Аннотация:The limbs of marine reptiles lack some function and characteristics essential
for most amniotes: they do no more support body weight in terrestria l conditions but are
modified into fins to steer or stabil ize underwater motion, and into fl ippers , primary propulsive
organs. Thereby their structure was partially reorganized in order to serve new f unctions.
ln some cases, this leads to ambiguous identifications of certain elements and makes the ir
homologization problematic.
The main controversy is the identification of mesopodial elements related to the emerge
of 'neomorphic' ossicles in advanced ichthyosaurs and plesiosaurs. According to some
researchers, the element located distal to the zeugopod ia and between them is the astragalus,
whereas anterior to it the enlarged centrale emerges (e.g. Caldwe ll 1997a, b), or even the
distal tarsal, shifted proximally to the tibia, fills the empty space anterior to the astragalus
(e.g. McGowan and Motani 2003: p. 53). Others prefer to interpret the mesàpodial elements
using amphibian-like pattern, thereby identifying a separated intermed ium and tibiale (e.g.
Andrews 1910; McGowan and Motani 2003: p. 44).
Recently published data on 're-evolution' of mesopodial elements in chameleons due to
'missing structures' (Diaz & Trainor 2015) could be successively appl ied for marine reptile
tarsus. Thereby lacking of an astraga lus and evolving of an 'ancest ral' structu re convergent
with amphibians seems to be preferable . This adaptation could be explained by strengthen ing
of a leading edge of a limb in order to resist a hydrodynam ic loading, which is essential for
basal aquat ic rept iles retaining the terrestrial hindlimb structure and lack ing ossifications in
the anterior part of the mesopodium.