Journal of the Geological Sociev, London, Vol. 144, 1987, pp. 5W-511, 3 figs. Printed in Northern Ireland The occurrence and diversity of Coal Measure insects E. A. JARZEMBOWSKI Booth Museum of Natural Hktory, Dyke Road, Brighton BN1 5AA, UK Abslrnd Insects are generally considered to be rare in the Upper Carboniferous Coal Measures. However, recent work in the Westphalian D of SW England suggests that many have been overlooked in the past. This is because wings, which are the most characteristic insect fossils, may be mistaken for detached ‘fern’ pinnules, which are much more common. The resemblance may be functional convergence rather than leaf-mimicry. The earliest members of the class Insecta in the strict sense occur in the Upper Carboniferous. Eleven major divisions or orders are represented in the Coal Measures of which only four are still living. Primitively wingless insects (Archaeognatha) are present, relatives of familiar living insects such as the silverfish. There are numerous winged insects, some ofwhich could fold their wings (Neoptera) and others which could not (Palaeoptera). Palaeopterous insects were more diverse than today. They include three extinct orders (Palaeodictyoptera, Megasecoptera, Diaphanopterodea) which were probably plant suckers like present clay bugs. Other extinct palaeopterous insects (order Protodonata) were probably aerial predators like modem dragonflies, and included some of the largest insects of all time (‘giant dragonflies’). By far the most common neopterous insects were cockroaches (Blattodea) which outnumber all other insects in theUpper Carboniferous. This abundance is perhaps less surprising when one considers the general picture of the coal forests as warm, humid, and rich in organic matter. Another important neopterous group was the extinct order Protorthoptera, which is probably related only in part to extant Orthoptera (grasshoppers, crickets and locusts). There is no evidence at this time of higher insects such as flies, fleas, beetles, moths and butterflies, ants, bees and wasps. If diversity is expressed as the total number of species, then enormous colliery tips in mining areas. Recently, systematic insects are the most successful classof organisms of all time. collecting on one such tip at LowerWrithlington, near Insects are also abundant animals, and it has been estimated Radstock in SW England, yielded over 300 specimens and that the current global population totals about 1 X 10” doubled the entire national collection of Carboniferous individuals(Wigglesworth 1964). The earliest evidence of fossil insects. This work suggests that the less spectacular true insects (Pterygota and Thysanura auctt.) isfrom the insect fossils from ‘roof shales’ may havebeen overlooked in Upper Carboniferous, and mainlyis from the Coal the past. There are severalreasons why this may be so. Measures. This is probably because the Coal Measures, like Dissociated insect remains are often small and not easy to insects, are essentially non-marine, and coal-bearing strata distinguish on beddingplanes crowded with dissociated are some of the most extensively mined sedimentary rocks plant remains. A detached wing bears a superficial in the geological record. Terrestrial plantremains are resemblance to certain types ofleaf on account of its extremely abundant in these deposits, andfossil evidence elongate shape and branchingvenation. Eight out of ten shows that the vegetation supported a considerable variety Carboniferous insects are cockroaches (Carpenter & of animal life. Burnham1985), the wingsofwhich are particularly pinnule-like. Diagenetic changes often impart a similar blackcolour to wings and plants in the CoalMeasures, Occurrence heightening the resemblance. Insect fossils are never abundant in the Coal Measures, and Jarzembowski (in press) lists criteria for distinguishing well-preserved (articulated) specimens have only been found insect wings of which the symmetry ‘rule’ has most general in quantity at a small number of localities. North (1931) application. If an imaginary line is drawn fromthe wing base attributed the scarcity of insect bodies to the feeding tothe wing apex (or lengthwise down the middle of an activities of other animals. This would account for the incomplete wing),then the venational pattern in the two preponderance ofwings over other parts found, because halves of the wing so produced is quite different, whereas in there is much more edible tissue in a body than in the wing a pinnule it is more or less the same (Janembowski 1985, of an insect. figs1-2). Wings, or evenfragments ofwings, may be A general examination of a representative insect identified by the exact configuration of the veins and, where collection from the Euramerican Carboniferous, such as in developed, an intricate network (archedictyon) between the the BritishMuseum (Natural History), shows that insects veins. The variety is considerable, but Jarzembowski (1985) have been found traditionally in small ferruginous nodules. gives somegeneral pointers, and Carpenter (1954)has However, the total number of specimens is not great, provided a key in English to many extinct families; more possiblybecause the area of bedding plane exposedin recent papers are reviewed by Wootton (1981).Specific nodules is comparatively small. Much larger areas may be identification of Coal Measure insects is complicated by the examined by splitting blocks of ‘roof shale’, of which there is comparativelysmall samples available: newly collected a plentifulsupply because it makesup the bulk of the specimens often represent new records, variants, or new 507 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/3/507/4889131/gsjgs.144.3.0507.pdf by guest on 02 October 2021 508 E.A. JARZEMBOWSKI Fig.- 1. Lithomantis carbonarius Woodward, Coal Measures, near Ayr, Strathclyde, Scotland (r, rostrum; p, prothoraciclobe). Scale in mm. taxa.Wings distinguish insects from all other arthropods, 1973). There is no evidence of higher Neoptera in the Coal but body parts have fewer diagnostic characters. However, Measures (Endopterygota or Holometabola auctt. : common wings found in the same beds provide valuable clues, and insects with a pupal stage or chrysalis during development some parts have a distinctmorphology, such as the and including nine extant orders). Other notable absentees cockroach head-shield or pronotum (Bolton 1930, p1 111, fig. at this time are the hemipteroid orders (five extant) which 2) * belong tothe exopterygote Neoptera or Hemimetabola auctt., insects whichlack the pupal stage. (Laurentiaux Vieira & Laurentiaux (1985) consider that Hemiptera are Diversity uniquely represented in the Namurian of Belgium). The Coal Measure insects can be considered as representing However, the blattoid-orthopteroid orders of exopterygote three grades of insect evolution (Carpenter & Burnham Neoptera are well represented, and the forewings in these 1985). In descending order, they are insects commonly serve as tegmina. The order Blattodea, or (i) wingedinsects which habituallyfold their wings cockroaches, (Figs 2 & 3) comprises 80%of all insects found when at rest (Neoptera); in the Upper Carboniferous (Carpenter & Burnham 1985). (ii) winged insects which usually do not fold their wings About loo0 specieshave been describedfrom the late (Palaeoptera); Palaeozoic (Schneider 1983). This predominance cannot be (iii) primitively wingless insects (Apterygota auctt). attributed to selective preservation becausenumerous Wings are important because, apart from being common hindwingsand whole bodies are knownin addition to insect fossils, they provide insects with a mode of dispersal detached tegmina; extant cockroaches are often foundin unique amongst the invertebrates. Insects which canfold warm, humid, well vegetated situations and perhaps their their wingswhen not flying enjoy additional advantages, abundance in the Coal Measures reflects the suitability of such as avoiding being blownabout, and being able to enter the coal swamp environment. Fossil cockroaches also show confined spaces. In the latter case, the forewings are often promise as biostratigraphical tools (Schneider 1983; Durden modified as hard wing covers (tegmina or elytra) whilst the 1984). The order Orthoptera (grasshoppers,crickets and hindwings remain membraneous and flexible to provide locusts) is scarce in the Coal Measures and it may be noted propulsion in flight. This specialization has palaeontological that crickets and locusts are not known to occurin the significance because the tougher forewings are more likely Palaeozoic, nor are any of the several extant families of the to survive transport in a recognizable form. suborder Ensifera which are referred to generallyas There are 11 orders of insects found in the Coal grasshopppers.However, the Carboniferous Orthoptera Measures compared with about 27 at the present day (see (Oedischiidae)were certainly grasshopper-like (Sharov proposed Treatise classification by F. M. Carpenter in Tasch 1971). Related to the Blattodea and Orthoptera, and found Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/3/507/4889131/gsjgs.144.3.0507.pdf by guest on 02 October 2021 COAL MEASURES INSECIX 509 Fig. 3. Blattodea nymph, Upper Coal Series, Lower Writhlington, 2. Archimylacris eggintoni (Bolton): Middle Coal Fig. eggintoni Avon, England (ST 703553) Note wing pads are contiguous with of Measures, Coseley, Staffordshire, England. Negative impression terga showing that a metamorphic instar was presentin