Page 393 - Introduction to Paleobiology and The Fossil Record

380 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

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.

(a)

(b)

(c)
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.)