Fossils can include not only the actual preserved remnants of organisms from times long past (like shells and bones), and the replacements of such organisms (such as infilled moulds and casts), but also the evidence of their existence – for example their modified habitats, trails, burrows, tubes, marks and structures left by their feeding activities. Such evidence of long dead creatures is referred to as trace fossilisation; and the study of fossil traces is termed ichnology.
Part of the process of identifying the exact nature of trace fossils and reaching an understanding of their significance, involves making observations of the behaviour of related or similar present day organisms. This includes a study of their activities and the impact of them on their immediate environment in life. It also involves making records of the way in which the organisms change – what happens to them and their habitat after death (taphonomy).
Many of the trace fossils which I have discovered while walking along seashores and looking at cliffs, outcrops and boulders on beaches were made by various invertebrate seashore creatures in soft intertidal sediments that have later been buried, compressed, and hardened into rock. Recent marine worm tubes of various kinds are a fairly common occurrence on beaches and also as fossils in the sedimentary rocks of, for example, the Dorset coast in England. A short while ago I posted some photographs of fossil worm tubes that I found at Winspit.
For these reasons I was particularly interested to see the millions of sand grain tubes of marine polychaete worms on the low tide beach at Whiteford Sands on the north coast of the Gower Peninsula in South Wales. Many could be seen in situ, no doubt with worms in residence, projecting from the surface of the wet sand like some kind of stunted crop with seashells scattered among them. On the shallow bank created by a beach stream wending its way seawards, it was fascinating to see how far down into the sediments the tubes extended, a thick layer of unknown depth. It reminded me of a fantastic illustration in a book I recently acquired by Wilhelm Schäfer called Ecology and Palaeoecology of Marine Environments, originally published in Edinburgh by Oliver & Boyd in 1972. Copyright considerations mean the book is not old enough for me to reproduce the drawing here for you without written permission but Figure 190 on page 326 shows a cross-section through bedded sands and muds and shell deposits with many-branched and frequently extended dwelling tubes of Lanice conchilega cutting across the layers – just like the deposits on Whiteford Sands. Even though the book is old it is still widely available secondhand and is a wonderful repository of information.