It was a relatively quiet year on the temnospondyl research front – I think we may be seeing the real effects of the pandemic accumulating now, as people have started running out of leftover projects. By my count, there were just 17 papers either focusing entirely on temnospondyls or with a substantial temnospondyl component, and four of those were published just in the past two weeks! Nonetheless, there was some very exciting work this year, including a disproportionate amount of metoposaurid studies; this seems to be in a trend in recent years, driven almost entirely by teams working on the Polish material, which is a real testament to Krasiejów. I think there is some exciting stuff coming up the pipeline in 2023 (not from me), and I am looking forward to hopefully a more productive year for temnos!
Put it on the map
Werneburg et al. (PalZ) described a new zatracheid, represented by an essentially complete skeleton, from the early Permian Chemnitz Fossil Lagerstätte in Germany, Chemnitzion richteri. This deposit records the dying moments of a terrestrial environment being buried in ashfall and the subsequent pyroclastic flow of a volcanic eruption, thus preserving a remarkable diversity of organisms (including plants and invertebrates) in 3D natural molds and casts. Despite a decent number of specimens, zatracheids remain relatively rare in the fossil record – both Acanthostomatops vorax (Germany) and Dascyeps bucklandi (England) are only known from a single site, which may reflect the lower frequency of dryland habitats recorded in the Permo-Carboniferous of Europe compared to North America. The postcranial record of the clade is also only known from Acanthostomatops, and the nearly complete skeleton of C. richteri provides further details on the variation within zatracheids.
Schoch & Sues (Journal of Paleontology) provided a long-awaited reassessment of the enigmatic early Permian temnospondyl Parioxys ferricolus. Despite being known from an appreciable sample size of specimens from Texas, the anatomy and taxonomy of this taxon have long been confusing because the original descriptions were limited to what are now grainy photographs and stylized reconstructions. The original descriptions from nearly 70 years ago were influenced by the descriptor's (Y. Shawki Moustafa) hypothesis that Parioxys was closely related to Eryops in how comparisons and reconstructions were made, but other workers have long-suspected that the taxon might belong to a different clade. Schoch & Sues' redescription and reassessment provided more compelling evidence for dissorophid affinities, specifically with the cacopines, based on features like a transverse nuchal ridge on the postparietals, a foreshortened posterior skull table, and modified features of the palate.
Fresh off the press from last week is a revision of the Middle Triassic trematosaur Trematolestes hagdorni by Schoch & Mujal (Neues Jahrbuch für Geologie und Palaöntologie). When originally described in 2006 by Schoch, the taxon was represented by a number of essentially complete specimens, but these were interpreted as belonging to immature specimens. New material described in this study greatly expands the ontogenetic range on both sides, from highly immature individuals to demonstrably mature adults. The ontogeny of many trematosaurs (and arguably most stereospondyls) remains very poorly known due to a lack of variably sized specimens, and the new material of Trematolestes represents the most completely known ontogeny among Trematosauria.
Under the microscope
Another hot off the press paper, Surmik et al. (BMC Ecology and Evolution) report the oldest unequivocal occurrence of osteosarcoma (bone tumor) in the vertebra of an "amphibian" (non-amniote). Given the scarcity of pathologies in the fossil record in general, it likely comes as little surprise that this osteosarcoma was identified from the extensive sample of Metoposaurus krasiejowensis from the Late Triassic Krasiejów locality in Poland. Through CT scanning and histology, the authors provided a detailed description of the tissue type and organization, which permitted their diagnosis.
One of the interesting discussion points raised by the authors is the scarcity of identified occurrences of cancer in fossil "amphibians" given their sample size; indeed, I have never seen such a malformed intercentrum out of perhaps >1,000 that I have seen in North American collections. Modern amphibians have a relatively low rate of reported bone cancer (most occurrences of cancer are reported from the skin), and the authors speculated that temnospondyls may have been similarly resistant to cancer through a variety of developmental and genetic mechanisms.
Figure 3 from Kalita et al., showing the black-and-white images of thin sections across the ontogenetic range of interclavicles of Metoposaurus krasiejowensis. Black represents bone, white represents empty space, and cyan represents taphonomic/artificial cracks. Specimens decrease in maturity to the right.
One of the distinctive features of stereospondyls is their massively enlarged pectoral elements, which form large, plate-like structures. It has often been assumed that such bones would serve as ballast to help these animals sink to the bottom of water bodies; increasing bone weight and/or density is a common feature among many aquatic animals. However, general size is not necessarily reflective of weight (although in fossilized form, it certain does) – bone compactness is the real metric that can be used to assess how a bone contributes to buoyancy. Kalita et al. (Journal of Anatomy) compared the compactness of the clavicle and interclavicle of Cyclotosaurus intermedius and Metoposaurus krasiejowensis and found a high degree of compactness in both taxa. However, differences in microanatomical structure hint at different lifestyles (Metoposaurus more benthic, Cyclotosaurus more active swimming), corroborating other research suggesting interspecific niche partitioning. At least among the several sampled specimens of Metoposaurus, there was no indication of intraspecific niche partitioning, indicating a benthic lifestyle was adopted quite early.
Weryński & Kędzierski (Geological Quarterly) used both histology and SEM to examine the external and internal microstructure of teeth of Metoposaurus krasiejowensis (yes, it's another year filled with research on Polish metoposaurids). In addition to the typical labyrinthodont infolding that has been known for over 150 years in temnospondyls, the authors also identified directional porosity in the canals making up this structure that they interpret as an adaptation for counterbalancing stress forces during biting. Additionally, they identified what they interpret as growth marks within the teeth, equivalent to four seasonal cycles, which adds to the data used to assess the perceived biological response to any local climatic periodicity.
The problem with growing up
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).