What Is an Ichnotaxon? Naming the Trace Instead of the Animal
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Footprints, burrows and borings get scientific names of their own, separate from the names of the animals that made them. That parallel naming system looks like duplication and is the only honest way to handle evidence that rarely identifies its own maker.
Why traces need their own names
A trace is the record of a behaviour rather than of a body, and the connection between the two is usually unavailable. A footprint identifies the shape of a foot and the way it moved and almost never identifies the species, since many animals have similar feet, since the same animal produces different prints on different substrates and at different speeds, and since the animal is not lying in the print. Assigning a track to a body fossil species would therefore assert something unsupported, so the convention is to name the trace on its own characteristics and keep the question of the maker separate. The consequences follow logically, since one animal can produce several differently named traces by walking, resting and burrowing, and several animals can produce traces sharing a name. Both facts look wrong until the purpose is clear, which is to name what is actually observed rather than what is inferred.
What the names describe
Traces are classified on features of the trace rather than on the maker's anatomy:
- •Shape and geometry, including whether a burrow is vertical, horizontal, branched, spiralled or U-shaped
- •Wall structure and lining, which records how the burrow was maintained
- •Fill, which distinguishes a burrow backfilled by the animal from one filled passively by sediment
- •Arrangement, since trackways record gait, speed and direction as well as foot shape
- •Behaviour represented, which groups traces by whether they record resting, feeding, dwelling, crawling or escape
- •Substrate relationship, since the same behaviour in firm ground, soupy mud or hard rock produces different structures
What traces are good for
The apparent weakness of the evidence conceals real advantages over body fossils. A trace is made in place and cannot be transported, so it records an environment directly rather than somewhere a carcass drifted to. It records behaviour, which body fossils do not, including locomotion, feeding strategy, social grouping and response to disturbance. It is produced during life rather than after death, so it samples living communities including soft-bodied animals that leave no other record, which is why burrows are the main evidence for whole groups of organisms. Traces are also abundant, since one animal makes many during a lifetime and only one body. And certain assemblages are reliable indicators of water depth and energy, which makes them useful to sedimentary geologists and, like microfossils, commercially valuable in interpreting subsurface rock.
What counts as a trace
The category is wider than footprints and the boundaries are argued over. Burrows, tubes and tunnels made by worms, arthropods and vertebrates are the largest group by volume and are the main subject of the field. Borings into shell, wood and rock by organisms that dissolve or rasp their way in form a separate category, since removing hard substrate is a different process from displacing sediment. Feeding traces include the scratches left by grazing on a surface and the distinctive patterns made by systematically working over an area. Coprolites, fossilised faeces, are usually included and sit awkwardly, since they are a product rather than a structure. Eggs and nests are sometimes included and sometimes treated separately. Bite marks on bone are traces of behaviour and are generally handled by a different literature. The unifying idea is that something was done rather than that something died, which is what the naming system is built to record.
Matching maker to trace
Connecting the two systems is possible occasionally and the cases are prized. The strongest evidence is an animal preserved at the end of its own trace, which happens rarely and settles the question for that instance, with several famous examples including arthropods at the end of their trails. Anatomical matching works where a foot is distinctive enough and a body fossil of appropriate size and age is available in the same rocks, which supports a probable rather than certain assignment and is how most dinosaur tracks are attributed to groups rather than species. Experimental work with living animals in controlled sediments establishes how substrate and speed alter a print, which is essential for interpreting variation and has corrected several mistaken identifications. The general rule is that assignment above the level of a broad group is speculative, and the parallel naming system exists precisely so that the trace record stays usable while the attribution stays uncertain.
The takeaway
Traces record behaviour rather than bodies and rarely identify their maker, so they are named on their own features and one animal can leave several differently named traces. They cannot be transported, so they record the environment directly, and they sample soft-bodied organisms that leave no other record. An animal preserved at the end of its own trail is the rare case that connects the two systems.