How the heart evolved has been, for the most part, a mystery. But a pair of ancient fish may change that.
In the journal eLife, a team of scientists from Brazil, France and Sweden reports fossil hearts in two 119-million-year-old specimens of the extinct bony fish Rhacolepis buccalis – and they seem to be more complex than hearts found in present-day vertebrates (animals with a backbone).
Like most soft tissues, hearts are quite squishy and don't tend to preserve well, so such fossils are few and far between.
But some tidbits have cropped up recently: last year, scientists stumbled upon blood cells in 75-million-year-old dinosaur bones. Could soft tissue be preserved in even older specimens?
A team of researchers, led by Lara Maldanis from Brazil's University of Campinas and Murilo Carvalho from the Brazilian National Sciences Laboratory also in Campinas, turned to R. buccalis, a bony fish that was alive more than 113 million years ago in what is now Brazil.
Bony fishes (Osteichthyes) have been around more than 400 million years and represent the first major vertebrate group to evolve.
They've been very successful over the aeons – today, Osteichthyes comprise the largest class of vertebrates.
The R. buccalis specimens used in this study were found in the Araripe Basin in northeastern Brazil, and dated to around 113 to 119 million years old, thanks to ancient pollen and spores found in the same rock layers.
To gain a 3-D look inside the fossils, the researchers used a non-invasive imaging technique called X-ray synchrotron microtomography, which takes "slices" through the specimens and reconstructs them on a computer. And when they looked at the hearts, they noticed something unusual.
R. buccalis hearts, like modern-day vertebrate hearts, contained valves in a major artery to prevent the blood from sloshing back into the heart.
But the fossil hearts contained five valves – more than any modern-day bony fish species. The researchers suggest vertebrate hearts gradually evolved to simplify and become the structures we see today.
Now they've demonstrated their technique on bony fish, similar non-destructive methods can be used to fill in the rest of the evolutionary gaps, they write: "we hope our results will open exciting new possibilities for research in cardiovascular palaeontology and evolution."
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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