X-rays bring extinct spiders back to life
5 August 2009, by Sara Coelho
Palaeontologists have brought an extinct group of spider-relatives back to life with a new 'look but don't touch' method which analyses fossils inside hard lumps of rock.
Digital visualizations of the trigonotarbid Eophrynus prestvicii
With this cutting-edge technique scientists can see tiny details of the critters' bodies without destroying the samples.
The method uses high-resolution X-ray images, taken by a machine similar to the CT-scanners employed in hospitals to detect tumours in patients.
As a part of his palaeontology PhD at Imperial College London, Russell Garwood used the Natural History Museum's CT-scanner to look at two species of spider-like animals from the trigonotarbid group.
'Trigonotarbids are extinct arachnids that lived more than 300 million years ago,' Garwood explains. 'They looked very similar to modern spiders, but they couldn't make silk and the back half of their body was split into small plates.'
'We can almost see them as they roamed the Earth over 300 million years ago.'
Russell Garwood, Imperial College
These critters are usually found within hard lumps of iron carbonate rock known as concretions. Until now, scientists had to rely on breaking the lumps in two to observe the fossil. As a result they could only see what was revealed on the crack, not the portion of the creature which went into the rock. 'We wanted to see if CT-scans could be an efficient alternative,' says Garwood.
To do this, Garwood and his colleagues focused on two trigonotarbid species, both about 2.5 cm long: the well-studied Eophrynus prestvicii known from exceptionally preserved fossils and Cryptomartus hindi, a member of a poorly-understood sub-group. The idea was to see if the new X-ray technique added to the body of knowledge of the first species or if it helped to solve the problems surrounding the second.
Garwood took over 3000 X-rays and combined the images in a three-dimensional model of the two fossils. The digital images and films, published today in the journal Biology Letters, reveal miniscule details as small as tiny claws and spines and many new morphological features.
Cryptomartus hindi (left) and Eophrynus prestvicii (right).
Thanks to the 3D-model, Garwood was able to identify for the first time defensive spikes in the well-known Eophrynus prestvicii. 'We also confirmed that their fangs face downwards and backwards, in a different orientation to modern spiders' fangs,' he adds.
In the Cryptomartus hindi fossil, Garwood discovered growths inside the legs and claws on the limbs - two features never seen before in this group of fossils. The former could be a hangover from an ancestor, which used the growths to process food, while the claws are similar to structures seen in rare, still living, arachnids that could be closely related. 'We could even tell that C. hindi had its limbs rotated slightly, allowing them to be held forward,' says Garwood. If this was a real life posture, then it hunted by ambush, he suggests.
'The amount of detail we can see in these images promises to revolutionise the study of iron carbonate concretion fossils,' says Garwood. This is important to understand life during the Carboniferous period (359-299 million years ago) - a crucial time, shortly after the evolution of land-based animals, he argues.
'And it's nice to see these in so much detail,' Garwood adds. 'It almost allows us to see them as they were when they roamed the Earth over 300 million years ago.'
Garwood's next step is to apply the method to other groups of fossils including cockroaches and millipedes.
Russell Garwood, Jason A. Dunlop and Mark D. Sutton. High-fidelity X-ray microtomography reconstruction of siderite-hosted Carboniferous arachnids. Biology Letters, published online 5 August 2009.
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