GIANT CRUSTACEAN BURROWS: BERGMANN'S PRINCIPLE AND ITS ICHNOLOGICAL SIGNIFICANCE

Modern high- and low-latitude settings display predictable trends in climatic conditions, many of which exert controls on the distributions, behaviors, and sizes of trace-making organisms. Resolving this signal in the ancient record, however, remains elusive, owing to the complex interplay of numerous environmental parameters.

Cooler annual temperatures, seasonal severe storms, and harsh conditions in inshore settings characterize high-latitude settings. Shallow marine subtidal suites from icehouse and greenhouse periods are largely indistinguishable in shelf, offshore, and lower shoreface positions. Early Permian shoreface deposits from Australia accumulated in a high-latitude setting, probably similar to that of the present day Ross Ice Shelf. The succession records a period of very cold climatic conditions at the close of the Late Palaeozoic Gondwanan ice age. These Australian post-glacial Permian units do display, however, marked size increases of possibly crustacean-generated Diplocraterion and Rhizocorallium. Such observations are consistent with Bergmann's Principle of gigantism in marine crustaceans. This suggests that an increase in the geographic latitude and depth of crustacean's habitat (correlating mainly with lower temperatures) lead to an increased cell size, life span of the animal, and, as a result, an increase in the body size. Timofeev proposed a unified principle, according to which the size of the crustacean's body increases along a declining temperature gradient. Similar observations have been recorded in the Clearwater Formation in Alberta, Canada, deposited in the Arctic Boreal Sea during the Albian.

The ichnology of high-latitude marine settings reveals that: i) gigantism does occur, especially in crustaceans; ii) increased bioturbation intensities across the shelf; iii) the Cruziana Ichnofacies (Cenozoic) or mixed suites of the Cruziana and Zoophycos (Mesozoic) ichnofacies should be present across the shelf; iv) the absence of crustacean burrows in intertidal deposits; and v) rapid sediment processors (represented by Macaronichnus segregatis) are generally absent in polar upper shoreface or intertidal deposits.

Continued integration of neoichnological analyses with observations from the ancient record is essential for elucidating the high-latitude signal.