Asterotrygon
Encyclopedia
Asterotrygon is an extinct genus
of stingray
from the Eocene
Green River Formation
in Wyoming
. Several complete skeletons representing juveniles, adults, males and females have been uncovered from the late early Eocene Fossil Butte Member of the formation. The type
and only species, A. maloneyi, was named in 2004 on the basis of these fossils. Another stingray, Heliobatis
, is also known from the formation. Asterotrygon is a primitive stingray closely related to the living family Urolophidae whose ancestors likely originated in the Indo-Pacific
. It lived in Fossil Lake, a body of water that existed in a subtropical mountainous region for only 5 million years.
83.25) to 65 centimetres (25.6 in) in the largest adult (FMNH
15166). The shape of the disk is rounder than those of other extinct stingrays such as Heliobatis, which are more diamond-shaped. The upper surface of the disk is covered in small dermal denticles, each with a small hook. Unlike Heliobatis and modern stingrays, it has a small dorsal fin
in front of its stingers. While most stingrays have a cartilaginous rod extending from the stinger to the tip of the tail, Asterotrygon retains separate vertebrae throughout the tail's entire length. The tail is also somewhat thicker at its base than those of other stingrays. Small fin-like tail folds are present at the tip of the tail. The puboischiadic girdle, an element to which the pelvic fins attach, is primitively narrow and arch-shaped. Like other stingrays, Asterotrygon lacks thoracic rib
s but possesses cartilage around the spine called thoracolumbar synarcual cartilage.
Several details of the neurocranium link Asterotrygon with modern stingrays. Like other stingrays, the hyomandibula
e, which allow the jaw to extend outward, are completely separated from the lower jaw. In other cartilaginous fish, the hyomandibulae and lower jaw are fused by the hyomandibular-Meckelian ligament. Despite this loss, Asterotrygon still retains some calcified cartilage in the space where the ligament once was. The postorbital processes behind the eye sockets are broad, flat, and shelf-like.
Before the description of Asterotrygon, Heliobatis was the only known stingray in the Green River Formation. American paleontologist Othniel Charles Marsh
named Heliobatis radians in 1877, and Xiphotrygon acutidens and Palaeodasybatis discus were subsequently named from the formation in 1879 and 1947, respectively. These stingrays were later synonymized with Heliobatis.
In 1980, a Green River fossil called AMNH
P 11557 was described that included a female stingray and two smaller individuals thought to be aborted fetus
es. A second specimen called FMNH PF 15166 included a female and a fetus that was still in an embryo
within the animal. The genus Asterotrygon was erected in 2004 along with the type species A. maloneyi, and FMNH PF 15166 was designated the holotype
specimen. AMNH P 11557 and many other complete fossils were also referred to A. maloneyi. Hundreds of fossils of Heliobatis have been found, while Asterotrygon is represented by 15 specimens. Asterotrygon means "star stingray" from the Greek
asteros ("star") and trygon ("stingray") after the star-like shape of the base of each denticle. The type species A. maloneyi was named after Thomas Maloney, who donated the paratype
specimen AMNH P 11557 to the American Museum of Natural History
.
Although Asterotrygon is not the earliest stingray (they were present as early as the Early Cretaceous
, it is one of the most well-preserved of the early forms. Most earlier stingrays are known only from small teeth, dermal denticles, or stinger
s, but not entire bodies.
stingray within Myliobatoidei. Only Hexatrygon, the living Sixgill stingray, is more basal. Despite living alongside each other, Asterotrygon and Heliobatis are not closely related. Neither are classified in any living family of stingrays, although a 2004 phylogenetic analysis found Heliobatis to be closely related to a clade
including river stingrays and whiptail stingray
s. Asterotrygon is more closely related to Urolophidae, the stingarees, and Plesiobatis, the Deepwater stingray. Both types are from the Indo-Pacific. Below is a cladogram
showing the phylogenetic relationships of Asterotrygon from Carvalho et al. (2004):
stingrays. Its ancestors probably originated in the western Pacific or Indo-Pacific
. In contrast, the contemporaneous Heliobatis probably evolved from stingrays that were already present in the Americas. Because the two genera are not closely related, their ancestors probably colonized the Green River Formation lakes in two separate but closely timed events. Neither Asterotrygon nor Heliobatis are known outside the Green River Formation, so they probably evolved within the lakes.
Asterotrygon and other organisms from the Green River Formation lived in subtropical lakes in a mountainous region. These lakes existed for a very long time, some up to 15 million years. The oldest modern lakes have existed for only a few million years, and the vast majority only a few thousand. Three major lakes are known: Fossil Lake, Lake Gosiute, and Lake Uinta. Although it was the shortest-lived, Fossil Lake preserved the most fossils, including all stingray fossils. Fossil Lake is one of two areas that preserve complete stingray fossils; the other, the Monte Bolca Formation in Italy
, preserves stingrays that lived in a marine lagoon
behind a coral reef
. Monte Bolca preserves a greater diversity of rays, including guitarfish
es and electric ray
s. More rays may have lived in Monte Bolca because it was a marine environment; today, most stingrays live in the ocean and only a few live in fresh water.
Fossil Lake was a freshwater lake in a semitropical environment. The modern Gulf Coast and southern Atlantic regions of the United States
have been used as modern analogues to the environment. Asterotrygon is found in two types of deposits. One group of fossil localities is called F-1 or the "18-inch layer" and includes limestone that was deposited in the middle of fossil lake over a few hundred years. These sediments were thought to have formed in a deep area where the lake bottom was anoxic. The F-2 layer is thicker, up to 4 metres (13.1 ft), and was deposited over a longer period of time, possibly several thousand years. F-2 limestone was deposited closer to the north and southeastern shorelines of the lake where the water was richer in dissolved oxygen and organic material.
Genus
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...
of stingray
Stingray
The stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae , Plesiobatidae , Urolophidae , Urotrygonidae , Dasyatidae , Potamotrygonidae The...
from the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
Green River Formation
Green River Formation
The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes. The sediments are deposited in very fine layers, a dark layer during the growing season and a light-hue inorganic layer in winter. Each pair of layers is called a varve and...
in Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
. Several complete skeletons representing juveniles, adults, males and females have been uncovered from the late early Eocene Fossil Butte Member of the formation. The type
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...
and only species, A. maloneyi, was named in 2004 on the basis of these fossils. Another stingray, Heliobatis
Heliobatis
Heliobatis is an extinct genus of ray in the Myliobatiformes family Dasyatidae. At present the genus contains the single species Heliobatis radians....
, is also known from the formation. Asterotrygon is a primitive stingray closely related to the living family Urolophidae whose ancestors likely originated in the Indo-Pacific
Indo-Pacific
The Indo-Pacific is a biogeographic region of the Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia...
. It lived in Fossil Lake, a body of water that existed in a subtropical mountainous region for only 5 million years.
Description
Asterotrygon had a typical stingray shape with a flat, rounded disk formed from the head and pectoral fins and a long, narrow tail with sharp stingers. It ranged in size from 7.97 centimetres (3.1 in) in the smallest juvenile (the specimen SMMPScience Museum of Minnesota
The Science Museum of Minnesota is an American museum focused on topics in technology, natural history, physical science and mathematics education. Founded in 1907 and located in Saint Paul, Minnesota, the 501 nonprofit institution is staffed by over 500 employees and over 1,600 volunteers...
83.25) to 65 centimetres (25.6 in) in the largest adult (FMNH
Field Museum of Natural History
The Field Museum of Natural History is located in Chicago, Illinois, USA. It sits on Lake Shore Drive next to Lake Michigan, part of a scenic complex known as the Museum Campus Chicago...
15166). The shape of the disk is rounder than those of other extinct stingrays such as Heliobatis, which are more diamond-shaped. The upper surface of the disk is covered in small dermal denticles, each with a small hook. Unlike Heliobatis and modern stingrays, it has a small dorsal fin
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...
in front of its stingers. While most stingrays have a cartilaginous rod extending from the stinger to the tip of the tail, Asterotrygon retains separate vertebrae throughout the tail's entire length. The tail is also somewhat thicker at its base than those of other stingrays. Small fin-like tail folds are present at the tip of the tail. The puboischiadic girdle, an element to which the pelvic fins attach, is primitively narrow and arch-shaped. Like other stingrays, Asterotrygon lacks thoracic rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...
s but possesses cartilage around the spine called thoracolumbar synarcual cartilage.
Several details of the neurocranium link Asterotrygon with modern stingrays. Like other stingrays, the hyomandibula
Hyomandibula
The hyomandibula, commonly referred to as hyomandibular [bone] , and Latin: mandibula, "jawbone") is a set of bones that is found in the hyoid region in most fishes. It usually plays a role in suspending the jaws and/or operculum...
e, which allow the jaw to extend outward, are completely separated from the lower jaw. In other cartilaginous fish, the hyomandibulae and lower jaw are fused by the hyomandibular-Meckelian ligament. Despite this loss, Asterotrygon still retains some calcified cartilage in the space where the ligament once was. The postorbital processes behind the eye sockets are broad, flat, and shelf-like.
History
Othniel Charles Marsh
Othniel Charles Marsh was an American paleontologist. Marsh was one of the preeminent scientists in the field; the discovery or description of dozens of news species and theories on the origins of birds are among his legacies.Born into a modest family, Marsh was able to afford higher education...
named Heliobatis radians in 1877, and Xiphotrygon acutidens and Palaeodasybatis discus were subsequently named from the formation in 1879 and 1947, respectively. These stingrays were later synonymized with Heliobatis.
In 1980, a Green River fossil called AMNH
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...
P 11557 was described that included a female stingray and two smaller individuals thought to be aborted fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
es. A second specimen called FMNH PF 15166 included a female and a fetus that was still in an embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
within the animal. The genus Asterotrygon was erected in 2004 along with the type species A. maloneyi, and FMNH PF 15166 was designated the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
specimen. AMNH P 11557 and many other complete fossils were also referred to A. maloneyi. Hundreds of fossils of Heliobatis have been found, while Asterotrygon is represented by 15 specimens. Asterotrygon means "star stingray" from the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
asteros ("star") and trygon ("stingray") after the star-like shape of the base of each denticle. The type species A. maloneyi was named after Thomas Maloney, who donated the paratype
Paratype
Paratype is a technical term used in the scientific naming of species and other taxa of organisms. The exact meaning of the term paratype when it is used in zoology is not the same as the meaning when it is used in botany...
specimen AMNH P 11557 to 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...
.
Although Asterotrygon is not the earliest stingray (they were present as early as the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
, it is one of the most well-preserved of the early forms. Most earlier stingrays are known only from small teeth, dermal denticles, or stinger
Stinger
-Biology:* Stinger, an organ or body part found in various animals that usually delivers some kind of venom.* Stinger , a minor neurological injury suffered by athletes.-Sports and entertainment:...
s, but not entire bodies.
Classification
Asterotrygon is classified as basalBasal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
stingray within Myliobatoidei. Only Hexatrygon, the living Sixgill stingray, is more basal. Despite living alongside each other, Asterotrygon and Heliobatis are not closely related. Neither are classified in any living family of stingrays, although a 2004 phylogenetic analysis found Heliobatis to be closely related to a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
including river stingrays and whiptail stingray
Whiptail stingray
The whiptail stingrays are a family, Dasyatidae, of rays in the order Myliobatiformes. They are found worldwide in tropical to temperate marine waters, and a number of species have also penetrated into fresh water in North America, Africa, Asia, and Australia. Members of this family have flattened...
s. Asterotrygon is more closely related to Urolophidae, the stingarees, and Plesiobatis, the Deepwater stingray. Both types are from the Indo-Pacific. Below is a cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
showing the phylogenetic relationships of Asterotrygon from Carvalho et al. (2004):
Paleobiology
Asterotrygon is closely related to PacificPacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
stingrays. Its ancestors probably originated in the western Pacific or Indo-Pacific
Indo-Pacific
The Indo-Pacific is a biogeographic region of the Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia...
. In contrast, the contemporaneous Heliobatis probably evolved from stingrays that were already present in the Americas. Because the two genera are not closely related, their ancestors probably colonized the Green River Formation lakes in two separate but closely timed events. Neither Asterotrygon nor Heliobatis are known outside the Green River Formation, so they probably evolved within the lakes.
Asterotrygon and other organisms from the Green River Formation lived in subtropical lakes in a mountainous region. These lakes existed for a very long time, some up to 15 million years. The oldest modern lakes have existed for only a few million years, and the vast majority only a few thousand. Three major lakes are known: Fossil Lake, Lake Gosiute, and Lake Uinta. Although it was the shortest-lived, Fossil Lake preserved the most fossils, including all stingray fossils. Fossil Lake is one of two areas that preserve complete stingray fossils; the other, the Monte Bolca Formation in Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, preserves stingrays that lived in a marine lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
behind a coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
. Monte Bolca preserves a greater diversity of rays, including guitarfish
Guitarfish
The guitarfish are a family, Rhinobatidae, of rays. The guitarfish are known for an elongated body with a flattened head and trunk and small ray like wings. The combined range of the various species is tropical, subtropical and temperate waters worldwide. They often travel in large...
es and electric ray
Electric ray
The electric rays are a group of rays, flattened cartilaginous fish with enlarged pectoral fins, comprising the order Torpediniformes. They are known for being capable of producing an electric discharge, ranging from as little as 8 volts up to 220 volts depending on species, used to stun prey and...
s. More rays may have lived in Monte Bolca because it was a marine environment; today, most stingrays live in the ocean and only a few live in fresh water.
Fossil Lake was a freshwater lake in a semitropical environment. The modern Gulf Coast and southern Atlantic regions of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
have been used as modern analogues to the environment. Asterotrygon is found in two types of deposits. One group of fossil localities is called F-1 or the "18-inch layer" and includes limestone that was deposited in the middle of fossil lake over a few hundred years. These sediments were thought to have formed in a deep area where the lake bottom was anoxic. The F-2 layer is thicker, up to 4 metres (13.1 ft), and was deposited over a longer period of time, possibly several thousand years. F-2 limestone was deposited closer to the north and southeastern shorelines of the lake where the water was richer in dissolved oxygen and organic material.