An updated classification of animal behaviour preserved in substrates

1. What they did

Fossil tracks, trails, burrows, borings and droppings are the only direct signs of how ancient animals moved, fed, hunted and lived. Since 1953, scientists who study these traces (ichnologists) have sorted them by the behaviour that made them, starting with five groups proposed by Seilacher: resting, dwelling, combined dwelling and feeding, locomotion, and grazing traces. Bromley (one of the authors) updated the scheme in 1996, but many more categories have been proposed since.

The authors:

• Reviewed every proposed category. They listed 34 behavioural categories proposed since 1953 and measured how often each is used by counting Google Scholar citations (Table 1).
• Brought in ideas from animal behaviour science. They adapted Tinbergen’s four classic questions about behaviour (its function, what controls it, how it develops during an animal’s life, and how it evolved) for fossil traces, and added a fifth: how was the trace made?
• Recommended how to use each category. Where a category described a rare or very specialised behaviour, they folded it into a broader category as a subcategory, to keep the total number small.
• Drew an updated diagram of the scheme, based on Bromley’s, with arrows showing how one behaviour can turn into another within a trace (Figure 1).

2. What they found

Thirteen categories recommended

• Farming traces (agrichnia): highly regular, patterned burrows, mainly in the deep sea, possibly used to grow microbes for food.
• Brooding traces (calichnia): structures built by adults to raise young, from insect brood chambers to vertebrate nests, including pits, mounds and birds’ nests.
• Chemosymbiotic traces (chemichnia): burrows of animals living with bacteria that need chemicals from deeper, oxygen-poor sediment.
• Resting traces (cubichnia): shallow hollows from short stays, such as resting, hiding or hibernating. These often show the outline and underside of the animal, so they give the best chance of identifying who made them. Landing and take-off traces of flying or leaping animals are included here.
• Digestion traces (digestichnia): droppings (coprolites), regurgitated pellets, gut contents and stomach stones (gastroliths). The authors note that some bird pellets, such as those of hawks and owls, have a recognisable shape. Because animals digest and excrete during almost any activity, this category links to all others.
• Dwelling traces (domichnia): permanent or semi-permanent burrows and borings, including aestivation chambers (aestivichnia), where earthworms, amphibians and lungfish wait out bad conditions.
• Moulting traces (ecdysichnia): traces left while moulting or shedding, including insect pupation chambers (pupichnia). The authors give the example of a deer rubbing newly grown antlers on a tree and scratching the bark.
• Attachment traces (fixichnia): marks on hard surfaces where an animal anchors itself for life.
• Feeding traces (fodinichnia): burrows of animals eating their way through sediment, including wood borings (xylichnia) and insect larval mines in leaves, bark and wood.
• Escape traces (fugichnia): made by animals digging up after sudden burial. Failed escapes (taphichnia) are included, because the behaviour is the same whether or not the animal survives.
• Grazing traces (pascichnia): meandering or spiral trails of animals feeding as they move.
• Predation traces (praedichnia): drill holes and chipped shells, and gnawing and biting marks on bones (mordichnia). Scavenging is included here, because it is often impossible to tell whether bone marks were made during an attack or long after death. Trapping traces (irretichnia), such as pitfalls and spider webs, are a subcategory.
• Locomotion traces (repichnia): all traces of moving, including trails, trackways of footprints (cursichnia), swimming traces (natichnia, navichnia) and wing marks of animals flying just above the ground.

Categories not recommended

• Above-ground structures (aedificichnia) and multipurpose social insect nests (polychresichnia), because they serve several purposes and fit better under their main one.
• Equilibration traces (equilibrichnia), where animals shift their burrows as sediment builds up or erodes. The underlying behaviour is dwelling or feeding, so the authors prefer “equilibrated dwelling” or “equilibrated feeding” traces.
• “Death struggle” traces (mortichnia), because they cannot be recognised without the body at the end, and many supposed examples from Solnhofen end in shed skins (exuviae) rather than corpses.
• Embedment structures in a host (impedichnia) and plant galls (cecidoichnia), which are not traces made by moving substrate; the authors suggest the neutral names impeditaxa and cecidotaxa.

Mixed behaviours are the rule

Most animals do more than one thing in their lives, and many traces record several behaviours at once, for example a grazing trail that includes resting hollows, or termite nests used for dwelling, farming and raising young. For practical reasons, each trace type is usually listed under its most distinctive behaviour.

3. What they concluded

• To keep the number of categories small, some newer categories are best treated as subcategories of well-established ones that reflect a broader behaviour.
• The updated diagram shares the strengths of Bromley’s 1996 version, showing transitions between behaviours to help classify and interpret traces.
• After more than 60 years, Seilacher’s behavioural classification is still the framework palaeoichnologists work in, and it remains useful for thinking about traces, although not everything needs to be categorised.
• What is needed is a clearer connection between ideas from biology (such as Tinbergen’s) and ideas from palaeontology (such as Seilacher’s).

4. How tracking was or was not used

No tracking fieldwork was done. The paper is about how to read animal behaviour from traces left in sand, mud, soil, wood, shell and rock, mainly fossil traces. It draws on modern traces for comparison, for example fiddler and ghost crab burrows, bird pellets (citing Elbroch and Marks 2001) and a deer’s antler rubs on a tree.

6. Other technologies, skills or innovations

• A fifth question added to Tinbergen’s four for anyone interpreting a trace: how was it made? This covers the overall shape, clues to how it was dug, marks left by the maker’s body (bioprint) and claws or other appendages (bioglyphs), and whether a burrow wall is lined.
• A worked example of applying all five questions to a fossil nest of the dinosaur Troodon.
• Google Scholar citation counts as a simple measure of which proposed terms scientists actually use.