Boby And Trace Fossil In Paleobiology

Treptichnu s

Curvolithus

First simple burrow systems. First traces with three-lobed lower surface

Archaeonassa

Helminthoidichnites Helminthorhaphe

Unbranched horizontal traces

?Prot I

Planolites

?First shallow infaunal traces

Figure 19.16 Trace fossils may help to define the Precambrian-Cambrian boundary, and to flesh out detail about the Cambrian explosion. Jensen (2003) identified seven trace fossil zones, each characterized by trace fossils of increasing complexity. Evidence for trilobites, and arthropods in general, is signaled first by trace fossils and then by body fossils. Prot, Proterozoic. (Drawing by Simon Powell.)

do not evolve. There are one or two exceptions, and one of these is the critical Precam-brian-Cambrian boundary interval.

The timing of the Cambrian explosion has been hugely controversial, and the story from body fossils and trace fossils is different (see p. 249). The base of the Cambrian System, and therefore the Precambrian-Phanerozoic boundary, has generally been placed at the first occurrence of trilobite body fossils. But the oldest known trilobite body fossils actually occur above the first trilobite trace fossils, such as Monomorphichnus, Rusophycus, Cruziana and Diplichnites. Below this boundary, in the Neoproterozoic, trace fossils document how early animals were becoming more complex in the lead-up to the Cambrian explosion (Fig. 19.16). First to appear were simple shallow burrows (Planolites), then unbranched horizontal traces such as Archae-onassa, Helminthoidichnites and Helmin-thorhaphe (Jensen 2003). In the third Neoproterozoic, trace fossil zone are the first records of simple burrow systems (Treptich-nus) and traces with a three-lobed lower surface ("Curvolithus"). The basal Cambrian is then marked by the Treptichnus pedum zone, characterized by Treptichnus, Gyro-lithes and Bergaueria, examples of branching burrow systems and sea anemone resting traces. The body fossils show a sudden explosion of marine animals at the beginning of the Cambrian. The trace fossils give richer detail: a longer-term build-up of complexity in the latest Precambrian, and then the explosion of new life forms.

There is rare evidence for the evolution of certain trace fossils through time. For example, pascichnia like Nereites and agrichnia like Paleodictyon seem to have become smaller and more regular through time, perhaps evidence for improvements in feeding efficiency.

Orr et al. (2003) found another time-related aspect of animal behavior on the ocean floor. They were puzzled by the abundance of certain kinds of exceptionally preserved marine fossil assemblages in the Cambrian. The Burgess Shale is the most famous example (see p. 249), but there are many other such continental shelf and deep marine conservation Lagerstätten, where soft tissues seem to survive without damage to become biomineralized. These assemblages occur throughout the Cambrian, and are rare after that. Orr et al. (2003) suggested that this was linked with an increase in bioturbation of the ocean floor after the Cambrian, and an increase in agrichnia and pascichnia in particular. The change in trace fossils suggests that the mobile infauna in deep waters had become more diverse and voracious, and more mobile. It seems that more invertebrates moved to the deep ocean floors after the end of the Cambrian, perhaps in search of new sources of food as the shallower waters became more crowded. They then searched out any dead organisms they could find to feed on, and no Burgess Shale-

type exceptional preservation could ever happen again.

Trace fossils and the oil industry_

Trace fossils are powerful tools in studying large sedimentary basins. Petroleum geologists frequently want to understand the architecture of large volumes of sediment in order to determine whether they might be oil reservoirs, and yet they usually have to work from geophysical soundings across these basins and isolated boreholes. Sedimentologists who study these boreholes have to understand the significance of often subtle and rather small indicators.

Many sediments are bioturbated, or churned up by animal activity. Under water, the sediments may be burrowed and re-burrowed, creating sometimes dense masses of cross-cutting burrows. On land, trampling by animals may also churn up the sediments. Several measures have been devised to report the degree of bioturbation in a vertical section of sediment, such as is seen in a core, and the most widely used is the ichnofabric index of Droser and Bottjer (1989). This is a flash card system that can be used to assign an index to any burrowed sediment (Fig. 19.17).

Sedimentologists working for the oil industry have to identify trace fossils from chance sections in narrow borehole cores, which can generic Skolithos Ophiomorpha "Deep sea"

shelf ichnofabric ichnofabric ichnofabric ichnofabric generic Skolithos Ophiomorpha "Deep sea"

shelf ichnofabric ichnofabric ichnofabric ichnofabric

Bioturbation Index

Figure 19.17 Ichnofabric indices for different sedimentary/ichnofacies settings. These diagrams show the proportions of sediment reworked by bioturbation, as seen in vertical section and numbered from top to bottom: 1 (no bioturbation) to 5 (intensely bioturbated). (Courtesy of Mary Droser and Duncan McIlroy.)

Figure 19.17 Ichnofabric indices for different sedimentary/ichnofacies settings. These diagrams show the proportions of sediment reworked by bioturbation, as seen in vertical section and numbered from top to bottom: 1 (no bioturbation) to 5 (intensely bioturbated). (Courtesy of Mary Droser and Duncan McIlroy.)

Diplocraterion Ichno

Figure 19.18 Interpreting trace fossils in borehole cores can be difficult. Vertical (a) and horizontal (b) cuts across a core may show rather obscure burrow impressions, but these make sense when interpreted in three dimensions (c). These are indications of the U-shaped burrow Diplocraterion, typical of the Skolithos ichnofacies, and so an indicator of the intertidal zone. (Courtesy of Duncan McIlroy.)

Figure 19.18 Interpreting trace fossils in borehole cores can be difficult. Vertical (a) and horizontal (b) cuts across a core may show rather obscure burrow impressions, but these make sense when interpreted in three dimensions (c). These are indications of the U-shaped burrow Diplocraterion, typical of the Skolithos ichnofacies, and so an indicator of the intertidal zone. (Courtesy of Duncan McIlroy.)

be extremely tricky (Fig. 19.18). They regularly record the following ichnological features of cores (McIlroy 2004):

• Ichnofabric index (= intensity of bioturbation).

• Diversity of trace fossils (high diversity usually means the water was well oxygenated and food was abundant).

• Relative abundance of trace fossils (does any ichnogenus dominate?).

• Trace fossil sizes (low-diameter burrows may indicate salinity stress or low oxygen levels).

• Infaunal tiering (deep tiers suggest well-oxygenated bottom-water conditions).

• Succession of burrows (did burrows pre- or postdate particular sedimentary events?).

• Colonization styles (softgrounds may be colonized from above or below, hardgrounds generally from above).

Putting this information together with sedi-mentological observations may allow the ich-nologist to interpret the original modes of deposition throughout a core in some detail. Petroleum geologists are often able to interpret sedimentary facies from the limited information available in a narrow core.

Ichnofabric studies may provide critical evidence for the identification of key strati-graphic surfaces, the basis of sequence stratigraphy (see pp. 34-7). For example, marine flooding surfaces or exposed erosive surfaces may be indicated by distinctive ichnofabrics that can be traced laterally across a whole basin.

Trace fossils may also be important in determining oil reservoir quality. The key factors are the porosity (void space between grains) and permeability (ability to transmit fluids) of the sediments. The economic value of an oil reserve may be reduced or enhanced by trace fossil activity: porosity and permeability may be reduced by intense burrowing and mixing of sand and clay grains, and so the value is lowered; or organisms may burrow through impermeable clay layers and link porous sandy layers, so increasing the value of the oil reserve. These interactions of trace fossils and sediments are only now coming to be understood and used extensively in the oil industry. Who would have thought the price of a barrel of crude oil, such a key factor in driving the world economy, might depend on the activities of some ancient worms!

1 What kinds of trace fossils could a human being leave behind? Think of examples of all the main categories shown in Fig. 19.10.

2 Do big dinosaurs run faster than small dinosaurs? Find 10 dinosaur track photographs on the web, check the scale and measure the stride lengths. Use the Alexander formula (see Box 19.5) to work out speeds, and plot these against estimated body sizes for the dinosaur track makers.

3 How well do the classic marine ichnofacies work (see Fig. 19.12)? Look up exam ples of the indicator trace fossils, such as Nereites and Zoophycos, from lakes and other settings. How many exceptions invalidate the general usefulness of a scheme like Seilacher's ichnofacies model?

4 If life has diversified through time, you might expect trace fossil diversity to increase too. Find 10 papers about deep-sea Nereites ichnofacies, trace fossil assemblages from the Cambrian, Ordovician, Silurian, Carboniferous, Jurassic, Cretaceous and Tertiary, and count the number of named trace fossils. Plot these against time: does deep-sea trace fossil diversity increase, decrease or stay the same through the last 500 million years? Why would your result need further work to be completely convincing?

5 Find examples of where trace fossil study has been useful in the oil industry. What ages of rocks and sedimentary facies most benefit from trace fossil studies of boreholes?

Further reading

Bromley, R.G. 1996. Trace Fossils: Biology, taphonomy and applications, 2nd edn. Chapman and Hall, London.

Donovan, S.K. (ed.) 1994. The Palaeobiology of Trace Fossils. Wiley, Chichester.

Ekdale, A.A., Bromley, R.G. & Pemberton, S.G. 1984. Ichnology; The use of trace fossils in sedimentology and stratigraphy. Society of Economic Paleontologists and Mineralogists, Tulsa, OK.

Lockley, M.G. 1991. Tracking Dinosaurs. Cambridge University Press, Cambridge.

Maples, C.G. & West, R.R. (eds) 1992. Trace Fossils. Short Courses in Paleontology, No. 5. Paleontologi-cal Society, Tulsa, OK.

McIlroy, D. (ed.) 2004. The Application of Ichnology to Palaeoenvironmental and Stratigraphic Analysis. Special Publication No. 228. Geological Society, London.

Miller III, W. 2005. Trace Fossils; Concepts, problems, prospects. Elsevier, Amsterdam.

Seilacher, A. 2007. Trace Fossil Analysis, Springer, New York.

References

Alexander, R.M. 1976. Estimates of speeds of dinosaurs. Nature 261, 129-30. Crimes, T.P. & Crossley, J.D. 1991. A diverse ichno-fauna from Silurian flysch of the Aberystwyth Grits

Review questions

Formation, Wales. Geological Magazine 26, 2764.

Droser, M.L. & Bottjer, D.J. 1989. Ichnofabric of sandstones deposited in high-energy nearshore environments: measurement and utilization. Palaios 4, 598-604.

Ekdale, A.A. & Bromley, R.G. 1991. Analysis of composite ichnofabrics: an example in the uppermost Cretaceous Chalk of Denmark. Palaios 6, 232-49.

Ekdale, A.A., Bromley, R.G. & Pemberton, S.G. 1984. Ichnology; The use of trace fossils in sedimentology and stratigraphy. Society of Economic Paleontologists and Mineralogists, Tulsa, OK.

Frey, R.W., Pemberton, S.G. & Saunders, T.D.A. 1990. Ichnofacies and bathymetry: a passive relationship. Journal of Paleontology 64, 155-8.

Gatesy, S.M, Middleton, K.M., Jenkins Jr., F.A. & Shubin, N.H. 1999. Three-dimensional preservation of foot movements in Triassic theropod dinosaurs. Nature 399, 141-4.

Jensen, S. 2003. The Proterozoic and earliest Cambrian trace fossil record; patterns, problems and perspectives. Integrative and Comparative Biology 43, 219-28.

Lockley, M.G., Hunt, A.P. & Meyer, C.A. 1994. Vertebrate tracks and the ichnofacies concept: implications for paleoecology and palichnostratigraphy. In Donovan, S.K. (ed.) The Palaeobiology of Trace Fossils. Wiley, Chichester, pp. 241-68.

McIlroy, D. 2004. Some ichnological concepts, methodologies, applications and frontiers. In McIlroy, D.

(ed.) The Application of Ichnology to Palaeoenviron-mental and Stratigraphic Analysis. Special Publication No. 228. Geological Society, London, pp. 3-27.

Milan, J. & Bromley, R.G. 2006. True tracks, under-tracks and eroded tracks, experimental work with tetrapod tracks in laboratory and field. Palaeogeog-raphy, Palaeoclimatology, Palaeoecology 231, 253-64.

Minter, N.J. & Braddy, S.J. 2006. Walking and jumping with Palaeozoic apterygote insects. Palaeontology 49, 827-35.

Orr, P.J. 1995. A deep-marine ichnofaunal assemblage from Llandovery strata of the Welsh Basin, west Wales, UK. Geological Magazine 132, 26785.

Orr, P.J., Benton, M.J. & Briggs, D.E.G. 2003. PostCambrian closure of the deep-water slope-basin taphonomic window. Geology 31, 769-72.

Pemberton, S.G. & Frey, R.W. 1984. Ichnology of storm-influenced shallow marine sequence: Cardium Formation (Upper Cretaceous) at Soebe, Alberta. Canadian Society of Petroleum Geologists, Memoir 9, 281-304.

Seilacher, A. 1964. Biogenic sedimentary structures. In Imbrie, J. & Newell, N. (eds) Approaches to Paleo-ecology. Wiley, New York, pp. 296-316.

Seilacher, A. 1967a. Bathymetry of trace fossils. Marine Geology 5, 413-28.

Seilacher, A. 1967b. Fossil behavior. Scientific American 217, 72-80.

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