I found this at Gerroa Beach today which measures 40-50mm across. It is bound together with a stone-like material which cannot be scratched with a fingernail.
What is it, and how did it form in this way?
Fossils. Congrats. :)
Dark Orange said:
Fossils. Congrats. :)
Thanks :)
I should add to my questions,
How did they likely find their way onto the beach after all that time?
The interface between the land and the ocean is extremely dynamic. Sand moves, exposing the rock layer that contains fossils, and some breaks off and gets washed up onto the beach, or washed somewhere else, or washed up onto the beach and covered in sand and then uncovered again.
I would take a guess by the amount of wear on that piece that is has been moving around for a while after being freed from its base rock.
(Not in any way an expert)
bivalves in a well cooked sandstone matrix?
Is the concretion of the sand caused by the calcium leaching from the shells?
Rule 303 said:
Is the concretion of the sand caused by the calcium leaching from the shells?
Having seen a lot of this stuff in opal mines, I’d have a guess that you are most likely on the money.
Probably need to lick it to be sure
Where is MV? He’ll know all about it.
I do know that plesiosaurs and pliosaurs would scoop up belemnites and vomit up masses of the bullet-shaped structures called guards. These were also internal structures, and they formed the tip of the phragmacone at the ‘tail end’ of the animals.
Just like owls pellets in a way.
I wonder if they also ate sea snails?
dv said:
bivalves in a well cooked sandstone matrix?
Who cooked it do you think?
Is there a possibility it was from a midden so old it stuck together?
roughbarked said:
I do know that plesiosaurs and pliosaurs would scoop up belemnites and vomit up masses of the bullet-shaped structures called guards. These were also internal structures, and they formed the tip of the phragmacone at the ‘tail end’ of the animals.
Just like owls pellets in a way.
I wonder if they also ate sea snails?
Sorry, bivalves.
roughbarked said:
dv said:
bivalves in a well cooked sandstone matrix?
Who cooked it do you think?
Is there a possibility it was from a midden so old it stuck together?
The shells are still in pairs, so probably not.
dv said:
bivalves in a well cooked sandstone matrix?
Brachiopods
Brachiopods, phylum Brachiopoda, are a group of lophotrochozoan animals that have hard “valves” on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection.
PermeateFree said:
dv said:
bivalves in a well cooked sandstone matrix?
Brachiopods
Brachiopods, phylum Brachiopoda, are a group of lophotrochozoan animals that have hard “valves” on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection.
For the benefit of those who didn’t know:
Fossil brachiopods are useful tools for the geologist. They are generally robust and relatively abundant, and display considerable morphological diversity. In some Palaeozoic rocks that formed in shallow water, brachiopods are very abundant and may make up the bulk of the rock in which they occur. They provide an exceptionally rich record of almost the whole history of an animal phylum, and are therefore valuable in the study of evolutionary processes as well as in palaeoecology, palaeobiogeography and stratigraphy.
They are particularly suitable for palaeoecological analyses because of their occurrence in different sedimentary facies. They lived on the sea floor where marine sedimentary rocks form. They were evidently susceptible to differences in environment as often recorded by the surrounding sediments. Their shell features reflect physiological differences relating to their way of life, and there are sufficient living brachiopods to provide useful evidence of functional morphology and lifestyle.
They can also be useful stratigraphical indicators and tools for correlation in sedimentary rocks because certain lineages evolved relatively rapidly. With a basic knowledge of brachiopods, a keen observer can identify relative ages of strata to at least the level of period, especially in Palaeozoic rocks. More specialised knowledge is needed for Mesozoic rocks because Mesozoic-aged brachiopods show less external morphological diversity and are relatively rare.
Influenced by such factors as water depth, salinity, oxygen levels and static lifestyle, the distribution patterns of fossil brachiopods provide a useful tool in deducing the position of ancient shorelines and the past distribution of land and sea.
Cambrian rocks characteristically contain a diverse and abundant brachiopod fauna, at first completely dominated by chitinophosphatic species but later joined by various calcite-shelled forms. These prelude a dramatic diversification such that brachiopods, together with trilobites, are the primary stratigraphical guide fossils in the shallow-water facies of the Ordovician; they are also important in the Silurian and Devonian. In the Carboniferous, they have been of some stratigraphical value, especially at a local level, when combined with corals. Many major groups of brachiopods became extinct at the end of the Palaeozoic.
Speedy said:
I found this at Gerroa Beach today which measures 40-50mm across. It is bound together with a stone-like material which cannot be scratched with a fingernail.What is it, and how did it form in this way?
roughbarked said:
For the benefit of those who didn’t know:Fossil brachiopods are useful tools for the geologist. They are generally robust and relatively abundant, and display considerable morphological diversity. In some Palaeozoic rocks that formed in shallow water, brachiopods are very abundant and may make up the bulk of the rock in which they occur. They provide an exceptionally rich record of almost the whole history of an animal phylum, and are therefore valuable in the study of evolutionary processes as well as in palaeoecology, palaeobiogeography and stratigraphy.
They are particularly suitable for palaeoecological analyses because of their occurrence in different sedimentary facies. They lived on the sea floor where marine sedimentary rocks form. They were evidently susceptible to differences in environment as often recorded by the surrounding sediments. Their shell features reflect physiological differences relating to their way of life, and there are sufficient living brachiopods to provide useful evidence of functional morphology and lifestyle.
They can also be useful stratigraphical indicators and tools for correlation in sedimentary rocks because certain lineages evolved relatively rapidly. With a basic knowledge of brachiopods, a keen observer can identify relative ages of strata to at least the level of period, especially in Palaeozoic rocks. More specialised knowledge is needed for Mesozoic rocks because Mesozoic-aged brachiopods show less external morphological diversity and are relatively rare.
Influenced by such factors as water depth, salinity, oxygen levels and static lifestyle, the distribution patterns of fossil brachiopods provide a useful tool in deducing the position of ancient shorelines and the past distribution of land and sea.
Cambrian rocks characteristically contain a diverse and abundant brachiopod fauna, at first completely dominated by chitinophosphatic species but later joined by various calcite-shelled forms. These prelude a dramatic diversification such that brachiopods, together with trilobites, are the primary stratigraphical guide fossils in the shallow-water facies of the Ordovician; they are also important in the Silurian and Devonian. In the Carboniferous, they have been of some stratigraphical value, especially at a local level, when combined with corals. Many major groups of brachiopods became extinct at the end of the Palaeozoic.
You found brachiopod fossils. Wow.
> they are also important in the Silurian and Devonian. In the Carboniferous
The timing may match the area around Gerroa Beach. Parts of the Illawarra are famous for Carboniferous fossils.
http://www.fossils.com.au/opalised.html
https://australian.museum/learn/australia-over-time/fossils/sites/common-fossils-of-the-sydney-basin/
Very interesting that Jstor doesn’t list the ANL as an institution able to access documents. https://www.jstor.org/institutionSearch?redirectUri=%2Fstable%2F27670396&refreqid=excelsior%3A8b4db7ef63f3dbe460f6d4c313a08c7a
roughbarked said:
Very interesting that Jstor doesn’t list the ANL as an institution able to access documents. https://www.jstor.org/institutionSearch?redirectUri=%2Fstable%2F27670396&refreqid=excelsior%3A8b4db7ef63f3dbe460f6d4c313a08c7a
I was able to access a lot from ANL, I was a member for a year.
mollwollfumble said:
roughbarked said:
Very interesting that Jstor doesn’t list the ANL as an institution able to access documents. https://www.jstor.org/institutionSearch?redirectUri=%2Fstable%2F27670396&refreqid=excelsior%3A8b4db7ef63f3dbe460f6d4c313a08c7a
I was able to access a lot from ANL, I was a member for a year.
Yes this was so.
Speedy said:
I found this at Gerroa Beach today which measures 40-50mm across. It is bound together with a stone-like material which cannot be scratched with a fingernail.What is it, and how did it form in this way?
As others have said: fossils in sandstone.
The yellowish sandstone is not one I remember from Gerroa.
Gerroa headland platform fossils (Permian marine) are in a dark grey siltstone, which contains a suite of brachiopods, crinoids and bryozoans amongst other things. Note that fossil collecting from the Gerroa Headland (Black Head) is now illegal, as it is a registered geological preserve site. But it’s well worth having a look at the exquisite preservation of some fossils there.
I wouldn’t be surprised if your fossils were much younger – maybe something Miocene or younger, from a near-shore sandy reef or shell-bank. From the photos, the sea-shell valves seem the same size. Brachiopods have a larger valve and a smaller valve; they have a different symmetry to bivalves. Below shows the symmetry differences:
I think they are likely bivalves – likely some type of clam, but I can’t be certain without having the specimen in my hand.
As for forming – shells in sand, followed by the deposition of a cement to bind all the grains together. That cement might be calcium carbonate or silica (you’d need to test for hardness – a pocket knife won’t scratch silica cement) along with some iron oxy-hydroxides – to give it the yellow colouration.
Thanks for all your replies.
I wondered at the time whether the taking of this specimen was legal, but only because I was unsure whether we had reached the Seven Mile Beach National Park boundary. Either way, it was just washing around in the loose sand on a long beach, and I figured if I didn’t appreciate it no-one ever would.
The shell is obviously much stronger than the sandstone which has worn down around it, so this is not original shell, right?
Speedy said:
Thanks for all your replies.I wondered at the time whether the taking of this specimen was legal, but only because I was unsure whether we had reached the Seven Mile Beach National Park boundary. Either way, it was just washing around in the loose sand on a long beach, and I figured if I didn’t appreciate it no-one ever would.
The shell is obviously much stronger than the sandstone which has worn down around it, so this is not original shell, right?
Not necessarily. The matrix of sandstone can be more labile than calcium carbonate.
dv said:
Speedy said:
Thanks for all your replies.I wondered at the time whether the taking of this specimen was legal, but only because I was unsure whether we had reached the Seven Mile Beach National Park boundary. Either way, it was just washing around in the loose sand on a long beach, and I figured if I didn’t appreciate it no-one ever would.
The shell is obviously much stronger than the sandstone which has worn down around it, so this is not original shell, right?
Not necessarily. The matrix of sandstone can be more labile than calcium carbonate.
Thanks dv. Labile is a new word for me.
The shell material seems so much more robust than any other shell that I found on the beach. Maybe they made ‘em tougher back in those days :)
Speedy said:
dv said:
Speedy said:
Thanks for all your replies.I wondered at the time whether the taking of this specimen was legal, but only because I was unsure whether we had reached the Seven Mile Beach National Park boundary. Either way, it was just washing around in the loose sand on a long beach, and I figured if I didn’t appreciate it no-one ever would.
The shell is obviously much stronger than the sandstone which has worn down around it, so this is not original shell, right?
Not necessarily. The matrix of sandstone can be more labile than calcium carbonate.
Thanks dv. Labile is a new word for me.
The shell material seems so much more robust than any other shell that I found on the beach. Maybe they made ‘em tougher back in those days :)
I mean there is a simple test you can do to find out if it is still calcareous but it will destroy/damage part of the shell.
Speedy said:
dv said:
Speedy said:
Thanks for all your replies.I wondered at the time whether the taking of this specimen was legal, but only because I was unsure whether we had reached the Seven Mile Beach National Park boundary. Either way, it was just washing around in the loose sand on a long beach, and I figured if I didn’t appreciate it no-one ever would.
The shell is obviously much stronger than the sandstone which has worn down around it, so this is not original shell, right?
Not necessarily. The matrix of sandstone can be more labile than calcium carbonate.
Thanks dv. Labile is a new word for me.
The shell material seems so much more robust than any other shell that I found on the beach. Maybe they made ‘em tougher back in those days :)
Without using a hand lens on the “shells” it’s difficult to tell whether the original calcium carbonate has been replaced. Even then it might still be difficult to tell.
dv said:
Speedy said:
dv said:Not necessarily. The matrix of sandstone can be more labile than calcium carbonate.
Thanks dv. Labile is a new word for me.
The shell material seems so much more robust than any other shell that I found on the beach. Maybe they made ‘em tougher back in those days :)
I mean there is a simple test you can do to find out if it is still calcareous but it will destroy/damage part of the shell.
Carbonate can be replaced by carbonate with a different-to-original crystal size and stacking structure. So using a weak acid solution is not really going to tell you much at all, unless it has been replaced by silica (and it doesn’t look like that has happened, judging by the image).
Michael V said:
Speedy said:
dv said:Not necessarily. The matrix of sandstone can be more labile than calcium carbonate.
Thanks dv. Labile is a new word for me.
The shell material seems so much more robust than any other shell that I found on the beach. Maybe they made ‘em tougher back in those days :)
Without using a hand lens on the “shells” it’s difficult to tell whether the original calcium carbonate has been replaced. Even then it might still be difficult to tell.
Thanks MV. I guess we will never know :)
Speedy said:
Michael V said:
Speedy said:Thanks dv. Labile is a new word for me.
The shell material seems so much more robust than any other shell that I found on the beach. Maybe they made ‘em tougher back in those days :)
Without using a hand lens on the “shells” it’s difficult to tell whether the original calcium carbonate has been replaced. Even then it might still be difficult to tell.
Thanks MV. I guess we will never know :)
Maybe if you can find the fossil bed it came from?
Speedy said:
Michael V said:
Speedy said:Thanks dv. Labile is a new word for me.
The shell material seems so much more robust than any other shell that I found on the beach. Maybe they made ‘em tougher back in those days :)
Without using a hand lens on the “shells” it’s difficult to tell whether the original calcium carbonate has been replaced. Even then it might still be difficult to tell.
Thanks MV. I guess we will never know :)
bring it to the next pud
dv said:
Speedy said:
Michael V said:Without using a hand lens on the “shells” it’s difficult to tell whether the original calcium carbonate has been replaced. Even then it might still be difficult to tell.
Thanks MV. I guess we will never know :)
bring it to the next pud
Next? I’ve never been to one :)
Speedy said:
dv said:
Speedy said:Thanks MV. I guess we will never know :)
bring it to the next pud
Next? I’ve never been to one :)
never too late to start
