Plesiosaurus Temporal range: Early Jurassic, | |
---|---|
Restored skeleton in Japan | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Superorder: | † Sauropterygia |
Order: | † Plesiosauria |
Superfamily: | † Plesiosauroidea |
Family: | † Plesiosauridae |
Genus: | † Plesiosaurus De la Beche & Conybeare, 1821 |
Species: | †P. dolichodeirus |
Binomial name | |
†Plesiosaurus dolichodeirus Conybeare, 1824 | |
Plesiosaurus (Greek: πλησίος (plesios), near to + σαῦρος (sauros), 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 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, the type, Plesiosaurus dolichodeirus. Other species once assigned to this genus, including P. brachypterygius, P. guilielmiimperatoris, and P. tournemirensis have been reassigned to new genera, such as Hydrorion , Seeleyosaurus and Occitanosaurus .
The first complete skeleton of Plesiosaurus was discovered by early paleontologist and fossil hunter Mary Anning in Sinemurian (Early Jurassic)-age rocks of the lower Lias Group in December 1823. [1] [2] Additional fossils of Plesiosaurus were found in rocks of the Lias Group of Dorset for many years, [3] [4] [5] "until the cessation of quarrying activities in the Lias Group, early in this [20th] century." [2] although less complete remains were used by Henry De la Beche and William Conybeare to name the species two years earlier in 1821, [6] and despite being discovered first, Conybeare's remains were not the holotype; Anning's were.
Plesiosaurus was one of the first of the "antediluvian reptiles" to be discovered and excited great interest in 19th-century 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. [6] The type species of Plesiosaurus, P. dolichodeirus, was named and described by Conybeare in 1824 on the basis of Anning's original finds.
Compared to other plesiosaur genera, Plesiosaurus has a small head. The skull is much narrower than long, [7] reaching its greatest width just behind the eyes (the postorbital bar). [8] The anterior portion is "bluntly triangular". [8] In lateral view, the skull reaches its highest point at the rear of the skull table. [9] "The external nostrils overlie the internal nares". [8] They are not positioned at the tip of the snout, but farther back, nearer the eyes than the tip of the skull. [7] Unlike the nostrils of Rhomaleosaurus , [10] they do not appear to be adapted for underwater olfaction. [8] The orbits (eye sockets) are roughly circular and are positioned about halfway along the length of the skull. [8] They face up and to the sides. [7] [9] Just posterior to the orbits are the supratemporal fenestrae, which are about the same size as the orbits and also roughly circular. [8] Between the four openings is the pineal foramen, and between the temporal fenestrae is a narrow sagittal ridge. [8] As in other plesiosaurs, the pterygoids of the palate are fused to the basioccipital of the braincase, [8] although the union is not as robust as in the pliosaurs Rhomaleosaurus and Pliosaurus . [8] [11] "The palatal bones are thin, but there is no suborbital fenestra." [8]
The two rami of the lower jaw make a "V" shape with an angle of about 45°. [7] 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. [12]
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. [7] There are 20 to 25 teeth per upper jaw tooth row, [8] and 24 per low jaw tooth row. [7] Up to four teeth of a lower jaw's tooth row are found in the symphyseal region. [12]
Plesiosaurus was a moderately sized plesiosaur that grew to 2.87–3.5 m (9.4–11.5 ft) in length. [13] [14] There are approximately 40 cervical vertebrae (neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae. [15] 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, [15] approximately 21 dorsal or back vertebrae, three or more sacral vertebrae, and at least 28 caudal vertebrae. [16] 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. [15]
Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads. [15] 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." [15] 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. [16]
The shoulder girdle is only partly known but appears to be typical for plesiosaurs. It includes fused clavicles at the anterior end, scapulae (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. [16] 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 sauropterygians. 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. [17] 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. [18]
The pelvis includes equant pubic bones, ischia, [18] and blade-shaped ilia connecting the pelvis to the vertebral column. [19] 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. [18] The hindlimbs are long and narrow, [19] and in adults, they are much smaller than the forelimbs. [18] 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". [19]
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. [2] Several other European Early Jurassic species have been assigned to new genera. P. brachypterygius, P. guilielmiimperatoris and P. tournemirensis, for example, were assigned to the new genera Hydrorion , Seeleyosaurus and Occitanosaurus .
The following cladogram follows an analysis by Benson et al., 2012, and shows the placement of Plesiosaurus within Plesiosauria. [20]
Plesiosaurus fed mainly on clams and snails, and is thought to have eaten belemnites, fish and other prey as well. [21] 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. Plesiosaurus gave live birth to live young in the water like sea snakes. The young might have lived in estuaries before moving out into the open ocean. It has been postulated that the long neck of Plesiosaurus would have been a hindrance when trying to speed up, any bend in the neck creating turbulences. [22] If that is the case then Plesiosaurus would have had to keep its neck straight to achieve good acceleration, something that would make hunting difficult. For this reason it may be possible that these animals would actually lie in wait for prey to come close instead of trying to pursue them.
Unequivocal specimens of Plesiosaurus dolichodeirus are limited to the Lyme Regis area of Dorset. [23] It appears to be the most common species of plesiosaur in the Lias Group of England. [24] Plesiosaurus is best represented from the "upper part of the Blue Lias, the 'Shales with Beef,' and the lower Black Ven Marls" the latter of which form part of the Charmouth Mudstone; using the Lias Group ammonite fossil zones, these rocks date to the early Sinemurian stage. Some other Plesiosaurus fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from late Rhaetian or early Hettangian rocks. [25]
Elasmosaurus is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, about 80.5 million years ago. The first specimen was discovered in 1867 near Fort Wallace, Kansas, US, and was sent to the American paleontologist Edward Drinker Cope, who named it E. platyurus in 1868. The generic name means "thin-plate reptile", and the specific name means "flat-tailed". Cope originally reconstructed the skeleton of Elasmosaurus with the skull at the end of the tail, an error which was made light of by the paleontologist Othniel Charles Marsh, and became part of their "Bone Wars" rivalry. Only one incomplete Elasmosaurus skeleton is definitely known, consisting of a fragmentary skull, the spine, and the pectoral and pelvic girdles, and a single species is recognized today; other species are now considered invalid or have been moved to other genera.
The Plesiosauria or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.
Liopleurodon is an extinct genus of large, carnivorous marine reptile belonging to the Thalassophonea, a clade of short-necked pliosaurid plesiosaurs. Liopleurodon lived from the Callovian Stage of the Middle Jurassic to the Kimmeridgian stage of the Late Jurassic Period. It was the apex predator of the Middle to Late Jurassic seas that covered Europe. The largest species, L. ferox, is estimated to have grown over 6 metres (20 ft) in length based on a large skull.
Macroplata is an extinct genus of Early Jurassic rhomaleosaurid plesiosaur which grew up to 4.65 metres (15.3 ft) in length. Like other plesiosaurs, Macroplata probably lived on a diet of fish, using its sharp needle-like teeth to catch prey. Its shoulder bones were fairly large, indicating a powerful forward stroke for fast swimming. Macroplata also had a relatively long neck, twice the length of the skull, in contrast to pliosaurs.
Elasmosauridae is an extinct family of plesiosaurs, often called elasmosaurs. They had the longest necks of the plesiosaurs and existed from the Hauterivian to the Maastrichtian stages of the Cretaceous, and represented one of the two groups of plesiosaurs present at the end of the Cretaceous alongside Polycotylidae.
Muraenosaurus is an extinct genus of cryptoclidid plesiosaur reptile from the Oxford Clay of Southern England. The genus was given its name due to the eel-like appearance of the long neck and small head. Muraenosaurus grew up to 5.2 metres (17 ft) in length and lived roughly between 160 Ma and 164 Ma in the Callovian of the middle Jurassic. Charles E. Leeds collected the first Muraenosaurus which was then described by H. G. Seeley. The specimen may have suffered some damage due to the casual style of Charles Leeds’ collection. The first muraenosaur was recovered with pieces missing from the skull and many of the caudal vertebrae absent. Because the animal was described from Charles Leeds’ collection it was given the name Muraenosaurus Leedsi. M. leedsi is the most complete specimen belonging to the genus Muraenosaurus and also the only species that is undoubtedly a member of the genus. Two other species have been tentatively referred to as members of the genus Muraenosaurus: M. reedii and Muraenosaurus beloclis Seeley 1892, which in 1909 became the separate genus Picrocleidus.
Peloneustes 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 species of Plesiosaurus by palaeontologist Harry Govier Seeley in 1869, before being given its own genus by naturalist Richard Lydekker in 1889. While many species have been assigned to Peloneustes, P. philarchus is currently the only one still considered valid, with the others moved to different genera, considered nomina dubia, or synonymised with P. philarchus. Some of the material formerly assigned to P. evansi have since been reassigned to "Pliosaurus" andrewsi. Peloneustes is known from many specimens, including some very complete material.
Thalassiodracon (tha-LAS-ee-o-DRAY-kon) is an extinct genus of plesiosauroid from the Pliosauridae that was alive during the Late Triassic-Early Jurassic (Rhaetian-Hettangian) and is known exclusively from the Lower Lias of England. The type and only species, is Thalassiodracon (Plesiosaurus) hawkinsi.
Leptocleidus is an extinct genus of plesiosaur, belonging to the family Leptocleididae. It was a small plesiosaur, measuring only up to 3 m (9.8 ft).
Augustasaurus is a genus of aquatic sauropterygian reptile belonging to the Pistosauria, a clade containing plesiosaurs and their close relatives. Pistosaurus and Augustasaurus were thought to be the only known members of the family Pistosauridae. However, some recent cladistic analyses found Augustasaurus to be a more advanced pistosaur, as a sister group of the order Plesiosauria. The only known species of Augustasaurus is Augustasaurus hagdorni, which was first described in 1997.
Eretmosaurus is an extinct genus of plesiosaur from the Early Jurassic Blue Lias of England. Only the type species is known, which is E. rugosus.
Sthenarosaurus is an extinct genus of rhomaleosaurid pliosauroid discovered in the Toarcian-aged 'Main Alum Shale' in Whitby, England. The type species, S. dawkinsi, was named and described in 1909. The type specimen is MMUM LL 8023, a set of postcrania discovered in Whitby. Other indeterminate specimens are known, including a pectoral girdle described in 1911.
Plesiopterys is an extinct genus of plesiosaur originating from the Posidonienschiefer of Holzmaden, Germany, and lived during the Early Jurassic period. It is thought to be the sister taxon to all other plesiosauroids including the Plesiosaurus, and is placed outside of the Plesiosauroidea group. Plesiopterys wildi is the one known species within the genus, and is 220 centimeters long, or about 7.2 feet, and its body and skull are both relatively small. It possesses a unique combination of both primitive and derived characters, and is currently displayed at the State Museum of Natural History, Germany.
Avalonnectes is an extinct genus of small-bodied rhomaleosaurid known from the Early Jurassic period of the United Kingdom. It contains a single species, A. arturi.
Stratesaurus is an extinct genus of small-bodied rhomaleosaurid plesiosaur known from the Early Jurassic period of the United Kingdom. It contains a single species, S. taylori. It was a small plesiosaur, with a skull length of 18 cm (7.1 in) and a body length of 2 m (6.6 ft).
Eoplesiosaurus is an extinct genus of basal plesiosauroid known from the Early Jurassic period of the United Kingdom. It contains a single species, E. antiquior.
Microcleididae is an extinct family of basal plesiosauroid plesiosaurs from the Early Jurassic of France, Germany, Portugal and the United Kingdom. Currently, the oldest and the most known microcleidid is Eretmosaurus from the middle Sinemurian of the United Kingdom. Microcleididae was formally named and described by Roger B. J. Benson, Mark Evans and Patrick S. Druckenmiller in 2012.
Anningasaura is an extinct genus of basal plesiosaur. It is known from a single type species, A. lymense, discovered in Early Jurassic rocks of Lyme Regis in the United Kingdom.
This timeline of plesiosaur research is a chronologically ordered list of important fossil discoveries, controversies of interpretation, taxonomic revisions, and cultural portrayals of plesiosaurs, an order of marine reptiles that flourished during the Mesozoic Era. The first scientifically documented plesiosaur fossils were discovered during the early 19th century by Mary Anning. Plesiosaurs were actually discovered and described before dinosaurs. They were also among the first animals to be featured in artistic reconstructions of the ancient world, and therefore among the earliest prehistoric creatures to attract the attention of the lay public. Plesiosaurs were originally thought to be a kind of primitive transitional form between marine life and terrestrial reptiles. However, now plesiosaurs are recognized as highly derived marine reptiles descended from terrestrial ancestors.
Atychodracon is an extinct genus of rhomaleosaurid plesiosaurian known from the Late Triassic - Early Jurassic boundary of England. It contains a single species, Atychodracon megacephalus, named in 1846 originally as a species of Plesiosaurus. The holotype of "P." megacephalus was destroyed during a World War II air raid in 1940 and was later replaced with a neotype. The species had a very unstable taxonomic history, being referred to four different genera by various authors until a new genus name was created for it in 2015. Apart from the destroyed holotype and its three partial casts, a neotype and two additional individuals are currently referred to Atychodracon megacephalus, making it a relatively well represented rhomaleosaurid.