 |
| Sauropods were huge animals, but why were they so massive - and did they really hold their necks like this? [Credit: Mark Witton] |
Why did they get so big?
Today saw the release of a collection of 14 papers under the banner Sauropod gigantism: A cross-disciplinary approach and published in the online open access journal PLOS ONE.
Many theories have been suggested, running to the wildly fanciful and improbable such as proposals that the Earth’s gravity was lower in the Mesozoic Era (around 252 to 66 million years ago).
Somewhat puzzling then is the paucity of proper scientific study that these magnificent beasts have attracted in the past. Why this might be so is not clear; perhaps it is partly to do with the sheer difficulty and expense of extracting and dealing with such large yet exceedingly fragile fossil bones.
 |
| An Apatosaurus has a drink [Credit: WikiCommons] |
Headed up by Professor Martin Sander at Bonn University, the unit includes 13 working groups from several different disciplines in science. So far they have published well over hundred papers and a comprehensive book summarising their work on the biology of sauropod dinosaurs - and today, add these 14 new papers to the literature.
This collection adds new research into several aspects of sauropod biology and takes a look at how the unit’s overarching model for the evolution of sauropod gigantism is faring with continued testing and investigation, from both within and without of the research unit.
Evolutionary cascades
At the heart of the research unit’s effort lies the “Evolutionary Cascade Model”, or ECM for short. This model posits that it was the sauropod ancestor’s unique mix of primitive and derived life history, physiological and functional anatomic traits that led to several evolutionary cascades of changes, that fuelled by positive feedback loops, that drove sauropod body size up beyond that of any other terrestrial animal group.
 |
| [Credit: epSos.de] |
These traits are then hypothesised to have initiated five interrelated evolutionary cascades:
- reproduction
- feeding
- head and neck
- birdlike lung
- metabolism.
To look at how just one these cascades might work, let’s look at the feeding cascade.
If we start with the primitive trait of little to no chewing of food (and I should add at this point sauropods were undoubted strict herbivores) early sauropods needed little time between the acquisition of the food and swallowing it, which meant that they could have a high food intake rate.
Indeed through the evolution of sauropods we see the evolution of several specialisations to support increased food intake rates such as very fast tooth replacement, widening gaps through broader jaws and loss of cheeks.
This produced a selective advantage of obtaining more energy from the environment, provided that there was a larger gut capacity to deal with the high input of poorly chewed food, and selecting for larger body size.
Tiny head, long neck
To show how different cascades were linked we can see that the feeding cascade was also intimately linked to the head/neck anatomy cascade. The lack of oral processing of the food meant that the head did not have to carry a massive set of chewing muscles to deal with the increased load of plant fodder.
 |
| Rearing Barosaurus skeleton [Credit: WikiCommons] |
By swinging their tiny head around on a very long neck, a huge amount of browse is available at little energetic cost, allowing the evolution of faster food intake rates, larger guts and larger body mass.
This is just one cascade chain in model that contains four others. In effect what the unit is proposing is a particularly complicated version of “correlated progression”, a model that has been proposed to explain a number of major transformations in macroevolution such as the origin of turtles and mammals. In correlated progression many traits are interrelated and evolution progresses by small changes in all of them occurring side by side in parallel.
Neck angle
So has a unified, monolithic picture of sauropod biology emerged? Not quite. It is interesting to see that even within this collection there is still dissention between various researchers on the question of just how those immensely long necks of sauropods were deployed.
 |
| … a little like this [Credit: WikiCommons] |
These conclusions are based largely on digital models of the skeleton where each bone is articulated with its neighbours and manipulated so that the joint facets are minimally or maximally overlapping. This yields both a total range-of-motion (ROM) and something known as the osteologically neutral position (ONP), where the joint surfaces are maximally overlapped and the bones fit together most “comfortably”.
The researchers finds that the ONP has the sauropods neck sticking straight out, rather than raised up in a swan-like curve, and that the ROM does not allow the head to be lifted very far in the vertical plane although it does allow for wide lateral sweeps of the neck.
This would imply that despite their superficially giraffe-like necks no sauropods were committed high browsers.
Not so fast, says a different paper, which argues that the ONP tells us nothing about the typical attitude that the neck was held in life and that these skeletal models fail to take in the effect that soft tissues such as joint cartilages and intervertebral discs would have had on both the ROM and the ONP.
Huge body mass
Central to the issue of sauropod gigantism is body mass, which is a surprisingly difficult thing to measure in extinct, incompletely known animals.