World’s oldest heart preserved in 380-million-year-old armored fish

A team of Australian scientists has discovered the world's oldest heart, part of the fossilized remains of an armored fish that died some 380 million years ago. The fish also had a fossilized stomach, liver, and intestine. All the organs were arranged much like similar organs in modern shark anatomy, according to a recent paper published in the journal Science.

As we've reported previously, most fossils are bone, shells, teeth, and other forms of "hard" tissue, but occasionally fossils are discovered that preserve soft tissues like skin, muscles, organs—or even the occasional eyeball. This can tell scientists much about aspects of the biology, ecology, and evolution of such ancient organisms that skeletons alone can't convey.

For instance, earlier this year, researchers created a highly detailed 3D model of a 365-million-year-old ammonite fossil from the Jurassic period by combining advanced imaging techniques, revealing internal muscles that had never been previously observed. Among other findings, the researchers observed paired muscles extending from the ammonite's body, which they surmise the animal used to retract itself further into its shell to avoid predators. 

And last month, British researchers described their experiments monitoring dead sea bass carcasses as they rotted over the course of 70 days to gain insights into how (and why) the soft tissues of internal organs can be selectively preserved in the fossil record. One of the best ways that soft tissue can turn into rock is when it is replaced by a mineral called calcium phosphate (sometimes called apatite). Specifically, muscles, stomachs, and intestines tend to "phosphatize" much more frequently than other organs like kidneys and gonads. The authors concluded that the phosphorus content of specific organ tissue contributes to this unusual selection bias for which soft tissues are preserved in the fossil record.

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