Data from: Microstructural and geochemical evidence offers a solution to the cephalopod cameral deposits riddle

Orthoceratoid cephalopods are common in the Palaeozoic rock record but
became extinct in the Late Triassic. Many orthoceratoids contain cameral
deposits, which are enigmatic calcareous structures within their chambered
shell that presumably balanced their straight conchs in a horizontal
position. Since the mid-19th century, palaeontologists have attempted to
understand the cameral deposit formation process. The various hypotheses
include growth from cameral fluids, precipitation by a cameral mantle or
even their dismissal as post-mortem structures. All of these previous
interpretations have in common that they are complicated by contradictory
evidence. Here, we present evidence from well-preserved Trematoceras
elegans specimens from the Late Triassic St. Cassian Formation (Dolomites,
northern Italy). We studied the specimens by using optical and electron
beam microanalysis techniques and argue that the cameral deposits consist
of primary aragonite and calcite fabrics. A fibrous microstructure, which
is bilaterally symmetrically arranged with irregularities, is documented.
Thin organic sheets originally delimited radial growth sectors. Based on
these observations, we propose a new growth model that explicitly involves
the cameral sheets. These sheets acted as an extension of the pellicle and
held a thin film of supersaturated liquid in the otherwise emptied
chambers by the cameral sheets via the capillary effect. Ions were
supplied through the siphuncle, such as in living Nautilus, and enabled
the precipitation of aragonite and calcite fabrics. This model goes beyond
previous interpretations, resolves contradictory observations and has
functional implications, suggesting that cameral sheets and deposits were
an adaptation to increased growth rates.