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. 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. 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,
Cryptoclidus eurymerus,
Elasmosaurus platyurus,
Leptocleidus superstes,
Pliosaurus brachydeirus or
Polycotylus latipinnis." Benson
et al. (2012) found strong support for Microcleididae, however their diagnosis focused on unambiguous
postcranial
synapomorphies since cranial material of basal microcleidids is unknown. The synapomorphies are: widely separated posterior
cervical rib 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
cryptoclidian were included while
elasmosaurids weren't included at all. The
cladogram below follows the topology from Benson
et al. (2012) analysis. }} ==References==