Phosphate-shelled brachiopods differ in filter-feeding lifestyle, with Lingula anatina an active infaunal burrower, and Discinisca tenuis a shallow marine epibenthic animal. The shells of these biomineralized organisms are built of organophosphatic constituents, the organic fibres/sheets reinforced with calcium phosphate (Ca-phosphate) to provide a sophisticated ultrastructural robustness. This investigation studied the nature of organic fibres in order to improve our understanding of how design principles are relevant to biominerals. Unlike the powdered samples that were commonly used in previous studies, for the first time organic fibres were isolated and the shell fractions were purified, in order to study the content and nature of the biopolymer fibres. Ultrastructural analysis, thermal gravimetric analysis and spectroscopic analyses reveal that the core polysaccharide framework is composed of layers of β-chitin sheets that are coated with a fibrous protein matrix. The chitin matrix in Lingula shells, 26.6 wt.%, is about two-fold higher wt.% compared to Discinisca shells,12.9 wt.%. After thermal gravimetric analysis, skeletal imaging of these shells shows important differences. Discinisca contains just a single layer that is ~21.0 µm thick. In contrast, Lingula exhibits two separate layers: the thicknesses of the primary- and secondary-layers are ~20.3 µm and ~8.1 µm, respectively. Taken together, the data shows that the chitin matrix contributes to increased skeletal strength, making Lingula highly adapted for life as an active burrower. In comparison, Discinisca contains less chitin and lives as attached epibenthos in a shallow marine environment.