Friday, 17 February 2012

Insect borings in Triasic wood.

Most ecologists would regard the most important organisms in terrestrial ecosystems as plants and arthropods (invertebrates with jointed exoskeletons, such as insects and spiders), and as far as we can tell this has been the case since at least the Devonian. However our knowledge of the history of these groups, and their interactions, is somewhat limited. Plants have quite a good fossil record, but are seldom preserved intact; thus we have good fossil records of leaves, wood and pollen, but assembling these into whole plants is a great skill. Insects and other terrestrial arthropods are even less well known. Traditionally palaeontologists have concentrated on large, glamourous animals such as dinosaurs in terrestrial ecosystems, the importance of smaller organisms has only recently began to be explored (this is not really a criticism; earlier palaeontologists were often reliant on wealthy private individuals to fund expeditions, such individuals could be impressed by naming dinosaurs after them, but might have reacted unpredictably to beetles).

In a paper published in the journal PLoS One on 15 February 2012, Lief Tapanila of the Department of Geosciences at Idaho State University and the Division of Earth Science at Idaho Museum of Natural History and Eric Roberts of the School of Earth and Environmental Sciences at James Cook University announce the discovery of a series of wood borings in a log from a Late Triassic Petrified Forrest in Horse Canyon, Utah.

Series of partially exposed borings, showing brown lining, and sparite (limestone) infill. Scale bar is in cm. From Tapanila and Roberts (2012).

The borings have been named as Xylokrypta durossi (Xylokrypta meaning 'hidden in wood' and durossi in honor of Christopher DuRoss, the discoverer of the borings). The borings are kidney-shaped in cross-section, with two entrance/exit shafts, one of which is blocked with filled wood. This is interpreted as an insect pupation chamber, where the larvae had entered through the first shaft, then blocked it with chewed wood, pupated hidden inside the chamber, then dug the second shaft to escape. There are numerous shafts on the log, arranged so as not to overlap.

Cross section through a Xylokrypta boring, (py) pyritized frass (mineralized chewed wood) lining, (qz) quartz spar filling part of the interior chamber, (cc) calcite spar, filling part of the interior chamber. From Tapanila and Roberts (2012).

Diagrammatic representations of the Xylokrypta borings. (A) Three dimensional view, (R) radial view, (Tr) transverse view, (Ta) tangential view. (B) Tangential view, (pa) proximal aperture, (ps) proximal shaft, (da) distal aperture, (ds) distal shaft, (e) pyrite encrustation. (C) transverse view, (da) distal aperture, (pa) proximal aperture. (D) Radial view. From Tapanila and Roberts (2012).

The positioning of the Xylokrypta borings on the log. From Tapanila and Roberts (2012).

Tapanila and Roberts are cautious about assigning borings to particular insect groups, but note that the borings are not dissimilar to those of Cupidid Beetles, which are known to have been abundant in Asia, South America and Australia the Late Triassic, and which therefore would not be a surprising discovery in North America. They also note that similar borings from the Late Triassic Petrified Forest National Park in Arizona have been attributed to borings by solitary bees (which would be truly amazing in the Late Triassic), and suggest that these probably have the same origin as the Utah borings.

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