The study of the morphology, long bone microanatomy, and histology of Placodontia clearly shows that extinct taxa are not ‘simply similar’ to modern taxa but that they could have had a variety of features, which are in certain combinations sometimes without a modern analogy so that the possibilities for interpretation are limited.
Clear evolutionary trends within Placodontia regarding microanatomical and histological features are not observable but identification might be hampered due to still limited sample size. Fibro-lamellar bone tissue already occurs in placodonts from the late early Anisian and persists until the early Carnian with Henodus. The lamellar-zonal bone type is so far only documented in stratigraphically younger placodonts from the Anisian-Ladinian boundary and the Rhaetian. A similar distribution of lamellar-zonal bone and fibro-lamellar bone is documented in pachypleurosaurs (Sauropterygia). Neusticosaurus ssp. from the Anisian-Ladinian boundary of the Alpine Triassic show lamellar-zonal bone (Sander, 1990; Hugi et al., 2011) whereas the stratigraphically older Anarosaurus heterodontus from the Lower Muschelkalk of Winterswijk (Germanic Basin) grow with incipient fibro-lamellar bone (Klein, 2010). This could indicate that the fibro-lamellar bone tissue was inherited from the unknown terrestrial ancestor of Sauropterygia and was later abandoned in some taxa.
In conclusion, all placodonts sampled followed an essentially or probably exclusive aquatic lifestyle. However, we observe distinct differences in histology and microanatomy that lead to different growth strategies and life (swimming) styles in Placodontia independent from phylogenetic relationships, ontogenetic stages or the presence or absence of armour. The differences imply a high inter- as well as intraspecific variability, most likely depending on the environment each individual lived in (developmental plasticity), and diverse lifestyles among the group.