Was the Atmospheric Pressure Different at the Time of Dinosaurs?
Chemical Engineering Department,
Oregon State University, Corvallis OR 97331
If we visit dinosaurland we will come across some of the most amazing and puzzling phenomena. For example: how can a flying creature as large as a giant quetzalcoatlus (12-15 m wingspan) actually fly when aerodynamic theory and biology both say that it cannot. Also, how can a giant dinosaur such as an apatosaur pump blood up to its brain (more than 13 m above its heart) when animal energetics and physics say that in no way can it do it.
Do we have to change the laws of physics and biology? I don't think so. Instead we have to accept that ancient Earth's atmosphereic pressure was very different from what it is today. This talk will explore this new picture of our earth's past. Come explore it with me.
Scientists have exhaustively studied Earth's surface extracting its history from rocks and ocean bottoms. However, little attention has been given to the history of Earth's atmosphere because its historical record is ephemeral. In fact, most scientists have just accepted that the atmosphere was not much different in the past from what it is today. True, some [1, 2] have speculated that the CO2 concentration was as much as 800 times larger than today's value, or about 0.25 bar, but little else is assumed to differ.
In the Cretaceous fossil record we find flying creatures which have an estimated mass between 86 and 100 kg . The Washington DC Museum of Natural History displays a full sized model of the Quetsalcoatlus having a 13-15 m wingspan, while a Texas find is estimated to have a wingspan of 15.5 m . This is about half the wingspan of a Boeing 737 commercial airliner, see Fig 1. How could such a large creature fly?
Today, the world's largest flying birds, the South American condor, the Australian kori bustard, and the largest European swan have wingspans no more than 4 m. Considering the limitation of skeletal and muscle structure, physiologists and aerodynamicists [5, 6] estimate that these birds which weigh up to 14.5 kg, represent the upper size limit of creatures that can support and propel themselves through air. How then could 86-100 kg creatures fly in the age of dinosaurs, 64-100 Mya? Let's see what explanations have been forwarded to date to resolve this anomaly.
1. The biology of these ancient flyers differs from today's flyers in that they were more efficient in their use of oxygen, living at a much higher metabolic rate. From biological considerations this is quite unlikely.
2. These creatures were not true flyers. They stay on the ground and waited for a strong wind. With a wind speed of over 5 m/s they would spread their wings and glide about.
3. They sat on top of hills peering down. When they spied dinner hopping about down below they would swoop down, snatch their meal and then trudge back up the hill, to rejoin their cousins there, see Fig 2.
However, an analysis by Bramford and Whitfield  raised all sorts of difficulties with these explanations. They suggest:
1. The pteranodon could not stand bipedally because its legs were positioned wrongly on its body, see Smith .
2. It probably slid along on its stomach by reaching forward with its legs gripping the ground with its feet and pulling itself along, as does a crawling bat. Hankin and Watson  and Abel  come to similar conclusions.
3. Most importantly, large pteranodons appear to have lacked the physical power to perform hovering, thus could not have taken off from level ground. So it probably lived at edges of cliffs, see Romer .
4. To counter this deficiency others proposed that the Andes in the southern half of South America somehow did not exist 60 Mya, so the strong westerly winds (the "roaring 40's") could sweep practically continuously across the low-lying continent unopposed by any mountain range. This allowed pterosaurs to fly, see Fig. 3.
All these difficulties lead to improbable scenarios. To have survived and thrived for millions of years, these flyers had to be fast, efficient, and well adapted to their environment.
Let's see what physics and physiology can tell us.
The power of resting warm-blooded creatures (in effect, their metabolic rate) is represented by the mouse-to-elephant curve , see Fig. 4, with its representative equation
where M is the creature's body mass.
Now the minimum power needed for the level flight of any creature was given by Renard  over a century ago as
where A is the wing area of the creature and p is the air density. It was pointed out by von Karman  that this expression is essentially what is used today by aerodynamicists and aircraft designers to represent the power needed to keep an aircraft aloft, from Piper Cub to the largest of passenger planes.
If L represents the size of the flying creature, then for creatures of different size but of similar geometry
Replacing eq. 3 in eq. 2, we find that the power needed for a creature to fly is given by
This is shown in Fig. 5.
Next compare the power needed to fly with the power available, see Fig 6.
This graph shows that there is always a maximum size above which no creature can fly. This limit today is the 4 m wingspan 14.5 kg bird. But how do we explain the existence of the 15 m wingspan , 86-100 kg quetzalcoatlus? Since the power needed is lower at higher atmospheric pressure, see eq. 2, let us propose that the atmosphere in the Cretaceous period was different from today's in that it was denser.
Comparing masses (86-100 kg vs. 14.5 kg) and assuming geometrical similarity, eq. 1 and eq. 4 combined leads us to conclude that the atmospheric pressure at the time of the quetzalcoatlus had to be about 3.2-4.8 bar. This is significantly greater than today's 1 bar. Graphically, we illustrate this conclusion in Fig 6.