Publication: Rhizolith balls from the Lower Cretaceous of Patagonia: Just roots or the oldest evidence of insect agriculture?
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Publication Date
2010
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Elsevier
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Abstract
A new type of trace fossil from the Lower Cretaceous of Patagonia that may record the oldest evidence of insect
agriculture is represented by biconvex, sub-spherical, carbonate balls composed of a dense mass of rhizoliths.
The best preserved balls show an external wall, a tunnel entrance, and vertical, large rhizoliths crossing from
top to base, from which a mass of smaller rhizoliths arise. In some cases there is a pelletal surface texture in
internal layers of the wall. Some specimens show meniscate tubes attributed to organisms different than the
constructors of the chambers, probably attracted by the original organic matter. Micromorphology, analyzed
by thin sections and SEM, shows strong evidence of high fungal activity associated to the original roots.
Four different hypotheses are analyzed to explain the origin of the balls. The less probable ones are either that
the balls were produced only by a self induced and localised overgrowth of secondary and tertiary rootlets from
a group of neighbouring primary roots, or that they were crayfishes' feeding chambers. Rhizolith arrangement,
as well as other characters, essentially rule out both hypotheses. The hypotheses involving social insects,
termites and ants, are more risky, considering the lack of support from the body fossil record, the biogeography
of these extant insects, and particularly the lack of definitive evidence of discrete fungus or bacterial gardens
in the balls. However, the trace fossils described herein partially resemble the nests and behaviour of the
African termite Sphaerothermes sphaerothorax, particularly because of the presence of roots inside a thick
lined chamber excavated around them to cultivate bacterial combs. The ant origin of these balls is at least
a hypothesis to explore considering the size, shape, wall, entrance tunnel, presence of traces of associated
organisms, and particularly roots. The roots and wall relate these balls to hypothesised scenarios for the origin
of fungus agriculture in ants. One hypothesis suggests that primitive Attini developed fungus agriculture from
the habit of feeding on fungi associated with roots (mycorrhizae) and still other proposes that adventitious
fungi on nest walls were responsible for the arise of fungiculture. The balls would supply physical evidence that
by the lower Cretaceous some social insects excavated chambers around uncut roots, which in turn provided
optimal conditions for fungi development. These rhizolith balls possibly constitute the oldest physical evidence
for the origin of fungiculture in social insects, and represent a critical contribution of ichnology to this hot topic
intensively discussed on the basis of a scattered body fossil record and molecular phylogeny