Do osteohistology, microanatomy or life history traits show patterns linked to neotenic lifestyles in extant Caudata and the Permian temnospondyl Apateon?

Neoteny is widespread in extant Caudata and has also been inferred in extinct temnospondyls. Here, we compare osteohistology, microanatomy, and life history traits of the Permian branchiosaurid Apateon spp. with those of modern obligate and facultative neotenic and not neotenic caudates to possibly identify common patterns separating groups. The osteohistology of these modern caudates is studied for the first time based on petrographic ground sections, which yield a better comparability between modern and fossilized bones than microtomized thin sections. We found in all adult individuals' avascular lamellar matrix, except for Necturus maculosus, which grew with avascular parallel-fibered bone. The presence of osteoblasts indicates prolonged bone mineralization process in caudates. Medullary cavity size varies widely in humeri of Ambystoma mexicanum and in humeri and femora of Apateon spp., whereas in all other taxa it is more uniform and tendentially compacter which, however, might be the result of lower sample sizes. Apateon spp. had numerous lines of arrested growth indicating strong seasonal growth as was described in detail before and which is in accordance with the inferred environmental conditions. The markedness of growth marks is well pronounced in Necturus maculosus, Amphiuma means, Triturus cristatus, Lissotriton vulgaris and Salamandra salamandra but not in others. Poor expression of growth marks might be related to constant climatic conditions (e.g., Ambystoma mexicanum, Proteus anguinus) or due to an early ontogenetic stage and a yet low mineralized matrix of some individuals in our sample (Ichthyosaura alpestris, Calotriton asper). Growth mark counts of captive A. mexicanum of known age considerably underestimated their actual age. The analysis of adult body mass, age at maturation and maximum longevity of a large dataset on obligate neotenic and non-obligate neotenic caudates indicated a high variability in the two latter traits and associations among each other and to adult body mass. Due to this large variability, we did not find any pattern that can be used to unequivocally infer whether a taxon is obligate neotenic or not. Overall, our results suggest that no shared pattern in osteohistology, microanatomy or life history traits exists among extant obligate neotenic, non-obligate neotenic and extinct neotenic Apateon.