Complex sentence: He strengthened the stone plate by adding pieces to its rim and on one of these he added a fake tail made from polyester resin as that of the fossil was largely lacking because he had failed to recover it completely.
Split into simpler sentences: He strengthened the stone plate by adding pieces to its rim. On one of these he added a fake tail made from polyester resin as that of the fossil was largely lacking because he had failed to recover it completely.