Since 1859, when Origin of Species was published, palaeontologists have known that the proof of Darwin’s pudding lay in large slabs of the fossil record’s pie. This turned out to be rather indigestible since the fossil record is notoriously gappy. Indeed, so poor is the record earlier than the Cambrian (more than 541m years ago) that this huge slice of Earth history is known as the Proterozoic, literally earlier life, because for many years it was inferred that there must have been life prior to the base of the Cambrian rather than regarded as proved.
It was in the Cambrian that fossils first became abundant in the fossil record and these comprise mostly hard materials such as calcium carbonate that are resistant to the processes of fossilisation.
There are exceptions, however – windows of exceptional preservation where the soft-tissue parts of fossilised organisms can be examined. These are known as Lagerstatten.
The Burgess Shale of British Columbia is one of the world’s most celebrated lagerstatten. At 505m years old (Middle Cambrian), it is one of the earliest fossil beds of the Phanerozoic that exhibits soft-part imprints.
The fossils are preserved as carbonaceous films. Since the soft parts of the body are preserved, it allows palaeontologists to examine the working of animals that are half a billion years old in extraordinary detail.
What is responsible for such exquisite preservation? Palaeontologists agree that the key is the speed of burial together with a lack of oxygen (anoxia). In the case of the Burgess Shale, it would seem that the animals were living at the margin of a shallow sea when a sudden mudslide entombed them. The animals suffocated almost immediately in an oxygen-poor environment that promoted the preservation of their soft parts.
For much of the 20th century, it was thought that the Burgess Shale was a one off occurrence; but from about the mid-1980s onwards, more startling finds began to emerge from Cambrian rocks. The Chengjiang fauna in China is slightly older than the Burgess Shale at 515m years old and the Sirius Passet faunas in Greenland are 518m years old. Both are exquisitely preserved too.
Peter Allison of Imperial College London, UK, says: ‘It was a real eye-opener, once is happenstance, twice is circumstance, but three times? The occurrence of so many deposits preserving soft bodied fossils within the same narrow time window argues against coincidence.’ Allison and his colleague Derek Briggs, both then at Bristol University, UK, wondered what all these sites, broadly in geological terms the same age but widely different in geographical locations, could have in common. ‘The answer was that all these fossil beds were deposited and preserved because the sea bed was anoxic,’ says Allison, ‘because the worms and molluscs that routinely scavenge the sea bed for food and “bioturbate” – and therefore allow oxygen into – the sediment had not yet evolved.’
Allison points out that in the modern day Santa Catalina basin, in California, US, a particle of sediment will pass through the gut of the average priapulid worm 17 times before it is buried below bioturbation depth. The depth of this disturbed layer in the modern ocean is 30cm. After the evolution of these bioturbators, as sediments built up, nearly all of them will be missing fossilised soft-parts simply because all the organisms have been eaten and/or oxidised. Only the indigestible shells are left behind. By contrast, in the Cambrian, in the absence of bioturbation, this disturbed layer was non-existent.
Further work by Robert Gaines at Pomona College in California, US, and colleagues in Europe and China, have refined this story. Sulphur isotope evidence from sedimentary pyrites reveals that the exquisite fossilisation of Burgess-age organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments because of this oxygen deprivation. Low sulphate concentrations in the global ocean and low-oxygen bottom water conditions at the sites of deposition resulted in reduced oxygen availability.
Another factor in this Burgess Shale-type preservation was the rapid entombment of fossils in fine-grained sediments and the rapid sealing of these sediments by impermeable carbonate cements at bed tops. This further restricted oxidant flux into the sediments.
This permeability barrier, a feature shared among Burgess Shale-type deposits, resulted from the unusually high alkalinity of Cambrian oceans. Thus, Burgess Shale-type preservation was a result of unique aspects of early Paleozoic seawater chemistry providing a unique record of the immediate aftermath of the ‘Cambrian explosion’ of animals with three body layers, the triploblasts, of which we vertebrates are the ultimate expression.
Further up the geological column, Roy Wogelius and colleagues at Manchester University, UK, have been applying inorganic chemical techniques to the study of fossils from the celebrated Solnhofen limestones, which contain fossils of some of the earliest known birds, including the famous Archaeopteryx.
‘We’ve been using synchrotron radiation to look at the distribution of sulphur in the holotype of Archaeopteryx,’ Wogelius says., ‘We can look at the different oxidation states of sulphur from completely oxidised sulphate to completely reduced sulphide and all the oxidation states in between. We can even map the spatial distribution of a single oxidation state.’
‘When we did that with Archaeopteryx, we found that the organic sulphur is concentrated in the leading edge of the flight feather, which is also the best preserved part of the feather. Copper and nickel are also concentrated in the same area. So clearly the concentration of these elements in this one area must be related to its better preservation. It turns out that these trace metals are the residue of a protein called eumelanin, which is also found concentrated in the leading edges of modern feathers.’
Eumelanin pigmentation has a number of functions, the most important of which is as a biocide to reduce infection from bacteria and mites. ‘So this distribution of eumelanin residues in Archaeopteryx and other fossil birds shows that this is an ancient evolutionary innovation that evolved almost immediately that birds did,’ Wogelius says. ‘It is a prerequisite for caring for your plumage if you are a bird.’
Eumelanin pigmentation has modern applications too, he continues. ‘The Chinese have been doing some experiments on microorganisms that make the enzyme keratinase, which breaks down feathers. What they are trying to do is to use it in chicken farms to turn the waste into protein that can be used as a foodstuff. One of the things they have shown is that the breakdown kinetics of black feathers is not as fast as the breakdown kinetics of white feathers and that’s because one of the things that eumelanin does is to protect the keratin from breakdown when it is attacked by microbes.’
Wogelius compares eumelanin with Cuprinol preservative used for wooden fencing, because it contains copper. ‘Eumelanin is made from an enzyme that is copper-based, which is why this element shows up so strongly in our synchrotron tests,’ he says. ‘In addition, the breakdown product of eumelanin is sulphur, which explains the anomalous concentrations at the leading edge of the wing which need the most protection.’
Wogelius and his collaborators are also looking at a group of animals closely related to birds – dinosaurs. Again using high energy synchrotron radiation, the group is able to identify the healing process in broken dinosaur bones. ‘There is a biochemical signal that controls the continuous process of bone recycling. In the event of a break, the body sends a signal to stop the dissolution process and just continue with the repair. The signal is sent via a biochemical chain, which has cofactors such as copper and zinc.’
By analysing the broken toe of the dinosaur Allosaurus, they found that the chemical markers in the bone showed it had a fracture that must still have been actively repairing when the animal died. ‘That tells us that the biochemical signals for osteoblast and osteoclast [bone cell] control have stayed the same for a long time. There’s no reason why they should change of course, because we are all vertebrates.’
But their research has a more general message for dinosaur biology. ‘We have found this pattern in so many different dinosaur remains that we think that the predatory dinosaurs used to break their feet a lot. Fracture and repair must have been a very important part of predatory dinosaur’s biology.’
One of the most remarkable lagerstatten, dating back to the Silurian (443–419m years before present) was found in Herefordshire in the mid-1990s. In the Silurian, today’s Herefordshire lay on the micro-continent of Avalonia in the southern subtropics.
The sediments containing the Herefordshire biota formed on the outer shelf or upper slope area of the sea-bed at a water depth of 150–200m and can be dated radiometrically to about 425m years ago.
The Herefordshire locality contains a variety of small marine invertebrates such as worms, molluscs, starfish, brachiopods, and arthropods, plus many intriguing animals of, as yet unknown, affinity.
All of the fossils are beautifully preserved in extraordinary detail in three dimensions and are unknown from elsewhere. Even completely soft-bodied animals, such as worms, have survived intact.
The fossils occur as calcite in-fills within carbonate nodules which are themselves entombed in a volcanic ash (bentonite). The presence of clay appears to have been crucial, surrounding the carcass almost immediately after burial and providing sufficient support to help maintain three-dimensionality until crystalline and fibrous calcite precipitated in the void that resulted from the decay of internal soft tissues.
It seems that calcium from the volcanic ash falling on the area, combined with the bicarbonate released from the decaying carcass to produce this calcite in-fill,were critical in preserving these creatures.
Because of their unique mode of preservation, the Herefordshire fossils cannot be extracted from the rock by conventional physical or chemical methods. Instead, each specimen is ground down 20 microns (ie 0.02mm) at a time and digitally photographed after each grind.
For a single fossil this provides hundreds of images that can be stacked together as a series of closely-spaced slices that can then be rendered in 3D as a ‘virtual fossil’ by computer software. Remarkable pictures including a brachiopod, a sea spider – and even a female ostracod complete with eggs – have been obtained.
Richard Corfield is a freelance science writer based in Oxford, UK