Herbivory in arthropods
Most of our knowledge of herbivory in arthropods comes from coprolites, feeding marks on leaves and other plant parts, wood borings (sometimes containing coprolites), and inferences based on comparisons of fossils with their living counterparts. Plant-bearing arthropod coprolites are oval to cylindrical in shape and can be as much as 1 mm long, although most are smaller. Those that have been analysed contain the remains of various plant parts (e.g. leaves, stems, spores, pollen) derived from a variety of land plants, including lycopods, ferns, gymnosperms, and angiosperms. The earliest plant-bearing arthropod coprolites occur in Upper Silurian and Lower Devonian strata and thereafter become increasingly common in younger formations.
The oldest clear indications of herbivory which can be linked with a particular group of arthropods are lesions on Early and Middle Devonian stems. These lesions are similar to those produced by modern piercing and sucking insects, but the oldest remains of such insects occur in Permian strata.
Some of the most abundant and credible evidence of herbivorous arthropods is provided by feeding marks on leaves. These consist of relatively small excisions where a portion of the lamina has been removed from the margins or inner parts of the leaves. A rim of reaction tissue is present along the edges of the undamaged lamina of most of the damaged leaves, confirming that feeding occurred while the leaf was alive and still attached to the parent plant. In some specimens the veins adjacent to the excisions have been dislocated, and in rare instances the growth of the leaf appears to have been stunted by the grazing. The excisions indicate that many of the leaf-feeding strategies used by modern insects appeared during the Palaeozoic (Labandeira 1998). These strategies include marginal feeding (Fig. 4.1.16.1a,b,d), which first appeared in the Late Carboniferous; hole feeding (Fig. 4.1.16.1c); skeletonizing; and window feeding (Fig. 4.1.16.1d), which developed during the Permian. Evidence of leaf mining, a feeding strategy which also provides the herbivore with shelter, typically consists of narrow irregularly sinuous bands on the lamina of fossil leaves (Fig. 4.1.16.1a). Such structures are well documented in Paleocene and younger strata, and have also been infrequently described from fossil leaves as old as Late Carboniferous (Stephenson and Scott 1992).
Small borings excavated in both living and dead tissues are common in the geological record, beginning in the Carboniferous. Some of them contain coprolites similar in morphology to those of termites and confirm that these specialized insects were present in the Cretaceous, if not earlier (Boucot 1990). Most of the borings, particularly the older ones, are usually attributed to the activities of beetles.
It seems clear from the fossil evidence that phytophagous arthropods were present during the Devonian, or perhaps in the Late Silurian, and had developed a variety of feeding strategies by the Late Carboniferous, which they continue to use. This stasis in feeding strategies is remarkable because many groups of arthropods became extinct at the end of the Palaeozoic, yet the phytophagous arthropod fauna which replaced the early arthropod fauna continues to follow the same feeding strategies. Furthermore, land plants have faced increasing attack by phytophagous arthropods since the Carboniferous, a trend which has accelerated from the Cretaceous to the present day.
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