Carboniferous tetrapod fauna of the fossil lycopsid trees at Joggins and the origin of the hollow tree guild

In their 1853 paper announcing the discovery of fossil tetrapods in the once-hollow trees at Joggins, Nova Scotia, Lyell and Dawson gave three possibilities for their entombment: the tetrapods may have been washed in after death, entered via a ‘crevice’ in the trunk, or fallen into partially buried, hollow trunks. The latter scenario, or ‘pitfall theory’, grew to be favoured by Dawson and to be depicted in most texts on the history of life, but is here scrutinized following recent discoveries by the authors and re-evaluation of historic finds. Nominally eleven tetrapod taxa have been identified from the fossil trees, including the earliest known amniotes — the richest single Carboniferous record of terrestrial tetrapods. Tetrapods co-occur with detrivorous invertebrates (land snails and millipedes), as well as putative insects. Here, we systematically evaluate scenarios for the occurrence of the tetrapod fauna, including accidental entry (pitfall, being washed in); temporary, opportunistic use (by predator, as refuge from fire); and denning. These scenarios have been weighed against salient aspects of their taphonomy, including their entry prior to infilling, disassembled skeletons, and ubiquitous association with charcoal. The broader role of wildfire is suggested by discovery of a tetrapod-bearing lycopsid with notched, charred base consistent with a fire scar, the oldest such example in the fossil record. Of the scenarios scrutinized, the most parsimonious is that tetrapods utilized lycopsid hollows as dens; compelling analogues are found in tree hollow fauna of modern forest biomes, many of which use fire scars as entry. Temporal shifts to seasonal, fire-prone drylands may have exerted ecological pressure on the Joggins fauna to avail themselves of tree hollows whilst providing access via fire scars. The tetrapod fauna at Joggins, which includes the earliest amniote, records the earliest known example of the hollow tree guild, well adapted to the ecological dynamics of the ‘Coal Age’ landscape.