Life cycles, plasticity and palaeoecology in temnospondyl amphibians

Abstract

In the largest early tetrapod clade, the temnospondyls, ontogenies were diverse and quite distinct from the life cycles of extant amphibians. Three well‐studied clades exemplify the diversity of these long‐extinct ontogenies, here analysed with respect to their bearing on developmental plasticity, reaction norms and evolution. Sclerocephalus readily adjusted by means of developmental evolution to different lake environments. In addition, plasticity (reaction norm) played a significant role, apparent both morphologically and by altered developmental traits. Size increase and extension of the ontogenetic trajectory gave larger predators, a phenomenon also found in the dissorophoid Micromelerpeton. Whereas Sclerocephalus was throughout preying on the same fishes, Micromelerpeton was able to fit into different trophic levels. In the branchiosaurid Apateon, a biphasic life cycle was established, with metamorphosis producing a terrestrial morph in some species; truncation of the ontogenetic trajectory gave a sexually mature larva as an alternative morph (neoteny). Plasticity was high in the larval morphs, permitting neotenes to live as filter feeders or small carnivores. Fine‐tuning of development permitted Apateon populations to adjust to specific lake properties and readily change from a filter‐feeding to carnivorous mode of life. In the nonmetamorphosing Triassic Gerrothorax, morphology was extremely conserved, but histology reveals much plasticity at the microscopical level, correlating with fluctuating salinity and water energy. In responding to environmental fluctuations by enhanced plasticity, the studied temnospondyls managed to populate lakes inhabitable to other tetrapods and fishes.

Carpus and tarsus of Temnospondyli

“…Other amphibamids such as Eoscopus lockardi have at least the tarsus sufficiently ossified to reveal details in individual bones for comparison with other temnospondyls (Daly 1994). Even among aquatic taxa, developmental plasticity can produce an altered pathway forming more terrestrial adults (Schoch 2014). Populations of Sclerocephalus in environmentally different lake systems can develop into large or small fully aquatic adults with poorly ossified postcranial skeletons or occasionally large terrestrial adults with robust postcranial skeletons including well-formed carpus and tarsus.…”

A Middle Triassic palaeontological gold mine: The vertebrate deposits of Vellberg (Germany)

“…For instance, even within the small temnospondyl clade of plagiosaurids, brackish (Plagiosternum), lacustrine (Plagiosuchus), and stream dwellers (Megalophthalma, see have been identified -along with a pioneer taxon (Gerrothorax) that dwelled numerous different habitats (Schoch, 2014). In addition, partial or complete drying of lakes was followed by the deposition of terrestrial faunas in close succession.…”

Developmental bias in the fossil record

“…In general, this is a difficulty faced when investigating the fossil record in search for support for PLE; macroevolutionary patterns have historically proven an inadequate source of discrimination between genetic variation and plasticity (Hopkins, 2014;Schoch, 2014;Webster, 2019). PLE predicts that novel environments lead to evolutionary change mediated by phenotypic and genetic accommodation.…”

“…Naturally, demonstrating this kind of intraspecific pattern requires a fossil organism or lineage with a highly resolved stratigraphy, sufficient abundance, and known presence across temporal and/or environmental gradients (Schoch, 2014). Such research is not uncommon (e.g., Hopkins & Webster, 2009;Schoch, 2014;Webber & Hunda, 2007), yet morphological variability is rarely the focal point, and phenotypic accommodation rarer still.…”