Subphylum Myriapoda

The myriapods are a varied group comprising the millipedes, centipedes, symphylans and pauropods. They first appeared during the Mid Silurian, when Kampecaris-like forms were responsible for a variety of terrestrial trails (Box 14.6). Some of the largest forms, for example the giant Arthropleura, nearly 2 m long, hoovered their way through the lush, green vegetation of the Late Carboniferous forests.

Box 14.6 Invasion of the land

The myriapods were the first animals to colonize the land. Heather Wilson and Lyall Anderson (2004) have described the few Silurian and Devonian taxa from Scotland (Fig. 14.18). One of the oldest genera from the Middle Silurian, Cowiedesmus, is named after Cowie Harbour near Stonehaven, near Aberdeen and occurs together with Pneumodesmus, which shows clear evidence of a respiratory system. Cowiedesmus is so distinctive and different from other millipedes that it forms the basis for a new order, the Cowiedesmeda. These animals suggest that terrestrializa-tion amongst the arthropods had already begun by the Mid Silurian and millipedes were breathing and scuttling across the emerging new landscapes of the Caledonian mountain belt in Scotland. But there may be older indirect evidence. Trackways from Middle Ordovician rocks in the English Lake District (Johnson et al. 1994) suggests that arthropods were on land about 50 myr earlier, while trace fossils from Cambrian rocks in Ontario push arthropod life on land back even further into the Cambrian (MacNaughton et al. 2002), suggesting the presence then of large, amphibious euthycarcinoids. Euthycarcinoids are an enigmatic group of arthropods with antennae and mandibles, that have been placed in phylogeny somewhere near the origin of myriapods and insects.

Silurian Myriapod

Figure 14.18 The millipedes: (a) Archidesmus (Lower Devonian), (b) Cowiedesmus (Middle Silurian) and (c) Pneumodesmus (Middle Silurian), from Scotland. Scale bars, 2 mm. (Courtesy of Lyall Anderson.)

Figure 14.18 The millipedes: (a) Archidesmus (Lower Devonian), (b) Cowiedesmus (Middle Silurian) and (c) Pneumodesmus (Middle Silurian), from Scotland. Scale bars, 2 mm. (Courtesy of Lyall Anderson.)

—■— Origination —I— Extinction —*— Diversity

Figure 14.19 Ranges of selected insect orders. Geological period abbreviations are standard, running from Silurian (S) to Recent (R). (Based on Jarzembowski, E.A. & Ross, A.J. 1996. Geol. Soc. Spec. Publ. 102.)

—■— Origination —I— Extinction —*— Diversity

Figure 14.19 Ranges of selected insect orders. Geological period abbreviations are standard, running from Silurian (S) to Recent (R). (Based on Jarzembowski, E.A. & Ross, A.J. 1996. Geol. Soc. Spec. Publ. 102.)

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