Plant–arthropod interactions in the Mesozoic of Poland

Plant–arthropod interactions developed gradually and became enriched with new types during the coevolution of plants and arthropods on land. Labandeira (2006) distinguished four phases of their evolutionary development: (1) a Late Silurian to Late Devonian phase characterized by myriapod, apterygote, and perhaps early pterygote insect herbivores feeding on primitive vascular plants; (2) a Late Mississippian to end-Permian phase with mites and apterygote and basal pterygote insects associated with pteridophytes and basal gymnosperms; (3) a Middle Triassic to Recent phase involving mites, orthopteroids, hemipteroids, and basal holometabolans on pteridophytes and gymnosperms; and (4) a mid-Early Cretaceous to Recent phase with modern orthopteroids, derived hemipteroids, and holometabolous insects exploiting angiosperms. He recognized seven principal functional types of plant–arthropod interactions: external foliage feeding, piercing-and-sucking, boring, leaf mining, galling, seed predation, and oviposition. Recent findings from Polish Mesozoic localities have provided additional evidence of such interactions, potentially yielding important new insights once described in detail.
Among several Upper Triassic sites in Upper Silesia yielding vertebrate, invertebrate, and plant macro- and microremains, only two – Patoka and Poręba – provided evidence of plant–animal interactions. The strata exposed at these localities belong to the upper part of the Patoka Member of the Grabowa Formation and are middle–late Norian in age based on sporomorph assemblages characteristic of the subzone Classopollis meyeriana b. Plant macroremains belong almost exclusively to conifers of the Brachyphyllum-Pagiophyllum-morphotype and are preserved as coalified compressions with well-preserved cuticles. Some leaves exhibit mining and contain elongate masses of frass within the mines, composed of very small, ovoid coprolites measuring approximately 36 × 45 μm. Previously the Triassic fossil record of leaf mining has been based primarily on galleries and frass in fossil leaf impressions (e.g., Labandeira, 2002). The Silesian material provides, for the first time in the Triassic, frass preserved within isolatable coalified leaf cuticles. However, this mode of preservation makes it difficult to determine the shape of the tunnels and classify them according to the types defined by Labandeira et al. (2007). Nonetheless, this type of preservation allows for more detailed observations of leaf mining, which first appeared in the fossil record just during the Triassic. The small size of the tunnels and coprolites may indicate that they were produced by mites. Additional evidence of interactions includes round holes smaller than 1 mm in diameter present on several isolated leaves, although not mined ones. These holes represent the most simple type of hole feeding. Several shoot apices are atypical, differing from the normal condition: they are shortened, swollen, and have very densely arranged leaves. Such alterations may represent galling by arthropods, possibly mites, analogous to galls induced on modern Picea (Pacyna et al., 2018). Moreover, several leaves from Poręba exhibit enigmatic modifications in cuticle micromorphology, potentially representing oviposition traces or stomatal alterations caused by an unknown agent. Under light microscopy (LM) these structures appear as elongated ovoid or circular bodies containing probable lenticular eggs inside, similar to closed egg capsules deposited by recent and fossil odonates. Under SEM, however, they manifest externally as grooves bordered by protruding papillae and internally as accumulations of amorphous or finely spherical material, possibly coprolites. Overall, traces of plant–arthropod interactions in the Polish Upper Triassic are rare, suggesting either that few types of interactions occurred and that arthropod herbivory was minimal, or that conifers were not commonly exploited by arthropods.
Polish Jurassic floras are abundant in both taxa and specimens, with Early Jurassic assemblages from the Holy Cross Mountains and Middle Jurassic ones from the Kraków region. Reinvestigation of putative galls on the Hettangian (Zagaje Formation) fern Dictyophyllum nilssonii from Gromadzice reported by Pacyna and Zdebska (2012) revealed that these structures do not represent traces of plant–arthropod interactions but are in fact very peculiar traces of fossil roots penetrating sediment and perforating leaves already buried in sediment. This taphonomic phenomenon – roots growing through leaves embedded in the substrate of fine-grained siliciclastic sediments and being in the process of fossilization – is not addressed in the literature and requires further research. Interestingly, roots and their associated soil-forming processes did not destroy or even damage buried leaves undergoing fossilization (except for the cavities and holes left by penetrating roots). Thus, no confirmed evidence of plant–arthropod interactions is currently known from the Polish Early Jurassic.
In the well-known Middle Jurassic (Bajocian–Bathonian) Grojec clays flora, leaves of the cycad Ctenis potockii show not only typical oviposition traces but also numerous other very enigmatic traces. These consist of oval bodies arranged in rows between leaf veins on the abaxial surface, varying slightly in size and bearing fine surface ornamentation. Their interpretation remains problematic; they may represent oviposition traces or alternatively scale insect (Coccoidea) body fossils. The Ctenis potockii leaves also show leaf undermining – feeding and resting traces produced by small invertebrates sheltering and moving immediately beneath fallen leaves (Jarzynka et al., 2018).
Late Cretaceous angiosperm leaves from Poland have not yielded so far any evidence of plant–arthropod interactions. Therefore, the Cretaceous part of fourth phase of plant-arthropod interactions evolution is currently absent from the Polish fossil record. The only Cretaceous plant with such traces is bennettitalean trunk Cycadeoidea sp. described by Reymanówna (1960) from Barremian Verovice shales of Przenosza near Limanowa, which still represents phase 3 of plant-arthropod interactions evolution. This specimen displays numerous tunnels filled with coprolites probably of insect origin in xylem, ramentum coat of stem and especially in the flower buds. Plant was heavily damaged by this activity: the regular arrangement of leaves bases and flower buds on stem was disrupted. As a defense, the plant secreted gum and abscised the attacked young flower buds.
Acknowledgements: The study was financed by funds from the National Science Centre, Poland (No 2021/43/B/ST10/00941 and 2022/45/B/NZ8/02000).