What is the real Megalodon?

[Max-fossils]

Hello everyone!

 

Something has been confusing me for a long time, so now I finally want to spit it out.

What is the "real" megalodon species? I am asking this because I have seen many different genera associated with the same species name: Carcharodon megalodon, Carcharocles megalodon, Megaselachus megalodon, Otodus megalodon, etc. And I know that two completely different genera can have the same species name (eg: Liopleurodon ferox and Titanosuchus ferox, etc.), but the thing is that with the megalodon all the teeth look a lot like each other (or as we say in French: comme deux gouttes d'eau). Now I wouldn't be surprised to learn that there is a lot of paleontological debate going over this topic, but I would still like to know what the "real" megalodon species is, or at leat according to you, and why.

 

What do you have to say?

 

Max

[FossilDAWG]

To start with, all those names apply to the same species.  The different generic names reflect various opinions about the taxonomic relationship of that species to other mega-toothed sharks.  Other posters are much better versed in the history of the various names and hopefully they will offer comments.  I'll just mention that "Carcharodon megalodon" reflected a belief that megalodon was closely related to the white shark.  This idea has been pretty much completely discounted due to the numerous differences between megalodon and white shark teeth; the similarities are mostly superficial, and the generic assignment is no longer regarded as valid.  I believe the currently favored combination is Otodus megalodon.

 

Although the frequent shifting of names can be irritating, it helps (maybe a little bit) to remember that Linnean taxonomic groupings above the level of species (so genera, families, orders etc) are artificial constructs, and in a way they are hypotheses about the relationships between species.  When someone names a new genus, they are making a statement that the species they are studying is sufficiently different from all other named species that it cannot be grouped with them in the same genus.  Of course there is no hard and fast rule for "how different is different enough".  Historically that has been a matter of judgement, hopefully by someone with a great deal of familiarity with the set of species under consideration.  However it is still fairly subjective, with some researchers defining genera very narrowly and splitting species between many genera (so-called splitters), and those taking a broader view and grouping species that cover a large amount of variation under a single genus (so-called lumpers).  These days tools for analyzing large matrices of measurements of morphological features can perhaps make the practice a bit more objective, in that you can reconstruct phylogenetic trees and look for natural clustering of species into higher categories.

 

Anyway, when two species are put into the same genus the scientist is making a hypothesis that the two species have a very close phylogenetic relationship; you might think of them as small twigs from the same small branch of a phylogenetic tree.  Species that are put in different genera but, say, the same sub-family are twigs that split off the same branch but further apart.  In this way the names come to reflect the evolutionary history of the species.  So putting species in the same genus reflects the researchers ideas about the closeness of the relationship between the species, coupled with her ideas about how narrowly or broadly to define genera.  This hypothesis is dependent on data (the morphology of the species, which can be reduced to measurements of lots of characters), but that data is also filtered by ideas about the relative importance of individual characters.  In the past much weight was given to the presence or absence of serrations on shark teeth, so species might be grouped based largely on that character.  Nowadays it is recognized that serrations can be gained or lost quickly in evolutionary time, so they can evolve independently (convergent evolution) multiple times and they may not reflect actual close relationships.  Other characters are now given more "weight", in other words they are thought to be less likely to evolve multiple times so their presence in two different species is likely to be due to actual close relationship, such as sharing a common ancestor that had evolved that feature.  An example might be pores that represent the pattern of blood vessels in the root.  Shifting hypotheses about relationships of "megalodon" to other sharks, based on more data (such as more transitional fossils, or better understanding of how to weight characters in phylogenetic analysis) will inevitably be reflected in changes in the choice of genus name to apply.

 

Don

[Max-fossils]

33 minutes ago, FossilDAWG said:

To start with, all those names apply to the same species.  The different generic names reflect various opinions about the taxonomic relationship of that species to other mega-toothed sharks.  Other posters are much better versed in the history of the various names and hopefully they will offer comments.  I'll just mention that "Carcharodon megalodon" reflected a belief that megalodon was closely related to the white shark.  This idea has been pretty much completely discounted due to the numerous differences between megalodon and white shark teeth; the similarities are mostly superficial, and the generic assignment is no longer regarded as valid.  I believe the currently favored combination is Otodus megalodon.

 

Although the frequent shifting of names can be irritating, it helps (maybe a little bit) to remember that Linnean taxonomic groupings above the level of species (so genera, families, orders etc) are artificial constructs, and in a way they are hypotheses about the relationships between species.  When someone names a new genus, they are making a statement that the species they are studying is sufficiently different from all other named species that it cannot be grouped with them in the same genus.  Of course there is no hard and fast rule for "how different is different enough".  Historically that has been a matter of judgement, hopefully by someone with a great deal of familiarity with the set of species under consideration.  However it is still fairly subjective, with some researchers defining genera very narrowly and splitting species between many genera (so-called splitters), and those taking a broader view and grouping species that cover a large amount of variation under a single genus (so-called lumpers).  These days tools for analyzing large matrices of measurements of morphological features can perhaps make the practice a bit more objective, in that you can reconstruct phylogenetic trees and look for natural clustering of species into higher categories.

 

Anyway, when two species are put into the same genus the scientist is making a hypothesis that the two species have a very close phylogenetic relationship; you might think of them as small twigs from the same small branch of a phylogenetic tree.  Species that are put in different genera but, say, the same sub-family are twigs that split off the same branch but further apart.  In this way the names come to reflect the evolutionary history of the species.  So putting species in the same genus reflects the researchers ideas about the closeness of the relationship between the species, coupled with her ideas about how narrowly or broadly to define genera.  This hypothesis is dependent on data (the morphology of the species, which can be reduced to measurements of lots of characters), but that data is also filtered by ideas about the relative importance of individual characters.  In the past much weight was given to the presence or absence of serrations on shark teeth, so species might be grouped based largely on that character.  Nowadays it is recognized that serrations can be gained or lost quickly in evolutionary time, so they can evolve independently (convergent evolution) multiple times and they may not reflect actual close relationships.  Other characters are now given more "weight", in other words they are thought to be less likely to evolve multiple times so their presence in two different species is likely to be due to actual close relationship, such as sharing a common ancestor that had evolved that feature.  An example might be pores that represent the pattern of blood vessels in the root.  Shifting hypotheses about relationships of "megalodon" to other sharks, based on more data (such as more transitional fossils, or better understanding of how to weight characters in phylogenetic analysis) will inevitably be reflected in changes in the choice of genus name to apply.

 

Don

Wow this is interesting...

What you explained here was something I already knew about, but you go a lot deeper and explain a lot better, in order to make it clearer.

Also, you said that species are classified according to their differences, but that all this is just artificial. That's something I have never thought of before, but it does make a lot of sense.

 

Thank you very much for your amazing explanation!

 

Best regards,

 

Max

[FossilDAWG]

To be clear (or at least a bit more so) species are grouped according to shared similarities (which are called synapomorphies).  For example all mammals have hair or fur and females produce milk to nurture their young.  No other vertebrates have hair/fur or produce milk, so these two traits are synapomorphies that unite all mammals.  It is assumed that synapomorphies evolved one time, and so their presence in multiple species indicates that those species evolved from a common ancestor that first evolved the synapomorphy.  Features that evolved over and over in otherwise different groups are not synapomorphies.  For example amongst vertebrates both bats and birds have wings, so "wings" are not a synapomorphic trait.  However when such structures evolve independently they will differ in many details of their construction, and those details may themselves be synapomorphies.  This process of emphasizing shared synapomorphies in grouping species produces groupings that reflect shared common ancestors, and so reflect evolutionary history (phylogeny).  As new data (such as new species) are added to the analysis, ideas about whether a structure is a good synapomorphy versus a feature that can evolve independently multiple times can change, and that will change ideas about phylogeny.  For example, at one time it was assumed that serrations on teeth were a good synapomorphy, but accumulating evidence indicated that serrations have been gained and lost independently in various lineages of sharks, and so it is not a good synapomorpy on which to base hypotheses about phylogeny.

 

Species are differentiated based on unique distinguishing features.  Differentiating species is distinct from grouping species into higher order classifications.

 

What is artificial is the creation of categories of higher order groupings into genera, families, and so on.  Of course we need such groupings in order to be able to name different levels of clustering of species.  If you think of a tree, the trunk gives rise to branches that give rise to smaller branches that give rise to still smaller branches all the way down to twigs at the end.  In naming species, genera, families, and so on we are starting at the twigs (species) and working our way back towards the trunk (Kingdom).  If you look at phylogenetic trees you'll notice that they don't have consistent levels where branching occurs.  Just like a real tree, phylogenetic trees branch at different rates and it may be somewhat arbitrary to decide where to draw the line between different categories.   The Linnean classification scheme is a way of putting a name on the different sizes of branches.  It is very useful for naming groupings that reflect the phylogenetic history of species.  It is arbitrary in that what is called a genus may differ from one cluster of species to another, there is no rigidly defined amount of similarity/difference between species that demands that they be grouped together in a genus, or that a genus be split up.

 

Don

[Max-fossils]

44 minutes ago, FossilDAWG said:

To be clear (or at least a bit more so) species are grouped according to shared similarities (which are called synapomorphies).  For example all mammals have hair or fur and females produce milk to nurture their young.  No other vertebrates have hair/fur or produce milk, so these two traits are synapomorphies that unite all mammals.  It is assumed that synapomorphies evolved one time, and so their presence in multiple species indicates that those species evolved from a common ancestor that first evolved the synapomorphy.  Features that evolved over and over in otherwise different groups are not synapomorphies.  For example amongst vertebrates both bats and birds have wings, so "wings" are not a synapomorphic trait.  However when such structures evolve independently they will differ in many details of their construction, and those details may themselves be synapomorphies.  This process of emphasizing shared synapomorphies in grouping species produces groupings that reflect shared common ancestors, and so reflect evolutionary history (phylogeny).  As new data (such as new species) are added to the analysis, ideas about whether a structure is a good synapomorphy versus a feature that can evolve independently multiple times can change, and that will change ideas about phylogeny.  For example, at one time it was assumed that serrations on teeth were a good synapomorphy, but accumulating evidence indicated that serrations have been gained and lost independently in various lineages of sharks, and so it is not a good synapomorpy on which to base hypotheses about phylogeny.

 

Species are differentiated based on unique distinguishing features.  Differentiating species is distinct from grouping species into higher order classifications.

 

What is artificial is the creation of categories of higher order groupings into genera, families, and so on.  Of course we need such groupings in order to be able to name different levels of clustering of species.  If you think of a tree, the trunk gives rise to branches that give rise to smaller branches that give rise to still smaller branches all the way down to twigs at the end.  In naming species, genera, families, and so on we are starting at the twigs (species) and working our way back towards the trunk (Kingdom).  If you look at phylogenetic trees you'll notice that they don't have consistent levels where branching occurs.  Just like a real tree, phylogenetic trees branch at different rates and it may be somewhat arbitrary to decide where to draw the line between different categories.   The Linnean classification scheme is a way of putting a name on the different sizes of branches.  It is very useful for naming groupings that reflect the phylogenetic history of species.  It is arbitrary in that what is called a genus may differ from one cluster of species to another, there is no rigidly defined amount of similarity/difference between species that demands that they be grouped together in a genus, or that a genus be split up.

 

Don

Again an incredible explanation, Don!

Thanks a lot.

One thing though: I only asked about megalodons, and you came up with a whole explanation about species and how they are sorted out. Now don't get me wrong, I really enjoyed your explanation, but it didn't answer my initial question. Maybe I should rename the topic

 

Best regards and thanks again,

 

Max

[FossilDAWG]

I'm not a shark tooth expert.  However we have several members who post regularly who are.  Hopefully they will comment on the specific reasons for the shifting genus names used for the megalodon shark.  Each change has a rationale, (for example as I explained Carcharodon megalodon was used when it was thought that megs were just really big white sharks), and it would be interesting to learn about the history of the names as that reflects the history of ideas about what megs are in relation to other sharks.

 

I thought you were also wondering about why names seem to change so much, and I tried to explain the more general reasons why such changes occur.

 

Don

[tmaier]

That explaination clears it up for me. Very well done. You should lecture.

[Phevo]

Very nicely explained

 

[FossilDAWG]

20 minutes ago, tmaier said:

That explaination clears it up for me. Very well done. You should lecture.

That's what I do (University of Georgia).

 

Don

[tmaier]

"That's what I do (University of Georgia). "

Doh! Your words seemed very concise and practiced, I should have guessed. Lecturing on the same topic causes a person to refine their presetation to a fine edge. Also, questions from the students keeps the edge honed. You can see from their faces if you are clicking with them, and that creates a feedback for refining the lecture. And the tests... good test scores come from good lectures.

I have a couple hundred lectures on electronics, programming, and math in my memory, finely honed. I pity the students who had to sit through it the first or second presentation, but by the fifth time around, I've got it nailed like a stand-up comedian act. =-)

[FossilDAWG]

To be fair, taxonomy (and paleontology for that matter) is not my research focus, though I find it very interesting.  My research is very different, think "molecular biology", "immunology", and "diseases transmitted by things that bite you and suck your blood".

 

I find repeating lectures leads to them being more "polished" to a point, but that can blend into "lack of passion", quickly followed by "boring".  It's a challenge to stay fresh enough that you convey passion for the subject.  I have many unpleasant memories of professors who clearly had no interest in the subject they were teaching.  I vowed never to be one of them.  I hope I'm succeeding so far.

 

Don

[JohnJ]

56 minutes ago, FossilDAWG said:

To be fair, taxonomy (and paleontology for that matter) is not my research focus, though I find it very interesting.  My research is very different, think "molecular biology", "immunology", and "diseases transmitted by things that bite you and suck your blood".

 

I find repeating lectures leads to them being more "polished" to a point, but that can blend into "lack of passion", quickly followed by "boring".  It's a challenge to stay fresh enough that you convey passion for the subject.  I have many unpleasant memories of professors who clearly had no interest in the subject they were teaching.  I vowed never to be one of them.  I hope I'm succeeding so far.

 

Don

 

Your commentary and "boring" have never come together in my mind.  Thanks for your input. 

 

[Al Dente]

As Don already stated, Otodus megalodon is the most current. Before that it was Carcharocles megalodon. There really isn't a difference between Otodus obliquus, Otodus subserrata and Carcharocles auriculatus beside the amount of serrations and age.

[piranha]

text from:

 

Shimada, K., Chandler, R.E., Lam, O.L.T., Tanaka, T., & Ward, D.J. (2016)

A new elusive otodontid shark (Lamniformes: Otodontidae) from the lower Miocene, and comments on the taxonomy of otodontid genera, including the ‘megatoothed’ clade.

Historical Biology, 29:704-714   PDF LINK

 

"The otodontid taxonomy has been in flux largely due to the introduction of multiple generic names aimed to include the iconic fossil megathoothed species, megalodon, such as Carcharocles Jordan and Hannibal, Procarcharodon Casier, and Megaselachus Glikman. Zhelezko and Kozlov (1999) considered many of these ‘megalodon-clade genera’ not to merit a genus status and placed them as subgenera under the genus Otodus Agassiz."

 

"Some workers have begun to consider taxa of the ‘megalodon-clade’ to belong to the genus Otodus (e.g. Andrianavalona et al. 2015; Malyshkina & Ward 2016). However, the previously used ‘megalodon-clade genera,’ such as Megaselachus and Carcharocles, continue to be used in literature to date (e.g. Ehret & Ebersole 2014; Laurito et al. 2014; Carrillo-Briceño et al. 2015; Pimiento & Balk 2015; Pimiento et al. 2016). We note here that not using any of those non-Otodus genera for the megalodon-clade taxa can be justified on the basis of systematic principles."

[FossilDAWG]

Thanks Scott!  That paper is exactly what this thread needed.

 

Don

[DenBoy]

Very interesting and informative thread, Thanks guys

[Max-fossils]

17 hours ago, piranha said:

excerpts from:

 

Shimada, K., Chandler, R.E., Lam, O.L.T., Tanaka, T., & Ward, D.J. (2016)

A new elusive otodontid shark (Lamniformes: Otodontidae) from the lower Miocene, and comments on the taxonomy of otodontid genera, including the ‘megatoothed’clade.

Historical Biology, (ahead-of-print publication) PDF LINK

 

"The otodontid taxonomy has been in flux largely due to the introduction of multiple generic names aimed to include the iconic fossil megathoothed species, megalodon, such as Carcharocles Jordan and Hannibal, Procarcharodon Casier, and Megaselachus Glikman. Zhelezko and Kozlov (1999) considered many of these ‘megalodon-clade genera’ not to merit a genus status and placed them as subgenera under the genus Otodus Agassiz."

 

"Some workers have begun to consider taxa of the ‘megalodon-clade’ to belong to the genus Otodus (e.g. Andrianavalona et al. 2015; Malyshkina & Ward 2016). However, the previously used ‘megalodon-clade genera,’ such as Megaselachus and Carcharocles, continue to be used in literature to date (e.g. Ehret & Ebersole 2014; Laurito et al. 2014; Carrillo-Briceño et al. 2015; Pimiento & Balk 2015; Pimiento et al. 2016). We note here that not using any of those non-Otodus genera for the megalodon-clade taxa can be justified on the basis of systematic principles."

 

 

This does indeed answer very well my question. Thanks a lot!

 

Max

[Max-fossils]

22 hours ago, tmaier said:

That explaination clears it up for me. Very well done. You should lecture.

Agreed.

22 hours ago, FossilDAWG said:

That's what I do (University of Georgia).

 

Don

And done!

 

It doesn't surprise me that you lecture, and it wouldn't surprise me either if you are a real professional at it!

It must be great to do something like that, as long as you lecture in front of an interested audience.

And @FossilDAWG, I'm sorry if my question wasn't very clear, but I'm glad it wasn't because I got to be a VIP listener of your lecture

 

Best regards, and thanks so much,

 

Max

[Max-fossils]

17 hours ago, Al Dente said:

As Don already stated, Otodus megalodon is the most current. Before that it was Carcharocles megalodon. There really isn't a difference between Otodus obliquus, Otodus subserrata and Carcharocles auriculatus beside the amount of serrations and age.

That's interesting, I didn't know that before. Thanks!

Also, Lamna obliqua is the old name for Otodus obliquus, right?

 

Best regards,

 

Max

[erose]

On December 4, 2016 at 2:54 PM, Max-fossils said:

... And I know that two completely different genera can have the same species name (eg: Liopleurodon ferox and Titanosuchus ferox, etc.), ...

 

Yes, different species can share the same trivial name such as ferox above. But as much as possible they will be from very different groups. By your example above one is a therapsid and the other a marine reptile and they are from different periods of earth's history as well.  In earlier days of paleontology there were sometimes very similar genera given similar species names for fossils found in association, but that is now frowned on. 

 

It is a bit dense but you can read the International Code of Zoological Nomenclature (http://www.iczn.org) which is the standard for naming species, both fossil and extant, by which scientists are supposed to follow. 

 

 

[Max-fossils]

1 hour ago, erose said:

 

Yes, different species can share the same trivial name such as ferox above. But as much as possible they will be from very different groups. By your example above one is a therapsid and the other a marine reptile and they are from different periods of earth's history as well.  In earlier days of paleontology there were sometimes very similar genera given similar species names for fossils found in association, but that is now frowned on. 

 

It is a bit dense but you can read the International Code of Zoological Nomenclature (http://www.iczn.org) which is the standard for naming species, both fossil and extant, by which scientists are supposed to follow. 

 

 

Yeah, that's what I meant with "completely different". But thanks anyways!

 

Best regards,

 

Max

[ashcraft]

22 hours ago, FossilDAWG said:

 My research is very different, think suck your blood".

 

 

 

So you are studying my children?

 

Another semester in the bag

Brent Ashcraft