A reconstruction approximating the size of Trierarchuncus compared to an average modern-day house cat (based on other alvarezsaurids) © Matt Dempsey.

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Dinosaur diaries: new 'Captain Hook' dinosaur with clawed hands discovered

A tiny dinosaur the size of a house cat has been discovered, dubbed the 'Captain Hook of the Prairie' because of its clawed hands.

Discover more about these dinosaurs, as well as the importance of flight for Microraptor and the repositioning of fanged dinosaurs on the family tree with the Museum's team of dinosaur experts.

When we think about dinosaurs, the first images that we conjure in our imaginations tend to be huge, almost monstrous scaly creatures striding across a prehistoric landscape. However, living within the shadows of these giants was an entire menagerie of strange and sometimes tiny creatures.

One such group was a small and enigmatic family of bird-like dinosaurs, known as the Alvarezsauridae, that lived throughout the Cretaceous Period, just before the asteroid hit.

While we know of several groups of dinosaurs that bore a strong resemblance to their modern-day birds, alvarezsaurids stand out because they brandished a large claw upon an otherwise small, but possibly strong, arms.

Perhaps because of their tiny and delicate builds, fossils of alvarezsaurids are quite rare, with the most complete skeletons known from geological formations in Asia and South America.

Reconstruction of the new dinosaur, showing that it is about the same size as a house cat.

A reconstruction of a pair of bird-like alvarezsaurid dinosaurs © Matt Dempsey.

In a new study, a team led by Denver W. Fowler from the Badlands Dinosaur Museum of Dickinson, present a new species of these dinosaurs unearthed from the Upper Cretaceous rocks of the Hell Creek Formation in Montana.

Named Trierarchuncus prairiensis, meaning 'captain hook of the prairie', this new species is important not only due to the relative scarcity of North American alvarezsaurid fossils, but also because it represents the youngest member of the family known thus far. 

This means that Trierarchuncus would have been among the last of the non-bird dinosaurs, existing alongside famous and much larger creatures such as Triceratops and Tyrannosaurus rex.

The new fossil material includes well-preserved hand claws, which appear to have come from differently sized, possibly differently aged individuals. Fragmented bones of the forearms, feet and hips might also belong to Trierarchuncus, or possibly another related species.

The claws of this new find are more strongly curved than in previously known alvarezsaurids. It is suggested that the claws would have been used for 'hook and pull' actions, which may have included tearing apart rotting wood or insect nests, as the diminutive teeth of previously discovered and more complete alvarezsaurids have suggested that they may have been insectivores.

Discoveries like Trierarchuncus help demonstrate that dinosaurs were far more diverse than popular imagery often portrays, filling a variety of body sizes and behavioural roles in ecosystems of the Mesozoic.

Dinosaurs moulted their feathers like birds do today

The most distinctive characteristic of birds  is their feathers, and to look after them, birds will periodically replace them through moulting. By looking at the feathers preserved on some dinosaurs, researchers have been able to see evidence that they moulted too, giving some insights into importance of flight to these extinct animals.  

Birds use feathers for a variety of things, such as keeping warm, to camouflage and communicating, in addition to letting them fly. Consequently, living birds must spend a lot of time caring for their feathers to ensure they are in tip-top condition.

Reconstruction of a microraptor, showing that it had four feathered wings, one on each limb.

Restoration of Microraptor, arms outstretched in a gliding pose. New research shows that Microraptor moulted its flight feathers sequentially, so its four wings would have retained their aerodynamic qualities throughout the year © Fred Wierum/Wikimedia Commons

An important part of this self-care regime is the moult, during which birds shed their old feathers and replace them with a new, healthy set. Despite this importance, until now the way in which the ancient ancestors of birds replaced their feathers has remained unknown.

A new paper led by Yosef Kiats, from the University of Haifa, presents a remarkable fossil that helps unlock this mystery.

Kiats and colleagues report a 120-million-year-old specimen of the crow-sized, four-winged dinosaur Microraptor caught in the process of moulting and then compared it to patterns of moulting in living birds. This has allowed the researchers to figure out just how important flight was to Microraptor.

Flying birds can be split into two categories. Some, such as ducks, shed and replace their flight feathers all at once. This leaves them temporarily flightless and vulnerable to land predators and so is seen in waterfowl or birds living in sheltered habitats. Most birds, on the other hand, shed their flight feathers piecemeal in what is known as a sequential moult. This is more gradual, but it means that they never lose the ability to fly and so is seen in birds for which flight is an essential part of their everyday lives.

The fossil of microraptor showing the feathers preserved on the wings.

A fossil of Microraptor clearing showing the preserved feathers surrounding the fossilised skeleton © David W. E. Hone et al./Wikimedia Commons

The Microraptor specimen shows a combination of flight feathers of different growth stages in its wing, demonstrating it also replaced its feathers in a sequential moult.

This means it would have retained its flight capabilities throughout the year. Whether Microraptor was capable of active flight or only gliding remains unclear, but nonetheless this specimen demonstrates that flight was crucial to its daily life, likely in helping it to find food and evade predators. It also provides us with our oldest look yet at how dinosaurs replaced their feathers and shows that the pressures of flight were key in the evolution of the moulting strategies seen in living birds.

The fanged dinosaurs climbing the bird-hipped dinosaur tree

Stegosaurus, Triceratops and Iguanodon have become iconic dinosaurs, capturing our imaginations for decades.

They are all bird-hipped dinosaurs, a group whose origins are hotly debated by scientists.

For the last decade, experts have thought that the most primitive bird-hipped dinosaurs were the fanged heterodontosaurs. These animals stood on two legs and were about the size of a turkey. They are among the oldest known bird-hipped dinosaurs.

Last week, Paul-Emile Dieudonné and colleagues published a new family tree that challenges our ideas about the evolutionary history of bird-hipped dinosaurs. They found that heterodontosaurids diverged from the other dinosaurs in their group much later than we thought. It means that they were the predecessors of the pachycephalosaurs, or dome-headed dinosaurs.

An evolutionary tree showing how the fanged dinsaurs are now nested within the cerotopsian dinosaurs.

Simplified ornithischian family tree showing the movement of heterodontosaurs (in black) from the base of Ornithischia (left) to within Pachycephalosauria (right). Silhouettes by S. Hartman and E. Willoughby

This idea isn’t new, but it is the first time someone has suggested that heterodontosaurs gave rise to the 'true' dome-headed dinosaurs.

The evolution of pachycephalosaurs is poorly understood and not many of their fossils have survived. While some suggest that they originated in the Late Jurassic period (163-145 million years ago) their fossils have only been found in Late Cretaceous rocks (100-66 million years ago), leaving over 70 million years of missing material.

This newly proposed position for heterodontosaurs may be the missing link that dinosaur palaeontologists have been looking for.