▁living ▁along ▁the ▁coast ▁of ▁this ▁sea . ▁Later ▁in ▁the ▁C ret ace ous , ▁local ▁North ▁Dak ot an ▁verte br ates ▁left ▁behind ▁foss il ▁b ones ▁and ▁on ▁rare ▁occasions ▁impress ions ▁were ▁preserved ▁of ▁leaves ▁from ▁the ▁local ▁fl ora ▁in ▁the ▁Hell ▁Creek ▁Form ation . ▁These ▁were ▁also ▁preserved ▁in ▁the ▁area ▁now ▁occupied ▁by ▁the ▁banks ▁of ▁the ▁Missouri ▁and ▁Little ▁Missouri ▁rivers . ▁The ▁C ret ace ous ▁Se qu o ia ▁d ak ot ensis ▁left ▁foss il ▁con es ▁in ▁the ▁area ▁near ▁Mand an . ▁The ▁same ▁site ▁preserved ▁sections ▁of ▁pet r ified ▁wood , ▁some ▁up ▁to ▁ 5 0   l bs . ▁Some ▁of ▁the ▁pet r ified ▁wood ▁at ▁this ▁site ▁bear ▁the ▁bor ings ▁made ▁by ▁ancient ▁ship ▁w orm s . ▁Din osa urs ▁included ▁the ▁arm ored ▁Ed mont onia , ▁du ck - bil led ▁Ed mont osa urus , ▁o str ich ▁din osa urs , ▁p ach y ce phal osa urs , ▁Tr ic er at ops , ▁and ▁Ty ran n osa urus . ▁ ▁During ▁the ▁en su ing ▁Pale oc ene ▁epo ch , ▁cor als ▁and ▁sn ails ▁were ▁preserved ▁by ▁sed iments ▁now ▁known ▁as ▁the ▁C annon ball ▁Form ation , ▁however , ▁such ▁foss ils ▁are ▁very ▁rare . ▁F resh water ▁cl ams ▁and ▁sn ails ▁have ▁also ▁been ▁found ▁in ▁other ▁contemporary ▁depos its ▁in ▁the ▁state . ▁On ▁land