The "Uniramia" do not exist: the ground plan of the Pterygota as revealed by Permian Diaphanopterodea from Russia (Insecta: Paleodictyopteroidea)

Abstract

The paleomorphology of extinct Paleozoic Diaphanopterodea (Paleodictyopteroidea: Paleoptera) in exceptionally well-preserved Lower Permian Uralia (Parelmoidea) and Paruralia (Paruraliidae) from Tshekarda in the Ural Mountains, Russia, was used to identify the basic morphological components of the pterygote head, body, and genitalia, as follows: a 6-segmented head with sutures between the somites and between the cephalic terga and epicoxae; antennae 2 with four enlarged basal elements and a flagellum; mandibles (shown as fully homologous to the gnathobase of other arthropods) with a sliding anterior articulation; leglike maxillary palps, labial palps, abdominal leglets, and gonostyli, all starting with prefemur, bearing a fully articulated patella (as in spiders) and ending in double claws; polyramous thoracic legs (i.e., with several outer rami or exites) and a prefemur not associated, as it usually is, with the trochanter; a thoracic pleuron (wall support) formed from a flattened subcoxa; an abdominal pleuron clearly composed of three flattened leg segments: the subcoxa, coxa, and trochanter; and genitalia revealing their derivation from legs bearing coxal and trochanteral endites. It is suggested that the homoeotic mutant ophthalmoptera is a morphologically acceptable indication that the cephalic epicoxa surrounds the eye. The presence of several rami (exites) on the legs of primitive Paleozoic and modern Crustacea, Chelicerata, and Insecta shows that they share a polyramous arthropod leg; "biramy," as well as "uniramy," is always secondary and does not define higher arthropod taxa. The Atelocerata Heymons, 1901 (= Myriapoda + Hexapoda) is a natural taxon. "Uniramia" was a taxon based mainly on an erroneous "ground plan" of the arthropod leg and mandible, and should be completely dropped from use. In marine arthropods, one of the upper exites often convergently develops into a long swimming oar ramus, and the legs become functionally "biramous." A complex aquatic epicoxal oar ramus of the ancestral Atelocerata may be the only appendage suitably preadapted to become a protowing, modifiable by selection for flapping flight. Paleodictyopteroids with sucking rostra (about 50% of Paleozoic entomofauna) are probably the most important selective agent that initiated the change from the Paleozoic plant community into the present plant community.