▁years ▁ago ) ▁and ▁became ▁virt ually ▁ext inct ▁at ▁the ▁close ▁of ▁the ▁C ret ace ous ▁( 6 6 ▁My a ) ▁along ▁with ▁the ▁din osa urs . ▁The ▁classification ▁of ▁am m ono ids ▁is ▁based ▁in ▁part ▁on ▁the ▁orn ament ation ▁and ▁structure ▁of ▁the ▁sept a ▁compr ising ▁their ▁shell s ' ▁gas ▁ch amb ers . ▁ ▁Or ders ▁and ▁sub orders ▁▁ ▁The ▁Am m ono idea ▁can ▁be ▁divided ▁into ▁six ▁orders , ▁listed ▁here ▁starting ▁with ▁the ▁most ▁primitive ▁and ▁going ▁to ▁the ▁more ▁derived : ▁ ▁Ag oni at it ida , ▁Lower ▁Dev onian ▁- ▁Middle ▁Dev onian ▁ ▁C ly men i ida , ▁Upper ▁Dev onian ▁ ▁G oni at it ida , ▁Middle ▁Dev onian ▁- ▁Upper ▁Perm ian ▁ ▁Pro le can it ida , ▁Upper ▁Dev onian ▁- ▁Upper ▁Tri ass ic ▁ ▁Cer at it ida , ▁Upper ▁Perm ian ▁- ▁Upper ▁Tri ass ic ▁ ▁Am mon it ida , ▁Lower ▁Jur ass ic ▁- ▁Upper ▁C ret ace ous ▁▁ ▁In ▁some ▁class ifications , ▁these ▁are ▁left ▁as ▁sub orders , ▁included ▁in ▁only ▁three ▁orders : ▁G oni at it ida , ▁Cer at it ida , ▁and ▁Am mon it ida . ▁ ▁Tax onomy ▁of ▁the ▁Tre at ise ▁on ▁In verte br ate ▁Pale ont ology ▁ ▁The ▁Tre at ise ▁on ▁In verte br ate ▁Pale ont ology ▁( Part ▁L , ▁ 1 9 5