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Fossil Focus: Chelicerata www.palaeontologyonline.com Title: Fossil Focus: Chelicerta Author(s): Jason A. Dunlop*1 Volume: 1 Article: 1 Page(s): 1-8 Published Date: 31/01/2011 PermaLink: http://www.palaeontologyonline.com/articles/2011/fossil-focus-chelicerata/ IMPORTANT Your use of the Palaeontology [online] archive indicates your acceptance of Palaeontology [online]'s Terms and Conditions of Use, available at http://www.palaeontologyonline.com/site-information/terms-and-conditions/. COPYRIGHT Palaeontology [online] (www.palaeontologyonline.com) publishes all work, unless otherwise stated, under the Creative Commons Attribution 3.0 Unported (CC BY 3.0) license. This license lets others distribute, remix, tweak, and build upon the published work, even commercially, as long as they credit Palaeontology[online] for the original creation. This is the most accommodating of licenses offered by Creative Commons and is recommended for maximum dissemination of published material. Further details are available at http://www.palaeontologyonline.com/site-information/copyright/. CITATION OF ARTICLE Please cite the following published work as: Dunlop, Jason A. 2011. Fossil Focus: Chelicerata. Palaeontology Online, Volume 1, Article 1, 1-8. Published by: Palaeontology [online] www.palaeontologyonline.com |Page 2 Fossil Focus: Chelicerata by Jason A. Dunlop*1 Introduction: sea scorpions) is also common in older Chelicerata is one of the main divisions of the publications, although most experts now feel arthropods, and essentially consists of that this is not a natural group, and that arachnids and their closest relatives. The eurypterids are probably more closely related name was coined in 1901 by the Berlin-based to arachnids. Currently, there are over zoologist Richard Heymons (Fig. 1). It means 100,000 living species of chelicerates, the the ‘claw-bearers’, in reference to the claw- or most diverse groups being mites and spiders. fang-shaped mouthparts that characterize the More than 2,000 fossil species, some of which group. In addition to the arachnids, go back to the late Cambrian period, have Chelicerata also includes the horseshoe crabs been recorded so far. (Xiphosura), the extinct sea scorpions (Eurypterida) and little-known chasmatapaids Morphology: (Chasmataspidida), and the sea spiders As noted above, chelicerates are defined by (Pycnogonida). their jaws. Whereas most modern arthropods FIGURE 1 - LEFT: RICHARD HEYMONS, WHO COINED THE TERM CHELICERATA. MIDDLE: AN EXAMPLE OF THE EXTINCT GROUP CHASMATASPIDIDA. RIGHT: AN EXAMPLE OF THE EURYPTERIDA. The inclusion of sea spiders within this group have chewing mouthparts called mandibles, is controversial, as we shall see below, and the jaws of chelicerates — the chelicerae, arachnids, horseshoe crabs, eurypterids and which give the group its name — are usually chasmataspids are sometimes grouped shaped like claws or pincers and are mostly together as the Euchelicerata. The name used for grasping and tearing up prey (Fig. 2). Merostomata (used for horseshoe crabs and In spiders, these chelicerae have been further Published on: 31/01/2011 | Published by: Palaeontology [online] | Authors: Jason A. Dunlop www.palaeontologyonline.com |Page 3 modified into fangs, which function more like walking legs. This body plan doesn’t quite a folding pocket knife. In sea spiders, the work for sea spiders, which — oddly — have chelicerae are sometimes called ‘chelifores’. an ‘extra’ pair of appendages. The sea spiders Chelicerates lack antennae, and recent work (Fig. 3) also cause problems for another has shown that the appendage pair that textbook character: the division of the body makes up the antennae of other arthropods is into two separate halves, a prosoma (or FIGURE 2 - THE CHELICERAE ON DIFFERENT GROUPS OF CHELICERATES (COLOURED IN RED). TOP IS SPIDER, BOTTOM LEFT IS A FOSSIL SCORPION SPECIES, AND BOTTOM RIGHT IS A HORSESHOE CRAB. A SECOND HEAD APPENDAGE, THE PEDIPALP, IS COLOURED IN BLUE FOR EACH. actually the same set of limbs that form the cephalothorax) and opisthosoma (or chelicerate mouthparts. Thus, insects feel abdomen). While this simple division of the with the first pair of limbs on their heads; body into two halves works for (most) chelicerates bite with them. arachnids, it does not really define Chelicerata as a whole. Further discussion of body plans Classic textbook accounts of the chelicerates can be found in the section on arachnids. usually refer to the presence of six pairs of limbs: chelicerae, pedipalps and four pairs of Published on: 31/01/2011 | Published by: Palaeontology [online] | Authors: Jason A. Dunlop www.palaeontologyonline.com |Page 4 Phylogeny: became accepted and Heymons offered the By the early 19th century, arachnids — as we name Chelicerata for the group. Sea spiders understand them today — were widely (despite their name) are not really spiders at recognized as a natural group (Fig. 4). The all, and only resemble them superficially. They FIGURE 3 - A MODERN MEMBER OF THE SEA SPIDERS (PYCNOGONIDA). position of the horseshoe crabs was more were later accepted as probably belonging to controversial. As their name implies, they Chelicerata too. were initially assumed to be crustaceans, The position of chelicerates, relative to other largely because they lived in water. Some early workers pointed out their arachnid-like arthropods, in the overall tree of life has features, but it was a classic paper entitled proved controversial. Essentially, three main hypotheses have been proposed. One idea ‘Limulus an arachnid’, written in 1881 by the London zoologist E. Ray Lankester, which that was once particularly favoured by convincingly showed where horseshoe crabs’ palaeontologists grouped together trilobites, crustaceans and chelicerates, calling them true affinities lay. Ill-tempered debate followed, but eventually Lankester’s ideas ‘Schizoramia’. This name effectively means Published on: 31/01/2011 | Published by: Palaeontology [online] | Authors: Jason A. Dunlop www.palaeontologyonline.com |Page 5 FIGURE 4 - EXAMPLES OF THE ARACHNIDS, FROM ERNST HAECKEL'S FAMOUS BOOK KUNSTFORMEN DER NATUR. ‘divided leg’, and refers to the branched limbs doesn’t really work for arachnids, but the last of crustaceans and trilobites. This feature leg of horseshoe crabs does show remnants of Published on: 31/01/2011 | Published by: Palaeontology [online] | Authors: Jason A. Dunlop www.palaeontologyonline.com |Page 6 this division. Some modern molecular studies the head region became more complex and have offered an alternative idea, in which structures like the mandibles and maxillae — chelicerates are most closely related to all of which are highly modified head limbs — millipedes and centipedes (Myriapoda). This appeared. combined group has been called ‘Myriochelata’ or, because the idea is so Tied to this debate are questions about the strange, ‘Paradoxopoda’. It remains origins of the chelicerates. For many years it was assumed that they were related to — or controversial since it is mostly supported by FIGURE 5 - A TRILOBITE (LEFT) AND HORSESHOE CRAB (RIGHT) WHICH ON THE BASIS OF THEIR SIMILAR APPEARANCE WERE TRADITIONALLY CONSIDERED RELATED. THIS IS LESS WIDELY SUPPORTED BY MODERN WORK. molecular data, and few convincing even evolved from — trilobites, since the anatomical features unique to chelicerates body shape of a horseshoe crab is superficially and myriapods have been uncovered. Perhaps quite similar to that of a trilobite (Fig 5). The the best-supported view sees chelicerates as fact that trilobites have antennae rather spoils primitive arthropods with a fairly simple this party; the troublesome sea spiders are ‘head’ region and pincer-shaped jaws also unwelcome gatecrashers, because they followed by numerous legs used for walking. do not resemble trilobites at all. In recent The more ‘advanced’ arthropods (including years, an alternative hypothesis has been trilobites) evolved antennae instead of claws; gaining ground. There is a group of enigmatic then, in crustaceans, myriapods and insects, Cambrian fossils that is often loosely referred Published on: 31/01/2011 | Published by: Palaeontology [online] | Authors: Jason A. Dunlop www.palaeontologyonline.com |Page 7 to as the ‘great appendage’ arthropods. These Lifestyle: creatures do not have antennae, but they do Given that arachnids make up the vast have a large pair of sometimes spiny limbs at majority of the modern Chelicerata species, the front of the body, which look as though most chelicerates can be classified today as they were adapted for grasping prey. terrestrial. As a group, they are found in all Conceivably, these ‘great appendages’ almost all habitats, even deserts, evolved into the more robust, claw-like mountaintops and arctic environments. They chelicerae; in this scenario, some of the ‘great have been on land since at least the Silurian appendage’ arthropods may turn out to be period, although a few spiders and certain the ancestors of arachnids and related forms. groups of mites have subsequently returned FIGURE 6 - LEANCHOILIA, AN EXAMPLE OF THE CAMBRIAN 'GREAT APPENDAGE' ARTHROPODS. IMAGE COURTESY OF DIEGO GARCIA-BELLIDO. Published on: 31/01/2011 | Published by: Palaeontology [online] | Authors: Jason A. Dunlop www.palaeontologyonline.com |Page 8 to the water. Horseshoe crabs and sea spiders onto their food and begin the digestion are found only in marine settings today, process in front of the mouth, sometimes in a although some fossil horseshoe crabs may special ‘preoral cavity’. Harvestmen have lived in freshwater
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