Actinoceras - Actinoceras

...The ancestors of the Nautilus appeared in the late Cambrian and differentiated in the Ordovician, and the species flourished in oceans around the world. They are thought to form a single phylogenetic tree with the differentiation of internal structures as the main direction of evolution. Looking at the large classification lineage of the Nautilus subclass, in addition to the foundational Ellesmeroceratida, there are the Endoceratida (endoceratida) and Actinoceratida (bead horn horn horn) which have shells with complex structures due to significant deposits in the tubules and chambers. The Orthoceratida (orthoceratida) also differentiated. ...

*Some of the terminology that mentions "Actinoceras" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…オウムガイ類の祖先はカンブリア紀後期に現れ，オルドビス紀に分化し，世界中の海で仲間が栄えたが，内部構造の分化をおもな進化方向とする単一系統樹をなすと考えられている。オウムガイ亜綱の大きい分類系統をみると，根幹をなすエレスメロケラス目Ellesmeroceratidaのほかに，連室細管や気房中に著しく沈殿物をつくり構造の複雑な殻をもったエンドケラス目(内角石類)Endoceratidaとアクチノケラス目(珠角石類)Actinoceridaがある。オルトケラス目(直角石類)Orthoceratidaも分化した。…

※「アクチノケラス」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報