Complex sentence: Dune records show traces of burrowing animals as far back as the lower Mesozoic (250 Million years ago), although bioturbation in other sediments has been seen as far back as 550 Ma.
Split into simpler sentences: Dune records show traces of burrowing animals as far back as the lower Mesozoic (250 Million years ago). Bioturbation has been seen as far back as 550 Ma.