Permian macroburrows as microhabitats for meiofauna organisms: an ancient behaviour common in extant organisms

Abstract

Meiofaunal organisms are indirectly influenced by the activity of benthic macroinvertebrates within the sediment, which plays a role in modifying physical and chemical characteristics of the habitat. The association of meiofaunal organisms and macroburrows is well known in modern environments, but the record of this relationship in the geological record is still incipient. This study documents diminutive burrows (Helminthoidichnites tenuis) associated with the surface of macroburrows (Palaeophycus tubularis) in Early Permian deposits. The cylindrical shape and meandering to loop trajectory of the diminutive burrows indicate that they were produced by small free-living meiofaunal nematodes. Apparently, P. tubularis (open burrow) constituted a favourable microhabitat for nematodes, providing the following: (1) protection against erosive processes and meiofauna predators; (2) oxygen access to more in-depth layers within the sediment; (3) temperature stabilization; and (4) food supply due to mucus impregnation in the macroburrow walls by the Palaeophycus tracemaker. The association between H. tenuis and P. tubularis constitutes the first fossil record of a symbiotic relationship between meiofaunal nematodes and macrobenthic organisms (polychaetes). It also suggests that ecological strategies such as mutualism or commensalism, which are common between extant nematodes and macrobenthic invertebrates, were available in the behavioural programme of these organisms since the Early Permian.