Amphibian burrows and ephemeral ponds of the Lower Permian Speiser Shale, Kansas: evidence for seasonality in the midcontinent
Abstract Burrows attributed to the lysorophid Brachydectes elongatus occur within a green mudstone lens in the Lower Permian (Wolfcampian) Speiser Shale (Council Grove Group) of eastern Kansas. The burrow-bearing unit consists of a 100 m long lens of massive, calcareous, silty mudstone with a maximum thickness of 35 cm. The lens also contains the fossils of fish, amphibians, reptiles, charophytes (Stomachara), and ostracodes (Carbonita). The mudstone lens pinches out laterally into red, pedogenically modified mudstone. The burrows occur in three layers. At the top of each layer is a subaerial exposure surface indicated by rhizoliths, mudcracks, and coarse-grained lags. The mudstone lens is interpreted as the deposit of a shallow, ephemeral pond. The burrows occur in three vertically and temporally separated clusters with concentrations up to 20/m2. The burrows show two types of architecture. Type I burrows are elongate, elliptical tubes 4–32 cm long and 2–7 cm in width. Type II burrows are short, elliptical tubes 1.5–3.5 cm in length and 2.5–5 cm in width. The burrow fill consists of a mudstone capped by sandy siltstone. Each burrow layer contains a varying number of articulated and disarticulated lysorophid skeletons within the burrows and surrounding matrix. While similar to Permian lungfish burrows, the architecture and surficial morphology of the lysorophid burrows permits their distinction even in the absence of body fossils. The occurrence of burrow layers capped by subaerial exposure surfaces and separated by layers of non-burrowed, massive mudstone indicates that burrowing occurred in response to episodic, perhaps seasonal, droughts on the Permian coastal plain. The lysorophid burrowing behavior is analogous to that of the extant amphibians Amphiuma sp. and Siren intermedia, which inhabit rivers and ponds in the southeastern USA.