4.1.5. Microbial farming hypothesis
In a variation of the deposit feeding hypothesis, marine burrowers have
been hypothesized to create a helix to optimize feeding by increasing
the sediment-to-burrow ratio; this is referred to as microbial farming
or gardening (Felder, 2001; Moosavizadeh & Knaust, 2001; Netto et al.,
2007). Netto et al (2007) argued that Permian Gyrolithes were
created by crustaceans (rather than by polychaetes, as proposed by
Powell, 1977; see also Gingras et al., 2008), possibly implicating
microbial feeding since the Paleozoic. Moosavizadeh & Knaust (2021)
considered microbial farming as the most likely function ofGyrolithes from Iran, which occurred in soft, low-energy
sediments. Microbial farming has also been hypothesized as a potential
function of helical burrows of trace fossil Helicodromites in
oligotrophic conditions in a well-oxygenated environment (Poschmann,
2015). The possibility of microbial gardening in Gyrolithes was
discussed by several authors (e.g., Seilacher, 2007; Wetzel et al.,
2010); however, the burrow morphology is not a good candidate for such a
behavior as it is also part of other burrow systems that are boxwork
structures (Mayoral and Muñiz, 1995, 1998; Dworschak and Rodriquez,
1997; Carvalho and Baucon, 2010). Furthermore, microbial farming is
indeed a trait of such marine trace fossils as Chondrites andPaleodictyon , which have much more complex morphologies, like
downward and outward dendritic branching patterns and intricate
horizontal tubular patterns with vertical tubes (similar in pattern to a
chain-linked fence), respectively (Ekdale et al., 1984; Bromley, 1996).
Direct observations in extant marine helical-burrowing species in
situ or in a laboratory setting are needed to provide indirect support
for this hypothesis.