Paleozoic Herbivory

The Palaeodictyopterida were the first major group of herbivorous insects. We tend to think of the impact of herbivorous orthopterans, phytophagan beetles, plant bugs, and caterpillars on modern ecosystems, but during the Paleozoic the palaeodictyopterids were among the primary herbivores. They probably caused much of the plant-tissue damage during the Permian and Late Carboniferous (perhaps even into the Early Carboniferous, although body fossils are lacking), and in fact feeding scars can be found readily on Paleozoic plants. In a comprehensive study of Permian plants, 83% of the leaves showed evidence of insect herbivory, and 4.5% of the leaf area was consumed by insect herbivores (Beck and Labandeira, 1998). Though all the insects that caused the damage are not definitively known, Palaeodictyopterida were certainly among them, as indicated by holes made with beaks.

Long before definitive evidence for winged insects, mites and other terrestrial arthropods probably exploited plants. Indeed, the damage observed in some Devonian plants is identical to that made by modern phytophagous mites. Early Devonian rhyniophytes and trimerophytes had herbivore damage, and they contained arthropod coprolites (presumably of myriapods and mites) that consisted of spores (e.g., Edwards, 1966; Banks, 1981; Trant and Gensel, 1985; Banks and Colthart, 1993) (e.g., Figure 2.21). Evidence of pierced plant tissues is also known from this time period (Kevan et al., 1975; Labandeira and Phillips, 1996b); perhaps the piercings were made by mites and springtails, the latter group of which is also known from this time period (e.g., Rhyniella). However, the frequency of herbivory was perhaps minor as terrestrial arthropod communities were dominated by predators, particularly among the arachnids. Basal insects (e.g., archaeognaths and zygentomans) were probably mostly detritivorous (although silverfish certainly also consumed spores or pollen) and probably presented little threat to the plants that radiated across the barren landscapes of the Early Devonian.

As we have already discussed, the rise of forests in the Devonian may have led to the development of flight and it is then, among the pterygotes, that plants finally felt the force of insects, most impressively from Palaeodictyopterida during the Paleozoic. The Odonatoptera, like their modern counterparts, were almost certainly predatory and played little part in these nascent plant-insect interactions.

Contemporaneous with the Palaeodictyopterida were numerous families of "Protorthoptera," which were clearly herbivorous. Unlike the paleodictopterids, protorthopterans had chewing mouthparts and must have fed on external foliage (e.g., Müller, 1982; Scott and Taylor, 1983; Beck and Labandeira, 1998; Labandeira and Beall, 1990; Obordo et al., 1994; Castro, 1997), or they consumed spores and pollen (e.g., Rasnitsyn, 1977c; Scott and Taylor, 1983; Rasnitsyn and Krassilov, 1996a,b; Krassilov and Rasnitsyn, 1997).

Palaeodictyopterids, however, could exploit tissues within the plant through the then novel means of piercing and sucking. To what extent the paleodictyopterid beak could take in fluids or was relegated to feeding on nutritious internal tissues is unknown (bear in mind, too, that the vascular systems of many Permian and Carboniferous plants were not as developed as plants are today). Damage from piercing mouthparts has been found on various Paleozoic seed ferns (e.g., Medullosales, Cordaitales), tree ferns (Marattiales), and lycophytes (Lepidodendrales) (e.g., reviewed by Labandeira, 1998), all presumably the activity of palaeodictyopterids owing to their physical proportions and trace morphology (e.g., Labandeira and Phillips, 1996b). Some Palaeodictyoptera (e.g., Eugeronidae, Homiopteridae) had particularly long, possibly flexible beaks reaching nearly 32 mm (1.3 in.) in length. Such insects could presumably insert their stylets into the inner tissues and extract phloem and xylem. The base of the beak was broad and resulted in a conical surface scar, indicative of palaeodictyopterid feeding. Similar piercing scars are also seen in fossilized seeds, in which the insect bored its beak through the protective layers and into the embryonic tissues (Sharov, 1973). Feeding punctures into plant stems are documented from the mid-Pennsylvanian (Scott and Taylor, 1983; Taylor and Scott, 1983). When palaeodictyopterids became extinct at the end of the Paleozoic, diverse and efficient new insect herbivores took their place.

ODONATOPTERA: DRAGONFLIES AND EARLY RELATIVES

The Odonatoptera are the most recognizable of the primitive pterygotes. The familiar dragonflies are conspicuous day-flying insects common to most parts of the world. They are also among the most ancient of winged insects, and with the Ephemeroptera and Palaeodictyopterida comprise the former "Palaeoptera" - an unnatural grouping that has been abandoned (see earlier discussion in this chapter).

Three orders are included in this group: the Geroptera, Protodonata, and Odonata, which are collectively defined by a reduction of the anal region of the wings; a distinctive form of bracing where there is a "kink" in CuP where it meets the anterior anal vein; and two articular plates at the wing base (Riek and Kukalova-Peck, 1984; Bechly, 1996; Bechly et al., 2001). The fusion of several axillary sclerites forming two large plates in the wing articulation is distinctive to Odon-tatoptera and is not found in any other group of flying insects. In Odonata, the points of fusion between the original, smaller plates are indistinguishable; in Geroptera and Protodonata suture lines (and purportedly some membrane, but this is based on compression fossils) still exist that demarcate the original, joining sclerites.

It is difficult to surmise when the Odonatoptera originated. The basal Geroptera and Protodonata occurred as early as the Late Carboniferous and, in fact, the records of the Geroptera are among the oldest of any winged insects (e.g., Brauckmann et al., 1996). Thus, the age of the last common ancestor of Odonatoptera is speculative, but may have been in the Early Carboniferous or even the latest part of the Devonian.

0 0

Post a comment