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Description
these are the ammonites of Devonian and Mississippian Michigan. ammonites probably didnt evolve here, but it looks like they started colonizing the area sometime in the late Devonian period. the first to arrive were Bactrites (the straight shelled ancestor of ammonoids and coleoids, possibly an Anarcestid), Tornoceras (a typical goniatite) and Koenenites (an Anarcestid, the most primitive family).

during the early Mississippian period these guys really took off. the number of species jumps from 3-5 in the Devonian to 6-14 in the Mississippian, despite the end-Devonian mass extinction and the fact that the Michigan Basin had shrunken somewhat.

im not sure if goniatites like Gattendorfia, Imitioceras, and Beyrichoceras evolved from Tornoceras (the average ammonite species lasts only around 14mys) but goniatites like Kazakstania, Beyrichoceras and Munsteroceras seem novel in form. Merocanites was in fact a prolecantid, the family from which Mesozoic ammonites should be descended from.

Description
these are the ammonites of Devonian and Mississippian Michigan. ammonites probably didnt evolve here, but it looks like they started colonizing the area sometime in the late Devonian period. the first to arrive were Bactrites (the straight shelled ancestor of ammonoids and coleoids, possibly an Anarcestid), Tornoceras (a typical goniatite) and Koenenites (an Anarcestid, the most primitive family).

during the early Mississippian period these guys really took off. the number of species jumps from 3-5 in the Devonian to 6-14 in the Mississippian, despite the end-Devonian mass extinction and the fact that the Michigan Basin had shrunken somewhat.

im not sure if goniatites like Gattendorfia, Imitioceras, and Beyrichoceras evolved from Tornoceras (the average ammonite species lasts only around 14mys) but goniatites like Kazakstania, Beyrichoceras and Munsteroceras seem novel in form. Merocanites was in fact a prolecantid, the family from which Mesozoic ammonites should be descended from.