CUBICHNIA REVISITED: WHEN 'RESTING' IS MORE THAN JUST 'RESTING'

Cubichnia (resting traces) are among the archetypical behavioral categories for traces erected by Dolf Seilacher in 1953. In his initial definition, 'resting' implied a temporary cessation of locomotion, and because of its linking with the trilobite trace fossil Rusophycus (which he considered as a variant of Cruziana), 'resting' became nearly synonymous with 'hiding,' presumably from predators. A more catholic application of 'resting,' especially when guided by modern examples of resting traces, reveals a myriad of behavioral motivations for why animals pause. To give an arthropod example from Georgia, some resting traces of the semi-terrestrial ghost crab Ocypode quadrata are related to predation avoidance, but others are formed as crabs rehydrate their gills after extended activity outside their burrows. Ghost crabs form such traces by settling into saturated sand and staying long enough to absorb pore waters, leaving ventral body impressions in intertidal sands. Juvenile limulids (Limulus polyphemus) also form 'resting' traces whose morphology owes as much to respiration as to hiding. Trilobites apparently did the same, as shown by the respiratory tubes of Rusophycus hartselleanus (Mississippian, Alabama). Vertebrate examples are also common. Some amphibians similarly rest in saturated sediments to retain body moisture, whereas some reptiles rest in warm sediments to absorb heat; fossil examples are rare but occur as well (Pennsylvanian, Alabama). Multiple, overlapping purposes for resting are exemplified by bison, which use wallows for transmitting their scent while also alleviating ectoparasites.

Equating 'resting' with 'hiding' from predators also can be flipped in meaning: Modern ambush-style predators, waiting motionless for their prey, also form cubichnia. With neoichnology thus serving as a guide, invertebrate and vertebrate trace fossils interpreted as cubichnia can take on new and nuanced meanings for an observer that extend well beyond mere 'resting.'