Some palaeontologists have argued that creatures such as these simply couldn’t have built such structures. They argued that the homes were far too sophisticated to have been created by what were effectively small blobs of jelly. Our evidence suggests that this is, in fact, what happened.
Dr Jan Zalasiewicz
A geologist from the University of Leicester has discovered that ancient organisms performed specific roles in order to construct intricate floating homes. Dr Jan Zalasiewicz, Senior Lecturer in Palaeobiology at the University’s Department of Geology, recognised evidence of this prehistoric division of labour whilst examining a graptolite fossil that had been preserved in the collections of the British Geological Survey (BGS) since 1882.
The fossil in question, which shows a planktonic colony from almost half a billion years ago, was first unearthed by nineteenth century geologists in the Southern Uplands of Scotland. Whilst graptolites from this era are fairly common, it is only the multi-storey floating homes of the creatures that are usually visible in these fossils. The organic matter pertaining to the creatures themselves decomposes before fossilisation can occur.
Dr Zalasiewicz, however, noticed something quite special. Whilst conducting a routine examination of the rock slab, he noticed a peculiar pattern on one of the graptolites. This pattern, although not created by the original creatures, seems to have been produced by the connecting ‘ropes’ that the animals used to build their intricate floating homes.
What’s more, Dr Zalasiewicz observed variations between these connections, suggesting that the creatures adopted an elaborate division of labour in which different members of the colony performed different tasks. This evidence contradicts the previously held consensus that members of these colonies were all basically similar. Instead, it seems that the animals would have varied in shape and organisation across different sections of the colony.
I spoke to Dr Zalasiewicz to find out more about this prehistoric division of labour…
Why did the connections that you spotted go unnoticed for such a long period of time?
I think that this is due to the fact that the fossil isn’t tremendously well preserved. It is actually one of a number of fossils that exist on the same slab and it isn’t the fossil for which the slab was originally collected. I wasn’t even examining that particular fossil when I noticed its connections. The light just happened to catch the graptolite in the right way. It was just sheer luck. I saw patterns that I had never seen before. Within a few minutes, I knew that there was something a bit special about what I had spotted. To find any trace of creatures living inside these fossils is very rare indeed.
So we are talking about fairly subtle patterns…
Yes. The light has to catch them in the right way as they are quite reflective. They are patterns which consist partly of shiny clay minerals and partly of the carbonised material of the animal itself. It is also important to remember that these patterns have not been observed in graptolites before. It is common for geologists only to notice the characteristics that they are looking for. One simply wouldn’t have expected to see these particular patterns.
You have said that these creatures had long, tentacle-bearing arms, yet they have never been directly observed in a fossil. How then do you know that this was the case?
We can say this because of the homes of these creatures. They are built constructions that to some extent resemble those of modern-day organisms called pterobranchia. These organisms live on the sea floor and have rather disorganised, spaghetti-like living chambers; they are just masses of tubes. Analyses of pterobranch limbs show that they are closely enough related to provide a reasonable model of what the graptolites would have looked like. Essentially, they would have been very delicate, gelatinous creatures less than a millimetre across with filter-feeding tentacles.
Pterobranchia also build living tubes for themselves. When compared with the tubes of pterobranchia, graptolite homes are immensely sophisticated Faberge eggs. Even so, both of these creatures are animal architects.
How do the connections that you observed differ from one another?
The first thing to note is that the pattern of these connections alters along the length of the colony. There are three different types of connection. At the beginning of the colony, there is an interweaving network of tubes. These tubes then turn into isolated, hourglass-shaped structures and this continues for most of the colony. At the very end of the structure, they change once again into a much simpler pattern of connected tubes. These changes clearly reflect the different physical interconnections that existed between the organisms.
What can these connections tell us about the shape and organisation of the organisms themselves?
Firstly, we know that these creatures had a number of tasks to perform. As well as feeding, reproducing and so on, they had to cooperate with one another to build their homes. They constructed rather delicate and precisely engineered living quarters by working together. In a sense, these structures were similar to a block of flats. Each flat must be seamlessly joined to its counterparts above and below. There is some speculation as to whether particular organisms performed specific tasks, or the organisms performed different tasks at different stages of their lives. This is the first evidence that points towards a division of shape, and in turn, this probably reflects a division of labour. It is a striking palaeontological example of colonial division of labour expressed in fossil form.
And I suppose that this evidence also goes some way towards answering the mystery of how these basic creatures managed to create such intricate structures…
Yes, I think that it does. This has always posed somewhat of a puzzle for our discipline. Some palaeontologists have argued that creatures such as these simply couldn’t have built such structures. They argued that the homes were far too sophisticated to have been created by what were effectively small blobs of jelly. Our evidence suggests that this is, in fact, what happened. I think that evidence of a division of labour represents a reasonably large step on the way to understanding this process.
Why do you think connections have not been observed in other graptolite fossils?
The simple answer is that these patterns are few and far between. There are, however, several theories on this subject. One possibility is that we are looking at a very rare species that for some reason, formed armoured connections. Another possibility is that we are witnessing some kind of pathology. Perhaps some sort of ‘skin disease’ caused usually very soft connections to harden.
So this pathology might have made the connections robust and thus durable…
Durable and fossilisable. On the other hand, we could be observing an immensely complicated parasite which mimicked the shape and pattern of the graptolite colony. The problem with this theory is that it only serves to shift the question. Even if this was a parasite, it still employed a division of labour.
I think that we are looking at the organisms themselves, and I think that what we have found is very rare. I have probably looked at over a million graptolites during the course of my career. Had these patterns been at all common, one of my colleagues or I would have noticed them before. What we have here is a genuine rarity. I was just the lucky geologist who stumbled upon it.
Congratulations on your discovery Dr Zalasiewicz and thank you very much for taking the time to speak with me today…