The largest specimen of &lt;i&gt;Apateon&lt;/i&gt; and the life history pathway of neoteny in the Paleozoic temnospondyl family Branchiosauridae

N B Fröbisch,R R Schoch

doi:10.5194/fr-12-83-2009

Abstract

Two distinct developmental trajectories, metamorphosis and neoteny (the retention of larval somatic features in adult animals), have been reported for the small gill-bearing branchiosaurids of the Late Carboniferous and Early Permian of central Europe. Based on a very large specimen of the species <i>Apateon caducus</i> (Ammon, 1889), anatomical features characteristic for the neotenic phenotype of branchiosaurids are described. Large neotenes lack changes that occur during a short phase of transformation into terrestrial adults (metamorphosis), such as ossification of the braincase and palatoquadrate and intercentra, further ossification of the girdles and formation of muscle attachment scars and processes on the limb bones. They also lack a distinct sculpturing of the dermal skull roofing elements with deep polygonal ridges and grooves. Instead, larval somatic features are retained including ossified branchial denticles indicative of open gill slits and accentuated larval-type sculpturing of the dermal skull roof. Large size, high degree of ossification as compared to the larvae, and the presence of uncinate processes on the ribs clearly demonstrate an adult ontogenetic stage. Neotenes remained in the aquatic environment throughout their life and were most likely not capable of effective terrestrial locomotion. The frequency distribution of the two phenotypes in modern salamander populations and the environmental cues that influence the development of them provide a comparative framework for the discussion of the evolution of the two life history pathways in branchiosaurids. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.200800012" target="_blank">10.1002/mmng.200800012</a>

Highlights

The small, gill-bearing Branchiosauridae represent the best-known clade within the diverse dissorophoid amphibians of the Late Carboniferous and Early Permian
The findings showed that the tempo and mode of metamorphosis is comparable to modern amphibians and represents the first evidence for a condensed metamorphosis outside the Lissamphibia
Changes in the ornamentation of the dermal skull roofing elements have frequently been suggested to be associated with metamorphosis in Paleozoic amphibians (Bystrow 1935; Boy 1974; Schoch 2001; Schoch & Fræbisch 2006) and metamorphosed specimens of Apateon gracilis display a pattern of pronounced polygonal ridges and grooves otherwise never found in branchiosaurids

Summary

Introduction

R.: Neoteny in Branchiosauridae suggested that the absence of metamorphosed adults that would have visited the lakes only seasonally for mating, could be a result of the incomplete fossil record, because very few specimens are known from the shoreline or near shore terrestrial environment (Boy & Sues 2000). We discuss the biological background and the available morphological evidence for neoteny in branchiosaurids in more detail, based on a previously undescribed, wellpreserved specimen of a large Apateon caducus with a skull length of 38 mm from the Niederkirchen locality in the Saar-Nahe region of western Germany (Figs 1, 2, 3B, C).

Results

Conclusion