New sail-backed dinosaur species unearthed on the Isle of Wight

A new species of sail-backed dinosaur has been named after a famous sailor.

Istiorachis macarthurae lived more than 120 million years ago in what is now the Isle of Wight. Weighing around one tonne and reaching the size of an American bison, the new species would have been one of many Iguanodon relatives, known collectively as iguanodontians, living on the island.

But what sets Istiorachis apart is the sail-like structure that ran along its back. Researchers believe the structure could have helped the dinosaur to be recognised by other members of its species, including any potential mates.

The sail also inspired the dinosaur’s name. It honours the sailor Dame Ellen MacArthur, an Isle of Wight resident who set the record for the fastest non-stop solo voyage around the world in 2005.

Dr Jeremy Lockwood, the lead author of the paper describing the dinosaur, says that the name of the new species recognises Ellen’s nautical accolades as well as the Isle of Wight’s heritage.

“The discovery of another iguanodontian from the Isle of Wight shows that this was a very diverse area in the Early Cretaceous,” Jeremy adds. “I’m sure there’ll be more discoveries in the years to come.”

The findings of the study were published in the journal Papers in Palaeontology.

Diverse dinosaurs on the Isle of Wight

The Early Cretaceous, which lasted from 143 to just over 100 million years ago, was a time when dinosaur diversity was changing. In the northern hemisphere, large herbivores like Diplodocus gave way to the iguanodontians which likely roamed in herds across the landscape.

Why exactly this happened, however, isn’t quite certain. Rocks from the end of the Jurassic and start of the Cretaceous aren’t as well studied in Europe as they are in other areas of the world, making it difficult to investigate what was happening to dinosaurs at this time.

“Exposed rocks showing what life on land was like during the Early Cretaceous are relatively rare,” Jeremy says. “One of the few regions in Europe that has deposits from this age are the Isle of Wight and southern England, in a formation of rocks known as the Wealden.”

“Historically, it was generally assumed that Iguanodon-like bones from these rocks were one of two species: Mantellisaurus atherfieldensis and the larger Iguanodon bernissartensis. Over the last few years we’ve shown that there were far more dinosaurs than that.”

The research carried out by Jeremy and other palaeontologists shows that the Isle of Wight was home to a diverse range of Early Cretaceous dinosaurs. As well as iguanodontians, there were ankylosaurs, Velociraptor relatives and one of the largest carnivores that’s ever lived in Europe.

Istiorachis further adds to the diversity of this ancient ecosystem. It also reveals a new feature that hasn’t been seen in this region before – its remarkable sail.

What did dinosaurs use their sails for?

While dorsal sails aren’t unheard of in nature, they’re quite rare. Today only a few reptiles have them, including crested chameleons, sail lizards and certain basilisks. They’re generally better known from now extinct animals, including dinosaurs such as Spinosaurus and an early mammal relative called Dimetrodon.

These sails are typically formed around bones known as neural spines, which stick out of the top of an animal’s backbones.

The team wanted to know why these sails might have evolved in iguanodontians and so searched the fossil record of Iguanodon-like dinosaurs for signs of the neural spines getting longer. They found that taller spines seem to have become more common during the transition between the Jurassic and the Cretaceous Periods.

“Partly, we think this was for muscle attachment,” Jeremy explains. “Iguanodontians were changing from small bipedal dinosaurs into much bigger animals that spent more time on four legs, so they would have needed stronger muscular support for their backbones.”

However, Istiorachis’ neural spines were more exaggerated than expected when compared to its relatives. While it’s previously been suggested that sails provided a way for animals to control their body temperature, the team aren’t so sure this was the case for Istiorachis.

“A sail with a lot of blood vessels would be very vulnerable to attack and could cause massive blood loss if it was damaged,” explains Jeremy. “Instead, we think that a role in signalling is more likely.”

“The sails could have helped different individuals to recognise each other as herds of iguanodontians roamed the landscape and might have deterred predators by making the animal look bigger.”

But Jeremy and his colleagues suspect that the most likely reason for the evolution of the sail was due to sexual selection. This is the theory which explains why exaggerated traits have evolved in animals, such as the extraordinary tail of male peacocks.

“Sexual signalling is the most probable explanation,” Jermey says. “When a characteristic is exaggerated beyond its practical function in living animals, it is invariably due to the evolutionary pressure to attract a mate. Istiorachis’ sail seems to be another example of that.”

Finding more fossils from this dinosaur could help to answer these questions. Unfortunately, other fossils of Istiorachis were stolen from a dig site more than 50 years ago and are yet to be found, despite the team’s best efforts.

Despite these setbacks, ongoing discoveries in the Wealden are offering new windows into life more than 120 million years ago.

“We know that this was an environment that was changing, with the sea increasingly intruding onto the land,” Jeremy adds. “At the same time, we can see patterns of diversity shifting across a variety of different dinosaurs.”

“The Wealden is one of the most scientifically important regions to help us discover exactly what was happening in this important time for life on Earth.”