UF 257195, the right dentary of a turtle, possibly Rhinoclemmys panamaensis (dorsal view). Photo © VP FLMNH.
Today’s Fossil Friday post is on the dentary, or lower jaw bone, of a turtle (possibly Rhinoclemmys panamaensis) found at the Hodges Microsite locality in the Cucaracha Formation of the Panama Canal Basin. The fossil is early Miocene in age. Its large size is notable as it is larger than any modern species of Rhinoclemmys.
To learn more about this specimen, read the publication on its discovery here.
Cadena, E., Bourque, J., Rincon, A., Bloch, J.I., Jaramillo, C., and MacFadden, B. 2012 New Turtles (Chelonia) from the Late Eocene through Late Miocene of the Panama Canal Basin. Journal of Paleontology 86: 539-557. doi: 10.1666/11-106.1
UF 242111, the right second peripheral of an podocnemidid turtle (genus indeterminate). Photo © VP FLMNH.
This week’s Fossil Friday features the peripheral of a podocnemidid turtle! This specimen was found in El Lirio Norte of the Culebra Formation and is early Miocene in age. The family Podocnemididae belongs to the suborder Pleurodira. The most common depiction of turtles shows them pulling their heads into their shells in order to escape danger (suborder Cryptodira); however, pleurodires are distinct in that they hide by pulling their head and neck to the side under an overhang of their carapace.
The twist-necked turtle Platemys platycephala is also a pleurodire (family Chelidae). In this photo, the turtle has pulled its head to the side and glances out from under the overhang of its shell. Photo © http://www.birdphotos.com, taken from Wikimedia.
PCP-PIRE intern Justy Alicea measuring turtle skulls for his morphometric analysis
In order to tell whether the turtle skull I am studying is different than the skulls of modern relatives, I needed to run a morphometric analysis. This means I needed to take measurements of many different features on many different specimens and then analyze the relationships among those measurements. There are a few ways to do this. Traditionally, measurements of lengths, angles, masses, etc are taken and compared between a set of specimens. These measurements are essential and are the first step in studying variation in a species. We can use this data to understand development, as in a growth series, or population differentiation across space. The main challenge for interpreting these relationships is that most of these measurements are correlated, for example, forearm length will vary with the length of the humerus. In order to make these comparisons informative, we have to remove the effects of size. We do this by standardizing the data.
As one may imagine, it is difficult to make comparisons as the data sets get larger and larger in terms of number of specimens and number of measurements. There are programs that can be used to help visually represent this variation. R is a statistical analysis program that allows you to write computer code to take the data and manipulate it in all kinds of ways. The manipulated data can be visually represented by any number of graphs and charts. As in all code writing, there is a lot of going back and forth, fixing code errors that can be as small as a misplaced comma. I started by looking at box plots of my measurements to see where the greatest variation was and then looked over bivariate plots to see which pairs of measurements have most interesting or unexpected relationships.
I am still working my way through a great R tutorial created by DataCamp (www.datacamp.com) and hosted by the swirl project (www.swirlstats.com). It’s not easy but it’s not unlike learning how to use a suped-up scientific calculator. I still have a lot to learn but I can see the power of adding a program like R to my Paleo toolkit.
UF 242076, a peripheral element of the plastron of Staurotypus moschus. (Photo © VP FLMNH)
This Fossil Friday we have a peripheral element of the plastron (ventral surface of the shell) of a kinosternid turtle called Staurotypus moschus. This fossil, which is early Miocene in age, was collected by Michael Kirby under the Centenario Bridge from sediments of the Cucaracha Formation. The specific name for Staurotypus moschus was chosen due to the turtle’s relatively deep anterior musk duct groove. In life, the musk duct groove would have held a gland that contained foul-smelling musk that the animal release when disturbed.
To read more about this specimen, read the publication on it here.
A photo of a modern Staurotypus salvinii, the Chiapas Giant Musk Turtle. The publication describing S. moschus claims that the fossil turtle has its closest affinities with S. salvinii. (Photo © L. A. Dawson)
UF 237887, the partially articulated shell of Rhinoclemmys panamaensis. (Photo © VP FLMNH)
This Fossil Friday I would like to introduce you to Rhinoclemmys panamaensis. This specimen was collected from the Cucaracha Formation under the Centenario Bridge and is early to middle Miocene in age. The genus to which this species belongs (Rhinoclemmys) is commonly called the Neotropical wood turtles and the members of this genus are the only living representatives of the family Geomydidae in the New World (although they have great diversity in the Old World).This species is distinguished from other members of the genus by its large size, and it might have looked most like the extant black wood turtle, or black river turtle, Rhinoclemmys funerea.
If you would like to find out more about this specimen and other turtles from Panama, read the publication here.