Saturday 10 May 2014

Traces of Insect oviposition on Ginko leaves from the Middle Jurassic Daohugou Biota of Inner Mongolia.

Plants and Insects are the most abundant organisms in modern terrestrial ecosystems, and generally considered to be the most important. It is thought that Plant-Insect interactions have driven terrestrial ecosystems from the Carboniferous onwards, though direct evidence for such interactions, such as feeding traces or borings, can be difficult to find and interpret. One distinctive form of interaction is oviposition, the laying of eggs by Insects either on the external surfaces of Plants (exophytic oviposition) or within their tissues (endophytic oviposition). Fossil records of exophytic oviposition are, for obvious reasons, rather rare, but endophytic oviposition produces clear scars, and is known in the fossil record from the Carboniferous onwards.

In a paper published in the journal Acta Geologica Sinica in February 2014, Na Yuling, Sun Chunlin, Li Tao and Li Yunfeng of the Key Laboratory for Evolution of Past Life and Environment in Northeast Asia and the Research Center for Palaeontology and Stratigraphy at Jilin University report the discovery of endophytic oviposition scars on the underside of leaves of the Ginko Sphenobaiera, from the Juilongshan Formation of Daohugou in Inner Mongolia; a fossil rich lucastrine (lake) deposit known as the Daohugou Biota or Daohugou Lagerstätte.

The traces are placed in the ichnogenus Paleoovoidus, which has previously been used to describe ovoid or lens shaped scars in Plant tissues with surrounding envelopes of darker reaction tissue, and given the specific name venustus, meaning beautiful and elegant (palaeontologists often give trace fossils binomial names as if they were biological species; however while this does reflect similarity in form it is not intended to necessarily imply a biological relationship, as different organisms may produce similar traces in response to similar environmental challenges). The scars are elliptic to oval structures with one end more pointed than the other, 0.7-0.9 mm long and 0.3-0.4 mm wide.

Paleoovoidus venustus. (a) Leaf of Sphenobaiera with ovipositional scar. (b) Enlargement of square area in (a) showing the arrangement of the ovipositional scars. (c) Enlargement of the square area in (a) showing the gross morphology of the ovipositional scars. Na et al. (2014).

Traces interpreted as ovipositional scars, including many assigned to the ichnogenus Palaeoovoidus, are known in Plant tissues from the Carboniferous onwards. These are found in a wide variety of plants from the Carboniferous-Triassic, predominantly the Sphenophytes (Horsetails), but also including the Ferns, Bennettitaceans and Conifers, whereas those from the Cretaceous onwards are found almost exclusively in Angiosperm (Flowering Plant) leaves, reflecting the rise to dominance of this group. Traces from the Jurassic therefore provide an important source of information on Insect behaviour immediately prior to this major shift. 

However the Jurassic record of Insect oviposition is surprisingly sparse, with a few traces on the leaves of Sphenophytes, Conifers, Ginkos and Bennettitaceans from the Early Jurassic of Europe and some scars on Ginko and Conifer leaves from the Late Jurassic of Russia. The discovery of such oviposition scars in the Middle Jurassic Daohugou Biota is therefore highly significant, particularly as these are highly productive strata from which numerous specimens are still being recovered, and with many leaf specimens already in the collections of various academic institutions, which may prove to have undiscovered oviposition scars.

Insects of the order Odonta (Dragonflies and Damselflies) are thought to be the most likely makers of scars of the ichnogenus Palaeoovoidus, as they have been inexistence throughout the recorded range of the traces, and still exhibit similar behaviour today. Nine species of Draginflies and Damselflies have been found in the Daohugou Biota to date, strongly supporting the theory that these scars had an Odontan maker. However a variety of other Insects with ovipositors (egg laying organs) at least in theory capable of producing such scars have also been found, and while these Insects typically lay their eggs in soil or other Plant tissues today, they cannot be ruled out as the makers of the Daohugou Biota traces.

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