Mako To Gw Transitional In North America?

[wRick]

I spent about 5 hours Monday working on the wedigdixie viaduct to redirect a small section of Green Mill Run in Greenville NC. During the creek re-engineering project, which is intended to redirect a channel of the creek into some new fossil bearing gravel and thus use the force of the raging waters to help uncover more buried treasures, I found what I initially thought was a 1 & 3/4" Mako, then I thought maybe its a Great White with almost all the serrations worn off.

Now I'm wondering if it could be a transitional tooth between the unserrated broad tooth Mako Isurus Hastalis to the Great White Carcharodon carcharias. Everything I've read says these transitional teeth only occur in the Pacific Especially in Peru. Does anyone know of any found in North America on the East Coast? Do the little ridges on the blade of this tooth look like the beginnings of serrations? Or am I crazy?

There are 3-4 ridges on one side of the blade near the root, and one ridge on the other side near the root. The ridges can be seen in person and you can feel them with your fingertip or fingernail. In trying to be objective about this I put the tooth in my pocket with a handfull of similar sized makos and great whites and I could definitely find this tooth everytime by feeling for those ridges.

So what do you think? Transitional? or minor pathology?

Also found 1 GW and four more Makos, some Crows, tiger, a decent mackerel shark and either a hammerhead or a requiem tooth as well as the usual assortment of Meg chunks, goblins, sand sharks and the rest. Wedidixie found 5 Great Whites and a handful of makos and the rest. I've been getting text messages the last few days that he found 9 more Great White's yesterday and a meg today, so I guess that means the viaduct is working!

[wRick]

Thanks for the reply, but that's Isurus escheri the narrow toothed Mako isn't it?

I know the classic transitionals have serrations all the way down, but some of the earlier ones just have serrations near the root. I'm thinking this could be one of the early ones. Afterall, what would the first serrated mako be except an odd pathological?

[FossilForKids]

I have seen a couple in Bakersfield. Although I've never seen a GW there I have found them less thjan 80 miles away.

FFK

[LeeCreek2thHntr]

The buena vista museum in bakersfield has a transitional there. I bought a mako tooth from megateeth that had ripples near the top. These were likely the early beginning of the transition. I can't get a pic of it right now because i'm on a trip.

[bmorefossil]

here are two of mine both broad form makos one upper and one lower, the lower has alot more serrations than the upper yet the upper serrations are larger.

[siteseer]

It's tough to say since your tooth shows some wear. Do you know the age of the bed? Has it been assigned a date within a half-million years or less? The earliest transitional teeth don't have fully-formed serrations. The cutting edge bears a slightly wavy not quite saw-like edge which is similar to that area on your tooth but wouldn't be as distinct in one spot without indication elsewhere on the edge, especially as you noted, toward the root. On some teeth there are nearly fully-formed serrations towards the root and the more wavy edge toward the tip.

Honestly, I would lean against it since the transitional tooth form has been reported only from Pacific Ocean sites (Peru, Chile, and California) but couldn't rule it out.

I spent about 5 hours Monday working on the wedigdixie viaduct to redirect a small section of Green Mill Run in Greenville NC. During the creek re-engineering project, which is intended to redirect a channel of the creek into some new fossil bearing gravel and thus use the force of the raging waters to help uncover more buried treasures, I found what I initially thought was a 1 & 3/4" Mako, then I thought maybe its a Great White with almost all the serrations worn off.

Now I'm wondering if it could be a transitional tooth between the unserrated broad tooth Mako Isurus Hastalis to the Great White Carcharodon carcharias. Everything I've read says these transitional teeth only occur in the Pacific Especially in Peru. Does anyone know of any found in North America on the East Coast? Do the little ridges on the blade of this tooth look like the beginnings of serrations? Or am I crazy?

There are 3-4 ridges on one side of the blade near the root, and one ridge on the other side near the root. The ridges can be seen in person and you can feel them with your fingertip or fingernail. In trying to be objective about this I put the tooth in my pocket with a handfull of similar sized makos and great whites and I could definitely find this tooth everytime by feeling for those ridges.

So what do you think? Transitional? or minor pathology?

Also found 1 GW and four more Makos, some Crows, tiger, a decent mackerel shark and either a hammerhead or a requiem tooth as well as the usual assortment of Meg chunks, goblins, sand sharks and the rest. Wedidixie found 5 Great Whites and a handful of makos and the rest. I've been getting text messages the last few days that he found 9 more Great White's yesterday and a meg today, so I guess that means the viaduct is working!

[siteseer]

I assume you mean the Kettleman Hills teeth (San Joaquin Fm.)? I don't think I've seen one from there. The weird thing I have seen is at least one C. carcharias juvenile tooth from there with lateral cusplets but an unserrated crown. I've seen one from Oceanside as well and Gordon Hubbell (Hubbell, 1996: 12-13) observed that this is sometimes seen in today's young great whites as well.

Hubbell, G. 1996.

Using Tooth Structure to Determine the Evolutionary History of the White Shark. In Klimley, A.P. and D.G. Ainley (eds). Great White Sharks. Academic Press.

I have seen a couple in Bakersfield. Although I've never seen a GW there I have found them less thjan 80 miles away.

FFK

Edited July 9, 2010 by siteseer

[siteseer]

I have seen that tooth but can't recall why I leaned against it showing actual serrations. It's too old for one thing but I don't want to use that as a crutch. One thing to consider is something an ichthyologist once told me. He pointed out that the development of serrations is a genetic expression that has appeared among many unrelated vertebrate groups (think about Dimetrodon, a synapsid, xiphodont crocodiles, some lizards, many dinosaurs, some sabercats, etc.) and it could have appeared more often than we have seen because it is less of a stretch for a fine-edged tooth to develop irregularities than it is to lose lateral cusplets. It is possible that we could find oddball Isurus hastalis teeth with a weak serration or two like wRick's tooth at different points in its history without it heralding the morphological trend that for some reason did catch on right around the beginning of the Pliocene.

Think about Isurus escheri. For whatever reason, the serrated edge didn't catch on in the Middle Miocene for that group even while it was working for several Carcharhinus species.

The buena vista museum in bakersfield has a transitional there. I bought a mako tooth from megateeth that had ripples near the top. These were likely the early beginning of the transition. I can't get a pic of it right now because i'm on a trip.

[siteseer]

here are two of mine both broad form makos one upper and one lower, the lower has alot more serrations than the upper yet the upper serrations are larger.

What is the age of the bed where than lower came from? Is it from a land site?

[bmorefossil]

miocene in age, calvert formaiton, beach site

edit: zone 11 or 13

Edited July 9, 2010 by bmorefossil

[wRick]

Thanks for your input Siteseer.

Unfortunately Green Mill Run, the creek where I found this tooth, is a reformulated hash of a number of different layers. You can simultaneously find Cretaceous, Miocene and Pliocene fossils in a single screen, so we're not going to gain much insight about the tooth from whatever formation it came from.

I agree with your comments below that serrations have appeared multiple times throughout shark and vertebrate history, and that the appearance of this trait is due to some underlying genetic mechanism that is certainly present in sharks and probably present in other vertebrates as well.

"One thing to consider is something an ichthyologist once told me. He pointed out that the development of serrations is a genetic expression that has appeared among many unrelated vertebrate groups (think about Dimetrodon, a synapsid, xiphodont crocodiles, some lizards, many dinosaurs, some sabercats, etc.) and it could have appeared more often than we have seen because it is less of a stretch for a fine-edged tooth to develop irregularities than it is to lose lateral cusplets."

But,I don't entirely agree with the last part:

"It is possible that we could find oddball Isurus hastalis teeth with a weak serration or two like wRick's tooth at different points in its history without it heralding the morphological trend that for some reason did catch on right around the beginning of the Pliocene."

To my mind the appearance, progression, and then further development of full-fledged serrations suggests that serrations in shark teeth are a quantitative trait, that is, multiple different genes(alleles) contribute to the observed trait, no one or two genes is enough to cause the complete phenotype, but enough of the right variations of several different genes(alleles) can produce the phenotype.

I suspect, based on the teeth that I've seen, that serrations are a quantitative trait, a few of the right genes and you get either small serrations everywhere, or some other combination of genes produces a few serrations elsewhere. As the frequency of these genes increases in the population you get more and more individuals with significant serrations over a significant portion of the tooth. This confers a competitiive advantage and so these genes continue to spread through the population.

In the case of the tooth I found in NC, I don't believe it's the beginning of the Mako to GW transition, I believe it is probably the offspring of some shark from the Pacific (Large pelagic sharks couldn't have stayed isolated in Peru for a million years) transformation who migrated to the east coast of N. America and mated with a member of the broad toothed Mako Isurus hastalis population there. This pairing produced offspring with a partial phenotype, not likely to confer a selective advantage on the individual, but not likely to hurt its fitness either, so the frequency of some of those quantitative serrated tooth genes(alleles) increases in the Atlantic population. Then, at some point once the fully serrated GW style tooth genes are fixed in the Pacific population and they begin expanding their range they encounter an Atlantic population that already has a high frequency of some of the serrated tooth genes and they're effectively primed to become Great Whites (by become great whites of course I mean: produce offspring over the next million year or so who end up as GWs), and the broad toothed Makos disappear, not because they go extinct but because they all end up with the advantageous serrated GW tooth design.

So, a tooth like this might not be an "oddball" tooth out of the mainstream of the evolution of the species, but rather, it may be the manifestation of "Gene Flow" between Atlantic and Pacific populations that leads to increasing frequency of the genes(alleles) necessary to support a fully serrated tooth phenotype in the Atlantic population. A tooth like this might not be an "oddball" but just the meagre expression of 10? genes(alleles) for a fully serrrated phenotype that requires 100?

[Boesse]

In the case of the tooth I found in NC, I don't believe it's the beginning of the Mako to GW transition, I believe it is probably the offspring of some shark from the Pacific (Large pelagic sharks couldn't have stayed isolated in Peru for a million years) transformation who migrated to the east coast of N. America and mated with a member of the broad toothed Mako Isurus hastalis population there. This pairing produced offspring with a partial phenotype, not likely to confer a selective advantage on the individual, but not likely to hurt its fitness either, so the frequency of some of those quantitative serrated tooth genes(alleles) increases in the Atlantic population. Then, at some point once the fully serrated GW style tooth genes are fixed in the Pacific population and they begin expanding their range they encounter an Atlantic population that already has a high frequency of some of the serrated tooth genes and they're effectively primed to become Great Whites (by become great whites of course I mean: produce offspring over the next million year or so who end up as GWs), and the broad toothed Makos disappear, not because they go extinct but because they all end up with the advantageous serrated GW tooth design.

Following occam's razor... I think a far more plausible hypothesis is that these specimens are pathologic. Bobby

[Scylla]

But,I don't entirely agree with the last part:

"It is possible that we could find oddball Isurus hastalis teeth with a weak serration or two like wRick's tooth at different points in its history without it heralding the morphological trend that for some reason did catch on right around the beginning of the Pliocene."

To my mind the appearance, progression, and then further development of full-fledged serrations suggests that serrations in shark teeth are a quantitative trait, that is, multiple different genes(alleles) contribute to the observed trait, no one or two genes is enough to cause the complete phenotype, but enough of the right variations of several different genes(alleles) can produce the phenotype.

I suspect, based on the teeth that I've seen, that serrations are a quantitative trait, a few of the right genes and you get either small serrations everywhere, or some other combination of genes produces a few serrations elsewhere. As the frequency of these genes increases in the population you get more and more individuals with significant serrations over a significant portion of the tooth. This confers a competitiive advantage and so these genes continue to spread through the population.

In the case of the tooth I found in NC, I don't believe it's the beginning of the Mako to GW transition, I believe it is probably the offspring of some shark from the Pacific (Large pelagic sharks couldn't have stayed isolated in Peru for a million years) transformation who migrated to the east coast of N. America and mated with a member of the broad toothed Mako Isurus hastalis population there. This pairing produced offspring with a partial phenotype, not likely to confer a selective advantage on the individual, but not likely to hurt its fitness either, so the frequency of some of those quantitative serrated tooth genes(alleles) increases in the Atlantic population. Then, at some point once the fully serrated GW style tooth genes are fixed in the Pacific population and they begin expanding their range they encounter an Atlantic population that already has a high frequency of some of the serrated tooth genes and they're effectively primed to become Great Whites (by become great whites of course I mean: produce offspring over the next million year or so who end up as GWs), and the broad toothed Makos disappear, not because they go extinct but because they all end up with the advantageous serrated GW tooth design.

So, a tooth like this might not be an "oddball" tooth out of the mainstream of the evolution of the species, but rather, it may be the manifestation of "Gene Flow" between Atlantic and Pacific populations that leads to increasing frequency of the genes(alleles) necessary to support a fully serrated tooth phenotype in the Atlantic population. A tooth like this might not be an "oddball" but just the meagre expression of 10? genes(alleles) for a fully serrrated phenotype that requires 100?

First off, nice tooth. As far as how many genes it takes to produce serrations, be careful in your assumptions. It may only require one or two gene loci to change, or the interaction of only a few genes to produce very different phenotypes. I remember when we thought that it took about 100,000 genes to make a human, but the real number is probably less than 1/3 of that. I'll bet that it only takes about 1-4 genes to create serrations.

As far as not going extinct because your species has evolved into something else? I consider Australopithicus to be extinct, even if they may have led to us. It is also very unlikely that a Pacific population evolved only serrations, then mixed in with an Atlantic population. Much more plausible is that the Pacific sharks replaced the Atlantic ones. Evolution would not just work on teeth alone.

PS just checked on wikipedia, they say c. 23,000 human genes.

Edited July 10, 2010 by Scylla