Were Sauropod Dinosaurs Responsible for the Warm Mesozoic Climate?

Journal of Palaeogeography 2012, 1(2): 138-148 DOI: 10.3724/SP.J.1261.2012.00011

Biopalaeogeography and palaeoclimatology

Were sauropod dinosaurs responsible for the warm Mesozoic climate?

A. J. (Tom) van Loon* Geological Institute, Adam Mickiewicz University, Maków Polnych 16, 61-606, Poznan, Poland

Abstract It was recently postulated that methane production by the giant Mesozoic sauropod dinosaurs was larger than the present-day release of this greenhouse gas by nature and man-induced activities jointly, thus contributing to the warm Mesozoic climate. This conclusion was reached by correct calculations, but these calculations were based on unrealistic assumptions: the researchers who postulated this dinosaur-induced warm climate did take into account neither the biomass production required for the sauropods’ food, nor the constraints for the habitats in which the dinosaurs lived, thus neglecting the palaeogeographic conditions. This underlines the importance of palaeogeography for a good understanding of the Earth’s geological history.

Key words sauropod dinosaurs, greenhouse conditions, methane, palaeogeography

1 Introduction* etc. Some of these compounds are thought to have contrib‑ uted to the global temperature rise that took place in the For a long time, geologists have wondered why highly 20th century, but the causal relationship is still hotly de‑ significant fluctuations in the global temperature occurred bated (Rothman, 2002). One of the reasons is that climate in the geological past. It was found by Milankovich (1930, models show significant shortcomings (especially when 1936, 1938) that the alternation of Pleistocene glacials and applied back in time). Another reason is that data (e.g. the interglacials can largely be understood on the basis of as‑ relationship between CO2 concentrations in the ice in cores tronomical factors. Much shorter‑lived temperature fluc‑ from Antarctica and Greenland on the one hand and the tuations can be related to the fluctuations in solar activity rising or falling of global temperatures on the other hand)

(Usoski et al., 2005; Ineson et al., 2011). Moreover the seem to indicate a post‑warming CO2 increase rather than relatively low temperature of the Middle Ages (Little Ice the reverse (Gildor and Ghil, 2002). In addition, the ‘pollu‑ Age) has been explained by the absorption of sunlight by tion’ of the atmosphere still goes on, whereas the rise in the tiny particles in the atmosphere that had been released as a global temperature seems to have stopped some ten years consequence of extensive volcanism (Miller et al., 2012). ago. This may imply that Man’s influence on System Earth All these factors have a natural origin, and man cannot re‑ is much smaller than most people are aware. ally influence them. This does not imply, however, that all factors affecting Since the beginning of the industrial revolution, man the climate on Earth are astronomical in nature. It has been has greatly contributed to changes in the atmospheric com‑ suggested, for instance, that some geological boundaries, position, by releasing huge quantities of aerosols, gases, characterized by large‑scale changes in fauna and/or flora, have been caused by the release of giant amounts of meth‑

* Corresponding author: Professor. ane (CH4) from the sea floor. This gas is now considered Email: [email protected]; [email protected]. as a much more (23 times) effective greenhouse gas than

Received: 2012‑08‑14 Accepted: 2012‑08‑31 CO2, although the effect of this gas on the climate depends A. J. (Tom) van Loon: Were sauropod dinosaurs responsible for the warm Vol. 1 No. 2 Mesozoic climate? 139 on the way of accumulation in the atmosphere, and on the is not possible. Several of the assumptions that led to their rate of its oxidation (under the oxygenic conditions that remarkable conclusion are not ‘best‑guesses’, as will be prevail in the atmosphere). The present‑day man‑made detailed below. methane production is largely due to the breeding of cattle The question of how much methane was produced by and the ‘wet’ growing of rice. The total amount of methane an ‘average’ dinosaur, was not estimated by Wilkinson et involved is considerable, indeed: livestock is considered al. (2012) themselves: they refer to Franz et al. (2011a), nowadays as the source of some 20% of the annual global who conclude that modern non‑ruminant herbivores pro‑ release of this gas. This makes it worthwhile to consider duce 0.18 liters × (body mass)0.97 of methane per day. the possibility that life forms in the past of the Earth have There is no good reason to doubt this value, and I also produced greenhouse gases in such a quantity that they agree with Franz et al. (2011b) that this may be taken, for might have affected the global temperature. Recently it has the sake of simplicity, as 0.18 × (body mass in kg) liters been postulated that this has been the case, indeed. per day. Wilkinson et al. (2012) estimate, as an example, the methane production by Apatosaurus louisae (Fig. 1), 2 Sauropods as a methane source which they consider to have weighted some 20 t; the indi‑ vidual adults would have produced 2675 liters of methane The question of whether methane produced by sauro‑ per day, which means 1.9 kg. This implies that only the pod dinosaurs could have helped drive Mesozoic climate global body mass of sauropods living at the same time has warmth, was recently raised by Wilkinson et al. (2012). to be determined to calculate their total methane produc‑ Particularly considering the currently hot debates regard‑ tion. Wilkinson et al. (2012) estimated this global body ing global warming and its causes, this is an important is‑ mass using arguments that are at least questionable. sue. As mentioned above, livestock is considered by nu‑ merous researchers as a huge source of the greenhouse gas 3 Global sauropod biomass methane. It is consequently an attractive hypothesis that the relatively high temperature during most of the Meso‑ For a calculation of the global body mass, one should zoic might at least partly result from the methane produced know how many sauropods lived on Earth simultaneously, by the giant sauropods that roamed the Earth during this and what was their average body weight. Both parameters era. are unknown, so that they have to be estimated. Wilkinson et al. (2012) tackled the question by cal‑ 3.1 Potential number of sauropods and their total culating how much methane may have been produced body mass by the sauropods. They recognize that there are two rel‑ evant questions: (1) how much methane did an average Farlow et al. (2010) stated that the number of the giant sauropod produce, and (2) how many sauropods lived on herbivores must be related to the available food. They pos‑ Earth? Both questions are difficult to answer as we do not tulated correctly that this partly depends on the animals’ know with certainty the dinosaur metabolism, and because metabolism. If it was endothermic (mammalian‑style), we have to estimate the number of sauropods on the ba‑ 11-15 sauropods‑with a total body weight of 42,000 kg/km2‑ sis of fossil finds and on comparison with modern‑day might have lived per square kilometer in the area where the ‘analogues’. On the basis of their assumptions concern‑ Morrison Formation (Fig. 2), which extends in the United ing particularly these aspects, they came to the conclusion States over a surface area of 1.5 million km2, accumulated that the Mesozoic‑ and more specifically the Jurassic and during the Late Jurassic (Farlow et al., 2010). They also Cretaceous‑global temperature was probably significantly calculated the population density of sauropods from the raised by the methane produced by sauropods. Morrison Formation if they had a reptilian metabolism This seems very unlikely, however, considering the as‑ and came to the conclusion that, in this case, the biomass sumptions of the researchers. The reason is that these as‑ density was 377,000 kg/km2. This is in itself remarkable, sumptions are difficult to fit in the ecological conditions because it means that a reptilian metabolism would al‑ that as far as we are aware prevailed during the Jurassic low a nine‑fold higher population density (i.e. ~100-130 and Cretaceous. It may be true that some basic figures had sauropods per km2) or a nine‑fold higher body weight per unavoidably to be estimated by Wilkinson et al. (2012) in specimen (25,000-34,000 kg vs. 2800-3800 kg). It seems a very rough way (because no precise data are available), strange that such different body sizes are under discussion, but such a lack of data does not imply that a ‘best guess’ as the numerous finds of sauropods from the Morrison For‑ 140 JOURNAL OF PALAEOGEOGRAPHY Oct. 2012

Fig. 1 Skeleton of Apatosaurus louisae at the Carnegie Museum. This mid‑sized sauropod, weighing some 20 t, may have produced 2675 liters (1.9 kg) of methane per day (Photo Tadek Kurpaski).

Fig. 2 Exposure of the Morrison Formation in Utah (USA). The reddish color suggests subaerial exposure. A. J. (Tom) van Loon: Were sauropod dinosaurs responsible for the warm Vol. 1 No. 2 Mesozoic climate? 141 mation (Mateus, 2006; Foster, 2007) are in themselves the dinosaurs, being 80,000-90,000 kg/km2 (McNab, 2009), a clue regarding the (average) sauropod body size! value that still is the double of what is mentioned (Farlow It is of interest in this context that the type of metab‑ et al., 2010) for sauropods with a mammalian‑type me‑ olism is, though not directly, connected with the warm‑ tabolism. or cold‑blooded nature of a vertebrate. Although we The estimate of 200,000 kg/km2 is equivalent to 10 in‑ do not know whether extinct vertebrates were warm‑ or dividuals of Apatosaurus louisae per km2. Considering the cold‑blooded, new techniques give some clues (Eagle et methane production per individual of 1.9 kg per day, this al., 2010), and an ever increasing number of studies indi‑ means 19 kg/km2 per day, or 6935 kg (~7 t) per km2 per cate that dinosaurs were most likely warm‑blooded (Buf‑ year. fetaut, 2004; Gillooly et al., 2006; Faux and Padian, 2007; 3.2 Restrictions Eagle et al., 2011; Seymour et al., 2011). This is at least an indication that it is more likely that the sauropods had a Apart from the dubious assumptions of Wilkinson et mammalian metabolism than a reptilian one. Consequent‑ al. (2012) regarding the potential sauropod biomass per ly, a sauropod total biomass of 42,000 kg/km2 seems more square kilometer, it is remarkable that these authors did not likely than one of 377,000 kg/km2. pay any attention to aspects that may (and certainly will) Wilkinson et al. (2012) were, as can be deduced from have lowered the number of sauropods. their references, aware of the above figures. Nevertheless In the first place, they had natural enemies, for instance they “conservatively assume sauropod biomass density, in the form of carnivorous dinosaurs. Although particu‑ averaged over the global vegetated land area, to be around larly the giant sauropods must have been capable to resist 200,000 kg/km2”. It is remarkable that they call this value attacks of individual (much smaller) dinosaurs, herds of ‘conservative’ , particularly taking into account that they such raptors (such as Velociraptor: Fig. 3) must have been themselves refer to other recent estimates of herbivorous able to attack much larger preys as shown by, for instance,

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Fig. 3 Skeleton in the Wyoming Dinosaur Center of the relatively small carnivorous dinosaur Velociraptor, who may have success‑ fully hunted in groups for huge sauropods (Photo Ben Townsend). 142 JOURNAL OF PALAEOGEOGRAPHY Oct. 2012 the find of a pterosaur bone in the guts of the skeletal re‑ lethal. mains of a Velociraptor (Hone et al., 2012). There are also fairly numerous sauropod bones that show bite marks, but 4 Sauropod habitat it may, obviously, be that many of them are due to scav‑ engers. Whatever may be the cause of these bite marks, Here we come to a point where palaeogeography plays sauropods must have represented attractive food for rap‑ an important role, since palaeogeographic data provide tor animals, and thus will have attracted large numbers of clues for a reconstruction of all kinds of environments, and them. This must, by definition, have resulted in a death thus for the areas where sauropods may have lived. This, toll, diminishing the population that must consequently obviously, also holds for the environments of the Morrison have been smaller than the theoretical potential size (see Formation and their suitability for sauropod life. Remark‑ also the section on food). It is interesting in this respect ably, Wilkinson et al. (2012) states that it was probably that particularly young animals seem to have served as a because the formation was deposited at least in part under prey (Hanna, 2002). semiarid conditions not an optimal megaherbivore habitat. In the second place, all animals (and thus also sauro‑ This is, however, only partly true. The Morrison Formation pods) suffer from time to time from diseases, infections, is described (Bilbey, 1998) as ranging from desert‑like to etc. that may shorten their lives and that reduce the number shallow marine, with extensive fluvial, more particularly of individuals in a population. Wilkinson et al. (2012) did fluvial‑plain deposits (Fig. 5), the habitat where most dino‑ not pay any attention to such potential threats, including saurs seem to have lived. The land was partly covered by the occurrence of flea‑like ectoparasites such as Pseudop- plants that formed some kind of savannah (Parrish et al., ulex jurassicus (Fig. 4) and Pseudopulex magnus, which 2004) though, obviously, no grasses existed at the time, lived in the Jurassic and Cretaceous, respectively (Gao et but the large desert‑like parts must have been almost de‑ al., 2012); Poinar (2012) mentions the occurrence of more void of life. Obviously, the river banks and alluvial plains similar Mesozoic animals that may have been a plague for were the places where life forms concentrated, and where sauropods, resulting in infections that may have become dinosaurs, including the large sauropods, flourished and

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Fig. 4 Reconstruction of Pseudopulex jurassicus, a huge flea‑like ectoparasite that fed itself with blood, most probably also from sauropods (Drawing Wang Cheng, Oregon State University). A. J. (Tom) van Loon: Were sauropod dinosaurs responsible for the warm Vol. 1 No. 2 Mesozoic climate? 143 felt at home, as indicated by, for instance, Camptosaurus context is that not all suitable areas were inhabited by all embryos (Camptosaurus is not a sauropod but belongs to kinds of herbivorous dinosaurs: it appears that they all had the Ornithischia) that were found in the Morrison Forma‑ more or less well defined territories (Lyson and Longrich, tion (Foster, 2007). The nests indicate that at least this di‑ 2010), which also implies that the average population of nosaur genus did most likely not belong to a migratory sauropods must have been considerably lower than eco‑ population. logically possible (cf. Van Loon, 2012). Whether sauropods were migratory or not, is important. It is known that, in contrast to Camptosaurus, other sauro‑ 5 Availability of food pods were migratory. Several species, such as Camarasau- rus, migrated from hilly terrains in the Morrison Basin to It is also known, on the basis of sauropod trackways, lower areas, depending on the season (Fricke et al., 2011). that some sauropods (but also other dinosaurs) lived This is ascribed to seasonal lack of food and water in the in herds (e.g. Day et al., 2002), implying that locally a uphill regions; during the dry seasons, these uphill areas high population density must have existed, but the con‑ were therefore probably not (or in a much lower density) sequence of such herds is unavoidably that they required populated by sauropods. This implies that not all areas that large amounts of food. It has been estimated (Fricke et al., were potentially suitable for sauropods were inhabited by 2011), for instance for Camarasaurus, which had a size of these giants all the time. This brings down the average ‘only’ some 14 m and a weight of some 6-7 t (Fig. 6), that population density. Another point that is important in this this animal needed some 450 kg per day. The largest sauro‑

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Fig. 5 The reddish Tidwell Member of the Morrison Formation in Wyoming represents lakes and mudflats, whereas the overlying more whitish Salt Wash Member represents a semi‑arid alluvial plain (with seasonal mudflats), where sauropods used to roam (Photo Ankyman). 144 JOURNAL OF PALAEOGEOGRAPHY Oct. 2012 pods, which may have weighted 70 t (Fowler and Sullivan, their food (in the form of plants) consisted of water and 2011) or even more, will have needed the ten‑fold amount. half of carbon (cf. Domenach et al., 1994), an equivalent The consequence is that herds of large sauropods must of 910 t of carbon. The sauropod concentration assumed have required so much food that even the climate‑related by Wilkinson et al. (2012) would then have consumed the locally lush vegetation would be able to support a herd complete available lush vegetation in 16 years. This re‑ only for a short time; then like the herds of large herbivo‑ sults, according to the calculations by Strandberg and Crill rous mammals like reindeer and buffaloes that used to mi‑ (2012), in insufficient time for the vegetation to recover: grate on their search for food before man made this come the net primary production of leaves should in this case largely to an end——they had to go further, leaving an have been at least 2.5 times higher than in modern‑day area without food behind them. This implies that, even tropical forests, where the gross primary production is ex‑ though herds of sauropods may have roamed the Earth, ceptionally high: some 3000 t of carbon per km2 per year this can not have resulted in a large average population (Malhi et al., 2009). Some researchers presume that the density. gross primary production during the Mesozoic may have Whether sufficient food can have been available for a been 65%-70% higher (= ~5000 t) than nowadays (Beer‑ 2 sauropod population of 200,000 kg/km , has been com‑ ling, 1999) due to a higher atmospheric CO2 level and a mented upon by Strandberg and Crill (2012). They men‑ warmer climate, but both models and experiments point tion that sauropods may have eaten 5% of their body mass at an increase of the net primary production of maximally per day (cf. McCullag, 1969), which would imply that sau‑ some 25% and 50%, respectively; a productivity of 2.5 ropods have eaten 3700 t/km2/y, which means, if half of times that of modern tropical forests (Fig. 7), required to

Fig. 6 Skeleton of Camarasaurus lentus at the Carnegy Museum of Natural History in Pittsburgh. This relatively small sauropod, which measured ‘only’ some 14 m and which had a weight of some 6-7 t, needed some 450 kg of food per day. A. J. (Tom) van Loon: Were sauropod dinosaurs responsible for the warm Vol. 1 No. 2 Mesozoic climate? 145 feed the sauropods, therefore cannot have been reached. markably enough, the decline did not affect the sauropods, Strandberg and Crill (2012) come on the basis of the but hadrosaurs and ceratopsids. This may indicate that the above considerations to the conclusion that, assuming that latter groups had ecological problems, were more vulner‑ 10% of the net primary production of leaves was available able or could no longer compete with the sauropods when for sauropod consumption (equivalent to 700 t/km2/y), the food became scarce, for instance because of overgrazing. sauropod population can have been 15 t/km2: less than 1 It may, however, also be because the huge sauropods with adult specimen of Apatosaurus louisae. their long necks could reach food that was out of reach for Such a concentration may have resulted in a methane other herbivores. Analysis of the skull of Diplodocus, for emission of 500 kg/km2, which implies a global emission instance (Figs. 8, 9), indicates that this species was well of 40,000 t per year. The large sauropods could, howev‑ adapted to strip leaves from trees (Young et al., 2012). er, not live in a dense forest because they were too huge. This has also important consequences for the calcula‑ They seem to have lived largely along rivers, possibly with tions by Wilkinson et al. (2012), who arrived at a total woods in the neighborhood (Fig. 7). They would, however, global methane production by sauropods of 520 million t have been capable of feeding only on the marginal trees, per year, which is roughly the same as the present‑day re‑ so that they can have used only a relatively small part of lease of methane (Dlugokencky et al., 2011); this figure is the available food. Consequently, their number must have composed of a pre‑Holocene (natural) value of ~200 mil‑ been proportionally smaller. lion t per year with an addition of 330 million t as a result It is in this context an interesting thought that the rapid‑ of human activities. Wilkinson et al. (2012) arrive at the ly increasing knowledge about dinosaurs indicates clearly amount of 520 million t per year because they presume that not all representatives of this taxonomic group be‑ that half of the land area (150 million km2) was vegetated came extinct at the K/T boundary, but that specific groups and that all this vegetated area was inhabited by sauropods started to decline earlier. Recent research indicates, for in the concentrations mentioned before. Why half of the instance, that the large herbivorous dinosaurs started to continental area would have been vegetated is not indicat‑ decline already 12 million years before the K/T boundary, ed by Wilkinson et al. (2012). It seems a significant over‑ whereas the mid‑sized herbivores and the carnivorous di‑ estimation considering the fact that the global temperature nosaurs were still flourishing (Brusatte et al., 2012). Re‑ was high. So high that sufficient plants grew in the polar

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Fig. 7 Reconstruction of the sauropod Puertasaurus reuili in the habitat where most dinosaur remains are found: alluvial plains sur‑ rounded by trees with an undergrowth that was accessible for the huge animals. 146 JOURNAL OF PALAEOGEOGRAPHY Oct. 2012

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Fig. 8 Skeleton of Diplodocus longus, a sauropod that could reach enough to eat leaves of trees that grew too high for other animals.

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Fig. 9 Skull of Diplodocus, showing the teeth with which he could easily strip leaves from trees. A. J. (Tom) van Loon: Were sauropod dinosaurs responsible for the warm Vol. 1 No. 2 Mesozoic climate? 147 regions to feed sauropods (Martin et al., 2011; Woodward tion. Nature Communications, 3: doi:10.1038/ncomms1815. et al., 2011). Buffetaut, E., 2004. Polar dinosaurs and the question of dinosaur The temperatures at lower latitudes must, however, extinction: a brief review. Palaeogeography, Palaeoclimatology, have been much higher, so that huge areas must have been Palaeoecology, 214: 225-131. barren or only scarcely vegetated (as shown by large parts Day, J. J., Upchurch, P., Norman, D. B., Gale, A. S., Powell, H. Ph., 2002. Sauropod trackways, evolution and behavior. Science, 296: of the Morrison Formation itself). Apart from that, many 53-62. more areas are unsuitable for large animals, for instance Dlugokencky, E. J., Nisbet, E. 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