▁se aw ater , ▁but ▁the ▁exposed ▁terrest rial ▁environments ▁were ▁very ▁hot ▁and ▁dry . ▁As ▁the ▁Tri ass ic ▁gave ▁way ▁to ▁the ▁Jur ass ic , ▁North ▁Dak ota ▁was ▁becoming ▁a ▁series ▁of ▁pla ins ▁covered ▁in ▁subt rop ical ▁for ests . ▁Event ually ▁these ▁for ests ▁were ▁also ▁sw allowed ▁again ▁by ▁the ▁sea . ▁This ▁sea ▁was ▁home ▁to ▁life ▁like ▁b ival ves , ▁e chin od erm s , ▁for amin ifer ans , ▁and ▁g ast rop ods . ▁Through out ▁the ▁C ret ace ous ▁North ▁Dak ota ▁was ▁either ▁fully ▁or ▁partially ▁sub mer ged ▁by ▁sea ▁water . ▁The ▁N io br ara ▁Form ation ▁of ▁the ▁P emb ina ▁Esc arp ment ▁and ▁Valley ▁City ▁region ▁records ▁the ▁presence ▁of ▁fish ▁during ▁the ▁C ret ace ous ▁with ▁foss ils ▁of ▁their ▁scales ▁and ▁b ones . ▁The ▁C ret ace ous ▁Pierre ▁Sh ale ▁can ▁like wise ▁be ▁found ▁in ▁the ▁P emb ina ▁Esc arp ment ▁and ▁Valley ▁City ▁area ▁as ▁well ▁as ▁the ▁banks ▁of ▁the ▁Missouri ▁and ▁Little ▁Missouri ▁rivers . ▁The ▁Pierre ▁Sh ale ▁pres erves ▁later ▁C ret ace ous ▁in verte br ates ▁like ▁am mon ites , ▁cl ams , ▁and ▁sn ails . ▁Contempor ary ▁fish es ▁also ▁left ▁foss ils ▁of ▁their ▁scales . ▁Mos asa urs ▁lived ▁in ▁the ▁Late ▁C ret ace ous ▁se as ▁of ▁North ▁Dak ota . ▁On ▁land , ▁din osa urs ▁were