Sunday, December 28, 2008

Among old corals & living spiders…

The winter holidays means that other than getting a break from the university I also get to go home where it is warmer than DC, get some good coffee and also get to do some fieldwork. But so far this has been a very unproductive field season. The weather has been very unstable with rain almost everyday, which is very unusual for this period of the year as it is supposed to be drier.

On the only good day of fieldwork so far, I went to a couple of road cuts in the northwest where the Lares Limestone of Late Oligocene age is exposed. These outcrops are very good and have produced so far, fishes (both bony and cartilaginous), sirenians (which are part of my thesis), unidentified crocodylians (described in Brochu et al. 2007), and pelomedusid turtles, which I mentioned in an older post. Invertebrates are also found, among them the crustaceans, which have been recently described (Schweitzer et al. 2006) and corals, which are well known and well preserved (Frost et al., 1983; Edinger & Risk, 1994). Picture below shows a close-up of a Montastrea sp. (left) and a large overturned coral (right).

Now that you have a general idea of what can be found in the Lares Ls, lets get back to my fieldwork; this time around the prospecting in these limestones was not the best. The only non-fish remains were an incomplete neural plate, most likely from a pelomedusid turtle and a large croc tooth. As I mentioned above crocs have been reported from the Lares Ls, nonetheless in contrast to the ones previously reported, this recent find (see picture below) differs from the ones described previously which are smaller, slender and similar to the ones found in longirostrine crocs (Brochu et al. 2007). This larger tooth is the third tooth of this type found in this formation, the other two already in the paleo collection a the Department of Geology at UPR-Mayagüez. Finding such teeth is actually a big tease, as they seem to indicate that during Lares time there were other crocs in addition to an unknown longirostrine form. That there might have been a longirostrine form during Lares time should not be a surprise, one species is already known from the underlying San Sebastián Formation of Early Oligocene age. The species, called Aktiogavialis puertoricensis, is related to South American gharials and together they form a monophyletic group called Gryposuchinae (Velez-Juarbe et al. 2007 [free pdf here]). As you see it should come as no surprise that there might have been a longirostrine croc during Lares time, but who was the owner of the other large teeth? A crocodyloid? An alligatoroid? There must have been something other than a longirostrine croc, but only by finding better specimens will this question be answer.

Croc teeth from the Lares Ls: the one on the left is similar to those reported in Brochu et al. (2007), the tooth on the right is the recent find notice the size & shape differences (scale bar = 1 cm).

I almost forgot about the spiders, well, since the fossil collecting was so crappy, the trip was at least good for taking photos of spiders that are found in this outcrop (see composite picture below). Notably among those are the southern black widows (Latrodectus mactans), one of the three species of black widows known to occur in Puerto Rico (Pérez-Rivera, 1980). This is one of the few outcrops where I really have to keep an eye out for these spiders, as they are very common here. The others are the silver argiope (Argiope argentata) and an orb weaver (Leucauge regnyi). So, enjoy the pictures and have a happy holiday!

 

References

Brochu, C. A., Á. Nieves-Rivera, J. Vélez-Juarbe, J. D. Daza-Vaca & H. Santos. 2007. Tertiary crocodylians from Puerto Rico: evidence for late Tertiary endemic crocodylians on the West Indies? Geobios 40:51-59.

Edinger, E. N. & M. J. Risk. 1994. Oligocene-Miocene extinctions and geographic restriction of Caribbean Corals: roles of turbidity, temperature, and nutrients. Palaios 9(6):576-598.

Frost, S. H., J. L. Harbour, D. K. Beach, M. J. Realini & P. M. Harris. 1983. Oligocene reef tract development, southwestern Puerto Rico. Sedimenta 9:1-144.

Pérez-Rivera, R. A. 1980. Distribución geográfica, potencial reproductivo y enemigos naturales de la viuda negra en Puerto Rico. Caribbean Journal of Science 15(3-4):79-82.

Schweitzer, C. E., M. Iturralde-Vinent, J. L. Hetler & J. Velez-Juarbe. 2006. Oligocene and Miocene decapods (Thalassinidea and Brachyura) from the Caribbean. Annals of the Carnegie Museum 75(2):111-136.

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.

Friday, December 12, 2008

What’s wrong with the hands of Steller's sea cow

When talking about species driven to extinction in historic times we automatically think of the Dodo, Carolina Parakeet, Tasmanian tiger, Caribbean monk seal among others. We might as well think of the Steller’s sea cow, Hydrodamalis gigas (picture below of one of the specimens at the NMNH).

H. gigas was a sirenian (sea cows: manatees & dugongs) that lived in the northern Pacific until about 240 years ago. This was one of the largest sea cows that have lived, only surpassed by Hydrodamalis cuestae from the Late Pliocene of California, which is estimated to have reaches up to 9.03 meters (~30 feet!) whereas one of the H. gigas measured by Steller (the first person to describe live specimens) was about 7.51 meters (~25 feet) (Domning, 1978).
The picture below is of a mounted skeleton of H. gigas at the Museum National d’Histoire Naturelle in Paris. It is most likely specimen A.14516, which is the only mounted skeleton at the MNHNP (Mattioli & Domning, 2006). It is a nice mount, but there is something wrong with it……

Look at the hand/flipper, its huge, and very dugong or manatee like (see the more detailed picture below). You see, G. W. Steller was one of the few persons to give an account of H. gigas from observing live (or recently killed) specimens (and hence the name Steller’s sea cow). His description of the hand is significant because the morphology is unlike that of any other known sirenian (Steller, 1899). According to Steller’s description, the forelimb of H. gigas had no fingers, in fact he describes the ends of the limb as having a posteriorly oriented hook-like structure made up of, most likely, stratified squamous keratinized epithelium (thickened hardened skin). The habitat of H. gigas were shallow waters where feeding would have exposed them to higher wave action, therefore the loss of fingers as well as having a hardened pad or surface would have provided more traction when using the forelimbs as propulsion or stabilization in these shallower waters. Domning (1978) concluded from the osteology and inferred myology that additional forelimb adaptations are also present in H. gigas. These adaptations such as reduction of some muscles and modifications to the elbow joint, made the limb better suited for movement in a more parasagittal direction (Domning, 1978).

To sum it all up, Hydrodamalis gigas had no fingers, it also had other forelimb adaptations that permitted it to “walk” in shallow marine substrates when feeding. The Paris mount is nice, but wrong, in that it has huge flippers instead of fingerless stumps.
You can see the second part of this saga here!!

Domning, D. P. 1978. Sirenian evolution in the North Pacific Ocean. University of California Publications in Geological Sciences 118:1-176.
Mattioli, S. & D. P. Domning. 2006. An annotated list of extant skeletal material of Steller’s sea cow Hydrodamalis gigas (Sirenia: Dugongidae) from the Commander Islands. Aquatic Mammals 32:273-288.
Steller, G. W. 1899. The beasts of the sea. (Translated by W. and J. E. Miller); pp. 179-218 in D. S. Jordan (ed.), The fur seals and fur-seal islands of the North Pacific Ocean. Part 3, Article 8. Government Printing Office, Washington, DC.

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.

Welcome to my blog, here I will post about fossils (mostly vertebrates) from the Caribbean region as well as other extinct critters that I find interesting. I’ll post mostly in English, but sometimes also en Español. Sometimes I might just blog about random subjects. So, once again welcome and enjoy.