Plesiosaurus
Encyclopedia
Plesiosaurus was a genus of large marine sauropterygia
n reptile
that lived during the early part of the Jurassic
Period, and is known by nearly complete skeletons from the Lias
of England. It was distinguished by its small head, long and slender neck, broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the order Plesiosauria
, of which it is an early, but fairly typical member. It contains only one species, Plesiosaurus dolichodeirus. P. brachypterygius, P. guilielmiiperatoris and P. tournemirensis were assigned to new genera, Hydrorion
, Seeleyosaurus
and Occitanosaurus
.
bar). The anterior
portion is "bluntly triangular". In lateral view, the skull reaches its highest point at the rear of the skull table
. "The external nostrils overlie the internal nares". They are not positioned at the tip of the snout, but farther back, nearer the eyes than the tip of the skull. Unlike the nostrils of Rhomaleosaurus
, they do not appear to be adapted for underwater olfaction. The orbits
(eye sockets) are roughly circular and are positioned about halfway along the length of the skull. They face up and to the sides. Just posterior to the orbits are the supratemporal fenestrae, which are about the same size as the orbits and also roughly circular. Between the four openings is the pineal foramen, and between the temporal fenestrae is a narrow sagittal ridge. As in other plesiosaurs, the pterygoid
s of the palate are fused to the basioccipital of the braincase, although the union is not as robust as in the pliosaurs Rhomaleosaurus and Pliosaurus
. "The palatal bones are thin, but there is no suborbital fenestra."
The two rami of the lower jaw make a "V" shape with an angle of about 45°. The specialized region where they meet, the symphysis
, is robust. The two rami are fused at the symphysis, making a pointed, shallow scoop-like shape.
The teeth of Plesiosaurus are "simple, needle-like cones" that are "slightly curved and circular in transverse section". They are sharply pointed with fine striations running from tip to base, and point forward (procumbent). This procumbency becomes more pronounced near the leading end of the skull, where they may be only 10-15° above horizontal. There are 20 to 25 teeth per upper jaw tooth row, and 24 per low jaw tooth row. Up to four teeth of a lower jaw's tooth row are found in the symphyseal region.
Plesiosaurus was a moderately-sized plesiosaur that grew to a length of about 3.5 metres (11.5 ft). There are approximately 40 cervical vertebrae
(neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae. Of the rest of the vertebral column, there are a handful (four or five in the holotype
specimen) of "pectoral" vertebrae from the neck-torso transition, approximately 21 dorsal or back vertebrae, three or more sacral vertebrae
, and at least 28 caudal vertebrae. Generally, the centra
of the cervical vertebrae are relatively elongated, being slightly longer than tall. The width, however, is usually greater than or equal to the length. The articular surfaces of the cervical centra are "slightly concave and kidney-shaped, with rounded, slightly rugose edges." Small holes called foramina subcentralia are found on the ventral surface of the centra. Some of the dorsals have rugose articular edges, like the cervicals; this feature is typically absent from the caudals.
Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads. Dorsal ribs are thick and have only one head. Sacral ribs are "short, robust, and blunt or knob-like on both ends." Caudal ribs have different morphologies
depending on their location along the tail, with anterior examples being pointed and more distal examples being "broad and blunt." Plesiosaurus also has gastralia
, also known as "belly ribs." Nine or more sets of gastralia are present between the shoulder and pelvis
. Each set is composed of seven elements: a bone on the midline flanked by three lateral elements.
s at the anterior end, scapula
e (shoulder blades), and large coracoids. The scapulae and coracoids both contribute to the glenoids
(arm sockets). A pair of oval holes called pectoral fenestrae are found midway along the scapular/coracoid contacts. The forelimbs are elongate and relatively narrow compared to those of most plesiosaurs. The humerus
(upper arm bone) has distinctive curvature, which appears to be a retained primitive feature
among sauropterygia
ns. Mature Plesiosaurus also have a distinctive groove along the ventral surface of the humerus. The forearm includes a flat, broad, crescent-shaped ulna
and a "robust and pillar-like" radius
. The wrist includes six bones. The hand paddle has five digits; the phalangeal formula
is uncertain, but the count for one large individual, from "thumb
" to fifth "finger", is 4-8-9-8-6.
The pelvis includes equant pubic bones
, ischia, and blade-shaped ilia
connecting the pelvis to the vertebral column. The acetabulum
is formed by surfaces on the pubic bones and ischia. Similar to the pectoral girdle, there is a pair of holes between the ischia and pubic bones. The hindlimbs are long and narrow, and in adults, they are much smaller than the forelimbs. The thigh bones
are straight. The lower hindlimb includes two roughly equal-sized bones, the robust tibia
and the semilunate-shaped fibula. There are six bones in the ankle. The foot paddle includes five digits. Like the hand, the phalangeal formula is uncertain, but is at least 3-7-9-8-7 from innermost to outer "toe".
reptiles" to be discovered and excited great interest in Victorian England. It was so-named ("near lizard") by William Conybeare
and Henry De la Beche
, to indicate that it was more like a normal reptile than Ichthyosaurus
, which had been found in the same rock strata just a few years earlier. Plesiosaurus is the archetypical genus of Plesiosauria and the first to be described, hence lending its name to the order. Conybeare and De la Beche coined the name for scattered finds from the Bristol region, Dorset
, and Lyme Regis
in 1821. The type species
of Plesiosaurus, P. dolichodeirus, was named and described by Conybeare in 1824 on the basis of a complete skeleton discovered by the early collector Mary Anning
. She found this specimen in Sinemurian
(Early Jurassic
)-age rocks of the lower Lias Group
in December 1823. Additional fossils of Plesiosaurus were found in rocks of the Lias Group of Dorset for many years, "until the cessation of quarrying activities in the Lias Group, early in this [20th] century."
Plesiosaurus has historically been a wastebasket taxon. This is due in part to few anatomical or taxonomic studies of the relevant fossils. Uncritical taxonomic work resulted in hundreds of species representing most of the world and most of the Mesozoic being assigned to Plesiosaurus. None of the younger Jurassic
or Cretaceous
species belong to Plesiosaurus. Review of the Early Jurassic species indicates that the only English species properly assigned to Plesiosaurus is P. dolichodeirus. Several other European Early Jurassic species have been assigned to new genera. P. brachypterygius, P. guilielmiiperatoris and P. tournemirensis, for example, were assigned to the new genera Hydrorion
, Seeleyosaurus
and Occitanosaurus
.
, the 'Shales with Beef,' and the lower Black Ven Marls"; using the Lias Group ammonite
fossil zones, these rocks date to the early Sinemurian
stage. Some other Pleisosaurus fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from late Rhaetian
or early Hettangian
rocks.
Plesiosaurus fed on belemnites, fish and other prey. Its U-shaped jaw and sharp teeth would have been like a fish trap. It propelled itself by the paddles, the tail being too short to be of much use. Its neck could have been used as a rudder when navigating during a chase. It is unknown if Plesiosaurus laid eggs on land like sea turtle
s or gave live birth in the water like sea snakes. The young might have lived in estuaries
before going in the open ocean.
Sauropterygia
Sauropterygia were a group of very successful aquatic reptiles that flourished during the Mesozoic before they became extinct at the end of the era. They were united by a radical adaptation of their shoulder, designed to support powerful flipper strokes...
n reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
that lived during the early part of the 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...
Period, and is known by nearly complete skeletons from the Lias
Lias Group
The Lias Group or Lias is a lithostratigraphic unit found in a large area of western Europe, including the British Isles, the North Sea, the low countries and the north of Germany...
of England. It was distinguished by its small head, long and slender neck, broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the order Plesiosauria
Plesiosauria
Plesiosauria is an order of Mesozoic marine reptiles. Plesiosaurs first appeared in the Early Jurassic Period and became especially common during the Jurassic Period, thriving until the K-T extinction at the end of the Cretaceous Period.The name "plesiosaur" is used to refer to the order...
, of which it is an early, but fairly typical member. It contains only one species, Plesiosaurus dolichodeirus. P. brachypterygius, P. guilielmiiperatoris and P. tournemirensis were assigned to new genera, Hydrorion
Hydrorion
Hydrorion is an extinct genus of plesiosaur. It was found in Jurassic formations in Germany. The only species is Hydrorion brachypterygius....
, Seeleyosaurus
Seeleyosaurus
Seeleyosaurus is an extinct genus of plesiosaur. It is known from a large almost complete skeleton from the Upper Lias of Württemberg. There seems to be the impression of a rhomboidal flap of skin in a vertical plane; if so, many plesiosaurs may have been equipped in this way....
and Occitanosaurus
Occitanosaurus
Occitanosaurus is an extinct genus of plesiosaur from the Early Jurassic of what is now France. The type species is Occitanosaurus tournemirensis , first named by Sciau et al. in 1990...
.
Skull and dentition
Compared to other plesiosaur genera, Plesiosaurus has a small head. The skull is much narrower than long, reaching its greatest width just behind the eyes (the postorbitalPostorbital
The postorbital is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some vertebrates, the postorbital is fused with the postfrontal...
bar). The anterior
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...
portion is "bluntly triangular". In lateral view, the skull reaches its highest point at the rear of the skull table
Skull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...
. "The external nostrils overlie the internal nares". They are not positioned at the tip of the snout, but farther back, nearer the eyes than the tip of the skull. Unlike the nostrils of Rhomaleosaurus
Rhomaleosaurus
Rhomaleosaurus is an extinct genus of Early Jurassic rhomaleosaurid pliosauroid known from Northamptonshire and from Yorkshire of the United Kingdom. It was first named by Harry Seeley in 1874 and the type species is Rhomaleosaurus cramptoni...
, they do not appear to be adapted for underwater olfaction. The orbits
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...
(eye sockets) are roughly circular and are positioned about halfway along the length of the skull. They face up and to the sides. Just posterior to the orbits are the supratemporal fenestrae, which are about the same size as the orbits and also roughly circular. Between the four openings is the pineal foramen, and between the temporal fenestrae is a narrow sagittal ridge. As in other plesiosaurs, the pterygoid
Pterygoid
Pterygoid can refer to:* Pterygoid processes of the sphenoid bone** The Lateral pterygoid plate by it* a muscle such as Lateral pterygoid muscle or Medial pterygoid muscle* a branch of the Mandibular nerve...
s of the palate are fused to the basioccipital of the braincase, although the union is not as robust as in the pliosaurs Rhomaleosaurus and Pliosaurus
Pliosaurus
Pliosaurus is a genus of extinct marine reptile. It is included in the family Pliosauridae. Its diet would have included fish, squid and other marine reptiles. This genus has contained several species in the past but it currently consists of the type species P. brachydeirus, P. macromerus and P...
. "The palatal bones are thin, but there is no suborbital fenestra."
The two rami of the lower jaw make a "V" shape with an angle of about 45°. The specialized region where they meet, the symphysis
Symphysis menti
The external surface of the mandible is marked in the median line by a faint ridge, indicating the symphysis menti, mandibular symphysis, or line of junction of the two pieces of which the bone is composed at an early period of life....
, is robust. The two rami are fused at the symphysis, making a pointed, shallow scoop-like shape.
The teeth of Plesiosaurus are "simple, needle-like cones" that are "slightly curved and circular in transverse section". They are sharply pointed with fine striations running from tip to base, and point forward (procumbent). This procumbency becomes more pronounced near the leading end of the skull, where they may be only 10-15° above horizontal. There are 20 to 25 teeth per upper jaw tooth row, and 24 per low jaw tooth row. Up to four teeth of a lower jaw's tooth row are found in the symphyseal region.
Vertebral column
Cervical vertebrae
In vertebrates, cervical vertebrae are those vertebrae immediately inferior to the skull.Thoracic vertebrae in all mammalian species are defined as those vertebrae that also carry a pair of ribs, and lie caudal to the cervical vertebrae. Further caudally follow the lumbar vertebrae, which also...
(neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae. Of the rest of the vertebral column, there are a handful (four or five in 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) of "pectoral" vertebrae from the neck-torso transition, approximately 21 dorsal or back vertebrae, three or more sacral vertebrae
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
, and at least 28 caudal vertebrae. Generally, the centra
Body of vertebra
The body is the largest part of a vertebra, and is more or less cylindrical in shape. For vertebrates other than humans, this structure is usually called a centrum....
of the cervical vertebrae are relatively elongated, being slightly longer than tall. The width, however, is usually greater than or equal to the length. The articular surfaces of the cervical centra are "slightly concave and kidney-shaped, with rounded, slightly rugose edges." Small holes called foramina subcentralia are found on the ventral surface of the centra. Some of the dorsals have rugose articular edges, like the cervicals; this feature is typically absent from the caudals.
Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads. Dorsal ribs are thick and have only one head. Sacral ribs are "short, robust, and blunt or knob-like on both ends." Caudal ribs have different morphologies
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
depending on their location along the tail, with anterior examples being pointed and more distal examples being "broad and blunt." Plesiosaurus also has gastralia
Gastralium
Gastralia are dermal bones found in the ventral body wall of crocodilian and Sphenodon species. They are found between the sternum and pelvis, and do not articulate with the vertebrae...
, also known as "belly ribs." Nine or more sets of gastralia are present between the shoulder and pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
. Each set is composed of seven elements: a bone on the midline flanked by three lateral elements.
Limbs
The shoulder girdle is only partly known but appears to be typical for plesiosaurs. It includes fused clavicleClavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
s at the anterior end, scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
e (shoulder blades), and large coracoids. The scapulae and coracoids both contribute to the glenoids
Glenoid cavity
The glenoid cavity is a shallow pyriform, articular surface, which is located on the lateral angle of the scapula. It is directed laterally and forward and articulates with the head of the humerus; it is broader below than above and its vertical diameter is the longest.This cavity forms the...
(arm sockets). A pair of oval holes called pectoral fenestrae are found midway along the scapular/coracoid contacts. The forelimbs are elongate and relatively narrow compared to those of most plesiosaurs. The humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
(upper arm bone) has distinctive curvature, which appears to be a retained primitive feature
Symplesiomorphy
In cladistics, a symplesiomorphy or symplesiomorphic character is a trait which is shared between two or more taxa, but which is also shared with other taxa which have an earlier last common ancestor with the taxa under consideration...
among sauropterygia
Sauropterygia
Sauropterygia were a group of very successful aquatic reptiles that flourished during the Mesozoic before they became extinct at the end of the era. They were united by a radical adaptation of their shoulder, designed to support powerful flipper strokes...
ns. Mature Plesiosaurus also have a distinctive groove along the ventral surface of the humerus. The forearm includes a flat, broad, crescent-shaped ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
and a "robust and pillar-like" radius
Radius (bone)
The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...
. The wrist includes six bones. The hand paddle has five digits; the phalangeal formula
Phalanx bones
In anatomy, phalanx bones are those that form the fingers and toes. In primates such as humans and monkeys, the thumb and big toe have two phalanges, while the other fingers and toes consist of three. Phalanges are classified as long bones.The phalanges do not have individual names...
is uncertain, but the count for one large individual, from "thumb
Thumb
The thumb is the first digit of the hand. When a person is standing in the medical anatomical position , the thumb is the lateral-most digit...
" to fifth "finger", is 4-8-9-8-6.
The pelvis includes equant pubic bones
Pubis (bone)
In vertebrates, the pubic bone is the ventral and anterior of the three principal bones composing either half of the pelvis.It is covered by a layer of fat, which is covered by the mons pubis....
, ischia, and blade-shaped ilia
Ilium (bone)
The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...
connecting the pelvis to the vertebral column. The acetabulum
Acetabulum
The acetabulum is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.-Structure:...
is formed by surfaces on the pubic bones and ischia. Similar to the pectoral girdle, there is a pair of holes between the ischia and pubic bones. The hindlimbs are long and narrow, and in adults, they are much smaller than the forelimbs. The thigh bones
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
are straight. The lower hindlimb includes two roughly equal-sized bones, the robust tibia
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
and the semilunate-shaped fibula. There are six bones in the ankle. The foot paddle includes five digits. Like the hand, the phalangeal formula is uncertain, but is at least 3-7-9-8-7 from innermost to outer "toe".
Integument
Some traces of skin discovered suggest that Plesiosaurus was smooth, not scaly.Discovery and classification
Plesiosaurus was one of the first of the "antediluvianAntediluvian
The antediluvian period meaning "before the deluge" is the period referred to in the Bible between the Creation of the Earth and the Deluge . The narrative takes up chapters 1-6 of Genesis...
reptiles" to be discovered and excited great interest in Victorian England. It was so-named ("near lizard") by William Conybeare
William Daniel Conybeare
William Daniel Conybeare FRS , dean of Llandaff, was an English geologist, palaeontologist and clergyman. He is probably best known for his ground-breaking work on marine reptile fossils in the 1820s, including important papers for the Geological Society of London on ichthyosaur anatomy and the...
and Henry De la Beche
Henry De la Beche
Sir Henry Thomas De la Beche FRS was an English geologist and palaeontologist who helped pioneer early geological survey methods.-Biography:...
, to indicate that it was more like a normal reptile than Ichthyosaurus
Ichthyosaurus
Ichthyosaurus is an extinct genus of ichthyosaur from the Early Jurassic of Europe . It is among the best known ichthyosaur genera, with the Order Ichthyosauria being named after it...
, which had been found in the same rock strata just a few years earlier. Plesiosaurus is the archetypical genus of Plesiosauria and the first to be described, hence lending its name to the order. Conybeare and De la Beche coined the name for scattered finds from the Bristol region, Dorset
Dorset
Dorset , is a county in South West England on the English Channel coast. The county town is Dorchester which is situated in the south. The Hampshire towns of Bournemouth and Christchurch joined the county with the reorganisation of local government in 1974...
, and Lyme Regis
Lyme Regis
Lyme Regis is a coastal town in West Dorset, England, situated 25 miles west of Dorchester and east of Exeter. The town lies in Lyme Bay, on the English Channel coast at the Dorset-Devon border...
in 1821. 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...
of Plesiosaurus, P. dolichodeirus, was named and described by Conybeare in 1824 on the basis of a complete skeleton discovered by the early collector Mary Anning
Mary Anning
Mary Anning was a British fossil collector, dealer and palaeontologist who became known around the world for a number of important finds she made in the Jurassic age marine fossil beds at Lyme Regis where she lived...
. She found this specimen in Sinemurian
Sinemurian
In the geologic timescale, the Sinemurian is an age or stage in the Early or Lower Jurassic epoch or series. It spans the time between 196.5 ± 2 Ma and 189.6 ± 1.5 Ma...
(Early Jurassic
Early Jurassic
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...
)-age rocks of the lower Lias Group
Lias Group
The Lias Group or Lias is a lithostratigraphic unit found in a large area of western Europe, including the British Isles, the North Sea, the low countries and the north of Germany...
in December 1823. Additional fossils of Plesiosaurus were found in rocks of the Lias Group of Dorset for many years, "until the cessation of quarrying activities in the Lias Group, early in this [20th] century."
Plesiosaurus has historically been a wastebasket taxon. This is due in part to few anatomical or taxonomic studies of the relevant fossils. Uncritical taxonomic work resulted in hundreds of species representing most of the world and most of the Mesozoic being assigned to Plesiosaurus. None of the younger 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...
or 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...
species belong to Plesiosaurus. Review of the Early Jurassic species indicates that the only English species properly assigned to Plesiosaurus is P. dolichodeirus. Several other European Early Jurassic species have been assigned to new genera. P. brachypterygius, P. guilielmiiperatoris and P. tournemirensis, for example, were assigned to the new genera Hydrorion
Hydrorion
Hydrorion is an extinct genus of plesiosaur. It was found in Jurassic formations in Germany. The only species is Hydrorion brachypterygius....
, Seeleyosaurus
Seeleyosaurus
Seeleyosaurus is an extinct genus of plesiosaur. It is known from a large almost complete skeleton from the Upper Lias of Württemberg. There seems to be the impression of a rhomboidal flap of skin in a vertical plane; if so, many plesiosaurs may have been equipped in this way....
and Occitanosaurus
Occitanosaurus
Occitanosaurus is an extinct genus of plesiosaur from the Early Jurassic of what is now France. The type species is Occitanosaurus tournemirensis , first named by Sciau et al. in 1990...
.
Stratigraphy and paleoecology
Unequivocal specimens of Plesiosaurus dolichodeirus are limited to the Lyme Regis area of Dorset. It appears to be the most common species of plesiosaur in the Lias Group of England. Plesiosaurus is best represented from the "upper part of the Blue LiasBlue Lias
The Blue Lias is a geologic formation in southern, eastern and western England and parts of South Wales, part of the Lias Group. The Blue Lias consists of a sequence of limestone and shale layers, laid down in latest Triassic and early Jurassic times, between 195 and 200 million years ago...
, the 'Shales with Beef,' and the lower Black Ven Marls"; using the Lias Group ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
fossil zones, these rocks date to the early Sinemurian
Sinemurian
In the geologic timescale, the Sinemurian is an age or stage in the Early or Lower Jurassic epoch or series. It spans the time between 196.5 ± 2 Ma and 189.6 ± 1.5 Ma...
stage. Some other Pleisosaurus fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from late Rhaetian
Rhaetian
The Rhaetian is in geochronology the latest age of the Triassic period or in chronostratigraphy the uppermost stage of the Triassic system. It lasted from 203.6 ± 1.5 to 199.6 ± 0.6 million years ago...
or early Hettangian
Hettangian
The Hettangian is the earliest age or lowest stage of the Jurassic period of the geologic timescale. It spans the time between 199.6 ± 0.6 Ma and 196.5 ± 1 Ma . The Hettangian follows the Rhaetian and is followed by the Sinemurian.In Europe stratigraphy the Hettangian is a part of the time span in...
rocks.
Plesiosaurus fed on belemnites, fish and other prey. Its U-shaped jaw and sharp teeth would have been like a fish trap. It propelled itself by the paddles, the tail being too short to be of much use. Its neck could have been used as a rudder when navigating during a chase. It is unknown if Plesiosaurus laid eggs on land like sea turtle
Sea turtle
Sea turtles are marine reptiles that inhabit all of the world's oceans except the Arctic.-Distribution:...
s or gave live birth in the water like sea snakes. The young might have lived in estuaries
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
before going in the open ocean.
Sources
- Andrews, C. W. 1896. "On the structure of the plesiosaurian skull". Quarterly Journal of the Geological Society, London, 52, 246-253.
- Brown, D. S. 1981. "The English Upper Jurassic Plesiosauroidea (Reptilia) and a review of the phylogeny and classification of the Plesiosauria". Bulletin of the British Museum (Natural History): Geology, 35, (4), 253-347.
- Cruickshank, A. R. I.; Small, P. G.; and Taylor, M. A. 1991. "Dorsal nostrils and hydrodynamically driven underwater olfaction in plesiosaurs". Nature, 352, 62-64.
- Lydekker, R. 1889. Catalogue of the fossil Reptilia and Amphibia in the British Museum (Natural History), Part II. Containing the Orders Ichthyopterygia and Sauropterygia. British Museum (Natural History)
- Richard OwenRichard OwenSir 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...
, Fossil Reptili of the Liassic Formations, pt iii. (Monogr. Palaeont. Soc., 1865) - Persson, P. O. 1963. A revision of the classification of the Plesiosauria with a synopsis of the stratigraphical and geographical distribution of the group. Lunds Universitets Årsskrift, N. F. Avd. 2. 59, 1-59.
- Storrs, G. W. 1991. "Anatomy and relationships of Corosaurus alcovensis (Diapsida: Sauropterygia) and the Triassic Alcova Limestone of Wyoming". Bulletin of the Peabody Museum of Natural History, 44, 1-151.
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- Storrs, G. W. 1997. "Morphological and taxonomic clarification of the genus Plesiosaurus". 145-190. In Callaway, J. M and Nicholls, E. L. (eds.). Ancient Marine Reptiles. Academic press. London.
- Taylor, M. A. and Cruickshank, A. R. I. 1993. Cranial anatomy and functional morphology of Pliosaurus brachyspondylus (Reptilia: Plesiosauria) from the Upper Jurassuc of Westbury, Wiltshire. Philosophical Transactions of the Royal Society of London, Series B, 341, 399-418.
- Torrens, Hugh 1995. "Mary Anning (1799–1847) of Lyme; 'The Greatest Fossilist the World Ever Knew'". The British Journal for the History of Science, 25 (3): 257–284
External links
- Genus Plesiosaurus - The Plesiosaur Directory
- Plesiosauroidea - Palaeos
- Plesiosauria – Mikko's Phylogeny Archive