Calcium carbonite can be precipitated from water into two polymorphs with different crystal structures, calcite and aragonite. The type of crystal precipitated by non-biological processes, such as abiotic marine cement and ooid formation, is determined by seawater chemistry, with aragonite being precipitated when magnesium ions are at least twice as numerous in the seawater as calcium ions in the water, and calcite being precipitated when magnesium ions are less numerous.
Some simple reef-producing organisms also produce calcium carbonate in either form, in response to seawater chemistry, but most organisms produce either calcite or aragonite, with the process occurring within their tissues, where they effectively control the seawater chemistry. Most marine organisms produce low-magnesium calcite rather than aragonite, which is probably advantageous, as calcite is far more stable to changes in seawater chemistry. However, the earliest small shelly fossils from the Lowest Cambrian Terreneuvian Epoch (between 538.8 and 521 million years ago) appear to all have been arogonitic in nature, with various groups of Animals having subsequently evolved calcitic skeletons, which has led to the assumption that the Earliest Cambrian had an 'Aragonitic Ocean', with a high magnessium content.
In a paper published in the journal Geology on 2 November 2022, Luoyang Li of the Frontiers Science Center for Deep Ocean Multispheres and Earth System at the Ocean University of China, and the Laboratory for Marine Mineral Resources at the National Laboratory for Marine Science and Technology, Timothy Topper of the Shaanxi Key Laboratory of Early Life and Environments at Northwest University, and the Department of Palaeobiology at the Swedish Museum of Natural History, Marissa Betts, also of the Shaanxi Key Laboratory of Early Life and Environments at Northwest University, and of the Palaeoscience Research Centre at the University of New England, Dorj Dorjnamjaa of the Institute of Paleontology of the Mongolian Academy of Sciences, Gundsambuu Altanshagai and Gundsambuu Altanshagai, also of the Institute of Paleontology of the Mongolian Academy of Sciences, and of the School of Arts and Sciences at the National University of Mongolia, Guoxiang Li of the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, and Christian Skovsted, again of the Shaanxi Key Laboratory of Early Life and Environments at Northwest University, and the Department of Palaeobiology at the Swedish Museum of Natural History, describe an apparent low-magnesium calcite-producing Mollusc from the Terreneuvian Bayangol Formation at Bayan Gol in the Zavkhan Basin of southwestern Mongolia.
The Bayangol Formation has produced hundreds of shells of the Helcionelloid Mollusc, Postacanthella voronini, considered to be a stem-group Conchiferan (i.e. an early member of the group that includes Bivalves, Gastropods, Cephalopods, Monoplacophorans, and Scaphopods). These come from the Purella shelly biozone, which has been dated to approximately 555 million years ago.
Seen under the scanning electron microscope, the structure of the shell organic matrix and prismatic crystalline microstructure of the shells of Postacanthella voronini can be seen, replicated in apatite, a phosphatic mineral which has replaced the mineral structure early in the preservation process. Apatite has the ability to adopt the structure of other minerals as it replaces them (most minerals form crystals of a specific shape, tending to destroy fine structures during replacement. This mimicking process gives apatite its name, which derives from the Greek ἀπατάω (apatáō), meaning 'to deceive', but also makes it extremely useful to palaeontologists wishing to study the microstructure of ancient tissues.
The crystalline microstructure of the shell of Postacanthella voronini is comprised of tightly-packed, parallel, columnar prismatic crystals with polygonal cross sections. Clusters of these columnar crystals are bound together by a sinuous intraprismatic organic membrane, similar to that seen in shells of modern Pearl Oysters, Pinctada spp., although the individual crystals of the Postacanthella shells are somewhat smaller than seen in Pinctada. The surface of the Postacanthella shells shows polygonal structures, derived from the cross-sectional shape of the columnar crystals, these being about 10 μm in diameter and slightly convex and cell-like, with the raised margins apparently corresponding to the intraprismatic organic membrane.
The preservation of prismatic microstructures of Cambrian shells in apatite is not a new concept; it is this preservation which tells us about the aragonite structure of many Cambrian shells (aragonite itself is unstable, and seldom survives for hundreds of millions of years). However, this is the first time that the organic matrix of a Cambrian shell has been observed in this way. This general absence is not surprising, as the organic matrix typically makes up less than 5% of the shell's structure, and is made of proteinaceous material which generally breaks down quickly after the death of the Animal. Fossil shell organic matrices have been recovered before, but only from Mesozoic or later specimens.
However, it is the nature of the crystals moulds preserved within the apatite microstructure that is remarkable. Aragonite and calcite crystals form in quite different ways, with aragonite crystals fanning outwards from a central radiant point, to form a flower-like structure, so that the shell structure comes to resemble a series of interlocking flowers. Calcite crystals, on the other hand, grow in a linear fashion, and bundles of parallel crystals are quite typical.
The widespread presence of aragonitic shells in Early Cambrian organisms has led to the conclusion that the Earliest Cambrian ocean had a high-magnessium, 'arogonitic' nature. Under such circumstances, it would be easier for organisms to evolve the ability to develop aragonite shells, which would then persist within evolutionary lineages.
Most organisms will produce either calcite or aragonite shells regardless of the water chemistry (as the crystal formation occurs within their tissues, where chemistry is under their control), with switches between the two systems being rare. This means that organisms will, in theory, develop the ability to form calcium carbonate crystals of a type that matches the water chemistry.
This is not, however, an absolute rule. The Micrabaciids, a group of Sclerectinian Corals today restricted to the deep oceans, produce aragonitic skeletons, but first appeared in the Cretaceous, when the seas had a high calcium, low magnesium chemistry (i.e. a 'calcite' ocean). Some calcite-producing Bivalves will produce aragonite shells if places in water with a high enough magnesium content. Furthermore, there has been a general tendency throughout the Phanerozoic for organisms in warm seas to produce aragonitic shells, while those in cold seas make shells from calcite.
Molluscs are unique in their ability to secrete calcium carbonate crystals from an organic membrane within the shell. In the aragonitic Terreneuvian ocean, Mollusc shells were built from aragonite in a variety of ways, including prismatic aragonite, foliated aragonite, and various regular-irregular fibrous microstructures. Subsequently, in the calcitic seas of the Stage 3 Cambrian and above, Molluscs developed the ability to secrete calcite from their shell membranes, forming foliated calcite and calcitic semi-nacre microstructures. Postacanthella voronini appears to be an exception to this rule, having already developed the ability to secrete a low-magnesium calcite shell in the high-magnessium shell in the high magnesium aragonitic Terreneuvian ocean.
This in turn suggests that the selection of a calcium carbonate polymorph when developing the ability to secrete a shell is influenced by, but not absolutely controlled by seawater chemistry.
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