Ancient harvestmen in 3D reveals early evolution

23 August 2011

3D models of two 305-million-year-old fossil harvestmen have been created and reveal that these arachnids were one of the first land-based animals to evolve modern bodies, according to a report today in the online journal Nature Communications.

Accurate 3D models of the daddy-long-legs, or harvestmen, were made using a CT (computed tomography) scanner at the Natural History Museum.

Image of one of the harvestmen (A. scolos) taken using the Museum's CT scanner.

Image of one of the harvestmen (Ameticos scolos) taken using the Museum's CT scanner. Slice images are used to produce an accurate 3D model.

The international team, led by scientists at Imperial College London, took over 3000 X-rays of the harvestmen fossils from France, dating to the Carboniferous Period, over 300 million years ago, before the time of the dinosaurs.

The scans were edited using computer software to produce highly detailed and accurate 3D models. They reveal 2 new species of ancient harvestmen, Macroglyon cronos and Ameticos scolos, which, unlike most land animals from this time, had bodies very similar to their modern relatives living today.

M. cronos had curvature at the ends of its long legs, a trait found in some current-day harvestmen who use it to grip vegetation allowing them to live just above the forest floor. Spikes found on the back of A. scolos, are used in living harvestmen for protection.

Drawing of how the harvestmen may have looked 300 million years ago

Drawing of how the 2 new species of harvestmen may have looked 300 million years ago (A. scolos at the top and M. cronus at the bottom)

Harvestmen bodies have changed very little over the last 300 million years, making them one of the first groups of animals on land to evolve modern body plans, the team says.

‘It is absolutely remarkable how little harvestmen have changed in appearance since before the dinosaurs,’ says Dr Russell Garwood, of the Natural History Museum’s computed tomography lab, who carried out the research while at the Department of Earth Science and Engineering at Imperial College London.  

‘If you went out into the garden and found one of these creatures today it would be like holding a little bit of prehistory in your hands.

‘We can’t yet be sure why harvestmen appear so modern when most land animals, including their cousins such as scorpions, were in such a primitive form at the time. It may be because they evolved early to be good at what they do, and their bodies did not need to change any further.’

This research is helping the team add detail to the harvestmen family tree, and supports previous studies suggesting that the 2 harvestmen groups to which they belong shared a common ancestor before 305 million years ago.

Rare fossils
Scan of harvestmen, M. cronus that lived 305 million years ago.

Scan of harvestmen, M. cronus that lived 305 million years ago. Its body and legs are extremely similar to modern living harvestmen

Harvestmen are one of the oldest groups of land arthropods (animals without backbones with exoskeletons and jointed legs) and some harvestman fossils found are from almost 400 million years ago. They are a very diverse group found on all continents except Antarctica.

However, their fossil record is sparse and it is very rare to find fossils because their relatively soft bodies are not preserved easily, and they live on land where conditions are rarely right to form fossils.

Harvestmen

Harvestmen belong to the scientific class Arachnida. Nearly all arachnids have 8 legs and 2 parts to their body. 

The second pair of legs on harvestmen is the longest and is packed with sensory organs that they use to feel around their environment.

Harvestmen are not spiders. Harvestmen and spiders are two of the 11 orders of animals belonging to Arachnida that also includes scorpions and ticks and mites.

In parts of the USA harvestmen are known as daddy-long-legs, the nickname given to craneflies in the UK (craneflies are members of the Diptera, the scientific name for the flies in all their forms).

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