title: 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 µm 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.
authors.affiliation: Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity, Humboldt University Berlin, Berlin, Germany. jason.dunlop@mfn-berlin.de
authors.forename: Jason A
authors.initials: JA
authors.lastname: Dunlop
authors.forename: Stefan
authors.initials: S
authors.lastname: Wirth
authors.forename: David
authors.initials: D
authors.lastname: Penney
authors.forename: Andrew
authors.initials: A
authors.lastname: McNeil
authors.forename: Robert S
authors.initials: RS
authors.lastname: Bradley
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Withers
authors.forename: Richard F
authors.initials: RF
authors.lastname: Preziosi
mesh_terms: D000053:Acari / Q000033:anatomy & histology* / Q000145:classification* / Q000502:physiology; D018647:Amber / Q000737:chemistry; D000818:Animals; D005075:Biological Evolution; D005580:Fossils; D010163:Paleontology / Q000379:methods*; D010802:Phylogeny; D013112:Spiders / Q000033:anatomy & histology / Q000502:physiology; D013559:Symbiosis; D014057:Tomography, X-Ray Computed
chemical_list: D018647:Amber
references.citation: Exp Appl Acarol. 2009 Dec;49(4):317-27
references.pmid: 19697142
references.citation: Mol Phylogenet Evol. 2010 Jul;56(1):222-41
references.pmid: 20060051