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The results, published today (7 May) in the journal Current Biology, identify a key point in the evolutionary transition from soft to hard bodies in early ancestors of arthropods, the group that contains modern insects, crustaceans and spiders.
The study looked at two types of arthropod ancestors – a soft-bodied trilobite and a bizarre creature resembling a submarine. It found that a hard plate, called the anterior sclerite, and eye-like features at the front of their bodies were connected through nerve traces originating from the front part of the brain, which corresponds with how vision is controlled in modern arthropods.
The new results also allowed new comparisons with anomalocaridids, a group of large swimming predators of the period, and found key similarities between the anterior sclerite and a plate on the top of the anomalocaridid head, suggesting that they had a common origin. Although it is widely agreed that anomalocaridids are early arthropod ancestors, their bodies are actually quite different. Thanks to the preserved brains in these fossils, it is now possible to recognise the anterior sclerite as a bridge between the head of anomalocaridids and that of more familiar jointed arthropods.
“The anterior sclerite has been lost in modern arthropods, as it most likely fused with other parts of the head during the evolutionary history of the group,” said Dr Javier Ortega-Hernández, a postdoctoral researcher from Cambridge’s Department of Earth Sciences, who authored the study. “What we’re seeing in these fossils is one of the major transitional steps between soft-bodied worm-like creatures and arthropods with hard exoskeletons and jointed limbs – this is a period of crucial transformation.”
Ortega-Hernández observed that bright spots at the front of the bodies, which are in fact simple photoreceptors, are embedded into the anterior sclerite. The photoreceptors are connected to the front part of the fossilised brain, very much like the arrangement in modern arthropods. In all likelihood these ancient brains processed information like in today’s arthropods, and were crucial for interacting with the environment, detecting food, and escaping from predators.
During the Cambrian Explosion, a period of rapid evolutionary innovation about 500 million years ago when most major animal groups emerge in the fossil record, arthropods with hard exoskeletons and jointed limbs first started to appear. Prior to this period, most animal life on Earth consisted of enigmatic soft-bodied creatures that resembled algae or jellyfish.
These fossils, from the collections of the Royal Ontario Museum in Toronto and the Smithsonian Institution in Washington DC, originated from the Burgess Shale in Western Canada, one of the world’s richest source of fossils from the period.
Since brains and other soft tissues are essentially made of fatty-like substances, finding them as fossils is extremely rare, which makes understanding their evolutionary history difficult. Even in the Burgess Shale, one of the rare places on Earth where conditions are just right to enable exceptionally good preservation of Cambrian fossils, finding fossilised brain tissue is very uncommon. In fact, this is the most complete brain found in a fossil from the Burgess Shale, as earlier results have been less conclusive.
“Heads have become more complex over time,” said Ortega-Hernández, who is a Fellow of Emmanuel College. “But what we’re seeing here is an answer to the question of how arthropods changed their bodies from soft to hard. It gives us an improved understanding of the origins and complex evolutionary history of this highly successful group.”
Did ocean acidification from the asteroid impact that killed the dinosaurs cause the extinction of marine molluscs?
Ammonites, which were free-swimming molluscs of the ancient oceans and are common fossils, went extinct at the time of the end-Cretaceous asteroid impact, as did more than 90 per cent of species of calcium carbonate-shelled plankton (coccolithophores and foraminifera).
Comparable groups not possessing calcium carbonate shells were less severely affected, raising the possibility that ocean acidification, as a side-effect of the collision, might have been responsible for the apparent selectivity of the extinctions.
Previous CO2 rises on Earth happened so slowly that the accompanying ocean acidification was relatively minor, and ammonites and other planktonic calcifiers were able to cope with the changing ocean chemistry. The asteroid impact, in contrast, caused very sudden changes.
In the first modelling study of ocean acidification which followed the asteroid impact, the researchers simulated several acidifying mechanisms, including wildfires emitting CO2 into the atmosphere (as carbon dioxide emissions dissolve in seawater they lower the pH of the oceans making them more acidic and more corrosive to shells) and vaporisation of gypsum rocks leading to sulphuric acid or ‘acid rain’ being deposited on the ocean surface.
The researchers concluded that the acidification levels produced were too weak to have caused the disappearance of the calcifying organisms.
Professor Toby Tyrrell, from Ocean and Earth Science at the University of Southampton and co-author of the study, says:
“While the consequences of the various impact mechanisms could have made the surface ocean more acidic, our results do not point to enough ocean acidification to cause global extinctions. Out of several factors we considered in our model simulation, only one (sulphuric acid) could have made the surface ocean severely corrosive to calcite, but even then the amounts of sulphur required are unfeasibly large.
“It throws up the question, if it wasn’t ocean acidification what was it? Possible alternative extinction mechanisms, such as intense and prolonged darkness from soot and aerosols injected into the atmosphere, should continue to be investigated.”
The study, which is published in the Proceedings of the National Academy of Sciences (PNAS), involved researchers from the University of Southampton and the Leibniz Center for Tropical Marine Ecology. The project received funding from the European Project on Ocean Acidification and funding support from NERC, Defra and DECC to the UK Ocean Acidification programme (grant no. NE/H017348/1).
The team, led by the University of Leiden, and including researchers from Historic England and the universities of Southampton, Birmingham, Surrey, and Swansea, examined a 1500 year old male skeleton, excavated at Great Chesterford in Essex, England during the 1950s.
The bones of the man, probably in his 20s, show changes consistent with leprosy, such as narrowing of the toe bones and damage to the joints, suggesting a very early British case. Modern scientific techniques applied by the researchers have now confirmed the man did suffer from the disease and that he may have come from southern Scandinavia.
Archaeologist Dr Sonia Zakrzewski, of the University of Southampton, explains DNA testing was necessary to get a clear diagnosis: “Not all cases of leprosy can be identified by changes to the skeleton. Some may leave no trace on the bones; others will affect bones in a similar way to other diseases. In these cases the only way to be sure is to use DNA fingerprinting, or other chemical markers characteristic of the leprosy bacillus.”
The researchers tested the skeleton for bacterial DNA and lipid biomarkers to confirm the man had definitely had leprosy and to allow them to carry out a detailed genetic study of the bacteria that caused his illness.
Professor Mike Taylor, a Bioarchaeologist from the University of Surrey, says: “Not every excavation yields good quality DNA, but in this case, leprosy DNA isolated from the skeleton was so good it enabled us to identify its strain.”
The results showed the leprosy strain belonged to a lineage (3I) which has previously been found in burials from Medieval Scandinavia and southern Britain, but in this case it originates from a much earlier period, dating from the 5th or 6th centuries AD.
The identification of fatty molecules (lipids) from the leprosy bacteria confirmed the DNA results and also showed it was different from later strains. Emeritus scientist David Minnikin, from the University of Birmingham, says: “With Leverhulme Trust support, we recorded strong profiles of fatty acid lipid biomarkers that confirmed the presence of leprosy. However, one class of the lipid biomarkers had distinct profiles that may distinguish these older leprosy cases from later Medieval examples.”
Isotopes from the man’s teeth showed that he probably did not come from Britain, but more likely grew up elsewhere in northern Europe, perhaps southern Scandinavia. This matched the results of the DNA, and raises the intriguing possibility that he brought a Scandinavian strain of the leprosy bacterium with him when he migrated to Britain.
Project leader Dr Sarah Inskip of Leiden University concludes: “The radiocarbon date confirms this is one of the earliest cases in the UK to have been successfully studied with modern biomolecular methods. This is exciting both for archaeologists and for microbiologists. It helps us understand the spread of disease in the past, and also the evolution of different strains of disease, which might help us fight them in the future. We plan to carry out similar studies on skeletons from different locations to build up a more complete picture of the origins and early spread of this disease.”
Although leprosy is nowadays a tropical disease, in the past it occurred in Europe. Human migrations probably helped spread it, and there are cases in early skeletons from western Europe, particularly from the 7th century AD onward. However the origins of these ancient cases are poorly understood. The study of the Great Chesterford skeleton provided an important opportunity to shed light on the early spread of leprosy.
The results of the study will be published in the journal PLOS ONE and copies of the paper can be requested from Media Relations.
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