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trilobite evolution


Walt

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"It seems likely that trilobites were preceded by soft-bodied ancestors: at several localities, sedimentary rocks with trace fossils of trilobite activity underlie the oldest rocks with trilobite body fossils." 

From http://www.ucmp.berkeley.edu/arthropoda/trilobita/trilobitafr.html

 

Hello all,

I'm looking for papers that support or dispute the above standard line about trilobite ancestors likely having soft bodies. 

 

I searched the forum but tags such as "soft body" produces papers that refer to the soft bits of hard bodied trilobites; not true soft bodied trilobites.  One or two papers, if you know of any, would be sufficient to get me on my way.

 

Also, if anyone has thoughts about this subject, I would love to hear them.  It is bothersome (at least to me) that these creatures seem to have no connection to the Proterozoic.  

Everything is generated through your own will power ~ Ray Bradbury
 

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It is My understanding that annelid worms are considered to be the ancestors of trilobites, but I can not site any papers on the subject.

Darwin said: " Man sprang from monkeys."

Will Rogers said: " Some of them didn't spring far enough."

 

My Fossil collection - My Mineral collection

My favorite thread on TFF.

 

 

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A lot of Cambrian organisms have no apparent ancestors from the Proterozoic, but they certainly did exist.

 

 

“But just in proportion as this process of extermination has acted on an enormous scale, so must the number of intermediate varieties, which have formerly existed, be truly enormous. Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against the theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.”
Charles Darwin

 

“No fossil is buried with its birth certificate. That, and the scarcity of fossils, means that it is effectively impossible to link fossils into chains of cause and effect in any valid way... To take a line of fossils and claim that they represent a lineage is not a scientific hypothesis that can be tested, but an assertion that carries the same validity as a bedtime story—amusing, perhaps even instructive, but not scientific.”
Henry Gee

 

“The idea that one can go to the fossil record and expect to empirically recover an ancestor-descendant sequence, be it of species, genera, families, or whatever, has been, and continues to be, a pernicious illusion.”
Gareth J. Nelson 

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Spriggina has been debated as a proto-trilobite or possible worm.

Seilacher's Vendobiont interpretation: a variant of Charniodiscus.

 

IMG1.jpg
Seilacher, A., & Gishlick, A.D. 2015
Morphodynamics. CRC Press, 514 pp.
 
 
Selden & Nudds: not a 'worm'
 
Spriggina is a small organism with a horseshoe-shaped ‘head’ followed by an elongate, leaf-like body composed of two rows of short segments either side of a medial line. At first, Spriggina was thought to resemble a polychaete worm such as Nereis, but a close look at the segmentation reveals that the segments do not match across the mid-line, just as in Dickinsonia. Seilacher (1989) turned the interpretation upside-down, suggesting that Spriggina could be another type of sea-pen, and that the ‘head’ was actually a holdfast.
 
Selden, P.A., & Nudds, J.A. 2012
Evolution of Fossil Ecosystems
Academic Press, 288 pp.
 
 
Seilacher: not a 'trilobite'
 
In Spriggina and related shorter forms (Parvancorina, Vendomia, Vendia, Fig. 2), in contrast, the polar difference between the two ends is very pronounced - in fact so much so that one is tempted to call them the ‘missing link’ between annelid worms and trilobite arthropods. However, there is the problem of functional transformation. The broad pleural lobes of trilobites served primarily as a rigid hood under which the legs could process the sediment for food (Seilacher 1985b). So why make such structures while there were still no legs and no rigid exoskeleton?
 
Seilacher, A. 1989
Vendozoa: organismic construction in the Proterozoic biosphere.
Lethaia, 22(3):229-239

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7 minutes ago, piranha said:

A lot of Cambrian organisms have no apparent ancestors from the Proterozoic, but they certainly did exist

Yes, most certainly they did. 

I find the beginning of the trilobite story as fascinating as most people find the end.

As always, thanks for the info!  

Everything is generated through your own will power ~ Ray Bradbury
 

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In addition to the body and trace fossils we in this Forum are familiar with, clues as to the history of every species are to be found in their  genome.  To put it simply, once two species are distinct (which means no or very limited exchange of genetic information) they will independently accumulate mutations, so the sequence of their DNA will diverge more and more over time.  Recent advances in DNA sequencing technology allows one to compare hundreds or thousands of genes, and these comparisons allow one to reconstruct the deep relationships between groups of taxa.  In addition, fossils allow us to put minimum dates on when certain groups diverged, and that allows us to put a number (within limits) on the rate at which mutations accumulate (= the "molecular clock").  In this way we can estimate when phylogenetic groups diverged. 

 

There has recently been a significant effort to do this with groups of arthropods.  In large part this is because arthropod classes and orders have had such a long evolutionary history that their relationships are difficult or impossible to determine confidently based only on morphology.  Below is a figure from one such paper (M. Schwentner, D.J. Combosch, J.P. Nelson, and G. Giribet, 2017.  A phylogenomic solution to the origin of insects by resolving crustacean-hexapod relationships.  Current Biology 27:1818-1824.)  This and other recent papers have disproven the long-standing hypothesis that insects are most closely related to the myriapods (millipedes and centipedes), and establish that the closest relative are the Remipedes, an obscure group of crustaceans that superficially look like centipedes but differ in having two pairs of antennae, gills attached to their legs, and mouthpart structure.  Anyway, if you look closely at the figure you will see that many of the branches occur before the Ediacarian/Cambrian boundary.  Arachnids (ticks, scorpions, spiders etc) diverged from the rest of the arthropods in the early Ediacarian.  Next the myriopods split from the other arthropods.  Subsequently several "crustacean" groups diverged in the middle and late Ediacarian.  The hexapods (insects, proturans, "silverfish") diverged from the remipedes in the Cambrian or early Ordovician.  Trilobites are of course not on the tree because we have no DNA from them to sequence.  However the point is that most of the major branches in the tree occur long before we have corresponding fossils.  It is clear that arthropods were in existence by Ediacarian time, and they had already radiated into a diversity of forms before they developed the size or the hard exoskeleton that caused them to be potentially fossilized.  These changes (increase in size, hard exoskeleton) are thought to be a response to more complex food webs, in particular predators, and their relatively sudden appearance gives the illusion of the "Cambrian explosion".  In reality a huge diversity of phyla, classes, orders etc were in existence for perhaps hundreds of millions of years before the Cambrian explosion.

 

Don

 

Here is the figure:

 

arthropod history.jpg

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4 minutes ago, FossilDAWG said:

...and their relatively sudden appearance gives the illusion of the "Cambrian explosion".

 

 

or an alternative turn of phrase: "Cambrian Explosion Illusion" :P

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24 minutes ago, FossilDAWG said:

their relatively sudden appearance gives the illusion of the "Cambrian explosion".  In reality a huge diversity of phyla, classes, orders etc were in existence for perhaps hundreds of millions of years before the Cambrian explosion.

Thank you, Don. 

I tend to lose sight of just how much time we are dealing with here. 

Everything is generated through your own will power ~ Ray Bradbury
 

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SPRIGGINA IS A TRILOBITOID ECDYSOZOAN

 

A pretty definitive statement.

This is from a presentation at the 2003 GSA conference.  Does anyone know if it was from a published paper?  I have searched the author but found no papers that seem to support his assertion.

Spriggina_is_a_trilobitoid_ecdysozoan.pdf

Everything is generated through your own will power ~ Ray Bradbury
 

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2 minutes ago, Walt said:

SPRIGGINA IS A TRILOBITOID ECDYSOZOAN

 

A pretty definitive statement.

This is from a presentation at the 2003 GSA conference.  Does anyone know if it was from a published paper?  I have searched the author but found no papers that seem to support his assertion.

Spriggina_is_a_trilobitoid_ecdysozoan.pdf

I believe the author makes the claim in his 2000 book, The Garden of Ediacara: Discovering the First Complex Life (Columbia UP).

...How to Philosophize with a Hammer

 

 

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The section most pertinent would be starting on page 34, "Spriggina and the Soft-Bodied Trilobite" where he covers some of the backstory controversy. Very interesting reading.

...How to Philosophize with a Hammer

 

 

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2 hours ago, Walt said:

SPRIGGINA IS A TRILOBITOID ECDYSOZOAN

A pretty definitive statement.

This is from a presentation at the 2003 GSA conference. 

 

 

As I mentioned above, the proto-trilobite theory for Spriggina is widely panned nowadays.  Spriggina does not have bilateral symmetry and remains an enigma to Ediacaran workers to this day.

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The author is known for other controversial hypotheses as well.  For example, he has published a hypothesis that the ichthyosaur bones at Berlin-Ichthyosaur State Park (in Nevada) are evidence of a Triassic giant cephalopod (kraken) that killed the ichthyosaurs and arranged the bones.

 

He has also suggested that the Cambrian trace fossil Paleodictyon is the nest of an unknown animal.  This would be 200 million years older than the oldest generally accepted nest trace fossil.

 

On the other hand he is apparently credited with naming the Precambrian supercontinent Rodinia.

 

Don

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2 minutes ago, piranha said:

 

 

As I mentioned above, the proto-trilobite theory for Spriggina is widely panned nowadays.  Spriggina does not have bilateral symmetry and remains an enigma to Ediacaran workers to this day.

Thank you.

Please understand that every question I have answered on here generates a 100 more in my head.  I wish the subject was more linear, and perhaps in a classroom it is, but for me it is a twisting and tangled path of competing theories and nearly incomprehensible scientific jargon.  (which I am gladly learning)

Some of my questions are elementary and tiring, I'm sure.  But I do try and find the answers on my own before posting.  And to understand why this theory, for example, is widely panned, I have to look at all sides of the subject.  

I will also say that it is a shame so many scientific papers are locked away from folks such as myself.  That is why I appreciate you and TFF so much.  

 

Everything is generated through your own will power ~ Ray Bradbury
 

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And there is certainly no harm in reading widely. :) Even discredited works have a value (if perhaps to help better determine the wheat from the chaff!). History has produced some great minds that do sometimes deviate into wild speculation or unhealthy obsessions. For example, the bulk of Newton's writings were on alchemy, and Georg Cantor (who pioneered set theory and the continuum hypothesis) spent a bit too much of his time subscribing to the Baconian theory of Shakespearian authorship. 

...How to Philosophize with a Hammer

 

 

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7 minutes ago, FossilDAWG said:

The author is known for other controversial hypotheses as well.  For example, he has published a hypothesis that the ichthyosaur bones at Berlin-Ichthyosaur State Park (in Nevada) are evidence of a Triassic giant cephalopod (kraken) that killed the ichthyosaurs and arranged the bones.

 

He has also suggested that the Cambrian trace fossil Paleodictyon is the nest of an unknown animal.  This would be 200 million years older than the oldest generally accepted nest trace fossil.

 

On the other hand he is apparently credited with naming the Precambrian supercontinent Rodinia.

 

Don

You know, I was going to ask about his reputation....when I googled him in Scholar he is all over the place with the subjects of his papers.

Everything is generated through your own will power ~ Ray Bradbury
 

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