One of the many amazing features of animal regeneration is that although broadly distributed throughout phylogeny, there is an enormous heterogeneity in the regenerative capabilities shown by closely related species. A typical example is the capacity shown by some vertebrates to regenerate their limbs. Whereas many amphibians (newts, axolotls, frogs) are able to regenerate their limbs and tails and some fishes regenerate their fins, mammals have lost this ability. This heterogeneity has raised the question whether regeneration was a basal condition at the root of animal evolution and has been subsequently lost in some lineages or, alternatively, it has appeared independently in some animal groups. The fact that many regenerative models share a wide number of features and conserved signalling and genetic pathways controlling key aspects of the regenerative process supports the homology of animal regeneration. On the other hand, some studies have reported the existence of salamander-specific genes required for regeneration, which has been used to propose that the regenerative abilities may have appeared independently in different lineages. However, despite that some specific features may exist in each of the current regenerative models it is also evident that most of them share many more other key properties of regeneration. Somehow it is similar to considering embryogenesis as a basal conserved feature (with the dozens of conserved genes and pathways playing homologous roles) that has acquired some specific traits in different species depending on the type of fecundation or type of egg, among others.
Going back to the case of limb regeneration in tetrapods, a recent paper from the laboratory of Nadia Fröbisch reports on evidences of limb regeneration in a 300-million-year-old-amphibian (http://www.ncbi.nlm.nih.gov/pubmed/25253458). These observations have been made in several very well preserved specimens of Micromelerpeton crederni, a basal member of the dissorophoid clade within the temnospondyl amphibians. Although the phylogenetic position of modern amphibians remains still under debate, the authors state here that most scientists consider that dissophoroid temnospondyls including Micromelerpton represent the stem lineage of modern amphibians.
Many current amphibians are able to regenerate their limbs in a very precise way, so the regenerated limb is undistinguishable from the original one. However, there are also cases in which such regeneration is not perfect and some abnormalities appear. These cases may include repetitive amputations of the limbs, interference of some key early steps of regeneration or amputation at different stages of the life cycle. What it has been shown is that the abnormalities that appear during limb regeneration are usually different from the abnormalities that normally appear during limb development. What the authors of this study have found is that the pattern and combination of abnormalities in the limbs of the Micromelerpeton fossils are directly comparable to the variant morphological patterns in the regenerated limbs of current salamanders. These patterns include fusions along the proximo-distal axis and abnormalities predominantly located on the preaxial side of the autopods. Thus, the most common variant caused by abnormal regeneration in salamanders is an increase or decrease in the count of phalangeal numbers, which is also the most frequently abnormality in Micromelerpteon fossils.
In summary, the results presented here suggest that Micromelerpteon was capable of regenerating its limbs further suggesting that limb regeneration was an ancient capacity of the dissorophoid lineage leading towards modern amphibians and that has been retained in some lineages (such as salamanders). Further studies should try to determine the causes that have lead to the maintenance or loss of such regenerative ability in the dissorophoid lineage.