Palaeos Invertebrates: Arthropoda: Insecta: Paleoptera

Classically, the Paleoptera have been defined both apomorphically and paraphyletically..... That's a mouthful. Perhaps we should try again. Paleoptera are usually defined as "primitive winged insects." So, all paleopterans, and no other arthropods, are supposed to have a particular physical characteristic (wings) in common. That's the "apomorphic" part. However, the Paleoptera don't include all winged insects -- just the "primitive" ones. That's the "paraphyletic" part. This sort of definition is hopelessly vague and invites problems of classification.

Those problems recently surfaced in this group with the discovery -- actually, the redescription -- of Rhyniognatha, an insect from the Pragian (Early Devonian) of Scotland. Engel & Grimaldi (2004). Rhyniognatha is like other Paleoptera in most respects, but it probably had no wings. On the other hand, it is not a silverfish (Thysanura). The mouth parts are all wrong for a silverfish. The mouth parts are entirely correct for a paleopteran -- but there are no wings and no reason to believe there ever were any wings.

The reaction to Rhyniognatha in the semi-scientific press has been interesting. It was speculated by Engel & Grimaldi that Rhyniognatha's early Devonian date might imply that insects began even earlier, in the Silurian. The press duly reported that Rhyniognatha was Silurian. Much more importantly, Engel & Grimaldi stated, quite correctly, that Rhyniognatha appeared to be a paleopteran. It was duly reported that flying insects were present in the Silurian ... .

In fact, there are no flying insects known for almost a hundred million years after the Ludlow (Late Silurian). Flying insects are first found in the Carboniferous, specifically, the Late Mississippian. The acquisition of flight in insects was followed -- just as in pterosaurs, birds, and bats -- by an explosive radiation of flying forms and non-flying taxa related to them. So, were the press out of line? Maybe, but the real problem is that silly definition: "primitive winged insects." It invites precisely the misunderstandings which followed the article by Engel & Grimaldi. The irony is that the Paleoptera are some of the very insects studied by Willi Hennig, the founder of cladistics, whose influence on evolutionary theory is exceeded only by Darwin. Here, if anywhere, we would expect to see the hand of the cladistic Reformation which Hennig began.

But we don't. However, in homage to Hennig, we will ignore traditional usage and give Paleoptera a proper phylogenetic definition: fire ants (Solenopsis) > fire brats (Thermobia). For an explanation of this terminology, see Cladograms. This makes Rhyniognatha a paleopteran because, very probably, it shares a more recent common ancestor with Solenopsis than with Thermobia -- whether or not Rhyniognatha had wings.

Paleopteran relationships are a contentious field. Fortunately there are relatively few who care enough to contend, so collateral damage to civilians has been slight. Our own take on the subject is summarized in the following cladogram:

Hennig himself favored a monophyletic Paleoptera. In this scheme, Paleoptera and Neoptera represent a fundamental divergence between two designs for flight, which diverged long ago, something like this:

Unfortunately, our principled defense of Hennigian historic primacy in the previous section has pretty much exhausted our meager supply of principle. Thus we unctuously defer to current orthodoxy and treat the relationship as paraphyletic.

The main groups of paleopterans are all treated on separate pages, with the exception of the Megasecoptera, which we now summarize in the following fashion.

Megasecoptera

Protohymen after Kukalova-Peck (1974) Pennsylvanian to Guadalupian. There are 22 families of megasecopterans and only about 35 known genera, so -- right out of the box -- we should be suspicious that the diversity of the group is undersampled and poorly understood.

Like many other paleopterans, the megasecopterans had sucking mouthparts, which doesn't fit the usual party line about detritivory. Instead, it is quite clear that the Megasecoptera were "plant vampires." The suctorial wouth parts were used to pierce plant casings and extract high-quality plant materials, such as spores and pollen.

Megasecoptera bore two pairs of wings (some very basal paleopterans had three), which are nearly of the same size. The wings were held horizontally, as in dragonflies (Odonata, Anisoptera). The wing bases tend to be very slender, as in damselflies (Odonata, Zygoptera). The body is long and thin. The illustrated Protohymen is rather stouter and shorter than a typical megasecopteran.

During their relatively brief existence, the Megasecoptera were extremely succesful. It has been estimated that this order accounted for 50% of the insect biomass in some locations. This may be an overestimate, because Megasecoptera tended to be rather large. Some reached almost unbelievable dimensions, with wingspans close to 100 cm. As a result they are over-represented in the fossil record. It has been suggested that insects of this size would be capable of soaring flight, like an albatross or large pterosaur.

Links: ELMO INSECTS IN BRIEF, Insect Flight- Evolution

Palæos:		Arthropoda
INVERTEBRATES		Paleoptera

Paleoptera

Paleopteran Relationships and Diversity

Megasecoptera