Wednesday, 25 May 2011

The Great Unknowns

When walking through the great dinosaur halls of the big museums throughout the world, we are often treated to the sight of huge animals that are represented by complete skeletons. And we marvel – not only because of their size but also because we try to imagine these creatures as living, breathing animals.

There are often models, diagrams and paintings showing what these animals were like and information describing what they ate, how they may have reproduced, how they fought, how they moved and even how they may have migrated. To reach these conclusions has been the results of decades of field work, preparation and research and it is fair to say that today we have a pretty sound basis in our understanding of these long vanished animals with a high degree of confidence.

From the early days of vertebrate palaeontological research right through to the present day, the basis for most research has been to compare the fossilised remains of extinct animals with those of extant animals and perform comparative analyses supported by direct behavioural observation of the modern animal. This is known as comparative functional morphology.

When faced with reconstructing an extinct animal, possibly with no extant analogue, it’s normal to start with the basics and try to figure out its habits. Did it live on the land, in the air or in the sea? If a land animal, we should be able to determine whether it was quadrapedal or bipedal, whether it was fleet footed or slow and whether the limbs possessed any other adaptations that may be indicators to its lifestyle.

An aquatic animal may be a permanent inhabitant of the water or may show adaptations that indicate it came onto land on occasion. Wings, of course, are indicative of an inhabitant of the sky but was it a glider or could it sustain a powerful flapping flight?

What did the animal eat? Teeth are the clearest indicator of dietary preference and we can normally specify right away whether the animal is herbivorous or carnivorous. Further study determines what the diet may consist of such as meat, fish, coarse or soft vegetation. Teeth can reveal a lot, even specialist diets can be surmised such as insectivory or omnivory. And, of course, there are the toothless species to take into account.

These are fairly basic assumptions that we can make just by looking at the bones. But pretty soon we start approaching the great unknowns. How would you know if animals lived in herds or were whether they were loners? Did they lay eggs or give live birth? If eggs were laid, were they abandoned or brooded? Were they endothermic or ectothermic?

Fortunately, over the years, there has been more and more evidence to help clear up these matters although there is still much controversy regarding some and, despite the physical evidence, the arguments are not always persuasive. We have many bone beds of several taxa now that are indicative of herding. There are literally thousands of fossilised eggs, some in huge breeding sites that suggest social interaction, brooding and parental care. Significant research into bone histology reveals that dinosaurs were obviously more than ectothermic but whether they were full endotherms is still under the most intense of scrutiny.

Flesh and muscle can be put onto our fossil bones and, fortunately, the scars, lumps and depressions that show where the muscle attachments were fossilise well. Then we come to another great unknown. What was the texture of the skin or hide like? Was it furry, reptilian or were there feathers? Yet again we have been fortunate in some cases. Mummified skin and scale impressions have been found with some dinosaur remains, there are pterosaur specimens with outlines of fur, feather impressions have been revealed in the lithographic limestone from Solenhofen and most recently, and probably most spectacularly, the remains of small theropod dinosaurs from China revealing a wonderful outline of “fuzz” or proto-feathers.

But what about colour? This is virtually impossible to speculate about and remains a great unknown but it is likely that dinosaurs were indeed coloured to some degree since their closest living descendants, the birds, are often brightly coloured. But even here, research moves ever forward (Vinther 2008, 2010 and Zhang 2010) and perhaps we are moving ever closer to the truth.


Ultimately, there will always be an element of uncertainty and great unknowns will remain. For example, if it wasn’t for the exquisitely preserved ichthyosaur specimens from Holzmaden, then it is quite possible that these great marine reptiles would still be displayed in restorations without their dorsal fins and, indeed, the upper tail fluke since only the lower fluke has bone running through it.

For me, great unknowns are best summed up by an example that I recall seeing as a kid and which stuck with me all my life. If our extant elephants were already extinct and we had nothing to compare their bones with, then how on earth would we know that the animal had a long prehensile trunk? The nasal openings are high on the front of the skull and situated between the eyes and there is nothing to suggest the existence of such a trunk. And, when you think about it, the outer ears also leave no physical evidence although I do understand that this is singularly a mammalian trait. Can you imagine our restorations of elephants today if we did not have the modern analogy to compare with?



Yes, I know there are frozen mammoths that have been preserved displaying trunks and ears but the point is essentially correct. It’s an amazing thought that animals such as the dinosaurs, in some cases, may have been even weirder and more spectacular than they already are.

References

 
• Vinther, J., Briggs, D. E. G., Prum, R. O. & Saranathan, V. 2008. The colour of fossil feathers. Biology Letters 4, 522-525.

• Vinther, J., Briggs, D. E. G., Clarke, J., Mayr, G. & Prum, R. O. 2010. Structural coloration in a fossil feather. Biology Letters 6, 128-131.

• Zhang, F., Kearns, S.L, Orr, P.J., Benton, M.J., Zhou, Z., Johnson, D., Xu, X., and Wang, X. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463, 1075-1078 .


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