|mhorn||Дата: Wednesday, 08.06.2011, 20:52 | Сообщение # 1|
|Klug C. (2010) Konnten Ammoniten schwimmen? // Fossilien. Bd.27. Nr.2. S.83-91. |
Although their shells are highly abundant, the mode of life of ammonoids is still debated. Their ability to swim is here discussed in the light of their phylogenetic position, their anatomy as far as it is known, their habitat as reflected in their host-rocks, and in comparison with their extinct and living relatives. The reconstruction of the living ammonoid animal is also considered; when taking the phylogenetic neighbours into account, two possibilities for the number of arms appear reasonable: 90 as in Recent nautilids or ten as in the earliest coleoids and belemnites. The latter hypothesis is here preferred because nautilid embryos begin with ten arm buds which split into many arms later in embryogenesis. The ability to swim is here suggested to be likely for ammonoids because (1) ammonoid trace fossils documenting benthic locomotion are missing, (2) the shells of ammonoid hatchlings probably were already fully equipped to reach neutral buoyancy, (3) ammonoid remains are sometimes abundant in sediments deposited under anoxic conditions, (4) aperture orientation and shell shape of ammonoids are well adapted to horizontal swimming, and (5) high evolutionary rates combined with rapid dispersal of ammonoids after speciation or after extinction events are incompatible with a benthonic mode of life but well consistent with a nektoplanktic to nektonic, i.e. swimming, mode of life.
Middle Jurassic - Lower Cretaceous ammonites & aptychi