Insects and Plants during the Carboniferous

An American professor used to frequently ask his students, "If you were able to use H. G. Wells's time machine, which epoch would you like to visit?" Some said Carboniferous, others Triassic, and still others wanted Eocene. Many, without any hesitation, chose Cretaceous, the era which witnessed the fall of dinosaurs, the time of the ferocious Tyrannosaurus and the enormous flying reptiles Quet%altcoatlus, with wingspan reaching 15 meters. It was the time for the expansion of the flowering plants, of the ants, and of the chrysomelid beetles, after their timid start during the Jurassic.

It seems probable that flowering plants, the Angiospermae, were derived directly or indirectly from the Bennetitales, a kind of cycads, with complete bisexual flowers, living in the Jurassic and the Cretaceous. This view is at least a reasonable hypothesis. Does it mean that we have finally solved what Darwin used to name "the abominable mystery of the origin of the flowering plants"? It is too early to answer that. Pre-angiosperm fossils are very old and some are not cycadoid. However, it is not forbidden to think that in the forests of that time, the last Bennetitales had coexisted with the Phanerogamia. Let us recall a prairie in Southern New-Guinea, where Cycas prospers near palm trees, the Coniferae, and the flowering plants, and we can get a fair idea of the flora of that old time. Only the proportion of these plant groups might have been different.

However, another fascinating period must have been the Carboniferous, one of the last stages of the Palaeozoic. An enormous quantity of fossils is available in coal deposits of that time, and it has been possible to reconstitute the ecosystem of the forests of that period, with its swamps and its inhabitants. This age lasted 70 million years, 236 million years ago. We can speak already of a certain coevolution between plants and animals, and there were already cases of mimetism, but not yet of myrmecophily, as the ants did not exist then. Also carnivorous plants and the phanerogams had not appeared yet.

In the Carboniferous amphibians were carnivorous and insectivorous, and reptiles, first carnivorous, and some became herbivorous at the end of the period. Beetles appeared a bit later, during the Permian. However, insects were diverse, some of them giants, and adapted to all sorts of diets. Most of them, however, were herbivorous. The giant myriapod Arthropleura was trilobite-like, two metres long, and was mostly feeding on lycopods. Many insects had then long proboscis, and were sap suckers, as are our bugs. Certain species were phytophagous or saprophagous, as are the present cockroaches; they were numerous and were detritivorous or truly phytophagous. Grasshoppers then were evidently phytophagous, and scorpions, having left water, were eating what they could find in forest litter.

One can imagine that lycopods, horsetails and giant ferns, mixed with primitive conifers, were forming great humid forests. In swamps, giant dragonflies were breeding, with Ephemeroptera and Perlidae. Meganeura monyi was living then with its 75 cm wingspan (see the chapter on "Damselflies, experts in ballistics"). It was so strong that no spider could catch it with or without its web. Meganeura must have escaped any predator in air.

It has also been discovered that among insects and plants of the Carboniferous there were strange cases of palaeomimetism, e.g. pinnules of ferns Neuroperis and Odontopteris closely resembled to the wing innervation of certain coackroaches, like Phylomylacris. That could explain why in the literature there was sometimes confusion between leaves and insects. Jeannel, in his time, reproached Nicolas Theobald (1937) for some confusion in his book on Oligocene terrains in France.

Pollen grains and spores were already carried by arthropods, including Arthropleura, which has been mentioned above. Fossils of this myria-

pod carry the proof it. Seeds probably carried attractive glands for attracting the disseminators. Plants protected themselves against her-bivory through sclerified layers, glandular hairs, etc., but they were even then attacked and eaten. Some leaves and stems of, for example Neuropteris of that period, strongly attacked by herbivores, have been found. But perhaps phyllophagy was still in its infancy. To suck the sap or to feed on soil detritus were probably the most current modes of nutrition. It was, however, noted that among the arborescent fern, Psaronius, entire stems were full of insect excreta, and were much eaten. There were also galleries inside wood, with coproliths or fossilised insect feces containing vegetal debris.

Carboniferous insects were mostly living on the ground, inside forest litter, on the plants and certainly on the forest canopy of that period. We don't know much more. There were carnivorous, saprophagous, and phytophagous insects. Those enormous arthropods moving two meters inside the litter must have been very impressive. Were they toxic and venomous? Probably yes, but that we will never know for sure.

It does not seem that large scale extinctions affected the insects, either because they multiplied rapidly, or because they escaped long cold periods, thanks to diapauses, or because the temperate zones, less affected, became readily repopulated. A great extinction at the Cretaceous-Tertiary (K-T), marked by the Alvarez iridium layer, does not seem to have much touched the insects (Whalley, 1987). At least, it was never found among the invertebrates indications of the effect of the catastrophic events, which are supposed to have killed the dinosaurs. Some people, e.g. Labandeira, however, have objected to the notion of non-disappearance of insects. It is very difficult to review the oldest extinctions, due to lack of definite data, and also because much of the rich amount of insect fossils available has not been examined and analysed by experts. One thing is certain, that no important insect group disappeared from the earth, the old orders being the ancestors to the present orders.

— Fig. 22.1. Diaphanopterodea, Uralia sp., a fossil species from the Permian, feeding on a primitive flower head (from Rohdendorf and Raznitsin, 1980; Jolivet, 1986).

— Fig. 22.2. Upper Carboniferous tropical forest, with Lepidodendraceae, Sigillariaceae, Equisetales, Gingkoales and tree ferns (Pteridospermae) (after Jeannel, 1946).

— Fig. 22.3: Meganeura monyi Brongniart, a primitive dragonfly (Meganeuridae). Carboniferous, from Commentry, France (after Jeannel, 1946).

References

Jeannel, R. 1946. Introduction à l'Entomologie. 3. Paléontologie et peuplement de la

Terre. Boubée & Co. Publishers, Paris : 101 pp. Jolivet, P. 1986. Insects and Plants. Parallel evolution and adaptations. Brill Publs., New York: 197 pp.

Rohdendorf, B. B. and Raznitsin, A. P. 1980. The Historical Development of the

Class Insecta. Trudy Paleont. Inst. Moscow 175 : 1-268. Theobald, N. 1937. Les Insectes fossiles des terrains Oligocènes de France. Nancy: 473 pp.

Whalley, P. 1987. Insect evolution during the extinction of the Dinosaurs. Entom. Gener. 13 (1-2): 119-124.

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