(thing) by Dialogue Fri Dec 08 2000 at 20:37:22

On Evidence for Endothermy in Dinosauria

This is an original work written in December of 2000. Reproduction in whole or part of this work without express authorization is a violation of the copyright I hold. So ask first, punk.

Dinosaurs and their relatives were the dominant forms of life on this planet for over 160 Million years. They filled every niche conceivable, from 2 kg chicken sized egg thieves to 80-ton herbivores. Fossils of Dinosaurs have been found on every continent on the planet.

The Dinosaurs came to dominate the landscape after the age of the Thecodonts in the late Triassic and became extinct in the late Cretaceous, ushering in the age of mammals (Bakker, 1978). Though at any one point there were only 30 to 40 species of Dinosaur extant these magnificent creatures were successful in maintaining ecological superiority (Ostrom, 1978). This superiority, like the superiority of mammals in the Cenozoic, was the result of a physiology evolved to take maximum advantage of the niches available and the opportunities present. One of the most controversial aspects of dinosaurian physiology is the method of temperature regulation they employed.

For the better part of this and the past century, Dinosaur thermoregulation was seen as a non-issue. Dinosaurs were reptiles, so the thinking went, and therefore were undoubtedly ectotherms, creatures that depend on the ambient environmental temperature for physiological heat regulation (Greenberg, 1978). As the century wore on, more and more paleontologists began to consider the possibility that at least some Dinosaur species may have had endothermic physiologies, self-regulating their body temperature through high and controlled heat production (Greenberg, 1978).

In the 70's and early 80's, Robert Bakker (1975, 1978) published a number of papers in which he claimed that endothermy was the only possible regulatory regime which Dinosaurs could have utilized and still maintained their ecological dominance. These papers spurred something of a paradigm shift in the discipline of paleontology. The view held by most paleontologists in the present day is that Dinosaurs likely had a homeothermic physiology (a relatively stable body temperature), with temperature regulation schemes existing somewhere between the ectothermy of reptiles and the endothermy of mammals and birds. Even still, there have been a number of papers published just recently examining Dinosaur temperature regulation. For many paleontologists, exactly how homeothermy was maintained, and exactly where in that zone of endo/ectothermy Dinosaurs lay is still in debate.

The purpose of this paper is to examine the arguments for and against endothermy in Dinosaurs. Evidence for endothermy will be examined primarily based on physiological data. Observations on the gross physical form, bone structure, circulatory system, and pulmonary system of Dinosaurs can provide clues as to which thermoregulatory scheme these animals employed. Recent data (Barrick/Showers 1994, Morrel 1994) regarding the oxygen isotopes present in the fossilized form of a Tyrannosaurus Rex will provide information about the temperature fluctuations in that particular specimen, likely giving us some clue as to what kind of thermostability Tyrannosaurs were capable of. I will conclude with a synthesis of my impressions on this issue and suggest future areas of investigation regarding this issue.

The physiology of Dinosaurs is an area of knowledge mostly consisting of detective work and educated guesses by paleontologists. That we know even as much as we know about the internal structures of these "Terrible Lizards" is a testament to the hard work of researchers in that discipline. In examining Dinosaur physiology for clues as to how the animal's body temperature was regulated, there are several aspects to consider.

The gross physical form of the animals provides the first clues as to what temperature regulation scheme they employed. Even as far back as the Early Permian, there existed reptiles that utilized a bipedal stance for quick locomotion in times of stress and for hunting (Berman, 2000). These reptiles, which existed 60 million years before the bipedal archosaurs we are all familiar with, likely contributed to the future generations of Dinosaurs that would use the bipedal stance. This upright stance left the species that adopted bipedality free to utilize their forelimbs for grasping, climbing, and hunting. It also gives us clues at to their scheme of thermal regulation (Berman, 2000).

In modern ecosystems, there is not a single bipedal animal that is not an endotherm. Some species of lizards and crocodilians have been known to raise up on their hind legs for short periods of time for reasons similar to the primitive Permian reptiles. However, modern day reptiles cannot sustain this erect stance, both because their hind limbs do not have the structural design for prolonged bipedal motion, and because the upright stance leaves them vulnerable to swift heat loss. The swift loss of body heat is one of the main structural reasons ectothermic creatures are sprawlers, close to the ground. Dinosaurs, even those who were not bipedal, all maintained erect, upright stances that left them exposed to breezes and rain showers. This physiological fact is one of the strongest arguments researchers employ when arguing endothermy in Dinosaurs (Ostrom, 1978).

Many workers who favor the ectothermic theory in Dinosaur temperature regulation argue that bipedality is not a strong indicator of endothermic temperature regulation, as it does not take into account the sheer size of the animals involved. Those scientists (specifically Spotila 1978) who do argue the ectothermic point of view usually do so based on the animal's sheer body mass. They argue that the size of the creatures, even those as "small" as the three-ton specimens, may have been sufficient to maintain homeothermy.

Mathematics teaches us that as an object increases in size, the object's surface area is squared, while the object's volume is cubed. They argue that the sheer body mass of the animals would have been sufficient to maintain a high, stable body temperature in conjunction with the tropics-like climate of the Mesozoic period. Spotila (1978) argued that in fact a three ton "Good Reptile" could maintain a body temperature of roughly 34 oC in a modern tropical landscape. He argues that this theoretical creature would have had roughly the same temperature variations on a day-to-day basis as mammals (roughly one degree). Additionally, Spotila states that this adaptive "coup" would be sufficient to explain how the Dinosaurs wrested control of ecological niches from the Thecodonts.

However, Bakker (1975, 1978) gives twofold reasons as to why that analysis is flawed. First, the Thecodonts likely had low body temperatures. Fossil evidence shows that the Thecodonts likely did not have physiologies capable of raising their body temperatures above 29 oC. Dinosaurs began assuming the niches the Thecodonts held from the top down. That is, the largest Thecodonts were the first to be replaced, and so on down the line to the smallest, 5 - 20 kg species. This indicates that the Dinosaurs, with their likely higher body temperatures, were better suited to bigger body sizes, quicker movement, and easier adaptation to new environments. This last point, adaptation to environments and their climatic dangers, was the main thrust of his second argument against large size being sufficient to maintain an ectothermic Dinosaur.

Bakker (1978) agrees with Spotila (1978) on his mathematics. Likely, the theoretical three-ton lizard could have maintained a fairly steady body temperature in the tropics. For just that reason, he finds it impossible that Dinosaurs could have been ectotherms. In modern ecologies, mammals consistently beat out other types of animals in niches requiring a large body plan. In fact, except for a single snake, all of the creatures on our planet in the 40 kg + range are mammals (Bakker, 1978). This strongly suggests that mammals, with their higher core temperatures and constant heat production, are better suited to large terrestrial forms than other types of animals. This also suggests, in Bakker's view, that Dinosaurs likely had high body temperatures, allowing them to rest control of large-body niches from the Thecodonts, and to keep mammals in small-body niches for over 160 million years. Additionally, Bakker (1978) notes, the fictional three-ton Good Reptile would have been vulnerable to catastrophic weather. A heavy downpour would likely have killed the fictional creature by sapping heat from the animal too fast for it to cope. Even if the reptile did survive the downpour, Bakker estimated that the creature would have required several days to restore itself to its ideal body temperature using ectothermic temperature control, likely allowing a swifter endothermic creature to end its life before it had a chance to recover.

Two recent revelations in the area of Dinosaurian gross anatomical features may add additional perspective to the endothermic argument. These revelations, the recent discovery that many Dinosaurs may have had feathers (Gibbons, 1998; Appenzeller, 1999), and a new theory suggesting that species of Dinosaurs once thought to have spiny sails probably had humps like a camel, connect Dinosaurs even more with the endotherms of today (Holden, 1998).

In addition to further linking Dinosaurs with Birds, the discovery of feathers on the T-Rex and some smaller theropods suggests that Dinosaurs may be the origins of the endothermic temperature regulation found in modern day members of the Aves clade. It has been estimated that Archaeopteryx, the famous bird-dinosaur, could not have led an ectothermic lifestyle and still had enough energy to utilize its wings for flight, or even gliding (Gibbons, 1998).

The feathers recently discovered on what appears to be a juvenile T-Rex also suggest that feathers may have been used in larger species for temperature regulation. The feathers may have been used by infant Rexes as a heat retention mechanism, indicating a possible endothermic body design. As the infants aged they likely shed their feathers, as sheer body mass quickly became sufficient for retaining the animals' constant output of heat. It is even speculated that a small number of feathers may have remained atop the animal's head for cooling purposes in hot climates, further possible evidence of constant body heat production (Appenzeller, 1999).

The spines that jutted from Dinosaurian predecessors called Pelycosaurs have been confirmed to be large sails used in helping to regulate the body temperature of the small reptiles (Holden, 1998). So when paleontologists discovered similar spine-like structures on several African species of Dinosaur, they assumed that the spines were support for similar sails. To scientists at the time, the spines seemed to be support for the ectothermic view of Dinosaurs.

A researcher named Jack Bailey recently came up with a different analysis of the large spines (Holden, 1998). Taking measurements of the spines of two African species, Ouranosaurus and Spinosaurus, he compared those figures with the depth of the animals' chests and the height of their vertebral discs. He then did the same comparisons with several Pelycosaurs and several hump bearing mammals. Bailey found that the African Dinosaurs had measurements much more similar to those of the modern Bison and Camel than those of the Pelycosaurs. Likely then, the African animals had large, fatty humps that could be used for long distance travel, as in modern African creatures (Holden, 1998). The possibility that these creatures did not rely on spines for temperature control, but instead utilized a hump for retention of vital fluids draws just one more correlation between Dinosaurs and mammalian/avian physiology.

A final aspect of the overall physical makeup of Dinosaurs to consider is the speed at which they attained adult size. A recent study (Stokstad, 1998) done on the childhood of the Apatosaurus indicates that it reached its full adult size of several dozen tons in under a decade. Upon drilling samples from the Sauropod's shoulder blades, the researchers

found regular changes in the density of microscopic canals that presumably once held blood vessels. The layers resemble the concentric rings laid down each year in manatee and sea turtle bones, so researchers assumed that they were annual and used them to age the Sauropod shoulder blades. Bones from half-sized individuals were 4 to 5 years old, while the largest Sauropods had apparently reached full growth in just 8 to 11 years. (Stokstad, 1998)

Thus, like mammals and Birds, Dinosaurs reached their adult proportions in a relatively short amount of time. Gross physical characteristics can allow us to draw correlations between the ancient creatures and modern day endotherms. However, for more concrete evidence of endothermy, it's necessary to better examine the internal structure of the animals.

Bone structure is one of the aspects of Dinosaurian physiology that workers have long hoped would reveal clues as to the creatures' method of temperature regulation. This hope stems from the ease with which, in modern species, endothermy or ectothermy can be distinguished based solely on bone structure. In mammals and birds, growth is swift. Most animals reach their adult size in a relatively short amount of time. As such, their bones grow quickly as well. This results in tightly woven collagen matrices being formed within the bone, creating what is known as fibrolamellar bone tissue, and trapping numerous blood vessels inside the matrix (Chinsamy, 1995).

In reptiles, which develop much more slowly, collagen is laid down in stately rings. This forms bone structures known as lamellar bone tissue. It is, in fact, possible to date a reptile based on the rings in its bones, much the rings of a tree (Chinsamy & Dodson, 1995).

With this in mind it should be noted that, unfortunately, what research has revealed about Dinosaur bones raises more questions than it answers. It seems that Dinosaurs, with their aforementioned quick growth rates, have a great deal of fibrolamellar bone structure (Chinsamy, 1995). However, as previously mentioned, they also have the concentric rings of lamellar bone structures. Dinosaurs likely went through growth spurts. Throughout each year, Dinosaurs probably took advantage of seasonal food surpluses to grow as quickly as they could. During leaner months they would likely slow to the more stately growth rate of reptiles. Some modern species, both endotherms (manatees) and ectotherms (crocodilians) fall into this pattern of on-again/off-again growth. However in the case of manatees and crocodiles, these growth spurts last only a year or two until the creature has reached its full size. Dinosaurs would continue this stop/start process for almost a decade in some cases in order to reach their adult proportions (Chinsamy, 1995; Chinsamy & Dodson, 1995). This data suggests that Dinosaurs may have had a method of body temperature regulation that lies somewhere between strict endothermy and strict ectothermy. These findings are further muddled by analysis of the circulatory systems of two species of Dinosaur.

In mammals and birds, the heart is a four-chambered organ, with two completely separated ventricles and a single systemic aorta. This ensures that only oxygenated blood is sent to organs and limbs, and is one of the main reasons mammals and birds can maintain such a high metabolism.

In reptiles, the heart is a two chambered organ separated by the foramen of Panazzi (Fisher, 2000). Oxygenated blood from the lungs and de-oxygenated blood from extremities mixes in the heart, with two systemic aortas distributing blood to the body. Thanks to a recent find of a Hypsilophodont in southwestern South Dakota, we have the first images of cardiac tissue from a creature of the order Ornithischian (Fisher, 2000).

The animal was put through a CT scanner to examine the inside of the thankfully uncrushed thoracic cavity. The heart the scan revealed was a curious mix of reptilian and mammalian structures. There only appeared to be two chambers, as in reptiles. This may have been due to tissue decay or the collapse of the atria upon death. However, the animal did appear to have a foramen of Panazzi. Unlike in reptiles, however, the foramen appeared to be fused shut. Additionally, there was only a single systemic aorta. This means that in at least one clade of Dinosaurs, advanced hearts (having only one systemic aorta) with possible chambered ventricles existed (Fisher, 2000).

Another circulatory system that may play a role in the debate over endothermy is that of the Barosaurus. Barosaurus was a large Sauropod that existed roughly 140 million years ago. It was pulled from the Morrison formation in Utah, and is one of the rarest Sauropod specimens currently known. Barosaurus, like most Sauropods, was exceedingly large once it reached its adult size. There has been a great deal of contemplation over whether this creature, with its incredibly long neck, could raise its tiny head to the tree line to feed, or whether it swept the ground like a giant vacuum cleaner (Choy, 1992).

A 1992 paper in the Lancet discussed the hydrodynamic implications of a Barosaurus that could raise its head to tree top level. For comparison, they used the longest necked creature currently alive, the Giraffe. In these African plains animals, many adaptations have led to a physiology capable of sustaining the long neck present in the Giraffe phenotype. The animal's blood has a much higher level of viscosity than the blood of other mammals. This means more blood can be pumped faster with less applied pressure (Choy, 1992).

Giraffe erythrocyte counts are double that of humans, which means that Giraffe blood can carry a great deal of oxygen. This allows blood fed to organs and extremities to impart more oxygen, again resulting in less pumping necessary. The pumping demands necessary even with these adaptations results in Giraffes having much larger hearts than animals their size would otherwise require. A bull Giraffe heart was found which weighed 11.3 kg; the left ventricular wall was 7.5 cm thick and the right 2.5 cm. The Barosaurus, a considerably larger animal than a Giraffe faces similar adaptive pressures, only to a much greater degree.

To pump blood 12 meters from the thorax to the top of the head, the heart of Barosaurus would need to achieve a systolic pressure of 12,000 millimeters of water, or about 880 millimeters Hg. Such an enormous pressure would require a very large and strong heart and very thick walls in the arterial system to prevent rupture. Indeed, zoologist Roger Seymour estimated the heart size of large Sauropods to have been more than 1.6 metric tonnes, or eight times that of a whale of similar size. (Choy, 1992)

Due to the large size of the creature's heart, it would have had to beat very slowly. Whale's hearts only beat between 30 and 40 times a minute (Choy, 1992). A Barosaurus would have had even fewer heartbeats than that. Additionally, because of the distance between the massive heart of the Barosaurus and the creature's head, the interval between heart beats would have allowed enough time for blood to actually fall back towards the animal's heart before the organ beat again.

Therefore, it is likely that the animal had additional "hearts" at intervals up the Dinosaur's neck. These small pumps would have allowed the animal's heart to only generate enough pressure to send the blood to the first pump. Each subsequent pump would have passed the blood further up the neck and stopped the flow of blood from being reversed (Choy, 1992). This arrangement of pumps and the amount of pressure required indicates that the Barosaurus likely had a heart similar to modern mammals and birds, which are the only creatures which have blood pressures high enough to make similar physiological comparisons.

Research into temperature regulation in Dinosauria has lately centered on the pulmonary system. Recent studies on pulmonary functions in these extinct creatures have provided support for both the endothermic and ectothermic theories of Dinosaur thermoregulation.

Until 1997, the pulmonary systems of Dinosaurs were largely unknown. A fossil specimen of the theropod Sinosauropteryx was discovered in China. Amazingly, some of the internal viscera were outlined in the rock. Under the scrutiny of an Ultraviolet light, many details about the animal's lungs were apparent.

This observation, combined with the occurrence among theropods of a distinct, relatively vertical, crocodile like, highly elongate pubis, as well as well-developed gastralia, provide evidence that theropod dinosaurs, like modern crocodiles, probably possessed a bellowslike septate lung and that the lung was probably ventilated, at least in part, by a hepatic-piston diaphragm that was powered by diaphragmatic muscles that extended between the pubic bones and liver. (Ruben, 1997)

This means that, unlike the sophisticated alveoli reliant lungs that mammals use, Dinosaur lungs were more similar to the lungs of reptiles today. However, Dinosaurs utilized the hepatic-piston diaphragm. In modern reptile forms, the only species to do so is crocodile (Wuethrich, 1999). In a more recent study, Ruben (1999) analyzed a find from Italy, Scipionyx samniticus. This theropod had well preserved soft tissues within its body cavity, similar to the Sinosauropteryx find. As with the Chinese find, evidence suggests that Scipionx did not have avian style air sac lungs. In this better-preserved specimen, however, Ruben and his associates were able to make further analysis of the hepatic-piston diaphragm and related structures. They found it was likely that the animals were "capable of sustaining rates and activity levels well beyond those of even the most active living reptiles." This is due to the effective method in which the small meat eaters utilized the hepatic piston diaphragm. Rates of oxygen crossing the lungs may have even matched the rates of some extant mammals (Wuethrich, 1999). Although Ruben's find has suggested to some an ectothermic lifestyle, the very data which ties Dinosaurs back to reptiles seems to allow for an active, mammal like existence.

Thus, nearly every aspect of Dinosaurian physiology can give us information on what type of thermoregulation scheme they employed. Their fossils can even give up interesting information on the animals they once were, just by analyzing their elemental makeup.

Everything Note: In the following section, I apologize for the weirdness with the B-P, B-BW, and (B^18)O. It was the best I could do to approximate the subscripts, Greek letters, and superscripts.

In the case of Dinosaurs, the fossil evidence we have consists of the bone-shaped rocks that are left in the wake of an animal's death. These rocks can give us clues as to the temperature of the animal that walked the planet millions of years go.

In a 1994 study, Reese Barrick and William Showers analyzed the fossilized remains of a Tyrannosaurus Rex skeleton. They used the oxygen isotopic composition of bone phosphate (B-p) to calculate the body temperature variability of that particular animal.

Vertebrate B-p is a function of the body temperature at which bone forms and of the isotopic composition of the body water. The isotopic composition of body water (B-bw) depends on the (B^18)O of water ingested during feeding and drinking as well as on the metabolic rate relative to water turnover rates. (Barrick & Showers, 1994)

As it is impossible to know exact (B^18)O values for the Tyrannosaurus Rex, we cannot calculate the animal's exact body temperature.

However, body compartments in vertebrate animals that contain water all have nearly identical (B^18)O values. Therefore, variations in B-p between different areas of the skeleton should reveal differences in body temperature. "The ratio of two oxygen isotopes within bony tissues, oxygen-16 and oxygen-18, changes according to temperature; there is relatively more oxygen-18 when it's colder." (Morell, 1994) Since the isotopes are created during bone formation, the relatively lightly fossilized bones of the T. Rex they examined would undoubtedly give a fairly accurate reading.

What the researchers found has been heavily attacked by those favoring ectothermic temperature regulation in Dinosaurs, but the results they revealed are compelling. The specimen they examined did not have large interbone or intrabone variation.

The small intrabone B-p variation indicates that this T. Rex was not an ectotherm. The lack of increased intrabone variation in the limbs and tail and the small interbone variation suggest that this T. Rex was also not a mass homeotherm using heat exchange mechanisms to dump heat through the extremities during warm periods and retain core body heat during colder periods. (Barrick & Showers, 1994)

Spotila's (1978) theory of the huge homeothermic ectotherm as well as the very notion of theropods using ectothermy (Ruben, 1997) is challenged by these findings. This controversial study is still being debated by experts in the field, and similar studies should likely be carried out to confirm or disprove the validity of the rather strong claims made by Barrick and Showers.

My own analysis of the data currently available regarding endothermy in Dinosaurs is somewhat biased. I am strongly of the opinion that the Aves clade is a direct descendent from Dinosauria (via the arboreal theory), and I find it hard to believe that modern day birds would have developed from "cold-blooded" creatures constantly seeking out the sun. While the large-bodied ectotherm theory seems to have some validity to it, it does not explain the numerous species of Dinosaur that came into direct competition with mammals in the 5 - 10 kg range.

For 160 million years, Dinosaurs managed to hold their own against our mammalian ancestors. This indicates that they likely had a metabolism as active as our own, or the ascension of mammals to large body sizes likely would not have begun only after the Dinosaurs had already become extinct. Additionally, I find the current evidence for endothermy in Dinosaurs to outweigh the evidence for ectothermy in Dinosaurs, both in literal amount of support and in the meticulousness and validity of the work.

As to where future research on this area of study should go, I believe the groundbreaking study done in 1994 by Barrick and Showers should be repeated again on other fossil species. As the main thrust of arguments against the technique are all the unknowns involved, utilizing the technique on fossilized species that are known to be endotherms or ectotherms can only assist the procedure in gaining respectability in the scientific community. Likewise, I believe more work is needed on the recent feather related research done in 1998. Proving the evolutionary link between Dinosaurs and Birds would likely solve the debate about endothermy and ectothermy in Dinosauria in the process.

Attempting to research a physiological process in a group of animals that has been dead for roughly 125 million years is an exercise in frustration. Everywhere we look, there exist pieces of the puzzle that could conceivably be telling us the answers to the questions we seek. Putting those pieces together correctly, and interpreting them in just the right way shall likely be a process that will be going on for a long time to come.

Bibliography

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Greenberg, Neil. 1978. "Physiological and Behavioral Thermoregulation in Living Reptiles." A Cold Look at the Warm-Blooded Dinosaurs. Thomas, Roger; Olson, Everett (editors). Westveiw Press, Boulder Colorado.

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Ruben, John A. 1999. "Pulmonary Function and Metabolic Physiology of Theropod Dinosaurs." Science, 01/22/99, Vol. 283 Issue 5401, p514.

Spotila, James. 1978. "Constraints of Body Size and Environment on the Temperature Regulation of Dinosaurs." A Cold Look at the Warm-Blooded Dinosaurs. Thomas, Roger; Olson, Everett (editors). Westveiw Press, Boulder Colorado.

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Wuethrich, Bernice. 1999. "Stunning Fossil Shows Breath of a Dinosaur." Science, 01/22/99, Vol. 283 Issue 5401, p468.

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