View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002known from mid-Cenozoic Baltic and Dominicanambers (Poinar, 1996); they exhibit features, suchas the absence of foot portions with claws, whichshow a closer link to the Paleozoic forms than toextant taxa. There is no evidence to indicate thatthis evolutionarily conservative lineage was everdiverse; and it may have been trophicallyoutcompeted by the ecologically equivalent andpredaceous centipedes.4. Pentastomids. As structurally distinctiveendoparasites of tetrapod respiratory systems,pentastomids are a unique phylum withresemblances to lobopods, crustacean naupliuslarvae, and annelids (Brusca and Brusca, 1990).Although recent molecular-based studies indicatecrustacean affinities with a divergence date of 350to 225 Ma (Abele et al., 1989), an alternativeinterpretation that suggests an independent, higherrankingorigin is supported by the occurrence ofmarine pentastomids 510-Ma deposits (Gould,1995). Currently, their typical host is a terrestrialtetrapod, often a reptile, but they are also knownto make use of bird and mammal hosts. Somespecies display a larval stage that temporarilyparasitizes an intermediate vertebrate host. In theseinstances the intermediate host with its pentastomidparasites is, in turn, consumed by the ultimate host,a predaceous tetrapod, completing the cycle. Adultsattach themselves securely to the tissues of thelungs and nasal passageways of their host, suckingblood with a simple mouth and a powerfulpharyngeal pump. Given their current obligateparasitism of terrestrial tetrapods, three occurrencesof pentastomids in upper Cambrian and lowermostOrdovician marine deposits (Andres, 1989;Walossek and Müller, 1994; Walossek et al., 1994)are intriguing, particularly as there are nosubsequent fossil specimens. Considering theirmarine origins during the early Paleozoic and theirpresence among abundant conodont faunas, it hasbeen speculated that they were parasites in the gillchambers of marine chordates or basal vertebrates(Walossek and Müller, 1994). This association thencontinued through the terrestrialization of tetrapodsduring the late Paleozoic, and is manifested todayin their amniote descendants.6. Arthropods. The four arthropod subphylawith carnivorous representatives—Chelicerata,Myriapoda, Crustacea, and Hexapoda—occuramong the earliest of continental deposits, from theuppermost Silurian of northwestern Europe to theMiddle Devonian of northeastern North America(Labandeira et al., 1988; Shear and Selden, 2001).This pattern is quite unlike the terrestrial record ofthe other eleven phyla, which occur moresporadically later in the Paleozoic (Nemertinea,Mollusca, Onychophora), or are confined to the lateMesozoic or Cenozoic (Rotifera, Annelida,Nematoda, Tardigrada), or have a very poor or absentfossil record (Platyhelminthes, Acanthocephala,Nematomorpha, Pentastoma). One explanation forthis is the greater preservability of cursorial,megascopic, and structurally distinctive arthropodsthat bear a chitinous or otherwise mineralizedexoskeleton and could be buried in mid-Paleozoicparalic and continental basinal environments.Several factors, in addition to incumbency (Wilson,1992), indicate that arthropods have always beendiverse and abundant in marine and particularly incontinental ecosystems during the Phanerozoic(Brusca, 2000); and thus their taxonomic dominancein modern ecosystems has a longstanding history.The chelicerates, with the exception of themany detritivorous and parasitic mites, arepredaceous; one apomorphic feature that has arisenin the group is the specialized use of silk by spiders(Araneida) to trap prey. Chelicerates use a pair ofclaw-bearing mouthparts, the chelicerae, forpredation (Snodgrass, 1952). In spiders, thechelicerae are modified into horizontally orvertically oriented structures with terminallyarticulating fangs; ducts in these fangs connect topoison glands (Foelix, 1996). A second, posteriorpair of typically grasping mouthparts, thepedipalps, are used to manipulate prey inScorpionida (scorpions), Ambylypygi (taillesswhipscorpions), and Opiliones (harvestmen). Thecontinental fossil record of chelicerates (Størmer,1969; Selden, 1996) begins in the latest Silurian,and by the mid–Early Carboniferous six orders arerepresented, including spinneret-bearing spiders(Shear et al., 1989a, 1989b). From the middle to226