million years ago, however, no track-making animals existed. Marine worms
burrowed into the bottom sediments or were attached to the seabed, living in
tubes composed of calcite or aragonite. The tubes were almost straight or
irregularly winding and attached to a solid object such as a rock, a shell, or
coral. Early forms of marine flatworms grew very large, up to several feet long.
The coelomic, or hollow-bodied, worms adapted to burrowing into the ocean
floor sediments and might have evolved into higher forms of animal life.
Sheetlike marine worms grew nearly 3 feet long but were less than a
tenth of an inch thick to provide a large surface area on which to absorb oxy-
gen and nutrients directly from seawater. Another reason for the unusual flat-Figure 33Stromatolite
beds from a cliff above the
Regal Mine, Gila
County, Arizona.
(Photo by A. F. Shride,
courtesy USGS)
Historical Geology