Dinosaurs

I have been looking through more of my old articles for DinoLore, the website I had before this blog, but which I decided to shut down. Some of them were pretty nice though, and, in times when I don`t have much time to write new `informative` posts, I thought I could revive some of these articles.

This one is about dinosaurs! It is a pretty brief, general overview of their main characteristis. There is of cours much much much more to each and every of these subtopics. If you find any bit particlarily interesting, please tell, and I can write something new and more in detail about that.

(Note: ignore all the bits about other articles and sections they refer to such in the old website, and some do not even exist I had planned to create them, but never got to it.)

Dinosauria
Most of us could easily recognise a dinosaur. But what features characterise all dinosaurs? And what makes them different from all other animals?


Size

The perhaps most apparent characteristics of dinosaurs is that they were big. Although there were a few species weighing around a kilo (e.g. Compsognathus), dinosaurs generally weighed more than a tonne. The largest animals on land the sauropods were dinosaurs, with the world record being held by Amphicoelias (it is only known from a single vertebra, or spine bone, and a femur, or thigh bone, but those were huge!), estimated to have weighed more than 100 tons!
Note, however, that this giant is still dwarfed by the now-living, marine blue whale, weighing close the the double! It is much easier for aquatic animals to grow large, and there is a simple reason why can you think of it? Nevertheless, dinosaurs were not less spectacular for achieving such gargantuan sizes on land!


Limbs

Dinosaurs are characterised more specifically by having an erect stance: their limbs are held beneath their bodies, unlike lizards, crocodiles and amphibians, which have their arms and legs in a sprawling posture. Such a stance is also called a parasagittal limb posture, because the legs lie on the sagittal plane of the body (the line that separates the body into a left and right half).

A parasagittal stance is generally much more efficient, partly because the body puts less weight on the limb muscles (we all know how strenuous it is to do push-ups!). Furthermore, a sprawling stance requires the whole body to move from side to side with each step: the left side of the trunk contracts to move the right forelimb forward, followed by the right side contracting, dragging the whole body forward. This way of moving has a serious disadvantage: when any side of the body contracts, doing so also compresses the lung on that side. Therefore, animals with a sprawling limb posture cannot breathe while running, and therefore tire very quickly. However, in a parasagittal stance, the limbs can move independently of the rest of the body, enabling breathing during exercise.

Dinosaurs are not the only animals with erect limb posture. Today, birds and mammals also have a parasagittal stance. The fact that birds and mammals are the only modern animals with a parasagittal stance, and are the only warm-blooded, or endothermic, animals today, suggests that dinosaurs also may have been warm-blooded. This correlation does have a biological explanation, which is outlined in the article on breathing and erect limb posture in the section Warm-blooded?

The parasagittal limb posture was achieved by making the process (bump) on the head of the femur (thigh bone) that articulates with the hip socket (or acetabulum) protrude at a right angle from the rest of the femur, which as a result will point directly downward.

However, this process is cylindrical, not ball-like as for most vertebrates. This means that the femur cannot roll around along the joint, but is restricted to rotating only. As a result, the leg can only be swung forward and backward, and not to the sides as, for example, we humans can. In addition, the ankle joint of the dinosaurs was relatively simple: in humans, there are seven ankle bones (or tarsals), and they form a synovial joint, a highly flexible joint that allows movement in several planes dinosaurs, however, only have two ankle bones, and the joint (referred to as the mesotarsal joint) forms a linear hinge joint, which only enables movement in a single plane only, in this case back and forward. Consequently, the dinosaur hindlimb had a highly restricted range of movement: it could only move along the sagittal plane the line parallel to the body. Thus, dinosaurs would not have been very agile regarding sidewise movement they were specialised for running forward.



All dinosaurs were digitigrade, which roughly means that they walked on their fingers and toes (jointly referred to as digits). Their wrist and ankle bones (carpals and tarsals) were held off the ground, leaving the finger and toe bones (metacarpals and metatarsals meta meaning after ) to bear all weight. If you try jumping around, you will notice that you are quicker and more agile if you do it on your toes only. The same would apply to dinosaurs (at least those that could run). Many forms that were incapable of running, such as the sauropods, instead evolved a fleshy pad on the back of the ankle bones, creating a soft heel that could help carrying some weight. Some modern, fast-running mammals, such as dogs, have also evolved a digitigrade stance. Other ways of walking on hands and feet include plantigrade walking on the whole foot and unguligrade walking on the outermost finger bones (called unguals).

Another feature common to all dinosaurs is that the forelimb always is shorter and slimmer than the hindlimb (with the exception of Brachiosaurus). This is true not only for the bipedal dinosaurs i.e. those that walked on two legs but also for the quadrupedal ones those walking on all fours. For the bipeds, this gave them balance by making the front part of the body lighter if the arms had been longer, they would fall on their noses! Regarding the quadrupeds, this meant that the horizontal line of the trunk would point slightly downward, placing the neck and head closer to the ground. This may have made grazing on low-growing plants and drinking water easier.

One main conclusion that can be drawn from all this information is that all dinosaurs were strictly terrestrial (ground-living) animals. They were not adapted to living in the water or the air (except for birds).


Teeth

Dinosaurs developed a wide variety of dentitions: from razor-sharp, serrated carnivorous teeth to sophisticated dental batteries designed to efficiently grind tough plant material. However, all dinosaur teeth had one thing in common: they were replaced continuously throughout their entire lives, in the same way as for sharks today. If T. rex took a big bite and lost a few teeth it did not matter they would grow out again, and again and again!


Defining features

Neither of the characteristics described above are truly unique to dinosaurs: they are present in other types of animals as well. Consequently, these features cannot be used to formally classify, or diagnose, an animal as a dinosaur. For this purpose, we must use so-called diagnostic features features that are seen only in one group, and in all members of that group. Some diagnostic characters that unite the Dinosauria include (if you are not familiar with the anatomical terms used here, please refer to the article related to the subject):

• epipophyses (a type of processes) on anterior neck vertebrae, except for the two first (called atlas and axis) probably helped to interlock the vertebrae, reducing flexibility but granting strength and stability
• distinct proximodistally oriented (i.e. in line with the length of the bone) ridge on the posterior surface of the tibia (shin bone) maybe the ridge to which the mesotarsal ankle (above) articulated
• cnemical crest (a crest-like process on the anterior head of the tibia, to which the largest extensor muscle of the thigh is attached) arcing anteriolaterally (forward and outward) sounds like an adaptation for running, perhaps granting more stability to the legs
• supratemporal fossa: a depression in front of the supratemporal fenestra the temporal region is the one behind the eye socket, so the supratemporal fenestra is a hole (fenestra is latin for window ) located above the temporal region, which on dinosaurs means on the top of the skull

The latter feature is a curious diagnostic character since it does not seem to have had any evolutionary significance other than showing that all who possessed it shared a common ancestor.


Physiology

The greatest question about the physiology of dinosaurs is whether they were warm-blooded, or endothermic, or not. One reason to suggest that at least some were endothermic is that the closest relatives to the dinosaurian ancestor, the crocodiles, are cold-blooded, or ectothermic, while the descendants of dinosaurs, the birds, are endothermic. Thus, endothermy ought to have appeared somewhere in between.

Furthermore, dinosaurs posses several features that are present only in the endothermic animals of today birds and mammals. These include a parasagittal limb posture, as described above. Dinosaurs also had a four-chambered heart, something only endothermic animals have today. Some dinosaurs intelligence was comparable to that of modern endotherms there is a clear distinction between endo- and ectotherms regarding intellect: endotherms are consistently around ten times more intelligent than other animals (based on their encephalisation quotient, or EQ, which measures brain size relative to body size see the article on dinosaur intelligence for more details), and some dinosaurs generally smaller forms could match the intellect of birds. The high metabolism of endotherms enables growth rates much faster than that of any ectotherm, and the sheer size of dinosaurs suggests that they ought to have had high growth rates. More formal studies on dinosaur ontogeny (the study of an organisms development from a fertilised egg to an adult) have shown different results, but the general consensus, or agreement, is that dinosaurs had growth rates exceeding those of modern ectotherms, but perhaps not matching those of living endotherms. Indeed, it has been suggested, for additional reasons, that dinosaurs were intermediate in between cold- and warm-blooded. For more details on this intriguing mystery, please go to the section titled Warm-blodded?

All dinosaurs, being reptiles, laid eggs. Several nests, eggs and even embryos have been uncovered. Nesting grounds have been discovered for many herd-living (or gregarious) dinosaurs, suggesting that they were social animals.

Other aspects of dinosaurian physiology are mostly left for mere speculation, since we have little clues about their soft tissues, which is rarely fossilised. We can be quite sure that they did have heart, lungs, intestines, kidneys, and so on, but their exact design and efficiency is hard to study. Still, it is fun to try! For instance, desert-living dinosaurs ought to have some adaptations to conserving water, such as efficient reabsorption of water in the kidneys, possibly achieved by consuming large quantities of salt. If you do not feel at home with animal physiology, please see its subsection under Biology.

Behaviour

Many types of dinosaurs lived in social groups. Evidence for this includes preserved trackways (foot prints) of several individuals marching together in the same direction, nesting grounds, and monotypic bonebeds (fossil sites with the vast majority of animals belonging to the same species). Walking together, breeding together, dying together. Living together, for short.

Dinosaurs known to have been gregarious include sauropods, ceratopsians, ornithopods, and smaller theropods. For the herbivores, living in a herd would have provided mutual protection to all its members, and the safety in numbers for every individual (when in a group, it is less likely that any predator will attack you). Regarding the carnivorous dinosaurs, hunting in a pack would allow them to subdue larger prey, as well as enhancing the probability of a successful kill.

Individuals in a group must, however, be able to communicate with the other members. Most social dinosaurs seem to have employed visual and/or sound display. Most ceratopsians and hadrosaurs had magnificently shaped (and probably coloured) head crests, which could have signalled an individual s state of health, or perhaps even its mood, if it could change colour, as in chameleons today. It has also been suggested that the sauropods could have waved their long tails in different patterns to send messages to the herd. It is also probable that many used sounds to communicate: certainly, any dinosaur could roar to show anger. Sound is superior as warning signals, since it is easier to stay alert for sudden noise while focusing the eyes on other things, such as finding food, and sound waves travel faster than odour. The lambeosaurines seem to have taken sound communication more seriously: they evolved hollow head crests, with complex tubes connected to the nose and throat, through which they could produce highly sophisticated sounds. Many modern animals also use scents to communicate. It is difficult to prove but not foolish to assume that dinosaurs also employed different scent signals.
Communication is always twofold: sending signals, and perceiving and processing signals. The latter requires fairly high intelligence. The sharpest brains among the dinosaurs are found within the smaller theropods, such as the raptors , commonly depicted as ingenious killing machines. Such an intellect would have enabled them to interact and hunt in sophisticated patterns.


Ecology

The tremendous size of most dinosaurs has one important ecological implication. A habitat can only provide a specific amount of nutrients. Therefore, the greater the energy requirements are for each individual of a population, the smaller the number of individuals that the habitat can sustain will be. Naturally, the larger an animal is, the more energy it consumes. Thus, the large dinosaurs would have had rather small populations: big but few. Unfortunately, a small population is very vulnerable to an unexpected catastrophe (anything from climate change to an infectious disease): it is more likely that the disaster will annihilate the entire population, and eventually cause the species to become extinct.

However, the time when the dinosaurs lived, the Mesozoic era, was a relatively stable period in Earth s history, with consistently warm and humid climate, something that presumably favoured plant growth, providing more food for the dinosaurs. They were lucky.

The dinosaurs also had an impact on the ecology of all other organisms. Being the dominant land animals, occupying all medium-sized to large niches, both herbivorous and carnivorous, they hindered the evolution of other creatures, such as the mammals, which were confined to tiny, insect-eating rodents. If they attempted to grow larger, they would become dinosaur prey before they had a fair start. They did better being small and easy to hide. For now


Evolution

To describe dinosaur evolution in short, simple terms: they are amniotes (lay eggs with waterproof shell and nutrient-rich amniotic fluid inside), diapsids (have two fenestrae, or windows , in their skulls, to which some jaw muscles are attached), archosaurs (have an additional fenestra in front of the eye, called the antorbital fenestra), and ornithodirans (more closely related to birds than to crocodiles). For further information regarding the biology and history of these groups, please visit the section about other organisms.

Dinosaurs probably evolved around 228 million years ago from a small, semi-bipedal ornithodiran resembling Lagosuchus. The very first dinosaurs would thus have been relatively small, slender, agile insectivores. The ancestral dinosaur has not been, and probably never will be found. The earliest fairly well-known dinosaurs include Eoraptor and Herrerasaurus, which were both tiny, dextrous, bipedal and digitigrade. They also had five digits on both their hands and feet, but, in Herrerasaurus, the two outermost fingers are short and clawless (and were eventually lost in subsequent theropods) its innermost and outermost toes were also reduced, so that only the three middle toes bore weight, as in all later theropods.

One intriguing conclusion that can be drawn from the earliest dinosaurs being bipedal and losing several digits on their limbs is that the later dinosaurs that were quadrupedal, and those that had more than three fingers and toes, would have evolved these features secondarily, or re-evolved them. This scenario also explains why the quadrupeds had shorter forelimbs than hindlimbs: it is a residual from their bipedal precursor.

Throughout their long history, dinosaurs diversified into a myriad of different forms, adapting to various different environments and niches. They gave rise to the largest land animals that have ever lived. They inhabited every continent and established in any habitat, from arid deserts to the frozen lands of Antarctica! The only power capable of subduing them was nature itself, which destroyed their rule 65 million years ago, leaving the birds (which are dinosaurs) as their legacy.