A new family, Mississaepiidae, from the Sepia–Spirula branch of decabrachian coleoids (Cephalopoda), is erected on the
basis of the following, recently revealed, morphological, ultrastructural and chemical traits of the cuttlebone in the late
Eocene Mississaepia, formerly referred to Belosaepiidae: (i) septa are semi−transparent, largely chitinous (as opposed to
all other recorded cephalopods having non−transparent aragonitic septa); (ii) septa have a thin lamello−fibrillar nacreous
covering (Sepia lacks nacre altogether, Spirula has fully lamello−fibrillar nacreous septa, ectochochleate cephalopods
have columnar nacre in septa); (iii) a siphonal tube is present in early ontogeny (similar to siphonal tube development of
the Danian Ceratisepia, and as opposed to complete lack of siphonal tube in Sepia and siphonal tube development
through its entire ontogeny in Spirula); (iv) the lamello−fibrillar nacreous ultrastructure of septal necks (similar to septal
necks in Spirula); (v) a sub−hemispherical protoconch (as opposed to the spherical protoconchs of the Danian Ceratisepia
and Recent Spirula); (vi) conotheca has ventro−lateral extension in early ontogenetic stages (as opposed to Sepia that has
no ventro−lateral extention of the conotheca and to Spirula that retains fully−developed phragmocone throughout its entire
ontogeny). Chitinous composition of septa in Mississaepia is deduced from (i) their visual similarity to the chitinous
semi−transparent flange of Sepia, (ii) angular and rounded outlines and straight compressive failures of the partial septa
and mural parts of septa similar to mechanically−damaged dry rigid chitinous flange of Sepia or a gladius of squid, and
(iii) organics consistent with −chitin preserved in the shell. The family Mississaepiidae may represent a unknown lineage
of the Sepia–Spirula branch of coleoids, a conotheca lacking a nacreous layer being a common trait of the shell of this
branch. However, Mississaepiidae is placed with reservation in Sepiida because of similarities between their gross shell
morphology (a cuttlebone type of shell) and inorganic−organic composition. In Mississaepia, as in Sepia, the shell con−
tains up to 6% of nitrogen by weight; phosphatised sheets within the dorsal shield may have been originally organic, like
similar structures in Sepia; accumulations of pyrite in peripheral zones of aragonitic spherulites and in−between the
spherulites of the dorsal shield may also indicate additional locations of organics in the shell of living animal.
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