Ammonoids

Evolutionary history

Ammonoids are widely thought to have originated from straight-shelled (orthocone) "nautiloids" belong to Bactritida during the early Devonian (Emsian) around 409-408 million years ago, with transitional fossils showing the transition from a straight shell, to a curved (cyrtoconic) shell to a relaxed (gyroconic) spiral and finally to a tight spiral. The Late Devonian mass extinction (Kellwasser event) at the end of the Frasnian led to a dramatic decline in ammonoid diversity, with only a handful of lineages belong to Tornoceratina (a subgroup of Goniatites) surviving, becoming ancestral to all later ammonoids. Ammonoids rediversified during the following Famennian, which also saw the radical shift of the siphuncle from a lower (ventral) to upper (dorsal) position. Ammonites were nearly completely exterminated by the Hangenberg Event (end-Devonian mass extinction) at the end of the Devonian, with only a handful of lineages surviving, with one of the surviving goniatite lineages becoming ancestral to all post-early Carboniferous and later ammonoids. Ammonoids again rediversified during the Early Carboniferous. During the Carboniferous ammonoids underwent alternating periods of diversification and decline, and during the late Carboniferous ammonoid diversity became concentrated in a few geographical regions.