Box Jumping bristletails

Paleontologists need keen eyesight. The slab in Fig. 19.1 shows a clean, slightly undulating surface, with some long cracks and obscure little markings here and there. But is there anything of importance on the slab? It might not at first seem so.

The slab comes from the Lower Permian Robledo Mountains Formation of southern New Mexico (Minter & Braddy 2006), where most surfaces show tracks of one sort or another: amphibians, reptiles, scorpions, spiders, millipedes and insects. Trace fossils are most commonly preserved in red-gray siltstones to fine-grained sandstones that were deposited in a flat tidal setting; mudcracks and raindrop imprints indicate periods of exposure to the air. On looking closely at this slab, you may be able to see three arrow-shaped markings, running from bottom to top of the slab. In close up, these arrow-shaped markings show three sharp, thin lines at the top, and a fainter marking below. This trace fossil is called Tonganoxichnus, and it has been noted before in both the Permian and the Carboniferous. But what could have made it?

Previous authors had suggested that Tonganoxichnus was produced by an extinct relative of a hopping insect like a jumping bristletail. Jumping bristletails, more properly called machilids, are primitive, wingless insects that are known today from moist coastal habitats of North America and Europe. They are closely related to silverfish, commonly seen in damp carpets inside houses, and they can jump up to 100 mm at a time, 10 times their body length. Fossil machilids and their extinct relatives such as Dasyleptus are known from the Permian, and they fit the tracks perfectly. The insect was hopping from the bottom of the slab upwards; the sharp grooves at the top of each marking are impressions of the feeding legs at the front, and the arrow-like marking behind is an impression of the abdomen as it hit the ground and propelled the animal forward. So this seemingly obscure slab from New Mexico tells a story of how a number of small wingless insects hopped across the damp sand near a lake 270 Ma.

Figure 19.1 Slab of fine sandstone from the Robledo Mountains Formation (Lower Permian) of New Mexico, showing the trace fossil Tonganoxichnus, the hopping trace of a basal wingless insect such as Dasyleptus (inset). (Courtesy of Nic Minter.)

Table 19.1 The main types of trace fossils, with definitions of the key terms.

A. Traces on bedding planes

Tracks: sets of discrete footprints, usually formed by arthropods or vertebrates Trails: continuous traces, usually formed by the whole body of a worm, mollusk or arthropod, either traveling or resting

B. Structures within the sediment Burrows: structures formed within soft sediment, either for locomotion, dwelling, protection or feeding, by moving grains out of the way

Borings: structures formed in hard substrates, such as limestone, shells or wood, for the purpose of protection, dwelling or carbonate extraction, by cutting right through the grains. Includes bioerosion feeding traces, such as drill holes in shells produced by gastropods

C. Excrement

Fecal pellets and fecal strings: small pellets, usually less than 10 mm in length, or strings of excrement Coprolites: discrete fecal masses, usually more than 10 mm in length, and usually the product of vertebrates

D. Others

Root penetration structures: impressions of the activity of growing roots Non-fecal pellets: regurgitation pellets of birds and reptiles, excavation pellets of crustaceans and the like

U-shaped burrow Arenicolites was named after the burrow of Arenicola, the lugworm, and the meandering deep-sea trail Nereites was named after another polychaete annelid, Nereis. However, most Arenicolites burrows and most Nereites trails have nothing to do with the modern worms Arenicola and Nereis. Footprints made by vertebrates, on the other hand, can often be matched more readily with their producers, and track names frequently indicate the supposed affinities of the track-maker. For example, the large three-toed dinosaur track Iguanodonichnus was supposedly made by the ornithopod dinosaur Iguanodon . . . or was it?

The principle that trace fossils should not be named after the supposed maker is based on two observations:

1 One animal can make many different kinds of traces.

Figure 19.2 One animal may make many different kinds of trace fossils. The modern fiddler crab Uca makes: (a) a J-shaped living burrow (domichnion; Psilonichnus), (b) a walking trail (repichnion; Diplichnites), (c) a radiating grazing trace with balls of processed sand (pascichnion), and (d) fecal pellets (coprolites). (Based on Ekdale et al. 1984.)

Figure 19.2 One animal may make many different kinds of trace fossils. The modern fiddler crab Uca makes: (a) a J-shaped living burrow (domichnion; Psilonichnus), (b) a walking trail (repichnion; Diplichnites), (c) a radiating grazing trace with balls of processed sand (pascichnion), and (d) fecal pellets (coprolites). (Based on Ekdale et al. 1984.)

2 One trace fossil may be made by many different kinds of organisms.

The fiddler crab Uca is observed today to make at least four quite distinct kinds of traces (Fig. 19.2): a J-shaped living burrow, a running track, a star-shaped feeding pattern and fecal pellets, each with its own name, as well as excavation pellets and feeding pellets. An example of one trace fossil made by many different animals is the ichnogenus Ruso-phycus, a bilobed resting impression marked by transverse grooves (Fig. 19.3). Rusophycus can be made by at least four different animals, belonging to three phyla, an annelid, a mollusk and two arthropods, but the traces are so similar that they must be given the same name.

An additional consideration is that, if trace fossils were named after their proposed makers, the name would depend on the validity of that interpretation: trace fossil names could not change at the whim of every paleo-biologist who proposed a different maker for the same trace. For example, Iguanodonich-nus, mentioned above as the supposed track of Iguanodon, turns out to have been made most likely by a medium-sized sauropod dinosaur. Should its name now be changed when the interpretation changes? Of course not -that would lead to endless confusion and instability. And this shows why it is best not

(b)
(d)

Figure 19.3 One trace fossil may be produced by many different organisms. Here, all the traces are resting impressions, cubichnia, of the ichnogenus Rusophycus, produced by (a) the polychaete worm Aphrodite, (b) a nassid snail, (c) a notostracan branchiopod shrimp, and (d) a trilobite. (Based on Ekdale et al. 1984.)

Ichnogenus

Figure 19.4 Variations in the physical nature of the sediment may create variations in the appearance of a trace fossil. Here, a subsurface, patch-feeding burrow develops different morphologies, and therefore has different names, when preserved: (a) in sand (Scalarituba), (b) at a sand-mud interface in firm sediment (Nereites), (c) at a sand-mud interface in wetter sediment (Neonereites), and (d) at a mud-sand interface, seen from below (Neonereites). (Based on Ekdale et al. 1984.)

Figure 19.4 Variations in the physical nature of the sediment may create variations in the appearance of a trace fossil. Here, a subsurface, patch-feeding burrow develops different morphologies, and therefore has different names, when preserved: (a) in sand (Scalarituba), (b) at a sand-mud interface in firm sediment (Nereites), (c) at a sand-mud interface in wetter sediment (Neonereites), and (d) at a mud-sand interface, seen from below (Neonereites). (Based on Ekdale et al. 1984.)

to use names for trace fossils that imply a particular producer.

The nature of preservation of a trace fossil may affect its appearance, but the name cannot necessarily take account of this. The appearance of trails and burrows may be altered significantly by the grain size, location with respect to a fine- and coarse-grained horizon, and water content of the sediment in which they are preserved (see p. 59). This can be seen clearly with the example of the Nere-ites-Scalarituba-Neonereites complex, a series of trace fossil forms produced by a single deep-sea grazing organism (Fig. 19.4). The situation is different for many vertebrate traces. For example, it is often possible to follow a single dinosaur trackway for some distance, and the shape of individual foot and hand prints might vary substantially, depending on the sediment type and the animal's behavior. It would clearly be crazy to give each variant print in a single trackway a different name.

In conclusion, trace fossil names should be based only on morphological features including shape and ornamentation, and not on the postulated maker or mode of preservation.

Preservation of trace fossils_

Trace fossils may be formed on bedding planes or within sedimentary horizons. The relationships of the trace fossils to the sediment, and the ways in which they are preserved must be established. Seilacher's terminology, developed in the early 1960s, is frequently used (Fig. 19.5). Burrows are three-dimensional structures, but they may be seen in different ways in the rocks: they are called full relief traces when they are seen in three dimensions, but semireliefs when just one side is seen projecting from the bedding plane. Semirelief burrows and trails may occur on the top of a bed, called epireliefs (epi, on), or on the bottom, termed hyporeliefs (hypo, under). Hyporelief preservation is very common in sedimentary sequences where sandstones and mudstones are interbedded - a feature of tur-bidite and storm-bed successions (Box 19.2). Here, the traces are best seen on the bottoms of sandstone beds as sole structures, because the mudstones often flake away.

It is important to realize that burrows and surface trails are not always easy to distinguish. Burrows are formed within sediment, and are thus endogenic (endo, within; genic, made), and they are seen both as full reliefs and as semireliefs along bedding planes. However, if subsequent erosion or weathering

Trilobite Lateral View
sediment horizons. (Based on Ekdale et al. 1984.)
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