Diet and Digestion as Factors for Large Size in Sauropods
Dinosaurs have captured the imagination since the very first fossils were discovered. The mystical creatures in many ancient cultures may be attributed to fossils weathering out of the earth. Nearly every culture had some form of dragon in their mythology, a uniformity best explained by fossilized dinosaurs. Some scientists believe the fossils of ceratopsians are the source of the mythological griffin. Part of the fascination with dinosaurs is their size. With few animals reaching such massive proportions, the creatures that produced single bones as tall as a human must have seemed other worldly to ancient peoples. They still draw fascination today; their features so different from anything living. As more and more fossils are found, explanations and extrapolations of the features, size, and behavior become more comprehensive. The unknown element of dinosaur life allows for wild projections and suppositions, and the drama of discovery and scientific discourse and hypothesis disproval continue to draw the interest of people. As more is discovered, more questions are raised. One field of questions pertain to sauropod size. Sauropods are distinct in their absolutely massive bodies. Far larger than anything known by living humans, they are fascinating to consider. Trying to imagine their size is in some ways like trying to imagine the vastness of space. Without actually standing next to a mounted skeleton, there is no living reference to fix the size of sauropods in the imagination. Some estimates place the largest sauropods at 80,000kg, or 170,000 lbs. Conservative estimates suggest weights of 40,000-50,000 kg. Sauropod height is in some specimens 60 feet, about 3 times the height of a two-story house. Some sauropods were a third of a football field in length! Such sizes are nearly unfathomable, raising the question of how they got to be so massive and how their bodies functioned. Modern herbivores are often much bigger than carnivores, because in general, their energy expenditure is less and the biomass of their food source is higher. The huge size of sauropods is definitely tied to their diet, and it is likely that their food source and digestive mechanisms were a major factor in their growth to such massive sizes.
The food available to sauropods was not very nutritive. Before the evolution and diversification of angiosperms 125 million years ago, herbivorous animals had to rely mainly on gymnosperms and ferns. The best measure of plant nutritional value is in the concentration of nitrogen and nonstructural carbohydrates like sugars. Protein levels largely correspond to the nitrogen levels in leaves. Because non-angiosperm plants don’t have the xylem transport network that evolved, there is less nutrient flow, and therefore less nitrogen concentration. Ferns, cycads, and conifers, some of the main plant types available to the sauropod dinosaurs, were low in nutrition, and gymnosperms had secondary chemical defenses that made them unpleasant to eat and harder to digest (Midgley, 2005). A study by Zvereva and Kozlov (2006) found that the nitrogen concentration in gymnosperms drops in environments with elevated temperature and CO2 concentration, compared to the modern environment. Sugar levels tended to stay the same. The toughness of leaves increases when CO2 levels are higher. The carbon-nitrogen ratio is significantly increased by elevated CO2. It is well-known that the environmental conditions during the age of dinosaurs was both higher in temperature and higher in CO2 concentration. This would have produced plant material significantly less nutritious than the plant matter today. The lesser food quality supports the evolution of large sauropod dinosaurs, as the Jarman-Bell Principle states that larger species can feed on diets of lesser quality food, evidenced by the relatively large size of modern grazing mammals (Clauss, et. al., 2009). The physiology of the dinosaurs shows evidence of partitioning. Some sauropods, such as diplodocids, were low browsers, who were unlikely to life their heads higher than a couple meters (Stevens and Parrish, 1999). This separated their food source from the mid to high browsers, limiting their available food source in some ways, yet also preventing competition amongst sauropod species. The lower plant quality requires that herbivorous animals consume more material in order to meet their nutritional and energy needs.
Given the quality of available plants, extended digestion would have been required. Of several methods to process plant material, sauropods digestion was extremely fermentation heavy. They had very little oral processing. Their teeth were either broad and leaf like, with serrations in the crown, or later more narrow and peg like. In most sauropods, teeth were present in the front part of the mouth but not the back. The body plan in the early Mesozoic seems to have been to maximize food intake through teeth adapted for cropping, stripping, and pulling plant material but with little oral processing. There is some evidence in the tooth replacement rate and wear that teeth farther back in the mouths of sauropods were not used in oral processing or food acquisition, but rather served the purpose of cheeks, to keep the food in the mouth (Schwarz, et. al., 2015). Prosauropods had some check development, but in the sauropod line, cheeks were lost early (Chure, et. al., 2010). There have been no teeth found in association with sauropods that had a large horizontal surface, indicative of use for chewing (Christiansen, 1999). The heads of sauropods were extremely small compared to their body size. Any dental batteries or cheeks would have increased the head mass and likely made it prohibitively heavy, especially at the end of a long neck. Christiansen also noted that the muzzle width in sauropod dinosaurs is proportionally wider than the width in herbivorous animals. This enables sauropods to intake more food per bite, increasing their intake rate. With relatively poor food nutrient quality, sauropods would need to consume a great amount of food to meet their daily needs. The length of the neck adds length to the digestive envelope, another way to maximize food consumption. The lack of oral processing, wide muzzle, and long neck work in conjunction to increase the consumption capability or sauropods, increasing their ability to meet their dietary needs and shortening their feeding time.
With such high consumption and such little processing, digestion would be almost entirely focused in the gut. The particle size would be very large, making digesta harder to break down, and the longer it takes to digest. There has been some debate about the presence of gastroliths and a gastric mill in sauropod dinosaurs. Gastric mills certainly would help break down the plant material into smaller particle sizes and speed the digestion process. Unfortunately, the fossil record seems to rule out the presence of a gastric mill for most sauropod dinosaurs. Wings and Sander (2007) tested the hypothesis of sauropod gastroliths by testing gastrolith usage in farm ostriches. They found that rose quartz, which has the same properties as the white vein quartz found in association with sauropods, last the longest. Using granite cubes, they found that the general shape of the stone stays the same. They also found that the gastric milling process quickly roughens the face of the stone. None of the stones that he used in his test retained a shine, unlike those suspected to be sauropod gastroliths. He also found that gastroliths are about 1% of the total body mass. According to them, the largest mass of gastroliths found in association with a sauropod is 15kg, much lower than 1% of the estimated sauropod body mass. Gastroliths likely scale with body mass simply because organ size roughly scales with body mass. The stomachs of sauropods could have been proportionally smaller than expected, although it is unlikely with the amount of food that they are estimated to have consumed. Using projections from living herbivores, it is estimated that the large sauropods would consume several hundred kilograms worth of food (Englemann, et. al., 2004). This estimate accounts for the proportional decrease in required food consumption for successively larger and larger organisms. In another study focusing on the distribution of gastrolith stones amongst sauropod remains, it was found that gastroliths were found with about 4% of sauropod fossils. There are some species of sauropod that have strong evidence of gastric mills, but they are not widespread enough (Wings, 2014). With little to no processing of plant material, and with no evidence of any other break down processes, sauropods must have heavily relied on fermentation to digest their food. The relatively low density of nutrients in the gymnosperms at the time already required longer fermentation times. In order to achieve longer fermentation time, the gut size needs to increase, or the amount of food eaten needs to decrease. As the food particles must have been large, the time taken for fermentation would need to be even longer. With no internal soft tissues preserved in the fossil record, it is hard to determine where fermentation took place in the gut. Fermentation demands a large digestive tract. Larger intestines allow for longer digesta retention, pulling more nutrients from the food. The large torsos of sauropods could certainly fit and extensive gut. Estimates based on living animals gives a digesta retention time between 8 and 16 days. Galapagos turtles, which do not chew their food, retain theirs for 11 days (Franz, et. al., 2009). Sauropod dinosaurs could have evolved to be so large because those individuals with larger guts had better survival chances than those who had smaller guts. As stated previously, the amount of food needed in relation to the body mass would likely have been much lower than other, smaller herbivorous organisms. As sauropods’ heads were evolved to take in the maximum amount of food, the intake of food would likely not have consumed most of the sauropods’ time. This time advantage would have been especially needed in semiarid environments, like the Jurassic area preserved in the Morrison formation. Although there is lush vegetation in the Morrison fossil record, it is not widespread instead found in clusters. The Morrison Formation suggests that the environment during the time of sauropods was savannah-like. The dense vegetation found was likely due to seasonal rains, or centered around areas of water, such as lakes or streams. The vegetation likely moved, growing in different areas depending on rainfall (Englemann, et. al., 2004). Large herbivores would need to follow the seasonal changes, and migrate in search of new food sources. The size of the sauropods and the advantages of that size would have been a major edge in a semi-arid environment. Such sizes would have also made locomotion more efficient. Longer strides afforded by the overall large body size decreases the amount of energy per unit of distance. This, too, increases the ability of sauropods to migrate in search of food. Sauropod reliance on fermentation was so great that their methane production has been linked to the warm climate of the Mesozoic era (Wilkinson, et. al., 2012). Assuming a more reptilian metabolism, one paper puts the global biomass of large sauropods at 200,000 kg/km2. Their total estimated annual methane emission is about 520 million tons. For comparison, modern day ruminants produce about 50-100 million tons of atmospheric methane, and the total modern day global emissions are about 500-600 million tons a year. The author notes that their estimate could have been overstated by a factor of two, but also understated by the same amount, depending on assumed metabolic function and density of sauropods. Sauropods large sizes and reliance on fermentation to digest food was a major influence on their environment, and created a positive feedback loop, where the temperature increase would push nutritive values of plants lower. Massive sauropod bodies were well adapted to their environment and digestion.
Of course, such large sizes bring their own challenges. The most debated question is whether or not dinosaurs were endothermic or ectothermic, and whether endothermy was even possible in mega dinosaurs like the sauropods. At body masses estimated between 20,000 kg and 80,000 kg, overheating would have been a huge problem in large sauropod dinosaurs. While it is unknown if they had special soft tissue adaptations in order to combat their size, it is speculated that their long necks and tails may have helped them keep cool by increasing surface area without adding too much internal volume (Eagle, et. al., 2011). The accelerated growth capable in endothermic animals is a major factor in favor of sauropod endothermy. Sauropods grew several size magnitudes, from hatchlings estimated to be about 10kg, to the hulking adult dinosaurs, in only a few decades (Sander & Clauss, 2008). But does the size of adult sauropods rule out endothermic metabolisms? According to Eagle and others (2011), endothermy was not impossible in large sauropods. In lower temperatures, more 13C-18O clumps form, which are preserved in the fossil record. The analysis of these clumps is not dependent on knowing the oxygen isotope composition of the surrounding water. These clumps can be observed in the tooth bioapatite of dinosaurs. The accuracy of this method is 1°C, with precision within 1°-2°C. The accuracy of this method comes from tests of modern taxa, in which the isotopic temperature agrees with the expected temperature of the organisms. Eagle found the average body temperature of Brachiosaurus to be 38.2°C Â± 1°C and the temperature of Camarasaurus to be 35.7°C Â±1.3°C including a sample from a different fossil site. These temperatures are within the range of modern mammals, and lower than the body temperatures of many birds, which can be greater than 40°C. Eagle does note that the temperature reflects the temperature of tooth formation, which may differ from the main body temperature. Body temperature is a product of metabolism, size, environmental temperature, and any special adaptations for the regulation of heat. The temperatures given for these sauropods is close to the temperatures estimated by earlier research done by Gillooly (2006). Such temperatures in such large animals suggests that they were either ectothermic, had low basal metabolic rates, or had some special methods of heat dissipation. Some researchers suggest that large sauropods were fermentative endotherms (Mackie, 2002). Because sauropods needed a lot of energy to reach their adult size in such little time, it is likely that they were endotherms who underwent some sort of change at maturation that prevented overheating as an adult. Metabolic changes through development is not unusual, so it is very possible that it also occurred in sauropods. They could have shifted from a higher metabolic endothermy during their rapid growth phase, to a lesser metabolic homeothermy supported by the fermentation heat output from their fully formed guts. Other evidence in support of endothermic sauropods are growth lines, or the lack there of, in sauropod bones (Kohler, et. al., 2012). Lines of arrested growth are normally associated with ectotherms, which have periods of rapid growth interspersed with periods of slow growth. These lines are found in mammals as well, and in the majority of dinosaurs. The pattern the lines leave are not found in sauropod bones. This suggests unbroken, stead growth rates, highly unlikely in ectotherms. The evidence suggests endothermy in sauropods, even in large ones. Because endothermy requires more energy to maintain, sauropods would have had to consume a massive amount of food, unless they had a low basal metabolism. Hippopotami, while mammals and clearly not the size of sauropods, have particularly long retention times because they have low food intake and enormous gut capacity. Their required energy for maintenance is remarkably low. This strategy is common in non-ruminant foregut fermentators and some small hindgut fermentators (Clauss, et. al., 2009). The metabolic process of sauropods is linked to their energetic needs and dietary restrictions. In some research, one of the byproducts of fermentation, heat, supports the endothermic theory, and endothermy in sauropods as juveniles helps explain how they managed to reach their massive adult size. The herbivorous, fermentative nature of sauropods is not an obstacle in understanding their ability to function at such large sizes.
Sauropods likely evolved to be large because of the abundance of plant materials, especially after many herbivores died out during both the Permian extinction, but also the Triassic-Jurassic extinction. Their adaptations allowed them to widely diversify and fill the newly opened ecological niches. Some have speculated that their large body size was driven by predation, as larger bodies, especially the size of sauropods, were a natural defense (Sander, et. al., 2011). Given the evidence, it is more likely that immunity to predation was a lucky side effect of size, not the driving factor. It seems more likely that the resource opportunities of plants drove the initial adaptation, especially with the diverse nature of sauropods and their apparent partitioning, than protection.
The size of sauropods is inextricably linked to their diet and digestive methods. Much of the discussion of sauropod feeding is based on conjecture is based on living animals, that are obviously very different from sauropods. As there is are no known records of internal tissues, it is hard to know anything about how sauropods functioned internally beyond comparing them to existing behaviors and traits in today’s animals. Even though today’s herbivores are different than sauropods, patterns of herbivory are similar in very different taxa. The circumstantial evidence offered by analysis of modern organisms still enables scientists to attempt to fit sauropods within the known herbivorous patterns, with allowances for the unknowns. Perhaps this analysis is just another element of the imaginative aspect of dinosaur life. Until more evidence is found, I believe that the evolution of the massive sauropods was in large part due to their diet and digestion. The nutrition offered by gymnosperms demanded higher levels of processing. The minimal oral digestion evidenced by small heads and non-chewing teeth led to greater digesta retention times in the gut. A greater gut size would have sped fermentation, compensating for the large particle size of the plant material and its low nutrient density. There is some evidence for resource partition amongst sauropods, both between different species and within the same species. Different tooth structure and browsing levels are some adaptations driven by available resources. The other benefits of the large body size of sauropods, in my opinion, do not seem likely to be as important in size evolution as the plants and their digestion. When the sauropods appeared, they filled an ecological niche left by previous extinctions, and quickly diversified, creating a hugely successful group of organisms, both in overall diversity, but also in longevity.
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