Not so Inferior Oolite

Some of the Jurassic Coast’s most spectacular fossils come out of the Inferior Oolite. In a previous post I talked about where the Inferior Oolite is found at Burton Bradstock on the Dorset coast. This layer of rock is found not only in southern England, where it is rather thin, but also west to Somerset and north to Yorkshire. This rock Group comprises up to about 100 m thickness of oolitic limestone, sandstone and mudstone deposited about 170 to 166 million years ago. The environment consisted of a shallow, warm sea with fringing reefs, similar to what we see in the Bahamas today.

The 3D fossil block above contains a variety of remains of creatures alive at the time the sediment was deposited, but only their hard skeletons remain. The most prominent is an ammonite called Parkinsonia bradstockensis. This is a common ammonite found across Europe, into North Africa and Eurasia, and because of the narrow window of time it is found in, it is used an an index fossil to characterise that geological layer. Parkinsonia was named in honour of the physician Dr James Parkinson, who lived from from 1755 to 1824 CE. Parkinson began his career as an influential political activist and was an advocate for representation of the people in the House of Commons, the institution of annual parliaments and universal suffrage. After being suspected of involvement in a plot to assassinate King George III, Parkinson turned to a quieter life as a medical doctor and researcher. He published on gout, appendicitis and first described and diagnosed what was latter to be called Parkinson’s disease. Parkinson gradually grew more interested in the natural sciences, specifically the emerging fields of palaeontology and geology. He was one of the first to write an introductory volume to palaeontology in English, called “Organic Remains of a Former World”, with two later volumes added. He was present at the inaugural meeting of the Geological Society of London and, like many early geologists, believed in Catastrophism.

Also present in the fossil block is an elongate clam (Pleuromya uniformis?). This clam was shaped to live in a deep burrow and to filter the seawater, and would have been mobile. There is also a high-spired snail (Pseudomelania bicarinata?). This snail would have lived on the surface of the sediment, eating fragments of detritus. Also present in the block is a belemnite (Belemnites canaliculatus?). This bullet-shaped fossil was part of a squid-like creature closely related to the modern cuttlefish, and would have been an active carnivore in open water.

What information can we derive about the environmental setting at the time that the sediment was deposited? Initially there seems to be two lines of conflicting evidence. Filter-feeding and bottom-feeding organisms are usually considered to require clear and relatively still water. However, here we have sand and broken shell fragments indicating there was a beach environment nearby. Clams and snails are actually quite resilient and can live on shallow low-energy beaches or tidal flats. Second, we have organisms that are generally considered to be relatively deep-water creatures (the ammonite and the belemnite). The likely explanation for this juxtaposition lies in the cross section at the end of the block. Various fossils are aligned in beds horizontally. Combined with the fact some fossils are damaged, the sediment was probably washed down from a shallower high-energy environment, by storms for example. Both ammonites and belemnites were fast-moving carnivores, and if similar to today’s nautilus, probably lived in deep (>100 m) water. How did the shells end up in shallow water? Like finding squid or cuttlefish on the beach today, when dead they probably filled with gas and were washed onto shore, to be combined with the shallow living shells once the soft parts had rotted away.

Fossils are notoriously difficult to photograph and the reasons for this are usually poor lighting, subtle details and because the fossil is represented in three dimensions. I decided to make a 3D model of my specimen of Inferior Oolite fossils to test the ability of photogrammetry to better represent specimens. Parkinson’s illustrations were drawn by hand and only available to a few, but now we have the opportunity to study and enjoy fossils in 3D glory from anywhere with an internet connection.


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