Perhaps one of the most frustrating — and most fascinating — facets of ichnology is the attempt to establish the zoological affinities of specific ichnofossils. It has been pointed out that difficulties arise in any systematic classification of ichnofossils, partly because extensive comparisons of trace morphology and ethology with the traces of most modern organisms has not yet been made. Furthermore, ichnofossils mostly reflect the behaviour of animals, and only to a small extent reflect their anatomy or morphology. The result is that more than one genus or species of ichnofossil may have been constructed by a single species of animal, or conversely, different species of animals may have made identical species or genera of trace fossils. For example, Skolithos linearis at one locality may show affinities to the phoronids, whereas at another locality they may show affinities to onuphid polychaetes.
Trace fossils record the activities of benthic organisms, many of which are soft-bodied and are not readily preserved. This less preservable group includes entire phyla (such as the nemerteans, nematodes, nematomorphs, annelids, sipunculids, echiurans, pognophorans, priapulids, phoronids, onychoporids, urochordates, nemertinids, chaetognathids, cephalochordates, and enteropneusts) or classes (i.e. anthozoans, aplacophorans, holothuroids, and demosponges). Many of these lineages are diverse (i.e. at present there are 18 000 extant species of annelids, 15 000 species of nematodes, 900 species of nemerteans, 2000 species of urochordates, and 320 species of sipunculids) and many are known to have originated at the start of the Phanerozoic. For example, annelids, echurians, pognophorans, priapulids, phoronids, onychoporids, and enteropneusts are known from deposits as old as Cambrian.
These groups are known from Konservat-Lagerstätten — exceptional preservational cases, where the soft parts of organisms are preserved as impressions — for example the Lower Cambrian Chengjiang fauna Qiongzhusi Formation (The Maotianshan Shale) of China, the Middle Cambrian Burgess Shale in British Columbia, the Upper Cambrian Orsten fauna of Sweden, the Lower Ordovician Fezouata Formation of Morocco, and the Lower Devonian Hunsrück Slate of Germany, among others. Traditionally, however, palaeontologists have relegated such groups to ‘minor phyla’ status and have ignored them in the following ways:
1. The analysis of diversity trends through time.
2. The taphonomic implications of the ‘incomplete fossil record’.
3. The evolution of infaunal suspension and deposit feeders.
4. The periodicity of mass extinctions.
5. The interpretation of population strategies.
The ichnological record of these minor phyla can have considerable significance. For example, Treptichnus pedum is regarded as the earliest widespread complex trace fossil. Its appearance is contemporaneous with the last of the Ediacaran biota and defines the dividing line between the Ediacaran and Cambrian Periods. Dzik (2005, 2007) has compared structures of Cambrian priapulids known from the Burgess Shale and the Chengjiang mudstones with horizontal burrows and bilobate traces and concluded that Treptichnus pedum was produced by priapulids. Likewise, during experimental ichnological studies with modern priapulids Vannier et al. (2010) were able to show that typical serial burrows of treptichnids were most probably produced by priapulids during shallow probing for infaunal or epifaunal prey.
We need a better understanding of the ichnological record of the so-called minor phyla that are not otherwise preserved in the usual shelly and bony fossil record. They can offer a more complete record of ancient biodiversity and behaviour, thereby enabling a better reconstruction of the palaeoecology of ancient aquatic communities.
Dzik, J (2005). Behavioral and anatomical unity of the earliest burrowing animals and the cause of the ‘Cambrian explosion’. Paleobiology 31, 507–525.
Dzik, J (2007). The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian-Cambrian transition. In Vickers-Rich, P and Komarower, P (eds.) The Rise and fall of the Ediacaran Biota. Special publications 286, Geological Society, London, 405-414.
Vannier, J, I Calandra, C Gaillard, and A Zylinska (2010). Priapulid worms: Pioneer horizontal burrowers at the Precambrian-Cambrian boundary. Geology 38, 711–714.