Nautilus and Ammonite

The Cephalopoda represent a monophyletic (including all descendants of a common ancestor) group of molluscs.
A simplified phylogenetic tree is presented at the end of the film. The Belemnoidea are excluded from this tree. They are assumed being the sistergroup of Decabrachia and Octopodiformes. When we present the tree of a monophyletic group, it is always important to ask for the next relatives. The Gastropoda might be the sister group of the Cephalopoda. This is not depicted in the film, but it would lead to the following interpretation: Cephalopoda and Gastropoda together form a monophyletic clade. They obviously are descendants of a common stem species.
Cephalopoda are characterized by a conspicuous head and tentacles, which evolved from the simple molluscan foot.
Contrary to the gastropods, the Cephalopoda are characterized by a bilateral symmetrical body.
As an old (plesiomorphic) character, basally branching cephalopods possess an external shell. As all molluscan shells, they are formed from the ectoderm (outer layer of the embryo).
Nautilus represents a "living fossil", a member of a widely extinct clade that "survived" until today and has no living very close relatives any more.
In the evolution of the cephalopod shells from ancestors similar to Nautilus on the stem line to the Ammonoidea, the complexity of internal and external shell-structures increased (stepwise as a result of evolution, but especially within the Ammonoidea). The suture-line of ammonites for example indicates this more complex morphology: It is the suture between septa and the wall of the shell. The shape of this suture is an important character with a systematic relevance.

Category:

Pets & Animals

Tags:

• Cephalopoda
• Kopffüßer
• Nautilus
• Natutilus pompilius
• Ammoniten
• ammonites
• Nautiloidea
• Ammonoidea
• Decabrachia
• Octopodiformes
• Gastropoda
• evolution
• monophyletic group
• plesiomorphy
• shell
• clade
• tentacles
• molluscan foot
• foot
• head
• living fossil
• suture-line
• Lobenlinie
• phylogeny
• systematics