''Bianchengichthys'' is an extinct

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 n ...

of maxillate placoderm

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% ...

from the late

Silurian Period The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

. Its fossils have been recovered from

Yunnan Province Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is Kunming. The province borders the C ...

, China, and it is represented by only one species: ''Bianchengichthys micros''.

Description

''Bianchengichthys'' is a small, somewhat dorsoventrally compressed placoderm fish. The mandible (made from

dermal bone A dermal bone or investing bone or membrane bone is a bony structure derived from intramembranous ossification forming components of the vertebrate skeleton including much of the skull, jaws, gill covers, shoulder girdle and fin spines rays ( l ...

) of this genus differs from ''

Entelognathus ''Entelognathus primordialis'' (“primordial complete jaw”) is a maxillate placoderm from the late Silurian (Ludlow epoch) of Qujing, Yunnan, 419 million years ago. A team led by Min Zhu of the Academy of Sciences' Institute of Vertebrate Pa ...

'' and ''

Qilinyu ''Qilinyu rostrata'' is a "maxillate" placoderm from the late Ludlow epoch of Qujing, Yunnan Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a populatio ...

''—two other maxillate placoderms from late Silurian China−in that the oral lamina is broad and carries a row of

tooth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, t ...

-like denticles, though the marginal flange is toothless. The pectoral fin, preceded by two small spines on its thoracic shield, is lobate in shape and situated along by a 'fringe' of scales similar to those of ''

Lepidotrichia Fins are distinctive anatomical features composed of bony spine (zoology), spines or Ray (fish fin anatomy), 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 bon ...

'' in bony fishes. Similarly to other maxillate placoderms, its eyes are anteriorly orientated and very close to its mouth.

Evolutionary significance

''Bianchengichthys

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bon ...

bears physical resemblance to both its relatives ''Qilinyu'' and ''Entelognathus'' as well as

arthrodire Arthrodira (Greek for "jointed neck") is an order of extinct armored, jawed fishes of the class Placodermi that flourished in the Devonian period before their sudden extinction, surviving for about 50 million years and penetrating most marine e ...

placoderms. It is likely that ''Bianchengichthys'' is closely related to the

common ancestor Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...

between

cartilaginous fishes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...

and

bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartila ...

, and represents a transitionary form between placoderms and extant jawed vertebrates. The evolutionary relationships of ''Bianchengichthys'' to other placoderms is thought to shed light on the early evolution of all jawed vertebrates—for instance, its potential arthrodire affinity may suggest that eugnathostomes are an extremely derived lineage of arthrodire placoderms.

Distribution and habitat

''Bianchengichthys'' has only been reported from the Xiaoxi Formation, which is from the

Ludlow Epoch In the geological timescale, the Ludlow Epoch (from 427.4 ± 0.5 million years ago to 423.0 ± 2.3 million years ago) occurred during the Silurian Period, after the end of the Homerian Age. It is named for the town of Ludlow in Shropshire, Engla ...

(427-423 million years ago). The Xiaoxi Formation primarily consists of

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

siltstones Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, pp ...

and

mudstones Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' ...

, and

ichnofossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

assemblages from the vicinity of

Guizhou Guizhou (; formerly Kweichow) is a landlocked province in the southwest region of the People's Republic of China. Its capital and largest city is Guiyang, in the center of the province. Guizhou borders the autonomous region of Guangxi to t ...

suggest that it was deposited in a shallow, subtidal, marine setting. Hui-zhen, M., & Xue-hua, W. (2014). A study of the sedimentary environment of the Xiaoxi Formation in NE of Guizhou, China. Advances in Earth Science, 29(7), 859. The presence of ''

Cruziana ''Cruziana'' is a trace fossil consisting of elongate, bilobed, approximately bilaterally symmetrical burrows, usually preserved along bedding planes, with a sculpture of repeated striations that are mostly oblique to the long dimension. It is fo ...

'' in the Xiaoxi suggests that ''Bianchengichthys'' may have lived alongside burrowing organisms such as

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

s. Another set of fish fossils belonging to the ''nomen dubium'' "'' Wangolepis''" have been recovered from this layer. "''Wangolepis''" material has also been recovered from the Kuanti Formation, also of China's

References

External links

Genus description
{{Taxonbar, from=Q110990752 Placoderm genera Silurian fish of Asia Prehistoric animals of China Placodermi enigmatic taxa Paleontology in Yunnan Transitional fossils Fossil taxa described in 2021 2021 in China

Description

Evolutionary significance

Distribution and habitat

See also

References

External links