Fish Distribution & Frequency
Over the course of meeting many local London Clay collectors, as well as having access to many museum collections, I’ve handled and reviewed over 1,250 different London Clay fossil fish skulls from differing exposures and divisions. I thought therefore it would be good to pull together some charts which showcase both the diversity of fish species as well as an indication of frequency of finds.
Whenever you undertake a form of distribution or frequency analysis, you have to consider the many biases which can significantly skew the data. These will include the ways in which people collect (it’s easy to miss the smaller fish craniums), preservation bias (larger fish tend to preserve better) as well as collection bias (where collections might not want more and more of the common ones, but they are more interested in keeping the rarer ones). In the graphs below, I’ve removed the data from some public collections (such as the NHM) where I think there is too much bias towards rarer species and focussed on finds from local collectors (particularly those who have retained the bulk of their collections). A big thanks for David & Martin Raynor, Dan Hogburn, Fred Clouter, Tony Vale, Tony Mitchell, Thomas Wells, Gary Walker, James Black and Sam Goldstone who’s collections, in addition to my own, have been collated together to produce this ‘directional’ frequency analysis.
Of all the London Clay exposures, the Isle of Sheppey offers the greatest diversity of fish species with over 80 bony fish species described to date from this location. It is however important to remember, that compared to other sites (like Tankerton and Seasalter), Sheppey has one of the largest exposures of divisions of the London Clay (covering lower and upper division D and E) and therefore covers a far greater time period – perhaps around 1+ million years. During that million years, sea levels will have changed significantly with multiple marine transgressions (rising sea levels) and marine regressions (falling sea levels). As such, fossil fish finds on Sheppey represent many differing marine ecosystems (and sea depths) each supporting different fish species occupying their respective ecological niches. In other words, Sheppey isn’t a snapshot of one marine ecosystem, it’s a snapshot of many over a long period of time.
Despite the diversity of fish that can be found, the following 5 species make up over half (52%) of all finds. These are ranked with the most common at the top:
As such, if you’ve found a skull on the beaches in Sheppey and you are looking for an identification, there’s a 50/50 chance it is going to be one of the species listed above.
The other notable point from the chart below is that ‘undescribed’ still represents around 5-6% of fossil fish finds (highlighted in bright red). This again illustrates the diversity of bony fish in these waters and also means that if you’ve a fish skull find, there is still a 1 in 20 chance that it might represent a fish that is yet to be described. There are a number of examples of undescribed fish within this website.
As with Sheppey, Seasalter offers significant diversity of fish finds but they cover a shorter time period. The below information is based on reviewing 134 clearly identifiable fish skulls found in Seasalter. The most frequent find at Seasalter, representing nearly half of fossil fish finds, is Halecopsis insignis – an extinct form of shoaling milkfish. Modern milkfish can be found in tropical waters, generally around the edge of the continental shelf at depths of 1-30m. In addition to Halecopsis, Eocoelopoma, Scombrinus nuchalis and Sciaenurus bowerbanki are some of the other fish that it is possible to find at this location. This is also clear evidence of Billfish present in these divisions.
Tankerton is an unusual site for fossil fish as it is dominated by one species – Rhinocephalus planiceps – which makes up nearly 90% of the fish skull finds from this location. Rhinocephalus planiceps is very comparable to modern Hake which generally live in towards the bottom of deeper water during the daytime (depths of 50-300m) and move to shallower waters at night. There are very few other species that have to date been found in this location.
Below is a work-in-progress table showing the species that have been recorded at each London Clay exposure. This is based on reviewing approximately 1,250 identifiable skull specimens across numerous public and private collections. Those highlighted in GREEN are those which are the most common in that location.
