Sunday, February 14, 2010

A river runs through an Oligocene sea: parte II

It’s been a while since the last post, mostly because of a class that takes most of my free time as well as some family situation back in December and January. This is the first post of the year and, hopefully, more will follow. Anyways, today I bring you a little about fieldwork that I did back in January while I was in Puerto Rico. I had written in a previous occasion about this locality (the reason why this is part II) and unlike that other time, I did find some vertebrate fossils.

The picture above is of one of the most productive spots along this locality. As you can see there are about four distinct units. I and III are paleosols (ancient soil horizons) whereas II seems to be shallow marine/brackish and IV shallow marine. Unit II has yielded good fossils in the past, including a sirenian skull and associated axial skeleton (currently under study), croc teeth, a rodent incisor and some nurse shark teeth (gynglymostomatids). These beds are part of the San Sebastian Formation of Early Oligocene age, which in the past have yielded other cool fossils such as the sirenian Caribosiren turneri (Reinhart, 1959) and the gharial Aktiogavialis puertoricencis (Vélez-Juarbe et al. 2007), among others.

Like I mentioned, this time around I did find some cool stuff!

Above is a closeup of unit II showing some of the fossils before I started digging. The red circles are for turtle shell fragments and the green is a sirenian rib. Yes, I know, they are somewhat difficult to see but click on the picture and look carefully, you’ll see them.

The sirenian rib was isolated and easy to collect; it is fairly normal to find isolated sirenian ribs in the San Sebastian Fm and other middle Tertiary localities. While digging around the turtle shell fragments shown in the picture above, I kept stumbling upon more fragments, until it was apparent that this represented a partially disarticulated turtle shell. Not only that, but there was also an associated left pelvis (shown in the picture below), not bad!!

One of my first posts was about the fossil side-neck turtles from Puerto Rico. In it I mentioned that some pelomedusid (a more technical name for side-necks) material from the San Sebastian Fm. had been described by Wood (1972) as an unknown taxon. In fact, Wood (1972) not only described an incomplete shell and plastron but also an associated pelvis. Maybe the material I recently collected represents additional material of that unknown taxon. But comparison with the description as well as with other turtle fossils from the overlying Lares Limestone will have to wait until the summer when I go back to Puerto Rico and open the jackets (you can see them in the picture below).

Oh, by the way, I’m pretty sure this is a pelomedusid, just by the morphology of the pelvis. Hope you enjoyed the pictures! Also, there was one other fossil collected that day (is in the jacket in the far left), it was both interesting and frustrating, but I'll leave that for next time!

Reinhart, R. H. 1959. A review of the Sirenia and Desmostylia. University of California Publications in Geological Sciences 36(1):1-146.

Vélez-Juarbe, J., C. A. Brochu & H. Santos. 2007. A gharial from the Oligocene of Puerto Rico: transoceanic dispersal in the history of a non-marine reptile. Proceedings of the Royal Society B 274:1245-1254.

Wood, R. C. 1972. A fossil pelomedusid turtle from Puerto Rico. Breviora 392:1-13.