Two Paleontology and Evolution students from the University of Bristol have undertaken the first ever study which describes the incredible range of fossils which were sucked into a neptunian dike, a deep, fissure-like cave on the ancient coastline.
These are long walls of mud and sand that cut into older rock that has been fractured, and do not usually contain fossils. Ancient Earth movements, probably connected with the initial unzipping of the Atlantic Ocean over 200 million years ago, caused long fault zones to form and mud with fossils was sucked into the fault because of the pressure.
Master’s students Oliver Weeks and Rebecca Cooper analyzed the fossil assemblage retrieved from a neptunian dike in the Mendip Hills, Somerset and found a mix of fossils of sharks and other fishes from the nearby ocean plus reptiles from the land. Usually, the fossils of land and sea are separate. Their findings are reported in a paper just published in Proceedings of the Geologists’ Association.
The Late Triassic seas, 200 million years ago around Bristol and in South Wales were full of life. These seas lapped on the shores of an archipelago of limestone islands, some of which were populated by dinosaurs, early mammals, and lizard-like sphenodontians. In the shallow waters offshore dwelled sharks, thick-scaled bony fishes, ichthyosaurs, and other marine reptiles.
But in the neptunian dikes that cut through the Mendip Hills here and there, the mix of strong faulting movements of the Earth’s crust, lubricated by the sea water, meant that sand, mud, and all the bones and teeth and other fossils scattered on the seabed were sucked into the dike zone along the fault.
Weeks and Cooper were students of the Paleontology and Evolution MSci at the University of Bristol’s School of Earth Sciences, and this was a summer project.
Together with academic staff, they investigated historical records, including detailed notes and collections made in the 1970s by posthumous author Charles Copp, and carried out excavations of one of the neptunian dikes at Holwell, on the south side of the Mendip Hills, at the time marking the coastline of the Mendip Island. They processed the rock and more than 3,000 identifiable fossils, mostly of fish species.
Weeks said, “The fossils showed close similarities to others found in the uppermost Triassic rocks around the Bristol area. This meant we could assign an age of about 202 million years ago, equivalent to the Westbury Formation of the surrounding area.
“We were excited when we started finding reptile bones alongside the fish teeth. To see this ecological story piece together before our eyes was fascinating. Comparing this site to other Rhaetian localities gives us some very interesting ecological insights, such as a separation of ecological communities between the north and south coasts of the Mendip Island.”
Cooper added, “During this time the area was submerged by a shallow tropical sea. The sea level was 50 meters higher than today, so what are now hills, such as the Mendips, were islands then.”
Professor Mike Benton, from the School of Earth Sciences, supervised the project. He said, “The marine reptiles include some bones and teeth of placodonts. These were bulky swimming reptiles that had broad teeth for crushing mollusks, so they lived quite close to the shore to feed on the oyster beds. But we didn’t expect to find a mix of marine and terrestrial fossils.”
Another Bristol supervisor, Dr. David Whiteside added, “The terrestrial fossils are jaws and other bones of sphenodontians. These would have looked like lizards, and they fed on insects and touched plant material. We didn’t find any complete skeletons, but somehow these bones of land dwellers got into the mix.”
Claudia Hildebrandt, Curator of Geology at the University, said, “We have to imagine a tropical island scene at the time. The whole of Europe was far further south than it is now, so Bristol was maybe at a latitude equivalent to North Africa, with dinosaurs and early mammals running about on the islands. It is always amazing when we get a rich fossil sample like this one to give a glimpse of what life was like then.”
Deborah Hutchinson of Bristol City Museum, where the collections of Charles Copp are curated, added, “We were delighted also to be able to honor Charles Copp, who did great work on the site a number of years ago. Charles was a huge enthusiast, and it was so sad he died young and couldn’t finish his work. Now, thanks to students Weeks and Cooper, we can see the job finished and honor his earlier work.”
More information: Oliver J. Weeks et al, Microvertebrates from a Rhaetian neptunian dyke at Holwell, Somerset: Dating the fissures, Proceedings of the Geologists’ Association (2025). DOI: 10.1016/j.pgeola.2025.101112
Provided by University of Bristol