Stereom is a

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

material that makes up the internal skeletons found in all

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

s, both living and fossilized forms. It is a sponge-like porous structure which, in a sea urchin may be 50% by volume living cells, and the rest being a matrix of

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

crystals. The size of openings in stereom varies in different species and in different places within the same organism. When an echinoderm becomes a

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

, microscopic examination is used to reveal the structure and such examination is often an important tool to classify the fossil as an echinoderm or related creature.

Evolution

Stereom was the first form of biomineralization to evolve in deuterostomes, predating the evolution of spicules in tunicates and

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...

in vertebrates. Stereom likely evolved before other distinctive traits of echinoderms, such as

radial symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symme ...

, as it is present in basal echinoderms with bilaterally symmetric or asymmetric body plans. Stereom may have evolved alongside a transition in

ocean chemistry Marine chemistry, also known as ocean chemistry or chemical oceanography, is the study of the chemical composition and processes of the world’s oceans, including the interactions between seawater, the atmosphere, the seafloor, and marine organ ...

from an

aragonite sea An aragonite sea contains aragonite and high-magnesium calcite as the primary inorganic calcium carbonate precipitates. The reason lies in the highly hydrated divalent ion, the second most abundant cation in seawater after , known to be a stron ...

to a

calcite sea A calcite sea is a sea in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carb ...

, which occurred late in

Cambrian Stage 2 Stage 2 of the Cambrian is the unnamed upper stage of the Terreneuvian Series. It lies atop the Fortunian and below Stage 3 of the Cambrian. It is commonly referred to as the Tommotian, after the Cambrian stratigraphy of Siberia. Neither the upper ...

. In the largely falsified

calcichordate hypothesis The calcichordate hypothesis, formulated by British Museum paleontologist Richard Jefferies, holds that each separate lineage of chordate ( Cephalochordates, Urochordates, Craniates) evolved from its own lineage of mitrate, and thus the echinoder ...

, stereom was believed to have been present in the common ancestor of echinoderms and

vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

. However, the

genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

which code for stereom are unique to echinoderms, indicating that it is likely that stereom is a

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...

of echinoderms, and that all stereom-bearing fossil taxa belong to the echinoderm

total group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

References

Echinoderm anatomy Calcium minerals {{echinoderm-anatomy-stub