GEOL 104 Dinosaurs: A Natural History

Fall Semester 2019
Dinosaur Physiology

Dinosaurs of the Middle Jurassic Xiashaximiao Formation of China: the sauropod Shunosaurus, the theropod Gasosaurus, and the stegosaur Huayangosaurus, by Gregory S. Paul

Key Points:
•Modern animals are often characterized as "warm-blooded" (mammals, birds) and "cold-blooded" (everything else)
•This is a simplification of several related phenomena: energy source (endothermy vs. ectothermy); metabolic rate (tachymetabolism vs. bradymetabolism); and temperature stability (homeothermy vs. poikilothermy)
•Dinosaur species were initially inferred to be "cold-blooded", but similarity in posture and other traits led Owen to suggest they might have been warm-blooded; since that time various researchers have examined the alternatives.
•Dinosaur posture, locomotion, feeding adaptations, growth rates, bone texture, inferred respiration, and predator-prey ratios point to elevated metabolisms relative to today's non-avian sauropsids.

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 temperatures largely independent of the outside temperature, so they feel warm to the touch. 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:

Note that it is not just mammals and birds that are "warm-blooded". For example, tunas, billfish (sailfish, swordfish, marlins), lamniform sharks (like great whites and makos), boid snakes (pythons, etc.; but only while brooding), and certain plants (which aren't "blooded" as such, but some can emit internally-generated heat).

When dinosaurs were first discovered, they were interpreted as being no more than gigantic cold-blooded lizards. However, as early as 1842 Owen (in the very paper in which he named "Dinosauria") speculated that dinosaurs may have been warm-blooded like mammals. During most of the 20th Century the model of dinosaurs as cold-blooded returned. Work by John Ostrom (of Yale University) and his colleagues and students (especially Robert Bakker) presented new information that dinosaurs were in fact warm-blooded. This hypothesis generated considerable research (both in support and in attempts to falsify it): this change in thinking about dinosaurs and renewed interest in dinosaurian studies has been termed the "Dinosaur Renaissance".

Among the lines of evidence supporting dinosaurian warm-bloodedness:

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 Respiration:

Dinosaur Hearts:

  • What about controlling heat?

    Keeping the Heat In: Insulation Issues: One problem that small-bodied organisms encounter is the fact that a small organism has a much higher surface area/volume ratio than a large one. Because of this, small animals tend to lose heat much faster than big ones. In contrast, large animals lose heat to or gain heat from the outside world only gradually. This has led some people to suggest the possibility that large dinosaurs exhibited "gigantothermy": effective homeothermy achieved because of large body size. However, this would not apply to small-bodied dinosaurs: either adults of small species or the hatchlings of giants. So how could these keep warm?

    There is strong evidence that many--if not most--of the theropods had a fuzzy body insulation over the body: true feathers in the advanced groups, simpler "protofeathers" in the primitive ones. Such fuzz would help keep the warmth in the body. In fact, this is the primary function of the fur of mammals, and one of the functions of body feathers in birds. The recent discovery of 1.4 t Yutyrannus demonstrates that even some giant theropods were fuzzy.

    Recent discovery of the early Late Jurassic Chinese ornithischian Tianyulong and the similar aged Kulindadromeus of Siberia showed they too had a fuzzy body covering over at least part of its body! If this is found to be homologous to the protofeathers of tetanurine theropod saurischians it would suggest that the concestor of all dinosaurs was fuzzy, and that dinosaurs were thus fuzzy ancestrally! (In the case of Kulindadromeus, there are also also scales, plates, and additional bizarre tufted plates.) At present, however, there is enough uncertainty to make the homology between Tianyulong's fuzz, Kulindadromeus's diverse integument, and theropod protofeathers suspicious. (But do not be terribly surprised if in the future we discover that most dinosaurs were fuzzy to some degree or another! All we need is a fuzzy primitive sauropodomorph, and it is basically a done deal!)

    Dumping Heat: Mechanisms to Remove Heat:

    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:

    It has just recently been recognized that the expanded frontopareital fossa (anteriorly-expanded sections of the supratemporal fenestrae) were filled by vascular tissue and fat rather than by muscle. So these areas may have helped to radiate heat to keep the brains of dinosaurs cool (they seem to have that function in modern crocodilians):

    Some 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 pseudosuchians. 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.

    Are Varanids (Monitor Lizards) a Model for Proto-Endotherms?
    Today's Varanidae (the monitor lizards, such as Australian goannas and Indonesia Komodo dragon) may give insight what the ecotherm-endotherm transition might have been like. Varanids have long been noticed to have a higher level of sustained activity (and consequently larger home range) than other lizards. They also have ziphodont dentition (recall that Megalosaurus teeth were specifically compared to monitor lizards'!) for tearing and shredding meat rather than swallowing their food whole, allowing for faster digestion. They even have a heart that "cheats" and acts like a proto-four chambered heart, so that the lung and body blood pressures are different! These changes could be what was present in the ancestral archosaur, giving an ectothermic animals the capacity for higher endurance and activity levels, as a stepping stone to becoming true endotherms.

    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.

    A relevant video:

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    Last modified: 12 November 2019

    The fuzzy neornithischian Kulindadromeus by Andrey Atuchin (2014)