With jaws like these, who needs big arms? A new analysis suggests dinosaurs like Tyrannosaurus rex had shrunken forelimbs because their massive, powerful heads became their primary tool for killing large prey, rendering their arms redundant. It is an evolutionary approach that five different lineages of large theropod dinosaurs took independently.

Researchers are well aware that a number of large, predatory theropods followed a trend towards bigger bodies, bigger heads and smaller, shorter arms over time. But it wasn鈥檛 known why this pattern repeated across multiple predatory dinosaur families, scattered across the globe and separated by many millions of years, says at University College London. There was also little understanding of how the bones in their ever-heftier skulls changed as their arms became proportionally smaller.

鈥淭his paper tackles one of the big evolutionary questions in theropod dinosaurs,鈥� says at the University of Bristol, UK, who wasn鈥檛 involved with the research.

Scherer and his colleagues compiled data on the proportions of the forelimbs and skulls of 85 theropod species, along with body-mass data. This allowed them to calculate a ratio between the skull dimensions and forelimb lengths, quantifying just how small the arms were compared with the head. The researchers then compared this ratio with other measurements of the dinosaurs鈥� bodies, along with a measure of the skulls鈥� strength based on factors such as bite force and skull rigidity.

The team found that skull durability was associated with smaller arms, regardless of where the species sat in the theropod evolutionary tree. 鈥淚f it鈥檚 a predatory theropod and has a very robust skull, it will most likely have relatively small forelimbs,鈥� says Scherer.

The researchers found this head-arm divergence evolved independently in five theropod groups: tyrannosaurids, the short-snouted abelisaurids, the knife-toothed carcharodontosaurids, ceratosaurids and megalosaurids. This evolutionary pattern hadn鈥檛 been identified in the last two groups until this study, points out at the Beipiao Pterosaur Museum of China, who wasn鈥檛 involved in the research. This shows how hidden evolutionary signals can be revealed when traits are quantified in this way, she says.

Free newsletter

Sign up to The Weekly

Sign up to newsletter

The findings provide clues as to why the dinosaurs鈥� arms kept shrinking. These predators鈥� increasing skull strength and body size coincided with the rising mass of their quarries. The theropods evolved huge, sturdy skulls for subduing their large, difficult-to-control prey. Their heads were clearly doing the majority of the work, says Scherer, reducing the need for strong, grappling arms.

鈥淣ature doesn鈥檛 like to have everything all at once,鈥� he says. A big, powerful head plus strong forelimbs would require a lot of energy to maintain.

This creates a trade-off between jaws and claws. Other theropods like the megaraptorans and spinosaurs were also very large predators, but they took the opposite route to dinosaurs like T. rex, coupling聽long arms with slender skulls.

Rowe is curious about the mechanical function of the jaw-centric theropods鈥� arms, even in their shortened state. 鈥淵es, tyrannosaurs had tiny, vestigial arms, but that does not necessarily mean they were completely useless,鈥� he says.

He adds that the study emphasises the evolutionary diversity of dinosaurs. 鈥淚t reminds me of why I fell in love with dinosaurs in the first place,鈥� he says. 鈥淭hey were some of the most innovative and successful animals to ever exist.鈥�

Fossil hunting in the Australian outback

Join this extraordinary adventure through the heart of Australia鈥檚 fossil frontier. Once a shallow inland sea millions of years ago, eastern Australia is now a hotspot for fossils. Over 13 unforgettable days, you鈥檒l travel deep into the outback, tracing the footsteps of prehistoric giants and uncovering the secrets of Earth鈥檚 ancient history.

Read more

(1979)

(1995)

(2001)

(2006)

(2011)

(2020)

(2022)

(2026)

Birds today have dinosaur DNA. They have dinosaur blood in their veins. Birds evolved directly from Velociraptor-type animals and are the only true dinosaurs that still exist. Steve Brusatte is a palaeontologist at the University of Edinburgh, UK, and has been studying 150 million years’ worth of bird evolution, from surviving the asteroid that wiped out their contemporaries to today鈥檚 adaptation into almost every niche. Brusatte introduces elephant birds, terror birds, demon ducks and penguins the size of gorillas, and why “when gulls dive bomb you and try to steal your pasty or your chips鈥�, says Brusatte, “you really can sense in the nastiness, the cunning, the agility, the feistiness. You can sense that inner Velociraptor.鈥�

Read more: The evolving science of dinosaurs

Dinosaurs ruled the Earth for over 165 million years and even now, we鈥檙e still uncovering new secrets about these extraordinary animals.
In this marathon, we explore the latest discoveries in paleontology, and examine where Hollywood gets dinosaurs wrong 鈥� and where it gets them surprisingly right. The truth is, real dinosaurs were often even more remarkable than their blockbuster portrayals, and our understanding of them is evolving all the time.

Forget what you鈥檝e seen in Jurassic Park: dinosaurs in real life were far more compelling, well adapted and intelligent than the movie monsters they are so often portrayed as. In this interview with renowned palaeontologist Dave Hone from Queen Mary University of London, we dive into the evidence revealing how dinosaurs and their contemporaries really behaved, exploring how iconic animals such as velociraptors, T. rex and Triceratops fought, hunted and raised their young. We also ask, was Spinosaurus truly aquatic? And how did pterosaurs, the extraordinary winged reptiles, fly?

We鈥檙e in a golden age of discovery, says Hone, where new fossils and technologies such as CT scanning and AI-based analysis are rewriting the dinosaur story every year, revealing animals that were richer, stranger and more spectacular than we ever imagined.

Read more: Massive Spinosaurus dinosaur swam through water propelled by its tail

An almost-complete skeleton of a dinosaur that weighed less than a small chicken has provided new insights into the evolution of alvarezsaurs, which are among the smallest dinosaurs that ever lived.

The 95-million-year-old fossil of Alnashetri cerropoliciensis was found at the La Buitrera site in northern Patagonia, Argentina, in 2014.

The first specimen of Alnashetri, found in 2012, was a set of incomplete hindlimb bones, says at the University of Minnesota, who was part of the study on the new fossil. With only fragmentary remains, it was impossible to say more than that it was probably an alvarezsaur. 鈥淲e were not even sure if it was a juvenile or fully grown,鈥� he says.

鈥淲ith a whole skeleton, we suddenly had all the information to understand how Alnashetri was similar or differed from other species, and a key to understanding how the unusual anatomy of alvarezsaurs evolved,鈥� says Makovicky.

The new fossil has very long, slender hind limbs and surprisingly long forelimbs that retain three well-developed fingers. Detailed analysis of the fossil bones revealed the dinosaur was an adult and at least 4 years old.

It is estimated to have weighed only 700 grams when it was alive. 鈥淭he specimen is truly tiny, smaller than a chicken,鈥� says Mackovicky.

Free newsletter

Sign up to The Earth Edition

Sign up to newsletter

Alvarezsaurs were once thought to be early ancestors of birds. However, it is now clear that, while Alnashetri might have had some superficial resemblance to a bird, it and all the alvarezsaurs were, in fact, non-avian theropods. 鈥淭he new discovery certainly underscores this,鈥� says Mackovicky.

Previously, it was thought that all the tiny alvarezsaurs had very short, stout forelimbs with a large thumb but shrunken side digits, and tiny teeth. Palaeontologists thought these anatomical features evolved alongside their shrinking body size because they only ate ants and termites, says Makovicky. 鈥淏ut Alnashetri does not fit that mould 鈥� it is among the smaller alvarezsaurs, but neither its teeth nor its forelimbs are reduced, because it represents a much earlier branch on the alvarezsaur evolutionary tree.鈥�

In fact, its forearms are more typical of other theropods rather than a specialist ant-eater, he says. 鈥�Alnashetri is tiny but is otherwise built like a more typical theropod 鈥� given its small size, it probably ate its fair share of invertebrates, but probably had a wider range of prey.鈥�

That means palaeontologists still don鈥檛 fully understand why these dinosaurs became so small. 鈥淲e鈥檙e left with only a vaguer sense that alvarezsaurs were successful at occupying the niches of very small predators,鈥� says Mackovicky.

Dinosaur hunting in the Gobi desert, Mongolia

Embark on an exhilarating and one-of-a-kind expedition to uncover dinosaur remains in the vast wilderness of the Gobi desert, one of the world鈥檚 most famous palaeontological hotspots.

Find out more