''Angustidontus'' is a genus of predatory
pelagic
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...
crustaceans from the Late
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, w ...
and Early
Carboniferous periods, classified as part of the subclass
Eumalacostraca
Eumalacostraca is a subclass of crustaceans, containing almost all living malacostracans, or about 40,000 described species. The remaining subclasses are the Phyllocarida and possibly the Hoplocarida. Eumalacostracans have 19 segments (5 cepha ...
.
Fossils of the genus have been recovered in relative abundance from
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tota ...
,
Germany
Germany, officially the Federal Republic of Germany (FRG),, is a country in Central Europe. It is the most populous member state of the European Union. Germany lies between the Baltic and North Sea to the north and the Alps to the sou ...
, the
Czech Republic
The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as Bohemia, it is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. Th ...
,
Poland
Poland, officially the Republic of Poland, , is a country in Central Europe. Poland is divided into Voivodeships of Poland, sixteen voivodeships and is the fifth most populous member state of the European Union (EU), with over 38 mill ...
,
Belarus
Belarus,, , ; alternatively and formerly known as Byelorussia (from Russian ). officially the Republic of Belarus,; rus, Республика Беларусь, Respublika Belarus. is a landlocked country in Eastern Europe. It is bordered by ...
,
Ukraine
Ukraine ( uk, Україна, Ukraïna, ) is a country in Eastern Europe. It is the second-largest European country after Russia, which it borders to the east and northeast. Ukraine covers approximately . Prior to the ongoing Russian invas ...
and large parts of the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...
, including
Oklahoma,
Ohio
Ohio () is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Of the List of states and territories of the United States, fifty U.S. states, it is the List of U.S. states and territories by area, 34th-l ...
,
Indiana
Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th ...
,
Kentucky
Kentucky ( , ), officially the Commonwealth of Kentucky, is a state in the Southeastern region of the United States and one of the states of the Upper South. It borders Illinois, Indiana, and Ohio to the north; West Virginia and Virgini ...
,
Montana
Montana () is a U.S. state, state in the Mountain states, Mountain West List of regions of the United States#Census Bureau-designated regions and divisions, division of the Western United States. It is bordered by Idaho to the west, North ...
,
Utah
Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...
,
Nevada
Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...
.
The major eumalacostracan lineages had already diverged from each other during the Devonian, but their early evolutionary history remains relatively unknown due to a poor fossil record, making fossils of ''Angustidontus'' and other early eumalacostracans important for scientific study.
Historically of uncertain classification, studies on the paleobiology of ''Angustidontus'' have allowed researchers to place it between the eumalacostracan orders
Amphionidacea and
Decapoda
The Decapoda or decapods (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, shrimp and prawns. Most decapods are scavengers. The order is es ...
.
Description
''Angustidontus'' was a predatory
angustidontid crustacean, measuring about 6 centimeters in length (9 centimeters if the large maxillipeds are counted).
It had one pair of grasping
maxillipeds and seven pairs of
pereiopod
The decapod (crustaceans such as a crab, lobster, shrimp or prawn) is made up of 20 body segments grouped into two main body parts: the cephalothorax and the pleon (abdomen). Each segment may possess one pair of appendages, although in various ...
s. The first to fifth pair of pereiopods were subchelate and short, ending in a hooked and enlarged
dactylus
The dactylus is the tip region of the tentacular club of cephalopods and of the leg of some crustaceans (see arthropod leg). In cephalopods, the dactylus is narrow and often characterized by the asymmetrical placement of suckers (i.e., the ven ...
. The sixth and seventh pairs were thinner and longer, ending in simpler and smaller dactyli. The fifth and sixth pleonal somites expanded laterally, with some partial overlap over the
telson
The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...
(the posteriormost division of the body).
''Angustidontus'' had large grasping appendages, modified from the first or second
thoracopod The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, plur ...
s, which represent some of the earliest
maxillipeds within the
Eucarida
Eucarida is a superorder of the Malacostraca, a class of the crustacean subphylum, comprising the decapods, krill, ''Amphionides'' and Angustidontida. They are characterised by having the carapace fused to all thoracic segments, and by the posse ...
. These maxillipeds were likely used through being folded downwards to strike at prey, then hold the prey and prevent it from wriggling itself free.
''
Schramidontus'', the only known close relative of ''Angustidontus'', also possessed a second smaller pair of maxillipeds that it could use to bring prey to the
maxillae
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 tw ...
, maxillulae and its large
mandibles
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 bone ...
, but these are absent in ''Angustidontus''.
Instead, ''Angustidontus'' used the next four pairs of thoracopods (which were short and possessed strong claws and serrated gnathobases) to tear apart prey and transport it to the mouth.
History of research
The genus ''Angustidontus'' was named by Chalmer L. Cooper in 1936, together with the family "
Angustidontidae". Cooper described the fossils, consisting of the serrated appendages, as fossil jaws of
actinopterygian fish. Since then, the appendages of ''Angustidontus'' have been the subject of much debate on its classification. In the 1950s, it was suggested that the fossils instead represented
eurypterid
Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is ...
chelicerae
The chelicerae () are the mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as " jaws", chelicerae may be shaped as either articulated fangs, or simila ...
, possibly of something closely related to ''
Pterygotus
''Pterygotus'' is a genus of giant predatory eurypterid, a group of extinct aquatic arthropods. Fossils of ''Pterygotus'' have been discovered in deposits ranging in age from Middle Silurian to Late Devonian, and have been referred to several di ...
''.
Copeland & Bolton (1960) considered the fossils to instead represent gill rakers of fish or be "claws similar to those on the second maxilliped of the
stomatopod
Mantis shrimp, or stomatopods, are carnivorous marine crustaceans of the order Stomatopoda (). Stomatopods branched off from other members of the class Malacostraca around 340 million years ago. Mantis shrimp typically grow to around in length ...
''
Squilla''", furthermore the appendage was noticed to have had some kind of "
ball and socket
The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...
" joint type of articulation. Though it was noted several times by prominent researchers that ''Angustidontus'' did likely not represent a eurypterid, it was subsequently treated as a eurypterid (albeit dubiously so) for decades by many authors.
This was not universal however, with Jean Berdan concluding in 1964, after studying multiple ''Angustidontus'' appendages, that the "rami of ''Angustidontus'' are almost certainly part of an arthropod rather than a vertebrate but are not necessarily part of a eurypterid".
''Angustidontus'' often occurs together with ''
Concavicaris'', another Devonian crustacean. ''Concavicaris'' fossils tend to lack the appendages, whilst ''Angustidontus'' fossils often lack the cephalothoracic shield because of its weak sclerotisation. This caused some confusion, and some researchers have suggested that the two would represent different parts of the same animal.
Expeditions to fossil localities in Nevada where ''Concavicaris'' and ''Angustidontus'' were reported to have occurred together by Cooper in 1936 yielded more information on the appendages of ''Concavicaris'' and allowed it to be determined that the appendages of ''Angustidontus'' did not represent appendages of ''Concavicaris''. With hundreds of specimens being collected, proper research could be conducted on ''Angustidontus'' for the first time with the discovery of the first complete specimens.
The new specimens allowed researchers to determine that ''Angustidontus'' was a
peracarid
The superorder Peracarida is a large group of malacostracan crustaceans, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a brood pouch, or ''marsupium'', formed from thin flattened plat ...
malacostraca
Malacostraca (from New Latin; ) is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobs ...
n crustacean, and that ''Concavicaris'' simply represented a separate animal that was part of a larger Late Devonian fauna including a large amount of different invertebrates, such as worms, cephalopods, bivalves, brachipods and sponges. The appendages which had caused confusion in the past were revealed to be the first
thoracopod The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, plur ...
s, but greatly elongated and adapted to be used in feeding.
Classification

''Angustidontus'' is classified as part of the extinct family
Angustidontidae together with the freshwater genus ''
Schramidontus'' from
Belgium
Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is a country in Northwestern Europe. The country is bordered by the Netherlands to the north, Germany to the east, Luxembourg to the southeast, France to ...
. This family is the only family classified as part of the
eucarid order
Angustidontida. Angustidontids are diagnosed as eucarids that possess carapaces and stalked eyes with "scale-like exopods" on the second antennae, an elongated pleion and a tail fan. These features make the group distinct from most eumalacostracan crustaceans and they are classified as part of the
Eucarida
Eucarida is a superorder of the Malacostraca, a class of the crustacean subphylum, comprising the decapods, krill, ''Amphionides'' and Angustidontida. They are characterised by having the carapace fused to all thoracic segments, and by the posse ...
due to their
carapace
A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the und ...
being fused to thoracic segments 1-7.
Some additional species of ''Angustidontus'' have been named other than the type species (''A. seriatus''), such as "''A. weihmannae''", but they were all considered synonymous with ''A. seriatus'' by Rolfe & Dzik (2006), with only the type species remaining as valid. Based on the morphology of different fossil mandibles, fossil material from the Early
Famennian
The Famennian is the latter of two faunal stages in the Late Devonian Epoch. The most recent estimate for its duration estimates that it lasted from around 371.1 million years ago to 359.3 million years ago. An earlier 2012 estimate, still used ...
of Poland may represent two possible additional species.
The cladogram below is based on the relationships of the Eucarida assumed by Gueriau, Charbonnier and Clément (2014),
based on the gradual modification of the first
thoracopod The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, plur ...
s into the maxillipeds seen in the
Decapoda
The Decapoda or decapods (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, shrimp and prawns. Most decapods are scavengers. The order is es ...
.
Paleoecology
Environment
The life environment of ''Angustidontus'' was low in diversity, but might have been very high in
biological productivity. Fossil animals found in association with ''Angustidontus'' are exclusively open sea
pelagic
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...
creatures, such as
conodont
Conodonts ( Greek ''kōnos'', " cone", + ''odont'', " tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, w ...
s,
cephalopod
A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...
s,
ostracod
Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...
s,
concavicarids and
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% ...
. The bottom regions were likely slightly
benthose with a soft and muddy environment. This environment was likely anaerobic and poisonous, with abundant
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The und ...
, which would have prevented decomposition and allowed for fossils to be preserved.
Concavicarids, pelagic crustaceans of uncertain classification found in association with ''Angustidontus'', also had
chelicerae
The chelicerae () are the mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as " jaws", chelicerae may be shaped as either articulated fangs, or simila ...
-like appendages and were covered in protective spines. They were predators, with known stomach contents including
cephalopod
A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...
remains as well as
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
and
teleost
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. Tel ...
fish vertebrae. The appendages of concavicarids like ''
Concavicaris'' differed in functionality from those of ''Angustiodontus'' and were thus likely used to catch other kinds of prey than what was eaten by ''Angustidontus''. Concavicarids may not only have occupied a different
niche
Niche may refer to:
Science
* Developmental niche, a concept for understanding the cultural context of child development
*Ecological niche, a term describing the relational position of an organism's species
*Niche differentiation, in ecology, the ...
, but an entirely different level in the
water column
A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
.
Prey
There are no taxonomically identifiable gut contents in any ''Angustidontus'' specimen. There are some known small fossil pieces found in association to ''Angustidontus'' specimens that may represent
cephalopod jaws or
conch
Conch () is a common name of a number of different medium-to-large-sized sea snails. Conch shells typically have a high spire and a noticeable siphonal canal (in other words, the shell comes to a noticeable point at both ends).
In North Am ...
es of thin-shelled
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 species of molluscs are recognized. The number of fossil species is esti ...
. The specimen NMNH 530451 preserves a spiral imprint that might represent the larval conch of a
goniatite
Goniatids, informally goniatites, are ammonoid cephalopods that form the order Goniatitida, derived from the more primitive Agoniatitida during the Middle Devonian some 390 million years ago (around Eifelian stage). Goniatites (goniatitids) survi ...
. ''Angustidontus'' might thus have fed on cephalopods. Another likely prey item based on its presence in Late Devonian pelagic environments are
conodont
Conodonts ( Greek ''kōnos'', " cone", + ''odont'', " tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, w ...
s.
References
{{Taxonbar, from=Q3617433
Prehistoric crustacean genera
Prehistoric Malacostraca
Devonian arthropods of North America
Carboniferous arthropods of North America
Devonian crustaceans
Carboniferous crustaceans