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Calvert Cliffs - Hemis for the HemiHunter

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HemiHunter

I reached out to Dr. Kent asking his opinion on the tooth.  I'll get back to everyone if I hear anything back.

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WhodamanHD

@MarcoSr Indeed, we often see what we want and it’s impossible to know exactly what the dentition looks like without a natural, associated dentition (and better yet an articulated one). I don’t think it is impossible one is found someday.
 

The reason I lean Isurus is because it seems more parsimonious, one makes less assumptions. It is much easier to assume Isurus retroflexus is descended from something like Isurus praecursor or perhaps more likely Isurus americana and gave rise to the similar looking Isurus paucus than to assume it came from some mystery alopiid into something which happens to look like I. paucus. Of course it is possible, convergence is always lurking. But it’s just as likely the similarities between I. retroflexus and alopiid are convergent (like the very Alopiid-esque intermediate teeth of I. paucus). Who knows what will come out tomorrow, it’s all up for a debate and merely an opinion at this point. 
 

P.S. Extant Alopias vulpinus has parasymphseals, see dentition below

ED59B3E7-39E6-4E79-A363-78213B39515A.jpeg

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MarcoSr
7 hours ago, WhodamanHD said:

@MarcoSr Indeed, we often see what we want and it’s impossible to know exactly what the dentition looks like without a natural, associated dentition (and better yet an articulated one). I don’t think it is impossible one is found someday.
 

The reason I lean Isurus is because it seems more parsimonious, one makes less assumptions. It is much easier to assume Isurus retroflexus is descended from something like Isurus praecursor or perhaps more likely Isurus americana and gave rise to the similar looking Isurus paucus than to assume it came from some mystery alopiid into something which happens to look like I. paucus. Of course it is possible, convergence is always lurking. But it’s just as likely the similarities between I. retroflexus and alopiid are convergent (like the very Alopiid-esque intermediate teeth of I. paucus). Who knows what will come out tomorrow, it’s all up for a debate and merely an opinion at this point. 
 

P.S. Extant Alopias vulpinus has parasymphseals, see dentition below

ED59B3E7-39E6-4E79-A363-78213B39515A.jpeg

 

 

Thanks for posting this.  That plate is from Bass 1975 and represents the dentition of a single female specimen.  Definitely a senior moment for me with the parasymphyseals and the extant threshers in my previous post.  The A. vulpinus can have both upper and lower parasymphyseals in some sharks as in the single female in Bass 1975 but can also have no parasymphyseals as Bass mentions for the Bigelow and Schroeder specimens.  My single jaw doesn't have parasymphyseal teeth.  A. pelagicus can have lower parasymphyseals in some sharks.  My A. pelagicus jaws do have lower parasymphyseal teeth which I totally forgot about.  The A supersiliosus jaws that I have don't have parasymphyseal teeth.  I haven't seen a paper that shows parasymphyseal teeth in their jaws.  The number of teeth in a symphysis can really vary within a species that can have teeth the the symphysis.  Looking at the two A vulpinus jaws in elasmo, one does show a set of only lower parasymphyseal teeth and the other jaw it is hard to tell from the pictures but there isn't a parasymphyseal  set for sure in the upper jaws.  Publications not being consistent on extant jaws is another problem that I run into.  That is why I like to have my own jaws.  But it doesn't help when I don't remember what teeth are in my jaws.  That is why I try to take pictures of the teeth in my jaws so I can quickly refresh my memory.

 

I just want to pass along what Robert Purdy from the Smithsonian used to say when people would argue that the fossil record proved a point.  Basically that less than 1% of fossil sharks had been described and that there were many missing links to be discovered.  I don't know if his percentage was right but I understand what he was saying.  Sometimes we think that we know a lot more than we really do and there are definite discoveries that will change what we firmly believed.  I can tell you that in the most studied sharks like in the Otodus lineage and the Carcharodon lineage what we understand today is totally different than what I believed when I first started collecting sharks teeth years ago based upon the accepted researcher opinions of the time.

 

Marco Sr.

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siteseer

Hi Marco Sr.,

 

I don't think those are retroflexus parasymphyseals either.  The roots don't look right for the position and they are too big.

 

I see what you're saying about the first laterals actually being anteriors.

 

It interesting to see someone make the attempt to build a retroflexus dentition.

 

Jess

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MarcoSr
5 hours ago, siteseer said:

Hi Marco Sr.,

 

I don't think those are retroflexus parasymphyseals either.  The roots don't look right for the position and they are too big.

 

I see what you're saying about the first laterals actually being anteriors.

 

It interesting to see someone make the attempt to build a retroflexus dentition.

 

Jess

 

Jess

 

Yeah the size and root features are wrong for parasymphyseals.

 

It's nice to see that many teeth together.  Although I don't think that they are all retroflexus.

 

Also I'm not sure how the number of files was determined.  You can't just eyeball the teeth or use relative size.  You should take tooth measurements and use measurement ratios, crown angles etc to group your teeth and then put the groups in files.

 

I'm waiting for a call back from Mel on the dentition that he reconstructed.  If he has pictures, I'll post them.

 

Marco Sr.

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HemiHunter

I reached out to Brett Kent at UMD as @siteseer suggested and this is what he had to say about all of this. Very interesting!  Thanks for the idea.

 

-Paul

 

"That's a very pretty tooth. It appears to be a lower posterior Isurus retroflexus. The root is somewhat incomplete, which could mean either that it was a replacement tooth (and the root was never completed) or that the root was eroded during fossilization. 
 
The problem of identification of fossils to species is a universal one. For living organisms we can use a biological species concept, where we can identify species by the degree to which they can freely interbreed to produce fertile offspring. But with fossils we are restricted to either typological or morphological concepts of a species where species are identified by similarity of shape. The difficulty with the older typological species concept is that it ignores the range of variation within species by assuming that such variation is trivial. This species concept was pretty much abandoned by paleontologists by the middle of the last century as being unworkable and unreliable. The more recent morphological concept accepts that variability is the norm within species and identifies species by statistically quantifying and analyzing this variation. The problem is that species, both living and fossil, frequently have some minor degree of overlap in shape. That means that some specimens cannot be unambiguously assigned to a single species, but could belong to one of two or more related species. 
 
The question of whether retroflexus belongs in the genus  Anotodus or the genus Isurus is an example of these difficulties. Using a typological approach either seems plausible. A morphological approach with statistical shape analysis finds that retroflexus teeth are very close to extant I. paucus, to the point that some researchers have suggested that they are the same species. The problem is that there are no extant Anotodus dentitions for comparison and leaves the issue incompletely resolved. But there are other forms of evidence that can be used. For example, the pattern of nutrient foramina in  retroflexus is like those of I. oxyrinchus and Carcharodon hastalis, and different from that in Alopias (both small-toothed and giant species). Since the distribution of nutrient foramina appears to be very conservative within a lineage we'd expect the foramina of retroflexus (if it belonged in the genus Anotodus) to be more like that of alopiids than those of the lamnids. There has also been a reasonable amount of research lately on tooth development within lamniforms. While not conclusive, retroflexus in a range of sizes appear to follow a developmental trajectory more like that of the lamnids than the alopiids. So in the end, we still can't say for certain that Anotodus or Isurus is the correct genus for retroflexus, although the evidence is favoring Isurus. 
 
I'd agree that studying extant dentitions is very important, but there are some pitfalls for the unwary. First, make certain the fossil teeth and the teeth in the dentition are the same size. Tooth shape changes during development and this can cause problems. For example, a colleague and I have been recently studying juvenile Carcharhinus dentitions in preserved sharks at the Smithsonian. Remarkably, every small juvenile bull shark ( C. leucas) we looked had teeth that were indistinguishable in shape from those of adult silky sharks ( C. falciformis). The teeth were very different in size, but were remarkably similar in shape. Second, look at multiple extant dentitions from each species, since there can be substantial variation between individuals. Some years ago I studied the Carcharodon carcharias jaws in Gordon Hubbell's collection and the variation in tooth shape was staggering. Finally, an overreliance on extant dentitions can produce a bias in your analysis. Stephan Bengtson has referred to this phenomenon as "the present is the keyhole to the past" and Mark Pagel has dubbed it the "everyanimal" effect: an overreliance on living species dooms extinct species to be generic versions of extant ones. We all have our own subconscious biases about how dentitions of extinct sharks should look based on extant dentitions. Unless we exercise caution we can reach conclusions that are consistent with our preconceived ideas, while ignoring variation that could be critical in understanding extinct species. The students in my lab and I are constantly challenging each other about whether the interpretation of a specific tooth is based on data or on a subconscious preconception. The resolution of that conundrum is not always clear, but it needs to be considered each and every time."

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