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Review
. 2011 Feb;86(1):117-55.
doi: 10.1111/j.1469-185X.2010.00137.x.

Biology of the Sauropod Dinosaurs: The Evolution of Gigantism

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Free PMC article
Review

Biology of the Sauropod Dinosaurs: The Evolution of Gigantism

P Martin Sander et al. Biol Rev Camb Philos Soc. .
Free PMC article

Abstract

The herbivorous sauropod dinosaurs of the Jurassic and Cretaceous periods were the largest terrestrial animals ever, surpassing the largest herbivorous mammals by an order of magnitude in body mass. Several evolutionary lineages among Sauropoda produced giants with body masses in excess of 50 metric tonnes by conservative estimates. With body mass increase driven by the selective advantages of large body size, animal lineages will increase in body size until they reach the limit determined by the interplay of bauplan, biology, and resource availability. There is no evidence, however, that resource availability and global physicochemical parameters were different enough in the Mesozoic to have led to sauropod gigantism. We review the biology of sauropod dinosaurs in detail and posit that sauropod gigantism was made possible by a specific combination of plesiomorphic characters (phylogenetic heritage) and evolutionary innovations at different levels which triggered a remarkable evolutionary cascade. Of these key innovations, the most important probably was the very long neck, the most conspicuous feature of the sauropod bauplan. Compared to other herbivores, the long neck allowed more efficient food uptake than in other large herbivores by covering a much larger feeding envelope and making food accessible that was out of the reach of other herbivores. Sauropods thus must have been able to take up more energy from their environment than other herbivores. The long neck, in turn, could only evolve because of the small head and the extensive pneumatization of the sauropod axial skeleton, lightening the neck. The small head was possible because food was ingested without mastication. Both mastication and a gastric mill would have limited food uptake rate. Scaling relationships between gastrointestinal tract size and basal metabolic rate (BMR) suggest that sauropods compensated for the lack of particle reduction with long retention times, even at high uptake rates. The extensive pneumatization of the axial skeleton resulted from the evolution of an avian-style respiratory system, presumably at the base of Saurischia. An avian-style respiratory system would also have lowered the cost of breathing, reduced specific gravity, and may have been important in removing excess body heat. Another crucial innovation inherited from basal dinosaurs was a high BMR. This is required for fueling the high growth rate necessary for a multi-tonne animal to survive to reproductive maturity. The retention of the plesiomorphic oviparous mode of reproduction appears to have been critical as well, allowing much faster population recovery than in megaherbivore mammals. Sauropods produced numerous but small offspring each season while land mammals show a negative correlation of reproductive output to body size. This permitted lower population densities in sauropods than in megaherbivore mammals but larger individuals. Our work on sauropod dinosaurs thus informs us about evolutionary limits to body size in other groups of herbivorous terrestrial tetrapods. Ectothermic reptiles are strongly limited by their low BMR, remaining small. Mammals are limited by their extensive mastication and their vivipary, while ornithsichian dinosaurs were only limited by their extensive mastication, having greater average body sizes than mammals.

Figures

Fig. 1
Fig. 1
The largest representatives of different terrestrial vertebrate clades, both extant and extinct. (A) Non-dinosaurian terrestrial vertebrates and birds: (a) the tortoise Geochelone gigantea, (b) the Komodo dragon Varanus komodoensis, (c) the Pleistocene Australian monitor † Varanus (Megalania) prisca, (d) the Eocene boid snake † Titanoboa cerrejonensis, (e) Homo sapiens, (f) the African elephant Loxodonta africana, (g) the long-necked Oligocene rhinoceros † Paraceratherium (Indricotherium) transouralicum, (h) Struthio camelus, (i) an unnamed Miocene † Phorusracidae. (B) non-avian dinosaurs: (a) the hadrodaur † Shantungosaurus giganteus, (b) the ceratopsian † Triceratops horridus, (c) the theropod †Tyrannosaurus rex, (d) the theropod † Spinosaurus aegyptiacus, (e) the sauropod † Brachiosaurus brancai, (f) the sauropod †Argentinosaurus huinculensis. Scale = 5 m.
Fig. 2
Fig. 2
Comparison of body masses of sauropod dinosaurs, theropod and ornithischian dinosaurs and mammals. The mass data for sauropods are found in Table 1, while those for the other dinosaurs are primarily from Seebacher (2001) with additional data from Christiansen (1997) and Anderson et al. (1985). The data for mammals were compiled from Janis & Carrano (1992), Fortelius & Kappelman (1993), and Spoor et al. (2007). With the exception of the two largest forms they represent extant mammals only. Mammals show a strongly right-skewed distribution, theropods and ornithischians show intermediate masses, and sauropods show a strongly left-skewed distribution. Not that the y-axis is logarithmic.
Fig. 3
Fig. 3
Body mass of the largest species inhabiting a land mass regressed against the size of the land mass in extant and Late Pleistocene terrestrial amniotes. The species are grouped by metabolism (bradymetabolic ectothermy versus tachymetabolic endothermy) and trophic level (herbivores versus carnivores). The two outliers of endothermic herbivores are island dwarf elephants. The largest species were ectothermic herbivores on only three land masses, precluding regression analysis of this group. Note that maximum body mass for a given land mass decreases with increasing metabolic rate and trophic level. Fossil mammal taxa adhere to the regressions while sauropod and theropod dinosaurs do not, being much larger than predicted. See text for details. Redrawn from Burness et al. (2001).
Fig. 4
Fig. 4
Simplified sauropod phylogeny compiled from Wilson (2002), Upchurch et al. (2007a), Yates (2007), Allain & Aquesbi (2008), and Remes et al. (2009). Only well-known taxa whose position in the phylogeny is relatively stable are shown. Arrows indicate stem-based taxa, and dots indicate node-based taxa.
Fig. 5
Fig. 5
The sauropod body plan and body size. The reconstruction of Brachiosaurus brancai (recently renamed Giraffatitan, see Taylor, 2009) is based on the mounted skeleton in the Natural History Museum Berlin. Sauroposeidon from the Lower Cretaceous of Oklahoma (USA), one of the recently described truly gigantic sauropods, is only known from a string of four neck vertebrae. Based on these, the animal can be estimated to have been about 30% larger in linear dimension than the Berlin Brachiosaurus. Modified from Wedel et al. (2000b).
Fig. 6
Fig. 6
Independent evolution of gigantic species (>40 t body mass) in several lineages of Sauropoda as shown by optimization of body size on a sauropod phylogeny (part of the supertree of Dinosauria published by Lloyd et al., 2008). Note that Turiasaurus, Paralititan, Puertasaurus, Futalognkosaurus, and Huanghetitan are not listed because they were not covered by this phylogeny. Body masses were taken from various sources (see Table 1). Lack of a colored box in front of the genus name indicates a lack of mass data.
Fig. 7
Fig. 7
Three factors, i.e., more resources available, fewer resources used, and the reproduction mode, potentially resolved the land area versus body size enigma of Burness et al. (2001) and thus contributed to the gigantism of sauropods and theropods. Specific hypotheses (discussed in the text) underlying each contributing factor are listed below each factor. Because very likely more than one factor was important, the relative contribution of each is best visualized in a ternary diagram. The symbols with the question marks indicate potential solutions to the gigantism enigma, and the relative importance of each factor can be read off the percentage scale leading up to its respective corner. Note that we do not offer a final solution but that this graph is meant to visualize the possibilities of interplay between the three factors.
Fig. 8
Fig. 8
Variation of atmospheric composition(O2, CO2) and body size through time. Each data point is located at the beginning of a stage, starting with the Carnian and ending with the Cretaceous-Tertiary boundary. The variation of body size through time is an extension of the Carrano (2006) data set with femur length as a proxy for body size. Missing data points for body mass are either due to lumping of data from two stages (i.e. the Kimmeridgian and Tithonian) or missing data (i.e. for the Berriasian, Barremian, and Aptian). Body size increases gradually from the Late Triassic to the Late Jurassic, forming a plateau in the Cretaceous. The two sharp drops in body mass in the Early and Late Cretaceous are probably due to a poor terrestrial fossil record at these times. Note the lack of correlation between atmospheric composition and sauropod body mass. CO2content of the atmosphere also determines global temperature, and this graph thus suggests that sauropod body size is not correlated with global temperature variations through time, either. The data for O2 and CO2 levels are from Ward (2006).
Fig. 9
Fig. 9
Flow chart of the evolutionary cascade leading to sauropod gigantism. The green boxes contain the biological properties of sauropods, and the black arrows indicate primary evolutionary causation. Theropod predation pressure is depicted as a representative selection factor for body size increase. In addition to primary evolutionary causation, sauropod gigantism was also driven by evolutionary feedback loops (blue arrows). The blue boxes indicate the selective advantage in the feedback loop. The boxes on the black arrows show the selective advantages conferred on sauropods by the biological properties. BMR, basal metabolic rate.

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