The Ordovician marks the last great diversification period amongst the trilobites , very few entirely new patterns of organisation arose post-Ordovician ; later evolution in trilobites was largely a matter of variations upon the Ordovician themes .
The Ordovician marks the last great diversification period amongst the trilobites : very few entirely new patterns of organisation arose post-Ordovician . 
 Later evolution in trilobites was largely a matter of variations upon the Ordovician themes .