|
''Hamites'' ("hook-like") is a genus of heteromorph ammonite that evolved late in the Aptian stage of the Early Cretaceous and lasted into the Cenomanian stage of the Late Cretaceous. The genus is almost certainly paraphyletic but remains in wide use as a "catch all" for heteromorph ammonites of the superfamily Turrilitaceae that do not neatly fit into the more derived groupings. In an attempt to identify clades within the genus, it has been divided up into a series of new genera or subgenera by different palaeontologists, including ''Eohamites'', ''Hamitella'', ''Helicohamites'', ''Lytohamites'', ''Planohamites'', ''Psilohamites'', and ''Sziveshamites''. The type species is ''Hamites attenuatus'' from the early Albian, named by James Sowerby in his ''Mineral Conchology of Great Britain'' of 1814, although the genus itself was created by James Parkinson in his 1811 book ''Organic Remains of the Former World''. This James Parkinson is best known as the first scientific description of a disease he called the ''Shaking Palsy'', now referred to as Parkinson's disease in his honour.〔 〕 ==Morphology and ecology== ''Hamites'' species are characterised by a shell that began with an open, sometimes helical, regular spiral that either opened into a single large hook (), or else formed three parallel shafts that gave the mature shell the approximate appearance of a paper clip (). No ''Hamites'' had spines or other such ornamentation on the shell, but several species appear to have developed apertural modifications when mature; that is, once the ammonite had grown to its final size, the aperture became constricted and was bounded by one or two thickened ribs, known as collars. These have been observed on other ammonites as well, and are assumed to be signs of sexual dimorphism.〔 The open shell of these ammonites would have made them poor swimmers because of drag, but beyond that fact, very little is certain about their mode of life. It is widely assumed that they were planktonic, perhaps catching small prey in the manner of jellyfish, but repaired shell damage apparently caused by crabs may indicate that they spent at least some time close to the sea floor. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hamites」の詳細全文を読む スポンサード リンク
|