Fossil arachnids

Dr David Penney (UK) and Dr Jason Dunlop (Germany) When it comes to fossils, arachnids are not a group that obviously springs to mind. However, with more than 100,000 described living species, Arachnida form the second most diverse group of primarily land-living organisms after the insects. And they probably made up a significant proportion of the Earth’s biodiversity in the past, just as they do in terrestrial ecosystems today. Despite this, arachnids have usually received only a cursory mention in palaeontology textbooks. In fairness, they are not as common as trilobites or brachiopods in the fossil record, and are usually found only under conditions of exceptional preservation. Yet, despite their rarity, we aim to show here that there are more fossil arachnids out there than is sometimes appreciated. What are arachnids? Arachnids are not insects and can easily be differentiated from them by the fact that they have eight legs and, in general, two principal parts to the body. Arachnids also lack both wings and antennae. In total, there are 16 arachnid orders (including four extinct) and all of them have a fossil record. Despite the advent of computer cladistic analysis and new molecular techniques, the relationships between the different arachnid orders continues to be debated and there is no universally accepted consensus regarding how they are all related to one another. Fig 1. Basic body plans of the fossil and extant arachnid orders. Note that the eye arrangement in Uraraneida is unknown.Arachnids as fossils Fossil arachnids date back more … Read More

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