Trace Fossils Ichnofossils

Some groups of insects don't readily fossilize, particularly soft-bodied ones, but remains of their activities persist. These largely involve structures on plants, like larval feeding mines on leaves, chew and puncture marks on leaves and stems, galls, and galleries in wood. Plants are abundant in the terrestrial fossil record, and because insects are the predominant group of herbivores, these trace fossils provide a unique and direct record of past plant-insect associations and remarkable persistence of some associations. Also involved are larval cases and burrows in soil (Figures 2.16 to 2.18). Fortunately, the insects that made trace fossils can frequently be identified (at least to order, sometimes to family) by the geometry of the burrows, galleries, and feeding traces, as well as by the type of substrate or plant in which they occur. Frass, or fecal pellets, also has been used in identification. Zherikhin (2003) reviewed types of insect trace fossils and proposed a nomenclature for them based on functional types. We prefer to not use that nomenclature because descriptive names of the traces are most recognizable.

Fossil Burrows and Nests. These occur in fossil soils, or pale-osols, and are usually formed by beetles, wasps, and bees, sometimes by ants and termites. In all cases the burrows are not mere excavations, but rather constructions consisting of corridors with brood cells that are provisioned with food for developing larvae. The architecture of some can be impressive. Genise et al. (2000) thoroughly reviewed all known 58 examples of fossil insect nests/burrows. Some types of burrows are abundant and widespread, such as Coprinisphaera (a form genus, or generic name used for a trace fossil). Though the actual beetle is unknown, Coprinisphaera burrows were made by scarabaeine dung beetles, modern species of which provision brood cells with balls of dung for development of their larvae. Nests of modern dung beetle species are usually distinctive (Halffter and Edmonds, 1982). Coprinisphaera lived from the Paleocene to the Pleistocene and was widespread from Antarctica to Ecuador, eastern Africa and Asia. Occurrence of these nests is believed to coincide with the evolution of ecosystems with abundant mammalian herbivores (Retallack, 1984), like savannas and pampas, since scarabaeines specialize on the dung of these animals. Scarabs even dined much earlier on dinosaur dung, as tunnels have been found in coprolites of herbivorous dinosaurs from the Late Cretaceous of Montana (Chin and

2.16. Subterranean bee cells replicated in calcite, from the Oligocene of Badwater, Wyoming. AMNH; length of largest cell 15 mm.
2.17. Subterranean nest of a bee replicated in sandstone from the Paleocene Ascencio de Palacio Formation of Uruguay. AMNH; greatest length 49 mm.
2.18. Portion of nest galleries of Macrotermes termites, replicated in phosphates, from the Pliocene of northern Tanzania. AMNH; greatest width 55 mm.

Gill, 1996). The fossil burrows provide a better temporal and spatial record of scarabs than do body fossils.

Nests of bees are another common type of fossil insect nest (Figure 2.17). Like scarabs, subterranean excavations of modern halictids are structurally diverse (Sakagami and Michener, 1962). Celliforma is a widespread form genus of halictid nests, occurring from the Late Cretaceous to the Pliocene, from North and South America, Europe, and Africa. Those from the Dakota Formation of Arizona (Elliott and Nations, 1998), if indeed bees, are among the oldest known evidence of bees. Not all the makers of fossil nests can be identified, though, even those nests with distinctive architecture.

Larval Cases. Various larval insects construct cases from gathered materials or feces, which they carry with them and in which they can retreat for protection. Some small moths produce larval cases from bits of their food woven together with silk, such as the "clothes" and scavenging moths (Tineidae), and "bag worms" (Psychidae) (Figures 13.34, 13.35, 13.37). Chrysomelid ("leaf") beetles also produce cases, such as many "tortoise" beetles (Cassidinae), which excrete strands of feces molded into a structure like a bird's nest, held over the larva. Tiny cryptocephaline beetles feed amongst decaying vegetation on the forest floor, dragging themselves around in a small sac of particulate debris (Figure 10.65). Because these

2.19. A caddisfly case constructed of sand grains, from the Early Cretaceous of Mongolia. AMNH; length 8.3 mm.

cases are composed of materials that readily decay, and the species are terrestrial, fossils of these preserve only in exceptional cirumstances.

Cases of larval caddisflies, or Trichoptera, however, are diverse and locally abundant in the fossil record (Figure 2.19). The larvae dwell on the bottom of lakes, ponds, and rivers and build cases from durable materials, like sand grains, minute pebbles, and bits of vegetation (Figures 13.2, 13.5). Moreover, the shape and composition of the cases are distinctive to most modern genera (Wiggins, 1977), allowing the separation of species and even identification of some fossil cases to genus and family. The oldest Trichoptera cases occur in the mid- to Late Jurassic, 160-145 mya (Sukatsheva, 1982, 1999; Ivanov and Sukatsheva, 2002); none are known from the Triassic, even though stem-group Trichoptera (Necrotauli-dae) are as old as the mid-Triassic (living families are no older than the mid-Jurassic). Caddisfly cases are diverse from the Cretaceous of Asia, with some 9 form genera and 200 ich-nospecies described. The most peculiar fossil caddis cases are in the ichnogenus Piscindusia, which are constructed of minute fish scales and bones (Jarzembowski, 1995b).

Leaf/Stem Feeding Damage. Traces of chewing and punctures on fossil plants made by feeding arthropods are the most abundant and diverse kind of insect trace fossil. Evidence of this, including galls and wood/stem boring, was reviewed by Scott and Taylor (1983), Taylor and Scott (1983), Scott (1991), Scott et al. (1992), Stephenson and Scott (1992), and Laban-deira (1998). The evidence is based mostly on leaf damage, particularly marginal and surface feeding, and mining. Virtually all groups of insects with chewing mandibles that feed on plants either leave chew marks along the leaf margins or rasp small holes in the surface. Feeding traces are sometimes distinctive, such as the large, circular pieces chewed from the margins of leaves by attine (fungus growing) ants (Figure 11.58) and megachiline (leaf-cutter) bees. Insects that feed continuously along the margin, like sawflies and caterpillars, leave indistinctive traces. Some insects skeletonize the leaf, eating just the epidermis among the veins and leaving the veins.

The earliest evidence of marginal and surface feeding on leaves involves both the Carboniferous seed fern Neuropteris and Glossopteris, small to large woody plants that were widespread throughout Gondwana from the Permian to early Mesozoic (Scott et al., 1992; Labandeira et al., 1998). The most comprehensive study involves herbivory on Gigan-topteridaceae from the Early Permian of Texas (Beck and Labandeira, 1998). Gigantopterids were plants of enigmatic relationship, having large, spreading leaves. Those from the Texas Permian had various kinds of feeding marks, and, remarkably, showed extensive herbivory: up to 83% of the leaves and some 4.4% of the leaf area for some species of the plants (Beck and Labandeira, 1998). By 300 mya herbivory was a routine lifestyle for terrestrial arthropods.

Mines. These blotches or meandering tunnels between the epidermal layers of a leaf are the pathways within which a larva feeds on the mesophyll layer. When the larva molts, the mine enlarges; at the end of the mine is a pupation chamber and exit hole. The size, shape, and path of the mine, the kind of leaf it is on, and even how the frass is deposited within the mine help determine the identity of the miner. Leaf mining is exclusively holometabolous, caused by the larvae of some Coleoptera, some Diptera (especially Agromyzidae), some Hymenoptera (especially Pergidae and Argidae sawflies), and especially by 10 major, basal families of "micromoths." Major reviews of leaf-mining insects are by Needham et al. (1928) and Hering (1951). Putative leaf mines are reported from the Carboniferous (Scott et al., 1992), which is doubtful, and possible mines exist from the Permian (Beck and Labandeira, 1998). Definitive mines first appear on the leaves of Triassic conifers and pteridosperms, but their identity is unknown. Like galls, leaf mines become diverse and abundant with the radiation of angiosperms in the Cretaceous and Tertiary (Figures 2.20, 13.32) (e.g., Rozefelds, 1988) and the evolution of the major mining groups of insects.

Traces of plant feeding by insects have documented apparent persistence of some plant-insect relationships over tens of millions of years. For example, there has been intimate association between various living genera of basal, leaf-mining families of moths on trees like Quercus (oaks) and Populus (poplars) for approximately 20 million years (Opler, 1973), and on Cedrela (Meliaceae) for approximately 40 million years (Hickey and Hodges, 1975). Leaf mines from the mid-Cretaceous Dakota Formation (100 myo) have been identified to living genera of Nepticulidae and Gracillariidae

2.20. An exceptionally well-preserved leaf from the Eocene of Angle-sea, Victoria, Australia, with the mine of a larval moth. Museum Victoria (VM) 180365; length of leaf 64 mm.

moths (Labandeira et al., 1994), which have been used to infer basal radiations of the Lepidoptera deep into the Early and mid-Jurassic. Other evidence of host plants use has been found on ginger (Zingerberaceae) leaves from the Upper Cretaceous of Wyoming (Wilf et al., 2000). Distinctive, parallel chew marks on the surface are indistinguishable from those made by modern hispine beetles on Heliconia plants: an association that has persisted for some 70 million years.

Galls. These are excessive growths of plant tissue on stems, leaves, cones, and flowers, created by a feeding insect (immature or adult) or an ovipositing female. Substances in the saliva or oviposition fluids cause plant tissue to grow around the developing insects, encapsulating them. Gall formers are scattered throughout arthropods, including some mites (Acari), some aphids and scale insects (Sternorrhyncha: Aphidoidea, Coccoidea), a few leafhoppers, some lacebugs (Auchenorrhyncha; Heteroptera: Tingidae), some thrips (Thysanoptera: Phlaeothripidae), a few Lepidoptera, various Diptera (some Tephritidae, Agromyzidae, and many Cecidomyidae), and various Hymenoptera (some sawflies and many Cynipidae). The largest groups of gall formers are several derived groups of gall midges (Cecidomyidae) and the gall wasps (Cynipidae: Cynipinae). The biology and diversity of galls has been extensively reviewed (Felt, 1940; Mani, 1964; Ananthakrishnan, 1984b; Meyer, 1987; Gagné, 1989, 1994; Williams, 1994). Fossil galls have been reviewed by Larew (1986, 1992), Scott et al. (1994), and Whittlake (1981). The size and shape of the gall, its location on a plant, and the type of plant the gall is found on are usually necessary to identify the galler. In an exceptional situation the inhabitants were preserved: galls on the seeds of a Sequoia from the Miocene of Germany contained larval and pupal Cecidomyidae (Möhn, 1960).

The oldest galls are well-described structures from the petioles of 300 myo Psaronius tree fern fronds (Late Carboniferous) of Illinois (Labandeira and Phillips, 1996a). The galls are elliptical structures with a central cavity about 3.5 X 0.50 cm, showing wound tissue and an exit hole and filled with cylindrical fecal pellets. Size frequency of the pellets grouped in four classes, the size of each class differing from adjacent classes by a factor of 1.3, which is similar to the size differences between instars of terrestrial arthropods. Thus, the inhabitant was a developing arthropod that spent at least four instars in the gall. By process of elimination, the gall maker was identified as the earliest known holometabolous insect (Labandeira and Phillips, 1996a), because more basal galling insects, hemipteroids, feed on plant vascular fluids and produce liquid feces. No Holometabola are known from the Carboniferous, the earliest records being body fossils of Neu-ropterida, Coleoptera, and Mecoptera from the Early Permian (Srokalarva, from the Carboniferous of Mazon Creek, was proposed as the earliest insect larva [Kukalova-Peck, 1991] but

2.21. Scanning electron micrographs of galleries in fossilized Prem-noxylon wood from the Carboniferous. The galleries were probably formed by mites, and they are filled with fossilized fecal pellets (frass). These are among the earliest remains of plant feeding. Photos: T. & E. Taylor, University of Kansas.

2.21. Scanning electron micrographs of galleries in fossilized Prem-noxylon wood from the Carboniferous. The galleries were probably formed by mites, and they are filled with fossilized fecal pellets (frass). These are among the earliest remains of plant feeding. Photos: T. & E. Taylor, University of Kansas.

is now believed to be a myriapod). Even though the Carboniferous gall and frass therein are much larger than ones made by living mites, it is quite possible that mites produced this gall. Some mites, including living ones, can be quite large (5 mm or more in size); wood-boring mites are known from the Carboniferous (Figure 2.21); and various Paleozoic arachnids were quite large.

Insect galls are encountered much more in the fossil record with the radiation of angiosperms, approximately 100 mya, with diverse galls found on angiosperm leaves from the Cretaceous and Cenozoic (e.g., Figure 2.22). Miocene galls on oak leaves from western North America reveal that for a minimum of 20 million years certain kinds of cynipid gall wasps have been intimately associated with trees in the genus Quercus (Waggoner and Poteet, 1996; Waggoner, 1999).

2.22. Gall on the petiole of a Populus leaf, from the Miocene of Oeningen, Germany. This gall is very similar to those made by some pem-phigine aphids on poplar leaves today. MCZ 16735; diameter of gall 5.5 mm.

Borings and Galleries in Wood. Thick, highly lignified stems and branches of plants (wood) preserve well in the fossil record and have also preserved a unique record of the workings of extinct arthropods. An early paper on these trace fossils was by Brues (1936). Labandeira et al. (1997) summarized the fossil record of wood borings.

Some arthropods merely excavate the wood and feed upon fungus that grows in the galleries or upon the nutritious cambial layer. These include most wood-boring beetles, some sawflies (Hymenoptera), and some Lepidoptera. A few groups of insects actually eat the wood, particularly termites and some closely related roaches. Beetles are the most diverse, though termites are the most ecologically significant group that excavates wood. The earliest records of wood borings were probably produced by mites (Acari) from the Carboniferous (Cichan and Taylor, 1982; Rothwell and Scott, 1983; Scott and Taylor, 1983; Rex and Galtier, 1986; Labandeira et al., 1991). Among these are exquisitely preserved borings in the outer layers of the wood of Premnoxylon, having chambers filled with frass (Cichan and Taylor, 1982) (Figure 2.21). The earliest possible beetle borings are in Permian glos-sopterids. The earliest definitive beetle borings are from the Triassic of Europe and Arizona, and later in the Mesozoic beetle borings become more diverse with the radiation of various families of beetles (reviewed in Labandeira et al., 2001) (Figure 2.23). An interesting report concerns engraved galleries of scolytid (bark) beetles in Eocene wood, ca. 45 myo (Labandeira et al., 2001). Body fossils of Scolytidae are abundant and diverse in the Tertiary, and their borings even occur in the Cretaceous (though not the body fossils) (Figure 13.32). The Eocene engraving is of the extant beetle genus Dendroc-tonus in larch (Larix) wood from Axel Heiburg Island in the high arctic.

Termite borings are found in the Cretaceous and Tertiary, which reflects their early history. Many of them are preserved three-dimensionally in silicified wood, the galleries filled with frass (e.g., Rogers, 1938; Rozefelds and de Baar, 1991) (Figure 2.24). The earliest termite workings are from the Late Cretaceous Javelina Formation of western Texas (Rohr et al., 1986) (Figure 7.84). They were attributed to Kalotermi-tidae, since the preserved wood (from Diospyros) appeared to be sound, and kalotermitids today excavate sound, often dry, wood. This is the oldest social insect nest. The only other kalotermitids from the Cretaceous occur in mid-Cretaceous amber from Myanmar.

Triassic fossil burrows attributed to termites and bees

2.23. Petrified wood studded with beetle bore holes, from the Eocene of central Queensland, Australia. Queensland Museum (QM) F. 14679; length of piece 38 mm.
2.24. Mineralized termite frass in wood from the Eocene of Queensland, Australia. QM ML511A; length of pellet ca. 2 mm.

(Hasiotis and Dubiel, 1995) illustrate how some ichnofossils are overly interpreted. Small burrows in wood from trees of the Late Triassic Petrified Forest (ca. 220 mya) of Arizona resemble those of termites. The order Isoptera, however, is known no earlier than Early Cretaceous, approximately 130 mya, and it is highly unlikely that body fossils of termites are missing from nearly 100 million years of the fossil record. Moreover, all Cretaceous termites belong to basal families, indicating that the order is probably no older than Late Jurassic in age. Bees almost certainly originated in the mid- to Late Cretaceous (Engel, 2001b), and the earliest aculeate (stinging) Hymenoptera occur in the Late Jurassic and Early Cretaceous, 150-140 mya. Triassic bees are likewise inconceivable, and it is most likely that the insect burrows from the Petrified Forest are from beetles.

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