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Ability of regenerate limbs ancient in tetrapods

Ben Creisler

A new paper:

Nadia B. Fröbisch, Constanze Bickelmann, Jennifer C. Olori & Florian
Witzmann (2015)
Deep-time evolution of regeneration and preaxial polarity in tetrapod
limb development.
Nature (advance online publication)

Among extant tetrapods, salamanders are unique in showing a reversed
preaxial polarity in patterning of the skeletal elements of the limbs,
and in displaying the highest capacity for regeneration, including
full limb and tail regeneration. These features are particularly
striking as tetrapod limb development has otherwise been shown to be a
highly conserved process. It remains elusive whether the capacity to
regenerate limbs in salamanders is mechanistically and evolutionarily
linked to the aberrant pattern of limb development; both are features
classically regarded as unique to urodeles. New molecular data suggest
that salamander-specific orphan genes play a central role in limb
regeneration and may also be involved in the preaxial patterning
during limb development. Here we show that preaxial polarity in limb
development was present in various groups of temnospondyl amphibians
of the Carboniferous and Permian periods, including the dissorophoids
Apateon and Micromelerpeton, as well as the stereospondylomorph
Sclerocephalus. Limb regeneration has also been reported in
Micromelerpeton, demonstrating that both features were already present
together in antecedents of modern salamanders 290 million years ago.
Furthermore, data from lepospondyl ‘microsaurs’ on the amniote stem
indicate that these taxa may have shown some capacity for limb
regeneration and were capable of tail regeneration, including
re-patterning of the caudal vertebral column that is otherwise only
seen in salamander tail regeneration. The data from fossils suggest
that salamander-like regeneration is an ancient feature of tetrapods
that was subsequently lost at least once in the lineage leading to
amniotes. Salamanders are the only modern tetrapods that retained
regenerative capacities as well as preaxial polarity in limb