The giant sauropod dinosaurs such as Diplodocus and Apatosaurus are familiar thanks to their huge sizes and unique body shapes of a long neck and tail with a tiny head perched on top. Among their ranks were the largest terrestrial animals of all time, and yet an enduring mystery remains: how did so many animals of this size get to be so big?

Take a look at modern faunas. There are rarely more than one or two megaherbivores – multi-tonne giants that can (and generally do) eat any plant material they can get hold of. Although we are lacking a number of big species now thanks to various relatively recent extinctions, it is still pretty normal for mammal faunas to have only a handful of giant herbivorous species over a tonne or so.

For the sauropods, though, there were often numerous species (many of which would easily top 10 tonnes, and plenty were many times this figure) in the same ecosystem. Even allowing for the propensity of dinosaurs to get very big, this is something of an ecological anomaly. Large species are typically not too selective about what they eat (their size allows them to effectively digest even tough things like wood, while they can’t afford to be picky eaters and go for only small, if high-quality, foods such as seeds) so we would only expect a few species to be able to coexist.

A new study published in the Proceedings of the Royal Society of London, however, tackles this problem. It had been noted that the skulls and teeth of sauropods could be used to break them down into two basic groups: those with relatively lightly built skulls and teeth suitable for nipping, and animals with stronger skulls and larger teeth apparently more suitable for tougher foods. So researchers sought to compare the two for their mechanical performance and see if these different heads really were well suited for different tasks.

Digital scans were made of the skulls of Diplodocus and Camarasaurus – animals that lived alongside each other in the Jurassic of North America and that are now preserved in rocks termed the Morrison Formation. The two sauropods selected represent the nipping and strong skull types, respectively, and are excellent examples of the two basic forms and, unlike many sauropods, both have good skull material available. Based on marks on the skulls and comparisons to living animals, the muscles of the jaws could be added to the model to give the heads real bite. This then allows researchers to assess how much power could be delivered and how the arrangements of the heads would resist the forces from biting.

Camarasaurus was seen to have a much stronger bite and was better able to deal with such stress in the bones of the cranium. This implies they were adapted to a more coarse diet and although they doubtless would take what they could get, they would have been able to effectively handle tough, and especially woody, food. This is also a good match for wear marks on the teeth that indicate a diet of primarily coarse foodstuffs, but the pattern contrasts sharply with Diplodocus. Here the skull was weaker and could not deliver so powerful a bite. Instead the head seems better adapted to softer vegetation, and both wear on the teeth and adaptations seen in the bones of the neck show that these animals would feed by stripping branches and twigs of their leaves rather than biting through them.

The lead author on the study, David Button of the University of Bristol, said: “The Morrison Formation represents a particularly interesting case in Earth’s history as it presents a staggering diversity of sauropods, but yet was a seasonally semi-arid environment not conducive to excessive plant productivity. The results from our various analyses corroborate previous works looking at gross anatomy and tooth wear, and together give us a decent grounding of the ecology of these animals. Although a lot of work still needs to be done to refine this picture, we can see that their ecological interactions were really pretty sophisticated, which would have been important in supporting the observed diversity.”

A second analysis in the study compared a whole series of sauropod skulls in terms of their shape and structure. This shows that other sauropods in the Morrison that lived alongside Diplodocus and Camarasaurus fell somewhere on the gradient between these two extremes, or even adopted very different skulls shapes entirely.

In short, although these huge animals would have been strongly competing for food, they had evolved rather different ways of tacking the available plant material, which would have reduced competition between them. This separation would have been enhanced (in adults at least) by different neck lengths and body sizes, which would have facilitated different species feeding at different heights on the available plants, just as giraffe, rhino and zebra typically feed at different levels today. As Button notes, “sauropod skulls also provide interesting data in the broader context of the evolution of sauropod feeding mechanisms, where we see the repeated evolution of similar functional complexes.”

There is clearly further complexity to be unravelled here, for example juveniles of large species might well have interfered with the adults of those with a lesser reach, and some sauropod faunas have such huge diversity that a few different head shapes do not seem to separate them far enough to have allowed so many species to coexist. Even so, this analysis is a major step forwards in our understanding of sauropod evolution and ecology and how this group of enigmatic giants ate their way across the Mesozoic landscapes.

Button, D.J., Rayfield, E.J., Barrett, P.M. 2014. Cranial biomechanics underpins high sauropod diversity in resource-poor environments. Proceedings of the Royal Society B.

Note : The above story is based on materials provided by  Dr Dave Hone for theguardian.com

How the largest animals to have ever walked the Earth fed, and how this allowed them to live alongside one another in prehistoric ecosystems is the subject of new research from the University of Bristol and the Natural History Museum, London.

The sauropods — large, long-necked plant-eating dinosaurs such as Diplodocus and Brachiosaurus — dominated the land between 210 and 65 million years ago. They were the largest land animals of all time, with the biggest weighing 80 tonnes (more than 11 elephants) and would have needed vast amounts of food.

Despite this, multiple sauropod species often lived alongside each other. The most notable example is the community of the Late Jurassic Morrison Formation, a distinctive sequence of sedimentary rock in the western United States from which over 10 species of sauropod are known.

How so many giant herbivores could have coexisted has long been a mystery: even the highly diverse faunas seen in modern Africa only support one truly gigantic species, the elephant. This is made even more puzzling by the harsh, semi-arid environment of the Morrison Formation during the Jurassic, which would have limited plant growth.

A study conducted by David Button, a PhD student in Bristol’s School of Earth Sciences and the Natural History Museum, and colleagues used a novel combination of approaches to investigate this problem.

Although sauropods were gigantic, their heads were comparatively very small and so how they ingested enough food has puzzled many scientists. The researchers focussed on the skull and jaws of sauropods, using a variety of biomechanical techniques to investigate how they functioned and what this would mean for sauropod ecology.

Using CT scans, the researchers digitally reconstructed the skulls of the sauropods Camarasaurus and Diplodocus, along with the jaw and neck muscles of both species from the traces left on the bones where these muscles were attached in life. These two species are very common in the Morrison Formation, and are known to have widely co-existed. From this data, a biomechanical computer model of the skull of Camarasaurus was built using Finite Element Analysis (FEA), a modelling technique often employed in engineering and design to calculate stress and strain distribution in complex shapes. This model was then compared to a pre-existing model of Diplodocus in order to investigate how the dinosaurs fed.

David Button said: “Our results show that although neither could chew, the skulls of both dinosaurs were sophisticated cropping tools. Camarasaurus had a robust skull and strong bite, which would have allowed it to feed on tough leaves and branches. Meanwhile, the weaker bite and more delicate skull of Diplodocus would have restricted it to softer foods like ferns. However, Diplodocus could also have used its strong neck muscles to help it detach plant material through movements of the head. This indicates differences in diet between the two dinosaurs, which would have allowed them to coexist.”

The researchers also used a series of biomechanical measurements from other sauropod species to calculate the functional disparity in their skulls and jaws and found that other Morrison Formation sauropods were also highly varied in feeding adaptations, suggesting different diets.

Co-author, Professor Emily Rayfield of the University of Bristol said: “In modern animal communities differences in diet such as this — termed ‘dietary niche partitioning’ — allow multiple similar species to coexist by reducing competition for food. Although, dietary niche partitioning has been suspected between Morrison Formation sauropods based on their structural features and patterns of tooth-wear, this is the first study to provide strong, numerical, biomechanical evidence for its presence in this fossil community.”

The research also helps to shed light on the evolution of sauropod feeding mechanisms and how these gigantic creatures managed to eat enough food to sustain their tremendous bulk. Whereas earlier sauropods would have been able to eat a wide range of plants, later lineages show the parallel evolution of traits suggesting that they were more specialized in their feeding habits.

Co-author Professor Paul Barrett of the Natural History Museum added: “Our study provides insight not only into the ecology of dinosaurs but more generally into the mechanisms supporting species-richness in other animal communities, both from the fossil record and in the present-day.”

Note : The above story is based on materials provided by University of Bristol.