出典:Wikipedia
出典:『Wikipedia』 (2011/02/13 20:32 UTC 版)
Arcestes is an extinct ammonite cephalopod genus belonging to the Ceratitida. Their shells were broad and rounded, giving them an almost spherical appearance. Unlike many other ammonites, the shells of Arcestes lack keels that would otherwise stabilize them while swimming. Because of this, some paleontologists have suggested that they were bottom-dwelling crawlers.