Microcleididae is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

of basal plesiosauroid

plesiosaur The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared ...

s from the Early Jurassic (middle Sinemurian to late

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarcian ...

stages) of

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

Portugal Portugal, officially the Portuguese Republic ( pt, República Portuguesa, links=yes ), is a country whose mainland is located on the Iberian Peninsula of Southwestern Europe, and whose territory also includes the Atlantic archipelagos of ...

and the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and North ...

. Currently, the oldest and the most known microcleidid is ''

Eretmosaurus ''Eretmosaurus'' is an extinct genus of plesiosaur. Taxonomic history ''Eretmosaurus'' was coined by Harry Govier Seeley for ''Plesiosaurus rugosus'' Owen, 1840. Richard Owen had coined the name ''P. rugosus'' for numerous vertebrae from th ...

'' 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.

History and Phylogeny

Großman (2007) referred informally to "microcleidid

elasmosaur 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 p ...

s", a clade comprising the three taxa referred to the Microcleididae by Benson ''et al.'' (2012). Ketchum and Benson (2010) found the traditional "microcleidid" genera to nest within a monophyletic Plesiosauridae, in a close position to '' Plesiosaurus''. However, Ketchum and Benson (2011) performed a phylogenetic analysis which found a monospecific Plesiosauridae, and a monophyletic Microcleididae. Smith ''et al.'' (in press) described a new "microcleidid" ''

Lusonectes ''Lusonectes'' (meaning "Portuguese swimmer") is an extinct genus of plesiosaurid from Lower Jurassic (Toarcian stage) deposits of Alhadas, Portugal. It is known from the holotype MG33, a partial skull and articulated mandible. It was found by ...

'', and obtained similar results in a different phylogenetic analysis. However, the family has not formally been erected until Benson ''et al.'' (2012) also found the same results in their analysis. Microcleidus1DB

To avoid further confusion between Plesiosauridae and Microcleididae, Benson ''et al.'' (2012) proposed a stem-based definition for this taxon, utilizing ''Plesiosaurus'' as an external specifier. Microcleididae was defined as "'' Microcleidus homalospondylus'' and all taxa more closely related to it than to ''

Plesiosaurus dolichodeirus ''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 ...

'', ''

Cryptoclidus eurymerus ''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 been ...

'', ''

Elasmosaurus platyurus ''Elasmosaurus'' (;) is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, about 80.5million years ago. The first specimen was discovered in 1867 near Fort Wallace, Kansas, US, and was sent ...

'', '' Leptocleidus superstes'', '' Pliosaurus brachydeirus'' or ''

Polycotylus latipinnis ''Polycotylus'' is a genus of plesiosaur within the family Polycotylidae. The type species is ''P. latippinis'' and was named by American paleontologist Edward Drinker Cope in 1869. Eleven other species have been identified. The name means 'muc ...

''." Benson ''et al.'' (2012) found strong support for Microcleididae, however their diagnosis focused on unambiguous

postcrania Postcrania (postcranium, adjective: postcranial) in zoology and vertebrate paleontology is all or part of the skeleton apart from the skull. Frequently, fossil remains, e.g. of dinosaurs or other extinct tetrapods, consist of partial or isolated sk ...

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

since cranial material of basal microcleidids is unknown. The synapomorphies are: widely separated posterior

cervical rib A cervical rib in humans is an extra rib which arises from the seventh cervical vertebra. Their presence is a congenital abnormality located above the normal first rib. A cervical rib is estimated to occur in 0.2% to 0.5% (1 in 200 to 500) of the ...

facets, posteriormost dorsal rib facets split between

centrum (Latin for ''center'') may refer to: Places In Greenland * Nuuk Centrum, a district of Nuuk, Greenland * Centrum Lake, Greenland In the Netherlands * Amsterdam-Centrum, the inner-most borough of Amsterdam, Netherlands * Rotterdam Centrum, a borou ...

and neural arch, medial surface of the iliac blade anteroposteriorly concave and a prominent flange extends anteriorly from the proximal half of the radius. This analysis focused on basal plesiosaurs and therefore only one derived pliosaurid and one

cryptoclidia Cryptoclidia is a clade of plesiosaurs. Cryptoclidia was named and defined as a node clade in 2010 by Hilary Ketchum and Roger Benson: the group consisting of the last common ancestor of ''Cryptoclidus eurymerus'' and ''Polycotylus latipinnis ...

n were included while elasmosaurids weren't included at all. The cladogram below follows the topology from Benson ''et al.'' (2012) analysis.

References

{{Taxonbar, from=Q6839368 Jurassic plesiosaurs Sinemurian first appearances Early Jurassic extinctions Plesiosauroids Prehistoric reptile families