Weighing in

There are two commonly used ways to estimate the weight of a dinosaur. The first is based upon a relationship between limb cross-sectional area and weight. This relationship has some validity, because obviously, as a terrestrial beast becomes larger, the size (including cross-sectional area) of its limbs must increase. The question is, does it increase in the same manner for all tetrapods? If so, a single equation could apply to all. It is clear, however, that it cannot. As noted by J. O. Farlow, weight is dependent upon muscle mass and muscle mass is really a consequence of behavior. Therefore weight estimates of dinosaurs are in part dependent upon presumed behavior. For example, reconstructing the weight of a bear would involve assumptions of muscle bulk and gut mass very different from those used in reconstructing the weight of an elk (Figure B12.5.1). Indeed, the cross-sectional area of their limb bones may be identical, but they may weigh very different amounts. Moreover, our knowledge of dinosaurian muscles and muscle mass is rudimentary. This method, although convenient and used by a number of workers (including R. T. Bakker), has the potential for serious misestimates of dinosaur weights.

sectional areas of bones.
Figure B12.5.2. Estimating the weight of a dinosaur using displacement. For explanation of (a) and (b), see the text.

A second method, pioneered by American paleontologist E. H. Colbert in the early 1960s, involves the production of a scale model of the dinosaur, and then the calculation of its displacement in water (Figure B12.5.2). That displacement could then be (a) multiplied by the size of the scale model (for example, ifthe model were 1/32 ofthe original, the weight of the displaced water would have to be multiplied by 32) and (b) further modified by some amount to a number corresponding to the specific gravity of tetrapods. But what is the specific gravity of a tetrapod? Based upon studies with a baby crocodile, Colbert determined that baby crocodiles, at least, have a specific gravity of 0.89. Unfortunately, there is no uniform specific gravity shared by all tetrapods. Studies with a large lizard (Heloderma) showed that the specific gravity of that lizard was 0.81. Among mammals, it would not be surprising to find the specific gravity of a whale differing from that of a cheetah, which might in turn differ from that of a gazelle. In short, while the displacement method is perhaps a bit more accurate than limb cross-sectional calculations, it is still dependent upon inferred muscle mass (and thus behavior), and therefore somewhat problematical.

This mix of animals is not particularly easy to interpret in terms of endothermy or ectothermy. Several members of the southern dinosaur assemblage had large brains and well-developed vision, potentially useful during periods of extended darkness. Others, however, were not so well equipped. Burrowing may have been an option for some, but not for all.

In the case of the North American faunas, only Troodon is of a size that could make burrowing feasible. Migration was potentially a solution to inclement winter weather, although the dinosaurs would have had to migrate for tremendous distances before temperatures warmed sufficiently.

As we have seen, birds are dinosaurs and modern birds are surely endothermic. As we asked before (see Chapter 11), at what point during theropod evolution did "avian" endothermy evolve?

An important clue comes with insulation. All small- to medium-sized modern endo-therms are insulated with fur or feathers. Indeed, pterosaurs are suspected endotherms, in part because they are known to have been covered with a fur-like coat. There is a certain sense to this; if an ectotherm depends upon external sources for heat, why develop a layer of protection (insulation) from that external source? And can a small endotherm, constantly eating to maintain its metabolism, afford to lose heat? Archaeopteryx, with its plumage, is therefore usually considered to have been endothermic.1 The discovery of non-avian, feathered (insulated) theropods from China (see Chapter 10) gives us a clue that endo-thermy occurred well within Coelurosauria, and perhaps at an even more basal level within Theropoda (Figure 12.12).

1. In 1992, J. A. Ruben suggested that Archaeopteryx could have been an ectotherm. His idea was based upon the amount of energy needed for flight, and upon the amount of energy available from an ectothermic metabolism. Since the bones of Archaeopteryx show limited adaptations for sustained, powered flight, Ruben argued that an ectothermic metabolism would have been more than sufficient for the kind of limited flight that apparently characterized Archaeopteryx. While powered flight may be possible in an ectothermic tetrapod, none (save perhaps Archaeopteryx) ever evolved it. Moreover, it seems to us that the presence of insulation (feathers) in Archaeopteryx is incompatible with an ectothermic metabolism.

Phylogeny

1. In 1992, J. A. Ruben suggested that Archaeopteryx could have been an ectotherm. His idea was based upon the amount of energy needed for flight, and upon the amount of energy available from an ectothermic metabolism. Since the bones of Archaeopteryx show limited adaptations for sustained, powered flight, Ruben argued that an ectothermic metabolism would have been more than sufficient for the kind of limited flight that apparently characterized Archaeopteryx. While powered flight may be possible in an ectothermic tetrapod, none (save perhaps Archaeopteryx) ever evolved it. Moreover, it seems to us that the presence of insulation (feathers) in Archaeopteryx is incompatible with an ectothermic metabolism.

Figure 12.12. Cladogram showing the inferred "depth" within Theropoda of endothermy. While we can be certain the avialans were all endotherms, it is not clear how far back - or how deep within the cladogram - endotherm extends. Many paleontologists suspect that could go back as far or further than coelurosaurs.

Geochemistry

Remarkably, fossil vertebrates carry around their own paleothermometers. These come in the form of stable istopes; that is, isotopes that, unlike their unstable brethren, do not spontaneously decay. Of particular interest to us are the istopes of the element oxygen. There are three: 16O, by far the most common2, 17O, and 18O. The last, 18O, is particularly interesting, because its proportion to

varies as temperature varies. Therefore, if a substance contains oxygen, one can learn something about the temperature at which that substance formed by the ratio 18O :16O. In the case of bone, the oxygen is contained in phosphate (PO4) that forms part of the mineral matter in the bone. Thus, knowing the oxygen isotopic composition of the bone, one can learn something of the temperature at which the bone formed.

If dinosaurs were poikilothermic, there should be a large temperature difference between parts of the skeleton located deep within the animal (that is, ribs and trunk vertebrae) and those located toward the exterior of the animal (that is, limbs and tails; Figure 12.13). If, however, dinosaurs were homeothermic, there should be little temperature difference between bones deep within the animal and those more external, because the body would be maintaining its fluids at a constant temperature. The difference in temperatures - or lack thereof - should be reflected in the proportions of 18O to 16O.

Studies reveal that bones from the cores of some of the dinosaurs tested -Tyrannosaurus, Hypacrosaurus, Montanoceratops, and a juvenile Achelousaurus) showed little temperature variation, suggesting that they were formed under homeothermic conditions. The small euornithopod Orodromeus and a nodosaurid ankylosaur that they tested, on the other hand, had an isotopic variation (hence, an inferred temperature variation) that pushed the limits of conventional homeothermy. The Jurassic dinosaurs Ceratosaurus and Allosaurus all showed an ectotherm-like variability in their core regions (the pelvis, in this case). In general, the extremities of these dinosaurs fell within 4 deg.C of the cores (Figure

2. 16O comprises 99.763%, 17O comprises 0.0375%, and 18O comprises 0.1905% of total atmospheric oxygen.

Figure 12.13. Core-to-extremity temperatures in a poikilothermic ectotherm. Because this tetrapod's temperature fluctuates with the ambient temperature, when it's cold outside, its extremities are much colder than its core.

■ known endotherm □ known ectotherm O dinosaurs

Komodo Pheasant Opossum without tail

Opossum/tail Deer

■ known endotherm □ known ectotherm O dinosaurs

Komodo Pheasant Opossum without tail

Opossum/tail Deer

Temperature difference (deg.C)

Figure 12.14. Estimated maximum temperature variations between bones located in the core of the body and those located at the extremities in living and extinct vertebrates, reconstructed with the use of oxygen isotopes. Living vertebrates are represented by the Komodo dragon (an ectothermic lizard) and a selection of mammals (endotherms). Note that the greatest variation between core and extremities occurs in the opossum (conventionally considered to be an endotherm), reinforcing the idea that even endotherms can undergo significant fluctuations between core and extremities (in this case the long tail). The researchers concluded that all dinosaurs tested, except the ankylosaur, matched the definition of homeotherms (Data from Barrick, R. E., Stoskopf, M. K. and Showers, W. J. 1997. Oxygen isotopes in dinosaur bone. In Farlow, J. O. and Brett-Surman, M. K. (eds.), The Complete Dinosaur. Indiana University Press, Bloomington, IN, pp. 474-490.)

Temperature difference (deg.C)

Lizard varanid Nodosaur

Ceratosaurus Allosaurus Camarasaurus Orodromeus Ceratopsian juvenile Montanoceratops Hypacrosaurus juvenile Hypacrosaurus Tyrannosaurus

-7 -6 -5 -4 -3 -2 -1 o Temperature difference (deg.C)

Figure 12.14. Estimated maximum temperature variations between bones located in the core of the body and those located at the extremities in living and extinct vertebrates, reconstructed with the use of oxygen isotopes. Living vertebrates are represented by the Komodo dragon (an ectothermic lizard) and a selection of mammals (endotherms). Note that the greatest variation between core and extremities occurs in the opossum (conventionally considered to be an endotherm), reinforcing the idea that even endotherms can undergo significant fluctuations between core and extremities (in this case the long tail). The researchers concluded that all dinosaurs tested, except the ankylosaur, matched the definition of homeotherms (Data from Barrick, R. E., Stoskopf, M. K. and Showers, W. J. 1997. Oxygen isotopes in dinosaur bone. In Farlow, J. O. and Brett-Surman, M. K. (eds.), The Complete Dinosaur. Indiana University Press, Bloomington, IN, pp. 474-490.)

12.14). The authors concluded that virtually all of these dinosaurs were homeotherms that experienced some "regional heterothermy."

Different strokes for different folks

The fact that the signal we receive from the fossil record is mixed may itself be a message: dinosaurian physiology appears to have been a complex mix of various strategies, relating to size, behavior, and perhaps environment.

Many paleontologists are now suggesting that some dinosaurs - particularly large orni-thopods and theropods - maintained something close to endothermic homeothermy as fast-growing juveniles, but became closer to homeothermic ectotherms as adults. Similarly, few paleontologists would argue that large sauropods maintained high, endothermic homeother-mic metabolic rates. Sauropods may have relied upon a strategy called gigantothermy: small surface : volume ratios (resulting from large size) retained core heat, allowing sauropods to maintain a homeothermic metabolism without the metabolic cost of being truly endother-mic. Small- to medium-sized theropods, and perhaps similarly sized ornithopods, may have been homeothermic endothermic throughout their active lives. What is becoming clear is that dinosaurs were neither endotherms in the mammalian sense nor ectotherms in the crocodilian sense. They were something else, and it is a virtual certainty that different strategies were adoped by different dinosaurs, including, of course, birds.

Was this article helpful?

0 0

Post a comment