Geologia Croatica The oldest plant-insect interaction in Croatia : Carboniferous evidence

The Carboniferous System and especially the Pennsylvanian Subsystem provides fossil evidence of the first major radiation of the winged insects, the dominant subclass of the Insecta (GRIMALDI & ENGEL, 2005). The record comprises various bodyand some trace fossils found in finer-grained deltaplain sediments (JARZEMBOWSKI, 1987). Insect fossils are, however, generally uncommon when compared with the abundance of plant compressions. Nevertheless, examination of the latter can reveal evidence of insect activity. In this contribution, the first discovery from the Croatian Pennsylvanian of the ichnospecies Phagophytichnus ekowskii VAN AMEROM, 1966, on the leaf of the cycadopsid Taeniopteris carnotii ZEILLER, 1888, is described and discussed. Animal folivory on leaves is known from the Croatian Miocene (ÐEREK & JAPUNDŽIĆ, 2010), but this find is considerably older (c.~290 Ma).


INTRODUCTION
The Carboniferous System and especially the Pennsylvanian Subsystem provides fossil evidence of the first major radiation of the winged insects, the dominant subclass of the Insecta (GRIMALDI & ENGEL, 2005).The record comprises various body-and some trace fossils found in finer-grained deltaplain sediments (JARZEMBOWSKI, 1987).Insect fossils are, however, generally uncommon when compared with the abundance of plant compressions.Nevertheless, examination of the latter can reveal evidence of insect activity.In this contribution, the first discovery from the Croatian Pennsylvanian of the ichnospecies Phagophytichnus ekowskii VAN AMEROM, 1966, on the leaf of the cycadopsid Taeniopteris carnotii ZEILLER, 1888, is described and discussed.Animal folivory on leaves is known from the Croatian Miocene (ÐEREK & JAPUNDŽIĆ, 2010), but this find is considerably older (c.~290 Ma).

GEOLOGY
The tectonic belt of Mt.Velebit and Lika is the best known and most completely developed Upper Palaeozoic outcrop in Croatia, showing more or less continuous sedimentation from the Middle Pennsylvanian (Moscovian) to the end-Originally referred to the Cibichnia or feeding (eating) traces, Phagophytichnus was then included in Praedichnia, trace fossils of predation, but more recently moved to Phagophytichnidea, feeding traces on plants (VASILENKO, 2007b), and even been given its own family, Phagophtyichnidae (VYALOV, 1975).ZHERIKHIN (2003) previously placed Pha gophytichnus in Trogichnia reserved for chew marks on plants.
Evidence that the living plant was predated is provided by a pronounced thickening (ridge) sometimes seen along the damaged margin and interpreted as callus ('scab') formation (see figure of damaged leaf of the pteridosperm Macroneuropteris scheuchzeri (Hoffmann) Cleal, Shute & Zodrow from the late Asturian of England in ATTENBO ROUGH, 2005: p.167).This was previously interpreted as leaf mining (MÜL-LER, 1982).
Distribution.Phagophytichnus has been described from the Late Mississippian to the Pliocene and has a worldwide distribution (IANNUZZI & LABANDEIRA, 2008;BRUS-TUR, 1997).It also represents the most common type of external foliage feeding in the fossil record (LABANDEIRA, 2006).

Phagophytichnus ekowskii
Description and Measurements: The Lika trace fossil is cuspate excisions (overall length c. 2 cm, width 3 mm) from the margin of the leaf of Taeniopteris carnotii ZEIL-LER, 1888(NĔMEJC, 1936) preserved as a compression in mudstone (Pl. 1, Figs. 1A-C).It comprises 2 to 3 semicircular, asymmetrically curved, successive and separate excisions, each of which is up to 9 mm long, and all approaching the midpoint of the distance between the leaf margin and midvein.
In the absence of cuticular studies, Taeniopteris carnottii may be a cycad or a bennettite within the Class Cycadopsida Barnard & Long (cf. CLEAL & REES, 2003), and possibly a synonym of Taeniopteris multinervis Weiss, 1869 (Chris Cleal, pers. comm., 2012).GEYER (1989) considered that holometabolous insects, especially the caterpillars of Lepidoptera (moths and butterflies) could also be responsible for this isp.as well as orthopterans (grasshoppers and crickets).Body fossils of Lepidoptera are known by the Lower Jurassic and Hymenoptera (sawflies, ants, bees and wasps), which also have a caterpillar larva, are known by the Upper Triassic although Orthoptera are known from the Pennsylvanian onwards (JARZEMBOWSKI, 2003).Millipedes are not considered to be a significant defoliator to-day but orthopterans are a large and diverse order with herbivorous and carnivorous forms (KEY, 1970).A bush cricketlike insect may therefore have been responsible for the feeding damage on European Taeniopteris, although it is unlikely to be always the same species due to a significant time gap (~c.75 Ma) between the Lika and Scheerich leaves, and an unknown stem-holometabolan be involved.BECK & LA-BANDEIRA (1998) also recognised orthopteroid damage on Taeniopteris sp. from the Early-Middle Permian of the USA differing from P. ekowskii in including leaf holes as well as marginal feeding, but supported by the presence of veinal strands as produced by recent short-horned grasshoppers.The latter are also present in chew marks on leaflets of Anomozamites villosus POTT, McLOUGHLIN, WU & FRIIS, 2012, a newly described cycadopsid from the Middle Jurassic of  China (pers.obs.).Examination of the Lika leaf under magnification was however inconclusive, revealing irregularities along the margin, but also embedding in an obscuring, grainy matrix (but see gross cf.below).

RECENT ANALOGUE
Grasshoppers and caterpillars are unrelated insects but share mandibulate (basic chewing) mouthparts.Modern lepidopteran (butterfly) caterpillars are known to defoliate some cycads, such as the Cycad Blue (Theclinesthes onycha HEWITSON) in Australia.Cycads are no longer native to Europe, but I have often observed caterpillars feeding on angiosperms in southeast England.The caterpillar's numerous paired legs are used to hold on to the leaf, often tenaciously.As in other mandibulate insects, the paired jaws are orientated beneath the anterior part of the head capsule and the edge of the leaf is harvested in a sideways cutting motion.The head moves progressively forwards and downwards, sometimes obliquely, to obtain purchase and then access to fresh tissue.This would produce a curved excision as seen on the fossil leaves.Slight body movements coupled with a break in feeding, often due to a disturbance, result in an excision with subordinate cuspules evident (cf.KELBER & GEYER, 1989: fig. 4; also, GRAUVOGEL-STAMM & KELBER, 1996: fig.6).The insect may resume feeding, or move on allowing wound reaction (callus) tissue and subsequent drying out to form a thickening inside the damaged margin as seen on fossils (e. g.Pl. 1, Fig. 2).My observations on bullace (Prunus insititia L.) point to this thickening of the callus being a gradual process, taking weeks to form a feature prominent enough to be readily seen on compression leaves.This could explain the apparent absence of callus in P. ekowskii in JARZEMBOWSKI (2004: pl. 1, fig. 3).
Bush crickets are less easy to watch, but such a great green 'grasshopper' fed on lettuce showed broadly similar chewing behaviour (MICHAËL, 2011).GANGWERE (1966) discussed feeding in more derived orthopteroids (acridiid grasshoppers): interestingly, some of the chew marks produced by the rattle grasshopper (Psophus stridulus (L.)), a recent forbivorous European species, show a comparable outline to the Lika fossil (Pl. 1, Fig. 3).Modern land snails, such as Helix aspersa (Müller), can also form marginal incisions and holes in leaves by rasping downwards and sideways with their radulae.Terrestrial molluscs are scarce in the Carboniferous-Triassic of Europe, but have been found in Middle Pennsylvanian nodules with plants and insects in the Upper Silesian Coalfield and in the Middle/Upper Pennsylvanian of the English Midlands, investigated during IGCP 469 (STWORZEWICZ, SZULC & POKRY SZKO, 2009).Where known, the affinities of Carboniferous European snails (ellobioid and Cerion-like, loc.cit.) suggest that they fed on detritus or non-vascular plants by analogy with recent thorn and peanut snails rather than the leaves discussed herein.The situation is similar in the Late Pennsylvanian of North America, such as at Joggins, in Newfoundland, also investigated during IGCP 469 (FALCON-LANG, BENTON, BRADDY & DAVIES, 2006; molluscan detritivory rather than folivory (leaf feeding) was also suggested by SOLEM & YOCHELSON, 1979).In the absence of slime trails and coprolites, stereo-electron microscopy on well-preserved fossil plants may help to distinguish the work of biting mandibles from rasping radulae in future studies.

CONCLUSION
Phagophytichnus ekowskii provides fossil evidence for a longand wide-ranging, generalised feeder with low host specificity, principally on pteridosperm leaf margins.GRAUVOGEL-STAMM & KELBER (1996) considered Phagophytichnus on Taeniopteris, however, as part of a novel insect-plant interaction on Mesozoic Cycadopsida following the Permo-Triassic extinction event.The Lika find shows that the Phagophytichnus-Taeniopteris association is considerably older, dating back to the late Palaeozoic (Upper Pennsylvanian).Insects are otherwise unknown from the Croatian Carboniferous and this discovery also helps to fill a gap in the Pennsylvanian record of southeast Europe.Its rarity is, however, not surprising because Taeniopteris was likely more resistant than pteridosperms to insect attack due to the cycadopsid's tougher cuticle and well-developed resin glands (BECK & LABANDEIRA, 1998).In contrast, the hirsute leaves of Macroneuropteris may have only slowed down chewing (ATTENBOROUGH, 2005, figure p. 167).
IGCP 575 is concerned with environmental change at the Middle/Upper Pennsylvanian transition.The Lika Flora needs to be examined further to determine the extent of insect herbivory, the current data predicting higher rates of pteridosperm folivory by insects unless carnivory was widespread.A search for associated fauna may provide clues to a more precise identification of the consumer.