Complex sentence: The age of a mammoth can be roughly determined by counting the growth rings of its tusks when viewed in cross section, but this does not account for their early years, as these are represented by the tips of the tusks, which are usually worn away.
Split into simpler sentences: The age of a mammoth can be roughly determined by counting the growth rings of its tusks when viewed in cross section. Such ring counting does not account for mammoths'early years, however; mammoths' early growth is represented in the tips of the tusks, which are usually worn away.