‘Austrian Salamander’ had a growth spurt in its youth

Most modern reptiles and amphibians grow slowly but steadily. But an ancestor of these animals evolved in no time from baby to apex predator.

It would just fit in your bathtub: it Whatcheria deltae. With its narrow head, long body and flexible legs, the animal – which lived around 325 million years ago – looked a bit like a large salamander. But with razor sharp teeth. And unlike modern salamanders, it quickly grew to its adult size. Paleontologists from Harvard University and the Field Museum in Natural History (USA) discovered this.

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Bone growth rings

Tetrapods (or quadrupeds) form a so-called infraphylum of vertebrates that includes reptiles, amphibians, birds and mammals. They arose between 385 and 320 million years ago, long before the dinosaurs, who lived 230 to 66 million years ago. Fossils from early tetrapods are rare. All the more special was the discovery of several skeletons of Whatcheria deltae in 1994 in a limestone quarry near Delta, Iowa (USA).

Researchers Megan Whitney, Stephanie Pierce and some colleagues recently examined the femurs of nine specimens of W. deltae break open and cut into thin slices. They then examined the slices under the microscope. Growth rings can often be seen in the bone tissue of a femur. Each growing season (spring and summer) a layer of bone is added. These growth rings give a good indication of the animal’s growth rate during its life.

A fossil of the skull of Whatcheria deltae (Front and back). Photo: © Kate Golembiewski/Field Museum

Grows like mammals

At the thighs off W. deltae these growth rings were also visible. But most striking, according to Whitney and Pierce, was that in the older layers, which were formed at a young age, they also found so-called fibrolamellar bone. This tissue characterizes a growth spurt during this period.

According to the research team, this finding proves that the ‘primordial salamander’ increased in size very quickly from young to adult – just like modern birds and mammals. The claim contradicts the current theory that early tetrapods, like modern reptiles and amphibians, gradually increased in size during their lifetime.

Different growth strategies

The studied tetrapod probably lived on the edge of a lake in what is now the US state of Iowa. According to Whitney and Pierce, this area formed a complicated environment in which dry periods alternated irregularly with wet ones. Then the advantage may have been to quickly grow to apex predator size, because you can always find food, the palaeontologists believe.

Previous research showed that a slightly earlier living ‘cousin’ of W. deltaeExactly Greererpeton burkemorani, had average growth until maturity. According to the researchers, the fact that the two salamander-like tetrapods had a different growth strategy indicates the diversity of this animal tribe in the past.

Strong hind legs?

“The discovery that an early tetrapod could grow much faster than, say, salamanders – to which they are often compared – is absolutely remarkable,” says palaeontologist Dennis Voeten from Sweden’s Uppsala University. “However, bone research has led to several surprises; it has been shown that, under ideal conditions, crocodiles can also grow much faster than previously thought.”

He also has a point of criticism. “The researchers only looked at the bone in the upper leg. This is not to say that the entire skeleton grew at the same rate. For example, it cannot be ruled out that the animal put a lot of energy into developing strong hind legs, while the rest of the body followed a slow growth curve, which is usual for the group.”

“However, the study certainly provides valuable information about innovation in growth rates in our early ancestors that was not previously recognized,” continues Voeten. “It provides good starting points for future research in this important animal group.”

Sources: Communications Biology, Field Museum via EurekAlert!, Harvard University via EurekAlert!

Image: © Adrienne Stroup/Field Museum

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