Pleurosternon
Encyclopedia
Pleurosternon is an extinct genus
of cryptodire
turtle
from the late Jurassic period to the early Cretaceous period. Its type species
, P. bullocki was described by the paleontologist Richard Owen
(noted for coining the word Dinosauria) in 1853. Since then, and throughout the late 19th century, many fossil turtles were incorrectly assigned to this genus.
.
after Tyler R. Lyson and Walter G. Joyce (2009).
, much flatter than similar genera, such as the North American Late Jurassic
and Early Cretaceous
Glyptops
. Adults show little or none of the nuchal emargination that is more visible in juveniles. The Xiphiplastras also have a large, V-shaped notch near the back of the bone. Together with Platychelys, it is one of the few turtle genera to exhibit the characteristics of both modern turtle suborders, the Cryptodira
and the Pleurodira
.
's Purbeck Group and Portland stone
, some specimens were even recovered in the Purbeck's type locality
. Several areas within the formation became noted by some for producing pleurosternon fossils. Among them were Swanage
, Durlston Bay
, Langton Matravers
, and Herston
.
The genus has also been found in several rock formations from the upper Jurassic in both France
and Spain
, and from the Cretaceous
Wealden group
of both England and northern Germany
. The Purbeck formation, at the time was a coastal region with a complex system of shallow lagoon
s that slowly lost their salinity over time. Similarly, the younger Wealden group was also coastal plain
dominated by lagoons but with the addition of braided streams. The Portland stone, however is a maritime deposit of similar age near the Purbeck, most bones found there are interpreted as having washed out to sea from the Wealden or the Purbeck. Because of this, Pleurosternon has been described as a "shore-turtle".
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 cryptodire
Cryptodira
Cryptodira is the taxonomic suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira in that they lower their necks and pull the heads straight back into the shells; instead of folding their necks sideways along the body under the shells' margins...
turtle
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
from the late Jurassic period to the early Cretaceous period. Its type species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, P. bullocki was described by the paleontologist Richard Owen
Richard Owen
Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...
(noted for coining the word Dinosauria) in 1853. Since then, and throughout the late 19th century, many fossil turtles were incorrectly assigned to this genus.
Taxonony
Pleurosternon fossils were first described by Richard Owen in 1841 under the living genus Platemys. It was not until 1853 however, that it was published under the name Pleurosternon in a paper Owen presented to the Palaeontographical SocietyPalaeontographical Society
The Palaeontographical Society is a learned society established in 1847 and specifically devoted to the advancement of palaeontographical knowledge, i.e., palaeontology...
.
Phylogeny
CladogramCladogram
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...
after Tyler R. Lyson and Walter G. Joyce (2009).
Description
Pleurosternon has a very depressed carapaceCarapace
A carapace is a dorsal 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 underside is called the plastron.-Crustaceans:In crustaceans, the...
, much flatter than similar genera, such as the North American Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
and Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
Glyptops
Glyptops
Glyptops is an extinct genus of cryptodire turtle dating from the Late Jurassic and Early Cretaceous periods 155 to 99 m.y.a. Fossils have been found in South Dakota, Wyoming, Colorado, Utah, New Mexico, Oklahoma, and Texas from both the Morrison and Cedar Mountain formations. The type species is G...
. Adults show little or none of the nuchal emargination that is more visible in juveniles. The Xiphiplastras also have a large, V-shaped notch near the back of the bone. Together with Platychelys, it is one of the few turtle genera to exhibit the characteristics of both modern turtle suborders, the Cryptodira
Cryptodira
Cryptodira is the taxonomic suborder of Testudines that includes most living tortoises and turtles. Cryptodira differ from Pleurodira in that they lower their necks and pull the heads straight back into the shells; instead of folding their necks sideways along the body under the shells' margins...
and the Pleurodira
Pleurodira
The Pleurodira are one of the two living suborders of turtles, the other being the Cryptodira. In many cases in the nomenclature of animals, ranks such as suborder are considered of little importance apart from nomenclatural or taxonomic reasons. However, this is not the case with the suborders of...
.
Distribution and habitat
In Europe, most species of Pleurosternon are best known from southeast EnglandEngland
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
's Purbeck Group and Portland stone
Portland stone
Portland stone is a limestone from the Tithonian stage of the Jurassic period quarried on the Isle of Portland, Dorset. The quarries consist of beds of white-grey limestone separated by chert beds. It has been used extensively as a building stone throughout the British Isles, notably in major...
, some specimens were even recovered in the Purbeck's type locality
Type locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
. Several areas within the formation became noted by some for producing pleurosternon fossils. Among them were Swanage
Swanage
Swanage is a coastal town and civil parish in the south east of Dorset, England. It is situated at the eastern end of the Isle of Purbeck, approximately 10 km south of Poole and 40 km east of Dorchester. The parish has a population of 10,124 . Nearby are Ballard Down and Old Harry Rocks,...
, Durlston Bay
Durlston Bay
Durlston Bay is a small bay next to a country park of the same name, just south of the resort of Swanage, on the Isle of Purbeck in Dorset, England...
, Langton Matravers
Langton Matravers
Langton Matravers is a small village on the Isle of Purbeck, in the county of Dorset in the south of England.Langton Matravers is part of the Purbeck local government district and is within the South Dorset constituency of the House of Commons and the South West England constituency of the European...
, and Herston
Herston, Dorset
Herston is a village in Dorset, England. It lies in the civil parish of Swanage....
.
The genus has also been found in several rock formations from the upper Jurassic in both France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
and Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
, and from 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...
Wealden group
Wealden Group
The Wealden Group is a group in the lithostratigraphy of southern England. The Wealden group consists of paralic to continental facies sedimentary rocks of Valanginian to Barremian age and thus forms part of the English Lower Cretaceous. It is composed of alternating sands and clays...
of both England and northern Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. The Purbeck formation, at the time was a coastal region with a complex system of shallow 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,...
s that slowly lost their salinity over time. Similarly, the younger Wealden group was also coastal plain
Coastal plain
A coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in eastern South America. The southwestern coastal plain of North America is notable for its species diversity...
dominated by lagoons but with the addition of braided streams. The Portland stone, however is a maritime deposit of similar age near the Purbeck, most bones found there are interpreted as having washed out to sea from the Wealden or the Purbeck. Because of this, Pleurosternon has been described as a "shore-turtle".