Having finally made it to Seattle and gotten some semblance of stability, I'm cautiously optimistic that I can reboot the blog on a more regular note again! As regular readers may notice, I'm a big fan of alliteration, so having missed the entirety of Metoposaurid May, which I did last year, August will be Amphibamiform August! This week will provide a general overview of amphibamiforms. Amphibami– what now?
Identity crisisSo what happened to Amphibamidae? Is it now the dreaded single-quote 'Amphibamidae,' indicating paraphyly (much like 'microsaurs')?? Nope! Amphibamidae got redefined by Schoch as well - it is now restricted to the eponymous Amphibamus (only one species) and Doleserpeton (also only one species). This pair is relatively high nested within Amphibamiformes and was long of interest in the study of lissamphibian origins; more recent discoveries like the "frogamander" Gerobatrachus have created some distance from crown Lissamphibia. Amphibamidae has a long and convoluted history far pre-dating the latest revision. The family was named in 1909 by Roy Moodie (although there is disagreement on the correct publication; some attribute it to his 1916 publication, which in turn attributed it to Cope, 1875 who named Peliontidae, which in turn in a synonym because Pelion is a junior synonym of Platyrhinops, which was at one point also Amphibamus...it's a lot...), but it was only given for Amphibamus; most of the amphibamiforms (especially the terrestrial ones) hadn't been named at this point. Part of this restrictive approach was due to the problems of associating small-bodied temnospondyls, which were often scarcer and less well-prepared than the classically large crocodilian-esque temnospondyls like Archegosaurus and Mastodonsaurus, with known groups. The same is true of Micropholis, the only amphibamiform from the southern hemisphere, for which Micropholidae was created. Other early "amphibamids" consisted of what are now recognized as junior synonyms of Amphibamus, like Miobatrachus and Pelion, although there was a time where the Carboniferous reptile Cephalerpeton was also considered an amphibamid. Above is a generalized schematic diagram showing the general consensus history on classification of dissorophoids (minus micromelerpetids). Note that this reflects general consensus, so fringe ideas such as that dissorophids and trematopids are not distinct clades (Eaton, 1973) are not captured here. Naturally there may be some disagreement on "consensus," especially in the pre-phylogenetics era; my infographic is just one interpretation! 1970 is marked by an asterisk because that's the decade where Dissorophoidea came into widespread use (Bolt, 1969); included taxa are bracketed by the dark grey box with red outline. For several decades, the affinities of real small-bodied temnos like these with larger dissorophoids like the armoured dissorophids wasn't apparent; Dissorophoidea wasn't created until 1969 by John Bolt. Even then, Amphibamus and the other known terrestrial amphibamiforms like Tersomius were actually considered to be dissorophids, just lacking in armour (e.g., Carroll, 1964). Thus, at least some of the early association of dissorophids, trematopids, and Doleserpeton, which also got its own monotaxic family, Doleserpetontidae, was based on similarities between Doleserpeton and these small "dissorophids" that were in fact amphibamiforms like Amphibamus. The idea that the small-bodied terrestrial forms might form a clade that warranted the resurrection of Amphibamidae (sensu Moodie) started gaining momentum thereafter (e.g., Boy, 1972). It only became "mainstream" in the 90s however (e.g., Clack & Milner, 1993; Daly, 1994), which in part had to do with a lot of revision of amphibamiform material, a lot of new dissorophoid taxa being described, and computer-assisted phylogenetics. It was then fairly stable into the 21st century, although patterns within amphibamids were constantly in flux because of the large number of new taxa being recognized, such as Georgenthalia, Pasawioops, Plemmyradytes, and Rubeostratilia. The idea that branchiosaurds might nest within the historical "amphibamids" has been around for decades now (e.g., Milner, 1982), but it didn't gain substantial traction until about a decade ago when phylogenetic analyses (e.g., Fröbisch & Schoch, 2009) started recovering this topology. Strange bedfellows
There are other examples of this discordance within Amphibamiformes - Amphibamus is one of the oldest taxa but is fairly highly nested with Doleserpeton in the new Amphibamidae - but generally the Carboniferous taxa are recovered in earlier diverging positions than the Permian taxa. Both examples are suggestive of a lot of missing records, especially in Gondwana and during the Carboniferous. General plan for the monthThis is a loose plan, subject to change pending dog stuff and other happenings of life, but the idea is to highlight some of the various amphibamiforms and to contextualize them in dissorophoid / temnospondyl evolution. More well-versed readers will note that I have said very little about their purported affinities with modern amphibians (Lissamphibia), but this is naturally a critical hypothesis that has led to the substantial body of work exploring amphibamiforms to date. For the less well-versed readers, the origin of modern amphibians remains very controversial, from the group(s) of origin to the timing of divergence to the evolution of lissamphibian features, but amphibamiforms have been widely posited as a potential ancestor since the 1960s when John Bolt described Doleserpeton as a "protolissamphibian"; some of this will be covered in some kind of cursory fashion down the line (it is a lot to cover and remains unresolved)! Refs
David Marjanović
8/16/2020 06:13:55 pm
The infographic is very informative! :-) Comments are closed.
|
About the blogA 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). Categories
All
Archives
January 2024
|