View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002exaptation for life in a subaerial environment wherewater conservation and osmotic regulation of bodytissues were critically important. Almost all phylaof Ecdysozoa have terrestrial members, namely theNematoda, Nematomorpha, Tardigrada,Onychophora, Pentastoma, and Arthropoda, thelatter synonymous with the Euarthropoda of someworkers. The exception is the more removed andexclusively marine Kinorhyncha + Priapulidasubgroup which was more diverse in the Cambrianthan it is today (Conway Morris, 1998). In terms ofpresent speciosity, most of Ecdysozoan phyla arenot very diverse—there are approximately 90species of Onychophora, 110 species of Pentastoma,and 450 species of Tardigrada—particularlyconsidering that all three have their origins inCambrian marine ecosystems older than 500 Ma.These conservative, basal lineages to the Arthropodahave been interpreted by Gould (1995) as exhibitiingprolonged stasis. Alternatively, drastic habitatchanges in the history of these groups suggests thecontinuous evolution of a durable body plan: freelivingonychophorans and tardigrades have shiftedfrom the marine to the continental realm; parasiticpentastomes have undergone profound shifts in hostsfrom unspecified marine fish or perhaps arthropodsto terrestrial vertebrates (Walossek and Müller, 1994;Müller et al., 1995; Poinar, 1996). Interestingly, threeof these phyla were present in the Cambrian,occupying an epibenthic habitat (Müller et al., 1995);and together with the lophotrochozoanAcanthocephala, they subsequently producedlineages that became continental parasites orpredators. The Nematomorpha may have had asimilar history, but lack an adequate fossil record(Poinar, pers. comm., 2002).Two phyla, nematodes and arthropods, dominatecontinental ecosystems today in terms of abundanceand diversity. Both phyla have invaded everypossible continental habitat, including the interiorsand exteriors of virtually all other organisms. About15,000 species of nematodes have been described,although an estimated one million species probablyexist. Arthropods in particular represent the ultimatesuccess story of life on land in terms of the breadthof their feeding strategies, their numerical andtaxonomic dominance in ecosystems, and estimatesof biomass (Wheeler, 1990; Wilson, 1992; Brusca,2000). Arthropods comprise about one milliondescribed species (Brusca, 2000), and estimates ofthe number of undescribed species range fromseveral million to 80 million species (Erwin andScott, 1980; Gaston and Hudson, 1994).1. Nematodes. Of the twelve continentalinvertebrate phyla, the Nematoda, or roundworms,are second only to the Arthropoda in number ofcarnivorous taxa documented in the fossil record.Most specimens occur in amber from the EarlyCretaceous of Lebanon, the middle Eocene of theBaltic region, and the early Miocene of theDominican Republic (Poinar, 1977; Poinar et al.,1994a, 1994b; Poinar and Poinar, 1999). The earliestdescription of these organisms cites elongate bodieson appendage exocuticle of the large EarlyMississippian scorpion, Gigantoscorpio willsi(Størmer, 1963). These minuscule structures werepreserved in a process akin to permineralization andretain some micromorphology. However, thesestructures are probably not nematodes (Wills, 1993;Poinar et al., 1994a). The earliest credible occurrenceis from the Late Carboniferous (Schram, 1973), andis followed by a long absence of specimens fromthe Permian to Early Cretaceous. During the EarlyCretaceous, the nematode fossil record resumes withseveral amber occurrences (Poinar and Poinar, 1999)and a few reports from compression deposits (Voigt,1957; Dubinina, 1972). The Cretaceous andCenozoic occurrences provide examples ofparasitism on known arthropod hosts, principallyinsects, by members of two most speciosenematode clades: the order Tylenchida of the ClassSecernentea and the order Mermithida of the classAdenophorea (Poinar, 1984a, 1993). These twoclades of nematodes represent two of the fourindependent originations of continental arthropodparasitism by nematodes (Blaxter et al., 1998).Individuals are excellently preserved inconjunction with infected beetles and flies trappedin amber, whose ruptured bodies have allowedrelease of nematodes into adjacent resin (Schleeand Glöckner, 1978; Larsson, 1978; Poinar 1984a).Among the several recorded families of224