• Agbaje, Oluwatoosin B. A.
• Brock, Glenn A.
• Zhang, Zhifei
• Holmer, Lars E.

Abstract

The shells of linguloid brachiopods such as <em>Lingula</em> and <em>Discinisca</em> are inorganic–organic nanocomposites with a mineral phase of calcium phosphate (Ca-phosphate). Collagen, the main extracellular matrix in Ca-phosphatic vertebrate skeletons, has not previously been clearly resolved at the molecular level in organophosphatic brachiopods. Here, modern and recently-alive linguliform brachiopod shells of <em>Lingula</em> and <em>Discinisca</em> have been studied by microRaman spectroscopy, Fourier transform infrared spectroscopy, field emission gun scanning electron microscopy, and thermal gravimetric analysis. For the first time, biomineralized collagen matrix and Ca-phosphate components were simultaneously identified, showing that the collagen matrix is an important moiety in organophosphatic brachiopod shells, in addition to prevalent chitin. Stabilized nanosized apatitic biominerals (up to ∼50 nm) permeate the framework of organic fibrils. There is a ∼2.5-fold higher wt% of carbonate (CO₃²⁻) in <em>Lingula versus Discinisca</em> shells. Both microRaman spectroscopy and infrared spectra show transient amorphous Ca-phosphate and octacalcium phosphate components. For the first time, trivalent moieties at ∼1660 cm⁻¹ and divalent moieties at ∼1690 cm⁻¹ in the amide I spectral region were identified. These are related to collagen cross-links that are abundant in mineralized tissues, and could be important features in the biostructural and mechanical properties of Ca-phosphate shell biominerals. This work provides a critical new understanding of organophosphatic brachiopod shells, which are some of the earliest examples of biomineralization in still-living animals that appeared in the Cambrian radiation.

Topics

• nanocomposite
• impedance spectroscopy
• mineral
• amorphous
• phase
• scanning electron microscopy
• Calcium
• Fourier transform infrared spectroscopy
• gravimetric analysis