The sun does unfortunately set on the dissorophid empire (arguably it's not even an empire to start with, but I'm treating it like one for the sake of being dramatic). To round out this five-week topical series on dissorophids, I'm wrapping up with the last-surviving dissorophids, which as suggested by the title, are found to the east in Eurasia.
This taxon is the only dissorophoid from Asia and one of the few temnospondyls that is well-documented from the region's extensive geologic record (too many people hunting those birds if you ask me). The only species, A. petrolicus, comes from Dashankou in Gansu Province, China and had a skull length estimated to be a whopping 40 cm (this is massive for a dissorophid)! Originally, this was only known from a pretty fragmentary part of the palate (Li & Cheng, 1999), but new material was published last year by Liu (2018) that includes a nearly complete skull and mandible and some postcrania. As a result, it's arguably now the best-known of the late dissorophids (and the only one to have published photographs of it).
Late stages of dissorophid evolution
Dissorophids hit their peak diversity and distribution in the early Permian, with taxa like the three species of Cacops, four species of Broiliellus, Dissorophus, and Platyhystrix all being well-documented. The same is true of the other dissorophoid clades, but following the early Permian, nearly all of them disappear. Amphibamiformes, the group closely linked to modern amphibians, has only one record after the early Permian: Micropholis from the Early Triassic of South Africa, and trematopids, branchiosaurids, and micromelerpetids have no record.
Dissorophids are the only other group to survive beyond the early Permian, and if their fossil record is taken at face value, there are a number of interesting observations (you can see their temporal / stratigraphic distribution in the figure above from May et al., 2011). In the late Carboniferous and the early Permian, dissorophids are virtually exclusive to North America and more specifically to the southwest and south-central regions of the United States. Thereafter, they are almost exclusive to Russia and China, with only one fragmentary record from North America. Additionally, most North American dissorophids are relatively small. The largest is Platyhystrix, which had a skull around 19 cm in length (Berman et al., 1981), and most of the other dissorophids are only known from specimens with skull lengths less than 10 cm. In essentially all ecosystems, they're not the biggest temnospondyls - there's always a larger slimeball like trematopids or eryopids hanging around as the top slimeball of the ecosystem. But trematopids go extinct after the early Permian, and the only surviving eryopid is Clamorosaurus from Russia. Overall, the the temnospondyl assemblages become increasingly aquatic, particularly with the evolution of Stereospondylomorpha, the predominantly Mesozoic clade that includes my favourite metoposaurids (among other large-bodied, aquatic taxa). Perhaps in coincidence with the disappearance of the larger clades, the youngest dissorophids are much larger than their earlier relatives. The smallest one is Zygosaurus at 20 cm in skull length, and Anakamacops is estimated to be have been twice that size. So it's quite possible that in the absence of any other large terrestrial temnospondyl (setting aside whether eryopids were fully terrestrial), dissorophids took over - at least until the amniotes finally did them in. What allowed dissorophids to persist isn't entirely clear; there are many anatomical differences between them and trematopids, the only other appreciably large dissorophoids, but who knows if things like having a normal-sized nostril were the make-or-break factor.
What about the shift in dissorophid distribution? That's a little trickier. The early Permian record is best in North America and is not so hot in Eurasia, but that pattern flip-flops in the middle and late Permian in which the terrestrial tetrapod record is dominated by basically anywhere other than North America. It is quite possible that dissorophids do shift eastward, perhaps following environmental patterns that made North America less hospitable for them. However, it can't be ruled out that dissorophids do still stick around in North America for some time and just aren't captured in the fossil record - there is one middle Permian dissorophid from Oklahoma, "Fayella chickashaensis," which was renamed last year in one of our papers (Gee et al., 2018) and which is mostly known from a large postcranial skeleton. Terrestrial temnospondyls don't show up in the fossil record of North America again until the Late Triassic when you get actual frogs (Stocker et al., 2019) and enigmatic small-bodied stereospondyls like Rileymillerus (Bolt & Chatterjee, 2000) and Chinlestegophis (Pardo et al., 2017), which seems very unlikely to represent the true nature of the amphibian assemblages. In all likelihood, we just haven't found them because they get much smaller and don't lend themselves well to being preserved in the more typical fluvial deposits.
This post wraps up five weeks of talking about dissorophids, which are a large part of my real dissertation (this is always news to someone who thinks I'm doing metoposaurids for my dissertation)! Dissorophids are a remarkably diverse group of temnospondyls, even if you lean conservative in your taxonomic approaches like I do, and they're very cool with respect to all of the different osteoderm types (which makes inferring osteoderm evolution very complicated). Hopefully you too have enjoyed reading about a select handful of these temnospondyls over the past month. If you're interested in learning about the ones I didn't talk about, Wikipedia is actually an okay start; the Dissorophidae page itself is a mess that I haven't cleaned up yet, but a lot of the genus or species pages are decent and at least list academic references for more reading. I kind of like doing these themed months - it's possible I will continue it for the next few months to give some semblance of order to this blog other than "that one sounded cool to me yesterday."
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).