Fish Jaw or Spine
Paleo mysteries can be fun, but if not answered they can be exasperating. One of the Falls of the Ohio State Park volunteers found what appears to be a small but extremely well-preserved bone at the famous Sulphur Exit Road Cut in Crawford Co., Indiana a month or two back. If you know anything about fish osteology, I would appreciate your comments on what this might belong to. I certainly have no idea!
1. Looks like a Xenacanthid occipital spine.
Click Here to a pic of one from a Permian age Orthacanthus sp.
2. I am no expert at all on Paleozoic fish, this is just logic. The "teeth" seem to be exactly the same material as the "jaw", and with no demarcation between the teeth and the jaw.. If this was a true jaw with true teeth, I would expect there to be some difference between the two, with a clear demarcation. My vote is for a spine.
After looking at the magnified images, I think I DO see demarcation and also perhaps the presence of enamel. Also, the perforations in the "jaw" suggest the former presence of blood vessels or possibly nerves. I hereby reverse my vote!! It would be easier, of course, if I were looking through the scope myself rather than at an image.
3. Notwithstanding the presence of a few fenestrae on the side, suggesting blood vessel supply along the Shaft the overall length to diameter ratio (very high) and the very gradual taper suggests spine vs. jawbone. The long shaft would require significant muscle attachments midway in order to have any reasonable mechanical advantage and avoid the risk of shearing off at the first bite. I see no evidence of structures to support use as a jaw.
I vote for spine. That said, it is pretty cool.
4. I agree the overall shape screams "spine"...seeing what appears to be teeth can create the illusion of a jaw bone. However, while working with the pictures I noticed the holes that appeared "in-line" running the length of the specimen, on the same ridge, with a groove on each side. If it is a spine then I'm sure there are those on the list which have seen these holes on a single ridge running the length of other spines. If the fossil is a spine...I'm curious as to what the holes might represent.
5. Im not even remotely acquainted with the fish/fauna or plants of the Carboniferous, but what Im looking at suggests some sort of grasping appendage rather than jaw or spine. The protrusions are only at one end and only on one side. The very end (left side) appears to have segments ala an insect leg (there is even a taper at the joints). Could the pores be for getting oxygen to the internal tissue? It just seems to be more insect-like than fish-like or even plant-like. Thats my two-cents worth and probably not even worth that.
6. Take a look at plate 22. Looks similar to 7a
Spine: Marracanthus rectus
Geological Survey of Illinois
Vol 6 Geology and Paleontology 1875
Click Here to view plate 22
7. The openings are either Haversian canals or a reservoir for poison. Both are possible. The spines were for defense and needed nutrients to remain viable and possibly to store or inject a poison into a predator.
8. This makes a lot of sense and it may be that the openings serve multiple purposes. Many of the so called sharks of the Mississippian are actually more closely related to ratfishes. The dorsal spine of modern ratfishes is venomous and is attached to the leading edge of the dorsal fin with only a portion protruding from the skin of the ratfish.
9. After examining (and didn't take long)...Plate 22 and reading the comments regarding the openings along the shaft...There's no question in my mind that the specimen is a fish spine.
Plate 22: Marracanthus rectus (fish spine)
Marracanthus St. John and Worthen 1875 (chimaera)
Here is a Smithsonian web page [ click here ] with a picture of a (Mississippian) Marracanthus fossil. The picture is clickable to view various levels of magnifications. [archived image if the website is no longer on-line]
10. Not a jaw bone, but an outstanding specimen of a Mississippian "fish spine". The first I've ever seen with the circular openings. Not only the first picture I've seen with the openings...but the first images of the openings coupled with very informative comments (above) pertaining to their purpose.
11. One more comment. While it is true that Marracanthus is a fish, a better description is that it is a chimaeroid similar to modern ratfishes or rabbitfishes.
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