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Isurus praecursor/ Macrorhizodus praecursor


BellamyBlake

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Are Isurus praecursor and Macrorhizodus praecursor synonyms? I have conflicting accounts on this and would like clarification please.

 

Thank you,

Bellamy

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3 hours ago, BellamyBlake said:

Are Isurus praecursor and Macrorhizodus praecursor synonyms? I have conflicting accounts on this and would like clarification please.

 

Thank you,

Bellamy

Yes, they are the same. The elasmo website has a long discussion on the classification of these teeth. 
 

 

 

4B09630C-6230-4D48-8C9C-B606DB1001A5.jpeg

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In my humble (unsolicited) opinion: M. praecursor is the ancestor to the Cosmopolitodus/Carcharomodus/Carcharodon lineage and M. americana which itself appears to be the ancestor to I. retroflexus/I. paucus. Isurus flandricus/desori/oxyrinchus seems to come from M. falcatus, which is itself the descendant of M. americana. This complex nesting and frequent speciation makes the group paraphyletic and problematic in terms of labels. I’m not sure you could ever get it where it makes real sense.

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“...whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved.” ~ Charles Darwin

Happy hunting,

Mason

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38 minutes ago, WhodamanHD said:

In my humble (unsolicited) opinion: M. praecursor is the ancestor to the Cosmopolitodus/Carcharomodus/Carcharodon lineage and M. americana which itself appears to be the ancestor to I. retroflexus/I. paucus. Isurus flandricus/desori/oxyrinchus seems to come from M. falcatus, which is itself the descendant of M. americana. This complex nesting and frequent speciation makes the group paraphyletic and problematic in terms of labels. I’m not sure you could ever get it where it makes real sense.

Some believe M. praecursor and M. americana are the same species. Here's an excerpt.

 

 

macro1.JPG

macro2.JPG

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10 minutes ago, Al Dente said:
50 minutes ago, WhodamanHD said:

 

Some believe M. praecursor and M. americana are the same species. Here's an excerpt.

Indeed, they are just chronospecies (same with M. falcatus) and one could lump them if they so desired. However, they do have different features and I believe gave rise to different species. To lump them would mean one species gave rise into more than two others (which is a problem in phylogenetic). 

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“...whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved.” ~ Charles Darwin

Happy hunting,

Mason

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3 hours ago, WhodamanHD said:

...To lump them would mean one species gave rise into more than two others (which is a problem in phylogenetic). 

This can be a problem in certain ways of analyzing phylogenies, but I'm not sure it is such a big problem in real world biology; it may be more of a mathematical artifact.  Let me give you an example, from Entomology not sharks but the principle is the same.  The locust species Schistocerca gregaria is ubiquitous and has huge populations in Africa.  Speciation has not occurred there because no population can remain reproductively isolated for enough time for genetic barriers to evolve.  However, on very infrequent (but historically documented) occasions locust swarms can be picked up by tropical storm systems/hurricanes that develop off the west coast of Africa, and at least some individuals may be carried across the Atlantic to islands in the Carribean.  Subsequently over time these invaders may island hop until they reach the mainland of North/Central/South America.  Each invasion is followed by rapid evolution due to founder effects, genetic drift, and adaptation to the new environment.  Invasions (or more properly vicariance events) occur at long enough intervals that more recent arrivals can not cross with individuals from earlier invasions due to accumulated genetic changes in the older populations, which are now species in their own right.  As a result of all of this, Africa has one species, Schistocerca gregaria, which has given rise to multiple species in the Americas such as Schistocerca americana, Schistocerca nitens, and 20-30 others.  In some approaches to phylogenetics (such as cladistics I think), Schistocerca gregaria would have to be given a new name or be treated as a new species every time one of these vicariance events spun off a new Americans species, even though absolutely nothing had changed in any biological sense in the African populations.  Biologically this makes no sense.

 

In general, it is thought that a common mechanism of speciation involves populations of a species becoming isolated by geographic or ecological barriers, with subsequent accumulations of changes until eventually the two populations can't interbreed if they do come back into contact.  There is nothing to say that the rate of accumulation of changes has to happen at the same rate in both populations, one could accumulate changes more rapidly than the other as a function of effective population size, gene pool diversity, and selection pressure.  There is also nothing to say that multiple isolated populations could independently arise from a larger population and so multiple species could be derived from the same common ancestor.

 

Don

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@FossilDAWG that is quite the example, a nightmare scenario for those who use cladistics! Thanks for sharing that, I’ll have to write it down for future reference. It has always seemed to me nature is far too complex to be explained by any one system or technique. Cladistics is a good tool but like anything one must pay attention to context.

“...whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved.” ~ Charles Darwin

Happy hunting,

Mason

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Just got done looking at some of these teeth from my collection and thought I would share them. This first batch is from the Eocene Castle Hayne in NC and is from a Lutetian outlier.

 

 

wells.jpg

 

 

This second tooth is also Castle Hayne but slightly younger Bartonian.

 

trent.jpg

 

 

This last group is Oligocene (Rupelian) River Bend Formation. Jerry Case described these as Isurus oxyrhincus back in 1980.

 

river.jpg

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