Was the Early Miocene a second "golden age" for the Chondrichthyans

[Joseph Fossil]

I recall not too long ago there was a pretty interesting scientific article published in the Journal Science describing how scientists analyzed the amount and diversity of chondrichthyan denticles in ichthyoliths from the North and South Pacific dating from around 20-19 million years ago and discovered a sharp drop in the number and diversity of denticles around 19 million years ago, indicating a massive extinction took place which took sharks 2 to 5 million years to somewhat recover from (it's still unknown if this impacted chondrichthyans worldwide or just in the pacific). Here's the paper:

 

https://www.science.org/doi/10.1126/science.aaz3549

 

E. C. Sibert, L. D. Rubin, An early Miocene extinction in pelagic sharks. Science 372, 1105–1107 (2021).

 

But after reading it, I remembered how the Mississippian and somewhat Pennsylvanian periods of the Carboniferous era have been called the golden age of sharks due to the sheer diversity of chondrichthyan genera alive at that time.

 

http://www.elasmo-research.org/education/evolution/golden_age.htm

 

What I'm wondering is does this paper, in revealing this early miocene extinction event, inadvertently also reveal that the period immediately prior to the event was a second "golden age" for the chondrichthyans?

[Doctor Mud]

Interesting - I’ve never seen that paper before.

 

It’s worth checking out the comment papers associated with the Science paper.

it seems it’s not even certain there was an extinction event in the early Miocene.

it’s always interesting to look at comment papers to see a snapshot of scientific debate over a theory or paper. Comment papers are accepted by the journal that published the article and are written by other scientists who feel the need to critique the paper in writing. You can see two comment papers on the same webpage as the Science article. 

 

A summary of the criticism from other scientists on the extinction theory:

 

- The paper uses dermal denticles to infer species diversity, but there is great variation in denticle morphology within a species 

- the conclusion is drawn from only 2 sites. Changes at these sites could just be a range shift.

- no shark teeth are preserved at these sites

- A reduction in denticle number can be explained by an increase in sedimentation rate

- we don’t see this extinction event in the fossil tooth record 

 

I think these criticisms are quite solid. So no extinction event. Science is one of the top journals, but from my experience, there can often be a lot of spin on the science. Pushing datasets to make an exciting story. 

[Crusty_Crab]

Why was the Carboniferous known as the golden age of sharks?

 

Is that mainly due to finds from the Bear Gulch Formation lagerstatte? The Elasmo site largely references finds of Chondrichthyans with a bewildering array of shapes and sizes, that seem so alien to the extant sharks and which are found in that formation. 

 

However, we only know of those because of their existence due to the exceptional preservation and documentation of the Bear Gulch Formation.  What if there was corresponding diversity in sharks in other periods (The Carboniferous is 60 million years, which is a really long time), but we just don't know about it because there hasn't been a corresponding lagerstatte to preserve them? Is it possible that we only call the Carboniferous the golden age of sharks because we are fortunate to have a locality that seems to preserve them so well, but there may have been other periods with similar abundance and diversity we don't know about just because they have not been so perfectly preserved as fossils?

 

The Science paper bases their case on a dropoff in diversity and abundance of denticles in their samples from the early Miocene. The assumption is that there is a correlation between fossilized denticles and shark diversity. Is that a sound assumption?

 

For me, its hard to justify large, sweeping conclusions about species diversity based on the fossil record because the fossil record is not an impartial record of life on earth. 

[Doctor Mud]

 

Here’s the webpage for the article, with the comment papers. The original authors usually have a right to reply to the comments and you can see those responses too. A great way to see “under the hood” of the scientific process. Not everything that is published is true even though it has passed peer-review. 
It’s hard for someone outside the field to tell usually, but my hot tip: look for comment papers. 

[jpc]

Paywalled

[digit]

The original paper may be paywalled but here is the response paper by Gavin Naylor (et al) who works here at UF.

 

 

Cheers.

 

-Ken

 

science.abj87233.pdf

[Joseph Fossil]

14 hours ago, Crusty_Crab said:

Why was the Carboniferous known as the golden age of sharks?

 

Is that mainly due to finds from the Bear Gulch Formation lagerstatte? The Elasmo site largely references finds of Chondrichthyans with a bewildering array of shapes and sizes, that seem so alien to the extant sharks and which are found in that formation. 

 

However, we only know of those because of their existence due to the exceptional preservation and documentation of the Bear Gulch Formation.  What if there was corresponding diversity in sharks in other periods (The Carboniferous is 60 million years, which is a really long time), but we just don't know about it because there hasn't been a corresponding lagerstatte to preserve them? Is it possible that we only call the Carboniferous the golden age of sharks because we are fortunate to have a locality that seems to preserve them so well, but there may have been other periods with similar abundance and diversity we don't know about just because they have not been so perfectly preserved as fossils?

 

The Science paper bases their case on a dropoff in diversity and abundance of denticles in their samples from the early Miocene. The assumption is that there is a correlation between fossilized denticles and shark diversity. Is that a sound assumption?

 

For me, its hard to justify large, sweeping conclusions about species diversity based on the fossil record because the fossil record is not an impartial record of life on earth. 

 

@Crusty_Crab You make an extremely good point about this, especially without how incomplete the fossil record can be. However, I would say it's not just the incredibly well preserved Bear Gulch Formation that could point to a large scale diversification event amongst chondrichthyans during the carboniferous as a whole, but other sites ranging from Mazon Creek in Illinois with forms such as Bandringa rayi to the recent discoveries from the Ste. Genevieve Formation in mammoth cave, Kentucky, the Mecca Quarry shale in Illinois-Indiana, and the Ballagan Formation in Scotland U.K.

 

http://www.fossilworks.org/cgi-bin/bridge.pl?a=collectionSearch&collection_no=132078

 

https://www.biodiversitylibrary.org/partpdf/10460

 

https://www.nps.gov/articles/000/the-ghosts-of-ancient-sharks-at-mammoth-cave-national-park.htm

 

https://www.mindat.org/paleo_strat.php?id=9838

 

https://www.mindat.org/taxon-P149833.html

 

ls/earth-and-environmental-science-transactions-of-royal-society-of-edinburgh/article/abs/diverse-and-durophagous-early-carboniferous-chondrichthyans-from-the-scottish-borders/2F94665815191C282145B5A0373D2A6E

 

Remember that this diversification of chondrichthyans came off the heels of the four extinction events that marked the end of the Devonian era (Lower Zilchov, Taghanic, Kellwasser, and Hangenberg events) and the end of the dominance of the Placoderm fish in aquatic ecosystems. Some even took over the role of top predator left vacant by the extinction of placoderms like Dunkleosteus. Just around 1 million years after the extinction Dunkleosteus 360.7 Million years ago saw, 359 million years ago the emergence of the gigantic ctenacanthiform shark Saivodus striatus. Saivodus striatus at 28 feet in length was even larger than the much later Cretaceous genus Leptostyrax with lengths reaching up to 27 feet (though the smaller sizes of Mesozoic sharks was likely due to the domination of marine reptiles of aquatic ecosystems at the time). With the placoderms demise, the chondrichthyans easily filled the ecological niches they left in fairly quick succession. For sharks, the Carboniferous counted 45 different shark families compared to the around 40 families alive today and the at least 30-31 different shark families from the entirety of the Cretaceous era.

[Joseph Fossil]

15 hours ago, Doctor Mud said:

Interesting - I’ve never seen that paper before.

 

It’s worth checking out the comment papers associated with the Science paper.

it seems it’s not even certain there was an extinction event in the early Miocene.

it’s always interesting to look at comment papers to see a snapshot of scientific debate over a theory or paper. Comment papers are accepted by the journal that published the article and are written by other scientists who feel the need to critique the paper in writing. You can see two comment papers on the same webpage as the Science article. 

 

A summary of the criticism from other scientists on the extinction theory:

 

- The paper uses dermal denticles to infer species diversity, but there is great variation in denticle morphology within a species 

- the conclusion is drawn from only 2 sites. Changes at these sites could just be a range shift.

- no shark teeth are preserved at these sites

- A reduction in denticle number can be explained by an increase in sedimentation rate

- we don’t see this extinction event in the fossil tooth record 

 

I think these criticisms are quite solid. So no extinction event. Science is one of the top journals, but from my experience, there can often be a lot of spin on the science. Pushing datasets to make an exciting story. 

 

@Doctor Mud Good point. I did actually read the comments other scientists posted on this paper and the subsequent rebuttal the papers original authors made and both made pretty good points. I do agree that the sample size should have been much much larger, incorporating samples from multiple sites in the Pacific along with the Indian and Atlantic Oceans. I also agree that these kinds of things do happen with Science occasionally. The Journal Science is extremely reputable and reliable, with probably something like 99% of their papers and articles, especially those dealing with topics like the impacts of Human induced Climate Change and the negative impacts of deforestation in the Amazon Rainforest, are incredible. 

 

But there are some I've seen as of recent - like the June 30th, 2022 article "Sea Change: Can former pirates help scientists study troubled waters" by David Grimm - that have truly horrified me.  

 

https://www.science.org/content/article/can-former-conservation-pirates-help-scientists-study-oceans

 

Basically it said (somewhat gleefully) how one of my favorite conservation groups is being reduced to the role of being an Uber for scientists studying the world's oceans (a job that Government organizations like NOAA should be doing) by the orgnization’s new leadership (which on the surface is good, but in reality takes away from the organization's mission of directly protecting marine wildlife and therefore is not a good thing at all). 

[Joseph Fossil]

16 hours ago, Crusty_Crab said:

Why was the Carboniferous known as the golden age of sharks?

 

Is that mainly due to finds from the Bear Gulch Formation lagerstatte? The Elasmo site largely references finds of Chondrichthyans with a bewildering array of shapes and sizes, that seem so alien to the extant sharks and which are found in that formation. 

 

However, we only know of those because of their existence due to the exceptional preservation and documentation of the Bear Gulch Formation.  What if there was corresponding diversity in sharks in other periods (The Carboniferous is 60 million years, which is a really long time), but we just don't know about it because there hasn't been a corresponding lagerstatte to preserve them? Is it possible that we only call the Carboniferous the golden age of sharks because we are fortunate to have a locality that seems to preserve them so well, but there may have been other periods with similar abundance and diversity we don't know about just because they have not been so perfectly preserved as fossils?

 

The Science paper bases their case on a dropoff in diversity and abundance of denticles in their samples from the early Miocene. The assumption is that there is a correlation between fossilized denticles and shark diversity. Is that a sound assumption?

 

For me, its hard to justify large, sweeping conclusions about species diversity based on the fossil record because the fossil record is not an impartial record of life on earth. 

 

@Crusty_Crab I do agree that there are better ways to test shark diversity in the fossil record than just utilizing denticles (fossilized teeth are a far better indicator to utilize), but the studying of the denticles does have slight benefits. Denticles, like teeth, are like fingerprints for chondrichthyans paleontologically. Each genera and likely each species having unique denticles that are unique to that genera or that species. 

 

https://www.eurekalert.org/news-releases/467114

 

Though in terms of identifying shark diversity from anyone geologic time, complete fossils and teeth are better bets than just denticles. After all, it was shark teeth that led scientists to discover not too long ago that some members of the Ctencanthiforms survived into the Cretaceous era.

 

https://www.nature.com/articles/ncomms3669

 

Guinot, G., Adnet, S., Cavin, L. et al. Cretaceous stem chondrichthyans survived the end-Permian mass extinction. Nat Commun 4, 2669 (2013). https://doi.org/10.1038/ncomms3669

 

https://www.tandfonline.com/doi/abs/10.1080/08912963.2018.1539971?journalCode=ghbi20

 

Iris Feichtinger, Andrea Engelbrecht, Alexander Lukeneder & Jürgen Kriwet (2020) New chondrichthyans characterised by cladodont-like tooth morphologies from the Early Cretaceous of Austria, with remarks on the microstructural diversity of enameloid, Historical Biology, 32:6, 823-836, DOI: 10.1080/08912963.2018.1539971

 

 

[Joseph Fossil]

1 hour ago, Joseph Fossil said:

 

@Crusty_Crab You make an extremely good point about this, especially without how incomplete the fossil record can be. However, I would say it's not just the incredibly well preserved Bear Gulch Formation that could point to a large scale diversification event amongst chondrichthyans during the carboniferous as a whole, but other sites ranging from Mazon Creek in Illinois with forms such as Bandringa rayi to the recent discoveries from the Ste. Genevieve Formation in mammoth cave, Kentucky, the Mecca Quarry shale in Illinois-Indiana, and the Ballagan Formation in Scotland U.K.

 

http://www.fossilworks.org/cgi-bin/bridge.pl?a=collectionSearch&collection_no=132078

 

https://www.biodiversitylibrary.org/partpdf/10460

 

https://www.nps.gov/articles/000/the-ghosts-of-ancient-sharks-at-mammoth-cave-national-park.htm

 

https://www.mindat.org/paleo_strat.php?id=9838

 

https://www.mindat.org/taxon-P149833.html

 

ls/earth-and-environmental-science-transactions-of-royal-society-of-edinburgh/article/abs/diverse-and-durophagous-early-carboniferous-chondrichthyans-from-the-scottish-borders/2F94665815191C282145B5A0373D2A6E

 

Remember that this diversification of chondrichthyans came off the heels of the four extinction events that marked the end of the Devonian era (Lower Zilchov, Taghanic, Kellwasser, and Hangenberg events) and the end of the dominance of the Placoderm fish in aquatic ecosystems. Some even took over the role of top predator left vacant by the extinction of placoderms like Dunkleosteus. Just around 1 million years after the extinction Dunkleosteus 360.7 Million years ago saw, 359 million years ago the emergence of the gigantic ctenacanthiform shark Saivodus striatus. Saivodus striatus at 28 feet in length was even larger than the much later Cretaceous genus Leptostyrax with lengths reaching up to 27 feet (though the smaller sizes of Mesozoic sharks was likely due to the domination of marine reptiles of aquatic ecosystems at the time). With the placoderms demise, the chondrichthyans easily filled the ecological niches they left in fairly quick succession. For sharks, the Carboniferous counted 45 different shark families compared to the around 40 families alive today and the at least 30-31 different shark families from the entirety of the Cretaceous era.

 

@JohnJ @digit I will definitely agree teeth are a better measure of shark diversity than denticles, but what do you guys think?

[JohnJ]

53 minutes ago, Joseph Fossil said:

 

@JohnJ @digit I will definitely agree teeth are a better measure of shark diversity than denticles, but what do you guys think?

 

I'll leave those conclusions to the shark specialists...which I am not.  

[digit]

Indeed. Even teeth don't tell the entire picture and denticles are even less diagnostic  (IMHO).

 

 

Cheers.

 

-Ken

[Joseph Fossil]

On 11/3/2022 at 11:09 AM, Joseph Fossil said:

 

@Doctor Mud Good point. I did actually read the comments other scientists posted on this paper and the subsequent rebuttal the papers original authors made and both made pretty good points. I do agree that the sample size should have been much much larger, incorporating samples from multiple sites in the Pacific along with the Indian and Atlantic Oceans. I also agree that these kinds of things do happen with Science occasionally. The Journal Science is extremely reputable and reliable, with probably something like 99% of their papers and articles, especially those dealing with topics like the impacts of Human induced Climate Change and the negative impacts of deforestation in the Amazon Rainforest, are incredible. 

 

But there are some I've seen as of recent - like the June 30th, 2022 article "Sea Change: Can former pirates help scientists study troubled waters" by David Grimm - that have truly horrified me.  

 

https://www.science.org/content/article/can-former-conservation-pirates-help-scientists-study-oceans

 

Basically it said (somewhat gleefully) how one of my favorite conservation groups is being reduced to the role of being an Uber for scientists studying the world's oceans (a job that Government organizations like NOAA should be doing) by the orgnization’s new leadership (which on the surface is good, but in reality takes away from the organization's mission of directly protecting marine wildlife and therefore is not a good thing at all). 

 

@Doctor Mud This article in particular featured interviews that bluntly disparaged the founder of the Sea Shepherd Conservation Society or Sea Shepherd US - interviews by acting members of the organization's own board. My second great passion alongside paleontology is conservation. I actually met Sea Shepherd's founder, Captain Paul Watson, at an environmental event hosted by my college a few years back and I have enormous respect for him. To see members of his own board disparaged him in this way, despite the fact he's led Sea Shepherd effectively and saved countless amounts of marine life with his non-violent direct action methods for 45 years, disgusted me.

 

A few days after this article was published, Watson resigned from Sea Shepherd Conservation Society and then (as a result of pressure from Sea Shepherd US) was kicked off the board of Sea Shepherd Global (without even all the board members or Watson even being there for the vote). He's currently founding his own group to continue Sea Shepherd's original mission, something that is truly the definition of awesome. I know this is not too Paleontology related but I feel it's important to highlight the impacts a politically motivated article in a journal like Science can have. Science is still an amazing journal and I still recommend readers to go to it at a foremost trusted source with at least 99% of their papers and articles being great and accurate, but this recent article demonstrates why we must be vigilant.

[hemipristis]

Given the explosion in diversity just within the family Carcharhinidae (aka "gray sharks) and the "makos" (the two groups that I particularly like to collect), I would say that there is something to this.  

 

Consider that in the Paleogene the genus Carcharhinus, as currently organized, possessed only two widely recognized species: C. ergotoni and C. gibbesi, which some consider the same species.  By the end of the Miocene, teeth from at least a dozen species are recognized as occurring in the Western Atlantic (where I"m most familiar) with many also identified as occurring in the Mediterranean and the Pacific. 

 

Also in the Paleogene, "makos" as we understand them as a group, were limited to Isurus (Macrorhizodus) praecursor.  Enter the Miocene, and the Cosmopolitodus lineage sprouts (C. hastalis, and later C. xiphodon---sorry, I don't accede to the idea that they're the same species), as well as Isurus oxyrinchus (aka I. desori) and I. retroflexus (aka I. paucus)

 

 

[hemipristis]

On 11/4/2022 at 2:43 AM, Joseph Fossil said:

 

@JohnJ @digit I will definitely agree teeth are a better measure of shark diversity than denticles, but what do you guys think?

Good question.  I really haven't dealved into shark denticles as they're so tiny, I don't collect them.  I'd be curious as to others' thoughts

[digit]

I find them picking micro-matrix but don't go out of my way to collect/study them.

 

 

Cheers.

 

-Ken

 

 

   

 

 

[siteseer]

On 11/4/2022 at 7:56 PM, hemipristis said:

Given the explosion in diversity just within the family Carcharhinidae (aka "gray sharks) and the "makos" (the two groups that I particularly like to collect), I would say that there is something to this.  

 

Consider that in the Paleogene the genus Carcharhinus, as currently organized, possessed only two widely recognized species: C. ergotoni and C. gibbesi, which some consider the same species.  By the end of the Miocene, teeth from at least a dozen species are recognized as occurring in the Western Atlantic (where I"m most familiar) with many also identified as occurring in the Mediterranean and the Pacific. 

 

Also in the Paleogene, "makos" as we understand them as a group, were limited to Isurus (Macrorhizodus) praecursor.  Enter the Miocene, and the Cosmopolitodus lineage sprouts (C. hastalis, and later C. xiphodon---sorry, I don't accede to the idea that they're the same species), as well as Isurus oxyrinchus (aka I. desori) and I. retroflexus (aka I. paucus)

 

 

 

There are at least a few other described Carcharhinus species known from the Oligocene including C. perseus, C. balochenisis, C. priscus and two unnamed species in Adnet et al. (2007).

 

I. desori and I. retroflexus were present in the Late Oligocene at least.

 

 

Adnet, T, S., P.O. Antoine, S.R.H.  BaqriI, J.Y. Crochet, I.  Marivaux, J.L. Welcomme, and G. MÉTAIS.  2007.
New tropical carcharhinids (Chondrichthyes, Carcharhiniformes) from the late Eocene-early Oligocene of Balochistan, Pakistan: Paleoenvironmental and paleogeographic implications.Journal of Asian Earth Sciences, 30: 303-323.

 

 

 

 

Edited November 7, 2022 by siteseer
added cited reference

[hemipristis]

On 11/7/2022 at 5:52 PM, siteseer said:

 

There are at least a few other described Carcharhinus species known from the Oligocene including C. perseus, C. balochenisis, C. priscus and two unnamed species in Adnet et al. (2007).

 

Adnet, T, S., P.O. Antoine, S.R.H.  BaqriI, J.Y. Crochet, I.  Marivaux, J.L. Welcomme, and G. MÉTAIS.  2007.
New tropical carcharhinids (Chondrichthyes, Carcharhiniformes) from the late Eocene-early Oligocene of Balochistan, Pakistan: Paleoenvironmental and paleogeographic implications.Journal of Asian Earth Sciences, 30: 303-323.

 

One publication on one locality does not an explosion in diversity make.

 

On 11/7/2022 at 5:52 PM, siteseer said:

I. desori and I. retroflexus were present in the Late Oligocene at least.

Close enough, LOL

[hemipristis]

On 11/17/2022 at 6:56 PM, hemipristis said:

Close enough, LOL

This was a joke.  What's a million years or so between friends?  lol.

 

Guess it kinda fell flat.  oops

 

[siteseer]

What I'm saying is that Carcharhinus was increasing in diversity before the Miocene and that continued into the Early Miocene, but even with that, we're still talking about just one genus.  There was no such explosion within other shark genera.  More importantly, there was no burst at the generic nor familial level.  Most of the genera we see today were already present by the Eocene with many others appearing back in the Cretaceous.

Also, little/nothing is known about which dermal denticles belongs to which extinct genera.  You'd need a skeleton with associated denticles and those are almost nonexistent.  I'm not sure there's been a paper referring isolated denticles to modern taxa because there might not be a paper on the range of variation across the body and within any modern shark species.

One other thing...I don't know if it was brought up in the paper but the Oligocene was a time of glaciation at the poles.  Antarctica was still well-forested in the Late Eocene but small glaciers were present.  The continent was mostly covered by an ice sheet in the Early Oligocene, retreating and then advancing again in the Middle Oligocene.  Increasing glaciation means that enormous volumes of seawater was getting locked up in that glaciation which led to a fall in sea level, draining many shallow seas worldwide.  This is reflected in the fossil record.  The average fossil collector might notice that you can trade for or buy fewer Oligocene marine fossils than Eocene or Miocene ones.  One could counter that the Oligocene lasted just about 10 million years while the other two were each about twice as long.  Even accounting for that, a shark tooth collector can tell you that Oligocene shark teeth aren't found in all the same areas as in the Early-Middle Eocene and that's because those areas became dry land.  It's more complicated than that but no less true in general.

Meanwhile, the Early Miocene was a warmer time in earth history.  Sea level rose so there are more localities for shark teeth again.  That gives the illusion that sharks and other marine forms had a comeback of sorts.

 

Anyway, I'm a little surprised there aren't more posts from shark collectors here but we've been getting into a busier time of year.