LOCOMOTION IN TRIADOBATRACHUS


crown-group anurans diversified, as is attested by the Jurassic salientians Prosalirus bitis (Shubin and Jenkins 1995) and Vieraella herbsti (Báez and Basso 1996). Since then, this general body plan has been highly conserved, despite extant taxonomic and ecological diversity observed in frogs, including a variety of locomotor capabil- ities and specializations (Emerson 1978; Roček 2000; Handrigan and Wassersug 2007). The latter includes strong jumping, hopping, burrowing, walking, swimming, climbing, and even gliding (Emerson 1978, 1979; Emerson et al. 1990), albeit a basic pattern of synchronous bilateral movements of the hindlimbs to propel a short, rigid body underlies several of these locomotor behaviors, both on land or underwater (Nauwelaerts and Aerts 2002). Locomotion in anurans has been investigated


for decades, with several studies converging, in that different aspects of postcranial morphology, mainly limb proportions and features of the sacro-caudo-pelvic complex, correlate to a varying degree with locomotor modes (Zug 1972, 1978; Emerson 1976, 1979, 1988; Gomes et al. 2009; Essner et al. 2010; Reilly and Jorgensen 2011; Jorgensen and Reilly 2013; Enriquez-Urzelai et al. 2015). Among locomotor modes, jumping has generally been considered to have evolved early in salientian history, together with most prominent features of the anuran Bauplan (e.g., Gans and Parsons 1966; Jenkins and Shubin 1998; Přikryl et al. 2009; Sigurdsen et al. 2012; but see Reilly and Jorgensen 2011 for an alternative view), although this hypothesis has derived from studies that did not incorporate an explicit phylogenetic framework. In any case, the anuran body plan and behavior depart considerably from an ancestral tetrapod condition with a locomotor pattern similar to that seen today in salamanders, which move with lateral bending of the spine and asymmetrical movements of their short limbs (Karakasiliotis et al. 2013). It is noteworthy that the fossil record


shows meager evidence of this evolutionary transition, with only two notable exceptions. The earliest known salientians Czatkobatrachus polonicus (Evans and Borsuk-Białynicka 1998) and Triadobatrachus massinoti (Piveteau 1936), both from the Early Triassic, have been


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considered as representing an intermediate stage in the acquisition of the anuran Bauplan (Jenkins and Shubin 1998; Roček and Rage 2000; Reilly and Jorgensen 2011). This is most apparent in the peculiar combination of features of T. massinoti, which includes a high number of presacral vertebrae (14) compared with anurans, several discrete caudal vertebrae (at least 6) instead of the urostyle, a sacral vertebra bearing free ribs, moderately elongated ilia, hindlimbs slightly longer than the forelimb, and unfused elements of the zeugopodia (Rage and Roček 1989). In this context, the origin of the anuran Bauplan and saltatory locomotion and the subsequent evolution in early anurans could not be fully understood without considering the basal locomotor capabilities of the earliest salientians in a phylogenetic context. Therefore, we aim to elucidate the main locomotormode of the early salientian T. massinoti with a multivariate approach in an explicit evolutionary context, using linear morphometrics of the major skeletal elements of the paired limbs. Correlation of these morphometric traits with locomotion in amphibians was tested using multivariate statistics and comparative phylogenetic methods. A discriminant func- tion analysis was performed to assess the ability of transformed morphometric variables to discriminate locomotor groups and to infer the locomotor capabilities of T. massinoti. Our findings, which contrast with some recent hypotheses (Jenkins and Shubin 1998; Přikryl et al. 2009; Sigurdsen et al. 2012), indicate that some distinct features of the anuran Bauplan were not originally linked to a saltatory locomotion.


Materials and Methods Taxonomic Sample.—Observations of


T. massinoti were based on a high-resolution mold of the holotype MNHN MAE126 kindly provided by Ana M. Báez (Fig. 1). The comparative sample (n=188 individuals) primarily included extant limbed amphibians (Anura + Caudata), representing 55 species and 21 families of frogs and 17 species and five families of salamanders (sensu Frost 2015;


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