Miniaturization and morphological evolution in Paleozoic relatives of living amphibians: a quantitative approach

Abstract

Miniaturization has been defined as the evolution of extremely small adult size in a lineage. It does not simply imply the decrease of the body size but also involves structural modifications to maintain functional efficiency at a strongly reduced size. Miniaturization has been proposed as a key factor in the origin of several major tetrapod clades. Current hypotheses propose that the living amphibians (lissamphibians) originated within a clade of Paleozoic dwarfed dissorophoid temnospondyls. Morphological traits shared by these small dissorophoids have been interpreted as resulting from constraints imposed by the extreme size reduction, but these statements were based only on qualitative observations. Herein, we assess quantitatively morphological changes in the skull previously associated with miniaturization in the lissamphibian stem lineage by comparing evolutionary and ontogenetic allometries in dissorophoids. Our results show that these features are not comparable to the morphological consequences of extreme size reduction as documented in extant miniature amphibians, but instead they resemble immature conditions of larger temnospondyls. We conclude that the truncation of the ancestral ontogeny, and not constraints related to miniaturization, might have been the factor that played a major role in the morphological evolution of small dissorophoids. Based on our results, we discuss the putative role of miniaturization in the origin of lissamphibians within Dissorophoidea.

Can We Reliably Calibrate Deep Nodes in the Tetrapod Tree? Case Studies in Deep Tetrapod Divergences

“…Lepospondyls are a morphologically diverse group of early tetrapods with little unifying morphology aside from small body size. Temnospondyls typically all share a common bauplan but exhibit a substantial disparity of body sizes, although putative lissamphibian outgroups within Temnospondyli are also small-bodied (Fröbisch and Schoch, 2009;Pérez-Ben et al, 2018). Phylogenetic support for the two hypotheses has traditionally been roughly within a statistical margin of error (Ruta and Coates, 2007;Marjanović and Laurin, 2019) with differing implications for both pattern of lissamphibian body plan assembly and timing of the origin of the tetrapod crown group.…”

“…The assembly of both the amphibian crown group and the amniote crown group are thought to have largely occurred at small body sizes (Carroll, 1982;Laurin, 2004;Kemp, 2007;Pérez-Ben et al, 2018), although the overall pattern of body size evolution in these clades is under some debate (Didier et al, 2019). Skeletal material from small vertebrates degrades more quickly than bones of larger vertebrates and is preferentially lost from the record (Behrensmeyer et al, 1979).…”

New material of the ‘microsaur’Llistrofusfrom the cave deposits of Richards Spur, Oklahoma and the paleoecology of the Hapsidopareiidae

“…Possibly concurrent with accommodation of musculature is the reorganization of both soft and hard tissues in the cranium that is frequently associated with miniaturization (Hanken & Wake, 1993). Miniaturization has been more thoroughly explored in the coeval amphibamid temnospondyls, with a focus on how this process could relate to the origin of modern lissamphibians (Fröbisch & Schoch, 2009; Schoch, 2013; Pérez-Ben, Schoch & Báez, 2018). With respect to “microsaurs,” miniaturization has been discussed with respect to the smaller brachystelechids Carrolla (Maddin, Olori & Anderson, 2011) and Brachydectes (Pardo & Anderson, 2016).…”

Descriptive skeletal anatomy of Blommersia transmarina (Amphibia: Anura: Mantellidae) from the Comoro Islands

“…Thus, we hypothesize that hyperossification of this structure may play an important role in the musculoskeletal functional performance of both males and females in the Blommersia genus. Furthermore, hyperossification of certain skeletal elements evokes the idea of structural compensation due to increasing individual size or improved mechanical performance at more developed stages (Pérez-Ben et al, 2018). This is especially true for structures that serve as muscle attachment sites such as the bony crest on the hyobranchial's posteromedial processes or the level of development of diverse processes on the transverse processes of presacral vertebrae II-IV.…”

“…However, there is currently no study of skeletal data that makes reference to size differentiation, especially in view that body size is known to influence dispersal and evolutionary rates within the genus (Wollenberg et al, 2011;Pabijan et al, 2012). Since B. transmarina is the largest Blommersia species identified to date (Glaw et al, 2019), it constitutes the perfect starting point to set a base to study the processes of miniaturization in the internal anatomy of mantellids on the grounds that miniaturization can lead to reduced ossification of post-metamorphic skeletal elements, hyperossification, and presents more structural trade-offs than enlargement (Hanken, 1993;Yeh, 2002;Hall, 2005;Pérez-Ben et al, 2018).…”