Abstract
BackgroundThe Subclass Coleoidea (Class Cephalopoda) accommodates the diverse present-day internally shelled cephalopod mollusks (Spirula, Sepia and octopuses, squids, Vampyroteuthis) and also extinct internally shelled cephalopods. Recent Spirula represents a unique coleoid retaining shell structures, a narrow marginal siphuncle and globular protoconch that signify the ancestry of the subclass Coleoidea from the Paleozoic subclass Bactritoidea. This hypothesis has been recently supported by newly recorded diverse bactritoid-like coleoids from the Carboniferous of the USA, but prior to this study no fossil cephalopod indicative of an endochochleate branch with an origin independent from subclass Bactritoidea has been reported.Methodology/Principal findingsTwo orthoconic conchs were recovered from the Early Eocene of Seymour Island at the tip of the Antarctic Peninsula, Antarctica. They have loosely mineralized organic-rich chitin-compatible microlaminated shell walls and broadly expanded central siphuncles. The morphological, ultrustructural and chemical data were determined and characterized through comparisons with extant and extinct taxa using Scanning Electron Microscopy/Energy Dispersive Spectrometry (SEM/EDS).Conclusions/SignificanceOur study presents the first evidence for an evolutionary lineage of internally shelled cephalopods with independent origin from Bactritoidea/Coleoidea, indicating convergent evolution with the subclass Coleoidea. A new subclass Paracoleoidea Doguzhaeva n. subcl. is established for accommodation of orthoconic cephalopods with the internal shell associated with a broadly expanded central siphuncle. Antarcticerida Doguzhaeva n. ord., Antarcticeratidae Doguzhaeva n. fam., Antarcticeras nordenskjoeldi Doguzhaeva n. gen., n. sp. are described within the subclass Paracoleoidea. The analysis of organic-rich shell preservation of A. nordenskjoeldi by use of SEM/EDS techniques revealed fossilization of hyposeptal cameral soft tissues. This suggests that a depositional environment favoring soft-tissue preservation was the factor enabling conservation of the weakly mineralized shell of A. nordenskjoeldi.
Highlights
The present-day shelled coleoid cephalopods, comprising the two genera Spirula and Sepia, have a long evolutionary history with indisputable earliest records of rostrum-bearing coleoids in the Early Carboniferous [1, 2]
This shell-wall type has been earlier known in the fossil spirulid genus Adygeya from the Early Cretaceous of north-western Caucasus [13] and is atypical for the externally shelled cephalopods, in which a nacreous layer forms the bulk of the shell-wall thickness [14,15,16,17,18,19,20]
The inorganic–organic shell-wall composition characterizes Recent Spirula, Sepia, and Eocene Mississaepia (Coleoidea), and is shown in A. nordenskjoeldi; these cephalopods are similar with respect to shell-wall material
Summary
The present-day shelled coleoid cephalopods, comprising the two genera Spirula and Sepia, have a long evolutionary history with indisputable earliest records of rostrum-bearing coleoids in the Early Carboniferous [1, 2]. Shimanskya demonstrates the evolutionary stability of the Spirula/Sepia shell-wall type through a period of about 330 million years Another Late Carboniferous coleoid genus, Donovaniconus (order Donovaniconida), retained a shell wall with nacreous layer, as in bactritoids, but secreted a rostrum-like sheath upon it and an ink sac, which is not the case with the bactritoids [4]. Recent Spirula represents a unique coleoid retaining shell structures, a narrow marginal siphuncle and globular protoconch that signify the ancestry of the subclass Coleoidea from the Paleozoic subclass Bactritoidea This hypothesis has been recently supported by newly recorded diverse bactritoid-like coleoids from the Carboniferous of the USA, but prior to this study no fossil cephalopod indicative of an endochochleate branch with an origin independent from subclass Bactritoidea has been reported
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
