Is solid always best? Cranial performance in solid and fenestrated caecilian skulls

Abstract

Accepted 23 November 2011 SUMMARY Caecilians (Lissamphibia: Gymnophiona) are characterized by a fossorial lifestyle that appears to play a role in the many anatomical specializations in the group. The skull, in particular, has been the focus of previous studies because it is driven into the substrate for burrowing. There are two different types of skulls in caecilians: (1) stegokrotaphic, where the squamosal completely covers the temporal region and the jaw closing muscles, and (2) zygokrotaphic, with incomplete coverage of the temporal region by the squamosal. We used 3-D imaging and modeling techniques to explore the functional consequences of these skull types in an evolutionary context. We digitally converted stegokrotaphic skulls into zygokrotaphic skulls and vice versa. We also generated a third, akinetic skull type that was presumably present in extinct caecilian ancestors. We explored the benefits and costs of the different skull types under frontal loading at different head angles with finite element analysis (FEA). Surprisingly, the differences in stress distributions and bending between the three tested skull types were minimal and not significant. This suggests that the open temporal region in zygokrotaphic skulls does not lead to poorer performance during burrowing. However, the results of the FEA suggest a strong relationship between the head angle and skull performance, implying there is an optimal head angle during burrowing. Supplementary material available online at

Evolution of Cranial Shape in Caecilians (Amphibia: Gymnophiona)

“…The evolutionary and functional implications of zygokrotaphic and stegokrotaphic crania have long been debated. Zygokrotaphy is reasoned to be the ancestral condition for caecilians because this occurs in rhinatrematids (sister group to all other living caecilians) and because frogs and salamanders have fully open-roofed crania (gymnokrotaphy) (reviewed in Kleinteich et al 2012;Maddin et al 2012a). While our findings cannot further resolve the ancestral condition in caecilians, the position of zygo-and stegokrotaphic crania in morphospace (Fig.…”

“…Conversely, zygokrotaphic crania are expected to be more kinetic (at least passively) and therefore less suited for burrowing, but perhaps better for handling larger and/or more muscular prey (Nussbaum 1983). Functional models examining patterns of stress and strain across the cranium during different loading regimes suggest however that there is no mechanical disadvantage to zygokrotaphy (Kleinteich et al 2012). Instead, Kleinteich et al (Kleinteich et al 2012) suggest that burrowing performance may be related to another factor, head angle during soil penetration.…”

“…Functional models examining patterns of stress and strain across the cranium during different loading regimes suggest however that there is no mechanical disadvantage to zygokrotaphy (Kleinteich et al 2012). Instead, Kleinteich et al (Kleinteich et al 2012) suggest that burrowing performance may be related to another factor, head angle during soil penetration. Given that zygokrotaphy is not only retained but likely re-evolved within extant caecilians, it seems unlikely that it represents simply the absence of a possible adaptation to improved burrowing.…”

“…Some of this information is becoming more available for caecilians (Müller 2006b;Kleinteich et al 2012) and also for fossil temnospondyls, an extinct group closely related to modern amphibians (Stayton and Ruta 2006;Fortuny et al 2011;Angielczyk and Ruta 2012). Such studies will also become increasingly feasible as caecilian phylogeny becomes even better resolved.…”

Cranial Morphology of the Carboniferous-Permian Tetrapod Brachydectes newberryi (Lepospondyli, Lysorophia): New Data from µCT

“…Early reports of lysorophian fossils identified these animals as early aquatic salamanders [22–23] or caecilians [24] but the first major attempts at describing lysorophian anatomy made it clear that lysorophian anatomy is inconsistent with direct ancestry to any specific lissamphibian group, and that lysorophians are either relatives of Lissamphibia more generally [25] or are morphologically-specialized early tetrapods without clear affinities to any modern tetrapod group [26–28]. A close relationship between lysorophians and modern lissamphibians has reemerged in some phylogenetic analyses [15–16, 29] and individual characteristics of lysorophians, such as the zygokrotaphic skull, have reemerged as possible support for a close relationship between lysorophians and caecilians [19] (but see [30] for further discussion), but a majority of analyses continue to find lysorophians to have no direct relevance to questions of lissamphibian origins [1–5, 21, 31]. …”