Australia: The Land Where Time Began
Dinosaurs - Success of the Sauropods
The first sauropod remains to be found were originally believed by Sir Richard Owen in 1842 to be those of an aquatic crocodile that he named Cetiosaurus (“whale lizard”). Then in 1871a skeleton of Cetiosaurus was discovered that was complete enough for John Phillips to suggest that it was an animal that spent at least part of its time on the dry land. Sauropods were still being argued among scientists to have flourished by default, having no other dinosaurs to compete with.in their habitat. They are now known to have been much more successful than previously believed, with more than 70 new species of sauropod being discovered since 2002, revealing that they were actually an extraordinarily resilient group that flourished for millions of years at the observed limits of body size for terrestrial animals. Scientists have finally begun to answer some key questions concerning the reproduction of sauropods, how they grew, their food sources and their strategy for adapting to dramatic changes in their environment. Almost everything that was believed about these iconic dinosaurs has been changed by recent research that has uncovered more specimens and the development of new analytical tools.
The origin and evolution of sauropods
The history of dinosaurs, their origins and diversification into so many forms over the 150 My of their existence, began in the Late Triassic, about 210 Ma, at the time of a worldwide extinction event of other reptile groups that made way for their rise and eventual dominance of terrestrial ecosystems.
Small-bodied bipedal animals that were recovered from 230 Ma sediments of the Southern Hemisphere are the oldest known dinosaurs to appear in the fossil record; they would eventually evolve into the sauropods and theropods. Characteristic trackways of true sauropods, showing their quadrupedal posture and elephantine feet, in North America and Argentina, which have been dated to about 225 Ma. The earliest indications of the massive skeletons of these large dinosaurs came in the form of finds of a number of fossils a bit younger than 225 Ma, such as Isanosaurus from Thailand, Gongrianosaurus from China and Vulcanodon from Zimbabwe. The sauropod signature, that have been described as “walking backbones”, as many of the sauropods had more than 100 vertebrae, with necks and tails that were long and tapering, with tiny skulls and pillar-like legs that were built for bearing their enormous weight. The sauropods were among the most recognisable of the dinosaurs, with a basic architectural plan that persisted for their entire evolutionary duration.
Rather than stagnate, the body plan of the sauropods became more elaborate with passing time, and plate tectonics helped drive the global diversification of their lineages. The diversity of the sauropods waxed and waned throughout the evolutionary history of this extraordinary group all the way to the close of the Cretaceous. There was a long-standing view that sauropods reached their prime for a short time in the Jurassic, barely limping through to the extinction event at the end of the Cretaceous. It is now known that was decisively not true, with important sauropod groups, such as the stocky titanosaurs and rebbachiasaurids, that were shovel-faced, that flourished long after their presumed heyday in the Jurassic. According to the authors the sauropods were still diverse and flourishing in the Late Cretaceous.
Based on recent research it is now known that sauropods were successful in a number of ways, diversity, lineage longevity and geographical range. It now seems they attained this success by combining a number of strategies, mixing characteristics of reptiles and mammals that were merged to make up a unique type of animal. The authors suggest that the term “whale lizard” applied to them by Owen ended up being a fairly accurate description of these magnificent animals, though he believed they were reptiles.
As with all dinosaurs and reptiles sauropods laid eggs. The first definite evidence of the reproductive biology of sauropods was found in 1977 by Luis Chiappe et al. in the form of a nesting ground that has been dated to about 80 Ma that contained thousands of eggs at Auca Mahuevo, Argentina. Contained in some of these eggs were the first known sauropod embryos, on some of which were fossilised skin and egg membranes. There is clear evidence from these nesting traces that titanosaur sauropods laid clutches of 20-40 eggs per nest, which was a depression they excavated. The spherical eggs were about the size of grapefruit, 13-15 cm in diameter. It can be seen from the large number of nests in the same area these titanosaurs associated in large numbers, nested in the same area at least 6 times. There is still no evidence that they brooded the eggs or cared for the resulting young after they hatched. According to the authors the proximity of the nesting structures to each other suggests that they didn’t, depositing the eggs then leaving. It seems that in their reproductive strategies they followed the typical reptile pattern of depositing their eggs then leaving the hatchlings to fend for themselves. There are exceptions to this pattern, the crocodile and at least 1 species of Australian desert lizard that care for their young for some time after hatching, though they don’t feed the young.
Once the young hatched they followed a pattern of growth much different from reptiles, and they had more growing to do than any other animal extinct or extant, starting from a length of less than 0.5 m and weighing less than 10 kg they needed to grow to 30 m long with a mass of 25,000-40,000 kg, or possibly more. This is compared with the baby African elephant that grows from 120 kg at birth to 2,268-6,350 kg. In the past dinosaur researchers extrapolated growth rates of reptiles to estimate the growth histories of sauropods, which yields an age of 60 before they reached their first growth plateau and eventually reached their adult size at more than 100 years of age. If they followed this pattern of growth they would remain at risk of predator attack for a very long time.
The pattern of bone mineral, the density and architecture of the cavities that remained that had been filled by blood vessels, as well as the degree to which the bone had been remodelled are all preserved in fossil bone. Sauropod growth rates are indicated by these features to have soared through most of life and were generally faster than those of reptiles and similar to those of extant large-bodied mammals, many of which reach maturity in decades, which suggests sauropods reached maturity considerably earlier than was believed when it was thought they followed a reptilian pattern of growth.
It is believed that as sauropods had such high rates of growth and they had so much growing to reach their adult size they must have needed to eat a large amount of plant food just to survive, and it is known they thrived for a very long time. Tooth shape, microscopic wear marks and jaw muscle reconstructions, and biomechanical analyses of the opening and closing of jaws have been the focus of research on the feeding strategies of sauropods. It has been found that distinct feeding methods were used by different sauropods, with some biting off tough vegetation, while others cropped or clipped plants that were less tough.
There has been general agreement that sauropods didn’t chew much and therefore must have used some other method of processing to break down their plant food to obtain the usable energy. Gastroliths, stomach stones, are the most commonly cited solution to this need. It is common to find polished stones in rock formations that also contain sauropod remains in western North America, and for a long time scientists have been hypothesising that these stones are analogous to the Gastroliths that are known to be used by some modern vertebrates such as some birds, the stones being swallowed to aid digestion by grinding the food. A study has been carried out (Wings & Sander, 2007) in which experimental analysis of how gizzard stones in modern birds were processed and degraded. It was shown by the results of their study that stones that had been suggested to be sauropod Gastroliths had a different surface texture than would be expected when the stones were compared with the Gastroliths of birds, and it is also not common and equivocal to find Gastroliths in sauropod body cavities.
Insights into the feeding behaviour of sauropods have come from the long necks of sauropods that are often suggested to be to allow them to feed high in trees. More recent research has disputed this, one study reconstructing the feeding strategies among diplodocoids, including Diplodocus and Apatosaurus, which survived from the Late Jurassic to the Late Cretaceous. It is indicated by the variation in snout shape and microscopic wear patterns that some of the sauropods specialised in particular plant types, while others were probably generalists, some feeding on ground level plants and some fed on trees. Other research groups have analysed sauropod neck postures, arriving at the same conclusion, their research showing that the vertebral flexibility in sauropod necks also constrained the feeding strategies of the various sauropods by varying their ability to crane their necks to reach plants that were either high or low. The authors suggest this variation in feeding habits helps explain how so many huge animals could share the same ecosystems.
During the Cretaceous the dietary adaptations of the sauropods flourished concurrently with the rise of the flowering plants that was occurring at this time. It had been suggested in the past that it may be the rise of the flowering plants that pushed the sauropods toward extinction as they were not adapted to new plant type. According to this view sauropods with weak teeth, small heads and giant bodies were not able to compete with the duck-billed dinosaurs and horned dinosaurs with their batteries of teeth that acted like conveyor belts acting as a single powerful tooth that grows continuously.
It is now known that the Cretaceous was the most diverse part of the evolution of sauropods, and the authors say it is arguably the most interesting part, and, rather than being outcompeted by highly adapted competitors, they diversified in booth form and function so they could exploit a wide variety of plants. An example is Nigersaurus from 115 Ma deposits in Niger, Africa, which was discovered by Paul S. Sereno in 1991. Nigersaurus had broad dental batteries that grew extremely fast for tooth replacement, an estimated rate of tooth replacement of a new tooth in each position per month, which make it up to twice as rapid as occurred in duckbill and horned dinosaurs, which kept the teeth sharp for efficient vegetation clipping. An unusual feature of Nigersaurus is that it is indicated by the orientation of the semicircular canals of the inner ear that its head was carried in such a way that its muzzle pointed directly downward, which is rotated far from the horizontal plane inferred for other sauropods. It is suggested by this position that it specialised in feeding on a food source close to the ground.
Flowering plants continued to diversify throughout the Cretaceous which allowed the sauropods to expend the range of their food choices. It has been found by microscopic analysis of coprolites that were produced by titanosaurs that they ate plants with phytoliths, silicified plant tissue, that have documented their consumption of at least 5 different grass types, as well as flowering plants such as magnolias, conifers and palms.
This discovery (Prasad, 2005) extended the origin and diversification of modern grasses back a further 30 My, and also supports the suggestion that at least some sauropods were indiscriminate feeders. It was not in the interests of such large animals as sauropods that grew so rapidly to be a picky eater. Rather than being pushed out of these emerging ecosystems as has been suggested in the past the evidence from coprolites suggests they took full advantage of the expanding range of food plants.
It is suggested by the authors that sauropod growth may have been aided by enhanced intake of oxygen. Air sacs that connect to outpocketings of the lungs of birds invade the hollowed out vertebrae of their spine, a structure which allows birds to extract more oxygen from each breath than is possible in mammals, and in birds a unidirectional airflow through the lungs is allowed by the air sac system, which allows even more oxygen to be extracted from each breath. Mammals have a method of oxygen extraction that is relatively inefficient in which the new air mixes with the old air in the lungs. As in modern birds, sauropods have an almost identical system of internal cavities in their vertebrae and a complex pattern of external hollows bounded by struts. The greatest extent of pneumaticity, or hollowness, is seen in the vertebrae in the neck and trunk, though in some sauropod species it extended to parts of the shoulders, hips and tail. A reduction of the overall weight of the vertebral column is the primary effect of pneumatisation, estimates indicating it may have reduced the body mass of sauropods significantly. An example is Sauroposeidon from North America which had a neck that was more than 75 % air. The authors suggest it is possible the pneumatised vertebrae of sauropods also housed an extensive system of air sacs as is present in birds, and possibly with flowthrough lung ventilation, and this extra oxygen extraction would have allowed them to maintain high metabolic rates, that were stable, as well as increased levels of activity which would be consistent with their rapid rate of growth and the massive body size of adults.
As with large animals of the present the size of sauropods had advantages, such as making the adults usually immune to attacks from predators, as is the case with elephants of the present, and many sauropods were bigger than adult elephants even before they were half grown. The authors suggest that once they reached adult size they would have been fairly immune to serious predators, giving Allosaurus as an example. There are some drawbacks of large size, such as in times of drought or food shortage, though the authors suggest some may have evolved solutions to these problems. Some titanosaurs evolved osteoderms, bizarre bony plates in their skin. Some animals of the present, such as crocodilians, lizards and armadillos have them, as did other dinosaurs such as Stegosaurus, where they were the plates on the back and spikes on the tail. There has been a problem with trying to determine the function of osteoderms on sauropods as it is not known were they were carried on the body of these large animals. In 2011 2 osteoderms were found in Madagascar beside the skeletons of a juvenile and an adult titanosaur Rapetosaurus. The adult specimen osteoderm was 57 cm long and more than 27 cm thick, having a volume of nearly 10 litres, is the most massive osteoderm ever found from a vertebrate. The Rapetosaurus osteoderm was found by CT scans and drill cores to have been hollowed out over the lifetimes of the animal, and at very large sizes, a volume of about 5 litres of internal bone was replaced, probably by soft tissue. Rapetosaurus, and probably as well as some other titanosaurs, had only a few large plates, and not the pavement-like covering they are used for in animals of the present.
A number of competing ideas about the function of osteoderms in titanosaurs were ruled out with the help of these features of the osteoderms of Rapetosaurus. The authors say little protection from predators would be provided by such elements that are hollow; they would be expected to be of little use as thermoregulators because of their low surface to volume ratio and sparse distribution in the skin. The authors argue that osteoderms of titanosaurs may have served as mineral reserves to sustain their high growth rates and egg-laying ability even in times of shortages, as occurs in some modern animals. Among modern vertebrates, including humans, blood calcium levels are maintained by resorbing it from bone mineral, a process which often increases seasonally when resources are scarce, and during egg laying as well as increasing age in human, in some of whom the loss of bone mineral leads to osteoporosis. There are rich blood supplies in the osteoderms that would be perfect conduits for the cells responsible for the resorbing and for the mineral resources unlocked from deep within the bones. It is suggested this idea would make sense for a huge sauropod in Late Cretaceous of Madagascar. During the Late Cretaceous Madagascar was a harsh place with regular, severe droughts that lead to carnivorous dinosaurs, such as Majungasaurus, cannibalising other members of their own species, and caused the demise of many animals from frogs and birds to sauropods. It is suggested that osteoderms may have helped sauropods in Madagascar, as well as elsewhere, to survive environmental perturbations, including frequent, intense droughts.
The authors suggest sauropods appear to test the laws of biological possibility both in their growth speed and their large adult size, though this could have painted them into an evolutionary corner, but increasing knowledge of them suggests the opposite is actually the case. Following a disaster they couldn’t escape, the sauropods finally went extinct after a spectacular 150 million years of existence during which they diversified extensively while keeping the basic appearance and structure.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|