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''Megamastax'' (meaning "big mouth") is a
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
lobe-finned fish Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includi ...
which lived during the late
Silurian 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 ...
period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...
, about 423
million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...
, in
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
. Before the discovery of ''Megamastax'', it was thought that jawed vertebrates (
gnathostomes Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...
) were limited in size and variation before the
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
period. ''Megamastax'' is known only from jaw bones and it is estimated that it reached about long.


Discovery and naming

Fossils of ''Megamastax'' were found in a layer of the Kuanti Formation in
Qujing Qujing () is a prefecture-level city in the east of Yunnan province, China, bordering Guizhou province to the east and the Guangxi Zhuang Autonomous Region to the southeast; thus, it was called "Key between Yunnan and Guizhou" () and "Throat of Yu ...
,
Yunnan Yunnan , () is a landlocked Provinces of China, province in Southwest China, 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 ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
. The
holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...
of ''Megamastax amblyodus'' is IVPP V18499.1, a complete left
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 tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
. Two additional specimens, IVPP V18499.2 (a partial left mandible) and IVPP V18499.3 (a right
maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The t ...
) have been referred to the species. The generic name of ''Megamastax'' is Greek for "big mouth", derived from ''megalos'' (big) and ''mastax'' (mouth). The species name, ''amblyodus'', translates to "blunt tooth".


Description

Although ''Megamastax'' can be safely considered a
sarcopterygian Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includi ...
(lobe-finned fish) due to having
cosmine Cosmine is a spongy, bony material that makes up the dentine-like layers in the scales of the lobe-finned fishes of the class Sarcopterygii. Fish scales that include layers of cosmine are known as cosmoid scales. Description As traditionally des ...
on its jaws, coronoid plates, a prearticular bone, and a biconcave
glenoid The glenoid fossa of the scapula or the glenoid cavity is a bone part of the shoulder. The word ''glenoid'' is pronounced or (both are common) and is from el, gléne, "socket", reflecting the shoulder joint's ball-and-socket form. It is a sha ...
, it is unique among early jawed fish for its jaw and teeth structure. Although most early
osteichthyans 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 cartilag ...
have only one row of sharp marginal teeth along the edge of the jaw, ''Megamastax'' has two rows of small marginal teeth. In addition, ''Megamastax'' uniquely has a row of large, blunt teeth fused to four coronoid bones on the inside edge of each mandible. Similar genera such as ''
Psarolepis ''Psarolepis'' (; ''psārolepis'', from Greek ψαρός 'speckled' and λεπίς 'scale') is a genus of extinct bony fish which lived around 397 to 418 million years ago ( Pridoli to Lochkovian stages). Fossils of ''Psarolepis'' have been fou ...
'' and '' Guiyu'' have sharp fangs on their five coronoids while poroplepiformes and
tetrapodomorphs The Tetrapodomorpha (also known as Choanata) are a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advance ...
have tusk-like teeth and three coronoids. Using the body plans of
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
osteichthyans as a guide, it can be estimated that the mandibles of ''Megamastax'' occupy between a fifth and a seventh of the total body length. If this is true, then IVPP V18499.1 had a length of and IVPP V18499.2 had a length of . Given these large estimates, ''Megamastax'' was the largest known Silurian vertebrate.


Paleobiology

With rounded coronoid teeth, ''Megamastax'' could have fed on hard-shelled animals. Extant fish with crushing teeth, such as
wolf eel The wolf eel (''Anarrhichthys ocellatus'') is a species of marine ray-finned fish belonging to the family Anarhichadidae, the wolf fishes. It is found in the North Pacific Ocean. Despite its common name and resemblance, it is not a true eel. It ...
s and horn sharks, generally have such teeth at the front of the jaw, separate from the coronoids. Although ''Megamastax'' differs from these fish by having its teeth and coronoids run the entire length of the jaw, this could simply be a result of having a different method of prey capture. While low body sizes for Silurian vertebrates has traditionally been seen as a result of the Silurian having low levels of atmospheric oxygen (as large vertebrates are susceptible to
hypoxia Hypoxia means a lower than normal level of oxygen, and may refer to: Reduced or insufficient oxygen * Hypoxia (environmental), abnormally low oxygen content of the specific environment * Hypoxia (medical), abnormally low level of oxygen in the tis ...
), recent analyses have shown that terrestrial plant life was more well established in the Silurian than once thought, providing a previously unknown source of oxygen. In addition, new Paleozoic atmospheric models indicate that oxygen levels were actually around modern levels by the end of the Silurian period. Although it is not certain that ''Megamastax's'' large size is a result of this atmospheric trend, the presence of such a large fish at this period does weaken the argument that a lack of large Silurian vertebrates can be used as evidence of low oxygen levels.


Paleoecology

Various hard-shelled animals were present in the late Silurian of China, including brachipods,
molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...
,
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 At ...
s, and even armored fish like
placoderms Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally 'Plate (animal anatomy), plate-skinned') is a Class (biology), class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devoni ...
and
galeaspids Galeaspida (from Latin, 'Helmet shields') is an extinct taxon of jawless marine and freshwater fish. The name is derived from ''galea'', the Latin word for ''helmet'', and refers to their massive bone shield on the head. Galeaspida lived in shallo ...
. Due to its large size and predatory lifestyle, ''Megamastax'' could be considered the first vertebrate apex predator.


References

{{Taxonbar, from=Q18434336 Prehistoric lobe-finned fish genera Silurian fish of Asia