In this story from the Jurassic Coast Collection, we take a deeper dive into the early Jurassic of East Devon and Dorset. It is no surprise really that this time period, one of significant environmental and ecological change, warrants a more detailed investigation of its place in the palaeontological story of our coastline. This time around we focus on ammonites – a favourite of many fossil collectors and something that everyone who visits the Jurassic Coast hopes to discover!
Ammonites are molluscs which means they belong to the same group of animals as snails, bivalves and squids. You will notice that they are remarkably similar to the modern day nautilus whose ancestors would have also inhabited the Jurassic seas. Ammonites have a coiled and chambered shell that was filled with water and air; by pumping water in and out they were able to adjust their buoyancy to float or sink within the water column. The soft parts of an ammonite are rarely preserved but scientists infer that they would have had tentacles, small round eyes and a beak for breaking apart prey. Ammonites swam backwards by propelling water from a hose-like structure in their bodies known as a siphon.
But have you ever considered the staggering diversity of ammonites found along the length of the coastline? They come in all different shapes and sizes! Well, this phenomenon is actually an important palaeontological feature and one of the reasons that the Site was given World Heritage status. Ammonites are really useful dating tools or ‘zonal fossils’ because they evolve and change over such short periods of time. By comparing different ammonite species found across the world, scientists are able to correlate rocks of the same age. The Jurassic Coast preserves a near-complete record of ammonite evolution throughout the Jurassic, as well as parts of the Cretaceous, and many important species were first found and described locally.
The rocks around Lyme Regis record the first significant diversification of ammonites in the early Jurassic – a crucial step towards their eventual domination of the Mesozoic. Only a million years after the Triassic-Jurassic mass extinction (the blink of an eye in the fossil record), ammonites had recovered to be one of the most abundant and diverse groups of marine creatures. The oldest ammonite fossils on the Jurassic Coast are known as Psiloceras planorbis – this smooth-shelled species is found at the base of the Jurassic, approximately 200 million years old, and are considered to be the ancestors to all of the ammonites that follow.
As ammonites adapt, recover, and evolve throughout the Blue Lias Formation, we see a number of new species introduced to the local fossil record including the large Coroniceras and Arietites that can be seen in loose limestone blocks on Monmouth Beach. At the Ammonite Graveyard, we can see evidence for a thriving ammonite population preserved in this unique death assemblage – hundreds, if not thousands, of Metophioceras ammonites are exposed for us to explore.
As water depths increased and became much less hospitable throughout the early Jurassic, the creatures that inhabited the seafloor became starved of oxygen, but ammonites in the aerated water column were able to thrive. In the mudrocks and shales east of Lyme Regis, near to Charmouth, we find ammonites preserved in hardened limestone nodules that formed around the fossil to protect it from destruction during fossilisation. These stones are highly sought after by collectors and, by looking at some of the specimens on show, I think you can see why! These fossils are generally replaced by calcite or pyrite, and the range of fantastic colours you can see is the result of tiny volumes of different trace elements and mud.
With the winter upon us, I cannot wait to see what new and exciting ammonite fossils are found along the length of the Jurassic Coast. One person that will be out there searching for his next big discovery is Kameran Sabbaghi. Kameran is part of an enthusiastic, passionate community who are leading the charge for a new generation of fossil collectors on the Jurassic Coast. The limitless potential for learning both on social media and in the field, coupled with a true intrigue and curiosity, means that Kameran has developed years’ worth of knowledge in a short space of time.