Australia: The Land Where Time Began
Sauropod Biology – What is known?
Clauss describes the sauropods as the ultimate outgroups among terrestrial vertebrates, based simply on their large size. Their huge body sizes challenge understanding of the maximum size that can be attained by terrestrial vertebrates. Sauropods are apparently so far outside the range of existing frameworks that they are a challenge.
Although the exact phylogenetic relationships of sauropods are not known, Clauss says they evolved from basal sauropodomorphs. A characterisation of sauropods describes them as having a quadrupedal stance with legs that are columnar, a neck and tail that are both long for the size of the body of the animal and with a head that is small compared to the remainder of the massive body. There is a large taxonomic diversity of the known sauropods, but there is little variation from the basic body plan. Sauropods can be grouped broadly into forms which have front legs that are longer, which are presumed to have had an upright neck, with a centre of gravity that is rather cranial, other forms in which the hind legs are longer, which are presumed to have carried their necks more horizontally, to have had a rather caudal centre of gravity. When the muscular and skeletal anatomy is reconstructed it reflects biomechanical particularities of the size and shape of their bodies, at both the macroscopic and the microscopic levels. It is possible to use differences in dental and cranial anatomy, length of neck, and posture, as well as isotope studies, to infer niche diversification of the taxa of sauropods. The sauropods were herbivorous, did not chew their food, and are believed to have most likely no other means of reducing the particle size of their food, such as a gizzard that contained gastroliths. Clauss suggests they probably had a massive hind gut containing a symbiotic microflora that fermented the plant material they swallowed, using the plant resources of their time as do present day herbivores. According to Clauss sauropods probably had heterogeneous “bird-like” lungs with air sacs and pneumatisation of various bony structures, in the neck vertebrae in particular. Sauropods are generally believed to have had a metabolic rate that was higher than that of extant ectotherms, though there is still a degree of debate about the actual rate difference; some (Sanders & Clauss, 2008) have suggested an ontogenetic decrease (decrease with age). The cardiovascular system of sauropods is being debated even more controversially, with consensus not yet being reached, with the exception that agreement has been reached that sauropods had 4-chembered hearts. Sauropods, which were oviparous, laid hard-shelled eggs, probably in many small clutches (Sanders et al., 2008), which also facilitated a fast rate of population regrowth. The young had a rapid growth rate, reaching sexual maturity in their 2nd year. It is believed they did not provide parental care to their young which probably contributed to a high rate of juvenile mortality, the various sauropod ontogenetic stages falling prey to a variety of different predators of the time (Hummel & Clauss, 2008). Clauss suggests it is likely sauropods lived in groups or herds, some of which appear to have been segregated by age (Coombs, 1990; Myers & Fiorillo, 2009).
According to Clauss this does not appear to be very exceptional, though as he points out they did this while achieving body masses when adults of 15-100 metric tons. Such a huge body size has never been attained by any other group of terrestrial vertebrates. As there were advantages to the sauropods of having such a large body size that apply to terrestrial vertebrates generally, what factors allowed only the sauropods to reach such huge sizes.
Characteristics independent of body mass, characteristics following on from body mass, and characteristics truly facilitating gigantism should be differentiated from one another in a functional approach to answering the question of how they achieved their very large sizes.
Many biomechanical adaptations of sauropods, e.g., were preconditions for, and the consequences of having such a large body size, though these adaptations appear to also be easy to achieve by other vertebrates by convergent evolution, so do not appear to be crucial factors that triggered gigantism. According to Clauss it is the universal applicability of the laws of static and dynamic mechanics that facilitates reaching an understanding of these convergent adaptations. According to mechanical principles the origin of these adaptations makes them particularly suitable for computer modelling investigations based on these principles. These studies are crucial to gaining an understanding of how a sauropod worked, though they cannot explain the origin and uniqueness of sauropod gigantism. Similarly the botanical and nutritional composition of potential sauropod food and the sauropod digestive tract, that was presumably enormous, can be described, though these factors do not set sauropods apart from other vertebrates. Clauss suggests that it seems the vertebrate musculoskeletal system and the digestive system can accommodate any given body size, however large, unless absolute limits to skeletal static due to gravity are being considered (Hokkanen, 1986; Alexander, 1989).
The intensive scientific debate concerning this can also be assumed for the cardiovascular system (Seymour, 2009a; Sander et al., 2009). The neck of sauropods, which is described by Clauss as peculiar, has been suggested to have been held in an upright position, distinctly inclined or curved posture, in many sauropods, based on skeletal reconstructions and in analogy with extant animals (Taylor et al., 2009). When the mechanics and energetics of the vascular system are considered this poses a dramatic conceptual problem (Seymour, 2009a, 2009b).
When considering thermoregulation in sauropods their gigantic size is believed to have guaranteed that their core body temperature would have been comparatively constant. Clauss suggests that an analogy with giant tortoises, “mass homoiotherms”, might suggest that the high level of their activity, which is inferred from sauropod trackways, e.g., can be accounted for by homoiothermy alone. It is hard to imagine that the growth rates of sauropods could be achieved without a high metabolic rate, and it has been suggested that it is not possible to achieve such high levels of gigantism, as has been observed in sauropods, in ectothermic animals (Head et al., 2009). Clauss suggests a convenient compromise between the different aspects of sauropod metabolism, the suggestions (Farlow, 1990; Sanders & Clauss, 2008), that there was an ontogenetic lowering of metabolic rate, which would facilitate the rapid growth rate in juveniles, but reduced heat stress and requirements for nutrition in adults, though this hypothesis is still to be corroborated.
Some factors that set the sauropods apart from giant terrestrial animals are their mode of reproduction, features of the anatomy and their physiology, long neck, respiratory system, and lack of mastication. Factors that appear to have been similar in sauropods and giant terrestrial animals are growth rates and possibly metabolism.
It is therefore suggested (Sanders & Clauss, 2008) the hypothesis that it was a combination of these factors made sauropod gigantism possible, however all of these factors will need to be investigated and tested if possible.
In extinct animals physiological testing is obviously difficult at best. Concepts of niche partitioning that are more precise are difficult to assess as there is insufficient resolution provided by the fossil record to associate particular plants with specific dinosaurs (Butler et al., 2009, 2010). Histological analysis of bone and dental tissue can yield information on growth, as well information on diet, thermoregulation and migration by isotope analysis (Tütken et al., 2004; Amiot et al., 2006; Fricke et al., 2009). Isotopic studies represent true tests as they have an advantage of presenting alternative approaches to questions that have been answered previously by other methods. To date such tests appear to be in accordance with hypotheses of Clauss.
It is rarely possible to generate hypotheses based on features of the skeleton that can be tested by other skeletal features alone. Such a rare example is represented by the association of features that facilitate a sauropod to rear on its hind legs and the mobility of its neck. Approaches that are more theoretical need to be used, that often involve allometric extrapolations, for hypotheses such as possible role of long neck, the presence of a respiratory system that is bird-like, and the absence of mastication. Such an approach must always remain speculative as sauropods are invariably outside the range of data from which the allometric regressions have been derived. The qualitative difference between vivipary and ovipary is the only feature evident enough that its relevance to population survival can be understood immediately.
According to Clauss it has been hotly debated within his research group whether a long neck represents an energetic advantage, as has been suggested, that might have favoured the evolution of a gigantic body size, or whether it is simply a feature that could be evolved independently of the size of the body by most nonchewing herbivores. Clauss suggests this issue will remain to be resolved as long as there are no model calculations on the energetic costs and benefits over the entire range of body size covered by juvenile to adult sauropods (Seymour, 2009a, 2009b; Sander et al., 2009). The potential advantage of having lungs that are bird-like will remain speculative while physiological models which take a comparative approach to quantifying a particular function of the lungs, such as that of heat exchange, for systems that are “mammal-like” and “bird-like” are lacking. The absence of bird-like lungs in both terrestrial mammals and the Ornithischia (Wedel, 2006), both of which failed to attain giant sizes of sauropods, is a strong indication for the relevance of such a system in the evolution of gigantism, even if the link between bird-like lungs and gigantism is not yet considered to be compelling. A model is still lacking that would demonstrate that mammal-like lungs constrained by body size.
Clauss suggests that the absence of mastication (Sander & Clauss, 2008; Sander et al., 2010a, 2010b) can be assumed to be associated with gigantism. Sauropods differ from mammalian and ornithischian herbivores in regard to the respiratory system. The percentage of time spent feeding by terrestrial mammalian herbivores, which have all evolved formidable adaptations for reducing the particle size of their food, increases in an allometric manner with body size that would require them to feed for more than 100 % of the day if the animals weighed in at more than approximately 18 metric tons. This threshold coincides with the estimated mass of the largest known terrestrial mammal (Indricotherium; Fortelius & Kappelman, 1993), the largest known ornithischian (Shantungosaurus; Horner et al., 2004), and with roughly a lower body mass range of the adults of many sauropod taxa, therefore the interpretation that herbivores, once they have evolved the adaptation for mastication that is very efficient, were then prevented from evolving giant size of the body as this would are required a secondary loss of mastication. It therefore seems a primitive feature of sauropods, never evolving mastication, allowed them to enter the niche of giants. The size of food particles will be determined by plant morphology alone, from a certain body size upwards, and therefore will remain rather constant, though the capacity of the gut will increase further with increasing body size. Clauss suggests sauropods might therefore represent a rare example of herbivores that actually benefit from increasing body size in terms of a larger gut and food being retained longer in that gut while avoiding the disadvantage of chewing efficiency decreasing.
Body mass will ultimately be restrained by the available food resources. The availability of biomass depends on climatic factors and the quality of the habitat, and when body size reaches gigantic proportions it is restricted by land mass. It is suggested by evidence that in some way the sauropods followed this pattern (Burness et al., 2001). Clauss suggest that by using the fossil record it is difficult to know if the number and diversity of smaller herbivores impacted on the available resources for the sauropods, and it has not been resolved for recent ecosystems either. Clauss suggests it might be possible to test whether the diversity of regular-sized and giant-sized herbivores is reciprocal across ecosystem, in parallel to the argument that an increased carnivore diversity could indicate more biomass available for secondary consumers in dinosaur ecosystems (Hummel & Clauss, 2008), which would indicate that the presence of giant herbivores can reinforce their own dominance via interference competition (Persson, 1985). Clauss suggests it will be through understanding the ecosystems, as has been advocated by a number of authors (e.g. Farlow, 2007), that the full dimension of gigantism will be understood.
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