Trace fossil classification
Trace fossils are classified in various ways for different purposes. Traces can be classified taxonomically (by morphology), ethologically (by behavior), and toponomically, that is, according to their relationship to the surrounding sedimentary layers. Except in the rare cases where the original maker of a trace fossil can be identified with confidence, phylogenetic classification of trace fossils is an unreasonable proposition. Taxonomic classification. The taxonomic classification of trace fossils parallels the taxonomic classification of organisms under the International Code of Zoological Nomenclature. In trace fossil nomenclature a Latin binomial name is used, just as in animal and plant  taxonomy, with a genus and specific epithet. However, the binomial names are not linked to an organism, but rather just a trace fossil. This is due to the rarity of association between a trace fossil and a specific organism or group of organisms. Trace fossils are therefore included in an "ichnotaxon" separate from Linnaean taxonomy. When referring to trace fossils, the terms "ichnogenus" and "ichnospecies" parallel genus and species respectively. The most promising cases of phylogenetic classification are those in which similar trace fossils show details complex enough to deduce the makers, such as bryozoan borings, large trilobite trace fossils  such as "Cruziana", and vertebrate footprints. However, most trace fossils lack sufficiently complex details to allow such classification. Ethologic classification. The Seilacherian System. Adolf Seilacher was the first to propose a broadly accepted ethological basis for trace fossil classification. He recognized that most trace fossils are created by animals in one of five main behavioural activities, and named them accordingly: Other ethological classes. Since the inception of behavioural categorization, several other ethological classes have been suggested and accepted, as follows: Over the years several other behavioural groups have been proposed, but in general they have been quickly discarded by the  ichnological community. Some of the failed proposals are listed below, with a brief description. Fixichnia is perhaps the group with the most weight as a candidate for the next accepted ethological class, being not fully described by any of the eleven currently accepted categories. There is also potential for the three plant traces (cecidoichnia, corrosichnia and sphenoichnia) to gain recognition in coming years, with little attention having been paid to them since their proposal. Toponomic classification. Another way to classify trace fossils is to look at their relation to the sediment of origin. Martinsson has provided the most widely accepted  of such systems, identifying four distinct classes for traces to be separated in this regard: Other classifications have been proposed, but none stray far from the above. History. Early paleontologists originally classified many burrow fossils as the remains of marine algae, as is apparent in ichnogenera named with the "-phycus" suffix. Alfred Gabriel Nathorst and Joseph F. James both controversially challenged this incorrect classification, suggesting the reinterpretation of many "algae" as marine invertebrate trace fossils. Several attempts to classify trace fossils have been made throughout the history of paleontology. In 1844, Edward Hitchcock proposed two orders: "Apodichnites", including footless trails , and "Polypodichnites", including trails of organisms with more than four feet.