Australia: The Land Where Time Began

A biography of the Australian continent 

Dinosaur Growth

According to the author¹, the growth rates of all land reptiles is low, applying also to the giant tortoises and big monitors that are energetic compared to other reptiles. The equatorial regions are the only parts of the world where the growth rate of reptiles can be increased, and they don't usually reach a tonne in weight even there. Growth rates are higher in marine reptiles, the author¹ suggesting that this results from the low energy costs of swimming that allows them to use more of their food intake for adding bulk. Reptiles have a tendency to grow slowly throughout their lives, but even the crocodilians, including the extinct giants that reached body masses of about 10 tonnes, grow more slowly than land mammals.

Among the mammals some, such as some marsupials and large primates, including humans, have growth rates not much higher than those of land reptiles, or even no higher. Other marsupials and placentals have growth rates that are moderate. There are some mammals that have high growth rates, growing to full adult size in less than 2 years. Some of the aquatic whales can grow to 50-100 tonnes in a few decades. It takes about 30 years for bull elephants to reach full adult size. Birds of the present all have high growth rates, especially altricial species and the big ratites. No known extant bird takes more than 1 year to mature, though some of the giant ratites living on islands are believed to have possibly taken a few years to reach full size. The author¹ suggests that an aerobic exercise capacity high enough, in either the juveniles or the adults caring for them, to gather enough food to keep the young growing at their maximum growth rate.

Land animals that lack armour are under pressure to grow as rapidly as possible because of the dangers of predation, disease and accidents, and it is statistically unlikely they will have long lives. There is a tendency for growth rates to decrease when reproduction begins while an animal is still growing as resources are diverted to the production of offspring. Among the birds of the present none begin breeding before they reach full adult size. Few mammals begin breeding before growth is complete. Elephants don't tend to live longer than about 50 years, and the medium-sized and large mammals, as well as birds, usually live a few years or sometimes a few decades. No known bird continues growing past maturity, the same applying to most mammals, though elephants and some marsupials continue growing slowly.

The speed of growth of an animal influences the bone matrix, and in this respect dinosaur bones tend to be more similar to the bones of mammals and birds than to those of reptiles that grow more slowly. A method that has been used to estimate the growth rate and lifespan of dinosaurs is the counting of the growth rings in the bone. The author¹ suggests this method may be inaccurate as some birds lay down more the 1 growth ring in a year leading to the growth rate being underestimated and the age being overestimated. Some animals don't lay down growth rings, and it is believed they probably grow rapidly, but the actual speed of growth growth is difficult to determine. Of the dinosaurs that have been subjected to the bone ring method nearly all appear to have grown somewhat faster than land reptiles. A very small troodont theropod that was bird-like is a possible exception in which the bone rings were possibly laid down at the rate of more than 1 per year, the author¹ suggesting this may have occurred because it was reproducing as it was growing.

It has been found that most small dinosaurs are in the lower end of the mammalian growth zone, possibly because they were reproducing before they reached full maturity and size. Bone rings are lacking completely from the bones of some small ornithopods, probably indicating that they were growing rapidly, possibly as fast as birds of the present, the author¹ suggesting they may have reached full size within a year. Of the gigantic dinosaurs most appear to have grown as fast as land mammals of similar size. Juveniles of hadrosaurs and ceratopsids didn't lay down bone growth rings, apparently indicating they grew particularly fast, though none of the dinosaurs grew as fast as the big rorqual whales. Large mammals are born only a few dozen times smaller than the adults, and for some time after birth are provided with highly nutritious milk by their mothers, but the offspring of the largest of the sauropod dinosaurs were so small compared with the adults of their species when they hatched they had to grow tens of thousand-fold in the space of a few decades to reach adult size, having little and more likely no nourishment being provided by their parents. The armoured dinosaurs appeared to have grown more slowly than the others.

No evidence has been found that the lifespan of dinosaurs were longer than those of similar-sized mammals. It is believed the giant theropods lived for about 30 years, possibly because they had an extremely dangerous lifestyle in which they attacked large adult prey that often had weapons with which they could defend themselves. According to the author¹ "The small-brained dinosaurs were throw-away organisms, unlike large, big-brained mammals that are major investments requiring extensive parental care and resources. The short life spans of these great dinosaurs were acceptable because they were expendable creatures, being early and fast-breeding r-strategists that could readily replace their losses. The cessation of significant growth of the outer surface of adult dinosaur bones indicates that most species did not grow throughout life the way many reptiles do".

Dinosaurs – Large Size²

It was previously believed that dinosaurs grew slowly as modern reptiles of the present grow, therefore very large dinosaurs must have been of very great age, though it was not known how old they were, as modern reptiles are much smaller than dinosaurs. The belief that dinosaurs grew slowly existed since the time of Sir Richard Owen the English palaeontologist.

At the time Owen named the Dinosauria in 1842 the group consisted of a very small number of species in the group, and they were not well known, apart from the fact that there were very large reptiles that were unusual. Marine reptiles, ichthyosaurs and plesiosaurs had been known since the early 1800s but these very large reptiles were terrestrial. Unlike extant reptiles that 2 2 vertebrae attached to their hip, dinosaurs had 5 vertebrae. Another feature that differed from extant reptiles is that their legs were beneath their bodies, whereas the legs of extant reptiles splayed out to the sides of the body. While recognising these anatomical difference, as well as others, such as their shapes, joints and muscle attachments, Owen insisted the dinosaurs were reptiles, therefore their physiology must have been of a reptilian type, i.e., typically “cold blooded”, with a slow metabolism. As a result of this, dinosaurs were considered to be sluggish, lumbering animals until the 1960s, that slowly grew to very large size in a climate that was a sort of hothouse.

In the second half of the 20^th century palaeontologists realised that the growth lines in dinosaur bones, similar to growth rings in trees, that had been known about for many years, could be used, together with other features inside their bones, to determine how dinosaurs actually grew.

These growth lines in dinosaur bones were laid down annually, as are tree rings, though the growth lines are somewhat more difficult to interpret. Unlike tree rings that record the entire growth of the tree often over many years, in the centre of the long bones osteoclasts break down existing bone to hollow out the bone, thereby allowing the constituents to be recycled. The centre of the bone becomes the marrow cavity where red blood cells are produced.

The bone grows constantly throughout life, with new tissue being deposited on the outside of the bone by osteoblasts and growth in the long bones occurs at the ends of the shafts. While the osteoclasts in the marrow cavity are eroding the bone that was deposited earlier in life, osteoblasts are depositing secondary bone tissue around the cavity perimeter or invading the cortex, the outer layer of the remaining bone to remodel it.

During this process in the centre of the bone the record of growth in the youngest stages of life of the individual is eroded, which makes it difficult to find a complete growth record by counting the rings. There are several methods for reconstructing the early history of dinosaur bones, one of which is to fill in the record by using the bones of younger individuals, which contain the tissues that are eroded later in development. The approximate number of years missing from older bones can be estimated by examining these tissues and counting the growth lines. The number of growth lines can be “retrocalculated” when there are no younger individuals available by examining the distances between growth lines that have been preserved.

The authors tried recalculating the bones of T. rex in 2004 by thin sectioning the hind leg bones from the collection at the Museum of the Rockies, Montana State University, which they examined microscopically.

Under the microscope the bones revealed 4-8 growth lines that had been preserved, though near the centre secondary bone tissue growth had obscured others. A more striking finding was that in these dinosaurs 2/3 of the original bone tissue had been eroded away, and in some individuals the space separating the growth lines became very small toward the outermost surface of the bone. They had previously found the same condition in Maiasaura an herbivorous duckbill dinosaur, among other dinosaurs. The narrowed spaces between growth lines indicate the growth phase had been completed at maturity.

As a result of their retrocalculations it was estimated that T. rex took 15-18 years to attain full size, at which time it was 3 m (10 ft) at the hip and was 11 m (34 ft) long, and weighed 5,000-8,000 kg (5-8 tonnes). The estimates arrived at by the authors matched those of Gregory M. Erickson, Florida State University. This seems to be rapid growth for a reptile, indicating that dinosaurs grew much faster than other reptiles, extinct or extant.

An example of an extinct reptile is Deinosuchus, a giant crocodile from the Cretaceous, about 75-80 Ma, that grew to an estimated length of 10-11 m, for which the growth was chartered by Erickson & Christopher A. Brochu. It was found by examining the growth lines that this species took almost 50 years to reach its maximum length. The African elephant has proved to be a closer match, reaching a similar mass of 5,000 to 6,500 kg in 25-35 years, which means T. rex grew even faster than an elephant.

It has been fund by further research that T. rex is not unusual for dinosaurs, though did grow slightly slower for its size than other large dinosaurs. It has been found (Anusuya Chinsamy-Turan) that Massospondylus reached a length of 2-3 m in 15 years. It has been found (Erickson & Tatyana A. Tumanova) that Psittacasaurus, a small ceratopsian, a horned dinosaur, matured at 13-15 years. The authors calculated that Maiasaura took about 7-8 years to reach adult size of 7 m long. It has been found that the giant sauropods grew faster than all other known dinosaurs, it was found by Martin Sander that Janenschia reached maturity in about 11 years, though it continued substantial growth after reaching maturity. It has been determined that Lapparentosaurus reached full size before the age of 20 years (Frédérique Rimblot-Baly). Apatosaurus (Brontosaurus) has been found to have matured at 8-10 years, (Kristina A. Curry Rogers) which means it gained almost 5,500 kg per year.

Dinosaur Bone Growth

In the typical long bone of a dinosaur the tissue is primarily of the fibro-lamellar type, which has a highly fibrous or “woven” texture that forms around a matrix consisting of collagenous fibres that are poorly organised. This type of tissue predominates in the bones of large birds and large mammals that reach full size faster than typical reptiles. The bone of crocodiles is mostly comprised of lamellar-zonal tissue, which is a compact, highly mineralised bone containing fibres that are more regularly organised and much sparser, smaller vascular canals. Also, in crocodile bones the growth lines are more tightly spaced than occurs in the bones of dinosaurs, which is another indication that crocodile bones are slower growing than those of dinosaurs.

In the 1940s it was recognised that the tissue type deposited in a bone at any given place or time during the growth of the bone was a function of the speed of the tissue growth at that point (Rodolfo Amprino). Wherever or whenever fibro-lamellar tissue is deposited reflects local growth that is rapid, while slower growth is indicated by lamellar-zonal tissue, any particular animal depositing either of these tissues at different times as is required by the growth strategy of that animal. The best guide to the growth rate of an animal is the predominant tissue type throughout the animal’s life.

 When comparing dinosaur bone with bone from crocodiles and other reptiles one difference is that in dinosaurs fibro-lamellar tissue is deposited throughout growth to adult size, whereas in other reptiles the switch to lamellar-zonal bone occurs much sooner. Based on this the authors inferred that dinosaurs sustained growth that was more rapid until adult size was reached as there was no other good explanation for the persistence and predominance of fibro-lamellar tissue.

Erickson, Rogers & Scott A. Yerby assessed the growth rate of dinosaurs in a different way, plotting the estimated mass of the animals against time, deriving growth curves for a variety of species then compared the curves with the curves for other groups of vertebrates. The results of this comparison showed that the dinosaurs grew faster than all living reptiles, with many dinosaurs growing at rates comparable to many extant marsupials, while the largest of the dinosaurs grew at rates that were comparable to those birds and large mammals that matured rapidly. These results were confirmed by the authors by their own studies that used length.

According to the authors these findings were unexpected in one sense. Ted J. Chase had shown many years ago the large species within any vertebrate group take a longer time to attain adult size though to reach it they grow much faster. The actual speed of the growth found for dinosaurs was surprising.

The authors plotted the estimated growth rates on a cladogram, a diagram of relationships, that had been built up by including hundreds of characteristics that were independent, from all parts of the skeleton. They then added estimated growth rates of pterosaurs, which are closely related to dinosaurs, which had growth that were similar to those of dinosaurs, as well as those of crocodiles and their extinct relatives, and lizards. The growth rates of birds were put among the dinosaurs as their growth rates were very similar, so they are technically included with them.

As extant birds have the same range of tissues that are expressed in dinosaurs bones, so the birds were included for added help in estimating dinosaur growth rates. Mallard ducks had been injected with a solution that stains the growing bones (Jacques Castanet et al.), using different colours at different times which allowed them to measure weekly rates of growth in the experimental birds. The authors determined that dinosaurs and pterosaurs grew much faster than other reptiles.

A different type of reptile

A great deal has been found about the evolution of some of the major features of dinosaurs by the study of dinosaur bones. The lineage that would produce dinosaurs, pterosaurs and related forms diverged from the lineage leading to crocodiles and their related forms in the Early Triassic, about 230 Ma. The sustained elevated growth rates that made the dinosaur lineage different from other reptiles were acquired soon after this split. The authors suggest this rapid growth rate may have had a role in the success of the dinosaurs and pterosaurs that occurred in the Late Triassic, a time of extinction for many of the species with more typical reptilian bone structure.

A firmer idea on the metabolic features of dinosaurs is gained from their high growth rates.  The speed of tissue growth changes in line with the speed of the metabolism, the faster the basal metabolic rate, the more energy that is devoted to forming and breaking down bone and other tissues, the higher the growth rate of the tissue. The animals in question are indicated to have had a relatively high basal metabolic rate, which is probably more in line with those of birds and mammals than those of extant reptiles, as indicated by the sustained rapid growth seen in dinosaurs into the late juvenile and subadult stages.

The suggestion is that they were much more likely to have been, in a general sense, warm-blooded than cold-blooded, though it remains difficult to know the details, such as the body temperature and the degree to which it varied. The more that is found out about dinosaurs the more unusual they appear to have been, being not quite any animals of the present, and they are known to have not been conventional reptiles. The authors quip that many of these questions will be settled if a 5-tonne bird is ever found.

Sources & Further reading

1. Paul, Gregory S., 2010, The Princeton Field guide to Dinosaurs, Princeton University Press.
2. John R. Horner, Kevin Padian and Armand de Ricqlès, 2014, Dinosaurs, Scientific American special Edition