Crabs Vs Trilobites

[Friar Broccoli]

CH4ShotCaller found this crab fossil:

http://www.thefossil...ttach_id=168029

in this thread:

http://www.thefossil...s-of-the-month/

When having "discussions" with creationists (who think Noah's flood explains fossil deposition), one of the points I often make is that trilobite and crab fossils are never found together (and/or the same for large mammals and large dinosaurs etc). In that context I have the following questions:

1) Is there anything obvious about CH4ShotCaller's crab that makes it obviously different from any crab that lives today?

2) Does anyone here know of any sites where rocks containing crab fossils lie directly above (or to be unbiased: below) trilobite fossils?

Thanks

[FossilDAWG]

Crabs are relatively advanced decapod crustaceans generally characterized by a broad flattened cephalothorax (carapace) and a reduced abdomen that is tucked tightly underneath the carapace. The first true crabs appeared in the upper Jurassic, long after the extinction of the trilobites at or near the end of the Permian. The only way strata with crabs would directly overlie strata with trilobites is if there was an unconformity such that all of the Triassic, and much of the Jurassic, was missing. Also, although crabs and trilobites are both crustaceans, crabs did not evolve from trilobites; rather, they both evolved from a common ancestor and the lineages diverged in the Precambrian or earliest Cambrian, before the oldest trilobite fossils.

By the Eocene/Oligocene, the age of the Washington State crab fossils, crabs had evolved their modern morphology. Although they differ from modern crabs at the level of species and genus, many can be assigned to still-living families. So, although they differ from modern crabs in their details, in most cases there isn't any feature that would be obvious to a non-specialist that marks Eocene and Oligocene crabs as different from modern crabs. Even Cretaceous crabs are easily recognized as crabs by non-specialists. The Jurassic ones do look weird, though, like strange flattened lobsters with really short tails.

Don

[DLB]

Crabs are relatively advanced decapod crustaceans generally characterized by a broad flattened cephalothorax (carapace) and a reduced abdomen that is tucked tightly underneath the carapace. The first true crabs appeared in the upper Jurassic, long after the extinction of the trilobites at or near the end of the Permian. The only way strata with crabs would directly overlie strata with trilobites is if there was an unconformity such that all of the Triassic, and much of the Jurassic, was missing. Also, although crabs and trilobites are both crustaceans, crabs did not evolve from trilobites; rather, they both evolved from a common ancestor and the lineages diverged in the Precambrian or earliest Cambrian, before the oldest trilobite fossils.

By the Eocene/Oligocene, the age of the Washington State crab fossils, crabs had evolved their modern morphology. Although they differ from modern crabs at the level of species and genus, many can be assigned to still-living families. So, although they differ from modern crabs in their details, in most cases there isn't any feature that would be obvious to a non-specialist that marks Eocene and Oligocene crabs as different from modern crabs. Even Cretaceous crabs are easily recognized as crabs by non-specialists. The Jurassic ones do look weird, though, like strange flattened lobsters with really short tails.

Don

Well said Don. Crabs rule trillos drool!!!!! Ha! Ha!

Edited November 22, 2012 by DLB

[MB]

Nothing else to say

[CH4ShotCaller]

The only way strata with crabs would directly overlie strata with trilobites is if there was an unconformity such that all of the Triassic, and much of the Jurassic, was missing. Also, although crabs and trilobites are both crustaceans, crabs did not evolve from trilobites; rather, they both evolved from a common ancestor and the lineages diverged in the Precambrian or earliest Cambrian, before the oldest trilobite fossils.

By the Eocene/Oligocene, the age of the Washington State crab fossils, crabs had evolved their modern morphology. Although they differ from modern crabs at the level of species and genus, many can be assigned to still-living families. So, although they differ from modern crabs in their details, in most cases there isn't any feature that would be obvious to a non-specialist that marks Eocene and Oligocene crabs as different from modern crabs.

Don

Excellent! Thank you for the information. I knew about the huge difference in time, but you really put pen-to-paper on this comment.

[Friar Broccoli]

Below I have played around with the html quote things so it may be a mess:

Crabs are relatively advanced decapod crustaceans generally characterized by a broad flattened cephalothorax (carapace) and a reduced abdomen that is tucked tightly underneath the carapace. The first true crabs appeared in the upper Jurassic, long after the extinction of the trilobites at or near the end of the Permian. The only way strata with crabs would directly overlie strata with trilobites is if there was an unconformity such that all of the Triassic, and much of the Jurassic, was missing.

Since unconformities of 100 mya+ are not uncommon I thought someone here might know of a relevant case. Since my practical knowledge of geology/fossils is next to non-existent I don't know how likely sedimentary layers on either side of such a gap are to contain any type of fossils (let alone trilobites and crabs in the relevant layers).

Also, although crabs and trilobites are both crustaceans, crabs did not evolve from trilobites; rather, they both evolved from a common ancestor and the lineages diverged in the Precambrian or earliest Cambrian, before the oldest trilobite fossils.

If my ancestors had had calcite lens eyes and evolution had taken them from me, I'd demand my money back.

By the Eocene/Oligocene, the age of the Washington State crab fossils, crabs had evolved their modern morphology. Although they differ from modern crabs at the level of species and genus, many can be assigned to still-living families. So, although they differ from modern crabs in their details, in most cases there isn't any feature that would be obvious to a non-specialist that marks Eocene and Oligocene crabs as different from modern crabs. Even Cretaceous crabs are easily recognized as crabs by non-specialists. The Jurassic ones do look weird, though, like strange flattened lobsters with really short tails.

Thanks. Interesting. I'll look at some Mesozoic crabs.

Don

[Friar Broccoli]

Nothing else to say

I briefly saw some interesting images in your site:

http://mbfossilcrab.multiply.com/

but now I'm seeing only error messages. I will try again.

Thanks

[piranha]

...Also, although crabs and trilobites are both crustaceans, crabs did not evolve from trilobites; rather, they both evolved from a common ancestor and the lineages diverged in the Precambrian or earliest Cambrian, before the oldest trilobite fossils...

Common ancestor yes, although trilobites are not placed in the Crustacea. They are in a distinct subphylum: Trilobitomorpha.

[Bullsnake]

So, where do Horseshoe crabs fit in? I've heard of them found in the Pennsylvanian. Same ancestor, different branch?

[piranha]

So, where do Horseshoe crabs fit in? I've heard of them found in the Pennsylvanian. Same ancestor, different branch?

Subphylum: Chelicerata

[Friar Broccoli]

So, where do Horseshoe crabs fit in? I've heard of them found in the Pennsylvanian. Same ancestor, different branch?

Subphylum: Chelicerata

I was extremely surprised to see that spiders are the nearest relatives to horseshoe crabs.

Also, what was your source for:

http://www.thefossilforum.com/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=168081

It looks like a scan from a book.

After an hour of looking, the impression I have of recent thinking is that Trilobites probably fit within Paradoxopoda as shown in the rightmost diagram here:

http://en.wikipedia.org/wiki/Chelicerata#Relationships_with_other_arthropods

Apparently within Myriapoda according to this:

http://en.wikipedia.org/wiki/Arthropod#Classification

This seems to agree with your tree which seems fossil rather than genetic based.

[Boneman007]

I have found that the best way to deal with creationists is to simply nod your head, smile, and walk away.

To a christian, anything should be possible. So to try to limit life on earth to 6,000, they are simply being closed minded to begin with, so there really is no point to having that conversation.

As a christian, I simply say, if pushed, "Darwin was right... God created evolution." It seams to end the conversation without room for further statements.

[Auspex]

Faith cannot disprove science, anymore than science can argue faith. Steven J. Gould published an essay called "Non-Overlapping Magisteria", which sums up, quite logically, the appropriate relationship between science and faith:

"The magisterium of science covers the empirical realm: what the Universe is made of (fact) and why does it work in this way (theory). The magisterium of religion extends over questions of ultimate meaning and moral value. These two magisteria do not overlap, nor do they encompass all inquiry (consider, for example, the magisterium of art and the meaning of beauty)."

The futility of argument between the magesteria is beautifully exposed by Gould's careful logic.

[Quarryman Dave]

^{Very interesting discussion when talking about religion and science}

[piranha]

I was extremely surprised to see that spiders are the nearest relatives to horseshoe crabs.

Also, what was your source for:

http://www.thefossil...ttach_id=168081

It looks like a scan from a book.

After an hour of looking, the impression I have of recent thinking is that Trilobites probably fit within Paradoxopoda as shown in the rightmost diagram here:

http://en.wikipedia....ther_arthropods

Apparently within Myriapoda according to this:

http://en.wikipedia....#Classification

This seems to agree with your tree which seems fossil rather than genetic based.

The Wiki-Cladograms are fine for a general overview but require additional cross-referencing to see the exact relationship of trilobites. The current template places chelicerates and trilobites in the Superclass: Arachnomorpha. The ‘Cheliceramorpha’ include all taxa more closely allied with Chelicerata than Trilobita. The Trilobitomorpha include all trilobite-like forms more closely related to the Trilobita than the Chelicerata. There has been some variance in the last 60 years among the specialists about the precise application of these terms but this basic formula seems to have overall consensus presently.

Here is the paper that was referenced above: LINK

[Boneman007]

Faith cannot disprove science, anymore than science can argue faith. Steven J. Gould published an essay called "Non-Overlapping Magisteria", which sums up, quite logically, the appropriate relationship between science and faith:

"The magisterium of science covers the empirical realm: what the Universe is made of (fact) and why does it work in this way (theory). The magisterium of religion extends over questions of ultimate meaning and moral value. These two magisteria do not overlap, nor do they encompass all inquiry (consider, for example, the magisterium of art and the meaning of beauty)."

The futility of argument between the magesteria is beautifully exposed by Gould's careful logic.

Pure Genius!

[Ramo]

I took this picture the other day when I was in DE. Hard to believe this is more closely related to a spider than a crab!!

Ramo

[piranha]

I took this picture the other day when I was in DE. Hard to believe this is more closely related to a spider than a crab!!

Ramo

The joy of phylogeny...

[CH4ShotCaller]

The joy of phylogeny...

LOL! Those are great emoticons! Didn't you have another one for the spammers with 30mm machine guns?! Good ones!

[FossilDAWG]

Common ancestor yes, although trilobites are not placed in the Crustacea. They are in a distinct subphylum: Trilobitomorpha.

D'oh! I was thinking "arthropod"as I typed "crustacean".

Don

[Friar Broccoli]

I was extremely surprised to see that spiders are the nearest relatives to horseshoe crabs.

Also, what was your source for:

http://www.thefossil...ttach_id=168081

It looks like a scan from a book.

After an hour of looking, the impression I have of recent thinking is that Trilobites probably fit within Paradoxopoda as shown in the rightmost diagram here:

http://en.wikipedia....ther_arthropods

Apparently within Myriapoda according to this:

http://en.wikipedia....#Classification

This seems to agree with your tree which seems fossil rather than genetic based.

The Wiki-Cladograms are fine for a general overview but require additional cross-referencing to see the exact relationship of trilobites. The current template places chelicerates and trilobites in the Superclass: Arachnomorpha. The ‘Cheliceramorpha’ include all taxa more closely allied with Chelicerata than Trilobita. The Trilobitomorpha include all trilobite-like forms more closely related to the Trilobita than the Chelicerata. There has been some variance in the last 60 years among the specialists about the precise application of these terms but this basic formula seems to have overall consensus presently.

Here is the paper that was referenced above: LINK

Thanks for the link to the 2002 paper. The Wikipedia authors specifically reference your Arachnomorpha description here:

http://en.wikipedia....p_to_other_taxa

and reference a 2005 paper (Heads, Hox and the phylogenetic position of trilobites by SCHOLTZ here:

http://tinyurl.com/cd489js

in support of a Mandibulata classification (which includes Myriapoda), so it's not obvious to me that there is overall consensus.

All this leads me to another question: You point out above that horseshoe crabs and spiders are now grouped together (although I assume their most recent common ancestor must be at least 400 mya). My guess is that that relationship was worked out from the DNA evidence - (Is that correct?). I further guess that prior to that DNA evidence, nobody had worked out from the morphologic evidence that spiders and horseshoe crabs were closely related - (correct?). Therefore since DNA (and probably protein) evidence for trilobites will never be available any classification of the group (more exact than within Arthropod) will forever be highly speculative. Comments?

[Auspex]

...My guess is that that relationship was worked out from the DNA evidence...

There is no ancient DNA available; the categorizations are accomplished the old-fashioned way, through comparative anatomy.

[Friar Broccoli]

I have found that the best way to deal with creationists is to simply nod your head, smile, and walk away.

To a christian, anything should be possible. So to try to limit life on earth to 6,000, they are simply being closed minded to begin with, so there really is no point to having that conversation.

As a christian, I simply say, if pushed, "Darwin was right... God created evolution." It seams to end the conversation without room for further statements.

Unfortunately,walking away doesn't seem like a workable strategy, at least not on a society wide level. Creationists are numerous and voting for politicians who what to teach creationism in schools (and unless stopped by the courts Louisiana appears to have worked out a way to actually do it http://www.huffingto..._n_1758003.html ) Leading to people that believe that Evolution and the Big Bang are 'Lies Straight From The Pit Of Hell' getting onto the House Science Committee. etc etc.

Since believing the earth is 6000 years old requires rejection of all of physics (speed of light, radio-active decay) geology and biology; rejection of all inconvenient science becomes easy. This,

it seems to me, is largely responsible for the lack of action on climate change in North America.

In short, their beliefs are affecting the rest of us.

[Friar Broccoli]

There is no ancient DNA available; the categorizations are accomplished the old-fashioned way, through comparative anatomy.

We can obtain DNA from living spiders and horseshoe crabs.

[piranha]

On 11/24/2012 at 8:47 AM, Friar Broccoli said:

...(although I assume their most recent common ancestor must be at least 400 mya)...

Actually the first trilobites appear in the fossil record at the lower Cambrian (Atdabanian) at about 525 Mya. Predating that, Parvancorina is a compelling basal arthropod from the Ediacaran interpreted as a sister group to the Arachnomorpha. The incipient cephalization of Parvancorina suggests an evolutionary path toward a division of cephalon and thoracopygidium by the early Cambrian as demonstrated in the naraoiids and further in the addition of thoracic tergites and pygidium to the basal arachnomorphs allied with trilobites.