amm oth s ▁and ▁m ast od ons ▁ro amed ▁the ▁local ▁wood lands . ▁ ▁Local ▁Native ▁Americans ▁interpreted ▁foss ils ▁as ▁the ▁remains ▁of ▁the ▁water ▁mon ster ▁Un kte hi ▁or ▁bur row ing ▁ser p ents ▁killed ▁by ▁the ▁th under bird s . ▁The ▁first ▁scientific ally ▁documented ▁local ▁foss ils ▁were ▁collected ▁during ▁the ▁Lewis ▁and ▁Clark ▁expedition . ▁Sh ip w orm - bor ed ▁pet r ified ▁wood ▁is ▁the ▁North ▁Dak ota ▁state ▁foss il . ▁ ▁Pre history ▁ ▁No ▁P rec amb rian ▁foss ils ▁are ▁known ▁from ▁North ▁Dak ota , ▁so ▁the ▁state ' s ▁foss il ▁record ▁does ▁not ▁begin ▁until ▁the ▁Pale oz o ic ▁era . ▁Lar ge ▁areas ▁of ▁North ▁Dak ota ▁were ▁under ▁the ▁sea ▁during ▁the ▁early ▁Pale oz o ic . ▁During ▁the ▁Sil ur ian ▁the ▁sea ▁briefly ▁with d rew ▁from ▁the ▁state , ▁although ▁it ▁was ▁quickly ▁re - in und ated . ▁Life ▁in ▁this ▁sea ▁included ▁bra chi op ods , ▁cor als , ▁fish es ▁and ▁m oll us cs . ▁Sea ▁levels ▁continued ▁to ▁fl uct uate ▁throughout ▁the ▁remainder ▁of ▁the ▁Pale oz o ic , ▁and ▁by ▁the ▁Perm ian ▁period ▁significant ▁areas ▁of ▁the ▁state ▁were ▁dry ▁land . ▁By ▁the ▁end ▁of ▁the ▁Pale oz o ic ▁the ▁state ▁was ▁position ed ▁near ▁the ▁equ ator . ▁ ▁During ▁the ▁Tri ass ic ▁much ▁of ▁the ▁state ▁was ▁covered ▁in ▁shall ow