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-19^th 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.