The location:
The nearby shopping centre was built originally in the 1970's and more recently remodelled, opening again in 2017. As part of the refurbishment, decorative stone panels were added to columns near the entrance close to Bonn Square. For this EarthCache, you are asked to examine the fossils on display in some of these panels.
Oxford offers many possibilities for public transport but car parking is scarce and expensive in the city centre. Many buses stop just around the corner from this location, making it just a short walk.
The EarthCache lesson:
A fossil is any preserved remains, impression or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood and DNA remnants. This EarthCache examines fossils that were formed under the sea, by considering three types of marine creature that are commonly preserved in this way (acknowledgements to the British Geological Survey website https://www.bgs.ac.uk).
1) Ammonites lived during the periods of Earth history known as the Jurassic and Cretaceous. Together, these represent a time interval of about 140 million years. The Jurassic Period began about 201 million years ago and the Cretaceous Period ended about 66 million years ago. The ammonites became extinct at the end of the Cretaceous, at roughly the same time as the dinosaurs disappeared. However, we know a lot about them because they are commonly found as spiral fossils, formed when the remains or traces of the animal became buried by sediments that later solidified into rock.
Ammonites were marine animals belonging to the phylum Mollusca and the class Cephalopoda. They had a coiled external shell similar to that of the modern nautilus. In other living cephalopods, e.g. octopus, squid and cuttlefish, the shells are small and internal, or absent.
The size of fossil ammonites varies from that of a small coin up to 6 feet in diameter and they are one of the most recognisable types of fossil.
2) Echinoids have lived in the seas since the Late Ordovician, about 450 million years ago, which is about 220 million years before dinosaurs appeared. The remains and traces of these animals were buried in sediment that later hardened into rock, preserving them as fossils. The living representatives of echinoids are the familiar sea urchins that inhabit many shallow coastal waters of the world. Fossil echinoids closely resemble some living sea urchins, which helps us to understand how they must have lived.
3) Corals (or more formally, Zoantharia) are marine animals related sea anemones that lack a free-swimming (medusoid) stage. They have mobile larvae that become sessile (fixed to one place) after a few days.
The oldest known corals lived during the Cambrian, more than 500 million years ago, and are still found living today. Some, like octocorals (which have eight ‘arms’) are soft bodied and rarely preserved as fossils, but others secrete a hard, calcarous (made of calcium carbonate) skeleton and are thus important rock-forming organisms. Three groups are best known and are most frequently found as fossils: Rugosa, Tabulata and Scleractina.
Rugose corals are solitary or colonial types with bilateral symmetry. They have a hollow in the top surface (calice) in which the polyp sits, together with numerous tabulae, septa (the major ones being arranged in groups of four), dissepiments and, in some, a central calcareous rod (columella). The picture below shows the conical structure of the coral, together with a cross-section, which is the view often seen in fossil corals.
The EarthCache:
Stand facing the entrance to the shopping centre, with your back to Bonn Square. To the left-hand side of the entrance, three stone columns are visible, framing two shops. Go to the middle column and examine the lowest panel. This is made of a sedimentary material and contains numerous fossils. Examine these fossils and identify the one that is circular in shape, then answer the following questions. Your answers should be sent to the CO via the Geocaching message service or by email.
1) What is the diameter of the circular fossil you have found?
2) Based on the descriptions above, what type of creature is preserved here?
3) What other evidence can you see that indicates that the main fossil was formed in a marine environment?