A day in the field, Tertiary

This time our field area is in northern Puerto Rico. We decided to visits a couple of outcrops of the Late Oligocene Lares Limestone. If the name of the formation sounds familiar you either know about the geology of Puerto Rico or, have read about it on a previous post.

One of these localities (see picture below), I have visited at least since 2000, and up until very recently, we thought that the only formations present there were the Early Oligocene San Sebastián Formation and the overlying Lares Limestone. Now, thanks to new information regarding the stratigraphy of the Tertiary limestones of the north coast of Puerto Rico (Ortega Ariza, 2009), we know that in this locality, overlying the Lares Ls, there are also units of the Montebello Limestone. The age of the Lares Limestone and Montebello Limestone were designated as Late Oligocene – lower Early Miocene and upper Early Miocene, respectively (Seiglie & Moussa, 1984). New data, using strontium isotopes obtained from tubes of the pelecypod Kuphus incrassatus, seems to indicate, instead, that both formations span the Late Oligocene (Johnson et al., 2006; Ramírez et al., 2006; Ortega Ariza, 2009). If this is correct (more samples need to be run, hint, hint!!) I will like this outcrop even more (sorry, can't hide my love for the Oligocene)!!

Here's the one of my favorite outcrops, where the Lares and Montebello limestones are exposed. The arrow points to a sirenian fossil that is yet to be collected.

Of course, what I’ve been mostly searching in these localities are sirenian remains, but like I mentioned on that previous post, other vertebrates have also been collected. Interestingly, the best sirenian remains have been collected from the upper Lares Limestone, with a total (so far) of two skulls, and a set of nine articulated vertebrae (see picture below). There are more fossils but those will be collected in due time. As for the sirenian skulls, well, they are an important part of my thesis work and I will discuss them at some point in the future.

Some articulated sirenian vertebrae, these have already been collected. This is an earlier picture, there were three more vertebrae behind the one labeled Ca1, the ones anterior to L3 were collected earlier.

References

Johnson, C. C., W. R. Ramírez, L. R. Mark, S. Y. Hernandez, E. A. Barrow, M. Hegewald & J. Velez. 2006. Oligocene reef deposits linked to OPD site 999 with strontium isotope stratigraphy. Geological Society of America Abstracts with Program 38:557.

Ortega Ariza, D. L. 2009. Establishing a high resolution sequence stratigraphy and sea-level curve for Tertiary limestones, Puerto Rico. M.S. thesis, University of Puerto Rico, Mayagüez, Puerto Rico, 132 pp.

Ramírez, W. R., C. C. Johnson, M. Martínez, M. C. Torres & V. Ortiz. 2006. Strontium isotope stratigraphy from Kuphus incrassatus, Cenozoic limestones, Puerto Rico. Geological Society of America Abstracts with Program 38:90.

Seiglie, G. A. & M. T. Moussa. 1984. Late Oligocene-Pliocene trangressive-regressive cycles of sedimentation in northwestern Puerto Rico. American Association of Petroleum Geologist Memoir 36:89-95.

A day in the field: Cretaceous

Cretaceous sedimentary rocks are found in Puerto Rico, especially in the southwest part of the island where several well-exposed limestone units are exposed. Our destination this time was a new outcrop of the Parguera Limestone. Located in the Southwest Igneous Province (Jolly et al., 1998; Schellekens, 1998), this formation, which ranges from Santonian to Campanian, has been divided into three units, the lower Bahia Fosforecente Member, the middle Punta Papayo Member and the upper Isla Magueyes Member (Almy, 1965). Like a lot of the Cretaceous limestone units in the Caribbean region, the age has been determined with the aid of the rudist bivalve assemblages, which have been divided into several biozones (Rojas et al., 1995).

A couple of rudist bivalves (red outline). During life the position of these was with the narrowest part semi-buried in the substratum (elevators). As we can see these are sideways.

There was some debate as to whether the outcrop we went to was part of the Bahia Fosforecente or Punta Papayo, the former which has been dated as Santonian whereas the latter as Campanian in age. Lithologically, this locality is most similar to the Bahia Fosforecente member. As we searched for fossils, we found several rudists (see picture above). These seem to have been transported, as these are elevators, but were found on their side. Although these were mostly complete, I must say I haven’t had the time to look in detail at their morphology, hence they remain nameless, for now.

One of the unknown rudist we collected (left); fragment of Macgillavryia nicholasi, notice the cell pattern (right).

Other rudists that were more fragmentary, were actually much more helpful for pinning down the age of the rocks here. Several fragments of the large* rudist Macgillavryia nicholasi were found and we were able to make an ID based on their diagnostic cell patterns (picture above) (Rojas et al., 1995). The occurrence of M. nicholasi indicates that these deposits are Campanian in age, as they are found in the Barrettia monilifera biozone of Rojas et al. (1995), meaning that these units are probably part of the Punta Papayo member.

*Some specimens reaching a diameter up to 1 meter!

In terms of the depositional environment, the Parguera limestone represents (mostly) slope to basin environments (Almy, 1965). This outcrop is different. The lithology here indicates that this was likely a nearshore deposit in a moderate/high-energy coast; sandy flat pebble conglomerates were the giveaway.

View of the outcrop of Parguera Limestone, rocks are dipping to the south (towards the left). To the far right, HSM & DLOA search for fossils.

Leaving what I think is most exiting for last; the whole reason for our visit to this outcrop was the search for fossils of tetrapods. One of us (DLOA) had found, on a previous visit, a non-fish vertebra*! We did not found anything else, but if we can get an id on what we have so far it would be a first! So, wish us luck!

*Update (Aug/2009): it most likely is an archosaur caudal vertebra!! Hat tip to MTC for the id!

Go here for a very good rudist database.

References

Almy, C. C., Jr. 1965. Parguera Limestone, Upper Cretaceous, Mayagüez Group, Southwestern Puerto Rico. Unpublished Ph.D. thesis, Rice University, Houston, 203p.

Jolly, W. T., E. G. Lidiak, J. H. Schellekens & H. Santos. 1998. Volcanism, tectonics, and stratigraphic correlations in Puerto Rico; pp. 1-34 in E. G. Lidiak & D. A. Larue (eds.), Tectonics and Geochemistry of the Northeastern Caribbean. Geological Society of America Special Paper 322.

Rojas, R., M. A. Iturralde-Vinent and P. W. Skelton. 1995. Stratigraphy, composition and age of Cuban rudist-bearing deposits. Revista Mexicana de Ciencias Geológicas 12(2):272-291.

Schellekens, J. H. 1998. Geochemical evolution and tectonic history of Puerto Rico; pp. 35-66, in E. G. Lidiak & D. A. Larue (eds.), Tectonics and Geochemistry of the Northeastern Caribbean. Geological Society of America Special Paper 322.

From land to sea

Some of the living marine mammals, like cetaceans, sirenians and pinnipeds* are so well adapted to a life in water that it might be difficult for us to relate them to their closest terrestrial relatives. However, we do know that the oldest members of these groups were indeed terrestrial. Here I’ll discuss some of the evidence known so far.

*(cetaceans = whales & dolphins; sirenians = manatees & dugongs; pinnipeds = seals, walruses & sea lion).

Cetaceans

Modern whales can be divided into two groups, odontocetes and mysticetes. Odontocetes are characterized for having teeth and using echolocation; mysticetes are characterized for having baleen instead of teeth (there are other adaptations that I won’t discuss now). Some examples of odontocetes are orcas and bottlenose dolphins; mysticetes include blue whales and right whales.

Based on molecular evidence, whales evolved from artiodactyls – a group that includes pigs, hippopotamus, camels, cows, lambs, etc – (Graur & Higgins, 1994; Shimamura et al., 1997), whereas, for a long time, morphological studies used to indicate a close relationship with mesonychids – a group of extinct terrestrial carnivores – (Luo & Gingerich, 1999). In part, the reason for this disagreement about the origin of whales was that the oldest fossils of cetaceans consisted of forms that were already completely adapted to a life in the water, or were only known from crania. This has already been resolved.

In 2001, two groups of paleontologist published papers where they described primitive cetaceans, including parts of the postcranium that corroborated an artiodactyl relationship (Gingerich et al., 2001; Thewissen et al., 2001). Gingerich and his team found remains of Artiocetus clavis and Rhodocetus balochistanensis, whereas the Thewissen team described Ichthyolestes pinfoldi and Pakicetus attocki (illustration above of Pakicetus by Carl Buell, taken from the Thewissen Lab webpage); the remains included one of the ankle bones, the astragalus, which was key to determine that cetaceans evolved from artiodactyls. All these fossils were found Middle Eocene (49-41 million years ago) deposits. More recently, Thewissen et al. (2007) describe postcranial material of the primitive artiodactyl, Indohyus, and show that it was an animal with aquatic adaptations, providing additional evidence about the origin of cetaceans. (Go here for a magnificent reconstruction of Indohyus).

Sirenians

The closest living relatives of manatees and dugongs are elephants, this relationships is supported by both, molecular and morphological evidence (Seiffert, 2007; Tabuce et al. 2007). Sirenians originated in northern Africa about 54 million years ago, where they last shared a common ancestor with proboscideans (elephants). Interestingly, the most primitive sirenian fossils have been found in Jamaica, which demonstrate that, very early, they seem to have been well adapted for life in an aquatic environment. For a long time, the most primitive sirenian known was Prorastomus sirenoides found in Jamaica in deposits that are between 51-49 million years old (Owen, 1855; Savage et al., 1994). Unfortunately, the postcranium was and it is still mostly unknown.

Another sirenian from Jamaica, found in slightly younger deposits – 49-45 million years old – was described by Domning (2001). The remains of this new sirenian, named Pezosiren portelli (illustration above from Domning, 2001), include cranial and postcranial material. The postcranial material include fore and hind limbs, pelvis and most of the vertebral column; all together, these indicate that Pezosiren was able to support its own weight on land, but at the same time, it had aquatic adaptations such as pachyosteosclerotic (enlarged & dense) ribs and in the cranium, retracted external nares (Domning, 2001). The combination of characters imply that Pezosiren spent time, both, in and out of the water.

Pinnipeds

Morphological and molecular studies show that pinnipeds belong to a group of mammals called arctoids (Deméré et al., 2003), that, along with pinnipeds, includes bears (ursids), weasels (mustelids), raccoons (procyonids) and skunks (mephitids), among others. Nonetheless, the origin of pinnipeds from one of these arctoids is not clear, and different studies place pinnipeds as originating from a common ancestor with mustelids or with ursids (Deméré et al., 2003). Other experts in the field support a multiple origin for pinnipeds, with seals sharing a common ancestor with mustelids and sea lion and walruses with ursids (Uhen, 2007 and references therein). Anyways, whatever is the group from which pinnipeds originated – this can only be resolved by finding more fossils – it is well known that these originate from a terrestrial ancestor. Interestingly, pinniped fossils that show a transitional morphology had not been found until recently.

The discovery of Puijila darwini in lacustrine sediments deposited between 23-21 million years ago, gives us an idea about the morphology of the earliest pinnipeds (Rybczynski et al., 2009). Although fossils of yet even older pinnipeds, such as Enaliarctos tedfordi and E. barnesi, have been found in rocks that date between 28.5-23.8 million years in Oregon (Deméré et al., 2003), these already show full adaptations to a life in the marine realm like those observed in modern taxa; this means that pinnipeds must have evolved previous to that date. So, even if Puijila (illustration above from Rybczynski et al., 2009) comes from younger deposits, its importance rests in that morphologically it is the most primitive known pinnipeds, providing evidence about the evolutionary steps that were taken in the transition from land to sea in this group of mammals.

Puijila the official website

Puijila in National Geographic

Also, here is the Spanish version of this post.

References

Deméré, T. A., A. Berta & P. J. Adams. 2003. Pinnipedomorph evolutionary biogeography. Bulletin of the American Museum of Natural History 279:32-76.

Domning, D. P. 2001. The earliest known fully quadrupedal sirenian. Nature 413:625-627.

Gingerich, P. D., M. ul Haq, I. S. Zalmout, I. H. Khan & M. S. Malkani. 2001. Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan. Science 293:2239-2242.

Graur, D. & D. G. Higgins. 1994. Molecular evidence for the inclusion of cetaceans within the order Artiodactyla. Molecular Biology and Evolution 11(3):357-364.

Luo, Z. & P. D. Gingerich. 1999. Terrestrial Mesonychia to aquatic Cetacea: transformation of the basicranium and evolution of hearing in whales. University of Michigan Papers on Paleontology 31:1-98.

Owen, R. 1855. On the fossil skull of a mammal (Prorastomus sirenoides, Owen), from the island of Jamaica. Quarterly Journal of the Geological Society of London 11:541-543.

Rybczynski, N., M. R. Dawson & R. H. Tedford. 2009. A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia. Nature 458:1021-1024.

Savage, R. J. G., D. P. Domning & J. G. M. Thewissen. 1994. Fossil Sirenia of the West Atlantic and Caribbean region. V. Prorastomus sirenoides Owen, 1855. Journal of Vertebrate Paleontology 14(3):427-449.

Seiffert, E. R. 2007. A new estimate of afrotherian phylogeny based on simultaneous analysis of genomic, morphological, and fossil evidence. BMC Ecolutionary Biology 7:224 Open access

Shimamura, M., H. Yasue, K. Ohshima, H. Abe, H. Kato, T. Kishiro, M. Goto, I. Munechika & N. Okada. 1997. Molecular evidence from retroposons that whales form a clade within even-toed ungulates. Nature 388:666-670.

Tabuce, R., L. Marivaux, M. Adaci, M. Bensalah, J.-L. Hartenberger, M. Mahboubi, F. Mebrouk, P. Tafforeau & J.-J. Jaeger. 2007. Early Tertiary mammals from North Africa reinforce the molecular Afrotheria clade. Proceedings of the Royal Society B 274:1159-1166.

Thewissen, J. G. M., E. M. Williams, L. J. Roe & S. T. Hussain. 2001. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature 413:277-281.

Thewissen, J. G. M., L. N. Cooper, M. T. Clementz, S. Bajpai & B. N. Tiwari. 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450:1190-1195.

Uhen, M. D. 2007. Evolution of marine mammals: back to the sea after 300 million years. Anatomical Record 290:514-522.

Field err… lab work

It’s field season of course, so I’ve been in Puerto Rico for a while now, unfortunately with my car dead, it has been difficult to do things. I actually wanted to leave fieldwork for latter in the summer and do some lab work now, and I have, to some extent. The lab work includes preparation of some crocodylian postcranial material and a sirenian skull. Of course, I haven’t really been able to keep myself from doing some fieldwork. So here’s some of what has been going on lately.

Lab work

The crocodylian material consists of vertebrae, ribs and dorsal osteoderms from the Early Oligocene San Sebastián Formation in northern Puerto Rico (see picture below). Fossil crocodylians have previously been collected from this formation (go here for a previous post on Caribbean crocs).  The material I am currently preparing has been slowly collected (thanks to a dangerous overhang) since 2006 from a unit that sits several meters below the unit where the cranium of Aktiogavialis puertoricensis was found (they were found the same day). With no known cranial material associated to this fossils, it cannot be referred to Aktiogavialis.

A couple of crocodylian ribs (center and lower left) from the San Sebastián Fm. This matrix is no fun.

The sirenian skull, originally found in 2003 and latter collected in 2005, belongs to an adult individual and it comes from the Late Oligocene Lares Limestone. Another skull, from a subadult individual that was collected in 2003 just meters away, both belong to the same dugongine taxon; these are part of my thesis project and will be properly described at some point. They represent the first sirenian cranial remains from this formation; sirenian fossils are known from the underlying San Sebastián Fm and overlying Cibao Fm (Reinhart, 1959; MacPhee & Wyss, 1990).

Trying to remove some annoying matrix from the Lares sirenian.

Field work

So far I’ve been to the field once. Colleagues from the Geology Department at UPRM and myself went to northwestern Puerto Rico, first to visit some people who had some fossils they wanted identified and then to do some actual fieldwork. The first part of the day was somewhat disappointing, as some of the fossils we saw were quite nice and useful, but, sigh, like on previous occasions, the owners of the fossils are unwilling to donate or loan these for research purpose. I always wonder what these people intend to do with the fossils?! Ignorance about geology and paleontology (probably science in general) as well as the lack of a natural history museum in the island is to blame (in part).

Anyways, after a very nice lunch and a lot of rain, we were able to finally visit a new outcrop of the San Sebastián Fm, which is very close to one of my favorite localities, Rio G. Although there is a very interesting exposure of the San Sebastián Fm, it was very unproductive in terms of vertebrates. Some jumbled fish bones, a shark tooth and a bunch of turtle shell fragments was all we found.

Left: tiger shark tooth, Galeocerdo sp.; Right: turtle shell fragments.

Invertebrates were more numerous and nicely preserved, especially crabs. The fossil crustaceans fauna of Puerto Rico was largely unknown until recently (see Schweitzer et al. 2006, 2008). Schweitzer and colleagues looked at fossil crustacean from Cretaceous, Oligocene, Miocene and Pleistocene and report about 13 species, seven which are new. In our new locality we found about three taxa (see picture below).

 Lower left: Scylla costata reported in Puerto Rico from the San Sebastián Fm and also from the Juana Díaz Fm; Upper left: cf. Necronectes collinsi also known from the Juana Díaz Fm and Lares Ls; Right: Portunus sp.

Well, that’s it so far. More fieldwork coming soon!

References

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

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

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.

Schweitzer, C. E., J. Velez-Juarbe, M. Martinez, A. Collmar Hull, R. M. Feldmann & H. Santos. 2008. New Cretaceous and Cenozoic Decapoda (Crustacea: Thalassinidea, Brachyura) from Puerto Rico, United States Territory. Bulletin of the Mizunami Fossil Museum 34:1-15.