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Petalodus12

Were the cephalic spines of Xenacanthid sharks venomous

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Petalodus12

Hi all,

I have seen and heard from multiple different sources that the cephalic spines of Xenacanthid sharks are considered to have been venomous. This is usually supported by the serrated nature of these spines and a canal that runs down the middle of them. Has any research been done to prove or disprove this hypothesis. I know that we can never know for sure but I am curious if there is any scientific support to these claims. Thanks in advance,

Zach

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Darktooth

I am curious to find this out myself.:popcorn:

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fossilsonwheels

I researched this for our education programs and I could not find a study that could provider any proof. I am not saying there is not one out there but as far as I could find, it is just a theory.

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connorp

I have seen this claim before but haven't found a reference either. From the perspective of modern fish, it is certainly a possibility. Several fish today have spines on the body which are venomous and can be extended to make the body harder to swallow, both of which discourage predators.

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fossilsonwheels

To expand on what Connor said, there are some extant sharks with venomous dorsal spines. The Spiny Dogfish, possibly other too I don’t know, and two Heterodontus sharks, Port Jackson and Horn Shark, are venomous. So it’s possible this evolved in ancient sharks. 

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connorp

Also, I don't think a venomous nature of the spines could ever truly be verified. I doubt any trace of a venom would be fossilized. Of course, we have to speculate a lot in paleontology, but my point is that I don't think there's a smoking "venomous" gun. For all we know, the canal in the middle could be a remnant from an ancient ancestor that served no purpose in xenacanths.

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fossilsonwheels
29 minutes ago, connorp said:

Also, I don't think a venomous nature of the spines could ever truly be verified. I doubt any trace of a venom would be fossilized. Of course, we have to speculate a lot in paleontology, but my point is that I don't think there's a smoking "venomous" gun. For all we know, the canal in the middle could be a remnant from an ancient ancestor that served no purpose in xenacanths.

I think that is an excellent point 

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Petalodus12

@connorp @fossilsonwheels @doushantuo thank you for the replies. I figured that there would be no way to prove or disprove this, because, as you guys said, the chemicals that would be responsible would not be preserved. Thank you all again for the information, it was very helpful

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jdp

Just a note about the figure-8 shaped cross sections: this is a structural feature of a lot of long and narrow structures that resist force in one primary axis. You see a similar figure-8/hourglass shape in the neural spines of Dimetrodon as well as a range of other things. There are mechanical reasons for this.  The fact it creates the appearance of a channel is just a side effect of this.

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Pemphix

As said: this will be not solved, cause there's no scientific prove (and probably will never).

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Petalodus12

So I guess that the general consensus is that the claim of venomous spines on Xenacanthid sharks is erroneous and impossible to actually prove or disprove. My thought is that this belief stemmed from the superficial resemblance of these spines to those of extant sharks and rays, which are venomous. The belief seems pretty widespread though, so hopefully others who are curious find this thread and have their questions at least somewhat answered. Thank you all again for the informative replies.

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Uncle Siphuncle

Are there any spines similar in structure on extant species known NOT to be venomous?  

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Coco

Recent Centrophoros squalosus has a spine on the dorsal fin, but I don’t think it’s poisonous. Squalus acanthias also has them.They may not look like Xenacanthid’s. I can do a pic if you are interested in but actualy it is the night.

 

Coco

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fossilsonwheels
4 minutes ago, Uncle Siphuncle said:

Are there any spines similar in structure on extant species known NOT to be venomous?  

Yes. Squalidae have dorsal fin spines and most are not venomous as far as I know. The Spiny Dogfish is venomous and that is the only one I can say I know is venomous. There could be others as it is a large family

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fossilsonwheels

Though I should note I am not sure how similar in structure they are. I think there may also be members of Heterodontus whose spines are not venomous. I know Horn and Port Jackson sharks are but I don’t know about the rest of that order. 

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Archie

In the specimens Ive found the canal runs down the length of the spine but does not seem to extend through the denticles/barbs. 

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