How Did Animals Get Their Skeletons?

SCIENCE

Animals with skeletons did not exist before about 550 million years ago. Then, suddenly, shells, exoskeletons, and skeletons showed up in the biological record. Why? (Science)

A few of these ancient, armored species are still around.

Teachers, scroll down for a quick list of key resources in our Teachers Toolkit.

Cloudina is one of the earliest skeletal animals. Cloudina, seen as a trace fossil on the left, had a characteristic “stacked cup” exoskeleton that presumably protected its soft insides from predators. (Some Cloudina fossils have holes that biologists think were made by predators attacking the animal by boring through its skeleton.) Photograph by O. Louis Mazzatenta, National Geographic Illustration by Graeme Bartlett, courtesy Wikimedia. CC-BY-SA-3.0

Cloudina is one of the earliest skeletal animals. Cloudina, seen as a trace fossil on the left, had a characteristic “stacked cup” exoskeleton that presumably protected its soft insides from predators. (Some Cloudina fossils have holes that biologists think were made by predators attacking the animal by boring through its skeleton.)
Photograph by O. Louis Mazzatenta, National Geographic
Illustration by Graeme Bartlett, courtesy Wikimedia. CC-BY-SA-3.0

Discussion Ideas

  • Life on Earth has been around for about three billion years, but skeletons only developed about 550 million years ago. What conditions made skeleton-building possible during the Ediacaran period?

 

  • Wait a minute. Skeletons aren’t made of oxygen. How does increased oxygen influence the evolution of skeletons?
    • Increased oxygen changed the entire chemical balance of the ocean, where almost all life existed at that time. The seabed came to be dominated by limestone, a carbonate rock which contains the minerals aragonite and calcite. Aragonite and calcite are crystal forms of calcium carbonate (CaCO3), the key chemical compound in shells, eggs, and exoskeletons.
    • Changes on land also contributed to changes in the ocean. “[A]gents of erosion like wind and rain bombarded the continents at unusually high rates. This caused one particular nutrient needed for the formation of calcite and aragonite—calcium—to flood the oceans, which further fueled the evolution of skeletons.”

 

  • Cloudina, one of the earliest skeletal animals, is extinct. Are early representatives of extant species a part of Ediacaran biota?
    • Yes! Sponges, algae, protists, and bacteria are all represented during this time period.

 

  • Were animals like Cloudina the only organisms that used chemical changes brought on by increased oxygen to build shells and skeletons? Take a look at our nice study guide on “armored amoebas” for some help.
    • Nope, plenty of organisms were adapting to changes in ocean chemistry. For instance, even though they look like snails or clams, forams—the “armored amoebas” in our study guide—aren’t actually animals at all. They’re protists, more closely related to slime molds and algae.
    • The first forams with tests (shell-like structures) appeared during the tail end or just after the Ediacaran period. These tests were not quite shell-like, but “agglutinated tubes.” Agglutinated just means the tests were made of marine sediment (such as aragonite or calcite) “glued” together by the soft body of the foram. Evolution is awesome, don’t let anyone tell you otherwise.

 

TEACHERS TOOLKIT

Science: How did animals get their skeletons?

Nat Geo: Armored Amoebas study guide

Wikipedia: Ediacaran Biota featured article

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