A minute fossil phoretic mite recovered by phase-contrast X-ray computed tomography

Abstract

High-resolution phase-contrast X-ray computed tomography (CT) reveals the phoretic deutonymph of a fossil astigmatid mite (Acariformes: Astigmata) attached to a spider's carapace (Araneae: Dysderidae) in Eocene (44-49 Myr ago) Baltic amber. Details of appendages and a sucker plate were resolved, and the resulting three-dimensional model demonstrates the potential of tomography to recover morphological characters of systematic significance from even the tiniest amber inclusions without the need for a synchrotron. Astigmatids have an extremely sparse palaeontological record. We confirm one of the few convincing fossils, potentially the oldest record of Histiostomatidae. At 176 mm long, we believe this to be the smallest arthropod in amber to be CT-scanned as a complete body fossil, extending the boundaries for what can be recovered using this technique. We also demonstrate a minimum age for the evolution of phoretic behaviour among their deutonymphs, an ecological trait used by extant species to disperse into favourable environments. The occurrence of the fossil on a spider is noteworthy, as modern histiostomatids tend to favour other arthropods as carriers.

Quantitative X-ray tomography

“…219 b and c Quantification performed using the 3D dataset (size, sphericity and aspect ratio) It should always be borne in mind that most of these measurements are performed using approximations calculated using discretised (voxelised) images. This discretised nature can have a strong influence on the results, especially in terms of surface length and surface 16 The description of a complex pore network can be simplified by the creation of a geometrical graph composed of nodes and tunnels. The specific dimensions of these simple elements have to be measured from the images.…”

“…Indeed it has proved invaluable in imaging fossils using lab sources. 15,16 However because the incident beam is polychromatic, phase retrieval is not as effective as for synchrotron X-ray imaging. 17 Analyser-based diffraction enhanced imaging 18,19 involves the reflection of the transmitted beam from a Bragg crystal which acts as an angular filter converting refractive effects caused by the object into intensity effects in the detector plane.…”

Phoretic springtail (Collembola: Sminthuridae) on a false blister beetle (Coleoptera: Oedemeridae) in Eocene Baltic amber

“…One such behaviour is phoresy, whereby a smaller organism uses a larger and more mobile organism for dispersal. In the Eocene Baltic amber it is predominantly seen in pseudoscorpions and the deutonymphs of several mite taxa (Uropodidae and Anoetidae) (WEITSCHAT & WICHARD 1998;DUNLOP et al 2012DUNLOP et al , 2013. Among extant springtails, phoresy has not been observed thus far, and only three cases of this behaviour are known to be preserved in fossils: one specimen on a mayfly in Dominican amber, five specimens of Sminthurus longicornis WOMERSLEY, 1932 on the leg of the opilionid Dicranopalpus ramiger KOCH & BERENDT, 1854 and two specimens on a not further determined opilionid, both in Baltic amber (POINAR 2010;PENNEY et al 2012).…”

Histiostomatids on common Dermaptera in Japan

“…Phoretic associations first occurred at least 49 million years ago (Dunlop et al, 2012). Japan is especially interesting in this respect because it is geographically distant from the Dermapteran ancestral area in Africa or southern Gondwana (Popham, 2000;Jarvis et al, 2005).…”