Following up on a comment I made over at Updates from the Vertebrate Paleontology Lab, I here bring you pictures of an unusual sirenian chevron from the Late Oligocene of Puerto Rico.
These fossils were collected as part of a partial articulated postcranium which I mentioned here. Chevrons, also known as hemal arches, are (normally) V-shaped* bones, consisting of two rami that meet ventrally, hence the shape. They protect blood vessels.
Chevrons, also known as hemal arches, are (normally) V-shaped* bones, consisting of two rami that meet ventrally, hence the shape. They protect blood vessels.
*More like Y-shaped due to the length of the symphysis in some specimens.
In the picture above you can see left lateral views of two of the chevrons I collected with that specimen. The chevron on the left seems to represent fused chevron 2 + 3 (missing part of its ventral edge), and the other one is chevron 4, which is normal.
Here is a posterior view, again, the one on the left likely represents fused chevron 2 + 3, and on the right is chevron 4, which is missing the right ramus.
To me this seems to have been a developmental anomaly rather than occurring due to an injury. Apparently, this is a first, at least for Sirenia! As I mentioned above these are part of an articulated partial postcranium, belonging to a new dugongine taxon from the Late Oligocene of northern Puerto Rico (you can see the skull here).So, leave a comment, let me know what you think about this unusual bone!
Very interesting bone!
Jorge, why do you think it's developmental rather than pathological? Is there no secondary bone growth? It does seem like the left side of chevron 3 is oddly shaped (or is that just postmortem deformation?).
I don't know if you've prepared the vertebrae yet, but there might be some effect on the corresponding caudal vertebrae, whether it's pathological or developmental.
The reason I think is developmental is that there is no secondary growth, at least none that is obvious. The bones are deformed, postmortem.
Yes, I've prepared the vertebrae and they seem normal. Actually, the whole specimen is a bit weird, the vertebrae and ribs are osteosclerotic but not pachyostotic. At least not to the degree seen in other specimens (even older ones or taxonomically different). Other postcranial material from the locality is similar. It seems this particular species (a dugongine) displays this plesiomorphic condition.
Sabes q las plantas nucleares tienen unos sistemas q se llaman Chevron? Ja, y tienen la misma forma de estos huesitos osea en forma de "V".
Chevron es un nombre para esa forma V, como el símbolo de la compañia petrolera de ese nombre.
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I agree that this seems to be a developmental anomaly, and not pathological sensu stricto. I think I've seen something similar in some modern odontocetes, but never a sirenian. It would be interesting to survey modern Trichechus and Dugong collections for this sort of thing to see if this is found in any sort of frequency. My first guess would be that it isn't.
Embryologically, this could probably be explained in the same way as the double-headed ribs found in some modern and fossil mysticetes, like in Butch Dooley's paper from several years back. Haemal arches, transverse processes, and proximal rib portions all come from related mesenchyme, and it would make sense to consider that developmental issues that could affect one would affect the others sometimes too, right?
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