Fang et al.—Late Triassic elcanid insect fossil from North America 92(6):1028–1034
thicknesses. The beam-energy image series displays notable changes in contrast with VA. In images acquired at low VA (2 keV), whereas the contrast between fossil and the substrate is high, contrast between the medial sector and radial sector (including pterostigma-like structure) veins is relatively low (Fig. 3.8). The contrast between the veins in the medial and radial sectors, however, increases with VA (Fig. 3.9–3.11), and in images acquired at VA of 10 keV, only the radial sector veins and pterostigma-like structure are evident (Fig. 3.11). Because mass-thickness contrast in BSE SSD Z-contrast images of the Solite Quarry fossils primarily corresponds to differences in the thicknesses of the carbonaceous material, and thicker carbonac- eous layers appear darker in BSE SSD images than thinner carbonaceous layers on relatively higher Z substrates (Muscente and Xiao, 2015), the beam-energy image series suggests that the pterostigma-like structure is preserved as a relatively thicker carbonaceous layer than those of veins in other parts of the wing. Such differences in the thicknesses of these features most likely correspond to taphonomic differences among the tissues because the ultimate thickness of a carbonaceous compression depends upon its original thickness and composition (Muscente and Xiao, 2015).
Systematic position.—Cascadelcana virginiana n. gen. n. sp. can be attributed to the Elcanidae based on the simple CuA+ CuPa1 and RP ending inMA1. The family Elcanidae is divided into two subfamilies: Archelcaninae Gorochov, Jarzembowski and Coram, 2006 and Elcaninae Handlirsch, 1906. Casca- delcana virginiana n. gen. n. sp. belongs to Archelcaninae in having a broad area between RA and RP, as well as free (rather than fused) distal parts of CuPa2, CuPb, and 1A. It clearly differs from Archelcana Sharov, 1968 and Sibelcana Gorochov, 1990 in its short CuA that is almost vertical against the posterior margin, and its RP+MA1 with fewer branches. It can also be distinguished from Parelcana Handlirsch, 1906 and Synelcana Zessin, 1988 by its MA with fewer branches before RP ending in MA1 (Parelcana with 4 branches). The relationship between CuA and CuPa1 was considered an important character for the classification of fossil orthopterans (Béthoux and Nel, 2002). The character of short but measurable CuA is also found in all ancient species of Permelcanidae, in contrast with some Elca- nidae species that have CuPa1 fused with the point of origin of CuA. Thus, C. virginiana n. gen. n. sp. may have a mosaic combination of features characteristic of the Permelcanidae and Elcanidae. These characters may help to clarify the phylogenetic relationship of the Elcanoidea (Fig. 4).
Elcanidae Elcanoidea
Permelcanidae 298.9 Ma
Permian Meselcaninae Permelcaninae 252.1 Triassic 1 201.3 Jurassic 145.0 Lower Cretaceous
Figure 4. Hypothesized phylogenetic relationships of the Elcanoidea. Modified from Gorochov (1995). The number ‘1’ marks the geological age (Upper Triassic, Norian, ~227–208Ma; Liutkus et al., 2010) of Cascadelcana virginiana n. gen. n. sp. from the Cow Branch Formation in the Solite Quarry.
100.5
1033
Pterostigma structure.—The pterostigma is a thickened, sclero- tized area on the anterior margin of the insect wing near the tip (Grimaldi and Engel, 2005). This structure is particularly notice- able on both wings of the Odonata and on the forewings of many species of the Mecoptera, Hymenoptera, Psocoptera, and Mega- loptera (Chapman, 1998). Similar to the Odonata,C. virginiana n. gen. n. sp. and some other elcanids (such as Panorpidium yixianensis Fang et al., 2015) also have thickened or colored parts close to the leading edge far out on the wing (Fig. 2). According to the Z-contrast image series (Fig. 3.8–3.11), this pterostigma-like area is characterized by relatively thick carbonaceous material, indicating that the leading edge on the wing of C. virginiana n. gen. n. sp. was probably thickened and sclerotized. The pterostigma structure in the wings of elcanids differs
from the pterostigmae of Recent dragonflies and hymenopterans in that it includes several crossveins (branches of RA) in the anterior margin of the wing. The normal pterostigma structure of Recent dragonflies and hymenopterans is often a thickened or highly pigmented cell in the outer wing with special micro- structures, such as a spinous protuberance and/or a network-like structure of the micro arrays (Bechly, 1995; Grimaldi and Engel, 2005). Nevertheless, some fossil species of these clades also have crossveins in the pterostigma area, including some species of the Aeschnidiidae (Neoanisoptera, Odonata) (e.g., Wigbtonia araripina Carle and Wighton, 1990; Bechly, 1998), and some species of the Nanosialidae (Siarapha, Panmegaloptera) (e.g., Hymega rasnitsyni Shcherbakov, 2013). The pterostigmae in those fossils differ from that of elcanids in that the pterostigma area of the latter is traversed by more crossveins (e.g., seven crossveins are visible in C. virginiana n. gen. n. sp. and more than fifteen in Panorpidium yixianensis Fang et al., 2015). In addition, the pterostigma area of elcanids is larger in size relative to the whole wing. Due to its relatively greater mass than other nearby sections of wings, the pterostigma may function to facilitate gliding. The mass and location of pterostigma has an important influence on the speed limits of
the pitching moments during the acceleration phases of wings flapping (Norberg, 1972). The pterostigma structures have not been previously described from other orthopterans. Elcanids may have evolved a particular flight mechanism distinct from those of other orthopterans.
Conclusions
The earliest known Elcanidae, Cascadelcana virginiana n. gen. n. sp., is described based on a forewing specimen from the
Archelcaninae Elcaninae
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