by Jason A. Dunlop*1 Introduction: Chasmataspidida (Fig 1) are rare, extinct arthropods known only from the early to mid Palaeozoic Era. They are probably closely related to either xiphosurans (horseshoe crabs; Fig 2) or eurypterids (sea scorpions; Fig 1); some chasmataspid fossils were originally misinterpreted as members of one of these two groups. They were first discovered in the 1950s, and were only recently recognized as a group distinct from horseshoe crabs. Chasmataspids are currently the oldest known examples of the Euchelicerata lineage —all Chelicerata except Pycnogonida (sea spiders) — and palaeontologists date the origins of these euchelicerates back to at least the late Cambrian Period. Morphology: Most chasmataspids are a few centimetres long. The body is divided int
by Ben Slater*1 Introduction: Coal swamps are the classical terrestrial (land-based) ecosystems of the Carboniferous and Permian periods. They are forests that grew during the Palaeozoic Era (encompassing the Carboniferous and Permian) in which the volume of plant biomass dying and being deposited in the ground was greater than the volume of clastic (grains of pre-existing rock) material, resulting in a build-up of peat. This was subsequently buried, and eventually turned into coal over geological time. These swamps gave rise to most of the major, industrial-grade coal reserves that are mined today. The palaeontology of these coal-forming ecosystems is well known from the Carboniferous rocks of Euramerica (modern day Europe and North America), owing to the history of coal exploitation ...
by Jason A. Dunlop*1 Introduction: Pycnogonida, or sea spiders, are not true spiders at all. They are in fact a group of — probably rather primitive — marine arthropods, characterized by a small, slender body and in many cases by correspondingly long legs (Fig. 1). So unusual is their morphology that many of their internal-organ systems have been displaced into the legs. Because of their strange appearance, older studies occasionally referred to them as ‘nobody crabs’ (literally crabs without a body) — although it is important to stress that they are not crustaceans, any more than they are spiders. Pycnogonids are thought either to have evolved right at the very base of the arthropod tree — and thus not to be closely related to any particular group of arthropods — or to be related to ara
by Jason A. Dunlop*1 Introduction: Chelicerata is one of the main divisions of the arthropods, and essentially consists of arachnids and their closest relatives. The name was coined in 1901 by the Berlin-based zoologist Richard Heymons (Fig. 1). It means the ‘claw-bearers’, in reference to the claw- or fang-shaped mouthparts that characterize the group. In addition to the arachnids, Chelicerata also includes the horseshoe crabs (Xiphosura), the extinct sea scorpions (Eurypterida) and little-known chasmatapaids (Chasmataspidida), and the sea spiders (Pycnogonida). The inclusion of sea spiders within this group is controversial, as we shall see below, and arachnids, horseshoe crabs, eurypterids and chasmataspids are sometimes grouped together as the Euchelicerata. The name Merostoma