Trace fossils are a record of life; examples are footprints, bite marks, and burrows. The earliest trace fossils are thought to have been produced by an amoeba and are about 2,000 to 1,800 million years old. A special form of taxonomy (classification) has been created for trace fossils based on behaviour as it’s rare to find a body fossil preserved at the end of its trace and different organisms may produce identical tracks. Originally five behaviours were recognised but this number has now been expanded. (To find out more a good starting point is here).
In this blog we are looking at boring trace fossils. No, not yawn boring! I mean the types of holes made by different animals in shells for predation, shelter and attachment. These will definitely not make you yawn. We are going to explore the three types of trace fossil that can be found in Charles Lyell’s collection. These represent two different types of behaviour and therefore taxonomy: Praedichnia, defined as “trace fossils that show evidence of predatory behaviour, such as borings and bite marks” and Domichnia defined as “dwelling structures reflecting the life position of the organism that created it”.
Gastropods have been boring into shells in the same way for the past 100,000 years. There are two main families of predatory snails, the Naticidae and the Muricidae. They can be identified to family level by the shape of the hole they create. The predation attempts are deemed successful when the hole has fully penetrated the shell.
The Naticidae create holes with slanted walls by secreting acid and scraping with their radula (rasp-like structure of tiny teeth used by molluscs for feeding). They feed on bivalves, scaphopods (tusk shells), and gastropods (including other naticids). The size of the holes they create varies depending on the species. This family evolved in the Late Triassic/ Early Jurassic and occur worldwide.
The Muricidae create holes with straight walls by using a softening secretion and scraping with their radula. They feed on bivalves, barnacles, and gastropods. This family evolved in the Early Cretaceous.
Unlike the gastropod borings, sponges do not create their borings to feed. Instead they use the shells they have bored into for shelter. Cliona celata is a boring sponge which creates holes up to 5 mm in diameter in mollusc shells and limestone. They use acid to bore into their chosen home. If they bore into a shell the animal usually dies because it has lost its protection as it is structurally weakened.
In The Boring-Sponge, Cliona written by Joseph Leidy in 1889 he describes the way “in which [the sponge] occupies the shells of oysters and clams with its sensitive papillae [small fleshy bumps] … protruding from the perforations of the surface of the shell”. Sometimes the sponge can outgrow the shell its living in, using other material from around it. This massive sponge was familiar to the fisherman of Beach Haven New Jersey, USA “under the name of Bay-Pumpkin ; often growing to the size of one’s head.”
Bryozoans are filter feeding aquatic invertebrates commonly known as moss animals. Some bryozoans encrust surfaces. This encrustation can cause a pattern of small pits to be etched into the substrate. To see the pits you need a microscope as they are approximately 0.1-0.9mm in diameter.
Originally this group of trace fossils were called Leptichnus, from the Greek leptos meaning “flimsy, delicate, subtle”, however this was found to be the name of a gastropod species. To keep the original meaning of the name, the name Finichnus was proposed, from the Greek finos meaning “fine, delicate”.