Consequently, the appropriate placement of this group in
one category or another is unclear. Egg cases are not usually
described as trace fossils, but eggs can be preserved within
a fossil nest, providing direct evidence of reproductive
behavior. In that sense, they fall within the realm of
paleoichnology and are often placed under “other evidence
of activity.”
The conceptual framework
The importance of paleoichnology in traditional fields
such as paleontology, paleoecology, sedimentology, and
stratigraphy derives from the peculiarities of trace fossils,
which reflect both their mode of formation and their taphonomic histories. Unfortunately, the limitations of trace
fossil also arise from these basic characteristics
(“ichnological principles” of Bromley and Fürsich 1980;
Ekdale et al., 1984; Bromley, 1996; Pemberton
et al., 2001). The examples are as follows: (1) A long stratigraphic range can limit the use of trace fossils in biostratigraphy. (2) A narrow environmental range may reflect
similar responses of tracemakers to a given set of paleoecological parameters, and therefore, biogenic sedimentary structures tend to occur preferentially in certain
depositional environments. The combination of (1) and
(2) greatly facilitates the comparison of rocks of different
ages formed in similar depositional settings. (3) The rarity
of secondary displacement means that trace fossils
are very rarely transported and therefore represent the
original environmental position of the tracemakers
(i.e., they are in situ fossils). This characteristic reveals
the strength of ichnofossils in paleoecological reconstruction. (4) Non-preservable soft-bodied trace producers
must be considered since many biogenic sedimentary
structures record the activities of soft-bodied organisms
that are usually not preserved because they lack hard parts.
This fact highlights once again the difference between
trace and body fossils. (5) Peculiar occurrences in otherwise nonfossiliferous sediments are very often the result
of diagenetic processes that, on the one hand, enhance
the potential preservation of trace fossils and, on the other,
may obliterate the tests and shells of body fossils.
(6) The same individual or species of organism may
produce different structures corresponding to different
behavior patterns; this characteristic can produce compound traces, where intergradational forms reflect the
transition from one behavior to another. (7) The same individual may produce different biogenic structures,
reflecting the same behavior on different substrates; this
peculiarity is attributable to variability in the substrate
conditions in terms of the degree of consistency, grain
size, and stratal position. (8) Conversely, identical
(or very similar) structures can be produced by systematically different organisms, where their behavior is similar;
this peculiarity makes it impossible to establish
a one-to-one relationship between tracemakers and biogenic structures. (9) A single structure may reflect the
activity of two or more organisms, living together or in
successive times, within the substrate (the “composite”
traces of Pickerill, 1994). Paleoenvironmental research
based on these characteristics represents the majority of
contemporary ichnological studies and applications.
Naming biogenic sedimentary structures
The use a formal taxonomy by ichnologists must accommodate the many difficulties that arise from both the
historical background and the intrinsic nature of
ichnofossils. In the early years of paleoichnology, a large
number of invertebrate trace fossils were named and
described as the remains of algae or other organisms
(Age of Fucoids by Osgood, 1975). However, based on
the priority law, many of these names are taxonomically
valid, such as Cruziana, Zoophycos, and Chondrites
erected as algae and Nereites as worms.
The 1964 edition of the International Code of Zoological
Nomenclature (ICZN) ruled that trace fossil names erected
after 1930 were to be accompanied by a statement on the
identification of the tracemakers. Because fulfilling that
requirement is essentially impossible, all post-1930 trace
fossil names (ichnotaxa) were formally unavailable,
whereas the pre-1930 taxa retained their valid names but
were treated on the same basis as body fossils. This is considered the beginning of the Dark Age of Ichnotaxonomy
(Bromley, 1996). Thanks to the long-lasting and determined activities of ichnologists and exhaustive scientific
debate, ichnofossils have finally been bounded by the ICZN
in 1985. The 4th edition of the ICZN (1999) includes in the
“work of animals” all trace fossils. This means that animal,
protistan, plant, and fungal trace fossils are considered in
exactly the same way as zoological taxa in terms of the
availability and validity of their names. However, they are
called “ichnotaxa” (“ichnogenera” and “ichnospecies”) to
distinguish them clearly from true biotaxa. The significant
departures with respect to body fossils (see also the previous section) further complicate trace fossil taxonomy. For
example, according to the ICZN, only fossil specimens
should be named, which prevents ichnologists erecting
ichnotaxa based on recent biogenic structures that might
be assigned very often to their producers on a case-by-case
basis. Under these circumstances, some authors prefer to
name the tracemaker associated with the recent structure,
whereas others opt to use the prefix “incipient” before the
ichnotaxon (e.g., incipient Thalassinoides) (Bromley and
Fürsich, 1980). A separate code for naming trace fossils,
as proposed by Sarjeant and Kennedy (1973), might be
a possible alternative to circumvent the aforementioned difficulties, but this prospect has never gained legal standing.
Classification of trace fossils
Although the recent ICZN explicitly encompasses
ichnofamilies, there is no true ichnotaxonomic superstructure above the rank of ichnogenus, and trace fossils can be
grouped together in several ways. Traditionally, the most
BIOGENIC SEDIMENTARY STRUCTURES
77
one category or another is unclear. Egg cases are not usually
described as trace fossils, but eggs can be preserved within
a fossil nest, providing direct evidence of reproductive
behavior. In that sense, they fall within the realm of
paleoichnology and are often placed under “other evidence
of activity.”
The conceptual framework
The importance of paleoichnology in traditional fields
such as paleontology, paleoecology, sedimentology, and
stratigraphy derives from the peculiarities of trace fossils,
which reflect both their mode of formation and their taphonomic histories. Unfortunately, the limitations of trace
fossil also arise from these basic characteristics
(“ichnological principles” of Bromley and Fürsich 1980;
Ekdale et al., 1984; Bromley, 1996; Pemberton
et al., 2001). The examples are as follows: (1) A long stratigraphic range can limit the use of trace fossils in biostratigraphy. (2) A narrow environmental range may reflect
similar responses of tracemakers to a given set of paleoecological parameters, and therefore, biogenic sedimentary structures tend to occur preferentially in certain
depositional environments. The combination of (1) and
(2) greatly facilitates the comparison of rocks of different
ages formed in similar depositional settings. (3) The rarity
of secondary displacement means that trace fossils
are very rarely transported and therefore represent the
original environmental position of the tracemakers
(i.e., they are in situ fossils). This characteristic reveals
the strength of ichnofossils in paleoecological reconstruction. (4) Non-preservable soft-bodied trace producers
must be considered since many biogenic sedimentary
structures record the activities of soft-bodied organisms
that are usually not preserved because they lack hard parts.
This fact highlights once again the difference between
trace and body fossils. (5) Peculiar occurrences in otherwise nonfossiliferous sediments are very often the result
of diagenetic processes that, on the one hand, enhance
the potential preservation of trace fossils and, on the other,
may obliterate the tests and shells of body fossils.
(6) The same individual or species of organism may
produce different structures corresponding to different
behavior patterns; this characteristic can produce compound traces, where intergradational forms reflect the
transition from one behavior to another. (7) The same individual may produce different biogenic structures,
reflecting the same behavior on different substrates; this
peculiarity is attributable to variability in the substrate
conditions in terms of the degree of consistency, grain
size, and stratal position. (8) Conversely, identical
(or very similar) structures can be produced by systematically different organisms, where their behavior is similar;
this peculiarity makes it impossible to establish
a one-to-one relationship between tracemakers and biogenic structures. (9) A single structure may reflect the
activity of two or more organisms, living together or in
successive times, within the substrate (the “composite”
traces of Pickerill, 1994). Paleoenvironmental research
based on these characteristics represents the majority of
contemporary ichnological studies and applications.
Naming biogenic sedimentary structures
The use a formal taxonomy by ichnologists must accommodate the many difficulties that arise from both the
historical background and the intrinsic nature of
ichnofossils. In the early years of paleoichnology, a large
number of invertebrate trace fossils were named and
described as the remains of algae or other organisms
(Age of Fucoids by Osgood, 1975). However, based on
the priority law, many of these names are taxonomically
valid, such as Cruziana, Zoophycos, and Chondrites
erected as algae and Nereites as worms.
The 1964 edition of the International Code of Zoological
Nomenclature (ICZN) ruled that trace fossil names erected
after 1930 were to be accompanied by a statement on the
identification of the tracemakers. Because fulfilling that
requirement is essentially impossible, all post-1930 trace
fossil names (ichnotaxa) were formally unavailable,
whereas the pre-1930 taxa retained their valid names but
were treated on the same basis as body fossils. This is considered the beginning of the Dark Age of Ichnotaxonomy
(Bromley, 1996). Thanks to the long-lasting and determined activities of ichnologists and exhaustive scientific
debate, ichnofossils have finally been bounded by the ICZN
in 1985. The 4th edition of the ICZN (1999) includes in the
“work of animals” all trace fossils. This means that animal,
protistan, plant, and fungal trace fossils are considered in
exactly the same way as zoological taxa in terms of the
availability and validity of their names. However, they are
called “ichnotaxa” (“ichnogenera” and “ichnospecies”) to
distinguish them clearly from true biotaxa. The significant
departures with respect to body fossils (see also the previous section) further complicate trace fossil taxonomy. For
example, according to the ICZN, only fossil specimens
should be named, which prevents ichnologists erecting
ichnotaxa based on recent biogenic structures that might
be assigned very often to their producers on a case-by-case
basis. Under these circumstances, some authors prefer to
name the tracemaker associated with the recent structure,
whereas others opt to use the prefix “incipient” before the
ichnotaxon (e.g., incipient Thalassinoides) (Bromley and
Fürsich, 1980). A separate code for naming trace fossils,
as proposed by Sarjeant and Kennedy (1973), might be
a possible alternative to circumvent the aforementioned difficulties, but this prospect has never gained legal standing.
Classification of trace fossils
Although the recent ICZN explicitly encompasses
ichnofamilies, there is no true ichnotaxonomic superstructure above the rank of ichnogenus, and trace fossils can be
grouped together in several ways. Traditionally, the most
BIOGENIC SEDIMENTARY STRUCTURES
77