Sirenian diversity in the past

Its been quiet here for a while as I’ve been busy working on the preparation of two sirenians skull, as well as getting ready for the upcoming field season.

It’s also been a while since I wrote something about sirenians so, here it goes.

Living sirenians can be divided into two families, Trichechidae (manatees) and Dugongidae (dugongs). Most people are probably more familiar with the manatees, after all, there are three species, West Indian, Amazonian and African, whereas there is only one species of dugong. The geographic distribution of extant sirenians is such that there is mostly no overlap between the different species. As the only living herbivorous marine mammals, it might be that by living in separate regions it reduced the chances of competing for the same resources (i.e. seagrasses). But what about in the past, what does the fossil record of sirenian tells us about their paleoecology.

When we look at the fossil record, sirenians were much more speciose, including multispecies communities in some regions (Domning, 2001). Now lets look at one good example.

The Late Oligocene of Florida

The Late Oligocene sirenian fauna of Florida includes at least three species of dugongids*. The dugongines, Crenatosiren olseni and Dioplotherium manigaulti, and the halitheriine Metaxytherium sp. (Domning, 1989, 1997, 2001). (See illustration below).

*The family Dugongidae includes three subfamilies: Dugonginae, Halitheriinae & Hydrodamalinae.

Illustration of known Late Oligocene sirenians from Florida (all at the same scale). Top, Crenatosiren olseni (modified from Domning, 1997); middle, Dioplotherium manigaulti (from Domning, 1989); bottom, Metaxytherium sp. (this last drawing based on a very similar skull from Puerto Rico, tusks not preserved, but presumed to be small as in the Fl specimen). The numbers in the circles are the degrees of rostral deflection. Mandibles absent in the middle and bottom specimens.

These three species, as you can see, differ in size, and to a lesser degree in rostral deflection. Also different from each other is the size of their tusks, increasing in size from Metaxytherium - C. olseni - Dioplotherium manigaulti. Taken as a whole, these differences (specially tusks size) could be indicators of different feeding habits, with small-tusked sirenians feeding of small rhizomes* and large-tusked sirenians feeding on larger ones (Domning, 2001; Domning & Beatty, 2007). Dugongids most likely used their tusks as a tool to dig out the rhizomes, with the most extreme specialization observed in the dugongines, including very large blade-like tusks as well as cranial adaptations that seemed to have help withstand the forces exerted when digging (Domning & Beatty, 2007).

*Rhizomes = the nutrient-rich, underground stems of seagrasses.

Other examples of sirenian multispecies communities are found in the Early Oligocene of Puerto Rico and the Early Miocene of India, among others (more on this sometime in the future). In addition, in the Pacific, sirenians were not the only herbivorous marine mammals. In the northern Pacific region, sirenians seem to have shared their resources with the desmostylians (see picture below), an interesting (and bizarre) group of mammals that lived from the Oligocene to the Miocene and were presumably feeding and spending time in the marine realm (Domning et al., 1986; Inuzuka et al., 1994). Whereas, in the southeastern Pacific, fossils of aquatic sloths (Thalassocnus spp.) have been found in the same formations as sirenians (Muizon & McDonald, 1995; Canto et al., 2008; Muizon & Domning, 1985; Bianucci et al., 2006; Domning & Aguilera, 2008).

Mounted cast of Palaeoparadoxia tabatai taken at the AMNH.

So, why is it so different in modern times, why do we see such a reduced diversity of sirenians and/or lack of any other herbivorous marine mammals? There has been, apparently, little change in the marine seagrass communities since the Eocene, so what happened? The answers for these and other questions could be answered with more fossils and more research. For now, we can certainly say that, like their close relatives, the proboscideans (elephants), sirenians are the last remnants of a once much more diverse group of animals.

References

Bianucci, G., S. Sorbi, M. E. Suárez & W. Landini. 2006. The southernmost sirenian record in the eastern Pacific Ocean, from the Late Miocene of Chile. Comptes Rendus Palevol 5:945-952.

Canto, J., R. Salas-Gismondi, M. Cozzuol & J. Yáñez. 2008. The aquatic sloth Thalassocnus (Mammalia, Xenarthra) from the Late Miocene of north-central Chile: biogeographic and ecological implications. Journal of Vertebrate Paleontology 28(3):918-922.

Domning, D. P. 1989. Fossil Sirenia of the West Atlantic and Caribbean region. II. Dioplotherium manigaulti Cope, 1883. Journal of Vertebrate Paleontology 9:415-428.

Domning, D. P. 1997. Fossil Sirenia of the West Atlantic and Caribbean region. VI. Crenatosiren olseni (Reinhart, 1976). Journal of Vertebrate Paleontology 17:397-412.

Domning, D. P. 2001. Sirenians, seagrasses, and Cenozoic ecological change in the Caribbean. Palaeogeography, Palaeoclimatology, Palaeoecology 166:27-50.

Domning, D. P. & O. A. Aguilera. 2008. Fossil Sirenia of the West Atlantic and Caribbean region. VIII. Nanosiren garciae, gen. et sp. nov. and Nanosiren sanchezi, sp. nov. Journal of Vertebrate Paleontology 28:479-500.

Domning, D. P. & B. L. Beatty. 2007. Use of tusks in feeding by dugongid sirenians: observations and tests of hypotheses. Anatomical Record 290:523-538.

Domning, D. P., C. E. Ray & M. C. Mckenna. 1986. Two new Oligocene desmostylians and a discussion of Tethytherian systematics. Smithsonian Contributions to Paleobiology 59:1-56.

Inuzuka, N., D. P. Domning & C. E. Ray. 1994. Summary of taxa and morphological adaptations of the Desmostylia. Island Arc 3(4):522-537.

Muizon, C. de & D. P. Domning. 1985. The first records of fossil sirenians in the southeastern Pacific Ocean. Bulletin du Muséum National d’Histoire Naturelle (Paris) (4)7, Sect. C, no. 3:189-213.

Muizon, C. de & H. G. McDonald. 1995. An aquatic sloth from the Pliocene of Perú. Nature 375:224-227.

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.

Cenozoic Caribbean Crocodylians

There are only a few naturally-ocurring modern species of crocodylians in the Greater Antilles, namely the endemic Cuban crocodile (Crocodylus rhombifer) and the American crocodile (Crocodylus acutus) which is widely distributed throughout the region. Here I briefly summarize what is known so far about the Cenozoic crocodylians of the Caribbean region. 

Eocene

The only Eocene crocodylian known so far from the Caribbean region is Charactosuchus kugleri from the Eocene of Jamaica (Berg, 1969). Described from a mandible missing posterior part, it represents the earliest record of a Tertiary crocodylian from the Greater Antilles. Other species of Charactosuchus come from geologically younger deposits in South America (Langston, 1965; Langston & Gasparini, 1997). Doubts about the generic affinities of Charactosuchus kugleri were raised by Domning & Clark (1993) who mention that it is more similar to the tomistomine Dollosuchus dixoni from the Eocene of Europe. Brochu (2007b) upon examination of material referred to D. dixoni, agreed with Domning & Clark (1993) in that C. kugleri might belong to Dollosuchus. In that same work Brochu also considered the name Dollosuchus as nomen dubium, as it is based on material that is too incomplete to offer real information on its affinities. A skull and associated skeleton from the Middle Eocene of Belgium that had been referred to D. dixoni has been redescribed and renamed as Dollosuchoides densmorei by Brochu (2007b).

Oligocene

The next time period from which crocodylian fossils are known from the Caribbean is the Oligocene of Puerto Rico. Early Oligocene remains have been collected from the San Sebastián and Juana Díaz formations, from northern and southern Puerto Rico, respectively. Further collecting efforts have yielded material from the Late Oligocene Lares Limestone and Early Miocene Cibao Formation, both found in northern Puerto Rico. Some of the material from the San Sebastián, Lares and Cibao formations was described by Brochu et al. (2007). The fossils from the San Sebastián Fm are from collections made in the early 1900’s by Narciso Rabell-Cabrero (probably the first Puerto Rican paleontologist) and by the AMNH in the late 1980’s (MacPhee & Wyss, 1990; Brochu et al., 2007). Although these are very fragmentary, and come from different localities, they are very interesting in that they show resemblance to gavialoids (Brochu et al., 2007), and were kind of a preview of what was soon to be found.

Dorsal view of skull of Aktiogavialis puertoricensis, while still under preparation; anterior end points downward. Large openings are supratemporal fenestrae.

It wasn’t until latter in 2006 when paleontologists from the Geology Department at the University of Puerto Rico, collected a partial cranium (see picture above) from deltaic deposits of the San Sebastián Fm. The fossil was described as a new gavialoid taxon, Aktiogavialis puertoricensis, that showed affinities to the South American gharials such as Gryposuchus colombianus, Ikanogavialis gameroi, Piscogavialis jugaliperforatus and Siquisiquesuchus venezuelensis (Vélez-Juarbe et al., 2007). An interesting aspects of the Puerto Rican gharial is its age, known from Early Oligocene deposits, it is the oldest gryposuchine*, a group whose other members are found in Miocene or Pliocene deposits (Langston, 1965; Gasparini, 1968; Sill, 1970; Langston & Gasparini, 1997; Kraus, 1998; Brochu & Rincón, 2004; Riff & Aguilera, 2008). Phylogenetic analysis show that gryposuchines are closely related to extant Gavialis gangeticus (shown below) and North African gavialoids (Brochu & Rincón, 2004; Vélez-Juarbe et al., 2007). This implies that they originated from a North African form that dispersed to the new world prior to the Early Oligocene (Vélez-Juarbe et al., 2007).

*Gryposuchinae: a monophyletic group that includes all the South American gharials (Vélez-Juarbe et al., 2007). The monophyly of this group has been supported by Brochu & Rincón (2004) and Riff & Aguilera (2008) as well; it has been challenged by Jouve et al. (2006; 2008).

Gavialis gangeticus photographed at the National Zoo, Washington, DC.

Not much has been collected from the Late Oligocene Lares Limestone in northern Puerto Rico. A vertebra was described in Brochu et al. (2007), but it is not informative enough; some isolated teeth resembling those of long-snouted croc have also been found. Other teeth, though, are much larger and seem to be from a short-snouted form (see picture below).

Crocodylian teeth from the Lares Limestone, northern Puerto Rico. Scale bar = 1 cm.

Miocene

The Miocene crocodylians of the Caribbean region are still a mystery. Early-middle Miocene crocodylian remains have been collected in Cuba, Dominican Republic and Puerto Rico (MacPhee & Wyss, 1990; MacPhee et al., 2003; Brochu et al., 2007; Brochu & Jiménez-Vázquez, 2014). The Dominican material is still undescribed, hence their affinities are unknown. The fossils from Puerto Rico, consisting of several associated cranial elements (shown below) were collected from the middle Miocene age Cibao Fm (MacPhee & Wyss, 1990; Brochu et al., 2007).  The Cibao fossils include a partial dentary, frontal and partial left squamosal, and show a combination of features that excludes it from being a gavialoid, alligatorid, Crocodylus or any other extant taxon (Brochu et al., 2007). Similarly, there are some unusual crocodylian remains from the early Miocene Lagunitas Formation in Cuba (Brochu & Jiménez-Vázquez, 2014). This implies that at least starting in the early Miocene (and probably even earlier) Puerto Rico, and other islands in the Caribbean , where probably home to an endemic group of crocodylians. 

Crocodylian remains from the Early Miocene Cibao Formation of northern Puerto Rico. From Brochu et al., 2007.

Other radiations of island-endemic crocodylians include the extinct Cenozoic Australasian Mekosuchinae and the extant Osteolaeminae, which includes two extinct island endemics: Voay robustus, from Madagascar (Brochu, 2007a; Bickelmann & Klein, 2009) and possibly Aldabrachampsus dilophus, from Aldabra (Brochu, 2006), both from Quaternary deposits.

Quaternary

Finally, during the Quaternary the Cuban crocodile, Crocodylus rhombifer (shown below) had a more widespread distribution. Restricted today only to the south-central coast of Cuba and adjacent Isla de Pinos, fossils referable to this species have been found in Quaternary deposits in Cuba (Varona, 1984), Grand Cayman (Morgan et al., 1993), and the Bahamas (Olson et al., 1990; Franz et al., 1995; Steadman et al., 2007). Based on this, we might ask some questions such as: was C. rhombifer present in the other Greater Antilles? Did the arrival of C. rhombifer replaced an extant group of endemics?  Only more fossils will help answer these question. 

Crocodylus rhombifer, photographed at the National Zoo, Washington, DC.

So, there it is, a summarized version of what is known about Cenozoic crocodylians from the Caribbean region. Additional, much older crocs are known from the Jurassic of Cuba, but I'll leave those for a future post.

References

Berg, D. E. 1969. Charactosuchus kugleri, eine neue Krokodilart aus dem Eozän von Jamaica. Eclogae Geologicae Helvetiae 62:731-735.

Bickelmann, C. & N. Klein. 2009. The late Pleistocene horned crocodile Voay robustus (Grandidier & Vaillant, 1872) from Madagascar in the Museum für Naturkunde Berlin. Fossil Record 12(1):13-21.

Brochu, C. A. 2006. A new miniature horned crocodile from the Quaternary of Aldabra Atoll, Western Indian Ocean. Copeia 2006(2):149-158.

Brochu, C. A. 2007a. Morphology, relationships, and biogeographical significance of an extinct horned crocodile (Crocodylia, Crocodylidae) from the Quaternary of Madagascar. Zoological Journal of the Linnean Society 150:835-863.

Brochu, C. A. 2007b. Systematics and taxonomy of Eocene tomistomine crocodylians from Britain and northern Europe. Palaeontology 50(4):917-928.

Brochu, C. A. & O. Jiménez-Vázquez. 2014. Enigmatic crocodyliforms from the early Miocene of Cuba. Journal of Vertebrate Paleontology 34:1094-1101.

Brochu, C. A. & A. D. Rincon. 2004. A gavialoid crocodylian from the Lower Miocene of Venezuela. Special Papers in Palaeontology 71:61-78.

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

Domning, D. P. & J. M. Clark. 1993. Jamaican Tertiary marine Vertebrata; pp. 413-415 in R. M. Wright & E. Robinson (eds.), Biostratography of Jamaica. Boulder Colorado, Geological Society of America Memoir 182.

Franz, E. F., G. S. Morgan, N. Albury & S. D. Buckner. 1995. Fossil skeleton of a Cuban crocodile (Crocodylus rhombifer) from a blue hole on Abaco, Bahamas. Caribbean Journal of Science 31:149-152.

Gasparini, Z. 1968. Nuevos restos de Rhamphostomopsis neogaeus (Burm.) Rusconi 1933, (Reptilia, Crocodilia) del “Mesopotamiense” (Plioceno Medio-Superior) de Argentina. Ameghiniana 5:299-311.

Jouve, S., M. Iarochene, B. Bouya and M. Amaghzaz. 2006. New material of Argochampsa krebsi (Crocodylia: Gavialoidea) from the Lower Paleocene of the Uolad Abdoun Basin (Morocco): phylogenetic implications. Geobios 39:817-832.

Jouve, S., N. Bardet, N.-E. Jalil, X. Pereda Suberbiola, B. Bouya & M. Amaghzaz. 2008. The oldest African crocodylian: phylogeny, paleobiogeography, and differential survivorship of marine reptiles through the Cretaceous-Tertiary boundary. Journal of Vertebrate Paleontology 28(2):409-421.

Kraus, R. 1998. The cranium of Piscogavialis jugaliperforatus n. gen., n. sp. (Gavialidae, Crocodylia) from the Miocene of Peru. Paläontologische Zeitchrift 72:389-406.

Langston, W. 1965. Fossil crocodilians from Colombia and the Cenozoic history of the Crocodilia in South America. University of California Publications in Geological Sciences 52:1-152.

Langston, W. & Z. Gasparini. 1997. Crocodilians, Gryposuchus, and the South American gavials; pp.113-154 in R. F. Kay, R. H. Madden, R. L. Cifelli & J. J. Flynn (eds.), Vertebrate Paleontology in the Neotropics: the Miocene fauna of La Venta, Colombia. Smithsonian Institution, Washington, DC.

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.

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.

Morgan, G. S., R. Franz & R. I. Crombie. 1993. The Cuban crocodile, Crocodylus rhombifer, from Late Quaternary fossil deposits on Grand Cayman. Caribbean Journal of Science 29:153-164.

Olson, S. L., G. K. Pregill & W. B. Hilgartner. 1990. Studies on fossil and extant vertebrates from San Salvador (Watlings) Island, Bahamas. Smithsonian Contributions to Zoology 508:1-15.

Riff, D. & O. A. Aguilera. 2008. The world’s largest gharials Gryposuchus: description of G. croizati n. sp. (Crocodylia, Gavialidae) from the Upper Miocene Urumaco Formation, Venezuela. Paläontologische Zeitchrift 82:178-195.

Sill, W. 1970. Nota preliminar sobre un nuevo gavial del Plioceno de Venezuela y una discusión de los gavialis Sudamericanos. Ameghiniana 7:151-159.

Steadman, D. W., R. Franz, G. S. Morgan, N. A. Albury, B. Kakuk, K. Broad, S. E. Franz, K. Tinker, M. P. Pateman, T. A. Lott, D. M. Jarzen & D. L. Dilcher. 2007. Exceptionally well preserved late Quaternary plant and vertebrate fossils from a blue hole on Abaco, The Bahamas. Proceedings of the National Academy of Sciences 104(50):19897-19902.

Varona, L. S. 1984. Los cocodrilos fósiles de Cuba (Reptilia: Crocodylidae). Caribbean Journal of Science 20:13-18. 

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.

Actualizado: 8 de febrero de 2021

De la tierra al agua

Algunos de los mamíferos marinos que existen hoy día, como los cetáceos, sirénidos y pinípedos*, están tan adaptados a una vida en el agua, que puede que se nos haga difícil relacionarlos con sus parientes más cercanos los cuales son terrestres. Aquí les voy a dar un poco de información sobre la evidencia que existe hasta ahora sobre la trancición de la tierra al agua en estos grupos de organismos.

*(cetáceos = ballenas y delfines; sirénidos = manatí y dugong; pinípedos = focas, morsas y leones marinos).

Cetáceos

Las ballenas que conocemos hoy día se pueden dividir en dos grupos, los odontocetos y misticetos. Los odontocetos se caracterizan por tener dientes y utilizar ecolocalización; los misticetos se caracterizan por tener barbas en lugar de dientes (ambos grupos tienen otras adaptaciones que discutiré en otro momento). Ejemplos de odontocetos son las orcas y delfines; las misticetos incluyen ballenas azules y ballenas pigmeas.

Estudios moleculares idicaban que las ballenas evolucionaron de los artiodáctilos – grupo que incluye a los cerdos, hipopótamos, camellos, vacas, ovejas, etc – (Graur & Higgins, 1994; Shimamura et al., 1997). Estudios morfológicos solían indicar que los cetáceos habían evolucionado de los mesoniquios – un grupo extinto de mamíferos terrestres carnívoros – (Luo & Gingerich, 1999). En parte, la razón para que existiera esa discrepancia sobre los orígenes de los cetáceos era que los fósiles de cetáceos más antiguos consistían de formas ya completamente adaptadas a una vida en el agua, o solamente se conocía el cráneo. Ya esto ha sido resuelto.

En el 2001, dos grupos de paleontólogos publicaron trabajos donde describían fósiles de ballenas primitivas, incluyendo partes del postcráneo que demostraban que estas estaban relacionadas a los artiodáctilos (Gingerich et al., 2001; Thewissen et al., 2001). Gingerich y su equipo recuperaron los restos de dos Artiocetus clavis y Rhodocetus balochistanensis, mientras que el grupo de Thewissen recuperaron los de Ichthyolestes pinfoldi y Pakicetus attocki (ilustración adyacente de Pakicetus por Carl Buell, tomado de la página del Thewissen Lab); entre las partes que encontraron incluía uno de los huesos del tobillo, el astrágalo, que fue la pieza clave para determinar que los cetáceos evolucionaron de los artiodáctilos. Todos estos fósiles fueron encontrado en sedimentos que datan del Eoceno Medio (entre 49 y 41 millones de años). Recientemente, Thewissen et al. (2007) describen los restos postcraneales del artiodáctilo primitivo, Indohyus, y demuestran que este era un animal con adaptaciones acuáticas y provee evidencia adicional sobre el origen de los cetáceos. (Una reconstrucción magnífica de Indohyus puede ser vista aquí.)

Sirénidos

Los parientes vivos más cercanos de los manatíes y dugones son los elefantes, esto está evidenciado tanto por información morfológica como genética (Seiffert, 2007; Tabuce et al. 2007). EL origen de los sirénidos fue en África alrededor de 54 millones de años donde compartieron un ancestro en común con los elefantes. Interesantemente, los fósiles de sirénidos más primitivos se han encontrado en Jamaica, lo cual demuestra que muy temprano en su historia evolutiva ya estaban adaptados a una vida en ambientes marinos. Durante mucho tiempo, el sirénido fósil más primitivo era Prorastomus sirenoides encontrado en Jamaica en depósitos que datan de 51-49 millones de años (Owen, 1855; Savage et al., 1994). Sin embargo, el postcráneo de este organismo todavía sigue siendo desconocido.

Otro sirénido fósil, también de Jamaica, pero encontrado en depósitos un poco más jovenes (49-45 millones de años), fue descrito por Domning (2001). Este nuevo fósil, llamado Pezosiren portelli (ilustración arriba tomada de Domning, 2001), consiste de partes del cráneo y postcráneo. Los restos postcraneales de Pezosiren incluye los brazos, patas, pelvis y casi toda la columna vertebral; en conjunto, estos indican que este organismo era capaz de soportar su cuerpo fuera del agua a la misma vez, también tiene adaptaciones acuáticas como costillas agrandadas y densas y la fosa nasal retractada (Domning, 2001). Esta combinación de adaptaciones indican que este organismo pasaba tiempo tanto dentro como fuera del agua.

Pinípedos

Estudios morfológicos y moleculares demuestran que los pinípedos pertenecen a un grupo de mamíferos llamado arctoideos (Deméré et al., 2003), que además de los pinípedos, incluyen a los osos (úrsidos), mustelas (mustélidos), mapaches (prociónidos) y zorrilos (mefítidos) entre otros. Sin embargo, el origen de los pinípedos de alguno de estos otros arctoideos no es clara y diferentes estudios ponen a los pinípedos originándose de un ancestro en común con los mustélidos o con los úrsidos (Deméré et al., 2003). Otros expertos en la materia apoyan la idea que los pinípedos tienen origenes separados, las focas evolucionando de un ancestro en común con los mustélidos y los leones marinos y morsas con los úrsido (Uhen, 2007 y referencias ahí). Cualquiera que sea el grupo del cual los pinípedos se originaron, algo que solo se podrá aclarar a medida que se descubren más fósiles, sabemos que se originan de un organismo terrestre. Interesantemente, fósiles de pinípedos que demuestren una morfología transicional, entre completamente marino / completamente terrestre, no fueron encontrados hasta hace poco.

El reciente descubrimiento de Puijila darwini en sedimentos lacustrinos depositados entre 23-21 millones de años, nos da una idea sobre la morfología de los primeros pinípedos (Rybczynski et al., 2009). Aunque ya se han econtrado restos de pinípedos más antiguos, Enaliarctos tedfordi y E. barnesi encontrados en rocas que datan de entre 28.5-23.8 en Oregon (Deméré et al., 2003), estos ya presentan adaptaciones a una vida marina como las que vemos en las especies modernas, lo cual significa que los pinípedos deben haber evolucionado previo a esta fecha. Así que aunque Puijila (ilustración adyacente tomada de Rybczynski et al., 2009) es más joven, su importancia resta en que su morfología es la más primitiva de los pinípedos conocidos, proveyendo además evidencia sobre los posibles pasos evolutivos que se llevaron a cabo en la transición de la tierra al agua en este grupo de mamíferos.

Gracias a MPT y YFS por ayudar con la gramática.

Página oficial de Puijila

Puijila en National Geographic

English version here.

Referencias

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 458:1021-102.

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.