Faculty of Biological Sciences, University of Leeds

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Evolutionary Developmental Biology Lecture 4

The curious history of the phyla (OVERHEAD,OVERHEAD)

 

 

So far so good. We have looked at ten different bodyplans out of the 35 that we think exist today ranging from 1 to more than a million species. Most of the examples that I have given you are modern, animals that we can see around us today. But we know that the distribution of animals changes. We have historical (written) records from England which mention bears a thousand years ago. We have records that there were wild boars and wolves around until fairly recently. If we go back before written history we find bones in caves belonging to lemmings and reindeer, of which we have no historical records. We have film sequences of Tasmanian wolves (marsupials) but there are none around today. Animals move about.

 

If we look at deposits in settlements or caves where men used to live the deposits nearest the surface contain 'modern' bones: chicken and rabbit, which indicate that this is post Roman stuff (because the Romans brought in both these species). As you dig deeper the bones no longer look like old bones, they become heavier, a different colour and more mineralised as the calcium and phosphorous is leached out and replaced by salts from the surrounding rocks and soils - fossilised. As you go deeper you find more exotic animal bones appearing, hippos, lions, elephant-like animals, big cats. When these bones were deposited animals lived here that no longer live here today, and animals lived here that don't live anywhere today. As well as moving about animals have changed. We find the same changes outside caves too: some rocks look like layers of sand on top of each other (SLIDE) - which is what they are - and in these layers we find fossils of familiar animals near the top, then less so, then quite bizarre things (like dinosaurs).

 

We never get a complete record of course, but bits of sediment from different places in the world tend to correspond, either in having the same set of animals together, or the same amount of C14 to C12 (radiocarbon date), or a variety of other indicators which all tell us about age. We can therefore put together a chronology for the earth. If we do this with the appropriate sorts of animals marked we can get a very convincing picture of a steady increase in numbers of more and more complex animals (OVERHEAD)

 

But this isn't really true. The types chosen here are, of course, mainly vertebrates -phylum Chordata. The group labelled 'seaweeds and invertebrate animals' includes representatives of 34 of the 35 animal phyla described (excluding the one found last year). These 34 phyla have been described in deposits 500 million years old. That doesn't square very well with our intuitive view of the past, because we would expect the phyla to crop up gradually over the millions of years. In fact, there they all are, very early in known metazoan history, and there have been no new ones for 500 million years. We find the three layered animals as far back as 550 million years and the two layered ones going back a bit further, to around 580 million years (OVERHEAD).

 

At first sight this massive increase in numbers of known animals seems to be an artefact and to coincide with the first formation of calciferous hard parts at around 575mya.. The fossil record consists mainly of skeletons: records of soft bodied animals or tracks have a very slim chance of survival. Nevertheless they do exist, and the massive increase in skeletal parts in the Cambrian is paralleled by an increase in these 'trace fossils' implying an increase in the number of soft bodied forms as well.

 

Before the Cambrian came the Vendian era, and fossils from this time are also known. The question is what are they? The most common lack skeletal parts, and are often frond like, perhaps a metre in length.(c on SLIDE). The best guess, based on rather tenuous links and circumstantial evidence, is that they are the ancestors of our cousins the diploblastic, 2 cell layered animals like Hydra and jellyfish.

 

But also present in the Edicarian deposits are undoubted articulated arthropods (a on SLIDE) The fossils aren't very good so we don't know what they are exactly, but we do know that they had exoskeletons, segmentation and bilateral symmetry.

 

Also in the Venetian is Halkieria (d on SLIDE): what is this? Well it looks like a slug and the shells undoubtedly grew by accretion around the margins. Could it be a mollusc?

 

By the time we reach the Cambrian we have more fossils. The best of these

are from a deposit called the Burgess shale (OVERHEAD). It is one of the earliest prolific fossil bearing deposits, dating from 530mya in Canada and has very favourable geology - it is sedimentary, fine grained rock made by the slow deposition of silt .

 

530 million years ago in British Columbia there was a limestone reef 500 metres deep and 20 k long. Alongside the reef lay deep water. Normally only the hard parts of animals are fossilised: but mud which periodically fell off the reef regularly and rapidly buried animals and plants in anoxic conditions. As well as hard parts the soft anoxic mud preserved the remains of many soft bodied animals and plants. A small region of the shales was excavated by Walcott in 1909 and the years following, and written up in the scientific literature.

 

Now in the Burgess shales there are 140 different recognisable species. 107 of these can be assigned to phyla known from elsewhere, but the other 33 look like nothing else on earth and very different from each other (SLIDE).

 

When the Burgess shales were first examined they were recognised to be very old, dating from just after the origin of the metazoa. The reasonable expectation was that there would only be a few phyla represented. It was also assumed that all the animals found would fit into existing phyla. But in the Burgess shale are representatives of all phyla we know, plus about 33 more phyla, 33 phyla we never see again. There was evidently a sudden diversification of life just after the metazoans appeared, followed by some pretty hefty pruning.

 

Interestingly, no-one believed this at first. We have a huge capacity for seeing what we wish to see. Because scientists expected Burgess shales animals to be members of existing phyla, they described them just like that. Lets just look at just two of them in a little detail.

 

 

Opabinia (OVERHEAD)

Bauplan: bilaterally symmetrical, straight gut, segmented, segmental appendages.

 

 

Opabinia is a good case in point. What do we see? Well, when first described by Walcott we saw a very funny animal - it made people laugh, because of the strange snout thing with teeth and the five eyes.

 

It's an arthropod of course - its segmented. But look closely - there are no appendages on the head apart from that proboscis: arthropods are good at mouthparts. There are five eyes, although the first reconstructions showed only two. Walcott said an interesting thing: if the head appendages were large they must have broken off, if they were small they must be hidden under shell plates deformed during fossilisation. Walcott knew that Opabinia was an arthropod, all arthropods have appendages on the head. The alternative, that they weren't there, didn't occur to him. Neither did it occur to other workers who later reconstructed it the other way up (OVERHEAD) or as an arthropod by ignoring three of the eyes and making the funnel, quite fallaciously in two parts - fused mouthparts . The body should also, of course, have paired jointed legs like all good arthropods: because we can't see them they must be small, or hidden under the plates on each skeleton.

 

Now we come to a very courageous act: A man called Whittington took one of the world's supply of these fossils (there were 10) and applied a technique of chipping off layers of the shale which had replaced the carapace. There was nothing underneath.

So this wasn't an arthropod, but it wasn't anything else we knew about either. Clearly some new thinking had to go on.

 

 

Hallucigenia (OVERHEAD)

The next nightmare was Hallucigenia, so called by Conway Morris 'because of its bizarre and dreamlike appearance' ; like something by Salvador Dali. This had a tube with a mouth on the end, a row of seven tentacles with claws, or something, on the end on one side and seven pairs of spines on the other. Because the spines were paired Conway Morris decided that they had to be unjointed legs. The claws on the tentacles perhaps passed food to the mouth.

 

Ramskold, a Swedish Palaeontologist didn't believe a word of this. He turned Hallucigenia upside down (OVERHEAD) and went looking for the claws on the second set of legs which must be there. He found them in the very squashed original. This makes Hallucigenia rather like a primitive arthropod found in rain forests, Peripatus (OVERHEAD). This guess looks even better in the light of a Chinese Cambrian version of Peripatus which also has (shorter) spines on its back. Fuller story in the handout.

 

So where does this leave the Burgess shales? I have given examples of one presumed arthropod moved into the funnies, and one presumed funny moved into the arthropods. There are other funnies too (OVERHEAD), and other respectable creatures like Pikaia (OVERHEAD) the first Amphioxus -like chordate, i.e. a vertebrate ancestor.

 

So we have some problems. First of all we looked at the Burgess shale partly because of the good preservation but also because it is old. I mentioned 530 million years. Now the first multicellular organisms with hard parts are found just before this, 570 million years ago. If we count the earth as 4.5 billion years old then multicellular organisms have been around for about 10% of the time. The first unicellular organisms left their remains in rocks 3.6 billion years old, in the oldest unmetamorphosed sediments we have i.e. they may even be older than this but the rocks before this date have been changed drastically by volcanic forces. Multicellular organisms arose only once. Why so late? And in those 40 million years, a geological eyeblink the 34 common phyla plus 33 others have turned up - apparently with no simpler intermediates.

 

The 34 phyla we have now are clearly not the only possible ones (OVERHEAD): all the Burgess animals that are not in these phyla show similar ideas, bilateral symmetry, segmentation etc. that we recognise, but grouped together in different ways. Are these animals failures that rapidly died out? How fast was evolution going at the time? Faster than it did after the Cambrian?

 

Lets take these one at a time.

 

Were these animals failures?(OVERHEAD) No. Once the Burgess animals had been properly described and interest had been aroused other sightings were made, and are being made. At first other local sites in Canada were examined, then deposits of the right age as far away as Utah. Local deposits had the same animals, but in different proportions: father away the deposits had mostly the same creatures. In earlier deposits (Lower - Middle Cambrian) Burgess representatives have turned up in Australia , Greenland and China (OVERHEAD). One species at least has a time range of 15 million years, with virtually no morphological variation.

 

How fast was evolution going? well in the middle -upper Cambrian not so fast that we can see new forms evolving. But before that there must have been a great flowering of diversity, coming perhaps from the first multicelled animals. This could have arisen in two ways

1. The first filling of the ecological barrel

Was there anything strange about the early Cambrian? Was there something odd about the environment?

Yes, of course, there were no multicellular animals - you didn't have to be the fittest to survive - there was, for a time, no competition. So any old strategy would probably work. We might picture the new, multicellular organisms proliferating quickly, then the rate of change slowing as the less efficient forms came into competition with others for the first time. Competition provides the regulator.

The drawback to this theory was that the barrel has been nearly emptied on other occasions since then, by great climatic changes for instance. We know about these, we know what expanded to fill the nearly empty earth - and it wasn't a new set of phyla.

 

2. Perhaps the lack of new phyla after the initial burst was due to genetic ageing or genetic or physiological constraint. Perhaps the complex genomes required for complex animals are so complex that they lose flexibility, and become gridlocked: and complex physiologies become so complex that they cannot be changed with impunity.

 

3. Perhaps our ideas about classification are wrong. One explanation is that the phyla that exist outside the Burgess shale are in fact part of a continuum, and those that exist in the shales form the missing parts of this continuum, the intermediate animals that were less successful and didn't stay the course.

 

If this is so then we can suggest some things about the appearance of the metazoa.

  • First almost as soon as the metazoan, multicellular body arose there was an almost immediate diversification into many types.
  • Secondly that some of these types survived until the present, and some were short lived
  • Third that there has been an increase in the number of species with each phylum since then. This increase is very large in a few phyla, but some are clearly dead ends.
  • Fourth that no new body plan or phylum has been added in the last 500 million years.

 

Gould had the following thoughts.

Our ideas about the world are governed by Philosophy - what we believe to be true. One very strongly held tenet is that of Progress - things getting more organised and better with time, although thermodynamics holds the other view, that chaos increases with time. The Burgess shales are not telling us that the world began with a very few simple models, then moved upward to many variations on these models due to the survival of the fittest. It is telling us that there was a sudden very rapid burst of possible multicellular forms, then a slowing and a suddenish end for most forms, probably due to climatic change. The forms that survived were not particularly numerous in the shales, not particularly successful: and some types that survived are nearly non starters, known today by only one or two species.

 

 

 

If we could record this event on videotape and play it again what would happen? If the cone model was right then sense would prevail and the best possible solutions would always survive : the chordates and the arthropods would always win. But if the other model is nearer to the truth and each phylum has the same chance of winning then there would be many possible sets of worlds, almost all of which would not include the human mind.


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