Polycotylus
Encyclopedia
Polycotylus is a genus
of plesiosaur
within the family
Polycotylidae
. The type species
is P. latippinis and was named by American paleontologist Edward Drinker Cope
in 1869. Eleven other species have been identified. The name means 'much-cupped vertebrae', referring to the shape of the vertebrae. It lived in the seas of North America, Russia and Australia toward the end of the Cretaceous
. One fossil preserves an adult with a single large fetus inside of it, indicating that Polycotylus gave live birth, an unusual adaptation among reptiles.
polycotylid because it has more vertebrae in its neck (a feature that links it with long-necked ancestors) and its humerus
has a more primitive shape. The long ischia of the pelvis are a distinguishing feature of Polycotylus, as are thick teeth with striations on their surfaces, a narrow pterygoid bone on the palate
and a low sagittal crest
on top of the skull.
in Kansas
in 1869. The holotype
bones from which he based his description were fragmentary, representing only a small portion of the skeleton. A more complete skeleton was later found in Kansas and was described in 1906. A nearly complete skeleton was found in 1949 from the Mooreville Chalk Formation
in Alabama
, but was not described until 2002.
and Elasmosaurus
, Polycotylus had a short neck. This led to it being classified as a pliosaur
, a marine reptile within the superfamily Pliosauroidea, closely related to true plesiosaurs (which belong to the superfamily Plesiosauroidea). Polycotylus and other polycotylids superficially resemble pliosaurs like Liopleurodon
and Peloneustes
because they have short necks, large heads, and other proportions that differ from true plesiosaurs.
As phylogenetic analyses became common in the last few decades, the classification of Polycotylus and other plesiosaurs have been revised. In 1997, it and other polycotylids were reassigned as close relatives of long-necked elasmosaurids. In a 2001 study, Polycotylus was classified as a derived
cryptocleidoid plesiosaur closely related to Jurassic
plesiosaurs like Cryptocleidus. Below is a cladogram from a 2004 study which supported a similar classification:
In 2007, Polycotylus was placed in a new subfamily of polycotylids called Polycotylinae. Another newly-described polycotylid called Eopolycotylus
from Glen Canyon
, Utah
, was found to be the closest relative of Polycotylus. Below is a cladogram from the 2007 study:
inside it. LACM 129639 was found in Kansas during the 1980s and was in storage at the Natural History Museum of Los Angeles County
until it was described in 2011. The length of the fetus is around 40 percent of the length of the mother. Gestation was probably two thirds complete based on what is known of the fetal development of related nothosaur
s. This fossil suggests that Polycotylus was viviparous, giving live birth (as opposed to laying eggs).
Vivipary
, or live birth, may have been the most common form of reproduction in plesiosaurs, as they would have had difficulty laying eggs on land. Their bodies are not adapted to movement on land, and paleontologists have long hypothesized that they must have given birth in water. Other marine reptiles such as ichthyosaur
s also gave live birth, but LACM 129639 was the first direct evidence of vivipary in plesiosaurs. The lives of Polycotylus and other plesiosaurs were K-selected
, meaning that few offspring were born to each individual but those that were born were cared for as they mature. Because it gave birth to a single large offspring, the mother Polycotylus probably gave it some form of parental care for it to survive. F. Robin O'Keefe, one of the describers of LACM 129639, suggested that the social lives of plesiosaurs were "more similar to those of modern dolphins than other reptiles." K-selected life-history strategies are also seen in mammals and some lizards, but are unusual among reptiles.
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 plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...
within the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Polycotylidae
Polycotylidae
Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to the Leptocleididae.With their short necks and large elongated heads, they resemble the pliosaurs, but closer phylogenetical studies indicate that they share many common features with the plesiosauridae and elasmosauridae...
. 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...
is P. latippinis and was named by American paleontologist Edward Drinker Cope
Edward Drinker Cope
Edward Drinker Cope was an American paleontologist and comparative anatomist, as well as a noted herpetologist and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested in science; he published his first scientific paper at the age of nineteen...
in 1869. Eleven other species have been identified. The name means 'much-cupped vertebrae', referring to the shape of the vertebrae. It lived in the seas of North America, Russia and Australia toward the end of the 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...
. One fossil preserves an adult with a single large fetus inside of it, indicating that Polycotylus gave live birth, an unusual adaptation among reptiles.
Description
Like all plesiosaurs, Polycotylus was a large marine reptile with a short tail, large flippers, and a broad body. It has a short neck and a long head, and was about 5 metres (16.4 ft) long. It has more neck vertebrae than other polycotylids, however. Polycotylus is thought to be a 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:...
polycotylid because it has more vertebrae in its neck (a feature that links it with long-necked ancestors) and its humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
has a more primitive shape. The long ischia of the pelvis are a distinguishing feature of Polycotylus, as are thick teeth with striations on their surfaces, a narrow pterygoid bone on the palate
Palate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
and a low sagittal crest
Sagittal crest
A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull of many mammalian and reptilian skulls, among others....
on top of the skull.
History
Edward Drinker Cope named Polycotylus from the Niobrara FormationNiobrara Formation
The Niobrara Formation, also called the Niobrara Chalk , is a geologic formation in North America that was laid down between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian ages of the Late Cretaceous. It is composed of two structural units, the Smoky Hill Chalk Member...
in Kansas
Kansas
Kansas is a US state located in the Midwestern United States. It is named after the Kansas River which flows through it, which in turn was named after the Kansa Native American tribe, which inhabited the area. The tribe's name is often said to mean "people of the wind" or "people of the south...
in 1869. 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...
bones from which he based his description were fragmentary, representing only a small portion of the skeleton. A more complete skeleton was later found in Kansas and was described in 1906. A nearly complete skeleton was found in 1949 from the Mooreville Chalk Formation
Mooreville Chalk Formation
The Mooreville Chalk Formation is a geological formation in North America, within the U.S. states of Alabama and Mississippi. The strata date back to the early Santonian to the early Campanian stage of the Late Cretaceous. The chalk was formed by pelagic sediments deposited along the eastern edge...
in Alabama
Alabama
Alabama is a state located in the southeastern region of the United States. It is bordered by Tennessee to the north, Georgia to the east, Florida and the Gulf of Mexico to the south, and Mississippi to the west. Alabama ranks 30th in total land area and ranks second in the size of its inland...
, but was not described until 2002.
Classification
Unlike some better-known long-necked plesiosaurs like PlesiosaurusPlesiosaurus
Plesiosaurus was a genus of large marine sauropterygian reptile that lived during the early part of the Jurassic Period, and is known by nearly complete skeletons from the Lias of England...
and Elasmosaurus
Elasmosaurus
Elasmosaurus + σαυρος sauros 'lizard') is a genus of plesiosaur with an extremely long neck that lived in the Late Cretaceous period , 80.5 million years ago.-Description:...
, Polycotylus had a short neck. This led to it being classified as a pliosaur
Pliosaur
Pliosauroidea is an extinct clade of marine reptiles. Pliosauroids, also commonly known as pliosaurs, are known from the Jurassic and Cretaceous Periods. The pliosauroids were short-necked plesiosaurs with large heads and massive toothed jaws. These swimming reptiles were not dinosaurs but distant...
, a marine reptile within the superfamily Pliosauroidea, closely related to true plesiosaurs (which belong to the superfamily Plesiosauroidea). Polycotylus and other polycotylids superficially resemble pliosaurs like Liopleurodon
Liopleurodon
Liopleurodon is a genus of large, carnivorous marine reptile belonging to the Pliosauroidea, a clade of short-necked plesiosaurs. Two species of Liopleurodon lived during the Callovian stage of the Middle Jurassic Period , while the third, L. rossicus, lived during the Late Jurassic...
and Peloneustes
Peloneustes
Peloneustes is an extinct genus of sauropterygian reptile belonging to the family Pliosauridae. It is known from the Callovian aged deposits of the Oxford clay formation of England...
because they have short necks, large heads, and other proportions that differ from true plesiosaurs.
As phylogenetic analyses became common in the last few decades, the classification of Polycotylus and other plesiosaurs have been revised. In 1997, it and other polycotylids were reassigned as close relatives of long-necked elasmosaurids. In a 2001 study, Polycotylus was classified as a derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
cryptocleidoid plesiosaur closely related to Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
plesiosaurs like Cryptocleidus. Below is a cladogram from a 2004 study which supported a similar classification:
In 2007, Polycotylus was placed in a new subfamily of polycotylids called Polycotylinae. Another newly-described polycotylid called Eopolycotylus
Eopolycotylus
Eopolycotylus is an extinct genus of plesiosaur....
from Glen Canyon
Glen Canyon
Glen Canyon is a canyon that is located in southeastern and south central Utah and northwestern Arizona within the Vermilion Cliffs area. It was carved by the Colorado River....
, Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
, was found to be the closest relative of Polycotylus. Below is a cladogram from the 2007 study:
Reproduction
A fossil of P. latippinis catalogued LACM 129639 includes an adult individual with a single fetusFetus
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...
inside it. LACM 129639 was found in Kansas during the 1980s and was in storage at the Natural History Museum of Los Angeles County
Natural History Museum of Los Angeles County
The Natural History Museum of Los Angeles County opened in Exposition Park, Los Angeles, California, USA in 1913 as the Museum of History, Science, and Art. The moving force behind it was a museum association founded in 1910. Its distinctive main building, with fitted marble walls and domed and...
until it was described in 2011. The length of the fetus is around 40 percent of the length of the mother. Gestation was probably two thirds complete based on what is known of the fetal development of related nothosaur
Nothosaur
Nothosaurs were Triassic marine sauropterygian reptiles that may have lived like seals of today, catching food in water but coming ashore on rocks and beaches. They averaged about in length, with a long body and tail. The feet were paddle-like, and are known to have been webbed in life, to help...
s. This fossil suggests that Polycotylus was viviparous, giving live birth (as opposed to laying eggs).
Vivipary
Vivipary
Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...
, or live birth, may have been the most common form of reproduction in plesiosaurs, as they would have had difficulty laying eggs on land. Their bodies are not adapted to movement on land, and paleontologists have long hypothesized that they must have given birth in water. Other marine reptiles such as ichthyosaur
Ichthyosaur
Ichthyosaurs were giant marine reptiles that resembled fish and dolphins...
s also gave live birth, but LACM 129639 was the first direct evidence of vivipary in plesiosaurs. The lives of Polycotylus and other plesiosaurs were K-selected
R/K selection theory
In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity or quality of offspring...
, meaning that few offspring were born to each individual but those that were born were cared for as they mature. Because it gave birth to a single large offspring, the mother Polycotylus probably gave it some form of parental care for it to survive. F. Robin O'Keefe, one of the describers of LACM 129639, suggested that the social lives of plesiosaurs were "more similar to those of modern dolphins than other reptiles." K-selected life-history strategies are also seen in mammals and some lizards, but are unusual among reptiles.