Thoracic and abdominal outgrowths in early pterygotes: a clue to the common ancestor of winged insects?

One of the fundamental questions in insect evolution is the origin of their wings and primary function of ancestral wing precursors. Recent phylogenomic and comparative morphological studies broadly support a terrestrial ancestor of pterygotes, but an aquatic or semiaquatic ancestor cannot be ruled out. Here new features of the branchial system of palaeodictyopteran larvae of several different instars of Katosaxoniapteron brauneri gen. et sp. nov. (Eugereonoidea) from the late Carboniferous collected at Piesberg (Germany) are described, which consist of delicate dorsolateral and lamellate caudal abdominal gills that support an aquatic or at least semiaquatic lifestyle for these insects. Moreover, the similar form and surface microstructures on the lateral abdominal outgrowths and thoracic wing pads indicate that paired serial outgrowths on segments of both tagmata presumably functioned as ancestral type of gills resembling a protopterygote model. This is consistent with the hypothesis that the wing sheaths of later stage damselfly larvae in hypoxic conditions have a respiratory role similar to abdominal tracheal gills. Hence, the primary function and driving force for the evolution of the precursors of wing pads and their abdominal homologues could be respiration.


List of examined specimens:
Holotype specimen No. Pal1242ab (Clay stone layer above the Dreibänke coal seam) -(previously considered as nymph -"Rochdalia type" with ovipositor), presumably exuvia due to distortion of metathoracic wing pads, prothoracic winglet lobe without distinctive articulation area -single vein precursor very faint in comparison to wing pads, meso-and metathoracic wing pads with well discernible pattern of lacunae, abdomen with terminalia.Structure of wing pads: WP1: ScP probably reaching RA behind midwing, precursors of RA, MA and CuA all simple, RP with 6 branches, MP with 7 branches, CuP with 4 branches, PCu deeply bifurcated, both probably secondary twigged, A with 6 branches; WP2: veins RA, MA and CuA all simple, RP with 8 branches, MP with 10 branches, CuP with 5 branches, anal area (including PCu) with 2? branches.
Specimen No. F375ab (Clay stone layer above the Dreibänke coal seam) -?exuviaprominent prothoracic winglet lobe, meso-and metathoracic wing pads (mesothoracic wing pad with keel and well discernible pattern of lacunae while in metathoracic only posterior part is discernible), abdominal segments I-IV with prominent lateral outgrowths (articulation of wing pads / abdominal outgrowths).Structure of wing pads: WP1: prominent keel, ScP running close to radius, three simple convex precursors of RA, MA, CuA, RP with 5? branches, MP with 6? branches, CuP simple or with two branches, anal area broad with numerous branches pectinated (about 5 branches); WP2: only distal part of wing pad showing the same as WP1, but with reduced costal area, three simple convex precursors of RA, MA, CuA, RP and MP similar to WP1, basal part of wing pad not preserved.
Specimen No. F429ab (Clay stone layer above the Dreibänke coal seam) -tip of one metathoracic wing pad and abdomen with prominent lateral outgrowths, caudal appendages not preserved (only fragments).Structure of wing pad: WP2: apex with discernible ScP running rather close along radius, RP with about 3 branches, convex MA probably simple.
Specimen No. F246ab (Clay stone layer above the Dreibänke coal seam) -exuvia based on distortion of meso-and metathoracic wing pads, thorax distorted with meso-and metathoracic wing pads, abdomen with prominent lateral outgrowths and caudal appendages, faintly preserved structures resembling valvulae of ovipositor discernible between abdominal segments VIII and IX.Structure of wing pads: WP1: ScP running very closely to RA in distal part of the wing, precursors of RA, MA, CuA all simple, RP with 3-4 branches, MP with 5? branches, CuP with number of branches not clearly discernible, broad anal area behind with about 5 branches incl.PCu, prominent widened posterior margin of wing; WP2: wing apex sunken (visibly jutting out of the line of the posterior margin), precursors of RA, MA, CuA all simple, RP with 6-7 branches, MP with about 7 branches, CuP with number of branches, anal area rather broad.
Specimen No. F148ab (Clay stone layer above the Dreibänke coal seam) -dorsoventral aspect, probably exuvia with metathoracic wing pads exposed, very well preserved abdominal lateral outgrowths, caudal appendages well preserved.Structure of wing pad: WP2: veins RA, MA, CuA all simple, RP rather reduced with 2 or max. 3 branches, MP with 4-5 branches, CuP with ?2-3 branches, PCu and anal area rather broad.
Specimen No. F427ab (Clay stone layer above the Dreibänke coal seam) -lateral aspect, presumably mesothoracic wing pad without preserved thoracic connection, but corresponding to the position.Distal part of abdomen showing segments III-X with well discernible caudal appendages and lateral abdominal outgrowths (presumably shortened epiproct and paired cone-shaped paraprocts, small reduced cerci are not well discernible -more distinct under film layer of ethylalcohol, abdominal segment X with a short ventral protrusion).Structure of wing pad: Basal part of mesothoracic wing pad on one slab while the distal part on the other one.WP2: Costal area broad but not triangular like in other specimens, ScA well discernible convex running towards costal margin, ScP running parallel to RA, convex veinal precursors of MA and CuA discernible, RP and MP with more branches, but impossible to count (MP area seems to be broader over RP), CuP not clearly discernible from anal area, anal area rather broad with numerous veins.
Specimen No. F320 (Clay stone layer above the Dreibänke coal seam) -aspect, thoracic segments distorted, abdomen with well discernible caudal appendages in form of paraprocts and perhaps also with epiproct, lateral flaps discernible only on some abdominal segments, mesothoracic wing pad with broad costal keel and presumably numerous branches of MP (4-5), otherwise vein precursors not well discernible.
Specimen No. Pal1243 (Clay stone layer above the Dreibänke coal seam) -dorso-lateral aspect, thoracic segments distorted, ?exuvia, abdomen with well discernible caudal appendages in form of paraprocts and perhaps also together with epiproct, lateral flaps not well preserved.
Specimen No. F139 (Clay stone layer above the Dreibänke coal seam) -dorsal aspect, thoracic segments with prothoracic lobe, meso-and metathoracic wing pads and proximal segments of abdomen, lateral outgrowths well discernible, wing pads with only partly discernible pattern of lacunae.
Specimen No. Pal cl4 (lake sediment between the coal seams Mittel and Johannisstein)larval exuvia in ventral aspect, showing abdominal lateral outgrows, valvular ovipositor between segment VIII and IX and caudal appendages.Branching pattern of wing pads not clearly discernible.