Dinosaur diaries: Spinosaurus, sauropod necks and a new 'starry lizard'
In our Dinosaur diaries series, the Museum's dinosaur experts share some of the latest science news and views from across the world of palaeontology.
This week they share their thoughts on three animals: Stellasaurus, Spinosaurus and Spinophorosaurus.
Spinosaurus: the first aquatic dinosaur?
During their long history, dinosaurs evolved a vast array of lifestyles: they burrowed, flew, and herded. Until now there has been little evidence of dinosaurs spending most of their lives in water.
Last week, a team of scientists led by Nizar Ibrahim from the University of Detroit Mercy published a paper that shows the giant meat-eating dinosaur Spinosaurus was probably aquatic.
Spinosaurus has always been a mystery. It was first described in 1915 from a partial skeleton found in Late Cretaceous rocks in Egypt, but the bones were destroyed in the Second World War while on display in Munich. Only a few bones thought to be Spinosaurus were found during the twentieth century, so palaeontologists relied on drawings of the destroyed skeleton.
Then in 2014 Nizar and colleagues discovered a partial skeleton in Morocco that bore all the hallmarks of Spinosaurus. It had unusually short back legs, broad feet, and dense bones similar to those of some modern aquatic mammals, like manatees.
The team suggested that these were adaptations for life in water, but that didn't go down well in the palaeontological world. Some people suggested that the limb bones were from another specimen, and computer models suggested that it was too light to sink, instead floating in the same way as land-living meat-eating dinosaurs.
But there is evidence that spinosaurs lived close to water, with long, low snouts and nostrils similar to a crocodile's. The British spinosaurid Baryonyx was also found with fish scales in its stomach.
Last week, Nizar and his team announced they had found more of the skeleton: this time the tail. Bony projections called neural spines point upwards from the tail vertebrae in dinosaurs, and other bones, called chevrons, hang downwards. The new tail shows that both are very long in Spinosaurus, much more so than in other dinosaurs, making the tail deep, like a paddle. The team showed that if Spinosaurus swished its tail from side to side, it could have propelled itself through water much better than other meat-eating dinosaurs, and almost as well as crocodiles.
It is exciting to see new evidence that suggests Nizar and his team were right all along: Spinosaurus had a fin-like tail, perfect for propelling the animal through the water. Since the paper has been published, some have pointed out that we have had evidence of swimming dinosaurs for some time in the form of tracks. But there's a difference between recognising that an animal that spent most of its life on land could swim, and the finding of a truly aquatic dinosaur.
It's hard to see why the idea of an aquatic dinosaur has been so controversial - why wouldn't these incredibly successful organisms have entered the water, as mammals and birds have today? We'll have to wait and see whether this evidence is enough to convince.
- Read the paper in the journal Nature: Tail propelled aquatic locomotion in a theropod dinosaur.
An evolutionary innovation in sauropod dinosaurs
Sauropods are famous for their long necks, but whether carried them horizontally or craned to reach the treetops has been debated. A paper published last month, led by Daniel Vidal from Complutense University of Madrid, believes it has the answer. Apparently it is all in the hips.
Vidal and colleagues scanned two skeletons of the early sauropod Spinophorosaurus and rebuilt them digitally. In doing so, they found that the sacrum, the set of backbones connected to the hips, was wedge-shaped.
This results in a giraffe-like posture, with tall shoulders and an inclined neck, reaching up to seven metres above the ground. Wedge-shaped sauropod sacra have been known for over 100 years, and this posture has previously been used for animals like Brachiosaurus. However, Vidal and colleagues suggest it defines the group more generally. Indeed, they argue it represents a critical step in sauropod evolution, allowing them to feed from tall trees early in their history.
The results show how the slope of the back and shoulder height are at least as important as neck flexibility in determining a sauropod's reach. However, measuring these relies on accurate estimates of arm length and orientation of the shoulder.
Unfortunately, the forearm of Spinophorosaurus is unknown, forcing the authors to rebuild it from related species, and the orientation of the shoulder blade in sauropods is uncertain. Without these crucial data, as well as consideration of joint cartilages and muscles, the posture of Spinophorosaurus and other early sauropods remains unclear.
Nonetheless, the paper does highlight how detailed consideration of the body beyond the neck - particularly the shoulder - is vital to understanding the tallest animals to have walked the Earth.
- Read the paper in the journal Scientific Reports: High browsing skeletal adaptations in Spinophorosaurus reveal an evolutionary innovation in sauropod dinosaurs.
A new horned dinosaur
The latest Cretaceous rocks of western North America host some of the richest dinosaur sites on Earth, but new discoveries continue to be made on a regular basis.
In this paper, John Wilson from Montana State University and colleagues describe a new ceratopsian (horned) dinosaur that adds to broader debates on their evolution.
This new species, dubbed Stellasaurus ancellae, is based on a partial skull. The name, literally 'starry lizard', refers to the characteristic shape of the frill on the back of its skull (and also the David Bowie song Starman).
Although fragmentary, the preserved skull parts show that Stellasaurus differs from all other named ceratopsian species, and is distinguished by a combination of features relating to the shapes and sizes of its nose horn and the spikes projecting from the sides and back of the frill.
More interestingly, Wilson and colleagues posit that Stellasaurus provides evidence for an evolutionary lineage of horned dinosaurs that started with the multi-spiked form Styracosaurus and culminated in the stubby nosed Pachyrhinosaurus.
This series of species spans around 2.5 million years of the Late Cretaceous. All were found in the same area of the USA and Canada and all lived at different times. Crucially, when lined up through time, several distinct trends in skull evolution emerge: the long nasal horn disappears to be replaced by a thick nose boss, the spikes on the frill reduce in number, and the remaining pair of spikes becomes very elaborate.
Such evolutionary sequences are rare in dinosaurs and it could be that Stellasaurus is a transitional fossil, helping to show how the ancestral Styracosaurus turned into Pachyrhinosaurus through time.
- Read the paper in the journal Royal Society Open Science: A new, transitional centrosaurine ceratopsid from the Upper Cretaceous Twon Medicine Formation of Montana and the evolution of the ‘Styracosaurus-line’ dinosaurs.