Peterborough's Jurassic Fossils

Alfred Nicholson Leeds recovered the remains of an enormous and deeply puzzling fish from the Oxford Clay brick pits around Peterborough.

Its bones were so unlike a conventional fish skeleton that early researchers struggled to understand what they were looking at. In 1889 Arthur Smith Woodward discussed material from Leeds's collection, and the name Leedsichthys problematicus was established for the extraordinary animal. The name was apt twice over: it honoured Leeds, while problematicus acknowledged just how difficult the fragmentary skeleton was to interpret.

Leedsichthys belonged to the pachycormids, an extinct group of ray-finned fishes. Unlike the great marine reptiles around it, it was not a giant predator. Its delicate gill rakers formed a filtering apparatus capable of straining small organisms from seawater. In ecological terms it occupied a niche more comparable with a whale shark or baleen whale than with a pliosaur.

So how big was it?

Size estimates have changed dramatically because no complete skeleton exists and much of the skeleton was poorly ossified. Older extrapolations produced spectacular estimates above twenty metres. Modern work has generally been more conservative. The Natural History Museum notes that the Peterborough tail suggests an animal around 16 metres long, while other specimens demonstrate that individuals varied greatly in size.

The important point is not a single headline number. Leedsichthys was unquestionably an enormous suspension-feeding bony fish, and some of the fossils most important to understanding it came directly from Peterborough's clay.

In 2001 students Marcus Wood and Matt Riley noticed bones projecting from a face at Star Pit, Whittlesey. Specialists recognised them as Leedsichthys. Surveying the layer revealed more bones spread across the cliff, suggesting a substantial associated individual rather than a few isolated fragments.

The excavation became a major undertaking. More than 10,000 tonnes of overburden had to be removed before fine excavation could begin. Thousands of fragile elements were recovered during successive seasons, producing a specimen nicknamed “Ariston” because the bones seemed to continue endlessly through the clay.

This modern excavation transformed understanding of Leedsichthys. It allowed researchers to compare associated parts of one animal, reassess older material and challenge some of the enormous size estimates that had grown up around the species.

It also produced a striking continuity: Star Pit was a place Alfred Leeds himself had collected fossils roughly ninety years earlier. Peterborough's nineteenth-century collecting tradition and twenty-first-century palaeontology met in the same clay face.