Carboniferous Curios: the Alveley Footprints

In 1914, now 111 years ago, thousands of fossil footprints from the Late Carboniferous were discovered by Doctor Frank Raw (Geology Department, University of Birmingham) at Butts Quarry in Alveley, southern Shropshire. By 1919, Raw had collected over 200 slabs of the red sandstone in which the footprints were preserved, now housed at the Lapworth Museum of Geology. This extensive assemblage of trace fossils provides a snapshot of one of the earliest tetrapod communities in the United Kingdom, despite a lack of corresponding body fossils, as well as insight into palaeoenvironmental conditions during the Carboniferous.

The fossil footprints from Alveley were made between 315 and 307 million years ago, during a time known as the Late Moscovian of the Carboniferous Period. At this time, the climate was tropical and the Midlands were covered by a humid, swampy rainforest and warm river floodplains. As animals wandered across the floodplain, their footprints were left as impressions in the mud, which were later covered by sand during flood events. This sand eventually became the footprint-bearing sandstone of the Alveley Member in the Salop Formation. As the sandstone represents a floodplain deposit that infilled the original footprints, the fossilised footprints are found as natural casts on the underside of the sandstone beds. Those at the base of the sandstone largely represent vertebrate tracks, whereas invertebrate trace fossils occur more abundantly on the rippled upper surfaces.

A range of different animals are represented by the Alveley footprints, with the vertebrate tracks being of particular importance for providing a record of early tetrapod (four-limbed vertebrates) communities. Although no body fossils from these animals have been found at Alveley, the various trackways indicate a diverse fauna of different amphibians, reptiles, and even reptile-like ancestors of modern mammals. Another comparable assemblage of tetrapod trackways was found in Hamstead in 1912 from slightly younger rocks, which contains a different faunal composition from Alveley and is consistent with an increasingly arid climate through the Late Carboniferous.

The reassembled Alveley sandstone slabs on display in the Lapworth Museum show two trackway types. The largest trackway is called Ichniotherium willsi and possibly represents the oldest known occurrence of Ichniotherium, though similarly aged tracks have been found in Germany. Ichniotherium tracks were likely made by heavily built tetrapods called diadectomorphs, which reached up to three metres in length and represent some of the earliest known herbivorous vertebrates. The smaller trackway across these slabs is known as Limnopus vagus, which was made by primitive amphibians called temnospondyls. 

The invertebrate trace fossils preserved at Alveley indicate a moderately diverse fauna dominated by arthropods, including crustaceans, arachnids, and other chelicerates. These are largely represented by repichnia, a trace fossil that indicates an organism’s locomotion, such as crawling across soft sediment. Another type of trackway known as Protichnites is noteworthy for preserving the path left by an invertebrate traversing a rippled surface. As the animal moved over the crest of the ripple, it created a medial groove towards this crest as it dragged its body along the ground, producing a trace indicative of a crustacean such as an isopod.

Besides exhibiting traces of prehistoric life, the Alveley sandstone preserves evidence of palaeoclimate. Such features include rain prints, indicative of periodic rainfall, and sun cracks, representing periods when the lakes dried out. Tetrapod communities and climate indicators from the Late Carboniferous provide evidence of a major environmental change around 307 million years ago, termed the ‘Carboniferous Rainforest Collapse’. This represented a transition from the moist environments of the Carboniferous to the drier, more desert-like ecosystems of the Permian, causing the forests to shrink and eventually disappear in the Midlands as plant species suffered a mass extinction. The onset of an arid climate further caused a decrease in tetrapod diversity, as the wetter environments amphibians require were greatly reduced, yet the surviving reptiles were able to travel more freely and subsequently dispersed worldwide. The trace fossils found at Alveley represent faunal communities that existed relatively shortly before this catastrophic transition, giving a fascinating and rare insight into life during the Late Carboniferous.