Carboniferous orthocerid-like coleoid Mitorthoceras with unique shell structure (new order Tuborthocerida, Coleoidea, Cephalopoda); comparison with bactritid-like coleoid Shimanskya. (3rd April 2019)

• Record Type: Journal Article
• Authors: Mutvei, Harry; Mapes, Royal H.
• Is Part Of: GFF. Volume 141:Number 2(2019)
• Page Start: 121
• Page End: 132
• Publication Date: 2019-04-03
• Subjects: Carboniferous USA -- orthoconic shell -- Tuborthocerida -- tube layer -- Mitorthoceras -- Shimanskya; Geology -- Scandinavia -- Periodicals; Geology -- Periodicals; 554.8
• DOI: 10.1080/11035897.2019.1572218
• Languages: English
• ISSNs: 1103-5897

Abstract

ABSTRACT: The Carboniferous genus Mitorthoceras from USA, previously assigned to orthocerids, is here restudied and assigned to the new coleoid order Tuborthocerida. Mitorthoceras has several orthocerid characteristics: longiconic shell with long body chamber; central or subcentral siphuncle containing endosiphuncular deposits; and a tubular, uni-layered connecting ring probably consisting of mixed calcareous and chitinous elements. Coleoid characteristics are: shell wall rich in a chitinous material, and lack of a nacreous layer. Characteristic for the new order is the unique structure of the outer layer of the shell wall; this layer is composed of numerous, narrow, transverse furrows, separated by distally expanding ridges and closed from the exterior by thin calcareous layers, transforming the furrows into tube-like structures. The apical end of the shell forms an acute tip. A similar tube-like furrow-layer probably occurs within the shell wall of the bactritoid-like coleoid Shimanskya that has a smooth shell surface.