Late Carboniferous Paleoichnology Reveals the Oldest Full-Body Impression of a flying Insect
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Late Carboniferous paleoichnology reveals the oldest full-body impression of a flying insect Richard J. Knechta,1, Michael S. Engelb,c, and Jacob S. Bennera aDepartment of Geology, Tufts University, Medford, MA 02155; bDivision of Entomology (Paleoentomology), Natural History Museum, and cDepartment of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66049 Edited by May R. Berenbaum, University of Illinois at Urbana–Champaign, Urbana, IL, and approved March 8, 2011 (received for review October 23, 2010) Insects were the first animals to evolve powered flight and did earliest mayflies and their relatives that wing fossils do not. More so perhaps 90 million years before the first flight among verte- significantly, the FBI somewhat blurs the usual distinctions brates. However, the earliest fossil record of flying insect lineages between trace and body fossils and the traditional dichotomy (Pterygota) is poor, with scant indirect evidence from the Devonian between paleoichnological and paleontological systematics and and a nearly complete dearth of material from the Early Carbonif- taxonomy. erous. By the Late Carboniferous a diversity of flying lineages is known, mostly from isolated wings but without true insights into Geological Context the paleoethology of these taxa. Here, we report evidence of a full- The geological context of the fossil locality is described in SI body impression of a flying insect from the Late Carboniferous Geological Context. Wamsutta Formation of Massachusetts, representing the oldest trace fossil of Pterygota. Through ethological and morphological Systematic Paleoichnology analysis, the trace fossil provides evidence that its maker was The following discussion is a systematic description of the trace a flying insect and probably was representative of a stem-group fossil morphology and its relation to the morphology of the in- lineage of mayflies. The nature of this current full-body impression sect that created it. Rather than a purely descriptive approach, as somewhat blurs distinctions between the systematics of traces and advised with most trace fossils (10), this paper takes a mixed trace makers, thus adding to the debate surrounding ichnotaxon- descriptive–interpretive tack, treating the specimen as if it were EVOLUTION omy for traces with well-associated trace makers. a body fossil. The material is housed in the Fossil Insect Col- lection, Division of Entomology, University of Kansas Natural Ephemeroptera | ethology | ichnology | Pennsylvanian History Museum, as SEMC-F97 (ATL-SM-2–31/32-SEMC-97) (Fig. 1). ull-body impressions (FBIs) of insects are rare in the fossil SEMC-F97 record, and most record the body plan and ventral mor- F ∼ × × phology of ground-dwelling, primitively wingless insects, such as Description. The specimen is on two opposing slabs, 10 6 1 fi GEOLOGY the ichnogenus Tonganoxichnus (1, 2). Even rarer are the FBIs of cm thick, of red, ne sandstone with thin mudstone drapes (Fig. flying insects, the Volichnia (3). FBIs are distinct from com- 1 A and B). The surfaces of the slabs exhibit portions of two pression fossils or natural casts of insect bodies, in that the for- slightly asymmetric ripple marks and a number of small, shallow mer are trace fossils and preserve evidence that the animal was burrows that are oriented parallel to bedding planes [Cochlich- alive before and after it made the impression. The same dis- nus ichinospecies (isp.) and Helminthoidichnites isp.]. The trace tinction can be made between mortichnia (4) and FBIs, because is preserved as a negative epirelief with corresponding positive the former give clear indication that the maker died while or hyporelief representing a natural cast of the primary surface (Fig. A B soon after making the trace. FBIs are unique among trace fossils 1 and ) and is nearly bilaterally symmetrical overall and 36 mm in total length. It is divided into three main sections longitudinally because they preserve detailed evidence of the maker’s ventral that are equivalent to the tagmata of the insect body plan, i.e., anatomy and provide reliable evidence of its identity in addition head, thorax, and abdomen (Fig. 1C). Most anteriorly, there is to ethological data. Special sedimentologic and taphonomic a bedding plane disturbance that does not seem to be related conditions are required to preserve FBIs; these requirements morphologically to the trace maker. Close to this disturbance, may be why they are so rare. Nonetheless, such traces represent a shallow, oval depression 3.5 mm wide represents an impression the only direct observation of otherwise rarely preserved of a portion of the head of the insect and may represent an im- organisms in the fossil record. pression of the labium (Fig. 1C, “Lab?”). This anterior depression An FBI from the Wamsutta Formation (Late Carboniferous) is separated from the thorax by a shallow, elongate, 1.5-mm im- of southeastern Massachusetts preserves details of the ventral “ ” fl pression made by the prothorax (Fig. 1C, Pt ). anatomy and behavior of a stem-group may y (superorder The thoracic impression is 3.8 mm at its widest between the Ephemeropterida) that landed in soft mud in a marginal fresh- front and middle pair of legs. Within the depression are smaller water habitat. This FBI is the earliest occurrence in the fossil features that divide it and give the appearance of separate plates. record of an FBI of a flying insect, and it provides enough fi fi The rst of these partitions is a small, transverse semicircular morphological detail to allow possible identi cation of the ridge that protrudes above the base of the main depression by fi maker. Believed to be among the rst pterygote lineages, ex- about 0.5 mm, representing sediment that squeezed between two cepting Rhyniognatha (5, 6), the earliest body fossil records of basal crown-group Ephemeroptera are from the Permian, par- ticularly Protereismatidae (6), and even more primitive stem- Author contributions: R.J.K. and J.S.B. designed research; R.J.K. and J.S.B. performed re- group taxa (e.g., Syntonopterodea) are from the Late Carbon- search; R.J.K., M.S.E., and J.S.B. analyzed data, and R.J.K., M.S.E., and J.S.B. wrote iferous (6, 7). Older evidence of winged insects exists in the form the paper. of body and wing compressions (5, 6, 8), as well as some indirect The authors declare no conflict of interest. trace evidence of putative orthopteroid feeding (9), all of line- This article is a PNAS Direct Submission. ages other than Ephemeroptera. The majority of the Ephemer- 1To whom correspondence should be addressed. E-mail: [email protected]. optera body fossil record is comprised of wings; therefore, the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. trace fossil provides information about the body plan of the 1073/pnas.1015948108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1015948108 PNAS Early Edition | 1of5 Downloaded by guest on October 4, 2021 Fig. 1. Views of specimen SEMC-F97. (A) Lower-facing side preserving trace fossil in concave epirelief. Two ripple mark crests are evident in this view running from left to right across the bedding plane. (B) Upper-facing side (in convex hyporelief). Possible incipient mud cracks are visible to the left of the trace fossil. (C) Macrophotograph of the trace fossil with key features labeled. Illumination is from due north to highlight small transverse features. See text for more information. Ab, abdomen; At, articulation; Ce, cerci; Fs, femoral striae; iss, intersegmental suture; Lab, labium; Lant, anteriorly directed legs; Lmid, antero- laterally directed legs; Lpos, posteriorly directed legs; Pc, pretarsal claw impressions; Pt: prothorax. (Scale bars: A and B, 20 mm; C, 10 mm.) adjacent sclerites (Fig. 1C, “iss”). Along the length of this tagma, stiff setae. Furthermore, these striae indicate the direction of six long, narrow impressions diverge and taper distally, repre- movement of the animal as it came to rest in the body impression. senting impressions of the legs. The first set is directed anteriorly Outside the reach of the limb impressions that correlate to the (Fig. 1C, “Lant”), the second set is directed antero-laterally (Fig. deepest impression of the body are a series of shallower 1C, “Lmid”), and the third set is directed posteriorly (Fig. 1C, impressions of all three right legs (Figs. 1 A and B and 3. Each “Lpost”). Multiple small impressions exist beyond the distal leg impression is in close contact with its predecessor, and the portions of the leg impressions that are comparable to pretarsal overlapping series of leg impressions form a pattern of em- claw impressions nearer to the body (Figs. 1C, “Pc,” and 2B). placement that begins distal to the body and becomes more The third and most posterior section of the trace is a seg- proximal to the deepest impression. In the case of the foreleg mented, elongate ovoid impression, 3.8 mm wide × 20.6 mm impressions, the pattern shows a translation to the left, then long, which does not taper significantly until its most distal rotation about an axis near the tibia–tarsus joint (Figs. 1C and 3). portion and which represents the abdomen (Fig. 1C, “Ab”). At Relative overlap is not as clear in the hindleg impressions, but least 10 segments can be observed within the abdominal im- these impressions indicate a pivot about an axis that may have pression. Differential curvature of tergite boundaries, convex– been represented by the coxa–trochanter joint or the femur–tibia anterior on segments 1–8 vs. convex–posterior on segments 8–10, joint and then a shift to the left to the position of deepest im- indicate a possible upward bend of the distal portion of the ab- pression. The middle leg did not make as many impressions, but domen midway between the eighth segment (Figs.