Doubts cast over dinosaur cold-blood theory

One of the strongest lines of evidence that suggested dinosaurs were cold-blooded, like modern reptiles, has been knocked down.

Previous studies of dinosaur bones uncovered what are known as “lines of arrested growth”.

The creatures were presumed to be cold-blooded because modern cold-blooded animals show these same lines.

But scientists have studied the bones of 41 modern mammal species from around the world, finding every one had these lines as well.

Many discoveries in recent years have challenged the 40-year-old notion that dinosaurs were cold-blooded.

But because soft tissues such as organs and skin are not preserved, much of what is known about dinosaurs has to be inferred from their bones, and comparisons made with modern animals that can be studied in greater detail.

Lines of arrested growth, or Lags, occur because organisms tend to suspend their growth and rally their resources during seasonal periods of environmental stress such as cold or dry conditions.

This forms a boundary from one season to the next as growth resumes when conditions turn favourable.

They are familiar in creatures such as molluscs, whose slow annual accumulations can be seen as ridges in their shells.

Lags have also been found in the bones of reptiles and amphibians and have until now been assumed to be limited to ectotherms – cold-blooded animals – that are more subject to the whims of harsh environments.

“Originally this was not a paper that we aimed to do,” the BBC quoted Meike Koehler of the Catalan Institute of Palaeontology in Barcelona, as saying.

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“We were very curious to know how environmental conditions and changes affect bone growth in fossil and extant mammals, to get a good idea about… how they may have coped with these changes in the past,” Koehler said.

As the team studied the thighbones of animals from all over the world – ranging from the Svalbard reindeer in the Arctic to muntjac deer species from South Asia – Lags showed up in every one.

“These lines of arrested growth have been used a lot in dinosaurs, but nobody has ever had a really deep look at mammals,” Dr Koehler explained.

“I don’t think that this debate is really settled,” she said.

“But this is the first time that you can say that Lags do not say anything about warm- or cold-bloodedness,” she added.

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