Basal cheliceriformes such as Aglaspidida and Chasmatasp-ida show an intuitive primitive similarity to the Trilobites but also resemble the early chelicerates such as xiphosurans (horseshoe crabs). Along with the marellomorphs, these lineages were at one time considered as a group, called Trilo-bitomorpha, that has since been recognized to be artificial (e.g., Wills et al., 1998). Little is known of the extinct Aglaspi-dida and Chasmataspida and the best understood lineage is that of the Chelicerata. These groups are primitively similar to the xiphosurans but lack traits such as chelicerae (Briggs et al., 1979), so they are likely to be stem-group Chelicerata.

Chelicerata. Included within the chelicerates are the sea "spiders," horseshoe crabs, arachnids, and their extinct relatives. Chelicerates are united by the presence of visible ecdysial lines and the loss of inner rami on the trunk appendages (Wills et al., 1998). However, the most prominent, defining feature of the chelicerates is the presence of che-licerae, a trait from which they derive their name. Chelicerae are modified appendages of the first body segment. The che-licerae serve a variety of roles, principally in feeding, and form the familiar "fangs" of lineages such as the spiders. The presumed basal lineage of the Chelicerata is the Pycnogo-nida, or sea spiders, a group difficult to place phylogeneti-cally and sometimes excluded from the chelicerates. The pycnogonids (also known as Pantopoda, mostly applied for the clade of living species only) are considered basal to a clade consisting of the "true" chelicerates (Euchelicerata): Xiphosura, Eurypterida, and Arachnida. There are around 1,000 living species known of pycnogonids, most of which are predators although a few feed on algae. The opisthosoma of the pycnogonids is dramatically reduced, and there is a short proboscis preceding the chelate segment of the prosoma. Pycnogonids are recorded from as far back as the Cambrian, and most paleontological work on the group has been undertaken by Hedgepeth (1955a,b), Bergstrom et al. (1980), and Waloszek and Dunlop (2002). Euchelicerates are united by the presence of six pairs of prosomal appendages (including the chelicerae), a 12-segmented opisthosoma, and the presence of a post-anal telson (Selden and Dunlop, 1998).

xiphosura. The horseshoe crabs are one of the classic examples of evolutionary stasis and "living fossils." Xipho-surans are the only living lineage of marine euchelicerates, the five extant species being the sole survivors of a once greater radiation. Modern species of Limulus are remarkably similar to Paleozoic fossils and attest not only to the longevity of this group but also to the success of their design, having survived several cataclysmic extinctions throughout evolutionary history. The head, or prosoma, is covered by a large dorsal shield, with lateral compound eyes, while the opistho-soma is similarly covered by a large shield, although primitively segmented in one order (the paraphyletic "Synziphosu-rina"). Numerous fossil genera are recorded for Xiphosura, dating from as far back as the Ordovician. Ordovician through Devonian xiphosurans had a segmented opistho-soma and are likely a stem group to true Xiphosura (Anderson and Selden, 1997). The Xiphosura is considered to be the sister group to all other Euchelicerata (Boudreaux, 1979; Paulus, 1979; Weygoldt and Paulus, 1979; Weygoldt, 1980; Wills et al., 1995; Selden and Dunlop, 1998).

eurypterida. The extinct "sea scorpions" were large, amphibious chelicerates that superficially looked like elongate xiphosurans with a segmented opisthosoma; indeed, they were once classified with them into an artificial group called Merostomata (e.g., Woodward, 1865). The eurypterids were among the first arthropods to venture onto land, although they remained principally marine. The legs were frequently modified into paddles for swimming, while the chelicerae could at times be dramatically altered into elon gate grasping "arms," useful for capturing prey during aquatic chases. The telson of the body was distinctly flattened and formed a terminal spine or paddle. The eurypterids could be quite large, exceeding two meters in total body length, and were likely terrifying predators in coastal waters. Eurypterids could easily have preyed upon early vertebrate lineages (no wonder the vertebrates quickly moved onto land!). The group persisted from the Ordovician until the Permian and had a described diversity of about 300 species. Recent classificatory treatments of the Eurypterida include the works of Plotnick (1983) and Tollerton (1989).

arachnida. The arachnids are entirely terrestrial chelicer-ates (except for water mites, which secondarily returned to an aquatic lifestyle), and they ventured on to land sometime during the Silurian, perhaps the Ordovician. The group is almost universally predatory or parasitic, principally victimizing other arthropods, like insects. With over 80,000 described species, the arachnids are certainly the most successful lineage of the cheliceriformes. In terms of numbers of species, the spiders (Araneae) and the mites (Acari) dominate the Arachnida; they are also the most diverse ecologically. The arachnids consist of numerous orders: Opiliones (the harvestmen), Scorpiones (scorpions), Pseudoscorpionida (pseudoscorpions), Solifugida (wind "scorpions"), Palpigradi (palpigrades), Araneae (spiders), Amblypygida (whip "spiders"), Uropygida (vinegaroons), Schizomida (schizomids), Ricinulei (ricinuleids), and Acari (mites and ticks), in addition to a few extinct orders known from the Paleozoic, Phala-giotarbida, Haptopoda, and Trigonotarbida (Figure 3.7). Arachnid phylogeny has been most recently treated by Selden and Dunlop (1988), Shultz (1989, 1990), Dunlop (1999), and Dunlop and Webster (1999).

The scorpions consist of approximately 1,900 species and are among the most ancient arachnids, today occurring in tropical and warm temperate areas worldwide. Modern species range from 8 mm to 21 cm in length and live in everything from xeric to tropical habitats. Scorpions tend to be nocturnal, remaining concealed during the day in crevices or under stones. Prey is captured with their large, chelate pedi-palps and is usually stunned with venom from a sting at the end of a narrow, five-segmented, tail-like metasoma. The anterior segments of the opisthosoma ("mesosoma") are relative broad and flattened, bearing the four pairs of walking legs. Scorpions exhibit maternal care, and the young, which are born live, are often carried on the back of the adult for several instars (Figure 3.8). Interestingly, the integument of scorpions will fluoresce under ultraviolet light and the use of blacklights is a standard method of collection. In fossilized forms where remains of the integument are preserved, some fluorescence may occur even after hundreds of millions of years. Fossils of the order are known from as far back as the early Silurian (e.g., St0rmer, 1977) and could reach nearly a

3.7. Trigonotarbids (here: Architarbus rotundatus, from the Carboniferous of Illinois) were a diverse group of Paleozoic, terrestrial arachinds that superficially resembled large mites. Note the segmentation on the opisthosoma. YPM 00185; length of opisthosoma approx. 15 mm.

meter in length. Even though scorpions today are, like all arachnids, terrestrial, some forms from the Silurian-Carboniferous were aquatic (e.g., Rolfe and Beckett, 1984), with terrestrial species first appearing in the Devonian (Selden and Jeram, 1989; Walossek et al., 1990). The earliest true scorpions are Proscorpius osborni from the Silurian (Figure 3.9). Mesozoic fossils are restricted to a single Triassic

3.8. A Centruroides scorpion from Panama with its young on its back. Photo: P. J. DeVries.
Triasic Scorpion

3.9 (left). The earliest scorpion, Proscorpius osborni, from the Silurian of New York. AMNH; length 38 mm.

3.10 (right). An Early Cretaceous scorpion from Brazil's Santana Formation, approximately 120 myo. Morone Collection; 43 mm. Photo: R. Larimer.

3.9 (left). The earliest scorpion, Proscorpius osborni, from the Silurian of New York. AMNH; length 38 mm.

3.10 (right). An Early Cretaceous scorpion from Brazil's Santana Formation, approximately 120 myo. Morone Collection; 43 mm. Photo: R. Larimer.

record (Gall, 1971) and several in the Cretaceous (e.g., Campos, 1986; Ross, 1998; Lourengo, 2001, 2002; Grimaldi et al., 2002) (Figure 3.10). Scorpions are also represented in Tertiary resins (e.g., Lourengo and Weitschat, 2001; Weitschat and Wichard, 2002). Scorpion biology has been reviewed by Polis (1990), the world species cataloged by Fet et al. (2000), and a phylogenetic treatment provided by Stockwell (1989) and Prendini (2001).

Most spiders (Araneae) are immediately recognizable. There are about 35,000 species of living spiders, making them the most diverse of all arachnids in terms of described species. The order is ubiquitous and includes taxa with a wide range of biologies. All species are predatory; however, their biology ranges from solitary to group hunters, or even commensals and cleptoparasites. Some species even live in large, social colonies. Certainly the principal factor in the success of spiders is their silk, which is used for prey capture as well as to construct elaborate retreats and protective cases for their eggs. The body of spiders is composed of a well-divided prosoma (sometimes called the cephalothorax) and opistho-soma, the former bearing the walking legs and mouthparts, the latter region being robust and bearing on its ventral surface near the apex a set of spinnerets. The spinnerets are perhaps the hallmark trait of spiders, which was how a very fragmentary Devonian fossil was identified (Shear et al., 1989). The order is divided into two suborders: the Mesothe-lae and Opisthothelae, the latter being further divided into the infraorders Mygalomorphae and Araneomorphae (Coddington and Levi, 1991). Mesothele spiders are the most primitive, living members of the order and are generally large. The suborder is immediately notable for having a segmented opisthosoma; all other spiders have the segments indistin-guishably fused. Mygalomorph spiders include the familiar tarantulas, trap-door spiders, and other large, hairy taxa that do not spin aerial webs (Figure 3.11) and are largely tropical but that are also well known in xeric habitats. The silk is used to construct burrows, either in the soil or in wood, and generally extends from the opening for some distance and is used as an extension of the spider's sensory area, detecting prey that walks across the mat of silk. Araneomorph spiders include all other lineages, from the common garden spider, to crab (Figure 3.12) and ground spiders. Many, but certainly not all, araneomorph species spin orb webs - the familiar nets used for sieving the air for prey (Figure 3.13). The oldest evidence of spiders is Attercopus fimbriunguis (Shear et al., 1987;

3.11. Tarantulas and other mygalomorph spiders are hairy, massive spiders, many of which spin trip lines along the ground to detect prey passing by their burrows. Photo: Valerie Giles.
3.12. A crab spider consuming a moth. Some spiders are sit-and-wait predators, such as this thomisid. It is cryptic among the blossoms, allowing it to ambush wary pollinating insects. Photo: Valerie Giles.

Selden et al., 1991), considered the sister group to all other Araneae, and an unnamed spinneret (Shear et al., 1989), both from the Devonian of New York. The earliest representatives of Araneae proper are Carboniferous representatives of the Mesothelae (Selden, 1996). The first opisthothele spiders are known from the Triassic and are of the Mygalomorphae (Selden and Gall, 1992), while araneomorphs are first known from the Jurassic (Eskov, 1984; Eskov and Golovatch, 1986). Spiders were numerous in the Cretaceous and Tertiary (e.g., Wunderlich, 1986; Selden, 1990, 2001, 2002; Johnston, 1993), particularly in ambers from throughout these periods (e.g., Wunderlich, 1988, 2000; Eskov, 1992; Eskov and Wunderlich, 1994; Penney, 2000, 2001, 2002). Foelix (1982) provided the most detailed account of spider biology, while Coddington and Levi (1991) summarized the higher classification of the order.

The pseudoscorpions are minute, predatory arachnids with large, chelate pedipalps. There are about 2,500 species, found in leaf litter and moss or under stones or bark. Many species are phoretic on other arthropods, grasping with their pedipalps to "hitch" rides. Species exhibit subsocial behavior with an extended brood care and even build brood chambers with silk extruded from glands in their chelicerae. Unlike other arachnids, the pseudoscorpions and solfugids, their nearest relatives, lack a patellar segment in the leg (Shultz, 1990). The Pseudoscorpionida is well represented in Tertiary (e.g., Schawaller, 1982; Weitschat and Wichard, 2002) and Cretaceous ambers (e.g., Schawaller, 1991; Azar, 2000; Judson, 2000; Grimaldi et al., 2002). Remains of pseudoscor-

pions have been recovered from the Devonian of New York (Shear et al., 1989; Schawaller et al., 1991). Pseudoscorpion biology is reviewed by Weygoldt (1969).

Wind scorpions (Solfugida) are moderate-sized (7-70 mm) arachnids, which are remarkably swift runners that chase down prey (Figure 3.14). There are about 900 species known from xeric regions of the world except for Australia. Perhaps the most notable feature of the solfugids are their enormous, stout, chelate chelicerae. These tremendous "jaws" allow solfugids to shred their prey, which for the larger species can also include small vertebrates. The pedipalps are rather stout relative to the walking legs and serve a tactile function. The oldest representative of the order is Proto-solpuga carbonaria (Petrunkevitch, 1913; Selden and Shear, 1996); otherwise, solfugids are unknown until the Early Cretaceous (Selden, 1996) and Miocene Dominican amber.

The harvestmen (Opiliones) are spider-like arachnids, noted for their rather short, robust bodies and long, thin legs. The approximately 5,000 species occur throughout the world and range in size from less than a millimeter up to about 23 mm in body length, although their leg span can be several times this length. Species are omnivorous, but, unlike other arachnids, they digest solid food (the other orders pre-orally digest their prey and then consume the dissolved fluids). The prosoma is broadly fused to the opsithosoma, giving them the appearance of having a single body tagma. Fossils of

3.13. A black widow (Latrodectus: Theridiidae) perched in her web, in the Dominican Republic. The genus is renowned for the potency of its venom and cannibalism of the males by females. Photo: D. Grimaldi.

3.14. A wind scorpion, or solpugid, in southern Texas, with a roach in its chelicerae. These are swift ground predators. Photo: P. J. DeVries.

3.15. A whip scorpion, or amblypygid, in Panama. They are flat and live under large rocks, on the walls of caves, on tree trunks, and under loose bark. Photo: P. J. DeVries.

3.14. A wind scorpion, or solpugid, in southern Texas, with a roach in its chelicerae. These are swift ground predators. Photo: P. J. DeVries.

3.15. A whip scorpion, or amblypygid, in Panama. They are flat and live under large rocks, on the walls of caves, on tree trunks, and under loose bark. Photo: P. J. DeVries.

harvestmen are known from the Early Carboniferous (e.g., bodied, soil- or humus-dwelling arachnids with a modern

Petrunkevitch, 1913; Wood et al., 1985), but their diversity in diversity of about 125 species. They have a long, jointed fla-

the fossil record is best documented from Cretaceous and gellum at the apex of the opsithosoma and superficially

Cenozoic deposits (e.g., Jell and Duncan, 1986; Weitschat and resemble minute schizomids (discussed later). The only fossil

Wichard, 1998, 2002). palpigrade is Palaeokoenenia mordax from the Pliocene of

The palpigrades are minute (less than 3 mm long), soft- Arizona (Rowland and Sissom, 1980).

Schizomida Amber Dunlop
3.16. A rare amblypygid in Miocene amber from the Dominican Republic. Morone Collection, M0699; body length 9 mm.
3.17. Representative mites (Acari). Mites are the most diverse lineage of arachnids, of which there are vast numbers of undescribed species. Scanning electron micrographs; not to same scale.

Amblypygida, or whip spiders, are moderate to large (15-47 mm) arachnids with a flattened, rather circular body and long, thin legs held close to the substrate (Figure 3.15). Species live in stone crevices, caves, and hollow trees; under loose bark; and in leaf litter, principally in tropical environments. The front pair of legs are particularly elongate and are not used in locomotion but instead are used as "antennae," which they sway back and forth to detect prey. The pedipalps are enlarged and beset with numerous, stout spines that allow them to snare prey easily, which are then consumed using the chelicerae. Approximately 80 species are recognized today. Definitive fossil whip spiders are known from the Late Carboniferous of North America and Europe (Dunlop, 1994) but are mostly represented in Tertiary deposits (e.g., Schawaller, 1979) (Figure 3.16). However, fragments of a putative amblypygid have been recovered from the Devonian of New York (Shear et al., 1984). Weygoldt (1996) has provided the most comprehensive treatment of the order.

The Uropygida, commonly referred to as vinegaroons, are superficially similar to scorpions because of their large,

3.19. A mite in Early Cretaceous Lebanese amber. AMNH.

3.19. A mite in Early Cretaceous Lebanese amber. AMNH.

chelate pedipalps and their defense posture of raising their opsithosoma. Their common name refers to the spray of acetic acid (essentially vinegar), which they disperse from pygidial glands when disturbed. Species live in subterranean burrows and hunt small arthropods. Vinegaroons are known from as early as the mid-Carboniferous of Europe (Brauckmann and Koch, 1983) as well as the Early Cretaceous of Brazil (Dunlop, 1998) but are otherwise unknown from the fossil record.

The order Schizomida is overall rather similar to the vinegaroons but they are smaller (1.5-15 mm), and indeed are essentially "miniaturized" uropygids (e.g., Shultz, 1990; Selden and Dunlop, 1998). The oldest fossils are a single species from the Oligocene of China (Lin et al., 1988) and three from the Pliocene of Arizona (Petrunkevitch, 1945).

Ricinulei are small, blind, tick-like arachnids that occur in leaf litter and caves in the equatorial tropics of the Americas and Africa. The most remarkable trait for the order is the presence of a "hood" (cucullus), which hinges to the front of the prosoma and effectively covers the chelicerae. The fossil ricinuleids were revised by Selden (1992) who revealed a dramatic diversity in the Carboniferous, significantly greater than that today, but otherwise fossils of the order are unknown.

Second in diversity for the numbers of described (named) species of arachnids are the mites and ticks (Acari), with approximately 30,000 species known (Figure 3.17). The number of mite species will eventually far exceed that of spiders owing to the remarkable number of undescribed species from virtually every habitat. Most mites are minute and, like

Myanmar Amber Spider
3.20. A spined mite in amber from Myanmar. AMNH Bu342; length 1.0 mm.

the spiders, have taken over a dramatic range of environments, including 5,000 species that are aquatic. Mites are particularly abundant in soil and organic debris (such as the forest floor), where the number of individuals can easily outnumber all other arthropods. Many mites are ectoparasitic on both vertebrate (e.g., ticks) and invertebrate hosts, in some cases co-evolving with their hosts. In addition, the order includes scavengers and the only herbivorous arachnids, some of which can be quite damaging to crops. Like the ricinuleids (their closest, extant relatives) the Acari have a

3.21. An argasid ("soft") tick, Carios jerseyi, in Late Cretaceous amber from New Jersey. It probably fed on birds or feathered dinosaurs. AMNH NJ8; length 520 |xm.

distinct gnathosoma that bears the chelicerae and pedipalps. Despite the assertions of van der Hammen (1972, 1989), mites are considered to be monophyletic (e.g., Shultz, 1990) and of two basic lineages - the Anactinotrichida and the Actinotrichida, the latter including the ticks (Ixodida). Anactinotrichid mites are known from as early as the Rhynie chert of Scotland (Hirst, 1923) and other Devonian sites (Norton et al., 1988, 2002; Kethley et al., 1989). Mites are also common as fossils in both Cretaceous and Tertiary ambers (e.g., Sellnick, 1931; Azar, 2000; Rasnitsyn and Ross, 2000; Grimaldi et al., 2002; Weitschat and Wichard, 2002) (Figures 3.18, 3.19, 3.20), and ticks have also been found as far back as the Cretaceous (Klompen and Grimaldi, 2001; Grimaldi et al., 2002) (Figure 3.21). General references on the biology, ecology, and evolution of Acari include Krantz (1970), Woolley (1988), Schuster and Murphy (1991), Evans (1992), Houck (1994), and Walter and Proctor (1999).

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