Nuevos sirénidos del Eoceno

Este año ha visto la publicación de dos artículos describiendo sirénidos del Eoceno (55.8-33.9 millones de años atrás [ma]). En adición a esos, otras dos especies más fueron descritos del Mioceno, los cuales mencioné en una entrada pasada. Creo que no está mal, cuatro nuevas especies de sirénidos fósiles en un año, especialmente para un grupo que hoy día solo cuenta con cuatro especies (tres de manatí y el dugong). El conocimiento sobre la diversidad de sirénidos en el pasado sigue aumentando a muy buen ritmo.

Durante el Eoceno existían al menos tres de las cuatro familias de sirénidos que se conocen, Prorastomidae (que incluyen los miembros más primitivos), Protosirenidae y Dugongidae. El registro fósil de los Trichechidae (la cuarta familia, que es la que incluye al manatí) es más escaso y hasta donde sabemos, estos se originaron durante el Oligoceno Tardío (28.4-23 ma). Los nuevos fósiles del Eoceno representan a los Prorastomidae y los Dugongidae.

Un protosirénido de India

En una entrada pasada mencioné que India es uno de los lugares más prolíficos en términos de hallazgos de fósiles de sirénidos. No debe sorprendernos que otra especie adicional ha sido descrita. Ashokia antiqua Bajpai et al. 2009 representa un nuevo género y especie de protosirénido del Eoceno Medio temprano (Lutetian: 48.6-40.4 ma). Esta nueva especie se distingue por una combinación de caracteres primitivos y derivados, morfológicamente se acerca más a otro protosirénido de Libya, aún sin describir (Bajpai et al., 2009). Ashokia se distingue de otros prorastómidos al tener una cresta sigmoidal prominente, la apertura auditiva externa más ancha, el puente zigomatico-orbital elevado, el borde del exoccipital más delgado y poseer un proceso zigomático del hueso temporal que disminuye en grosor gradualmente en dirección al rostro.

En la figura arriba Ashokia antiqua en vista lateral (foto e ilustración hechas por mi).

Este no es el primer prorastómido que se conoce de India. Bajpai et al. (2006) refirieron un cráneo incompleto al género Protosiren sp., este, al igual que Ashokia, provienen de la Formación Harudi. Otros sirénido del Eoceno indio son Eotheroides babiae y Eosiren sp. también de la misma formación, estos sin embargo son dugónguidos (Bajpai et al. 2006). Los protosirénidos se distinguen de todas otras especies conocidas de sirénidos por ciertas características craneales y por tener las epífisis* cartilaginosas, incluso en los adultos (Sickenberg, 1934; Zalmout et al., 2003; Bajpai et al. 2006). Aún no se han reportado protosirénidos en rocas del Oligoceno, por lo que parece que este grupo fue uno exclusivo del Eoceno.

*Los extremos de los huesos.

Un dugónguido de Madagascar

Poco se conoce de los animales que habitaron Madagascar en el pasado. No fue hasta en años recientes que se comenzó a descubrir fósiles de los antiguos pobladores de esta isla, especialmente durante el Mesozoico y Cenozoico. Uno de los descubrimientos más recientes es el cráneo de un dugóngido proveniente de estratas del Eoceno Medio. Estos han sido descritos por Samonds et al. (2009) quienes han identificado el fósil como una especie nueva del género Eotheroides. E. lambondrano Samonds et al. 2009 pertenece a un género de dugónguidos que están se han encontrado en rocas del Eoceno de Egipto e India (Domning, 1996; Bajpai et al., 2006), esta nueva especie es la única dentro de ese género que posee el rostro completo, permitiéndonos conocer su morfología en mayor detalle. Único entre otras especies de Eotheroides por la morfología distinta de los huesos nasales, tener procesos supraorbitales bien desarrollados y el puente zigomatico-orbital de la maxilla corto, también se distingue de otras especies de dugonguidos al poseer una fórmula dental primitiva.

En la figura arriba Eotheroides lambondrano en vista lateral, escala = 4 cm (compuesto de fig. 3A y 4A de Samonds et al., 2009).

El largo del cráneo de E. lambondrano es de 270 mm (~10 pulg.), haciendo de este uno de los dugones de menor dimensión que se conocen. Posiblemente se acercaba en tamaño a las especies de dugónguino Nanosiren los cuales tenian dimensiones similares y cual largo total de cuerpo se estima en alrededor de 2 metros (~6’6”) (Domning & Aguilera, 2008), lo cual es considerado pequeño para un sirénido adulto. Asi que según se ha escrito en otros lugares (como en este artículo de National Geographic) puede que la nueva especie de Madagascar sea el dugónguido más pequeño que conocemos.

La importancia de estos nuevos fósiles resta en que demuestran cuan diverso eran los sirénidos durante el Eoceno. El nuevo protosirénido añade otra especie a un interesante grupo de sirénidos, que incluso se ha postulado como el grupo que dio origen a los Trichechidae. El hallazgo de Eotheroides lambondrano en Madagascar nos muestra cuán amplia era la distribución de este género y de los dugónguidos durante el Eoceno; en adición nos puede ayudar a entender la evolución de estos en la región del Tethys. Y por supuesto, aquellos de nosotros que están al tanto de lo que ocurren en el mundo de la paleosirenología saben que aún quedan más por ser descritos, asi que pendientes!

Otras entradas sobre sirénidos

Prep work: update II and a note on sirenian periotics

Prep work: update

Sirenian diversity in the past

De la tierra al agua (English version here)

Domningia and other Indian sirenians

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

Bajpai, S., D. P. Domning, D. P. Das & V. P. Mishra. 2009. A new middle Eocene sirenian (Mammalia, Protosirenidae) from India. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 252/3:257-267.

Bajpai, S., J. G. M. Thewissen, V. V. Kapur, B. N. Tewari & A. Sahni. 2006. Eocene and Oligocene sirenians (Mammalia) from Kachchh, India. Journal of Vertebrate Paleontology 26(2):400-410.

Domning, D. P. 1996. Bibliography and index of the Sirenia and Desmostylia. Smithsonian Contributions to Paleobiology 80:1-611.

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(2):479-500.

Samonds, K. E., I. S. Zalmout, M. Irwin, D. W. Krause, R. R. Rogers & L. L. Raharivony. 2009. Eotheroides lambondrano, new Middle Eocene seacow (Mammalia, Sirenia) from the Mahajanga basin, northwestern Madagascar. Journal of Vertebrate Paleontology 29(4):1233-1243.

Prep work: update II and a note on sirenian periotics

This has been a long hiatus! I’ve been really busy doing some more prep work on the Puerto Rican Dioplotherium and the Yucatán skull (another new species of dugongine). On top of that I’ve been preparing a couple of manuscripts describing some sirenian remains from PR, which I hope to submit sometime next year.

The subject of this post is to show you more of the Puerto Rican Dioplotherium, which was featured on the previous post. I have done additional prep work on the left squamosal, which was detached from the skull.

The composite picture above shows the skull as it was back in 2006 (top picture) and an outline of the enlarged area below. All that was visible of the sqamosal were the zygomatic arch, post-tympanic process and the mastoid part of the periotic (bottom picture). It was exciting knowing that part of the ear bones were preserved, even if it was only the periotic. Fortunately I got more than I bargained for.

In this figure we see the squamosal, now free of matrix, in lateral (A) and medial (B) views. Notice that the tympanic bone was preserved as well as the periotic. And there is more!

Additional removal of matrix revealed the three auditory ossicles, in articulation! The picture below shows a posteroventral view into the middle ear (anterior is to the right, medial towards the top of the picture). This is really neat as these bones are easily lost in most fossils (they are, apparently, missing on the right side of the skull).

A little on sirenian periotics

The periotic can be divided into three parts tegmen tympani, pars mastoidea and pars petrosa. The latter can be subdivided into pars canalicularis and pars cochlearis (see picture above) (Robineau, 1969). In the pars cochlearis, the structure labeled perilymphatic foramen, is uniquely found in (most) sirenians, (most) proboscideans and Arsinoitherium (hinting at their tethytherian affinity?). The homologous structure in other mammals consists of two openings known as the fenestra cochleae (rotunda) and aqueductus cochleae (Court, 1994).

The occurrence of a perilymphatic foramen in some tethytheres (I’m not sure what is the condition in desmostylians) seems to indicate that it might be a unique derived character of the group. Nonetheless, when we look at the fossil record, primitive proboscideans (Phosphatherium escuilliei) and sirenians (Prorastomus sirenoides) do have fenestra cochleae and aqueductus cochleae (Gheerbrant et al., 2005; Court, 1990; Savage et al., 1994). Meaning that this condition is homoplasic in tethytheres (Court, 1994, Gheerbrant et al. 2005). Whether resulting from multiple origins or multiple reversals, I still think it is an interesting characteristic that is found in at least some tethytheres.

Previous post about sirenians:

Prep work: update

Sirenian diversity in the past

De la tierra al agua (English version here)

Domningia and other Indian sirenians

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

Court, N. 1990. Perotic anatomy of Arsinoitherium (Mammalia, Embrithopoda) and its phylogenetic implications. Journal of Vertebrate Paleontology 10(2):170-182.

Court, N. 1994. The periotic of Moeritherium (Mammalia, Proboscidea): homology or homoplasy in the ear region of Tethytheria McKenna, 1975? Zoological Journal of the Linnean Society 112:13-28.

Gheerbrant, E., J. Sudre, P. Tassy, M. Amaghzaz, B. Bouya and M. Iarochène. 2005. Nouvelles données sur Phosphatherium escuilliei (Mammalia, Proboscidea) de ‘Éocène inférieur du Maroc, apports à la phylogénie des Proboscidea et des ongulés lophodontes. Geodiversitas 27(2):239-333.

Robineau, D. 1969. Morphologie externe du complexe osseux temporal chez les sireniens. Mémoires du Muséum National d’Histoire Naturelle, Série A, Zoologie 60(1)-1-32.

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

Prep Work: Update

Wow! It’s been a while since I posted something. Working with some of the material collected back in August when I went to Puerto Rico to do fieldwork with my advisor has kept me busy. In addition, I had no computer for a while, just after posting the previous post, my computer’s hard drive died! Luckily, not much was lost.

Back in June I wrote about some prep work I had been doing on a sirenian skull from Puerto Rico. Well I am glad to say that four years after I collected said skull (in 2005), it is nearly done! So, here are some pictures, from the time it was collected until now.

In the picture above (from 2005) I am in the outcrop with my hand next to where the fossil is. This is a Late Oligocene limestone unit from northern Puerto Rico.

Here is a dorsal and right lateral view of how the fossil looked around 2006 (and actually it looked like that for the last 2 years). You might notice that on the top picture there is a bone floating in the matrix next to the braincase, this is the left squamosal, which is disarticulated.

Here is how the fossil looks like now (2009), with most of the matrix gone and the left squamosal removed. Beautiful, don't you think?!

If you know something about sirenians, you might have noticed that this is a dugongine (large tusks [broken, unfortunately], thickened supraorbital process of frontal, etc.). It is actually quite similar to Dioplotherium manigaulti from the Early Miocene of South Carolina and Florida (Cope, 1883; Domning, 1989). Nonetheless, the Puerto Rican skull is older, Late Oligocene, and it also has some primitive characters that sets it apart from D. manigaulti. This skull along with another one from the same locality make up an important part of my thesis. Fortunately, some postcranial material that was collected this summer, from the same outcrop and same unit, is referable to this taxon. This material also displays differences from other known sirenian postcrania. Pretty cool stuff!!

Previous post about sirenians:

Sirenian diversity in the past

De la tierra al agua (English version here)

Domningia and other Indian sirenians

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

Cope, E. D. 1883. On a new extinct genus of Sirenia from South Carolina. Proceedings, Academy of Natural Sciences of Philadelphia 1883:52-54.

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

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.