LOCOMOTION IN TRIADOBATRACHUS


with Hecht (1962) and Rage and Roček (1986), we consider that the latter interpretation is unfounded and that the holotype would instead represent a postmetamorphic individual. This is evidenced by the extent of cranial bones that begin to ossify late in amphibian metamorphosis, or even after metamorphosis, such as the sphenethmoid and some dermal bones of the suspensorium (Maglia et al. 2007; Weisbecker and Mitgutsch 2010), together with the well-ossified posteromedial processes of the hyoid (Rage and Roček 1986; R. O. Gómez personal observation). Also, well-ossified carpals and tarsals of the holotype contrast with the cartilaginous condition of anuran larvae (Rage and Roček 1986), which is also that of many juvenile frogs and adult salamanders (Marjanović and Witzmann 2015; R. O. Gómez personal observation). On the other hand, unossified long bone epiphyses, as in the holotype of T. massinoti, have previously been considered indicative of immaturity (Rage and Roček 1986). Never- theless, basal salientians such as Notobatrachus degiustoi and species of Liaobatrachus are represented by articulated and well-preserved specimens also lacking ossified epiphyses that have been interpreted as mature individuals (Báez and Basso 1996; Gao and Wang 2001; Gao and Chen 2004; Dong et al. 2013). Addi- tionally, in several extant amphibian species, particularly salamanders, the epiphyses ossify late during ontogeny or even remain cartilagi- nous throughout most of their life (Marjanović and Witzmann 2015; R. O. Gómez personal observation). Therefore, morphology of the holotype of T. massinoti is consistent with that of adults of many amphibian species, and interpreting it as an immature individual requires additional ad hoc assumptions. Triadobatrachus Morphology and Locomotion.—


Our results show that the limb proportions of T. massinoti, though somewhat intermediate, are more similar to those of salamanders than of frogs, which is also reflected in the high posterior probability with which it was assigned to the LU group. These findings would indicate that salamander-like asynchronous lateral undulatory movements were an important part of the locomotor skill


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repertoire of this taxon. This hypothesis is consistent with that of previous authors, who considered that T. massinoti’s axial and appendicular morphology “could have allowed quick crawling … but certainly not effective jumping” (Rage and Roček 1989: p. 15). In contrast, it has recently been suggested,


based primarily on the morphology of the humeral deltopectoral crest, allegedly similar to that of jumping frogs, “that hopping or jumping was an important form of locomo- tion” of T. massinoti, “perhaps combined with salamander-like crawling” (Sigurdsen et al. 2012: p. 87). Nevertheless, this morphology (Sigurdsen et al. 2012: Fig. 4) is also present in nearly all anurans, regardless of their loco- motor mode. Besides, humeral morphology might also be related to other aspects of anuran behavior, including feeding and mating (Duellman 1992; Grey et al. 1997; Sigurdsen et al. 2012). In this regard, the use of forelimbs during amplexus is ubiquitous among anurans and unique within amphibians (Wells 2007). If this mating behavior evolved early in salientian history, this might provide an alternative scenario for the humeral morphol- ogy seen in T. massinoti. Other authors, based on sacro-caudo-pelvic


complex morphology, have proposed that T. massinoti was able to move through frog-like synchronous limb movements, either by jumping on land (Shubin and Jenkins 1995; Jenkins and Shubin 1998) or swimming in an aquatic environment (Hecht 1962; Estes and Reig 1973). It has to be noted that the sacro-caudo-pelvic complex of T. massinoti shows a distinctive mosaic of features not seen in any other amphibian (Fig. 5C,D), which makes it difficult to interpret this structure functionally. This complex includes a pelvis with a derived iliac morphology similar to that of frogs, but unlike in frogs, the ischia are not fused. In turn, sacral morphology exhibits the plesiomorphic condition today seen in salamanders, with recurved ribs articulating with robust transverse processes (Rage and Roček 1989; Fig. 5A,B). This configuration suggests that the sacroiliac joint would be like that of salamanders, in which the sacral rib laterally contacts the medial surface of the iliac


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