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How did the biggest dinosaurs get so big?

Alongside Tyrannosaurus rex, the basic sauropod dinosaur is one of the most iconic and instantly recognisable of prehistoric animals. Not only is their elegant shape with four columnar limbs, a long muscular tail and a hugely long neck with a relatively tiny head perched atop very well known, so is their prodigious size.

At masses that were equivalent to those of large baleen whales (about 85 tons), the largest sauropods were far and away the largest land-living animals that have ever lived. This of course prompts the obvious question:

why did they get so big?

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Today saw the release of a collection of 14 papers under the banner Sauropod gigantism: A cross-disciplinary approachand 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.

Whatever the reason for past neglect, the tide has definitely been turning in the past decade or so. In particular, we have a major collaborative research unit, funded by the German federal government, looking into sauropod biology and in particular the evolution of their gigantism.

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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.

What is this proposed mix of traits? Put simply: a high basal metabolic rate and bird-style respiratory system including unidirectional airflow through the lungs (derived traits) combined with the production of many small offspring and very limited oral processing of food (primitive traits).

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. Wikimedia Commons
Rearing Barosaurus skeleton. Wikimedia Commons

 

In modern mammals, chewing muscles and the heads that have to support them have to grow larger relative to body size as absolute body size increases. Thus free from this constraint, sauropods were able to evolve relatively small heads that required far less energy to carry and to move around, thus allowing necks to elongate and feeding envelopes – the amount of food (“browse”) that an animal can reach without having to walk – to increase.

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.

One contribution argues strongly that all sauropods held their necks straight out in front of their bodies in a horizontal, or near-horizontal pose.

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.

One contribution argues strongly that all sauropods held their necks straight out in front of their bodies in a horizontal, or near-horizontal pose.

The method used was to scan in the complete skeleton and in the computer construct “convex hulls”, which are simpler three-dimensional shapes that enclose major regions of the skeleton.

From these an estimate for the volume of the animal can be made and from that an estimate of mass. The technique has been applied to animals of known masses with good results so the estimate of 85 metric tons for Argentinasaurus may not be too far off the mark.

Argentinosaurus femur. Wikimedia Commons
Argentinosaurus femur. Wikimedia Commons

 

However, it is worth remembering that much the scanned skeleton that the convex hulls were built around was itself modelled from scaled-up remains of types of related sauropods because the original remains of Argentinasaurus are so incomplete.

The method used was to scan in the complete skeleton and in the computer construct “convex hulls”, which are simpler three-dimensional shapes that enclose major regions of the skeleton.

From these an estimate for the volume of the animal can be made and from that an estimate of mass. The technique has been applied to animals of known masses with good results so the estimate of 85 metric tons for Argentinasaurus may not be too far off the mark.

However, it is worth remembering that much the scanned skeleton that the convex hulls were built around was itself modelled from scaled-up remains of types of related sauropods because the original remains of Argentinasaurus are so incomplete.

Contributing Source : theconversion

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Mark Milligan
Mark Milligan
Mark Milligan is multi-award-winning journalist and the Managing Editor at HeritageDaily. His background is in archaeology and computer science, having written over 7,500 articles across several online publications. Mark is a member of the Association of British Science Writers (ABSW), the World Federation of Science Journalists, and in 2023 was the recipient of the British Citizen Award for Education, the BCA Medal of Honour, and the UK Prime Minister's Points of Light Award.
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