Photo 1. An ichnofossil, about 50 million years old, from the Eocene in the sandstones of Lopar Peninsula (Photo by M. Randić)
WHAT ARE ICHNOFOSSILS?
Lopar Peninsula is made up of Eocene marlstone and sandstone, formed around the estuary of a primordial river, some 40 to 50 million years ago. Today in the Lopar rocks we can find numerous fossils, the petrified remains of ancient animals. However, in addition to the usual fossils of animals with hard shells or skeletons, such as foraminifera, snails and bivalves, we also come across ichnofossils, the remarkable trace fossils of animals.
Ichnofossils are the “petrified” traces left by animals as they bored, burrowed or dug into, crawled, sprinted or sprawled across, or fed in, soft substrates, such as mud or sand. The tunnels, cavities or imprints they made might later become filled with materials of different characteristics and, over time, could harden into solid rock. In this way, the traces of the activities of ancient animals remain visible in rock in the form of ichnofossils.
Ichnofossils vary in form, size and properties, depending on the species and habits of the animal that made the traces. Most often they are tubular forms, made by the burrowing of worm-like animals in loose and soft material. Tracks were often left by soft-bodied animals that had no solid parts, such as a shell or skeleton which are preserved more easily. Ichnofossils are, therefore, all that is left of the activities of those soft-bodies animals as the Earth’s crust evolved over geological time.
On a cliff in the region of Sturić Cove on Lopar Peninsula, interesting ichnofossils can be found in the form of spiralling tubes filled with lighter-coloured, hardened sand that is clearly visible against the darker mother rock (Photo 1).
Our guides along the geological trails of Rab Island were Ljerka Marjanac, Phd, Assistant Professor, and Prof. Tihomir Marjanac, Phd. They pointed out the interesting fact that these curved ichnofossils are probably the traces – fossilized remains – of holes dug by crabs into the sandy beach that was here in the distant past. Similar or identical crabs of the genus Callianassa can be found even today, digging deep spiralling tunnels on the sandy shores of Lopar, just like their predecessors did 50 million years ago. This makes the shores of Lopar Peninsula one of the rare places where we have the opportunity to observe “in person” what was happening so long ago in the Eocene.