''Polycotylus'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

plesiosaur The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...

within the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Polycotylidae Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. They are known as false pliosaurs. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widesprea ...

. The

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

is ''P. latippinis'' and was named by American paleontologist

Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontology, paleontologist, comparative anatomy, comparative anatomist, herpetology, herpetologist, and ichthyology, ichthyologist. Born to a wealthy Quaker fam ...

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

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...

of North America toward the end of the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is 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.

History

Edward Drinker Cope named ''Polycotylus'' from the

Niobrara Formation The Niobrara Formation , also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is com ...

Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...

in 1869. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

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 The Mooreville Chalk is a geological formation in North America, within the U.S. states of Alabama and Mississippi, which were part of the subcontinent of Appalachia. The strata date back to the early Santonian to the early Campanian stage of th ...

Alabama Alabama ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South, Deep Southern regions of the United States. It borders Tennessee to the north, Georgia (U.S. state), Georgia to the east, Florida and the Gu ...

, but was not described until 2002. A new species, ''P. sopozkoi'', from Russia was described in 2016.

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 a medium-sized plesiosaur, with the type species (''P. latipinnis'') measuring long. It has more neck vertebrae than other polycotylids, however. ''Polycotylus'' is thought to be a basal polycotylid because it has more vertebrae in its neck (a feature that links it with long-necked ancestors) and its

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

has a more primitive shape. The long

ischia Ischia ( , , ) is a volcanic island in the Tyrrhenian Sea. It lies at the northern end of the Gulf of Naples, about from the city of Naples. It is the largest of the Phlegrean Islands. Although inhabited since the Bronze Age, as a Ancient G ...

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 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 sep ...

and a low

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are excepti ...

on top of the skull.

Classification

Unlike some better-known long-necked plesiosaurs like ''

Plesiosaurus ''Plesiosaurus'' (Greek: ' ('), near to + ' ('), lizard) is a genus of extinct, large marine sauropterygian reptile that lived during the Early Jurassic. It is known by nearly complete skeletons from the Lias of England. It is distinguishable by ...

'' and ''

Elasmosaurus ''Elasmosaurus'' () is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, at about 80.6 to 77million years ago. The first specimen was discovered in 1867 near Fort Wallace, Kansas, US, and ...

'', ''Polycotylus'' had a short neck. This led to it being classified as a

pliosaur Pliosauroidea is an extinct clade of plesiosaurs, known from the earliest Jurassic to early Late Cretaceous. They are best known for the subclade Thalassophonea, which contained crocodile-like short-necked forms with large heads and massive toot ...

, 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'' (; meaning 'smooth-sided teeth') is an extinct genus of carnivorous pliosaurid pliosaurs that lived from the Callovian stage of the Middle Jurassic to the Kimmeridgian stage of the Late Jurassic period (c. 166 to 155 mya). T ...

'' and ''

Peloneustes ''Peloneustes'' (meaning ) is a genus of pliosaurid plesiosaur from the Middle Jurassic of England. Its remains are known from the Peterborough Member of the Oxford Clay Formation, which is Callovian in age. It was originally described as a sp ...

'' because they have short necks, large heads, and other proportions that differ from true plesiosaurs. As

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

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 The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

plesiosaurs like ''

Cryptocleidus ''Cryptoclidus'' ( ) is a genus of plesiosaur reptile from the Middle Jurassic period of England, France, and Cuba. Discovery ''Cryptoclidus'' was a plesiosaur whose specimens include adult and juvenile skeletons, and remains which have bee ...

''. Below is a cladogram from a 2004 study which supported a similar classification: Polycotylus latipinnis paddle Williston

In 2007, ''Polycotylus'' was placed in a new subfamily of polycotylids called

Polycotylinae Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. They are known as false pliosaurs. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widesprea ...

. Another newly described polycotylid called '' Eopolycotylus'' from

Glen Canyon Glen Canyon is a natural canyon carved by a length of the Colorado River, mostly in southeastern and south-central Utah, in the United States. Glen Canyon starts where Narrow Canyon ends, at the confluence of the Colorado River and the Dirty ...

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

, was found to be the closest relative of ''Polycotylus''. Below is a cladogram from the 2007 study:

Paleobiology

Reproduction

A fossil of ''P. latippinis'' catalogued LACM 129639 includes an adult individual with a single

fetus A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic development, embryonic stage, the fetal stage of development takes place. Pren ...

inside it. LACM 129639 was found in Kansas during the 1980s and was in storage at the

Natural History Museum of Los Angeles County Nature is an inherent character or constitution, particularly of the ecosphere or the universe as a whole. In this general sense nature refers to the laws, elements and phenomena of the physical world, including life. Although humans are par ...

until it was described in 2011. The length of the fetus is around , which is about 32 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 Nothosaurs (superfamily Nothosauroidea) were Triassic marine sauropterygian reptiles. 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 power the animal when sw ...

s. This fossil suggests that ''Polycotylus'' was viviparous, giving live birth (as opposed to laying eggs).

Viviparity In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juv ...

, 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 Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides. Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...

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 In ecology, selection theory relates to the natural selection, selection of combinations of Trait (biological), traits in an organism that trade off between quantity and quality of offspring. The focus on either an increased quantity of offsprin ...

, 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. Examination of the fetus of ''Polycotylus'' indicates that while in the womb, plesiosaurs sacrificed fetal bone strength for accelerated growth rates. Histological analysis and comparisons with another plesiosaur, ''

Dolichorhynchops ''Dolichorhynchops'' is an extinct genus of polycotylid plesiosaur from the Late Cretaceous of North America, containing the species ''D. osborni'' and ''D. herschelensis'', with two previous species having been assigned to new genera. Definitiv ...

'', showed that some plesiosaur infants were up to forty percent the length of the mother when born, and that infant plesiosaurs may have had some compromised swimming abilities.

References