Lyme Regis Ammonite Pavement

To the west of Lyme Regis, Dorset, England (https://goo.gl/maps/rqXHp) is a very unusual rock pavement made of limestone - up to 40 % of its surface is covered by ammonite shells, ranging in size from less than 1 cm to over 70 cm wide. 

A section of the ammonite pavement, photo showing area approximately 60 cm wide

The carbonate-rich muds which formed the limestone were laid down around 200 million years ago during the Early Jurassic when large areas of Britain and Europe were covered by a shallow tropical sea (Dorset was around 35 degrees north of the equator at this time).

But why are there so many ammonites in this one limestone pavement?

The muds, which formed this limestone, accumulated over a very long period of time, possibly hundreds or thousands of years. This allowed time for the number of ammonite shells to build up resulting in the concentration seen today.  Within the bed are also numerous fragments of ammonite shell (best seen if the limestone surface is made wet), as well as other fossilised creatures including shellfish (both bivalves and brachiopods), snails (gastropods), crinoids and burrows made by worms and other burrowing organisms.

How are the ammonites preserved?

The ammonites are mainly preserved lying on their sides and have either been filled by carbonate-rich mud or spar crystals, which have grown within voids in the shell.

Ammonites lying on their sides, preserved in the Ammonite Pavement.  Most are filled with carbonate-rich mud, but some are filled with milky-white crystals (spar) 

Although the shells of the ammonites were originally formed from aragonite, they are now preserved as either calcite, or less commonly pyrite.  However, some of the fossil shells are missing their upward-facing side!

A spar-filled ammonite on the side of the Ammonite Pavement.  A dark line can be seen picking out the shell on the bottom of the ammonite, but it is absent at the top of the fossil

If you look carefully you can often find large fossils and worm burrows within the inner whorls of the ammonites which could only have got there through a hole in the side of the shell. The mud filling a single ammonite often varies in colour (best viewed when the pavement is wet), which suggests the ammonites were infilled gradually, one section at a time.  Also, some ammonites are filled with spar crystals, but are also missing the upper side of their shell.  Spar crystals only grow in cavities when a shell is buried, so when these crystals grew, the ammonite shell must have been buried and complete, with the upper half of the shell removed at a later time.

These observations all provide clues about the history of this unusual bed.

So how did this happen?

Aragonite (the material ammonites formed their shells from) is not stable over geological timescales. If left lying on the sea floor it naturally dissolves back into the sea water, unless the creature is still alive and maintaining its shell. In this bed, however, the shells did not completely dissolve away, which allowed time for the aragonite to be replaced by calcite (which dissolves less easily), and for the number of shells to build up over time.

For aragonite to dissolve, the sea water needs to become slightly more acidic than usual. This generally occurs when hydrogen sulphide, which is produced during rotting, combines with oxygen, which animals need to live.  With no oxygen, the hydrogen sulphide bubbles away and does not make the water more acidic.  Within the beds at Lyme Regis there is evidence for periods of time when the oxygen level was lower (such as during the formation of black shales); when the oxygen levels were low the aragonite would have been safe from dissolution.  However there is also evidence that at times the oxygen level was high, allowing animals to live on and burrow into the sea floor.  During these times, the aragonite would have been at risk.

After the ammonites died and their shells sank to the sea floor, they would have come to rest on carbonate-rich mud.  Carbonate-rich mud is alkaline and can produce a buffering affect, resisting increases in acidity.  Therefore, any aragonite which was covered by the carbonate-rich mud would be protected from dissolution, even if there was oxygen in the water.  This explains why the top half of the ammonite shells are sometimes missing within the Ammonite Pavement as the upper sides of shells were more likely to be exposed by water currents.

Summarised history of the Ammonite Pavement

1. When the Ammonite Pavement pavement formed, Lyme Regis was covered by a shallow tropical sea.  The amount of mud being washed into this area of sea was very low and the amount of oxygen in the water was variable
2. During periods of low oxygen ammonite shells would sink to the sea floor, but not dissolve as there was no oxygen in the water to combine with hydrogen sulphide, so the water was not acidic. The shells would become either partially or fully buried in carbonate-rich mud, allowing time for spar crystals to grow
3. When the oxygen levels in the water increased again, animals colonised the sea floor, but any aragonite exposed from the carbonate-rich mud would be dissolved, occassionally removing the top-half of shells allowing sediment and animals into the ammonite.  For any ammonites which died during periods of higher oxygen, unless buried by carbonate-rich mud, their shells would completely dissolve leaving no trace in the fossil record
4. After time the sea would become less oxygen-rich and aragonite dissolution would decrease
5. This cycle continued for a very long time allowing the number of ammonite shells to build up gradually, and become preserved, before the amount of sediment washing into the area increased again and the beds above the Ammonite Pavement were formed

But why didn't this happen in every limestone bed?

For the Ammonite Pavement to have formed, you had to have a combination of factors coinciding at the same time, including a low supply of carbonate-rich mud (too high and it would dilute the number of ammonites, too low and there would not be enough to protect the ammonites), a very long period of time allowing the build-up of ammonites, and a varying level of oxygen in the water. Without any of those factors we would not have the pavement we see today.

This new model for fossil preservation has been created based on evidence observed at Lyme Regis, but is applicable to other fossil deposits around the globe of any age in carbonate environment with variable levels of oxygen.

Thank you for reading this blog post. If you would like to find out more information, please read the open-access academic paper (http://onlinelibrary.wiley.com/doi/10.1111/let.12126/epdf), and/or leave a comment for discussion.