We begin the amphibamiform showcase with the eponymous Amphibamus, one of the oldest known dissorophoids (both in terms of temporal occurrence and history of study)!
A brief history of study
Above we have the holotype of "Raniceps" lyelli, as figured by Clack & Milner (2009) in their revision of Platyrhinops lyelli (their scale bar is 1 cm). Below we have some ancient (maybe colourized) artwork of Raniceps raninus attributed to an old (19th century) book by Paul Gervais titled "Les Poissons." Raniceps the fish pre-dates Raniceps the temnospondyl by over 40 years, which is why the fish keeps the name (something that Cope noted in his Amphibamus description). Wikipedia says that the fish is popular in the aquarium trade, which I find hard to believe...
Most of the first temnospondyls to be described (pre-1900) have complicated taxonomic histories because they often encompass many historic names that were created for purportedly new taxa (usually based on differences now recognized to be ontogenetic or taphonomic in nature). This is particularly true for a lot of Carboniferous tetrapods, which are typically flattened and disarticulated in ways that can muddle interpretations ("roadkill specimens"). In the case of Amphibamus, it has not only several junior synonyms, but also various non-Amphibamus taxa that have since been reassigned to other taxa. Among the synonyms are several taxa named in the early 20th century from Mazon Creek specimens: Micrerpeton, Mazonerpeton, and Miobatrachus (not to be confused with Myobatrachus, or the extant turtle frog). At least some of the confusion may have resulted in from the loss of the holotype, which as first noted by Moodie (1916:126), was apparently destroyed by a fire. What we term the neotype specimen (a replacement for a destroyed holotype) is housed at Yale's Peabody Museum.
Two other valid taxa that were originally placed under Amphibamus are not even amphibamiforms! The trematopids Mordex calliprepes and Mattauschia laticeps are Carboniferous taxa from the Czech Republic. Each has their own complex history (summarized in Milner's 2018 revision of these taxa), but both are named for type specimens that are very small relative to other trematopids, hence the confusion and their previous referral to Amphibamus as distinct species (this will be another week's topic). Below you can find the exhaustive taxonomy of both of these taxa from Milner's paper:
CIn some respects, Amphibamus was a bit of what we call a "wastebasket taxon," which is a sort of 'catch-all' for taxa that generally share some features but that may not represent a monophyletic genus (Tersomius and Aspidosaurus are current wastebasket taxa among dissorophoids). In this instance, Amphibamus was a well-recognized, small-bodied dissorophoid, and the persistent problem of whether small dissorophoids are larvae of large-bodied taxa or adults of small-bodied taxa led to a whole lot of muddled taxonomy! Amphibamus as a single-species genus is fairly stable now, having been sorted out over the years with subsequent revisions (e.g., Gregory, 1950; Bolt, 1979; Milner, 1982; Hook & Baird, 1984; Clack & Milner, 1994, 2009; Daly, 1994).
Like many other Mazon Creek animals, Amphibamus is known from more or less complete skeletons, albeit ones that have to be studied from latex or plaster peels to visualize the three-dimensionality of the natural mold preserved by secondary minerals inside of nodules, a common preservation style at the locality. For those unfamiliar with how latex peels work, there are some good details in this publicly accessible tribute to one of the pioneers, Donald Baird (you may have to CTRL + F around a little bit). Baird's original paper published in Science(!) in 1955 on his methods can be found here. This method has been huge for providing information on historically inaccessible material preserved in nodules and other similar fashions and has provided what is probably the first or second-best characterization of a terrestrial amphibamiform via Amphibamus (perhaps surpassed only by Doleserpeton, which is known from a huge amount of material and which doesn't need latex peels).'
Amphibamus has long been linked to the origin of amphibians, although the general topic remains disputed and the nuances of morphological features linking various extinct tetrapods to living amphibians have constantly shifted over time. One of the key features are bicuspid teeth (two points) – most early tetrapods have simple monocuspid teeth, as do most temnospondyls – but many modern amphibians have bicuspid teeth. They're a far cry from the very complex teeth that mammals have, particularly omnivores like humans which have heterodont dentition (different shape throughout the mouth). Amphibamus (L; SEM image from Bolt, 1979) is generally considered to have bicuspid teeth, although the definition of said teeth is not as clear as in Doleserpeton (R; SEM image from Sigurdsen & Bolt, 2010). Bolt also interpreted the teeth of Amphibamus as possibly pedicellate, which is to say that there could be a dividing zone between the base and the tip of the tooth that would cause the tip to fall off post-mortem, but this is more controversial, again by comparison with Doleserpeton. In general, bicuspidity and pedicelly have been variably reported in early tetrapods, and a fair number of those reports are considered somewhat dubious or perhaps pre-biased by authors looking for more evidence of lissamphibian affinities. Bolt also considered bicuspidity to perhaps be a hallmark of early ontogeny in dissorophoids more broadly; this conjecture hasn't come to bear much fruit in part because of a limited fossil record, but this does occur in some modern amphibians in which juveniles have bicuspid teeth and adults have monocuspid teeth.
Together with other features like cylindrical pleurocentra (one of two 'central' elements in each vertebral position), Amphibamus and Doleserpeton were long considered stem lissamphibians, or as it was put decades ago, "proto-lissamphibians." Shifting dissorophoid phylogeny has resulted in a slight displacement of these taxa away from modern lissamphibians (assuming their origin lies within dissorophoids), as has the discovery of the much more batrachian-like Gerobatrachus (Anderson et al., 2008). Nonetheless, Amphibamus remains one of the oldest known dissorophoids and possesses a number of derived features that suggest there may be more highly nested amphibamiforms out there in the Carboniferous that we haven't yet discovered!
Below are a few different amphibamid / amphibamiform phylogenies from the past two decades or so, with Amphibamus being highlighted. All of these are strict consensus trees from parsimony analyses; if a study did a few different permutations, I picked the one that had more resolution for the sake of non-expert readers. Each of these has a different taxon sample (I simplified most of them), but there are different and sometimes very distinct topological differences, although Amphibamus usually comes out close to Doleserpeton. It's interesting that it can sometimes be very distant from Platyrhinops, which was previously considered a junior synonym of Amphibamus by some workers.
About the blog
A blog on all things temnospondyl written by someone who spends too much time thinking about them. Covers all aspects of temnospondyl paleobiology and ongoing research (not just mine).