Box Undertracks of the emu

Experts on dinosaur tracks have been aware for a long time about undertracks. Huge animals such as dinosaurs made very deep footprints when they walked over soft sediment. Sometimes the print shape was transmitted for a meter or more down through the layers of sediment below the layer on which the animal walked, and this means that many dinosaur tracks are actually undertracks, if they are viewed on a lower layer.

There have been many experiments to show how tracks are altered by the consistency of the sediment (grain size, water content) and the weight of the animal. Obviously, larger animals make deeper prints. Also, the larger the grain size of the sediment, the less well defined the prints are. Also, if the sediment was entirely dry when an animal moved across, the tracks might be lost. Too wet, and the sand or mud just flowed back into the footprint or trail, leaving a gloopy mess. If the sediment was just slightly wet, then an excellent impression might be preserved.

Jesper Milan, a graduate student at the University of Copenhagen, decided to try to understand tracks and undertracks of dinosaurs by experiments with an emu (Milan & Bromley 2006). The emu is a large flightless bird from Australia, known for its cussedness - the animal pretty much refused to run across the carefully prepared sand beds at a local emu farm, and the experimenters were soundly pecked for their efforts (Fig. 19.7a). In the end, Milan managed to make some clean emu tracks on prepared "sediment" layers; these show very clearly how the undertrack shape changes down through the sediment (Fig. 19.7b). This is a warning to ichnologists, to be clear about identifying tracks and undertracks, and not to overinterpret the anatomy of the track-maker from a deep undertrack.

Figure 19.7 Experimental ichnology: (a) graduate student Jesper Milan, trying to persuade an emu to walk where he wants it to walk, and (b) the tracks and undertracks of the emu - results of an experiment where an emu stepped on a package of alternating layers of concrete and sand. After the concrete hardened, the sand was flushed out and replaced with silicone rubber. The top print (left) made an impression on several layers below, shown as undertracks at depths of up to 40 mm. Notice how the impressions of the digits become wider and less well-defined along each subjacent horizon. (Courtesy of J. Milan.)

Figure 19.7 Experimental ichnology: (a) graduate student Jesper Milan, trying to persuade an emu to walk where he wants it to walk, and (b) the tracks and undertracks of the emu - results of an experiment where an emu stepped on a package of alternating layers of concrete and sand. After the concrete hardened, the sand was flushed out and replaced with silicone rubber. The top print (left) made an impression on several layers below, shown as undertracks at depths of up to 40 mm. Notice how the impressions of the digits become wider and less well-defined along each subjacent horizon. (Courtesy of J. Milan.)

Figure 19.8 Trace fossils of the deep ocean floor. The patch-feeding trace (pascichnia) Helminthopsis meanders on one horizon, and the network burrow system (agrichnia) Paleodictyon is seen at a different level, in this field photograph from the Lower Silurian Aberystwyth Grits, Wales. (Courtesy of Peter Crimes.)

Figure 19.8 Trace fossils of the deep ocean floor. The patch-feeding trace (pascichnia) Helminthopsis meanders on one horizon, and the network burrow system (agrichnia) Paleodictyon is seen at a different level, in this field photograph from the Lower Silurian Aberystwyth Grits, Wales. (Courtesy of Peter Crimes.)

this works very well are in the deep sea and on land. Very little is known from body fossils of the history of life in deep abyssal oceans, and indeed very little is known about life in these zones today because they are inaccessible. Trace fossils, however, are abundant in many deep oceanic settings (Fig. 19.8), and they show the diversity of trail-making and burrowing soft-bodied organisms, how many of them built complex shallow burrow systems and efficient patch-feeding trails, and how these assemblages evolved through the Pha-nerozoic. On land, some continental sequences preserve very few body fossils, and the only indications of animal life are abundant dinosaur and other vertebrate tracks (Box 19.4), as well as tracks and burrows made by insects and pond-living animals.

One of the major advances in trace fossil studies was Seilacher's (1967a) classification of behavioral categories. He divided trace fossils into seven behavioral types, depending on the activities represented (Fig. 19.10). Tracks and trails representing movement from A to B, such as worm trails or dinosaur trackways, are termed repichnia (repere, to creep; ichnos, trace). Grazing trails that involve movement and feeding at the same time are called pascichnia (pascere, to feed). These are typically coiled or tightly meandering trails found in deep oceanic sediments, where the regular pattern is an adaptation to feeding on restricted patches of food. Some unusual deep-sea horizontal burrow systems appear to have been maintained for trapping food particles, or for growing algae. These are termed agrichnia (agricola, farmer). Feeding burrows, such as those produced by earthworms, as well as many marine examples, are called fodinichnia (foda, food). Living burrows and borings are termed domichnia (domus, house). Escape structures, or fugichnia (fugere, to flee) are traces of upward movement of worms, bivalves or starfish seeking to escape from beneath a layer of sediment that has been dumped suddenly on top of them. Fugichnia are found in cases of rapid sedimentation, in beach, storm-bed and turbidite sediments. Resting traces, or cubichnia (cubare, to lie down), may be of many types, and can include impressions of the undersides of trilobites, starfish and jellyfish.

Tracks and trails can sometimes be assigned to their makers, and then it may be possible to carry out quantitative studies of their modes of locomotion. Arthropod tracks, for example, show the often complex patterns of movement of their numerous legs. Dinosaur tracks can show how fast the dinosaur was running (Box 19.5).

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