CHAPTER 26 – Material Constraints on Infaunal Lifestyles: May the Persistent and Strong Forces be with You

: Physics, materials sciences and mechanical engineering provide constraints that can be used to understand what infaunal animals do and did in the contexts of both making and destroying traces. As a primary example, on the temporal and spatial scales of burrow extension, many muds behave as linearly elastic solids that fail by cracking. These cracks are flattened, discoidal features that previously would not have been recognized as animal traces. Dominant guilds of deposit feeders in the oceans today are equipped with wedges, gills and feeding appendages to make, breathe in and feed in cracks. As a second example, feeding in different orientations with respect to gravity poses different problems in mixing and transport of digesta, and the varied solutions may leave different fossil signatures. Evolution of vertical bioturbation in mud required innovations in both crack making and digesta handling, suggesting that burrowing may have occurred first in sand, where burrowing at a shallow angle near the sediment–water interface is relatively easy and where detritus of high food value may have been easier to ingest and digest selectively.