Bringing Dinosaurs Back To Life

I thought of apologising for my recent absence from the blogosphere – then I realised, I'm not certain anybody actually reads any posts that are not about camera traps or that do not display pretty pictures of birdies. Hopefully somebody does; if not, it is still writing practice for if I ever do produce something worth reading!

Bringing an extinct animal back to life is never a simple task. Of course without implying it in the literal sense of "The Lost World", the biology of extinct animals can be reconstructed on hypotheses and theories – even if often one tries to exclude another or they change over time as more evidence comes to light. How extinct animals lived inspires the same fascination as how living species do, with the added charm of mystery and plenty of space for imagination. Dinosaurs are a clear and classic manifestation of the desire to investigate the biology of extinct animals. Here I want to give a (relatively?) brief overview of some of the main points of interest for zoologists and palaeontologists when trying to bring the biology of extinct reptiles back to life, and some of the ways in which they do, all from little more than fossils and an understanding of today's natural world.

Size matters

The size of dinosaurs has been the focus of countless studies. This is unsurprising given the massive fossils which have been unearthed, but it is important to study the biology of any animal, extant or extinct: it can tell us about the way it moved, what kind of lifestyle it led, how it could or could not reproduce, etc.

However, calculating the mass of living giants is more often then not very difficult, and sometimes involves chopping up their carcasses (and losing fluids – and a great amount of weight – in the process) and weighing those. This is obviously not an option with dinosaurs. It is not as straight forward as simply inferring a dinosaur's mass from measuring bones. Aside from the various modifications bones will have undergone during the process of fossilisation, organs, tissues, and fluids all contribute significantly to an animal's weight. So, all these components must be considered, and volume and density have to be taken into account.

However, there are some seemingly plausible ways to infer weight from skeletons. One method involves looking for a skeletal measurement that correlates with body size in other animals. For example, Anderson et al. (1985) obtained measurements of the thigh bone and upper arm bone circumference from 33 species of quadrupedal mammals. The circumferences of the long bones combined correlate quite well with the size of the corresponding animals, and from this the authors derive equations to calculate the weight of quadrupedal and bipedal dinosaurs (Anderson et al., 1985). Figure 1 shows this logarithmic correlation with added points, including one for Brachiosaurus (Alexander, 1985).

Some authors have dealt with the problem of volume by creating downsized models based on more or less complete skeletons, and placing them in a container full of a known quantity of sand, then removing the model and refilling the space with sand until packed (Colbert, 1962). The amount of sand necessary to fill the space is then scaled up to obtain the volume of the dinosaur. This seems a reasonable approach were it no for the fact that the proportions of models vary very much from author to author (Alexander, 1989).

What's more, the body mass of an animal depends on its animal. Some authors have used crocodiles as a comparison when estimating dinosaur densities (Cott, 1961; Colbert, 1962). Crocodilians are one of two groups of living archosaurs, the reptilian branch dinosaurs belonged to. Birds are actually more closely related to dinosaurs than are crocodiles – they technically are dinosaurs, in fact –, but the pneumaticity and flight adaptations of birds hardly render them a fair comparison for the purpose of calculating mass. According to Cott's (1961) measurements, crocodiles are slightly denser than water; according to Colbert's (1962), crocodiles are less dense than water. Given Crocodiles can rest just beneath the surface of the water with nothing but their nostrils and/or eyes above it, Alexander (1985) assumed the density of crocodiles and dinosaurs was be that of water, 1000g/L. However, when scaled to the sizes dinosaurs would have attained, even when considering the most conservative estimates, results in impossibly high masses. Estimations like these have in the past led to suggestions that perhaps such massive creatures as sauropods would have waded in lakes, with the additional support of water to enable them to move. Such ideas can be discredited on the basis of strata composition, not to mention the enormous pressure differences which would have to be created to allow the animals to breath. The heart would need to generate equally improbable blood pressures in order to distribute blood to the brain (as small as this organ may have been) of such a massive creature.

Tracking dinosaurs

All in all, the above are some of the variables which account for the inconsistency among dinosaur size estimations between different authors. Size is not the only characteristic that can be predicted (albeit an unsettled matter) about dinosaurs. Nor are bones the only place to look for clues.

Numerous sites of dinosaur tracks have been discovered, and there are possibly more types of tracks than actual fossilised feet to fit them. That said, it is possible to divide them into related groups - for example sauropods, theropods, stegosaurs, etc. Which can potentially reveal some clues to as whether they lived in groups or solitarily, or whether species travelled or shared spaces with other types of dinosaur. This of course is somewhat scuffled by the fact that tracks would not have been fossilised instantly and thus all the tracks of one site may not all pertain to one set of animals at the same time.

Nonetheless, tracks are useful for other predictions as well: they can contribute to revealing an animal's posture which, in combination with the shape of leg bones, is now well known to resemble the erect gait of mammals and birds rather than the sprawling gait of other reptiles. The speed of dinosaurs can also be estimated from tracks, and in order to compare the travelling speed of different dinosaurs to one another's or to that of extant animals, the dimensionless speed can be calculated by divided the estimated speed between the square root of the product of leg length and gravitational acceleration, thus accounting for the differences in size between species (Alexander, 1976, 1989).

Top speed of dinosaurs can now be estimated using robotic computer simulations which – despite claims of skeptics who regard computer simulations as free to produce what they will and not what is real – take into account issues of weight, shape, physical forces, etc. to produce reasonable estimations of the maximum speed these animals would have been capable of (Sellers and Manning, 2007).

The size and shapes of tracks enable us to dismiss claims that locomotion on land would have been impossible for such giants: the amount of estimated pressure each extremity would have had to sustain can be compared to that which the limbs of living species sustain – again, a comparison with mammals or with birds for bipedal dinosaurs is more appropriate than a comparison to sprawling animals as today's reptiles; indeed limbs with a sprawling gait could hardly evolve to withstand the immense pressure dinosaur limbs would have had to. Comparisons made by Alexander (1989) between the estimated pressures per surface area (so the base of the foot) that dinosaurs and extant species such as cattle, would exert on mud and on sand ("normal" or average ground considered as in between the two) have shown that the forces exerted on dinosaur feet fall within the ranges of those of extant species, and so locomotion would not have been as improbable as some have suggested.

Lungs of a bird

A topic which has received more attention in recent years in how dinosaurs breathed. Birds are directly descended from dromaeosaurs and so are the closest living relatives of dinosaurs (essentially because this means they are dinosaurs). They have a unique and highly efficient breathing system, composed by a pair of fixed lungs filled with hundreds to thousands of parallel tubes called parabronchi, and numerous air sacs which act as bellows during ventilation. It would be, of course, hasty to assume on the basis of blood that dinosaurs possessed a similar system, but crocodiles – who, as aforementioned, share an archosaurian ancestor with dinosaurs – also possess a complex and efficient, unidirectional flow breathing system similar to that of birds, but without the air sacs. It therefore seems reasonable to recognise the likelihood of this breathing system arising before the last common ancestor of crocodilians and dinosaurs (including birds), an idea made even more plausible when considering the relatively hypoxic atmosphere of the Mesozoic, when more efficient breathing systems would have been strongly selected for (Ward, 2006). 

Furthermore the presence of air-sacs may have been a pre-requisite for the evolution of avian flight, which would necessitate at least their dinosaurian ancestors to have modelled the system. What's more, fossae that have been described as "air pockets" for the insertion of air sacs have been found in the vertebrae of the dinosaur Aerosteon (Sereno et al., 2008). As far as breathing accessories go, uncinate processes are present in at least maniropteran dinosaurs (Codd et al, 2008); these extensions of the vertebrae serve as a mechanical brace for the appendicocostalis and external oblique muscles in birds, musculature involved in assisting aspiration. As well as that, dinosaurs possess what are known as gastralia, which in the case of theropods were highly modified and may have also been breathing accessories to enable greater tidal volumes (Claessens, 2004).

"Cold-blooded monsters or warm-hearted giants?" *

The question of whether dinosaurs were "warm-blooded" or "cold-blooded" seems to still be highly controversial. The stem of the argument comes from the indications that dinosaurs were highly active and thus needed to sustain a high metabolic rate. What's more, dinosaur nests have been found where the brooding parent has been preserved with the eggs, lending more support to the idea they were "warm-blooded".

Before I go into these, a note on some thermobiological terms. Homeothermic animals are those which maintain a constant body temperature, often higher than that of the environment. Endothermic animals produce their own body heat, thanks to a high metabolic rate. Most endotherms are homeothermic, but not all homeotherms are endothermic. The term "warm-blooded" refers to animals who maintain a warm body temperature; therefore, it can be a confusing term as it can refer to homeothermic or endothermic animals, without having to be both. It is the same for the term "cold-blooded": ectothermic animals need to soak up heat from their environment because they cannot generate it themselves. They are usually poikilotherms, meaning their body temperature fluctuates a lot. However, there are endotherms which at times you could call cold-blooded, because their body temperature drops bellow that of the environment – some hibernating mammals for example.

So, if a homeothermic animal can be "warm-blooded" without being endothermic, how does a warm-blooded homeothermic animal maintain a constant body temperature above that of the environment?

Back to our beloved dinosaurs, given the enormous size of what seem to be the majority of dinosaurs (although perhaps it's that big ones may be preserved better) and the relatively warmer conditions of the Mesozoic era throughout which they reigned, it's easy to imagine how many giants could have faired perfectly well by gigantothermy. Gigantothermy works because by the time the temperatures are low enough to start cooling the entire mass of the giant down, the sun is rising and the environment is warming back up again.

Indeed, the largest land animal alive today – the African elephant, which doesn't approach the dimensions of many dinosaurs and lives in a hot environment – faces the risk of overheating and has adaptations to aid cooling: for example, they're large ears expand the surface area through which they can loose heat. This implies that, were it not for cooling mechanisms like this, they would conserve far too much heat (especially since, as mammals, they are endotherms). So, dinosaurs could have been ectothermic like today's living reptiles and maintained a high body temperature by gigantothermy, at least the big ones.

On the other hand, birds are, as we've discussed, essentially just a type of dinosaur and they are endothermic. This implies dinosaurs will have had to have been endothermic at some point, especially given the high energy demands of flight that may make endothermy a pre-requisite. What's more, the fact that many theropod dinosaurs had feathers, long before flight evolved in the lineage, shows that feathers were a pre-adaptation to flight: they may evolved as a means of conserving the body heat produced, further supporting endothermy.

Thus, the real question is at what point dinosaurs became endothermic or whether their ancestral archosaurs were endotherms, which has been suggested based on the "over-engineered" respiratory and cardiac systems of crocodilians (Seymour et al., 2008). Histologic analysis of bones have revealed the growth rates of dinosaurs were superior to those of reptiles, but didn't show the same pattern of variation when body mass is plotted against growth rate as mammals or reptiles do, but had a unique pattern of its own (Erickson et al, 2001), suggesting that perhaps some dinosaurs lineages were endothermic while others were ectothermic (Seebacher, 2003).

Multicoloured dinosaurs and other weirdoes

The colour of dinosaurs is something which was long down to the imagination of researchers and palaeoartists. Now, the colour of some dinosaurs has been determined by examining the melanosomes of feathers (for example, the ginger Sinosauropteryx) or determining their pigmentation from their relationship with trace metals such as copper, when even melanosomes have been degraded to an uninterpretable extent (Wogelius and Manning, 2011); the latter approach is taken at the synchrotron in the USA.

Other morphological features of dinosaurs, directly preserved in the fossil record, also give us clues about their lifestyles. Rarely do they receive unanimously accepted interpretations, but a comparison between their features with those of modern animals can reveal much. For instance, it has been suggested the horns of Triceratopsids were weapons for male-male competition; however, it has been calculated they would have been significantly smaller and and weaker relative to body mass than the features of most living animals used in direct male-male competition. Pachycephalosaurus had an incredibly thickened skull, suggesting males may have rammed each other. Ankylosaurs had modified vertebrae and protective "armour", though whether this evolved primarily as a defence mechanism against predators or during male-male combat (or both) is unclear.

Conclusion

Various structures and features of dinosaurs can tell us more about how these extinct animals may have lived, and the same rules apply to other extinct groups, certainly vertebrates. However, without an understanding of extant animals as a point of reference and comparison, the attempt would seem futile. Then again, it is not to be expected that everything can ever be known about extinct animals, as the natural world provides us with an abundance of evidence that evolution often takes strange and seemingly bizarre twists. One cannot, after all, know what mutations would have arisen to allow selection, or all the variables and selection pressures in their palaeoenvironments.

* The subtitle "Cold-blooded monsters or warm-hearted giants" is the title of a previous blogpost and essay on thermoregulation on dinosaurs. If you do look at it, bear in mind that the only knowledge I possessed on thermobiology was from my own reading, and so there are additional points I would have covered if I were to re-write it now, especially in regards to cardiac and respiratory physiology.

Cited literature:

Alexander, R. M. (1976). Estimates of speeds of dinosaurs. Nature, 261: 129–130.

—— (1985). Mechanics of posture and gait of some large dinosaurs. Zoological Journal of the Linnean Society, 83 (1). doi: 10.1111/j.1096-3642.1985.tb00871.x

—— (1989). Dynamics of dinosaurs and other extinct giants. Cambridge: Cambridge University Press.

Claessens, L. P. A. M. (2004). Dinosaur gastralia: origin, morphology, and function. Journal of Vertebrate Paleontology, 24: 89–106.

Codd, J.R., Manning, P.L., Norell, M.A., Perry, S.F. (2008). Avian-like breathing mechanics in maniraptoran dinosaurs. Proceedings of the Royal Society of London B, 275: 157–161.

Colbert, E. H. (1962). The weights of dinosaurs. American Museum Novitates, 2076: 1–16.

Cott, H. B. (1961). Scientific results of an inquiry into the ecology and economic status of the Nile crocodile (Crocodilus nitoticus) in Uganda and Northern Rhodesia. Transactions of the Zoological Society of London, 29: 211–356.

Erickson, G. M., K.C. Rogers, and Yerby, S. A. (2001). Dinosaurian growth patterns and rapid avian growth rates. Nature, 412: 429–432.

Seebacher, F. (2003). Dinosaur body temperatures: the occurrence of endothermy and ectothermy. Paleobiology, 29: 105–122.

Sellers, W. I. & Manning, P. L. (2007). Estimating dinosaur maximum running speeds using evolutionary robotics. Proceedings of the Royal Society of London B, 274: 2711–2716.

Sereno, P.C., Martinez, R.N., Wilson, J.A., Varricchio, D.J., Alcober, O.A., Larsson, H.C.E. (2008). Evidence for avian intrathoracic air sacs in a new predatory dinosaur from Argentina. PLoS ONE 3, e3303, doi:10.1371/journal.pone.0003303.

Seymour RS, Bennett-Stamper CL, Johnston SD, Carrier DR, Grigg GC. (2004). Evidence for endothermic ancestors of crocodiles at the stem of archosaur evolution. Physiol. Biochem. Zool., 77 (6): 1051–67.

Ward, P. (2006). Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere. National Academies Press, Sep 26, 2006.

Wogelius R. A., Manning P. L., Barden H. E., Edwards N. P., Webb S. M., Sellers W. I., Taylor K. G., Larson P. L., Dodson P., You H., Da-qing L., and Bergmann U (2011). Trace metals as biomarkers for eumelanin pigment in the fossil record. Science, 333 (6049): 1622–1626.