In another example of slow-cooked science, this paper was the culmination of over three years worth of work collecting data on tegus. For the study, the authors looked at adult black and white tegus (Salvatore merianae). Tegus are an interesting group of lizards. They are the largest members of the family Teiidae and are often referred to as the monitor lizards of the new world, due to their convergent lifestyles (highly predaceous, active foragers). Besides their varanid-like demeanor, tegus are also known for their enormous jowls, especially in the males. The jowls hold the pterygoideus muscles, the big jaw snappers, which have been shown to increase in size for males during the breeding season (Naretto et al. 2014). As reptiles, tegus have been assumed to follow the standard ectothermic lifestyle of requiring external sources of heat to warm their bodies and maintain stable body temperatures. Looking at the natural history of the animals, tegus appear to fit the mold pretty well. They have distinctive winter and summer activity levels. In the summer, the animals regularly maintained body temperatures of 32–35°C, and in the winter they let their body temperatures drop to the temperature of their burrows (15–20°C). This is all fine and good for a bradymetabolic, ectothermic lizard, but when the researchers tracked body temperatures over time they discovered something completely unexpected.
Actually, I’ve never thought that sauropods were grey. Mammals in general tend to be rather bland in their colour schemes. Reptiles don’t have that problem. With xanthaphores (yellow pigmented cells), erythrophores (red pigmented cells) iridophores (iridescent cells) and melanophores (dark pigmented cells), the range of colour available to reptiles, and by extension – dinosaurs, is quite vast.
That said, I always pictured sauropods as either a brownish green colour, or maybe a very pale blue (blue is generally rare in tetrapods, hence the thought of it being a weak blue).
But I digress.
I grew up during an interesting time for dinosaur research. Unlike the majority of paleontologists working right now I didn’t grow up learning about dinosaurs being slow and sluggish mistakes of nature. I also didn’t grow up with the “hummingbirds on crack” version of dinosaurs that is currently pervading popular culture. Rather, I grew up during that strange transitory phase of the Dinosaur Renaissance where dinosaurs were sometimes viewed as sluggish beasts and other times as racecars of the Mesozoic.
The result, I think, has been a slightly detached and objective look at how perceptions of dinosaurs have changed over time.
A “Brontosaurus” getting attacked by Allosaurus during a sojourn on land to lay her eggs. Ah, the classics.
One book I remember fondly was the Golden Book of Dinosaurs (shown above). It featured these beautiful drawings of dinosaurs living life as best we thought at the time. One picture that really stuck in my head, was a shot of two Brachiosaurus; one on land and the other so deep in a lake that one could only make out the crest on the head. I found that page to be so immersive and atmospheric. My knowledge of physics was not so good at the time, so it never dawned on me that this poor sauropod was basically breathing through a straw with its lungs separated by at least 2 atmospheres from the air entering (as best it could) the nostrils.
Then around the early nineties when Jurassic Park the book came out I started to note a distinct change in how dinosaurs were being portrayed. No longer were sauropods swamp bound behemoths. Now they were fully terrestrial titans that could not only support their weight on all four legs, but could even do so on 2 (well 3 if one counts the tail). It was around this time that Robert Bakker’s infamous “Dinosaur Heresies” started making the rounds.
Now, admittedly, Heresies came out in 1986 and the changing view of dinosaurs actually started in the seventies. However, it wasn’t until the early nineties that the full effects of Bakker’s work could truly be appreciated. If anything this gives one an idea of the kind of inertia one must deal when it comes to getting scientific ideas out into the public.
Again I digress.
It was around the early nineties when I first read The Dinosaur Heresies. The first few chapters were amazing. I had never seen dinosaurs portrayed this way. They walked better and were more active. In many ways they better fit the concept I had in my head all along.
Then I came up to the end of chapter 3. The thesis of this chapter was to explain why reptiles should not be viewed as inferior to mammals. In order to do so Bakker explained all the various ways in which extant reptiles outshine extant mammals. The end of the chapter features a beautifully drawn shot of the “panzer croc” Pristichampsus snatching a Hyracotherium (formerly Eohippus). The caption read:
Pristichampsus hunted during the Eocene Epoch, about 49 million years ago, but it was very rare, much rarer than big mammalian predators, proof that cold-bloodedness was a great disadvantage.
That’s when the real thesis of the book hit me. The argument wasn’t: “Dinosaurs weren’t slow and stupid, because of the following.”
Rather the argument was: “Dinosaurs weren’t cold-blooded because the facts show the following.”
In order to pull dinosaurs out of the mire, Bakker had to change their fundamental thermophysiology. The general concept, that cold-bloodedness is inferior to warm-bloodedness, remained the same. This despite Bakker’s initial attempt to explain how “cold-blooded” reptiles outshine “warm-blooded” mammals.
Bakker’s book was just the start. From there, we had Adrian Desmond’s “The Hot Blooded Dinosaurs” (okay, technically Desmond was first by 7 years, but he largely stole Bakker’s work to make the book so it evens out) and Gregory S. Paul’s infamous: “Predatory Dinosaurs of the World.” Each new book taking the “dinosaurs can’t be cold-blooded” argument a little further. By the time we hit Predatory Dinosaurs of the World, Tyrannosaurus rex was running along at 40mph, dromaeosaurs were practically flapping around and every species of dinosaur was reaching adult size by between 4-10 years of age.
Sadly it was at this point that Jurassic Park was written. As hardcore fans know it was Greg Paul’s erroneous sinking of Deinonychus antirrhopus into Velociraptor that gave us the JP “raptors.” It was also at this point that the pendulum of dinosaur physiology officially swung the other way.
The thing that had always bugged me about this view of dinosaurs was the sheer lack of supporting data for it. The assumption was always that dinosaurs were so vastly different from “typical reptiles” that they had to have been doing something different. Yet when one looked at the actual data dinosaurs came out looking slightly odd at best. For the most part dinosaurs fit the reptile mold quite well. It was these elusive “classic reptiles” that didn’t appear to exist.
Most reptiles don’t fit the “typical reptile” mold at all. Yet despite numerous papers over the past 30 years depicting reptiles doing things normally thought un-reptile like (e.g. caring for their young, competing with large mammals, etc), most of this was dutifully ignored in favour of an older, more outdated view.
It was a problem that Nicholas Hotton III (1980) aptly called: The “endothermocentric fallacy.” Basically, the assumption that being an endotherm is inherently superior to being an ectotherm. Part of that superiority included the ability of endotherms to do everything faster and “better” than similar sized ectotherms. This problems with this way of thinking warrants an entire blog post to itself. So rather than get bogged down with this particular I’ll touch more on the endothermocentric fallacy at a later date. For now all that one needs to keep in mind is that the thinking of the time was that if dinosaurs were going to be active at all then they had to be endotherms.
By the late nineties we had the first evidence of feathers in a small branch of the theropods (Maniraptora). Birds were officially adopted into the dinosaur family tree and the fully endothermic concept of Dinosauria was completely entrenched.
The funny thing, of course, is that this dogmatic view of dinosaur metabolism was just as bad as the early 20th century’s “cold-blooded” swamp bound view. Sure dinosaurs were more active now, but the data supporting it was just as nebulous as the stuff that was used to keep dinos in the swamp.
Enter the 21st century, and the late…um, 0’s (does anyone have a name for this decade yet?). Biomechanic work on dinosaurs has started to reveal amazing insights into the physical limits of what dinosaurs could do, and the results have started to pull the pendulum back again.
Work by John Hutchinson and Mariano Garcia (2002) on T. rex showed that not only could T. rex not hit 40mph, but it technically couldn’t run either. A biomechanical assessment of theropod forelimbs by Ken Carpenter (2002) has shown that the “bird-like” dromaeosaurs could not fold their arms up like birds after all.
Work by Rothschild and Molnar (2005) on sauropod stress fractures showed no signs of rearing activity in sauropods, while work by Kent Stevens and J. Michael Parrish (2005) pulled the swan-like curve out of sauropod necks, placing things far more horizontally.
Work by Gregory Erickson and others (2001) on micro-slices of dinosaur bone has indicated that very few dinosaurs hit adult size in less than 15 years.
Now we have a new study by Lehman and Woodward (2008) which follows up on Erickson et al’s work and actually shows that even this toned down version of dinosaur growth is probably too fast as well. Lehman and Woodward focused on sauropods and studies on their bone microstructure. What they did was compare bone growth data to a well used equation for growth in animals.
Deemed the Bertalanffy equation; it states that the mass at any given age is an exponential function limited by the asymptote of adult body mass. This equation has been used extensively in studies on bird and elephant growth among others. An example of the equation is given to the right for fish.
When the authors did this they discovered something quite interesting. Instead of taking 15 years to reach adult mass, sauropods like Apatosaurus excelsus took closer to 70 years!!
Other sauropods measured took between 40 and 80 years! This is a substantial decrease in growth rate estimated before. Mind you this is data taken, in some cases, from the same piece of bone that Erickson et al had used. So one can’t suggest anomalous bones being used as the reason behind the surprising results. The authors also went to great lengths to take into account differences in mass estimations as well as allometric growth of body parts. In each case the changes had little affect on the overall outcome (in many cases, it made growth go even slower).
Now keep in mind we are talking about the time it took sauropods to reach full adult size. This is not the time taken to reach sexual maturity. Earlier studies by Erickson et al (2007) had already discovered that dinosaurs didn’t wait to grow up before engaging in sex, so there is no issue here of 80 year old sauropods finally “doing the nasty.”
What this does show is that growth in dinosaurs might not be as determinate as initially thought. An 80 year old sauropod might just have been close to the edge of its lifespan at this point (though the possibility of bicentennial sauropods does still exist). It also shows that dinosaurs had growth rates far closer to the realm of reality (before it was hard to imagine how an Apatosaurus excelsus was able to pound down enough food daily to add 13.6 kg of new mass a day. Especially given their small mouths).
Thermophysiologically what does this all mean? Were dinosaurs “cold-blooded” after all?
That’s one of those questions that will never be fully answered (short of a time machine). What this does do is pull dinosaurs ever further away from the “definitely warm-blooded” category and push them right back into the middle again. When/if the dust settles on this metabolism debate I suspect that dinosaurs will probably remain in the middle somewhere.
Of course while all of this is going on with dinosaurs we have other studies, like those from Tumarkin-Deratzian (2007) showing the existence of fibrolamellar bone growth in wild alligators, that are finally moving the rusty pendulum of reptile metabolism out of the “classic reptile” category and much closer to the middle.
So in the end dinosaurs will still probably wind up being “good reptiles.” Thankfully the exact definition of what that entails will have probably changed by then.
Bakker, R. 1986. The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and their Extinction. William Morrow. New York.
Carpenter, K. 2002. Forelimb Biomechanics of Nonavian Theropod Dinosaurs in Predation. Concepts of Functional Engineering and Constructional Morphology. Vol. 82(1): 59-76.
Desmond, A. 1976. The Hot Blooded Dinosaurs: A Revolution in Paleontology. Dial Press.
Erickson, G.M., Curry Rogers, K., Varricchio, D.J., Norell, M.A., Xu, X. 2007. Growth Patterns in Brooding Dinosaurs Reveals the Timing of Sexual Maturity in Non-Avian Dinosaurs and Genesis of the Avian Condition. Biology Letters Published Online. doi: 10.1098/rsbl.2007.0254
Erickson, G.M., K. C. Rogers, and S.A. Yerby. 2001. Dinosaurian Growth Patterns and Rapid Avian Growth Rates. Nature 412: 429?433.
Hotton, N., III. 1980. An Alternative to Dinosaur Endothermy: The Happy Wanderers. In A Cold Look at the Warm-Blooded Dinosaurs (R.D.K. Thomas and E.C. Olson Eds.), pp. 311-350, AAAS, Washington, DC
Hutchinson, J.R., Garcia, M. 2002. Tyrannosaurus was not a fast runner. Nature 415: 1018-1021.
Lehman, T.M., and Woodward, H.N. 2008. Modeling Growth Rates for Sauropod Dinosaurs. Paleobiology. Vol. 34(2): 264-281.
Rothschild, B.M., and Molnar, R.E. 2005. Sauropod Stress Fractures as Clues to Activity. In Thunder Lizards: The Sauropodomorph Dinosaurs. (Virginia Tidwell and Kenneth Carpenter eds). Indiana University Press. pp 381-394.
Stevens, K.A., and Parrish, J.M. 2005. neck Posture, Dentition, and Feeding Strategies in Jurassic Sauropod Dinosaurs. In In Thunder Lizards: The Sauropodomorph Dinosaurs. (Virginia Tidwell and Kenneth Carpenter eds). Indiana University Press. pp 212-232.
Tumarkin-Deratzian, A.R. 2007. Fibrolamellar bone in adult Alligator mississippiensis. Journal of Herpetology. Vol. 41. No.2:341-345.