — [[Pristella maxillaris]].thumb|right|250px|Golden Pristella tetra, a [[morph |morph]] of [[Pristella maxillaris]].thumb|right|250px|[[Silvertip tetra]] — Hasemania nana....
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 date from the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
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 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
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
(Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
) to the present day.
Taxonomic history
Santanichthys diasii was first described within the genusGenus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Leptolepis
Leptolepis
Leptolepis is an extinct genus of teleost fish that lived in the Mesozoic era. It was one of the first teleosts.- Appearance :...
as Leptolepis diasii in 1958. The specimens, fossilized fish from the Santana Formation
Santana Formation
The Santana Formation is a geologic Lagerstätte in northeastern Brazil's Araripe Basin where the states of Pernambuco, Piauí and Ceará come together. The geological formation, named after the village of Santana do Cariri, lies at the base of the Araripe Plateau...
no more than thirty centimeters long apiece, were simply described as "prehistoric teleosts" 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
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
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 Weberian Apparatus
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 collectively referred to as the Weberian ossicles...
, prompting reclassification of the taxon as a basal otophysan and within the Characiformes
Characiformes
The Characiformes are an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, there are a few thousand different species, including the well-known piranha and tetras.-Taxonomy:...
.