Plate of Jurassic crinoid "Pentacrinites fossilis"

[Bruce Williams]

Hi folks,

This is a plate of a 200 million year old crinoid called Pentacrinites fossilis collected from the beach at Charmouth in Dorset (south coast of England). This area is the World Heritage site called "Jurassic Coast". The fossil was prepared by Michael Harrison.

At maximums it measures 134mm x 53mm.

Crinoids were/are echinoderms related to the sea urchin and the star fish and they exhibit the same five-part topology. In life this Pentacrinites fossilis would have been anchored to the sea-floor substrate by a long stem of similar structure to the arms (ie, a stack of disk-like ossicles). At the top of the stem was a cup shaped head or calyx from which grew 5 groups of branching, articulated arms. The arms carried fine pinnules that bore food gathering tube-feet (these bore the food to the mouth).

See this drawing (A) to get an idea of what the whole animal looked like. See also drawing (B) of a related species.

This particular fossil includes 2 incomplete crinoids. It can be a bit difficult to make out the structure so I have uploaded a colour coded version of the image to help make sense of the structure.

Unfortunately with both crinoids the cup shaped calyx is missing as are the stems. There is however one large calyx plate (coloured yellow) supporting one of the main arms. I have coloured one complete arm blue. I have also coloured a selection of pinnules green. Although they look circular, the ossicles are in fact 5 sided (see drawing A) and see also the red coloured disk on my coloured image.

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[beetleman]

A really fantastic looking fossil Bruce. I love crinoid fossils. The modern day feather starfish are very interesting also. I posted a fossil of mine a while back also (yours is much nicer)
http://www.photomacrography.net/forum/v ... ht=crinoid

[Ken Ramos]

An interesting piece of history there Bruce. A few hours ago, I finished watching BBC "Space," I forgot the name of the narriator, but I am sure some of you may have seen or watched this program. It is all about our planet and what we can expect in the future of it, which is pretty dire by the way, we are all going to die if we don't get out of here but not for another 28 or 29 million years yet, so take yer time packing. Any way this fellow said that the earth has a history of being "whacked" (my terminology not his) every 30 million years or so by some very large astroid or a comet traveling at around 40 kilometers per sec., which in turns wipes out almost entirely all life on the planet, the dinosaurs for instance. So, I wonder now, how these things survived the cataclysims that came about every 30 million years or so, though they are more than likely carbon dated back to 200 million years, carbon dating which of course from what I have read is really not all that accurate, going on assumptions of half lifes and such.

[Bruce Williams]

Thanks for your comments guys,

Doug - I had a good long look at yout posting. Wow that plate has got so much on it - nice detail too!

I am always on the lookout for a reasonably priced (well detailed) crinoid complete with stalk, head and branches - but I don't think they exist - reasonably priced ones that is

Ken - Yes it would be interesting to know which animal species alive today has existed the longest. In fact it would be interesting to know the same for genus and family. I know in the case of the ammonite that individual species typically lasted no more than 2 million years (so are useful for accurate dating of rock strata). In the case of my crinoid fossil I would imagine the reverse process was used, ie that it would have been dated from the Jurassic rock strata that it was found in.

I believe that radiocarbon dating can only be used to date back about 60,000 years at which point the radioactivity of the residual carbon 14 becomes too close to background radiation. Yes you're right carbon 14 dating needs to be calibrated to take into account the varing levels of solar radiation (and other factors) over the 60,000 years period. I have no idea what the degree of error is with this method of dating? This is one for Rik I think?

Bruce

[MacroLuv]

Beautiful piece of fossil Bruce! And very well photographed.

... over the 60,000 years period. I have no idea what the degree of error is with this method of dating? This is one for Rik I think?

Bruce

Ohh really? I thought Earth was created some 6000 years ago, about 1000 years after Sumerians invented glue in Mesopotamia.

[Mike B in OKlahoma]

Thanks for posting these, I spent many hours collecting fossils of these in sandstone as a kid....Hmm, I think I still have a couple of those (not as attractive or clean specimens as these) I may have to pull them out and take some shots.

Carbon 14 has a half-life of 5730 years, and after ten half-lives, an isotope is completely gone (for practical purposes) so there's nothing left to measure (and it gets less precise as you go back past a couple of half-lives). They can do some dating for longer periods by analyzing other isotopes with large half-lives that are created by cosmic ray bombardment of atoms in the atmosphere (added later, I think that's the Argon method Rik mentioned). I just looked it up, and for very old rocks, looks like the decay of Uranium is used. This looks like a pretty good article:

http://en.wikipedia.org/wiki/Radiometric_dating

The part on "modern dating techniques" looks particularly relevant for this type of dating.

[Mike B in OKlahoma]

By the way, aquarium displays on horsehoe crabs invariably mention that they are one of the oldest living species of non-microscopic critter. I just checked wikipedia, and it says "they haven't changed much in the last 350 or 400 million years". What's 50 million years more or less among friends? :-)

[rjlittlefield]

Ken - Yes it would be interesting to know which animal species alive today has existed the longest. In fact it would be interesting to know the same for genus and family. I know in the case of the ammonite that individual species typically lasted no more than 2 million years (so are useful for accurate dating of rock strata). In the case of my crinoid fossil I would imagine the reverse process was used, ie that it would have been dated from the Jurassic rock strata that it was found in.

I believe that radiocarbon dating can only be used to date back about 60,000 years at which point the radioactivity of the residual carbon 14 becomes too close to background radiation. Yes you're right carbon 14 dating needs to be calibrated to take into account the varing levels of solar radiation (and other factors) over the 60,000 years period. I have no idea what the degree of error is with this method of dating? This is one for Rik I think?

Bruce

Bruce,

First, let me say that's a very clever trick, cutting a single large image into 3 chunks that individually fit within the forum limits. I might have to use that one myself on occasion!

About dating, the writeups at Wikipedia look pretty good. Look particularly at http://en.wikipedia.org/wiki/Radiocarbon_dating , http://en.wikipedia.org/wiki/Potassium-argon_dating , and http://en.wikipedia.org/wiki/Argon-argon_dating .

Short story is that carbon dating works directly on carbon-containing specimens (not fossils), works back to 60,000 years or so, and has maybe 1-2% accuracy for the first 25,000 of those, after proper calibration. The signal-to-noise ratio changes by 2X every half-life, so those 6 half-lives between 25 and 60 thousand years represent a factor of 64X increase in error.

For long-term stuff, you have to date the rocks, not the fossils. The techniques that I usually hear about are potassium-argon and argon-argon. The basic scheme of potassium-argon is that a particular isotope of potassium decays to a particular isotope of argon with a half-life of 1.26 billion years. When a rock is melted, any argon in it escapes, so the ratio of potassium to argon measures the time since the rock was last molten. In this case, the half-life is so long that large ages are not a problem, but short ages can be because not enough argon accumulates to be accurately measured. Ballpark, again, is generally considered to be a few percent error.

All of these dating techniques are based on assumptions, of course, and you can get wildly different results depending on the assumptions you throw in. I am always intrigued to note that the top Google hit for radioisotope dating is to www.creation-science-prophecy.com . There must be a lot of links to that site! But I digress. Let's not get a debate started.

--Rik

[Danny]

Older than me..........almost. Great 3D like quality presentation Bruce. The depth from the subtle shadows work and one very interesting post. Thanks Bruce

Danny.

[Bruce Williams]

Thanks for the comments guys.

Nikola - 6000 years e... The Biblical Age of the Earth - bit of a "sticky" subject that!

Mike - Unfortunately I didn't collect this one myself - it would be even more of a treasure if I had. Thanks for the info and link. Your horseshoe crab suggestion sent me on a Google browse that lasted a good hour and alerted me to the (to me) new concept of the "Lazerus Taxon" - so thanks for that one

Hey...and how come you knew that Rik was going to mention the argon method of dating rocks?

Rik - Thanks for your clear and interesting explanation on radiometric dating methods and limitations - still following up on your leads.

Danny - Very encouraging and much appreciated comments.

Bruce

[MacroLuv]

Bruce, from now I will be more careful picking up shore pebbles, who knows how many fossils I threw back to the sea.
By the way, did you see (in nature) or even have stromatolite fossil? They are the most old fossils albeit there are "fresh" stromatolites on some limited places.
Here is link for stromatolite fossils but almost all nice pieces already sold.

[cactuspic]

Beautiful job Bruce. The lighting makes the image sparkle and pop. Wonderful photo with a great story.

Irwin

[Mike B in OKlahoma]

Mike - Unfortunately I didn't collect this one myself - it would be even more of a treasure if I had. Thanks for the info and link. Your horseshoe crab suggestion sent me on a Google browse that lasted a good hour and alerted me to the (to me) new concept of the "Lazerus Taxon" - so thanks for that one

Hey...and how come you knew that Rik was going to mention the argon method of dating rocks?

I originally thought the argon was the main method for dating rocks, so wrote the phrase about atmospheric gasses, but then went and googled and saw reference to Uranium being used (which made perfect sense once I thought about it) and mentioned that, but forgot that I already had mentioned atmospheric gasses....I went back later and looked, reread what I'd written, and read what Rik wrote, so added the phrase in parentheses about "that's the argon method Rik wrote about".

No psychic powers here, or I'd be off playing the lottery instead of posting on the internet!

I hadn't heard of "lazarus taxon", but it was interesting to look up. That lead me to "Elvis taxon", which made me laugh out loud. :-)

[Bruce Williams]

Nikola - Until your posting I had not realised you could get beautiful looking, polished stromatolite fossils. The only ones I'd seen before following your link were like these currently for sale on eBay which, although interesting for what they tell us about the distant past of our planet, had not inspired me to part with hard cash. To be honest they reminded me of burst bubbles in a hot mud pool. But now I'm very tempted...

Irwin - Thanks for your kind and encouraging words.

Mike - Now why hadn't I figured that out for myself

Bruce