Saturday, November 29, 2008

Fossil side-necked turtles from Puerto Rico

As I have learned through my many fieldtrips around Puerto Rico one of the most common fossils in the Tertiary rocks are fragments of turtle shells. In fact one of the first vertebrate fossils I collected (back in 2001) was a turtle shell from the Juana Díaz Formation (Early Oligocene age), which is now in the paleo exhibit at the Geology Museum at the Department of Geology, University of Puerto Rico, Mayagüez Campus. As I latter found out this fossil represented a shell of a pelomedusoid turtle, more commonly known as side-necked turtles (image below of Podocnemis expansa taken at the Museo de Historia Natural Javier Prado in Lima, Perú). Nonetheless it wasn’t the first time that fossil side-necks or other kinds of turtles were found as fossils in Puerto Rico.

One of the first accounts about fossil turtles from Puerto Rico is found in Rabell-Cabrero (1914) where he briefly mentions the occurrence of turtle costal and neural plates among other vertebrate remains that he had collected. This material was collected in Salto Collazo in the town of San Sebastián (northwestern PR), which means that the fossils came from Oligocene age rocks (Rabell-Cabrero, 1914; MacPhee & Wyss, 1990). Most of the vertebrate material collected by Rabell-Cabrero was later donated to the American Museum of Natural History (MacPhee & Wyss, 1990).

Some of the Rabell-Cabrero material was subsequently described by Wood (1972). In this paper Wood describes a shell, plastron and pelvis collected from the San Sebastián Formation (Early Oligocene age) but due to the incompleteness of the material no binomial was given, although similarities with living and extinct South American pelomedusids were noted (Wood, 1972).

In the late 1980’s an expedition from the AMNH recovered side-necked fossils from Early Miocene deposits in northern PR including a shell, proximal humerus and pubis (MacPhee & Wyss, 1990). Although, as they mention, the material is well preserved, a proper description is still pending. This might be in part because the conservative shell morphology of most pelomedusoids makes identification below family level difficult (Gaffney & Zangerl, 1968; Wood & Diaz de Gamero, 1971; MacPhee et al. 2003).

More recently a pelomedusoid skull from Puerto Rico was described by Gaffney & Wood (2002). This skull is now the type species of Bairdemys harsteini, a new genus that also includes three species from Venezuela, B. venezuelensis (formerly known as Podocnemis venezuelensis Wood & Diaz de Gamero, 1971), B. sanchezi and B. winklerae (Gaffney & Wood, 2002; Gaffney et al. 2008). Bairdemys harsteini was collected from Early Miocene deposits of the Cibao Formation in northern Puerto Rico, in addition to the skull, some shell fragments collected by MacPhee & Wyss (1990) from a nearby locality have been referred to this taxon (MacPhee et al. 2003). One interesting aspect of the Venezuelan species B. venezuelensis is that the shells do not have neural bones in the carapace, which might be an autapomorphy of that taxon (the carapace of the other Venezuelan species is still unknown) as neurals seem to be present in the shell material assigned to B. harsteini (Wood & Diaz de Gamero, 1971; Gaffney & Wood, 2002; Gaffney et al., 2008).

            Now going back to the Juana Díaz pelomedusoid I mentioned at the beginning, this shell, although missing approximately the anterior half, does have preserved the last three neurals, and seems to have been approximately the same size as the material described by Wood (1972). No additional material from the Juana Díaz or San Sebastián formations has been collected recently (at least not by me or anyone else that I know of). Most of the turtle material I have collected lately actually comes from the Lares Limestone (Late Oligocene age-northern PR). So far the best material from the Lares Ls seems to be from side-necks as well and fairly similar in size and morphology to the Juana Diaz and San Sebastián forms (see drawing comparing these with a Cuban shell mentioned below). Additional preparation (one plastron is still in a plaster jacket and a shell is in a carbonate concretion) might give more information as to whether they represent the same unknown taxon.

            More fossil side-necks are known from the Caribbean region, such as Caribemys oxfordiensis from the Jurassic of Cuba, an unknown pelomedusoid from the Early Miocene of Cuba, as well as the many species from Colombia and Venezuela including the largest freshwater turtle Stupendemys geographicus.

Gaffney, E. S. & R. C. Wood. 2002. Bairdemys, a new side-necked turtle (Pelomedusoides: Podocnemididae) from the Miocene of the Caribbean. American Museum Novitates 3359:1-28.

Gaffney, E. S. & R. Zangerl. 1968. A revision of the chelonian genus Bothremys (Pleurodira: Pelomedusidae). Fieldiana Geology 16:193-239.

Gaffney, E. S. T. M. Scheyer, K. G. Johnson, J. Bocquentin & O. A. Aguilera. 2008. Two new species of the side necked turtle genus, Bairdemys (Pleurodira, Podocnemididae), from the Miocene of Venezuela. Paläontologische Zeitschrift 82(2):209-229.

MacPhee, R. D. E. and A. R. Wyss. 1990. Oligo-Miocene vertebrates from Puerto Rico, with a catalog of localities. American Museum Novitates 2965:1-45.

MacPhee, R. D. E., M. A. Iturralde-Vinent & E. S. Gaffney. 2003. Domo de Zaza, an Early Miocene vertebrate locality in south-central Cuba, with notes on the tectonic evolution of Puerto Rico and the Mona Passage. American Museum Novitates 3394:1-42.

Rabell-Cabrero, N. 1914. Notas paleontológicas. Revista de las Antillas 2(1):66-69.

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

Wood, R. C. & M. L. Diaz de Gamero. 1971. Podocnemis venezuelensis, a new fossil pelomedusid (Testudines, Pleurodira) from the Pliocene of Venezuela and a review of the history of Podocnemis in South America. Breviora 376:1-23.

2 comments:

Brian Lee Beatty said...

This is really interesting, Jorge, thanks for sharing it. I wonder, are cryptodires also known from these rocks?

J. Velez-Juarbe said...

Hi Brian, thanks!! No cryptodires known so far from these rocks. However, there are some Pleistocene records of Geochelone, such as Geochelone monensis from nearby Mona Island (Williams, 1952) as well as Trachemys cf. decussata from northern PR (Pregill, 1981). According to Hedges (2006) the origin of Trachemys extends back to the Late Oligocene. But, this is based on molecular data and Oligocene or Miocene fossils belonging to this genus have not been found yet anywhere in the Antilles to support this estimate. So maybe they were there and the fossils are still out there waiting to be found, or these arrived more recently to the region.

Hedges, S. B. 2006. Paleogeography of the Antilles and origin of West Indian terrestrial vertebrates. Annals of the Missouri Botanical Garden 93:231-244.

Pregill, G. K. 1981. Late Pleistocene herpetofaunas from Puerto Rico. Miscellaneous Publications University of Kansas Museum of Natural History 71:1-72.

Williams, E. E. 1952. A new fossil tortoise from Mona Island, West Indies, and a tentative arrangement of the tortoises of the world. Bulletin of the American Museum of Natural History 99:541-560.