the petalodonts are an abundant, diverse and wide-spread order that was almost entirely marine.  despite their abundance, they are poorly undrestood because, with few exceptions, they are known only from isolated teeth.   The systematic position of the petalodonts is not determinable at the present time. 
Approximate age: 310-312 million years old.
Location of find: Peabody Coal Co. Marissa Mine, Washington County, Illinois