Willo the Dinosaur Loses Heart

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At first glance, Willo was not an especially impressive dinosaur. A well-preserved Thescelosaurus, this herbivorous dinosaur was one of the mid-sized ornithischians that lived about 66 million years ago. What made Willo special was its heart. Preserved inside a concretion cradled within the dinosaur's ribcage were the remains of its major cardiac muscle. But not everyone has been in agreement that Willo has a heart at all.

Though discovered in 1993, Willo's major public debut came via a Science paper published in April, 2000. Written by Paul Fisher, Dale Russell and colleagues, the study concluded that inside the iron concretion in Willo's chest were the preserved remnants of a four-chambered heart. Overall, the heart of this Thescelosaurus appeared to be more bird-like than reptile-like—a finding that had important implications for studies of dinosaur physiology. If dinosaurs, even those distantly related to birds, had avian-type hearts, then they probably had bird-like metabolisms, too.

The discovery of a dinosaur heart excited many paleontologists—perhaps soft-tissue preservation was more common than had been thought—but not everyone was so enthused. In February of 2001, Timothy Rowe, Earle McBride and Paul Sereno critiqued the other team's analysis in the same journal, concluding "the object is not a fossilized heart but an ironstone concretion" that only superficially looked like a heart. Russell and co-authors responded that the structure was most consistent with the shape of a heart, and therefore a heart it must be.

A study just published in Naturwissenschaften by Timothy Cleland, Michael Stoskopf and Mary Schweitzer throws new support to the contentions of Rowe and other paleontologists. After reexamining the fossil by way of high-resolution CT scans, X-rays and close-up study of the purported soft tissues, the team could not find any good evidence of Willo's heart being preserved. Instead, the organ is really a concretion formed when sand was washed inside the body and became cemented into place.

But there was one surprise inside the concretion. Although Willo's heart was not preserved, Cleland and co-authors found several small scraps of "cell-like material." These may have come from plant debris washed into the skeleton, or they might have come from the dinosaur itself. Perhaps, the scientists hypothesize, the decaying body of the dinosaur triggered the beginning of concretion formation and some of the sand formed around the rotting tissues before they had a chance to fully decay. Frustratingly, there is not yet a way to detect the origin of these scraps, but new technologies may eventually provide means to do so. After all, the improvements in technology since 2000 allowed paleontologists to thoroughly reexamine Willo to a deeper degree of detail. Who knows what kind of insights new technologies will allow us to derive from long-dead dinosaurs?

If you want to see Willo yourself, the dinosaur is on display at the North Carolina Museum of Natural Sciences.


Cleland, T., Stoskopf, M., & Schweitzer, M. (2011). Histological, chemical, and morphological reexamination of the “heart” of a small Late Cretaceous Thescelosaurus Naturwissenschaften DOI: 10.1007/s00114-010-0760-1

Fisher, P. (2000). Cardiovascular Evidence for an Intermediate or Higher Metabolic Rate in an Ornithischian Dinosaur Science, 288 (5465), 503-505 DOI: 10.1126/science.288.5465.503

Morell, V. (2000). PALEONTOLOGY:Revealing a Dinosaur's Heart of Stone Science, 288 (5465), 416-417 DOI: 10.1126/science.288.5465.416b

Rowe, T. (2001). Dinosaur with a Heart of Stone Science, 291 (5505), 783-783 DOI: 10.1126/science.291.5505.783a

Stokstad, E. (2001). PALEONTOLOGY: Doubts Raised About Dinosaur Heart Science, 291 (5505), 811-811 DOI: 10.1126/science.291.5505.811

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