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Australia: The Land Where Time Began |
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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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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |