Conserving Darwin’s marine fossils | Digital Collections Programme

A guest blog by Lucia Petrera

While Charles Darwin was travelling on the Voyage of the Beagle, he found thousands of shells and hard parts of marine animals on land, far away from the sea. This shaped his and our current understanding on how the South American continent has changed overtime. In this blog, Lucia Petrera takes us behind the scenes to explore how these fossils will be protected and shared with the world.

The project to conserve and digitise the fossils collected by Darwin began with the fossil mammals collection. Thanks to the generosity of the Hartnett Conservation Fund we are now able to include the other fossils collected by Darwin which form part of the Museum collections. We are currently working to conserve and digitise around 255 marine fossil invertebrates collected in South America from 1831-1836.

They include:

• Brachiopods, or lampshells, which first appear in the fossil record 550 million years ago and are still alive today
• Ammonites, which are coiled shelled molluscs related to octopuses and squids, which first appear in the fossil record 420 million years ago and are extinct
• Bivalves, the group of molluscs that includes mussels and clams, which first appear in the fossil record 520 million years ago and are still alive today
• Gastropods, the group of molluscs that includes slugs and snails, which first appear in the fossil record 500 million years ago and are still alive today

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First ammonite in South America

Image of NHMUK PI C 2612, an ammonite in the genus Maorites one of the first ammonites recorded from South America with the original red rectangular Darwin labels.

Many of the specimens that Darwin collected are ‘type’ specimens – they were a new species, unknown to science at that time. Darwin collected one of the first ammonites ever recorded from South America, from Mount Tarn in Tierra del Fuego, Chile. Ammonites are excellent ‘index fossils’ – fossils that are often able to be used to identify the geological time period they existed in. Darwin enlisted assistance with the identification of the material from experts at the time, including Alcide D’Orbigny, a French naturalist. Initially this specimen was identified as Ancyloceras simplex, a European ammonite. However, more recent re-identification has placed it in the genus Maorites. Maorites is from the Late Cretaceous period (100-66 million years ago) and found only in the southern continents such as South America, Australia, and Antarctica, the continents that were part of the ancient super continent Gondwana. Gondwana gradually broke up 180 – 80 million years ago, carrying its cargo of ‘Gondwanan’ fossils on each of its constituent continents.

Miles of seashells

Fossils can be also indicators of past habitats and geological processes, and some of the marine shells collected by Darwin told a fascinating tale. Darwin observed marine shells along 1200 miles (1900 km) of coastline on what he called ‘successive beaches’- geological features now known as ‘marine terraces’. A marine terrace is any relatively flat, horizontal, or gently inclined surface of marine origin, bounded by a steeper ascending slope on one side and by a steeper descending slope on the opposite side. Darwin theorised his ‘successive beaches’ extended to 1600 miles (2600 km) or more. He collected his thoughts in an essay entitled ‘The Elevation of Patagonia’, documenting his model for the formation of successive beaches. This challenged his geological learning from Charles Lyell’s Principles of Geology, that had supposed elevation change to be much more localised. Darwin’s conclusions are confirmed by geologists today, and his successive beaches were most likely formed during ice ages alternating roughly every 100,000 years between glacial and warmer periods. During the glacial periods much of the world’s water was locked away and sea level fell by about 100m. In warmer times, the sea rose and areas were flooded again. This interplay of sea level change and regional uplift is why Darwin found so many marine fossils at high elevations and in some cases far from the sea. Some specimens Darwin found were species that could still be found living on local coasts, which told him that this land change happened relatively recently. One of Darwin’s major achievements on the Beagle voyage was his discovery that much of the southern half of South America had been uplifted in relatively recent geological times.

Preserving Darwin’s Legacy

NHMUK PI G 26369-26370-26371 Three specimens of the Plio-Pleistocene carnivorous gastropod Chorus blainvillei (d’Orbigny) from Coquimbo, Chile. Before and after restorage.

To conserve these specimens, we need to keep them in a stable environment to minimise the risk of damage to them during handling and storage. We use conservation grade inert materials to re-house these specimens so that they will be kept in the same condition for many years to come. Due to the historic nature of this collection, some of this work has involved the fabrication of bespoke supports suited for long-term preservation. For example, some of Darwin’s brachiopods were stored on old wooden boards using nails to keep the specimen in place. At the bottom of each board there is a handwritten label with the specimen’s data. As we want to both conserve the specimen and the historical label, this required some creativity. These nails are ordinary builders’ nails, however due to the metal in the nails being lot harder than the soft rock of the fossil shells, this can cause abrasion and  leave grooves where they touch the specimen, so needed to be removed. After removing the nails, the board was partially padded using Plastazote® foam which was cut to the exact size needed to support the specimens. This prevents movement, further abrasion and damage, while leaving the label area uncovered so that the important collection data, such as species name, collections site and date can be read.

In addition to conserving and re-storing the specimens, we are also photographing and capturing associated data on the specimens to be released via the Museum’s Data Portal. High resolution photographs are being taken so that all angles and defining characteristics of the specimen, as can be seen with the image of Terebratula above. Terebratula had a fleshy stem protruding from its shell that it used to attach to something hard and firm such as a rock. To feed, they open their shell and the sea water (containing small food bits) circulates and the animal uses its filter feeding organ to get the nutrients it needs. Researchers use these example specimens as reference material and will want to be able to clearly see the characteristics unique to that species.

All of these specimens had a significant impact on Darwin’s thinking. By fully digitising the specimens, researchers all over the world working in taxonomy, geology, biogeography, ecology and more will be able to access high resolution photographs and specimen data from these historically and geologically important specimens without travelling, continuing the long process of study of those specimens started by Darwin himself. Read more about other parts of Darwin’s collections that we are digitising in our previous blogs or stay up to date the Darwin digitisation project by following @NHM_Digitise and @NHMfossilmammals on twitter.