RT Journal Article
T1 Rhizolith balls from the Lower Cretaceous of Patagonia: Just roots or the oldest evidence of insect agriculture?
A1 Alonso-Zarza, Ana María
A1 Genise, J.F.
AB A new type of trace fossil from the Lower Cretaceous of Patagonia that may record the oldest evidence of insectagriculture 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 fromtop to base, from which a mass of smaller rhizoliths arise. In some cases there is a pelletal surface texture ininternal layers of the wall. Some specimens show meniscate tubes attributed to organisms different than theconstructors of the chambers, probably attracted by the original organic matter. Micromorphology, analyzedby 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 thatthe balls were produced only by a self induced and localised overgrowth of secondary and tertiary rootlets froma 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 biogeographyof these extant insects, and particularly the lack of definitive evidence of discrete fungus or bacterial gardensin the balls. However, the trace fossils described herein partially resemble the nests and behaviour of theAfrican termite Sphaerothermes sphaerothorax, particularly because of the presence of roots inside a thicklined chamber excavated around them to cultivate bacterial combs. The ant origin of these balls is at leasta hypothesis to explore considering the size, shape, wall, entrance tunnel, presence of traces of associatedorganisms, and particularly roots. The roots and wall relate these balls to hypothesised scenarios for the originof fungus agriculture in ants. One hypothesis suggests that primitive Attini developed fungus agriculture fromthe habit of feeding on fungi associated with roots (mycorrhizae) and still other proposes that adventitiousfungi on nest walls were responsible for the arise of fungiculture. The balls would supply physical evidence thatby the lower Cretaceous some social insects excavated chambers around uncut roots, which in turn providedoptimal conditions for fungi development. These rhizolith balls possibly constitute the oldest physical evidencefor the origin of fungiculture in social insects, and represent a critical contribution of ichnology to this hot topicintensively discussed on the basis of a scattered body fossil record and molecular phylogeny
PB Elsevier
SN 0031-0182
YR 2010
FD 2010
LK https://hdl.handle.net/20.500.14352/41885
UL https://hdl.handle.net/20.500.14352/41885
LA spa
DS Docta Complutense
RD 2 may 2024