I think what you are looking at is some form of coral but I could not tell you which one, or an age for it I'm afraid.
The circular structure is an individual polyp. Around the mouth of the polyp in the centre of the circle shape there would have been dozens of tentacles pulling food into the mouth. Some corals are solitary, others group together in colonies such as with this very similar one in a pebble I found on Deal beach many years ago. Deal is also on the south coast of the UK; my fossil looks so much like yours I wonder if it is the same species and date?
Here's a nice little diagram showing the structure of a coral polyp for you:
Thanks for the info Phil! You are right, your pebble looks very much like mine. I found another one too, with the fossil in a strange angle:
I thought it might have been where a sea urchin had been attached, but your theory sounds much better. Speaking of urchins I found this pebble which I think might have been an urchin judging by the rows of dots, what do you think? It's only 1 cm across. (sorry, it's a bit dark.)
Absolutely Christian, that certainly is a sea urchin. You can usually tell by looking for the five fold symmetry characteristic of most of the members of the echinoids (and the echinoderms in general). The dots on the surface were tubercles and would have held the ball-and-socket arrangement at the point of attachment for the spines. The spines are not only a defensive measure, many forms of sea urchin use them for walking. The mouth would have been located on the underside of your fossil as with modern species.
Is that urchin preserved in flint? If so, it is probably of Late Cretaceous date, i.e 100-65 mya (I think) when chalk and flint was being deposited right across the southern UK. Sussex was alternately covered by sea and tropical deltas at that point in time. A lucky find; did it come from Brighton too?
Attached is a picture of various fossils of sea-urchins all preserved in flint. All were picked up on local beaches or in the fields around here in SE England and would be of about this date.
By the way, those pictures are really clear. What sort of camera do you use?
I think it is flint, I found it at the same spot as the other two. I was looking at every stone within an arms reach for about 1.5 hours while my girlfriend was bathing and working on her tan. :)
I held the small (see photo) urchin up to a light and it is somewhat translucent so I guess it is flint. Don't you agree? Your urchins are a tad bit bigger than mine... Here's a photo for size comparison. I'm a bit amazed myself I managed to found it.
Thank you for the dating of the fossil and the additional information, it really gives a perspective on things. (Not that I can grasp how long 80 million years is...)
I noticed a couple of weeks ago that if I use the macro mode on the camera and put the camera lens right agains the lens of the magnifier I can get really cool photos. You can see the clear plastic at the edges of the first photo I posted. This one is from the back of my hand, not resized, just cropped, so as you can see I can get good close-ups.
This really got me going! I was fiddling with the zoom on my camera and I managed to get this extreme close-up of the coral polyp fossil from my second post (the one that is dug halfway down into the pebble. Note that the mouth (see, I'm learning) is only 1.5 mm across! I was really amazed to see the thing sticking out from the center of the mouth! I hadn't seen that before and it is missing from the other fossil (first post). But when the light fell just right it became visible!
Sorry for the large photo and my uncontrolled enthusiasm :) , but I think I'm becoming really interested in fossils (more than earlier). I will now do a Google image search for coral fossils to see if I can find something similar to this one.
You've got me intrigued now too Christian. I did not know one could get magnifiers like this for digital cameras; I'll see if I can get hold of one myself.
I'm still looking to try to identify this thing. In the meantime here's a better close up of my pebble and I'm convinced that yours and mine are the same thing. We really need a coral expert here, but to be honest I'm having real second thoughts about that identification. I'm starting to wonder if these are plates from the exterior of a sea urchin, the central disk being the tubercle, or socket of attachment, of a spine.
Come to think of it I think this trace may be a negative, or cast of the exterior of the skin of the sea-urchin. The pebble could be a worn down concretion containing an imprint of the surface of the echinoid. That would explain the concave shape of my specimen. The tiny spike structure you noticed could be a negative impression of a hollowed out spine....
Out of curiosity I rolled up some Blue-Tac, pushed into the socket to see what a positive image would look like. I've upped the contrast a bit and turned the image to black and white. I'm now pretty convinced it's a sea-urchin plate and so by default is yours. So I think you were right all along, Christian!
This is an interambulacral plate belonging to some type of Archaeocidaris. The thing's from the Pennsylvanian (found near Brownwood, Texas), and so it's a wee bit older than Late Cretaceous. However, I think it does hint at the possibility that your fossil(s) above are impressions of some bits of cidaroid sea urchins.
Cidaroids? I'm not terribly familiar with the whole echinoderm scene, and only learned about the subclass this evening from Invertebrate Palaeontology and Evolution. According to Clarkson:
The interambulacral plates are very large, with a single large tubercle in the centre of each plate, to which a strong spine is attached.
The mamelon, or central boss, is surrounded by a wide smooth areola around which is a ring of tiny scrobicular tubercles. Outside these a series of small secondary tubercles is irregularly dispersed.
Thanks guys for all the work you've put in trying to identify the fossil! Funny I probably was right from the beginning with my urchin theory. Phil, the Blue-Tac idea was a good one. I'll try it myself on my specimen.
The magnifier I use is of a regular kind you can get almost everywhere (at least here in Sweden) where they sell things like that, I think it's meant to be used to look at stamps, insects and things like that. I seem to recall having used one of those in school when I was younger. It's not meant to be used with digital cameras, my camera lens just happens to fit very well in the magnifier.
Wow! What a resonse! Yep I agree it's certainly an Echinoid impression, and your welcome to use the Echninoid pic... put credit should be to Robert Randell www.robertrandell.discoveringfossils.co.uk - Chalk fossils.
I find lots of these impressions of Littlehampton beach (West Sussex).