This post is for Jose Canseco.
Let me explain.
Earlier this week I wrote about how the largest dinosaurs – 100-foot-plus titans such as the sauropod Supersaurus – got to be so mind-bogglingly huge. The post was a response to baseball player Jose Canseco’s tweets that weaker Jurassic gravity allowed such enormous creatures to exist in the distant past.
As astronomer Matthew Francis elucidated in a piece coordinated with mine, gravity hasn’t significantly changed since the heyday of enormous dinosaurs. And in my own contribution, I outlined how unique aspects of dinosaur biology – such as air sacs and reproducing by laying eggs – explain why immense sauropods were so much larger than any land animals before or since. Here’s the kicker – Canseco actually enjoyed the post, and asked a follow-up question about dinosaur blood pressure!
Here’s Canseco’s question:
On the surface, the idea that long-necked sauropod dinosaurs had pseudo-hearts or blood accelerators seems like a fantastic sci-fi invention. But it’s an idea that paleontologists have actually entertained and addressed in their quest to understand the huge animals. There’s no evidence that dinosaurs of any sort had bizarre accessory hearts, but the idea has still played a small role in the ongoing investigation into how giant dinosaurs actually lived. To start, we have to go back to ancient bones and the ways in which paleontologists put them together.
We all know, as the fictional fossil expert Anne Elk once described, that sauropod dinosaurs were “thin at one end; much, much thicker in the middle; and then thin again at the far end.” True enough. But how the bulky dinosaurs managed to hold up their anterior “thin end” – the neck – depends on aspects of bones, soft tissue, and biology that are still being debated by paleontologists. And, of course, sauropods were not a totally uniform group of dinosaurs that all did the same thing. Still, to get at how these dinosaurs kept blood pumping to their brains, we first have to consider how sauropods held those elegant necks in the first place.
Some researchers, such as Kent Stevens and J. Michael Parrish, have proposed that the best fit for the cervical vertebrae of sauropods such as Apatosaurus and Diplodocus creates a neck posture held almost straight out and “gently declined” from the rest of the body. In this reconstruction, the dinosaurs would have been giant Mesozoic vacuums – snarfing up ferns and other low-lying plants within the reach of their necks.
Other paleontologists disagree. In 2009, Mike Taylor, Matt Wedel, and Darren Naish made the case that sauropod dinosaurs typically held their necks aloft. The evidence not only came from the way dinosaur bones fit together, but from the anatomy and posture of living animals.
Among mammals, birds, lizards, crocodylians, and other land-dwelling vertebrates alive today, “the neck is maximally extended and the head is maximally flexed, so that the mid-cervical region is near vertical,” wrote Taylor and collaborators. The same was probably true of sauropods, too. “Unless sauropods behaved differently from all extant amniote groups,” Taylor and coauthors argued, “they must have habitually held their necks extended and their heads flexed.” The dinosaurs could have lowered their necks to graze, but, according to the study by Taylor and colleagues, the dinosaurs held their necks high as they tromped around the Mesozoic. Even though the neck posture debate isn’t over, I believe that Taylor, Wedel, Naish, and other authors have made a strong case that sauropods typically held their necks high.
Neck posture matters for estimates of blood pressure. Just think of Supersaurus, with its 50-foot neck. The sauropod would have required a considerably more powerful heart and higher blood pressure to pump blood to a head held aloft than to one that was kept lower down, closer to the height of the heart. Researchers such as Roger Seymour and Harvey Lillywhite have used this physical constraint to argue that sauropods must have maintained their necks in a low-slung posture, similar to what Stevens and Parrish proposed.
After calculating the pressure that a sauropod such as the approximately 80-foot-long Barosaurus would have required for blood to reach the dinosaur’s head (estimated at 700mmHg), Seymour and Lillywhite wrote that the left ventricle of the dinosaur’s heart alone would have weighed two tons, or about fifteen times heavier than the left ventricle of an equally-long fin whale. And even if such an enormous heart existed, the researchers reasoned, the organ would have been grossly inefficient. Such a monstrous heart seemed unlikely. Based upon their estimates and calculations, Seymour and Lillywhite concluded that sauropod dinosaurs probably had more reasonably-sized hearts and, therefore, did not have the pumping power to hold their heads high.
Could sauropods have had some kind of workaround to escape the constraints that Seymour and Lillywhite outlined? Robert Bakker thought so. In a 1978 article speculating on dinosaur feeding behavior and the origin of flowering plants, Bakker suggested that sauropods “could have used contractions of neck musculature as a relay pump to carry the cranial arterial supply.” Yet no evidence of such a feature has ever been found in sauropods or, for that matter, any vertebrate. More than that, there’s no developmental mechanism that would explain parts of blood vessels being co-opted into heart-like pumping stations, not to mention that such a series would have required a great degree of coordination between the hearts and the dinosaur’s brain. Lacking a living sauropod to study, the idea is untestable. Likewise, a proposal that big sauropods had a special “siphon” system – in which falling blood facilitated rising blood – failed to gain acceptance. So far as researchers have been able to discern, sauropods did not have accessory hearts or a special siphon system to help them circulate blood.
So what did sauropods do? Were they permanently restricted to a low-slung lifestyle of sucking down low-lying plants? Probably not. As Andreas Christian pointed out in a 2010 study of the dinosaur Euhelopus, the anatomy of some sauropods very clearly demonstrates that there were species which kept their necks habitually elevated. And as costly as this might be in energetic terms – such as fueling a powerful heart – the advantage gained by being able to feed high and low may have circumvented the drawbacks. In terms of expending energy, Christian noted, standing in one place to feed over a large vertical swath is less expensive than having to walk great distances when you’ve got a high-running metabolism to nourish. Some sauropods undoubtedly held their heads aloft, as much as 26 feet above the level of their heart in a dinosaur like Giraffatitan. The puzzle is how their bodies were capable of achieving such a wonderful feat. We need to know much more about dinosaur organs and physiology to solve the question.
Despite anatomical clues indicate that some, and perhaps most, sauropods held their heads high, the fact is that paleontologists have yet to solve the mystery of how the dinosaurs solved biological problems involving bloodflow. As Bergita Ganse and coauthors relate in a recent review of sauropod physiology, “The cardiovascular system is still a field of speculation.” Barosaurus and even larger dinosaurs probably required big hearts and exceptional blood pressures, but, lacking soft tissue structures, researchers can only approach such biological details in outline.
And all of this is to say nothing of other circulation problems that come with being big. When a Supersaurus lowered its neck to feed at ground level, the dinosaur would have risked a terrible rush of blood to the head. Although very, very distantly related – seeing as they are mammals – giraffes hint at one solution for coping with this problem. The long-necked mammals have a web of little arteries called the rete mirable, Ganse and coauthors point out, which helps prevent blood pressure from skyrocketing while the giraffe’s neck is lowered. Sauropods could have evolved a similar structure. No one has yet found evidence of such an arrangement for sauropods, but it’s still a hypothesis worth entertaining since the feature exists in another long-necked vertebrate.
Then there’s the question of how sauropods prevented blood from pooling in their limbs. Once the blood went out to these extremities, how did it overcome gravity to go back to the heart? Again, mammals hold possible answers. In horses, Ganse and coauthors point out, there’s a kind of cushion that sits between the sole and the foot bones. Blood collects in the cushion, and is squeezed out into the veins when the horse steps. Giraffes have a different way of getting around the same problem. In addition to fluid pressure and details of the capillaries, the tight skin of giraffe legs helps keep blood moving through spaces where it might otherwise accumulate. We may never know how sauropods got around the same problem, but, given their size and facts of their biology, they must have had some mechanism to prevent edema. Foot cushions or tight skin are reasonable possibilities.
Sauropods are familiar dinosaurs. Apatosaurus, Brachiosaurus, and Diplodocus are some of the first dinosaurs we meet as children, and, for over a century, the awesome size of these dinosaurs has sparked our imagination. Despite our close acquaintance with Supersaurus and kin, though, there are many aspects of basic sauropod biology that still confound us. In many ways, we’re still only getting to know these giants. Paleontologists are assembling a more refined look at sauropods than has ever been possible before, yet it’s the questions – the persistent mysteries – that keeps sending scientists to pore over bone and wonder about long-lost soft tissues. We may not know as much as we would like about the sauropod cardiovascular system, but the monumental dinosaurs have certainly found their way into our hearts.
Bader, H., Hicks, J. 1996. Circulation to the head of Barosaurus revisited: Theoretical considerations. Comparative Biochemistry & Physiology A. 114:197-203
Bakker, R. 1978. Dinosaur feeding behaviour and the origin of flowering plants. Nature. 274: 661-663
Christian, A. 2011. Some sauropods raised their necks – evidence for high browsing in Euhelopus zdanskyi. Proceedings of the Royal Society B. 6, 6: 823-825
Ganse, B., Stahn, A., Stoinski, S., Suthau, T., Gunga, H. 2010. Body mass estimation, thermoregulation, and cardiovascular physiology of large sauropods, pp. 105-115 in Klein, N., Remes, K., Gee, C., Sander, P. Biology of the Sauropod Dinosaurs. Bloomington: Indiana University Press.
Seymour, R. 2009. Raising the sauropod neck: it costs more to get less. Biology Letters. 5, 3: 317-319
Seymour, R., Lillywhite, H. 2000. Hearts, neck posture and metabolic intensity of sauropod dinosaurs. Proceedings: Biological Sciences. 267, 1455: 1883-1887
Stevens, K., Parrish, J. 1999. Neck posture and feeding habits of two Jurassic sauropod dinosaurs. Science. 284, 5415: 798-800
Taylor, M., Wedel, M., Naish, D. 2009. Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica. 54, 2: 213-220.