LISSAMPHIBIA

Marjanović, David & Laurin, Michel, 2013, The origin (s) of extant amphibians: a review with emphasis on the “ lepospondyl hypothesis ”, Geodiversitas 35 (1), pp. 207-272 : 221-222

PHYLOGENY OF LISSAMPHIBIA

As if the confusion about the origin(s) of the extant amphibians were not enough, there is no broad consensus in the current literature on whether the frogs or the caecilians are the extant sister-group of the salamanders. The first hypothesis recognizes Batrachia Latreille, 1800, a clade formed by anurans and urodeles; the second recognizes Procera Feller & Hedges, 1998, formed by urodeles and gymnophionans. The position of the albanerpetontids is even less clear, with all possible positions except a sister-group relationship to the frogs having been supported by phylogenetic analyses within the last ten years. To some degree, as pointed out in the literature, these hypotheses interact with the abovementioned hypotheses on the origin of Lissamphibia, because several character states present in amphibamids (especially Doleserpeton or Gerobatrachus ) or lepospondyls (especially lysorophians) are present in some but not all lissamphibians.

BATRACHIA OR PROCERA?

Anderson (2008) portrayed the Procera hypothesis as part of the LH. This is indeed the topology that best fits the results of Vallin & Laurin (2004; Fig. 4A ) and earlier installments of the LH, as well as (arguably) those of the morphology-only analysis by Pyron (2011), but the Batrachia hypothesis is strongly supported by the Bayesian analyses with combined data by Pyron (2011) and the bootstrap analyses of Marjanović & Laurin (2008b: fig. 6d; 2009: supplementary figure; see also Fig. 4D, E ). Likewise, Germain (2008a) found the Batrachia hypothesis (frogs and salamanders as sister-groups to the exclusion of caecilians) to be better supported ( Fig. 4C ), as did Marjanović (2010; Fig. 4F ). Pawley found the same result in some ( Pawley 2006: figs 90-92; Fig. 4B ) but not others of her analyses (fig. 89 shows Procera; fig. 88 shows a polytomy between Salientia, the gymnophionomorph Eocaecilia , Caudata , and Albanerpetontidae ). In fact, as pointed out by Bolt (1991), Ruta et al. (2003) and Schoch & Milner (2004), the Procera hypothesis is more compatible with the TH because certain character states are shared only by salientians and dissorophoid temnospondyls; a possible example is the tympanic middle ear, of which any trace is lacking in Caudata and Gymnophionomorpha (and, as far as can be determined, Albanerpetontidae ), but which several authors believe to have been present in many or most temnospondyls, including all terrestrial and amphibious dissorophoids (e.g., Bolt & Lombard 1985). Indeed, Maddin & Anderson (2012) found the Procera hypothesis in combination with the TH (although Maddin et al. [2012] did not). Under the Batrachia hypothesis combined with the TH, homology of the salientian and the putative dissorophoid tympanum would require two (or, depending on the position of the albanerpetontids, more likely three) independent losses that resulted each time in convergence with the stapedial morphology and spatial relationships seen in lepospondyls and early amniotes but not any temnospondyls. In any case, the presence of a tympanum in temnospondyls is debatable; Laurin & Soler-Gijón (2006) reviewed evidence that most temnospondyls lacked a tympanum, and Witzmann & Schoch (2006) showed that if the terrestrial dissorophoid temnospondyl Acanthostomatops Credner, 1883 , possessed a tympanum, it must have been rather different in size, shape and position from that seen in frogs and commonly reconstructed in Doleserpeton .

The Batrachia hypothesis appears to be better supported than the Procera hypothesis by both morphological ( Ruta & Coates 2007; Germain 2008a; Marjanović & Laurin 2008b, 2009; Marjanović 2010; Sigurdsen & Green 2011; Maddin et al. 2012) and, to a lesser extent, molecular data ( Marjanović & Laurin 2007; Anderson 2008: table 2; San Mauro 2010). The combined analyses by Pyron (2011) concur – this is important because combined analyses do not necessarily yield results supported by any of their constituent data sets when these are analyzed in isolation; sometimes, three ( Lee 2009) or even “forty-five wrongs make a right” ( Gatesy & Baker 2005). Except for Maddin & Anderson (2012), which we consider superseded by Maddin et al. (2012), no analysis which included albanerpetontids has ever found Procera; but all of these have so far lacked molecular data altogether.

THE PHYLOGENETIC POSITION

OF ALBANERPETONTIDAE

Unfortunately, the confusion summarized by Marjanović & Laurin (2008b: 169) still reigns. The albanerpetontids were long interpreted as stem-caudates ( Trueb & Cloutier 1991) and were again found in such a position by Anderson (2007), Anderson et al. (2008b) and Maddin et al. (2012).Arguing against this hypothesis, McGowan & Evans(1995) and McGowan (2002), as well as Marjanović &Laurin (2009:ESM 2 [supplementary figure]; Fig.4E ) and Marjanović(2010; Fig. 4F ), found Albanerpetontidae and Batrachia as sister-groups. Ruta & Coates (2007) and Maddin & Anderson (2012) recovered Albanerpetontidae and Gymnophionomorpha as sister-groups.Finally, Pawley (2006: app. 16; Fig. 4B ) and Marjanović & Laurin (2008b; Fig. 4D ) found Albanerpetontidae and Lissamphibia as sister-groups,an arrangement called“only slightly longer” than an albanerpetontid-batrachian clade by McGowan & Evans (1995: 145) and contained in at least one of the 64 most parsimonious trees found by Ruta et al. (2003).

Potential reasons for this lack of consensus are easy to find: despite their vast stratigraphic distribution, most albanerpetontids are known only from isolated frontal or jaw bones. Complete skeletons (with fully articulated scales, possible femoral glands, and a body outline; McGowan 2002) have only been found for the Early Cretaceous Celtedens McGowan & Evans, 1995 , and these are preserved in two dimensions; worse yet, all of these specimens have been split through the bone, which hampers interpretation of certain features (such as most of the skull).

Figures

FIG. 4. — Crown-group tetrapod phylogeny according to analyses that support the LH, showing diversity within the LH. All are simplified from their sources (by collapsing suprageneric taxa), and the names are made comparable. Internal topology of Microsauria in B and C not shown. Colored boxes: yellow (Œ), Lissamphibia; pastel green ( ), Lysorophia; cyan (Ž), Amphibia; olive ( ), “microsaurs”; violet ( ), “nectrideans”; blue (‘), Aïstopoda; dark green (’), amniotes and diadectomorphs.A, Vallin & Laurin (2004:fig.6);B, Pawley (2006: fig. 92), with the taxon sample and cranial characters of Ruta et al. (2003) but Pawley’s own set of postcranial characters, and reweighting; C, Germain (2008a: fig. 5.15); D, Marjanović & Laurin (2008b: fig. 6c), Tuditanidae, and Hapsidopareiontidae are OTUs that may not be monophyletic,their composition was not tested,but carried over from McGowan (2002);E, Marjanović & Laurin (2009: supplementary figure); F, Marjanović (2010: chapter 5), unconstrained analysis without added taxa. The majority-rule consensus is shown; internodes absent from the strict consensus are gray.