A 120-million-year-old mystery: what killed the Hypsilophodon?

The Early Cretaceous rocks on the southern coast of the Isle of Wight are famous for hosting fossils from a diverse range of dinosaurs, earning it the nickname of “Dinosaur Isle”. A particularly curious case can be found within a thin layer of rock on the southwest of the island, where the remains of tens of small, plant eating dinosaurs called Hypsilophodon have been found. However, why we find so many Hypsilophodon in this one horizon (known as the Hypsilophodon Bed) is not clear. Previous suggestions have focused on mass death events, suggesting herds were killed by floods or by getting stuck in mud or quicksand. Trying to unravel this 120-million-year-old puzzle has been the focus of one of my PhD thesis chapters.

To do this, I focused on two key areas. Firstly, I examined the sedimentary rocks of the Hypsilophodon Bed, as this can provide insight as to what the area was like when the dinosaurs died. After taking the ferry to the Isle of Wight, I recorded the rocks by taking photographs, samples and by logging the outcrop. By examining features like grain size, colour, microfossils and structures within the sediment, we can work out what conditions were like when the sediment was deposited.

Secondly, along with one of our former MSci students Josh Gunn, I investigated the state of Hypsilophodon fossils from the Hypsilophodon Bed, as this could provide clues as to what had happened to the dinosaurs from their time of death through to their burial and subsequent fossilisation. The Natural History Museum in London has over 50 Hypsilophodon specimens. By looking at things like the remnants of rock on the outside of fossils, how complete skeletons are, and how worn-down fossils are, we can make inferences about where in the Hypsilophodon Bed fossils came from, how quickly they were buried, and how much the dinosaurs’ remains were transported between their death and subsequent burial.

What we found is that the sedimentary rocks of the Hypsilophodon Bed were deposited as a river floodplain, which flooded repeatedly as the river broke its banks. Over time, rising sea levels meant the floodplain became waterlogged and marsh-like. Overall, Hypsilophodon fossils were distributed throughout the bed, suggesting not all the dinosaurs died at a single time or location. While some skeletons were relatively complete, most were very incomplete with many represented by isolated bones only. This incompleteness may be the result of carcasses lying on the floodplain for some time between death and burial, where decomposition and scavengers removed some of the bones. Alternatively, incompleteness may result from skeletons weathering slowly out of cliffs bone-by-bone – or a mix of both! While the exact cause of death is difficult to pinpoint, we can rule out some possibilities such as forest fires or volcanic eruption due to a lack of burned wood or ash. Flood related death and miring in mud are both possibilities, as are other causes such as disease or drought.

This research is currently published as a preprint from the Journal of the Geological Society, co-authored by Richard Butler, James Wheeley, Ian Boomer and Josh Gunn at the University of Birmingham; Susannah Maidment and Stephen Stukins at the Natural History Museum; Gary Nichols at the University of Portsmouth; and Catherine Russell at the University of Leicester and Louisiana State University.