By Aodhán Ó Gogáin, Inventory Assistant, National Museum of Ireland  

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Fossils provide a record of the history of life and allow us to understand how organisms evolved and went extinct over time. On rare occasions, fossils can provide a glimpse at how different organisms interacted millions of years ago. In the collections of the National Museum of Ireland - Natural History (NMI-NH) there are several of these specimens that show a snapshot of two organisms interacting.  

Encrusters 

Animals that live in aquatic environments and have immobile lifestyles tend to encrust onto hard surfaces for stability. These hard surfaces generally consist of rock or compact sediment, but on occasion other animals can provide an ideal surface on which to encrust. This can be seen in Figure 1 where a colonial coral has attached and grown on top of a brachiopod (lamp shell). The brachiopod was likely alive while the coral grew on top of it. 

One of the most prolific encrusters are a group of small colonial animals called bryozoans, that use a fan-shaped appendage to filter feed. These are commonly found encrusting onto seaweed, rocks and boats and have been found in the fossil record hitching a ride on the shells of ammonites. Figure 2 shows a tooth of the shark Megalodon which has bryozoans encrusting on the root of the tooth. Once the shark shed its tooth, it made an ideal hard surface to support a colony of bryozoa. 

Feeding 

Feeding behaviours and predator–prey relationships can be interpreted from the fossil record with evidence coming from various sources, including fossilised stomach contents to bite marks on bones.  

Bivalves (clams, cockles, mussels) have a pair of valves that make up their hard shells that act in support and protection of the animal. However, some animals have adapted methods for breaking through these shells to feed on the juicy insides. One example is seen in gastropods (snails) which use a combination of acid, to dissolve the shell, and physical scraping of the shell to “drill” a small hole, leaving the interior of the bivalve vulnerable. These drill holes are commonly found in shells washed up on the beach, but are also found throughout the fossil record, as seen in the two specimens of Figure 3. Because the holes in both these specimens goes right through the shell, gastropod predation was the case of death for these bivalves.  

Some fossils also record evidence of feeding by detritivores (organisms that feed on decomposing organic matter). This can be seen in fossilised wood (Fig. 4) which has tunnels that were burrowed into it. We don’t know which animal made these traces, but it is highly likely that they were made by insects, because similar borings are made by modern insects in rotting wood.  

Visit the Natural History Museum on Merrion Square in Dublin, or visit virtually (https://www.museum.ie/en-IE/Museums/Natural-History/Visitor-Information/3D-Virtual-Visit), to see more fossils and to learn about the history of life!