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 group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

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 appea ...

s from the

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...

(middle

Sinemurian In the geologic timescale, the Sinemurian is an age and stage in the Early or Lower Jurassic Epoch or Series. It spans the time between 199.3 ± 2 Ma and 190.8 ± 1.5 Ma (million years ago). The Sinemurian is preceded by the Hettangian and ...

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 Toar ...

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 betwee ...

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 th ...

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 ...

. 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 the E ...

'' 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 elasmosaurs", 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 ''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 b ...

''. 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'', 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 ...

'', '' Cryptoclidus eurymerus'', ''

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 s ...

'', '' Leptocleidus superstes'', '' Pliosaurus brachydeirus'' or '' Polycotylus latipinnis''." 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 ha ...

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 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

elasmosaurid 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 ...

s weren't included at all. The

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

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