In tropical regions, a lot of times, good outcrops are along riverbanks. Puerto Rico is no exception and that is why the Rio G locality is so good, the exposure is kept “fresh” because of the nearly constant river erosion. Of course the drawback is that fossils are also lost if nobody visits these type of localities at least once a month or after big rainstorms (at least that’s what I used to do).
The picture above shows one such example. I spotted this bone fragment on the wall, very close to water level (a little more than half a meter). And, as you can see in the inset, the surface facing away from the rock looks freshly broken. I did collect the fossil, and unsuccessfully looked for additional fragments nearby.
Above is the picture of the fossil, in dorsal view (anterior towards the top), and the interpretative drawing. As it turns out the fossil was part of a sirenian skull. What was left of the fossil, is the anterior part of the frontals (Fr) and the nasals (N), the supraorbital processes of the frontals are missing. The convex frontal roof and shallow nasal incisure (the concave area between the frontals) are some characters that identify this fossil as belonging to a halitheriine dugongid. In fact, it is very similar to the same part of a much more complete skull that I collected from that locality several years ago (see below).
The figure above shows the more complete skull. A and B show a close-up dorsal view (anterior towards the top) of the area that was preserved in the fragmentary fossil. C is a dorsal view of the skull (anterior to the right) with the outline of the enlarged area in A. As you can see the frontal and its relationship with the nasals look much the same as the fragmentary fossil. It also displays a shallow nasal incisure at the posterior end of the mesorostral fossa (MRF) and convex frontal roof. In the more complete skull the premaxilla (Pmx) partially cover the nasals and the supraorbital processes (SOP) are preserved. (In A and C it is missing the left nasal process of the premaxilla which is loose and needs to be re-attached, but I was able to put it in the drawing).
I’m pretty certain that the fragmentary fossil belongs to the same species as the more complete specimen; it was a pity that part of it was lost previous to it being found. If you’re well acquainted with extinct sirenians, you can probably guess what genus this skull belongs to. So, go ahead and make a guess!