Necks question... how did the biggest dinosaurs get so big?

Oct 31, 2013 by Adam Yates
Sauropods were huge animals, but why were they so massive - and did they really hold their necks like this? Credit: Mark Witton

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?

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.

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.

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 (primitive traits).

These traits are then hypothesised to have initiated five interrelated evolutionary cascades:

  • reproduction
  • feeding
  • head and
  • 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.

Credit: epSos.de

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.

In modern mammals, chewing muscles and the heads that have to support them have to grow larger relative to body size as absolute 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 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.

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.

An Apatosaurus has a drink. Credit: Wikimedia Commons

Huge body mass

Central to the issue of sauropod gigantism is , which is a surprisingly difficult thing to measure in extinct, incompletely known animals.

Many estimates for different sauropods have been published using different methods and results have varied wildly even those based upon the same specimen.

In this collection of papers there is an attempt to estimate of one of the largest of all known sauropods: Argentinasaurus (see the video below).

This video is not supported by your browser at this time.

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.

… a little like this. Credit: Wikimedia Commons

This is a persistent problem in trying to discover the upper limits of mass that sauropods reached: all of the supergiant sauropod remains that have been found so far are frustratingly incomplete. Some of the researchers suggest a way out of this problem - rather than trying to estimate volume, and subsequently mass, from what is left of their dead remains, why not try to estimate the force applied to the ground when they walked and thus calculate mass in that manner?

Put very crudely the deeper the print, the heavier the trackmaker. Unlike skeletons, the footprints of supergiant sauropods are moderately well-represented in the fossil record, and trying that method with footprints left by elephants of known weight gets good results.

However, the researchers do not yet take the next step and apply their method to fossil prints. For the method to work properly, the physical properties of the substrate, and how it deforms when a heavy animal steps on it, must be known precisely. I suspect that estimating these properties for substrates that have been compacted and lithified by geological processes will present a great challenge - but I look forward to seeing future attempts.

There is a lot more contained in these papers that I haven't discussed here. What is clear is that sauropods are fascinating and extreme examples of biological engineering. We have made big inroads into understanding how they worked as living organisms and how they got to be that way.

Yet there remains much left to be discovered and these awesome beasts from Earth's past will keep scientists of all sorts busy for many years to come.

Explore further: New evidence of ancient rock art across Southeast Asia

Related Stories

A sauropod walks into a bar. 'Why the long neck?'

Oct 30, 2013

A new PLOS Collection featuring research on the complex evolutionary cascade theory that made the unique gigantism of sauropod dinosaurs possible launched on October 30th. This Collection features new research ...

Ostrich necks reveal sauropod movements, food habits

Aug 14, 2013

A new analysis of ostriches reveals that a computer model of long-necked sauropods used to simulate the dinosaurs' movements, featured in BBC's Walking with Dinosaurs and the focus of an installation at the ...

Some sauropods really did hold their long necks high

Jun 03, 2010

(PhysOrg.com) -- A new study suggests the long necks of sauropod dinosaurs really were held high, in spite of theories suggesting they were more likely to keep their necks low because of the very high blood ...

Plant-eating dinosaurs replaced teeth often, carried spares

Jul 17, 2013

Some of the largest herbivorous dinosaurs replaced their teeth at a rate of approximately one tooth every 1-2 months to compensate for tooth wear from crunching up plants, according to research published July 17 in the open ...

Found: First complete remains of early sauropod dinosaur

Oct 28, 2010

Scientists have discovered in China the first complete skeleton of a pivotal ancestor of Earth's largest land animals – the sauropod dinosaurs. The new species, tentatively dubbed Yizhousaurus sunae, lived ...

Recommended for you

Woolly mammoth skeleton sold at UK auction

12 hours ago

The skeleton of an Ice Age woolly mammoth fetched £189,000 ($300,000, 239.000 euros) at auction Wednesday as it went under the hammer in Britain with a host of other rare or extinct species.

Recreating clothes from the Iron Age

20 hours ago

A few years ago, the oldest known piece of clothing ever discovered in Norway, a tunic dating from the Iron Age, was found on a glacier in Breheimen. Now about to be reconstructed using Iron Age textile techniques, ...

User comments : 21

Adjust slider to filter visible comments by rank

Display comments: newest first

Egleton
1.5 / 5 (8) Oct 31, 2013
Very interesting thank you. My thought was that the flesh of the Sauropods might have had large cavities to lessen the weight. Even with this lessened weight the bones would need to be robust.
Perhaps the size was the result of sexual selection in the same way that a peacocks feathers are.
The females would not have known that the object of her desire was naught but a bag of wind.
Mr_Science
1 / 5 (13) Oct 31, 2013
It would seem that natural selection is a large part female selection.
Lurker2358
1.6 / 5 (14) Oct 31, 2013
I thought they might be good at living in the water. The water could help support some of the weight through buoyancy, and they have these long, serpentine bodies which also seem suited for movement through water, much like a crocodile, except just much fatter and taller.

I was not aware of such huge specimens as depicted in that diagram. When I last seriously was interested in Dinosaurs, as a child, the "Supersaurus" was at that time the largest specimen I knew about. The ones depicted in that diagram even dwarf that thing. I didn't realize there was a specimen estimated to be 85 tons. Imagine a 40 ton bulldozer, and then imagine that creature.* see below.

What about life cycles? How do they prove some of these aren't just the same species in a different stage of life: Juvenile, healthy adult, middle age, very old, etc.

Perhaps they never stop growing until they die, so the really successful ones just keep getting bigger and bigger.
Lurker2358
1.6 / 5 (14) Oct 31, 2013
A bulldozer leaves deep groves in clay even though it's treaded, well it's intended to cut in for grip, and then stop when the flat surface hits, but if you've ever watched, the entire tread really sinks in under the insane weight of the machine.

Now imagine an 85 ton creature having all of it's weight on just it's 4 feet, with very low relative surface area to distribute all that weight.

I find it hard to believe even a 6 ton predator would have dared to bother one of these things. It'd get squashed like a bug with a kick or that big tail.
Megapixel
1.3 / 5 (16) Oct 31, 2013
All of the characteristics mentioned would have contributed to the massive size of the sauropods. However, we must remember that certain mammals, such as indricotherium reached enormous proportions without these characteristics.

Lower surface gravity is the only logical explanation for dinosaur and mammal gigantism. The theory that offers a logical explanation is the Gravity Theory of Mass Extinction:
www.dinoextinct.com/page13.pdf
Humpty
1.3 / 5 (15) Nov 01, 2013
Godzilla was even bigger than all the ones here.

I know - I saw it in several Japanese documentaries.

http://sadhillnew...news.jpg
Mr_Science
1.3 / 5 (12) Nov 01, 2013
It would seem that natural selection is a large part female selection
It would lead into evolution of big tits instead of necks...;-)

I ment the females selections of males. Lets face it, guys are not nearly as picky.
Megapixel
1 / 5 (15) Nov 01, 2013
In response to Franklins, let me clarify.

Gravity within the solar system did not change. Surface gravity on the Earth changed due to the movement of the core elements, i.e., inner core, outer core and the densest part of the lower mantle. With respect to dinosaurs, surface gravity was lower on the supercontinent of Pangea and surface gravity would have been higher antipodal to the supercontinent.
Megapixel
1 / 5 (15) Nov 01, 2013
Franklins,
If the Earth's core elements move away from Earth-centricity (and away from you), you will weigh less because of Newton's Law of Gravitation. Of course the magnitude of the reduction in surface gravity depends on the distance moved from the Earth's center. No Earth expansion or collapse is necessary.

When a supercontinent, such as Pangea, forms and moves to high latitudes, which it did periodically, a scientific principle known as the conservation of angular momentum comes into play. When Pangea moved to higher latitudes, the distance from Pangea to the spin axis of the Earth decreased. This would have lower the Earth's angular momentum but the law cited above cannot be violated. The offsetting action that preserved the Earth's angular momentum was the core elements moving away from Earth centricity.

The core element movement was sufficient to lower the surface gravity on Pangea significantly due to the increased distance from Pangea to the shifted core elements.

Zephir_fan
Nov 01, 2013
This comment has been removed by a moderator.
barakn
4.2 / 5 (5) Nov 01, 2013
Megapixel's theory was aired and debunked here: http://phys.org/n...tic.html No need to waste any more time or energy on this.
Megapixel
1 / 5 (15) Nov 01, 2013
Zephir_fan and barakn,

If you want to challenge this theory, you are welcome to. But, I doubt whether either of you can.
Zephir_fan
Nov 01, 2013
This comment has been removed by a moderator.
Lurker2358
1 / 5 (14) Nov 02, 2013
Dude that is about the stupidest idea anyone has offered up on this site in at least the last week. That means it is competing with LurkingAtSchoolyards, HannesThePhiloAlfven, Can'tDriveOrDoScience, BenniThereAin'tDoneThat and TuxTheDuckMan. You are in first place for the dumbest comment of the week. Where do you people get these silly "theories" from? They sound like something you got watching cartoons on the Idiot's-R-Us-Channel.


Reported for defamation.
Zephir_fan
Nov 02, 2013
This comment has been removed by a moderator.
Megapixel
1 / 5 (15) Nov 02, 2013
Franklins and teech2,

The theory I provided a link to explains how surface gravity can change. It is based on a simple principle which is, the Earth's core elements can move off-center. I think you would agree that if that happens the surface gravity around the globe will vary.

During the reign of the dinosaurs all continents had coalesced to form a supercontinent, Pangea. When the core elements moved away from Pangea, its surface gravity lowered.

The underlying reason for the movement of the core elements in the case of Pangea is the supercontinent's latitudinal movement. As I mentioned previously, this would change the Earth's angular momentum if nothing else compensated for the change. The theory in question posits that the offsetting of the core elements was the compensating factor, and therefore, the cause of changes in surface gravity.
continued in next post.......
Megapixel
1 / 5 (15) Nov 02, 2013
continued.....
Gigantism during the Holocene occurred because, in a similar way, ocean water was transferred to the poles and formed ice. In other words, instead of continental mass (e.g., Pangea) moving to higher latitudes, ocean mass (i.e., water) moved to very high latitude. How this mass was distributed at the poles determined which surface of the Earth experienced lower surface gravity. And, this more recent change in surface gravity would develop and change much more rapidly than during the Mesozoic because the formation and dissipation of polar ice will occur on a much shorter timescale than continental movement.

worddigger
1.2 / 5 (13) Nov 02, 2013
If you rule out less gravity as the cause of the Sauropod gigantism, then the more obvious cause could have been a much denser atmosphere and greatly increased surface pressure on the face of the earth at that time. Is anyone aware of any kind of paleo-proxy method that could be employed to test such a hypothesis?
Megapixel
1 / 5 (13) Nov 03, 2013
Franklins,

Your last post is straying away from the topic of why dinosaurs got so large.

When ice accumulates over land it does compress the underlying mantle. This could cause very minor, local changes to the surface gravity. Scientists have used satellites to measure these gravity anomalies which are caused by differences in mantle densities near the surface. This is not related to the theorized offsetting of the core elements causing potentially large changes in surface gravity.

worddigger,

Changes in atmospheric pressure has been suggested as a cause of dinosaur gigantism but there is little support for this. If air pressure was high, for example, sauropods would have extremely narrow bodies so that the atmospheric force (air pressure X horizontal surface area) would be at a minimum.
Also, the largest pterosaurs, e.g. Quetzalcoatlus northropi, would not have developed huge wingspans if air pressure was high. Birds that can live in water, a high density medium, evolved smalle
Megapixel
1 / 5 (13) Nov 03, 2013
continued.......
Birds that can live in water, a high density medium, evolved smaller wings (e.g. penguins).
Zephir_fan
Nov 03, 2013
This comment has been removed by a moderator.
Zephir_fan
Nov 03, 2013
This comment has been removed by a moderator.
Humpty
1.3 / 5 (12) Nov 03, 2013
The earth used to spin way faster back then so everything got kind of flung towards outer space - not enough to throw them off the planet, but enough to make them way longer - like the brontosauraus should be drawn with it's head and neck as vertical, along with it's tail.
cantdrive85
1 / 5 (12) Nov 03, 2013
The earth used to spin way faster back then so everything got kind of flung towards outer space - not enough to throw them off the planet, but enough to make them way longer - like the brontosauraus should be drawn with it's head and neck as vertical, along with it's tail.

The reverse of black hole spaghettification? I think I saw a Plastic Man episode where he suffered from such an event. Call it dinospinplastification stretching.

BTW, the only logical explanation is lower gravity. This is another aspect of the EU paradigm providing for a more complete explanation of the physics required to enable such large flora and fauna.
Urgelt
5 / 5 (1) Nov 04, 2013
Is the moon full? The cranks are swarming, for some reason.

A very interesting article. Thank you.

I was wondering what selection mechanisms might be operating to *limit* growth of sauropods. What sets the upper limits as to body mass? Bone, cartilage, and muscle characteristics? (I often wonder if sauropods enjoyed some structural advantages over smaller animals, derived from some marvelous differences in the composition of their bones and soft tissues.) Metabolic efficiency of mitochondria? Oxygen content of the atmosphere? Available fodder? Heat dissipation mechanisms? Vulnerability to predation? (Get too large and the animal becomes too slow to avoid predators?) There's so much we don't know about these impressive animals!

Gigantism in mammals tends to be (but is not always) related to climate (colder = larger). Does that dynamic apply to sauropods?
Humpty
1 / 5 (10) Nov 05, 2013
They lived in the plasticine error, a time when stretching taller was much pronounced.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.