Eryopid temnospondyls were large apex predators in Carboniferousand Permian stream and lake habitats. The eryopid life cycle is exemplifiedby Onchiodon labyrinthicus from Niederhäslich (Saxony, Germany), which is represented bynumerous size classes from small larvae to heavily ossified adults.Morphometric and principal component analyses provide new insights intoontogenetic changes in O. labyrinthicus, and comparison with adults of other eryopidsdocuments phylogenetic patterns in the occupation of morphospace.

Compared with small specimens of Sclerocephalus spp., immature O. labyrinthicus occupies a neighboring butmuch larger space, corresponding to a broader range of variation. Adults ofActinodon frossardi map with some juveniles of O. labyrinthicus, whereas other juveniles of the latter lieclose to adults of O. thuringiensis, Glaukerpeton avinoffi and Osteophorus roemeri.

Morphospace occupation of adult eryopids is partly consistent with cladistictree topology, which gives the following branching pattern: Actinodon frossardi forms thebasalmost eryopid, followed by Osteophorus roemeri, Glaukerpeton avinoffi and the genus Onchiodon (O. labyrinthicus + O. thuringiensis); then Clamorosaurus nocturnus; and finally themonophyletic genus Eryops. The presumably juvenile skull of Eryops anatinus falls well outside thedomains of both adult eryopids and immature O. labyrinthicus, showing a unique combinationof juvenile and adult features. Instead, Onchiodon langenhani and the Ruprechtice specimensreferred 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 broadvariation was possibly caused by developmental plasticity responding tofluctuations in lake hydrology and nutrients in this small, short-livedwater body.