is where consensus ends. The fragile bones of the lissamphibians are extremely rare as fossils, and the modern amphibians are highly derived, making comparison with fossil labyrinthodonts difficult. Traditionally, the Lepospondyli has been favored as lissamphibian ancestors. Like the modern amphibians, they were mostly small with simple vertebrae, resembling lissamphibians in many aspects of external anatomy and presumably ecological niches. At a subclass level, it was thought that labyrinthodonts gave rise to lepospondyls, and lepospondyls to lissamphibians. Several cladistic studies also favour the lepospondyl link, though placing Lepospondyli as close relatives or even derived from reptile-like amphibians. One problem with this position is the question of whether Lepospondyli actually is monophyletic in the first place. Temnospondyl affinity for the Lissamphibia is suggested by other works. The temnospondyl family Amphibamidae has been considered a possible candidate for the ancestors of lissamphibians. The amphibamid Gerobatrachus, described in 2008, was proposed to be a transitional form between temnospondyls and anurans (frogs and toads) and caudatans (salamanders). It possessed a mixture of anuran and caudatan features, including a broad skull, short tail, and small pedicellate teeth. Complicating the picture is the question of whether Lissamphibia itself may be polyphyletic. Though a minority view, several variants have been forwarded through history. The "Stockholm school" under Gunnar Säve-Söderbergh and Erik Jarvik argued during much of the 20th century that Amphibia as a whole is biphyletic, based on details of the nasal capsule and cranial nerves. In their view lepospondyls are ancestors of frogs, while salamanders and caecilians have evolved independently from porolepiform fish. Robert L. Carroll suggested the tailed amphibians (salamanders and caecilians) are derived from lepospondyl microsaurs and frogs from temnospondyls. The cladistic analysis of Gerobatrachus suggested salamanders and frogs evolved from temnospondyl stock and caecilians being the sister group of the reptile-like amphibians, rendering Lissamphibia itself an evolutionary grade relative to the remaining tetrapod classes. The cladistic analysis of Chinlestegophis by Pardo et al. (2017) does recover Lissamphibia as polyphyletic, but with all groups of lissamphibians falling within Temnospondyli; Batrachia is recovered in the analysis as part of Dissorophoidea, while Gymnophonia falls within Stereospondylomorpha. Origin of the amniotes The fossil sequence leading from the early Carboniferous labyrinthodonts to the amniotes has traditionally been seen as fairly well mapped out since the early 20th century, mainly leaving only the question of the demarcation line between the amphibian and reptilian grade of reproduction. Work by Carroll