Original paper

Internal ventral keels in Triassic ceratid ammonoids: description and functional interpretation as muscle scars

Dagys, Algirdas S.; Keupp, Helmut

Kurzfassung

Zwei interne, durch die innere Prismenschicht gebildete, ventrale Schalenleisten beidseitig des Mediankieles auf Phragmokon und hinterem Wohnkammerabschnitt der Ammonoideen-Arten Czekanowskites tumaefactus und Arctohungarites involutus (Ceratitida, Czekanowskitidae) aus der Mitteltrias (Anisium) verschiedener Fundorte Arktisch Sibiriens (Umgebung der Laptev See) werden als Ansatzstellen des Trichtermuskelsystems gedeutet. Die durch die internen Leisten scheinbar tricarinate Ausbildung der Externseite von Steinkernen vergleichbarer Arten führte zu taxonomischen Fehlbeurteilungen. So wird Lenotropites? tricarinatus TOZER 1994 als Synonym zu Groenlandites kummeli TOZER 1994 erkannt.

Abstract

The Middle Triassic (Anisian) ammonoid species Czekanowskites tumaefactus and Arctohungarites involutus (both Ceratitida, Czekanowskitidae) were collected from several localities in the vicinity of the Laptev Sea (Arctic Siberia). The internal shell of the two species exhibits two parallel ventral ridges, running at short distance along both sides of the ventral, external, median keel. The internal ridges can be traced back into the phragmocone and terminate in the posterior third of the body chamber of adult specimens. Position and structure of the ridges suggest that they represent attachment scars reflecting part of the retractor muscle system. In other ammonoideans, e.g. of the Middle Jurassic Hecticoceras, usually a single ventral scar or ridge had been formed. Some taxonomic confusions result from the paired construction of internal ridges in adult C. tumaefactus and A. involutus due to the tricarinate ventral side of steinkerns.

Keywords

Ceratitida (Czekanowskitidae)shellsinternal ridgespaleobiologyfunctional morphologysystematicsAnisian. North Siberian PlainOlenek BayRussia.