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Ausich and Bottjer, (1982)
and (1986), (click on "Tiering" link to left) recognize tiering
as a common community structure and strategy of the geologic past. They
demonstrate how different suspension feeding organisms were distributed
vertically in space above and just below the sediment surface. They propose
that organisms used their vertical position in the water column to acquire
a food supply and maintain the ambient conditions necessary for their
life. These tiering settings and organisms were:
Primary Tierers – suspension feeders that have a body or burrow
that intersects the seafloor.
Secondary Tierers – suspension feeders that maintain a position
above or below the sediment-water interface and utilize a support structure
or a burrow of a primary tierer or a plant living in that setting. Ausich
and Bottjer associate this tiering with an epifauna of benthic organisms
living on or above the sediment-water interface and infauna of benthic
organisms living below the sediment-water interface.
Using the concepts of Ausich
and Bottjer, (1982), Bromley and Ekdale (1986) record how burrowing organisms
have a strategy of vertical tiering their burrows at and below the sediment
surface that is in response to their individidual sensitivity to sediment
dynamics, coherence of the sediment, the local salinity, oxygen levels,
food sources and predation.
They showed how shallow burrowing organisms lived in the watery uncompacted
sediment while the deeper burrowers occupied a stiffer, compacted firm
ground sediment. Bromley and Ekdale (1986) established that this vertical
tiering of the ichnofacies can be modified if more sediment buried the
existing faunal assemblages. This would tend to force the burrowing organisms
to migrate up through the tiered suites of trace fossils overprinting
earlier shallower burrows. Featured below are some examples of ichnofossil
tiering and how it manifests itself through time.
Cross cutting relationships
of juxtaposed shallow and deeper burrowers enable geologists to determine
the order in which the burrows were formed and so trace the evolution
of this shallow subsurface milieu. Bromley and Ekdale (1986) note that
the best preserved and best displayed trace fossils are associated with
the assemblages found in the deepest tiers.
The use of the concept of tiering, combined with the identification of
the ichnofacies assemblages proposed by Seilacher (1967) enhances our
ability to determine how stacked parasequences accumulated in ancient
depositional systems and acquired their geometries. Frey and Pemberton,
(1985) and the Ichnology Research Group (IRG), lead by George Pemberton,
at the University of Alberta in Edmonton Canada have detailed strategies
on how to do this: link by clicking on the IRG Logo to left.
References
Ausich, W. I. and D. J. Bottjer. 1982. "Tiering in suspension feeding
communities on soft substrata throughout the Phanerozoic." Science.
216:173-174.
Bottjer, D. J. and W. I. Ausich. 1982. "Tiering and sampling requirements in paleocommunity reconstruction." Proceedings of the Third North American Paleontological Convention, 1:57-59.
Bottjer, D. J. and W. I. Ausich., 1986. "Phanerozoic development of tiering in soft substrata suspension-feeding communities": Paleobiology, 2:400-420.
Bromley, R. G. and A. A. Ekdale. 1986. "Composite ichnofabrics and tiering of burrows." Geological Magazine, 123:59-65.
Seilacher, A. 1967 "Bathymetry of trace Fossils" Marine Geology v 5 p 413-428
