"An endocochleate experiment" in the Silurian straight-shelled cephalopod Sphooceras

Abstract

Sphooceras truncatum (Barrande, 1860), a Silurian straight-shelled cephalopod with a short finger-shaped shell, is one of a few cephalopods in which natural truncation of the apical part of the phragmocone from the rest of the conch is confirmed. Periodic natural removal of the apical part of the shell (4 to 5 phragmocone chambers) preceded formation of a terminal callus and a calcareous plug closing the septal foramen. The apical callus probably originated by fusion of the truncation septum with episeptal deposits. These structures temporarily formed the new apex on which two additional calcareous layers had been secreted. A unique specimen preserves a colour pattern in the convex apical region, which proves that the shell in Sphooceras was temporarily completely surrounded by mantle extending from the body chamber, i.e. the cephalopod was at least temporarily endocochleate. The co-occurrence of different growth stages of S. truncatum together with one type of short juvenile orthoceracone shell, with a maximum of eight phragmocone chambers and a very small subglobular initial chamber indicates that these embryonic shells may belong to Sphooceras. Two other genera are discussed, both previously included in the family Sphooceratidae: Disjunctoceras Gnoli in Kiselev, 1992 and Andigenoceras Gnoli in Kiselev, 1992. The newly discovered thickening of the apex in "Disjunctoceras" disjunctum, the type species of Disjunctoceras, indicates that this species does not differ substantially from Sphooceras and should be reassigned to this genus. Similarly, representatives of Andigenoceras also possess characteristic features of Sphooceras. Sphooceras has many features characteristic for modern cephalopods: short, thin-walled, semi-internal shell; phragmocone reduced to only a few chambers; uncalcified connecting rings; apical callus (a structure analogous to the belemnite rostrum); retractor muscle scars situated dorsally; very small protoconch without cicatrix. In some exceptionally well-preserved cephalopods with orthoceracone shell radula with seven rows of teeth were observed. All these features support the thesis that some straight-shelled cephalopods are evolutionarily closer to coleoids than nautiloids and their separation from nautiloids is legitimate. Vascular imprints on the surface of the cameral deposits provide further support for their primary origin and the existence of a cameral mantle. The character of cameral deposits in Sphooceras demonstrates that the systematic value of these structures in other straight-shelled cephalopods, a subject of controversy, has limited value. The morphology of Sphooceras also demonstrates that the boundary between endocochleate and ectocochleate cephalopods is not sharp, i.e. internalisation of the shell in cephalopods occurred repeatedly.•

A revision of the Ordovician cephalopod Bactrites sandbergeri Barrande: Systematic position and palaeobiogeography of Bactroceras

“…In their view, Orthocerida now constitutes a monophyletic taxon and of the characters listed, the form of the embryonic shell is the most important for elucidating the systematics of the order Mapes, 2004, 2007;Evans, 2005;Turek, 2010;Turek and Manda, 2012).…”

Colour patterns on Silurian orthocerid and pseudorthocerid conchs from Gotland – palaeoecological implications

“…Shell colouration in the orthocerid Dawsonoceras annulatum Sowerby was described in a single specimen from Wales (Blake 1882) where the colour pattern is restricted to only part of the shell. Similarly, only a single species of Lituitina (= Palliocerida, Astrovioidea; group of externally shelled cephalopods with lamellar cameral deposits, which was previously included in Orthocerida and now is considered to be a distinct evolutionary lineage ;Dzik 1984;Zhuravleva & Doguzhaeva 2002) retaining shell colouration is known (Sphooceras truncatum Barrande;Turek & Manda 2012). The colour pattern on oncocerid and disosorid cephalopods from the Silurian of Gotland has been described by Stridsberg (1985) and Turek & Manda (2011).…”

The Additional External Shell Layers Indicative of “Endocochleate Experiments” in Some Ammonoids

“…The aberrant shell wall structures, characterized by extra layers on the surface of the standard shell wall, supposedly indicate the "endocochleate experiments". This expression was introduced by Turek and Manda (2012) and applied by these authors for the Silurian straight-shelled cephalopod Sphooceras truncatum (Barrande 1860). The relevant data on the umbilical membrane of the Devonian ammonoid Prolobites Karpinsky 1885 and the helicolateral deposits of the Carboniferous goniatite Clystoceras globosum Nassichuk 1967 are discussed as well as the wrinkle layer on the external shell surface of the goniatites Platygoniatites molaris Ruzhencev 1956 (Carboniferous) and Erfoudites rugosus Korn 2000 (Devonian).…”