Sunday, February 14, 2010

A river runs through an Oligocene sea: part II

This is the first post of the year and, hopefully, more will follow. Anyways, today I bring you a brief overview of fieldwork that I did back in January while I was in Puerto Rico. I had written in a previous occasion about this locality (you can read part I here) and unlike back then, I did find some vertebrate fossils.
The picture above shows one of the most productive outcrops 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 deposits. Unit II has yielded good fossils in the past, including a sirenian skull and associated axial skeleton, croc teeth, rodent teeth and some nurse shark teeth (gynglymostomatids) (update: you can read about newer discoveries at this site and other nearby ones here, here, here, here, and here). These beds are part of the San Sebastian Formation of early Oligocene age, which in the past have yielded other interesting fossils such as the sirenian Caribosiren turneri (Reinhart, 1959) and the gharial Aktiogavialis puertoricencis (Vélez-Juarbe et al. 2007), among others.
Like I mentioned in the intro, this time around I did find some cool stuff!
In the photo above you can see a closeup of unit II showing some of the fossils as I found them and before I started digging. The red circles are for turtle shell fragments while 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 Oligocene and Miocene localities in Puerto Rico. In contrast, 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 the first posts on this blog was an overview of what's known of 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 this material I collected represents additional specimens of that unknown taxon. However, comparison with the description as well as with other turtle fossils from the overlying Lares Limestone will have to wait until after the specimens are prepared (you can get a glimpse of the preparation process here). Below you can see the jackets with the specimens inside.
Preliminarily, I am quite certain that this San Sebastian turtle is a pelomedusoid just by the morphology of the pelvis. Also, there was one other fossil collected that day (is in the jacket in the far left), which was both interesting and frustrating, but I'll leave that for next time!

Recommended Literature

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, and 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.

This post was updated April 28, 2020