Macrofossils and ichnofossils can be used to generate reconstructions of ancient environments based on the nature of preservation. Both of these allow us to interpret different aspects of the environment the fossils were preserved in and its controls, however advantages and disadvantages of each must be considered so that the data collected is treated with the appropriate caution. Macrofossils, ranging from bones to shells, with exceptional preservation allowing for the analysis of soft tissues, focuses on the analysis of the organism itself to interpret the conditions that it lived in, whereas ichnofossils, or trace fossils as they are more commonly known, provide a more useful indicator as to how the organism lived in its environment.
There are several approaches to palaeoenvironmental reconstructions, including the actualistic approach (using Recent relatives), geochemical approach (trace element record of biogenic hard parts), functional approach (function interpreted from morphology), ecological principle approach (ecological concepts applied to fossil assemblages), and finally the integrated approach. This latter approach combines all of the above as well as using the sedimentary record to interpret a palaeoenvironment from the fossils present, and will have a higher degree of reliability than each individual method (Fürsich, 1995). This is mainly applied to the analysis of macrofossils due to the nature of the data available to be analysed.
Macrofossils
With the preservation of macrofossils ranging from 7-70% (Lawrence, 1968), it is key to note that those preserved may not give a representative overview of the original environment. It is unusual that more than 50% of a living community consists of organisms with bio-mineralised skeletons (Brenchley & Harper, 1998), with soft bodied organisms such as algae, and many others at the base of the food chain lacking sufficient preservation potential, therefore skewing reconstructed trophic models with their absence. Amongst those fauna that have been preserved, many processes during subsequent geological events can affect the final assemblage – the most important of which being chemical dissolution (Brenchley & Harper, 1998). When considering small shelly fauna in marine environments, in most cases it is likely that preservation will favour older, more developed organisms, as these will often be of sufficient size and have grown the appropriate body parts to be preserved. Nevertheless, the sizes that are preserved can sometimes be indicative of the environmental conditions during the organisms’ lifetimes –the range of sizes in the environment can be used to determine whether there were any limiting controls on growth when comparing to datasets from control environments. When the situation arises that the environment in which communities are living changes, this can also affect the fossil record. Most likely to occur in nearshore marine environments, this involves the replacement of the living community which can be difficult to recognise in the fossil record, especially true of oxygen or salinity gradients as they are gradual changes (Fürsich, 1995).
Understanding taphonomy is key to the interpretation of macrofossils in the fossil record; with information loss as the norm (Fig. 1), a critical eye needs to be cast over any specimen or assemblage to analyse whether the information present hinders, or helps the reconstruction of ancient environments. Sometimes, the preservation of the organism can provide an insight into the environment of deposition, and also any diagenesis that has occurred during burial (Fig. 1f). This can be very useful when considering environmental reconstructions, as it gives a good indicator of what happened in an environment and how it has been altered over time.
Defined by Seilacher, et al. (1985), exceptionally well preserved fossils can be defined as a Lagerstätten; “bodies of rock unusually rich in palaeontological information, either in a qualitative or quantitative sense”. This can be shown either by in situ life assemblages, or soft-bodied faunas (Brenchley & Harper, 1998), which allow for the detailed study of the life mode of the organisms, allowing us to reconstruct how they lived, and therefore the type(s) of environment(s) that they lived in. The Wenlock Limestone (Fig. 2) shows characteristics of exceptional preservation, which allows for accurate palaeoreconstructions, as the macrofossils present allow the determination of the environment and some environmental conditions under which the organisms lived. A specific example of how exceptional preservation can be used to give information on how an organism lived is infaunal bivalves. These shelly organisms use a siphuncle to get food from the sediment surface, meaning the length of the siphuncle can give indications of how deep the bivalve was buried and therefore allows us to infer environmental controls such as sediment firmness and predation potential at the time the bivalve was living.
The Middle Cambrian Burgess Shale in Canada is a good example of exceptional preservation – more than 90% of the living fauna preserved was soft bodied, recording rarely preserved biota that allow for key observation and interpretation of predator-prey relationships, along with evolutionary relationships in the community (Brenchley & Harper, 1998). However from this, another cautionary note has to be added in that the assemblage is transported – when this is the case, not all of the fauna may originate from the same community, or even environment if the transportation distances are large. This can be where disarticulation and fragmentation becomes a problem, as it is not always possible to tell from simple observation whether fragments are from one, or many organisms (Fig. 1a-b). These barriers make it harder to reconstruct ancient environments due to the nature of the uncertainty.
Firmness of substrate, rates of sedimentation and palaeolatitude can all be determined from macrofossil assemblages (Brenchley & Harper, 1998), as well as general climate reconstructions. In a study by Aarnes, et al. (2012), plant macrofossils were found to give accurate temperatures that matched other climate proxies within the relevant uncertainty ranges. During cold periods, especially at high latitudes where the fossil record is lacking, climate reconstructions can be tricky. The study showed that these plant macrofossils in otherwise treeless periods found in lacustrine settings can be used for accurate climate reconstructions during this time period.
Ichnofossils
Ichnofossils present a whole new set of benefits and problems to environmental reconstructions. They are often associated with particular sedimentary facies, especially in the maine realm where the majority are defined (Fig. 3), adding ease to palaeoreconstructions. Due to the nature of their occurrence, they cannot be transported and reflect the exact environment in which the organism was living – this is of use when placing certain organisms in certain environments with the two most common trace fossils found falling into the Zoophycos and Ophiomora ichnofacies (Bottjer, et al., 1988), however terrestrial ichnofossils are assigned to the singular ichnofacies, Scoyenia. Being associated with particular facies can become a problem when identical structures are produced from a variety of organisms – this can lead to misinterpretation. This is why the study of ichnofossils needs to be coupled with sedimentology, which can provide a more accurate picture of where the organism lived. These traces are very useful to interpret the life mode of an organism – how it travelled, fed and/or rested, leading onto the major disadvantage of ichnofossils – the general lack of the organism that created the trace means that it can be extremely difficult to determine the identity of the trace maker for the ichnofossil that can be seen in the rock record. Nevertheless, the shape of the trace is not only a function of an organism’s activities and sedimentological conditions for preservation, but also its morphology (Seilacher, 1964), which can indicate the type of organism to create the trace. This can lead to a variety of innaccuraces, including;
• Different behaviours of individual organisms can result in different traces produced.
• A variety of organisms may produce identical traces (discussed above).
• A single structure may be produced by the movements of two or more organisms (Brenchley & Harper, 1998).
Boring traces, however, often have the boring organism preserved along with the trace, which reduces this major problem through the coupling of fossils types.
Features of the substrates of preservation may lead to innaccurate interpretations of the organisms that made the traces, with the main quality being the moisture content. Different preservation potentials within a substrate may lead to traces that look very different, when they are in fact a result of substrate quality rather than prints traces of numerous species. This, coupled with erosion may lead to an environmental interpreation that is too diverse. Nevertheless, trace fossils can provide a more accurate ecological reconstruction than macrofossils – the lack of the issue of limited preservation potential due to the composition of an organism, highest for mineralised skeletons, reducing the rarity of some fossils in a record constructed primarily from macrofossils.
Terrestrial trace fossils can provide information on specific behavioural features of individual organisms, for example tracks can show an eveven gait, which could be interpreted as being due to injury, with a special area of reseach developed to track dinosaurs (Lockley, 1991). Formulas have been derived to work out the speed of the dinosaur based on the traces that can be seen in the fossil record, with different stride lengths suggesting whether the dinosaur was walking or running. The traces can also aid our understanding of their behaviour individually and as a group, as well as contributing to the interpretation of the environments in which they lived through the trace sizes and shapes (Brenchley & Harper, 1998).
Ichnofossils can provide significant information on environmental conditions, even indicating factors such as the change in grain size of a sedimentary unit, and developmental patterns of organisms (Parcha & Pandey, 2011). In the study by Parcha & Pandey (2011), ichnogenera including Cruziana and Nereites (Fig. 3, 4) can be assigned to the different behaviours that they show, and with the lack of trilobites in this area, they prove to be extremely useful in dating the ‘Debsakhad Member’ (Fig. 4). The behavioural characteristics shown here range from suspension to deposit feeders, and this in itself can give us information as to the type of substrate present in the environment at this time, and can also allow for the reconstruction of relationships between the organisms of this community.
Palaeoenvironmental reconstructions have a heavy focus on ichnofacies (Seilacher, 1953), with 13 temporally and geographically consistent ichnofacies defined, and marine softground ichnofacies adding precision to reconstructions through their facies continuum along the depositional profile (MacEachern, et al., 2007).
To conclude, both macrofossils and ichnofossils provide vital information for reconstructing ancient environments; the nature of the reconstruction depends on what is best for the palaeoecologist to use. The use of ichnofossils is very popular for understanding the sedimentary environment that the trace was constructed in as well as giving extra behavioural information of the organisms such as how they fed, buried and rested, with traces grouped into particular facies depending on the behaviour that they showed as this can indicate ket environmental factors. Macrofossils however, can provide some extra pieces of information that ichnofossils cannot, with their size and shape confirming periods of unusual environmental conditions, for example lower than normal levels of salinity or storm reworking. Geochemical information from marine fossils can be used to provide information on the dominant mineral of preservation, and whether this had replaced the original skeleton/shell. However a fully integrated approach needs to be used with macrofossil analysis to generate the best possible interpretation when taking into account both the short comings and advantages of the information provided, especially through taphonomic processes.
Fossilised data can be used in conjunction with sedimentary data from the area, which will enhance the accuracy of the environmental reconstruction through interpretation of the environmental conditions at the time, and any changes that have taken place over time to preserve the fossils that can be seen in hand specimin today.
Trace fossils are not subjected to some of the inaccuracies such as preservation potential and reworking that are associated with macrofossils, and can therefore be determined to have a higher reliability as a data source. They are, of course, also limited in the information that they can provide, therefore when completing an environmental reconstruction, the best method of analysis involves using data from macrofossils and ichnofossils, along with any additional information that can be provided from sedementological and geochemical analysis.