Biogenic sedimentary structures do not represent the burrowing animals
themselves, but show their behaviour. Each trace fossil tends to be
attributed to its trace-making organism, but they can tell more
interesting things than the name of the "architect".
Here are the basic ichnological principles:
1.Same Species Different
1.The same species can produce different structures corresponding to
different behaviour patterns
The modern fiddler crab Uca produces a:
J-shaped living burrow (Psilonichnus domichnion - 1),
a walking (repichnia) trail (Diplichnitesrepichnion - 2),
a radiating grazing trace with balls of processed sand(pascichnion -
2.The same burrow may be differently preserved in different substrates
according to the average grain size, sediment stability, water content,
and chemical conditions in sediment.
A subsurface patch-feeding burrow develops different morphologies, and
therefore has different names, when
preserved in sand (Scalarituba - 1),
at a sand-mud interface in firm sediment(Nereites - 2),
at a sand-mud interface in wetter sediment (Neonereites - 3),
and at a mud-sand interface, seen from below(Neonereites - 4)
(after Benton & Harper, 1997)
3.Different tracemakers with similar behaviour may produce identical
Resting traces (cubichnia) of the ichnogenus Rusophycus, produced
polychaete worm (1),
a nassid snail (2),
a notostracan branchiopod shrimp (3),
and a trilobite (4)
(after Benton & Harper, 1997)
4.Multiple individuals may produce a single structure.
5.Burrowing Organisms Low
5.The burrowing organisms usually have a low potential of preservation as
A bivalve mollusc preserved as a mold into a network of burrows (Osmington
Mills, Dorset - photo M.A. Kaminski)
6.Open Burrow High
6.The open burrow is an environment with a higher preservation potential
than the rest of the sea floor in general, protecting the fallen in
skeletons against the most destructive phases of early diagenesis.
Trace fossils are hard to be connected to the zoological taxonomical
system and must be treated according to the ichnological principles.
Types of Traces
There are many kinds of trace fossils and also a variety of cryptic
fossils and sedimentary structures that may be regarded as trace fossils.
The main trace fossil types belong to four main groups:
Traces on bedding planes Tracks - sets of discrete footprints, usually formed
by arthropods and vertebrates Trails - continuous traces, usually formed by the
whole body of a traveling or resting worm, mollusc or arthropod
Structures within the sediment Burrows - structures formed within soft sediment by
moving grains out of the way, either for locomotion, dwelling, protection
or feeding Borings - structures formed in hard substrate by
cutting right through the grains, either for protection, dwelling,
carbonate extraction or even for feeding purpose (bioerosion drill holes
Excrements Faecal pellets and faecal strings - small pellets or
strings of excrement, usually of less than 10 mm length Coprolites - discrete faecal masses produced by
vertebrates, usually more than 10 mm length
Others Root penetration structures - impressions of activity
of growing roots Non-faecal pellets - regurgitation pellets of
vertebrates, excavation pellets of crustaceans
Many ichnologists might also include other biological interactions
with sediments as trace fossils, such as stromatolites, some mud mounds,
dinosaur nests, etc.
Trace fossils can be classified according to Linnaean taxonomy, ethology
(inferred life history and behaviour of trace producing organisms), or
Linnaean classification involves the using of a binomial
system similar to that employed for body fossils and living species.
Ichnotaxa include ichnogenera and ichnospecies, simply used to define
In many cases is difficult to prove a direct relationship between trace
producer and trace (e.g.. the trace produced by the genus Lithophaga is
called, according to its shape, Gastrochaenolites), therefore is
desirable an independent name for a particular trace morphology.
Ethological (Behavioural) classification is a mechanism of
distinguishing groups of traces on the basis of the behaviour of a group
The main types include: resting traces - cubichnia - created by active
organisms while at rest or hiding from prey; locomotion traces - repichnia - tracks and trails
created while moving across the sediment surface; dwelling traces - domichnia - three dimensional
dwelling structures created by burrowing; grazing traces - pascichnia - horizontal feeding
traces on the surface of the substrate; feeding traces - fodinichnia - three dimensional
networks characterized by the combined functions of deposit feeding and
dwelling. traps and gardening traces - agrichnia - regular,
patterned branching structures used as traps for migrating meiofauna or as
gardening systems where microbes might be cultured for food. predation traces - praedichnia - are common on hard
substrates, as round drill holes in shells and shell damage by predators.
equilibrium traces - equilibrichnia - are
characterized by burrows that must be constantly adjusted within the
substrate due the agradation and degradation processes. escape traces - fugichnia - occur as the animal flees
to the new sea floor edifices built above the substrate - aedificichnia -
are structures built of sediment, more or less cemented by the architect structures made for breeding purposes - calichnia -
are structures built for raising larvae and juveniles.
Major categories of trace fossils according to behavioural classification
and some illustrated ichnogenera:
11. Asteriacites; Rusophycus
(after Benton & Harper, 1997)
Preservational classification uses the relationship of the trace with the
According to this criterion the traces can be exogenic (on the sediment
surface) or endogenic (within the sediment itself).
Traces can be found in full relief (with the boundaries of circular
burrows clearly defined within the enclosing sediment) or semirelief,
either on the upper surface (epirelief), or on the lower surface
(hyporelief) of a sedimentary layer.
Terminology of trace fossil preservation depending on the relationship to
sediment horizons (after Benton & Harper, 1997)