Triadobatrachus massinoti, the earliest known lissamphibian (Vertebrata: Tetrapoda) re-examined by μCT scan, and the evolution of trunk length in batrachians

Abstract

Triadobatrachus massinoti is a batrachian known from a single fossil from the Early Triassic of Madagascar that presents a combination of apomorphic salientian and plesiomorphic batrachian characters. Herein we offer a revised description of the specimen based on X-ray micro-tomography data. We report previously unknown caudal vertebrae, possible mentomeckelians, and hidden parts of other structures. We also confirm the presence of a ventrolateral ledge on the opisthotic, and we rectify some previous interpretations. There are no cervical ribs and the jaw may have had an angular. The presacral region is composed of 15 vertebrae with a unique atlas-axis complex instead of 14 vertebrae with a bipartite atlas. The configuration of the pelvic girdle is not very clear, although it is likely more plesiomorphic than the anuran-like condition previously assumed. Our re-assessment of the saltatorial performance of Triadobatrachus supports the traditional interpretation that this animal was not a specialised jumper. In order to assess the sequence of events in the early evolution of the salientian morphotype, we estimated the ancestral length of the trunk region of batrachians under different hypotheses of lissamphibian relationships and divergence times. Most of our results suggest that some trunk reduction took place before the divergence of caudates and salientians (presumably in the Permian), and that the trunk of Triadobatrachus mostly reflects this ancestral condition. Thus, trunk reduction possibly preceded the anteroposterior elongation of the ilia and the shortening of the tail seen in Triadobatrachus. We also provide an updated review of the data relevant for the use of Triadobatrachus as a calibration constraint in molecular divergence age analyses that meets recently-suggested standards.

Morphological Variation in Anuran Limbs: Constraints and Novelties

“…Triadobatrachu s is believed to have inhabited riparian forests or woodlands in or near floodplains, a suitable environment for a walker/hopper (Sigurdsen et al., ; Lires et al., ; Ascarrunz et al., ). Many additional distinct anatomical features involved in the locomotion of modern anurans appear by the Early Jurassic, as found in Prosalirus bitis (Shubin and Jenkins, ).…”

Evolution of morphology and locomotor performance in anurans: relationships with microhabitat diversification

“…Ascarrunz et al . () re‐assessed saltatorial performance of the basalmost and oldest stem‐anuran Triadobatrachus massinoti , from early Triassic. These authors suggested that the trunk of these animals mostly reflects the nonelongated ancestral condition and confirmed previous hypotheses that they were probably not specialized jumpers (Ascarrunz et al ., ).…”

“…Anuran body shape presents specializations associated with jumping performance that are old and phylogenetically conserved, being present in their most basic aspects in fossils from the Jurassic period (Shubin & Jenkins, 1995). Ascarrunz et al (2016) re-assessed saltatorial performance of the basalmost and oldest stem-anuran Triadobatrachus massinoti, from early Triassic. These authors suggested that the trunk of these animals mostly reflects the nonelongated ancestral condition and confirmed previous hypotheses that they were probably not specialized jumpers (Ascarrunz et al, 2016).…”

“…Ascarrunz et al (2016) re-assessed saltatorial performance of the basalmost and oldest stem-anuran Triadobatrachus massinoti, from early Triassic. These authors suggested that the trunk of these animals mostly reflects the nonelongated ancestral condition and confirmed previous hypotheses that they were probably not specialized jumpers (Ascarrunz et al, 2016). The origin and maintenance of adaptations to jumping performance in anuran amphibians are probably related to selective pressures associated with prey capture and escape from predators (Gans & Parsons, 1966;Zug, 1972;Emerson, 1978;Gomes et al, 2002).…”

How informative is vertebral development for the origin of lissamphibians?

“…Despite an increase in knowledge on the fossil record of lissamphibians (e.g. Gao & Shubin, 2003;Jenkins, Walsh & Carroll, 2007;Skutschas & Martin, 2011;Ascarrunz et al, 2016), their interrelationship as well as their origin (or origins) from the vast range of early tetrapods remains a matter of debate (Laurin & Reisz, 1997;Meyer & Zardoya, 2003;Schoch & Milner, 2004;Ruta, Coates & Quicke, 2003;Ruta & Coates, 2007;Sigurdsen & Green, 2011;Marjanovi c & Laurin, 2013;Schoch, 2014;Pardo, Small & Huttenlocker, 2017a;Pardo et al, 2017b). Most authors consider lissamphibians as a monophyletic group of extant tetrapods including caecilians (gymnophionans), salamanders (urodeles) and frogs (anurans) that plesiomorphically possess a biphasic life cycle with an aquatic larval stage that metamorphoses into a mainly terrestrial adult stage.…”