Nematology Monographs & Perspectives, 2011, Vol. 9, 19-25 Nematodes from the Palaeozoic Much continental area was covered by the sea during the Cambrian, which began 542 mya, and marine nematodes were probably wide- spread, feeding on bacteria, fungi and protozoa among decomposing or- ganic matter along the bottom of the sea or in brackish inlets. Predaceous nematodes, as well as those that established symbiotic associations with bottom-dwelling invertebrates, were probably also present. Taking the well known fauna of the Burgess Shale (530 mya) as an example of marine life occurring during that period, possible nematode habitats could have included the following: the gill chambers of the small, snail-like Opabinia regalis Walcott, 1912 or the tiny Marrella splendens Walcott, 1912 or among the body setae of the polychaete worm, Canadia spinosa Walcott, 1911. Nematodes could have even inhabited the burrows of the priapulid worms, Louisella pedunculata Walcott, 1911 and Ottora prolifica Walcott, 1911 (a portion of the gut of O. prolifica shows what could be a coiled nematode) (Whittington, 1985, see Fig. 4.21). Additional habitats could have been in the gut or on the body surface of slow-moving, bottom-feeding detritivores such as trilobites (Fig. 12), the gregarious small crustacean Canadaspis ovalis (Walcott, 1912), or the slug-like Amiskwia sagittiformis Walcott, 1911. Some nematodes may have developed in the egg nests of the above groups. Members of the widespread nematode family Monhysteridae are not only free-living in the sea, but also occur on the gills, under the body plates and in the body cavities of various crustaceans (Poinar et al., 2009). Some forms even appear to burrow into the shell of horseshoe crabs (Leibovitz & Lewbart, 2004), organisms that have a fossil record extending back to the Silurian. The diverse habitats found during the Cambrian (marine, brackish water, freshwater, terrestrial) probably led to nematode diversification and associations were established with other organisms. Certainly by the Ordovician, when the first terrestrial plants appeared, nematodes were exploring the land, and by the Late Silurian, they were forming symbiotic and parasitic associations with the earliest known terrestrial © Koninklijke Brill NV, Leiden, 2011 19 The evolutionary history of nematodes, Poinar Fig. 12. Nematodes could have lived between the segments or in the alimentary tract of Cambrian trilobites similar to these Devonian Phacops megalomanicus (Struve, 1982). invertebrates (arachnids, eurypterids and myriapods) (Jeram et al., 1990). Reproducing populations of Palaeonema phyticum Poinar, Kerp & Hass, 2008 in the stomatal chambers of the land plant, Aglaophyton major (Kidson & Lang, 1920) preserved in the Rhynie Chert are evidence that nematodes had formed trophic associations with plants by the Lower Devonian (396 mya) (Figs 13-16; Plate 1). These early plant parasites reproduced within substomatal cavities, intercellular spaces and cavities containing disrupted cortical tissue, as well as inside the cortex cells of A. major. The nematodes probably entered the stomatal openings much as present-day plant-parasitic nematodes enter the stomata of angiosperms. In order to obtain nourishment, Palaeonema used its buccal teeth to crush cortical plant cells. The environment around Palaeonema was quite harsh with a number of hot springs and geysers. Periodic floods resulted in hot mineral water inundating the minute Aglaophyton plants with their nematode fauna. Aside from these physical conditions, Palaeonema had to contend 20 Nematology Monographs & Perspectives.