The life cycle in late Paleozoic eryopid temnospondyls: developmental variation, plasticity and phylogeny

Abstract

Eryopid temnospondyls were large apex predators in Carboniferous and Permian stream and lake habitats. The eryopid life cycle is exemplified by Onchiodon labyrinthicus from Niederhäslich (Saxony, Germany), which is represented by numerous size classes from small larvae to heavily ossified adults. Morphometric and principal component analyses provide new insights into ontogenetic changes in O. labyrinthicus, and comparison with adults of other eryopids documents phylogenetic patterns in the occupation of morphospace. Compared with small specimens of Sclerocephalus spp., immature O. labyrinthicus occupies a neighboring but much larger space, corresponding to a broader range of variation. Adults of Actinodon frossardi map with some juveniles of O. labyrinthicus, whereas other juveniles of the latter lie close to adults of O. thuringiensis, Glaukerpeton avinoffi and Osteophorus roemeri. Morphospace occupation of adult eryopids is partly consistent with cladistic tree topology, which gives the following branching pattern: Actinodon frossardi forms the basalmost eryopid, followed by Osteophorus roemeri, Glaukerpeton avinoffi and the genus Onchiodon (O. labyrinthicus + O. thuringiensis); then Clamorosaurus nocturnus; and finally the monophyletic genus Eryops. The presumably juvenile skull of Eryops anatinus falls well outside the domains of both adult eryopids and immature O. labyrinthicus, showing a unique combination of juvenile and adult features. Instead, Onchiodon langenhani and the Ruprechtice specimens referred to O. labyrinthicus map within the domain of immature O. labyrinthicus. Raised levels of variation in O. labyrinthicus coincide with evidence of a stressed habitat, in which limiting factors were fluctuating salinity, absence of fishes, enhanced competition and seasonal algal blooms. The documented broad variation was possibly caused by developmental plasticity responding to fluctuations in lake hydrology and nutrients in this small, short-lived water body.