The nature of the arachnid fossil record
The origins of the arachnids during the Lower Palaeozoic remain uncertain, however, fossil arachnids are known from the Silurian. A growing number of arachnid orders are known from the Devonian, including spiders and scorpions, which demonstrates that arachnids had made the transition on to land and began to radiate contemporaneously with the insects.
Most extant arachnid orders had become established by the Upper Carboniferous.
Orders such as Triginotarbida, Phalangiotarbida and Haptopoda had become extinct by the Permian.
The Mesozoic fossil record is poor but some examples demonstrate that Mesozoic arachnids had already assumed a modern appearance and many Mesozoic species can be placed within extant families.
The Cenozoic record is dominated by fossil speciemens preserved in amber and spiders, mites, opilionids and pseudoscorpians appear to be the most dominant orders preserved.
(after Dunlop. J.A)
|Eotarbus jerami with segmented abdomen, 4 pairs of walking legs and long pedipalps|
The earliest known trigonotarbid is from the Silurian Ludlow Bone Bed.
The group itself ranges form the the Silurian to the early Permian and was most diverse in genera and species during the late Carboniferous. During this time the group occupied a place in some of the earliest terrestrial ecosystems such as that preserved in the Rhynie chert of Scotland. Trigonotarbids were also an important faunal component of coal swamp ecosystems.
Unlike spiders, trigonotarbids lacked spinnerets and displayed a segmented opisthosoma as can be seen from the above diagram.
The Kustarachnida may represent opilionoids where as the position of the orders Phalangiotarbida and Haoptopoda remains obscure, though Phalangiotarbida resembles opilioacarid mites and Haptopda may belong in the sub-order Tetrapulmonata
The Rhynie Chert
The Rhynie Chert of Scotland is early Devonian in age (400 and 412 million yeras old) and contains exceptionally preserved fossil plants and arthropods which have enabled palaeontologists to make reconstructions of these ancient terrestrial ecosystems.
To date, 7 groups of arthropods have been identified from the Rhynie Chert including arachnids such as trigonotarbids, mites and opiliones
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Author: Ceri-Wyn Thomas
Last updated: 22.11.05
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