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Plesiosaur
Thời điểm hóa thạch: Trung Trias to Creta muộn
Phân loại khoa học
Giới (regnum)	Animalia
Ngành (phylum)	Chordata
Lớp (class)	Reptilia
Liên bộ (superordo)	Sauropterygia
Bộ (ordo)	Plesiosauria
Phân bộ (subordo)	Gray, 1825
Họ
Pliosauridae Plesiosauridae Cryptoclididae Elasmosauridae

Plesiosaurs là một bộ các bò sát biển lớn, ăn thịt. Chúng tồn tại và phát tiển từ khoảng 245 triệu ănm trước (mya) tới 65 mya.

Mary Anning là người đầu tiên phát hiện ra plesiosaur. Bà dã tìm thấy nó ở 'Jurassic Coast' tại Dorset, Anh trong mùa đông cuối năm 1820 đầu năm 1821. Hóa thạch này đã bị mất hộp sọ, nhưng vào năm 1823 bà đã tìm thấy một hóa thạch khác, lần này thì đó là một bộ hoàn chỉnh cùng với hộp sọ. Cái tên Plesiosaurus được đặt bởi Rev. William Conybeare.

Những plesiosaur sớm nhất xuất hiện chủ yếu từ Trung Trias,^p128 và phát triển cực thịnh trong kỉ Jura và kỉ Creta. Chúng có hai cặp vây chèo lướn,đuôi ngắn, cổ ngắn hoặc dài, và cơ thể to lớn. Chúng tuyệt chủng trong sự kiện tuyệt chủng K-T vào 65 triệu năm trước.

Classification

Taxonomy

Kronosaurus, a pliosaurid

Thalassomedon, an elasmosaurid

Dolichorhynchops, a polycotylid

The taxonomy presented here is mainly based on the plesiosaur cladistic analyses proposed by Ketchum and Benson, 2011 and Benson et al., 2012 unless otherwise noted. In this rank-based list, the traditional suborders Plesiosauroidea and Pliosauroidea are maintained but the short-necked Leptocleididae and Polycotylidae, which traditionally had been seen as Pliosauroidea, have been placed in the Plesiosauroidea, reflecting their evolutionary position according to these studies.

• Clade Pistosauria
  • Augustasaurus
  • Pistosaurus
  • Yunguisaurus
  • Order Plesiosauria
    • Bobosaurus
    • Suborder Plesiosauroidea
      • Family Elasmosauridae
      • Family Microcleididae
      • Family Plesiosauridae
      • Clade Cryptoclidia
        • ? Family Aristonectidae
        • Family Cryptoclididae
        • Clade Leptocleidia
          • Family Leptocleididae
          • Family Polycotylidae
    • Suborder Pliosauroidea
      • ? Family Rhomaleosauridae
      • Family Pliosauridae

Phylogeny

In modern phylogeny, clades are defined groups that contain all species belonging to a certain branch of the evolutionary tree. One way to define a clade is to let it consist of the last common ancestor of two such species and all its descendants. Such a clade is called a "node clade". In 2008, Patrick Druckenmiller and Anthony Russell in this way defined Plesiosauria as the group consisting of the last common ancestor of Plesiosaurus dolichocheirus and Peloneustes philarchus; and all its descendants. Plesiosaurus and Peloneustes represented the main subgroups of the Plesiosauroidea and the Pliosauroidea and were chosen for historical reasons; any other species from these groups would have sufficed.

Another way to define a clade is to let it consist of all species more closely related to a certain species that one in any case wishes to include in the clade, than to another species that one to the contrary desires to exclude. Such a clade is called a "stem clade". Such a definition has the advantage that it is easier to include all species with a certain morphology. Plesiosauria was in 2010 by Hillary Ketchum and Roger Benson defined as such a stem-based taxon: "all taxa more closely related to Plesiosaurus dolichodeirus and Pliosaurus brachydeirus than to Augustasaurus hagdorni". Ketchum and Benson (2010) also coined a new clade Neoplesiosauria, a node-based taxon that was defined by as "Plesiosaurus dolichodeirus, Pliosaurus brachydeirus, their most recent common ancestor and all of its descendants". Neoplesiosauria very likely is materially identical to Plesiosauria sensu Druckenmiller & Russell and was meant to be a replacement of this concept. The following cladogram follows an analysis by Ketchum & Benson, 2011.

Cast of "Plesiosaurus" macrocephalus, yet to receive a valid genus name

Benson et al. (2012) found the traditional Pliosauroidea to be paraphyletic in relation to Plesiosauroidea. Rhomaleosauridae was found to be outside Neoplesiosauria, but still within Plesiosauria. The early carnian pistosaur Bobosaurus was found to be more advanced than Augustasaurus in relation to the Plesiosauria and therefore it represents by definition the oldest known plesiosaur. However, the authors excluded Bobosaurus from this clade without any explanation, although that exclusion might be a typo in the article. This analysis focused on basal plesiosaurs and therefore only one derived pliosaurid and one cryptoclidian were included, while elasmosaurids weren't included at all. The cladogram below follows the topology from Benson et al. (2012) analysis.

"Rhomaleosaurus" megacephalus, yet to receive a valid genus name

Đặc điểm

Plesiosaurs had many bones in their flippers, making them flexible. No modern animal has this four-paddle anatomy: modern turtles use their forelimbs for swimming. They were mainly piscivorous (fish-eaters).

Pliosauridea

The pliosaurs were a group of mostly large submarine predators with short necks and large heads. Their sizes ranged from two to 15 metres, and they were predators of large fish and other reptiles. Their streamlined body shape suggests they swam and ate under water.

• Liopleurodon.

Plesiosauridea cổ dài

One of Mary Anning's finds: Rhomaleosaurus in the Natural History Museum, London.

There were three families of long-necked plesiosaurs, who evidently had a different life-style from the pliosaurs. It was suggested by D.M.S. Watson that their method was as surface swimmers, mostly eating with their head above water, darting down to snatch smaller fish which were feeding on plankton. It is hard to see the benefit of a long neck under water; aquatic mammals operating under water all have a streamlined torpedo-shape, as did pliosaurs and ichthyosaurs. All the longer-necked familiers were, from the setting of the teeth and jaws, eaters of small fish. However, some at least were bottom-feeders, consuming various prey. Digestion of shellfish was aided by gastroliths.

• Plesiosaurids: neck not so long as the other two families, and not so flexible: a more general all-round plesiosaur. Head of medium size, neck fairly thick and strong, up to 30 vertebrae.
  • Plesiosaurus

• Cryptoclidids: longer necks, with more than 30 vertebrae.
  • Cryptoclidus

• Elasmosaurids: very long necks; some later forms have as many as 76 cervical (neck) vertebrae and quite small skulls.^p30 Watson and Alexander's ideas apply especially to this group.
  • Elasmosaurus
  • Thalassomedon
  • Mauisaurus, the longest plesiosaur ever found.

Gastroliths

Plesiosaur gastroliths

Plesiosaurs have been found with fossils of belemnites (squid-like animals), and ammonites (giant nautilus-like molluscs) associated with their stomachs. But plesiosaurs could not crack shells. Instead, they probably swallowed them whole. In the belly of a plesiosaur were "stomach stones", which are called gastroliths. These stones moved around in the plesiosaur's stomach and cracked or crushed the shells of the animals it ate. One plesiosaur fossil found in South Dakota had 253 gatroliths weighing a total of 29 pounds.

Live birth?

Live birth has been proved for ichthyosaurs, but is uncertain for plesiosaurs.