Article by: J. D. Dixon
Edited by: Adam Manning and Harry T. Jones
Recently we were lucky enough to tag along to the Isle of Wight field course. In our first piece we explored Hanover Point, and in our second piece on this adventure, we explore what happened to a number of unfortunate dinosaurs.
Hypsilophodon foxii is a small-bodied, herbivorous ornithischian. It has been studied since the 19th century, when palaeontologists first recovered the dinosaur from the southwest Isle of Wight. The unit that yielded this species, a one-metre-thick layer in the Upper Wessex Formation (Early Cretaceous, Barremian, ~132-125 Ma), has since been termed the ‘Hypsilophodon Bed’. Hypsilophodon here have been recovered in large groups (with over 100 individuals excavated), and many are found near fully articulated, disturbed only by moderate burrowing.
The Wessex Formation is a sequence of laterally variable, alternating mudstones and sandstones. The Hypsilophodon Bed sandstones contain asymmetrical ripples, laminations, Beaconites (a burrow ichnofossil), fossilised wood, and climbing ripples. Sandstone in the upper succession preserves dinoturbation and Diplocraterion. The first mudstones are clastic and laminated with sandstone, but the most notable are those which fine upwards, terminate at a series of plant fossils, and exhibit mottling. Also those that are marked by calcrete nodules at the lower boundary and terminate with a transitional boundary at the overlying sandstone are significant.
The Hypsilophodon Bed’s plant material and Beaconites burrows imply this was a non-marine, herbaceous, oxygenated environment, yet the small-scale asymmetrical ripples were produced by flowing water. Therefore, it is thought that these sandstones were deposited during high-energy crevasse-splay events across a floodplain or distal of levees, with enough temporal exposure post-deposition for colonisation by living organisms (such as plants and small invertebrates).
The interbedded mudstones are floodplain alluvium deposits, formed proximal to infilled ponds. The mottled sediments are mature palaeosols (ancient soils) from a seasonally waterlogged area, with elevated areas subject to milder flooding forming redder vertisols. The calcrete nodules most likely developed due to high evaporation rates in a warm climate, and have been used to estimate CO2 levels of ~ 560ppmV during the Early Barremian.
Overall, this sequence is interpreted as a riparian floodplain, susceptible to seasonal submergence. The sandstone of the upper succession then shows deepening of this system as dinoturbation implies terrestrial access, while Diplocraterion is a U-shaped marine ichnofossil found in epicontinental settings. Therefore, this layer marks a sea-level transgression of brackish water and development of a shallow shoreface ecosystem during the Cretaceous.
The range of individual body sizes, extensive articulation, preservation of tendons, and lack of pre-burial scavenging implies that the Hypsilophodon herds were killed quickly by their burial or buried shortly after death. The Hypsilophodon Bed is unlikely to show the accumulation of dead individuals at a discharge basin via fluvial transport as the remains are not accompanied by equal concentrations of other vertebrates and the exceptional skeletal preservation is consistent between individuals.
So how did the Hypsilophodon die?
Soft sediment deformation structures within the Hypsilophodon Bed have been used to suggest that the dinosaurs became mired in quicksand, dying due to starvation or exhaustion. Most mired animals do not sink deeply due to buoyancy, and there is often predilection for adults due to their increased weight meeting the miring threshold. Therefore, more adult Hypsilophodon would be expected, with their upper bodies exposed to scavenging and disarticulation, yet there is a range of individual body sizes preserved, with no preference to the posterior regions or any preservation of scavenger remains.
It is more likely that the Hypsilophodon Bed was deposited when significant amounts of water and sediment were transported during overbank floods, entombing the herds alive. Due to the Hypsilophodon Bed’s wide extent, and the accompanying sedimentological data discussed previously, it is suggested this unit accumulated across a floodplain. The classically assigned Hypsilophodon Bed includes two units, the lower mudstone and upper sandstone, meaning a minimum of two burials at distinct horizons and the lack of demineralisation/disarticulation imply these animals were buried almost immediately.
Reduction halos associated with some of the skeletons indicate decomposition post-burial, which produced locally anoxic conditions. In this scenario, a destructive flood buried the first herd of Hypsilophodon and deposited the lower mudstone. Then a distal river burst its banks, and sand-ridden sediment covered the floodplain while waterflow carved channels into the lower Hypsilophodon Bed. A period of calm ensued and allowed burrowing organisms to colonise the unit, which produced the Beaconites and enabled the growth of plant material. At a later stage, another flood from a distal levee coated the region during a crevasse-splay event that deposited the climbing ripple-filled sandstone of the upper Hypsilophodon Bed and preserved the second layer of skeletons. The lack of sedimentary breaks and lateral thinning of this sandstone implies this was a catastrophic, rapid, single event, with diminishing lateral water flow.
While research does exist on this enigmatic unit, there are limited synthesis papers for the Hypsilophodon Bed in particular. Instead, studies focus on the entire Wealden Group, so an outline of previous work along with contemporary research may be needed to finally understand the true nature of this site and what killed these unlucky dinosaurs.
 Hypsilophodon reconstructed at Dinosaur Isle. Image by J. D. Dixon.
 A sedimentary log of the stratigraphic succession we examined on our 2021 field course. Created by J. D. Dixon.
 Asymmetrical ripple marks seen in Bed 3. Image by J. D. Dixon.
 Beaconites burrows seen in Bed 3. Image by J. D. Dixon.
 Plant material seen in Bed 4. Image by J. D. Dixon.
Information References and Further Sources
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