Although plant–arthropod relationships underpin the dramatic rise in diversity and ecological dominance of flowering plants and their associated arthropods, direct observations of such interactions in the fossil record are rare, as these ephemeral moments are difficult to preserve. Three-dimensionally preserved charred remains of Chloranthistemon flowers from the Late Albian to Early Cenomanian of Germany preserve scales of mosquitoes and an oribatid mite with mouthparts inserted into the pollen sac. Mosquitoes, which today are frequent nectar feeders, and the mite were feeding on pollen at the time wildfire consumed the flowers. These findings document directly arthropod feeding strategies and their role in decomposition.
Over the last three decades, rich assemblages of Cretaceous flowers and other plant mesofossils have provided a unique resource for understanding diversification patterns and the reproductive biology of early angiosperms [1,2]. Concomitantly, knowledge has also accumulated concerning fossil arthropods that may have promoted angiosperm diversification through their role as pollinators . Direct angiosperm–animal associations are, however, rare in the fossil record . A recent discovery of exquisitely preserved Late Albian–Early Cenomanian flowers with attached arthropod remains is therefore of considerable significance. The new fossils were recovered from karst infillings in Upper Devonian reef limestone at the northern rim of the Rhenish Massif, Germany. The charcoalified fossils are three-dimensionally preserved. After wildfires, they were washed from the soil surface into a system of caves and shafts . The exceptional preservation of the fossils provides not only important insights into Cretaceous plant–arthropod interactions, but also adds new evidence for reconstructing terrestrial decomposer food chains.
2. Material and methods
The fossils described in the present article were recovered from Late Albian–Early Cenomanian karst infillings exposed in the Prangenhaus quarry (locality ‘Karstspalte’) near Wülfrath, Germany. The ‘Karstspalte’ is an at least 200 m deep and 12 m wide vertical shaft, filled with weakly consolidated fusinite-rich clay and large blocks of Late Devonian age which represent a collapse breccia. Based on palynostratigraphical analyses and the angiosperm flower association, the sediments are supposed to be of Late Albian–Early Cenomanian age (for a detailed description of the locality, see [5,6]). All specimens have been isolated from the clay and show pristine anatomical preservation owing to charcoalification during wildfires . To clarify the systematic affinity of the insect that lost the scales, we compared the surface scale structure of extant representatives of possible basal angiosperm flower visitors (table 1) with the two scales found attached to the fossil Chloranthistemon flower and bract .
The fusinite-rich clay was disaggregated by soaking in hydrogen peroxide (17%). After breakdown of the matrix, the slurry was fractionated using 60, 200 and 630 µm sieves. From the air-dried residue, plant fossils were handpicked under a dissecting binocular microscope. Chloranthistemon specimens were cleaned from adherent sediment by using hydrogen fluoride (40%) and hydrochloric acid (10%), mounted on aluminium stubs with nail varnish, coated with gold and then examined using a Hitachi S-4300 scanning electron microscope (SEM). Accelerating voltage of the electron beam was 2.0 kV.
SEM stubs with specimens 203-03 and 204-03 were deposited in the palaeobotanical collection of the Geological Survey of North Rhine-Westphalia, Krefeld, Germany. For the SEM examinations of modern Chloranthaceae pollinators, specimens were dissected and transferred to 99% ethanol in an ethanol series. The specimens were then mounted on SEM stubs. Voucher specimens (table 1) are deposited in the collection of the Steinmann Institute of Geology, Mineralogy and Palaeontology, Division of Palaeontology, University of Bonn, Germany.
The arthropod remains were found on the surface of flowers and inflorescence fragments representing a new species of the extinct chloranthaceous genus Chloranthistemon . The naked flowers consist merely of a large three-lobed androecium attached to a unicarpellate and uniovulate ovary. Typically, the androecium is detached from the ovary and inflorescence axis. The androecium has an extensive connective, small pollen sacs and valvate dehiscence; characters that strongly suggest insect pollination  as in the living counterpart, Chloranthus .
One Chloranthistemon flower bears an oribatid mite nymph fossilized in life position with the mouthparts anchored in a pollen sac while feeding on pollen (figure 1a,b). This plicate immature nymph belongs to the widespread Achipteriidae, which are known since the Jurassic . The androecium is heavily infested by fungal hyphae, implying that decomposition had already started when the nymph fed on the flower. Hence, prior to wildfire and charcoalification, the nymph did not act as a pollinator but rather as a decomposer of the plant litter. This is also illustrated by faecal pellets of the mite on the connective surface (figure 1a–c). Today oribatid mites are important decomposers of plant material in soil ecosystems .
On the same flower and on a bract, two scales of mosquitoes (Culicidae) were found (figure 1e,f,i). They can be placed within the Culicidae because the scales are characterised by a series of evenly spaced parallel ridge-like features connected at regular intervals by thin cross-ribs (figure 1g,h). This is significant, given the fact that the scale morphology from other members of possible pollinators of extant basal angiosperms is considerably different (see electronic supplementary material).
The fossil culicids most likely lost their scales while feeding on pollen  rather than nectaries, as within the Chloranthaceae the present dry-type stigma does not function as a nectary to attract pollinators [17,18].
The study of charcoalified fossil flowers provides invaluable information on the past structural and taxonomic diversity of terrestrial ecosystems. Coupled with an exceptional quality of the preservation of morphological and anatomical details, these charcoalified mesofossils provide a key piece of evidence for tracking the origin of the angiosperm lineage . While fusainized plant remains have been frequently reported from the fossil record as old as the Late Silurian , charred arthropod remains are extremely rare [20–22]. If at all preserved, then only the heavily sclerotized parts of the exoskeleton survive a wildfire which, however, show minute details. As usually weakly sclerotized arthropods are destroyed by fire, elytra of Coleoptera are over-represented in charred arthropod assemblages . The pristine three-dimensional preservation of fusainized thin insect scales and of a delicate and relatively weakly sclerotized oribatid mite nymph is exceptional and hitherto unreported. These findings allow us to directly document arthropod feeding strategies and the role of arthropods in decomposition that are rarely preserved in the fossil record.
The earliest fossil insects which presumably fed on pollen and/or floral tissues are Coleoptera from the Cretaceous period . These early flower-visiting beetles, together with other insects with generalized feeding habits, such as mandibulate moths, sphecid wasps and short-tongued flies, mark the evolutionary onset of insect–angiosperm pollination. In particular, the variety of early Diptera is regarded as decisive for the pollination and evolution of Early Cretaceous flowering plants .
Pollinators of extant basal angiosperms include Coleoptera, Diptera, Thysanoptera, Hymenoptera and Lepidoptera (table 1). The early flowering plants were probably pollinated and visited by a variety of insects [8–10]. Today, three of the four genera of Chloranthaceae are species poor (ca 65 species) and are more restricted in their distribution [24,25]. Flowers are known to be visited by beetles, bees, Hemiptera, flies and rarely ants, but only little is known about their pollination and breeding system. Recently, two species of Chloranthus were found to be pollinated by thrips , whereas species of Sarcandra are visited/pollinated by insects belonging to several different clades .
It is possible that the fossil mosquitoes also pollinated the Chloranthistemon flowers, in the same manner as some modern culicids can be important pollinators , but this assumption cannot be definitively determined from the available fossil evidence. The fact that the scales are adhering to two different floral remains of the same Chloranthistemon species, and found inside a flower-supporting bract, suggests that the scales were left by flower-visiting mosquitoes rather than being washed into the flowers while the inflorescence was in the litter. The scales are currently the oldest known fossil remains of mosquitoes. A possible contemporaneous occurrence is a specimen from the Burmese amber [16,28,29] that has a presumed Late Albian–Early Cenomanian age .
The investigations have been financed by grants from the German Research Foundation (DFG VI 225, WA 1492/6-1), the Ministerium für Wirtschaft, Energie, Bauen, Wohnen und Verkehr des Landes Nordrhein-Westfalen, the LVR-Amt für Bodendenkmalpflege im Rheinland, from SYNTHESYS (SE-TAF-3519, 1553) and the Swedish Natural Science Research Council (E.M.F.; grant no. 80373001). Access to the fossil site and technical support during the excavations were provided by Rheinkalk GmbH, which is gratefully acknowledged. We wish to thank several institutions and people for the loan of extant material for comparison of surface scale structures: N. Dorchin (ZFMK, Bonn), C. Lienhard (Geneva), R. zur Strassen (Senckenberg, Frankfurt), M. Schmidt (ZFMK, Bonn), D. Stüning (ZFMK, Bonn) and K. Yoshizawa (Sapporo, Japan). We are also grateful to C.C. Labandeira, S. McLoughlin and M.S. Engel for helpful critiques of drafts.
- Received July 8, 2011.
- Accepted August 8, 2011.
- This journal is © 2011 The Royal Society