GEOL 104 Dinosaurs: A Natural History

Fall Semester 2015
Dinosaur Physiology

Hot- and Cold-Running Dinosaurs?
Among modern vertebrates, some gross generalizations:
Birds and mammals are warm-blooded; that is, they are warmer than the environment around them in typical temperate and colder environments. Crocodilians, lepidosaurs, turtles, amphibians, most fish, and almost all invertebrates are cold-blooded: their bodies are generally only about as warm as the general environment around them, so consequently they feel cool to the touch outside of tropical situations; in contrast, warm-blooded animals have temperaturs largely independent of the outside temperature, so they feel warm to the touch.

Old debate in dinosaur studies: were they warm-blooded or cold-blooded?

Owen in 1842 suggested dinosaurs might have been warm-blooded, or at least more warm-blooded than typical modern reptiles.

Need to be precise as to definitions of terms. "Warm-blooded" and "Cold-blooded" actually encompass several different (although related) topics:

A typical cold-blooded animal is an ectothermic bradymetabolic poikilotherm: needs to get its energy from the sun and fluctuates with external environment (but can moderate fluctuations by moving from sunlight to shade and vice versa); however, needs very little food (snakes can go weeks without feeding, for example). Cold blooded animals become torpid at night and in colder weather.

A typical warm-blooded animal is an endothermic tachymetabolic homeotherm: its body temperature is stable and activity levels can remain high for long periods of time, at night, and in colder weather; however, needs a LOT of food or will die (imagine the effects of not feeding a cat or dog for weeks...).

Additional issues to consider:

Why evolve such an expensive trait as endothermy? Some suggestions have included:

How can people determine the thermal physiology of extinct animals like non-avian dinosaurs?

Owen suggested dinosaurs might have been warm-blooded because:

Many late 19th Century paleontologists considered dinosaurs to be more similar to modern warm-blooded animals in terms of activity levels.

During early 20th Century, shift to lizard-like concept for dinosaurs.

Concept of warm-blooded dinosaurs revived in late 1960s by Ostrom because of a number of lines of evidence:

Colleague from France: Armand de Ricqlès added additional line of evidence:

Ostrom's undergrad student Robert T. Bakker: main advocate for the "hot-blooded" dinosaurs model. Added his own observations:

Additional lines of evidence (primarily from 1980s and 1990s):

Not everyone convinced that dinosaurs were fully endothermic tachymetabolic homeotherms.

Two main types of evidence to the contrary:

Lines of evidence supporting dinosaurian ectothermy:

And now, some 21st Century evidence:

Hearts, Lungs, and Faces: So You Want to Be An Endotherm?

Let's consider the equations of life. First, the aerobic respiration equation, the primary means by which animal cells operate:

C6H12O6 + 6O2 yields 6CO2 + 6H2O + Energy

(That is, food (glucose) plus oxygen yields waste carbon dioxide and waste water, plus energy).

If an animal's cells can't get enough oxygen, there is a second way of getting energy: the anaerobic respiration equation:

C6H12O6 yields 2C3H6O3 + Energy

(That is, food yields lactic acid plus energy (although much less than the aerobic respiration.) Lactic acid itself needs oxygen to break down, so you cannot run on anaerobic respiration for very long.

If you want to evolve endothermy, you need to:

So, where do we stand on dinosaur metabolism?

What would be necessary to justify the above observations?

Is there evidence for these features in dinosaurs? YES!

Dinosaur Breathing:

Dinosaur Hearts:

Dinosaur Temperature Regulators:

The enlarged narial regions may support tissues for a different function: recovery of moisture. In living endotherms, rapid rate of respiration would dry out lungs if not for some specialized tissues called nasal turbinates:

An secondary advantage of using respiratory turbinates to dump heat is that it helps direct some of the air flow onto the part of the nasal chamber associated with olfaction:

Eat or be Eaten: Dinosaur Paleoecology

Bakker used his interpretations of trophic relationships to try and determine the thermophysiology of dinosaurs and other extinct forms. His technique:
Predator-Prey ratios:

So, P/P ratios are problematic, at best. Some ways in which dinosaurs are distinctly different from modern mammalian communities:

Complications

Some additional possibilities:

  • Gigantothermy:
    From geometry, as linear dimensions double, the surface area goes up by squares, and the volume by cubes:
    Side Length Surface Area (SA) Volume (V) SA/V
    1 6 1 6/1 = 6
    2 24 8 24/8 = 3
    3 54 27 54/27 = 2
    4 96 64 96/64 = 1.5

  • Heterometabolism:

    A Complication: Ancient Atmospheres
    Even as dinosaurs were evolving, the atmosphere they were breathing was evolving, too. Geochemists have seen that the ratios of various gases, including oxygen, have varied over geologic time. At least some models suggest that the Middle Jurassic though the end of the Cretaceous had oxygen levels exceeding the present 20%. This would mean that every breath a dinosaur took would have more oxygen, making it easier to power a high metabolism.

    Furthermore, experiments of growing plants of Mesozoic varieties under Mesozoic-style atmospheres suggests that their productivity (essentially, the amount of nutrients they produce per area per unit time) could go up 2 to 3 times present day conditions. If so, then there would have been more food available per unit area for the herbivores (and from this up the energy pyramid), again making it easier to be an endotherm in these conditions.

    Another Complication: Is Crocodilian Ectothermy a Reversal?
    Most studies assume that endothermy evolved sometime after the bird lineage (Ornithodira) and the crocodilian lineage (Pseudosuchia) diverged from each other. This is because crocodilians are ectotherms, as are all the next several outgroups (lepidosaurs, turtles). However, what if crocodilians were not ancestrally ectotherms, but instead reverted to a cold-blooded physiology from warm-blooded ancestors?

    There is some evidence that this is the case:

    This has led to speculation that the ancestral archosaurs were in fact more warm-blooded than crocodilians, and that the latter evolved "cooler blood" after the divergence of their lineage from other types of crurotarsans. Thus, the origin of avian warm-bloodedness would not have occurred within Dinosauria, but at least in part before the bird line-croc line split.

    A New(-ish) Idea: Mesothermy
    In 2014 a study came out proposing that dinosaurs were intermediate between endotherms and ecotherms, and the authors termed them "mesotherms". (In fact, Dr. Scott Sampson had proposed the concept and the name "mesothermy" years earlier...). The particular study estimated both the maximum growth rate of fossil dinosaurs and their inferred metabolic rate (based in part on growth rate, so the whole study may wind up being a circular argument...). They found that most Mesozoic dinosaurs (including Archaeopteryx) fell in a range intermediate between where modern endotherms and modern ectotherms plotted (but in the same region as such animals as tunas, sharks, echidnas, etc.)

    The authors interpreted this to mean that dinosaurs had the ability to generate internal heat, but did not greatly regulate their body temperature. So in fact, what they call "mesothermy" is technically not intermediate between endothermy and ectothermy, but between homeothermy and poikiliothermy. And thus dinosaurs in their interpretation would be in terms of this course endothermic mesotherms. In their interpretation, the rise of actual warm-bloodedness in the bird lineage occurred somewhere well within Pygostylia. Future analyses will have to be done to see if this model is upheld.

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    Last modified: 15 August 2016