"Behold the mighty Dinosaur,
Famous in prehistoric lore,
Not only for his weight and strength
But for his intellectual length."
-- from Bert Leston Taylor's 1903 poem "The Dinosaur"
BIG QUESTION: How did giant size and large clutch size affect dinosaur ecosystems?
So How Did (Some) Dinosaurs Reach Tremendous Size?
It isn't just that some dinosaurs were large. It is that giant (>1 t) size is fairly common in dinosaur groups, and that >5 t size occurs in both carnivorous and ornithischian dinosaurs (with many independent cases), and that >10 t is actually a STANDARD body size for the sauropod dinosaurs! Indeed, the very largest dinosaurs reached masses of 80-100 t (or, in other words, the mass of a herds of elephants!). In contrast, where there have been a few 10 t land mammals, most mammals were much smaller. (Of course, even dinosaurs were dwarfed by the modern blue whale). So how did dinosaurs get so big?
Why Evolve Gigantism?
We have asked about some issues of how they might have grown so big, but not why.
The second aspect is probably easier to deal with. There are several possible advantages:
As with models of mass extinction, there are lots of unsupported, less-than-reasonable, pseudoscientific explanations:
So, let's take a look at other possibilities.
Was It Limb Posture (i.e., the Parasaggital Stance)? It is true that all the supergiant land animals (dinosaurs, elephants, deinotheres, indricothere rhinos, etc.) all have an upright stance. While there are giant 10-15 t crocodilians contemporaneous with and after the Age of Dinosaurs, these semi-upright animals were primarily aquatic and in a sense don't count.
But other groups with parasaggital stance--such as most pseudosuchians, marsupial mammals, modern birds, etc.--never reach these sizes. So it might be that a parasaggital stance is necessary but not sufficient for the >10 t size.
Was It From a REEAALLLLYYYYY LOOOOONNNNNGGGGG Period of Growth? Giant tortoises and big modern crocodilians live a very long time (centuries in the case of tortoises), growing throughout their life. Perhaps dinosaurs just grew for a tremendously long time? Let's take a look.
Dinosaurs went through a tremendous size increase over their life. No Mesozoic dinosaur egg has been found greater than 4 liters volume. Thus, even dinosaurs as massive as a herd of elephants arise from babies that fit inside a soccer ball.
How long did a dinosaur take to grow from tiny baby to giant adult? In the 20th Century, some researchers suggested that they grew rapidly, like mammals and birds. However, others thought that they grew more slowly like crocodilians, lizards and snakes, and turtles. (In fact, some tortoises have lifespans of centuries and it takes them many, many decades to reach sexual maturity.) Since the late 1990s a series of methods were developed to help resolve this.
Like many animals, dinosaurs deposited Lines of Arrested Growth (LAGs for short). These are basically growth rings like tree trunks, one per year. (Okay, technically speaking the broad term is cyclical growth marks (or CGMs). This includes rings (which show temporary cessation of growth) as well as more diffuse annulus marks (which simply show slow downs.) In practice, we typically just use the acronym "LAG". Various paleontologists use that information to "age" individual dinosaur fossils. (Note that since bone is a growing tissue, LAGs can get obscured by later bone growth. So for any individual multiple bones should be sampled to find the maximum number of LAGs, which would be a minimum age for the individual.) By comparing the age of the dinosaur to its size, and plotting all individuals of the species on a graph, you can figure out the growth pattern and growth rate of that dinosaur species: that is, we can develop a skeletochronology for some species.
Actual rate was VERY HIGH compared to typical reptiles, particular in big dinosaurs. Estimates based on bone "growth rings" (and other features) indicate small dinosaurs were full grown at only a couple of years, hadrosaurids (e.g. Maiasaura) at only 7 years for, and only 15-20 years for big sauropods (e.g., Apatosaurus) and theropods (e.g., Tyrannosaurus) to reach adult size.
Could this rapid growth have been the product of the warmer conditions of the Mesozoic? A test is comparing dinosaur growth rates to those of other animals in the same environment. Giant crown-group crocodilians from the Late Cretaceous DO have elevated growth rate compared to Cenozoic crocodilians, but still grow at much slower rates than dinosaurs in the same time and place.
Dinosaurs--like many vertebrates--have an external fundamental system (EFS) upon reaching the full adult size. The EFS is produced by LAGs stacked one upon another, showing that the rapid growth of adolescence has ended. Animals with EFS still add new LAGs until death. This gives us the ability to determine how old even fully-grown adults were at time of death, and thus an estimate for the longevity of dinosaur species. It has been found that many smaller dinosaurs died in their first or second decade; giant theropods like Tyrannosaurus only to about age 28-30; and even the largest sauropods only to 60 or so. This is in contrast to BOTH large bodied modern non-avian reptiles (such as crocodiles and tortoises) and large bodied mammals (rhinos, elephants, whales): these typically live to age 80, or a century, or more.
Unlike typical non-avian reptiles, dinosaurs seem to have determinate growth: reaching a fully adult size, then stopping growth (or at least slowing it WAY down: all the LAGs bunch up together).
So we can REJECT the idea that a long growth phase led to giant dinosaur size!
Very Efficient Dinosaur Hearts?:
But again, while probably a necessity for gigantism, it isn't sufficient.
Very Efficient Dinosaur Lungs?:
Was it Exceedingly Short Food Chains? When we watch nature specials on TV that focus on large charismatic modern land mammals, we get the impression that the simple food chain of plant → large herbivore → large carnivore (e.g., grass → zebra → lion) is typical. In fact, while it does apply to the world of Cenozoic hoofed mammals and their predators, it actually is not the case for most terrestrial life. As in the marine realm, most terrestrial food chains have many steps from primary productivity to apex carnivore, often through insects and other herbivorous invertebrates, to carnivorous arthropods, to small-bodied insectivorous animals, to predators of those, and so on. (For example, plants → aphids → ants → spiders or mantids → small insectivorous birds → snakes → hawks). And of course there is loss of energy every step up the food pyramid.
Ecosystems of protomammal synapsids of the Carboniferous, Permian, and Triassic, and perhaps even protocroc pseudosuchians of the Triassic, seem to have had relatively few large bodied herbivores, and may have had these longer, energy-leaking food chains. And thus, there is less energy available towards the tops. In contrast, dinosaurs produced a broad variety of herbivores able to capture the energy at the base of the pyramid, with a set of carnivores feeding on these. These short food chains may have allowed much more of the solar energy to make its way into meat.
Of course, there is still the observation that even mega-mammals with similarly short food chains fail to achieve dinosaurian size, so this cannot be the entire story. However, it is worth noting that said mega-mammals are the ONLY other case of terrestrial vertebrates which get anywhere near dinosaurian size.
Was it Changing Atmospheres? Even as dinosaurs were evolving, the atmosphere they were breathing was evolving, too. Geochemists have seen that the ratios of various gases, including oxygen, have varied over geologic time. At least some models suggest that the Middle Jurassic though the end of the Cretaceous had oxygen levels exceeding the present 20%. This would mean that every breath a dinosaur took would have more oxygen, making it easier to power a high metabolism.
Furthermore, experiments of growing plants of Mesozoic varieties under Mesozoic-style atmospheres suggests that their productivity (essentially, the amount of nutrients they produce per area per unit time) could go up 2 to 3 times present day conditions. If so, then there would have been more food available per unit area for the herbivores (and from this up the energy pyramid), again making it easier to be an endotherm in these conditions.
Environmental changes in Mesozoic discussed above may have supported gigantism:
However, we may be asking the wrong question. The problem might not be "how did dinosaurs get so big?"; it might be "why DON'T placental mammals get so big?" This may in part be due to the lower oxygen level and productivity during the Cenozoic. However, it may have to do with mammalian vs. dinosaurian reproduction:
In contrast, even giant dinosaurs could could lay clutches of a couple dozen eggs a year or more. Thus, they could survive environmental change much more easily than big mammals. So mammals may not be able to reach giant size because they become extremely vulnerable to extinction.
Additionally, the thicker cartilage of dinosaurs was present at all body sizes, whereas the size of the articular cartilage in mammals decreases with increasing body size. As a result, larger mammals have less cushioned joints, which may have been a hinderance to activity. So it may simply be that larger land mammals could not actually move effectively, but that dinosaurs continued to be mobile.
Of course, not all dinosaurs were giants, as we will see next lecture.
Bringing Up Baby: Dinosaur Babies and Growth
Fossil dinosaur eggs and nests found in the rock record, as have embryos of most major dinosaur clades.
ALL non-avian dinosaur eggs are basketball-sized or smaller: NO dinosaur hatched from eggs the size of people!!
Different types of organisms have different life history strategies: different ways of dealing with how many young are born at a time; how many survive to adulthood; how long they live; etc. There are two general types of strategies that we see commonly in nature:
Non-avian dinosaurs are definitely more r-selected than typical larger placental mammals, but more K-selected than sea turtles.
Even the largest dinosaurs came from eggs of only a few liters at most. So all dinosaurs started as small animals. As a consequence of this, any large or giant dinosaur species had to go through major ontogenetic niche shifts: that is, they played different roles in their ecosystems at different growth stages.
We do not see these much in modern mammals or birds. But this has to do with their particular forms of reproduction. Most crown-birds typically reach full body size and capabilities within the first year (or really just a season or two!) of growth; almost all live with and are provisioned by their parents until they are able to occupy their adult niche.
While big mammals do not necessarily achieve full body size this quickly, they normally do stay with their mother (or a herd/pack) all the way to essentially-full body size. So they, too, are provisioned while they are learning how to hunt or hide or otherwise live on their own.
In contrast, most non-avian reptiles have no parental care, and even crocodilians (which do) only have it for several months. So baby non-avian reptiles are on their own from a very small body size in life styles radically different from the adult. For instance, baby Komodo dragons (Varanus komodoensis) are tiny tree-dwelling hunters of insects, and only move back onto the ground when they are bigger; after that, they have relatively small hunting ranges and go after small prey until reach a threshhold when they are big enough to patrol huge territories and chase after large prey. Similarly, alligators (Alligator mississippiensis) have very different prey and different habitat ranges as they get older.
Were dinosaurs more like modern birds and mammals or more like non-avian reptiles? There is substantial evidence that like living dinosaurs (birds) and their crocodilian cousins, Mesozoic dinosaurs were watched over by their parents for several months at least. In a few groups (the duckbilled hadrosaurids and horned ceratopsids, as well as a few others), dinosaurs seem to have lived in large assemblages (herds or packs or flocks), and might have been provisioned to some degree. But there are also many cases where baby dinosaurs are found to have left the nest and travelled together, with no sign of parents. Packs/herds of young dinosaurs are known; in at least the case of the ceratopsian Psittacosaurus and the ornithomimosaur Sinornithomimus these herds can include individuals of different ages, including adults. (The parents may have been with the other baby herds, but were too big to be buried. So it isn't entirely certain that the "young only" herds are due to that being the actual social structure: it might be the byproduct of taphonomy.)
Niche Assimilation: Did the Combination of Giant Body Size, Small Hatchling Size, and Rapid Growth Allow Dinosaurs to Crowd Out the Competition?
Many dinosaurs show different adaptations at different growth stages (to the point that some youngsters were commonly considered different species than the adults!) For instance, tooth shape and snout shape in some herbivores show that baby dinosaurs fed on different plants and in a different style than the adults.
Paleontologist Michael Brett-Surman describes this as niche assimilation: one species occupies many different roles, and thus effectively crowds out what could have been multiple species in a different version of the ecosystem. So an duckbill hadrosaurid like Edmontosaurus was a 5 kg baby, a 500 kg animal later on, and an 8 ton adult: in the Serengeti today, that covers the entire range of hoofed mammals from the little dik dik through all the antelopes and zebras past rhinos up to a full grown African elephant.
As a consequence, dinosaur ecosystems could potentially have fewer species overall than a Cenozoic mammalian one, but still have as complex an ecosystem.
Dinosaurs large body size might have been extremely useful when times are fine. And their high rate of reproduction made them capable of surviving many types of environmental change to a greater degree than large placental mammals. But what about when times REALLY get bad?
It has been proposed that the large degree of changes in body size from hatching to adult in most dinosaurs actually made them more vulnerable to the Chicxulub impact effects than other groups were. After all, most other groups of animals (including birds) have basically two body sizes: baby and adult. Non-avian dinosaurs, in contrast, go through MANY different sizes, each essentially its own ecological niche. So a mammal or bird will go extinct if either the baby or the adult niche is disrupted in the new environmental condition, but otherwise will survive. In non-avian dinosaurs, however, there are many potential "points of failure": if any one of those niches becomes nonnviable during the events, the whole species will crash into extinction.
As a final aspect, here is a parade of animated dinosaurs of different sizes, to give you a sense of scale:
And one incorporating non-dinosaurs for comparison:
A video which addresses some of these issues:
And another:
