Bad weather, ancient oceans and modern seagrasses… a brief visit to Puerto Rico

Summertime is normally when geologist and paleontologist engage in fieldwork. For me the last two summers have been different; I’ve spent both teaching in the human anatomy lab at USUHS, which has been a fantastic experience. However, being busy in the summer months hasn’t really stopped me from doing a little fieldwork, fortunately, I work mostly in the Caribbean region where the weather is generally nice and the only thing to worry about is hurricane season…

And so it was that I had the chance to go to Puerto Rico at the end of the summer and in the midst of hurricane season. Every time I go there is both for pleasure and work; I get to see my family, enjoy the food and scenery and with a healthy dose of fieldwork, it makes for the perfect vacation. This time I arrived two days before hurricane Irene (then still a tropical storm) hit the island. I was fortunate to be staying in the northwestern part of the island, which did much better than most other parts. We got rain, lots and lots of it, which kind of messed up my plans for fieldwork a little bit.

Meanwhile, while it rained, I stayed busy looking through fossils I had collected during my years as an undergrad; mostly fish teeth (bony and cartilaginous) and otoliths, which I’ll use for some projects on the works. It wasn’t until five six, days after my arrival that it was sunny and dry enough to go out to the field. Without much time to go to different places, I decided that my best chances for fieldwork were in the vicinity of my hometown, basically here and here.

The early Oligocene "mystery" bone I found.

So first I went to the Río G locality where early Oligocene deposits are exposed along the banks of the river. Its one of my favorites and almost always I find something of interest. With all the rain, I was both worried and hopeful. Worried that cool stuff was being eroded away, but hopeful in that new stuff would be exposed. After prospecting for a while and not finding anything more than some sirenian ribs, I made it to the “bonebed” (previously featured here). This time around all that I found was an interesting piece of bone (see picture above), and as it turned out, I had good reasons be worried. If you look at the picture, the part of the bone towards the bottom had broken off, recently, probably during the rain and increased water levels earlier in the week. It doesn’t seem to be a rib, which is why I think its interesting, but due to its incompleteness I’m still not sure what it is… it sounds a bit frustrating, but the best thing to do is to turn that frustration into motivation to keep looking!

 My favorite beach in the Atlantic coast of Puerto Rico. Just how I like it, early in the morning and nearly devoid of people.

My other field excursion was to the beach. Working with marine organisms that thrive in shallow marine environments is always a good excuse for a trip to the beach (specially in the tropics). Staying in my hometown meant that my favorite beach on the Atlantic coast of Puerto Rico was only a 15-minute drive away! I didn’t see any manatee (wasn’t expecting to anyways) but did get to see some seagrass beds (see picture below).

It was wavy and shallow (~1m) hence all the suspended sediment. The turtle grass blades (leaves) are about 0.5-1 cm wide and ~10-12 cm long.

Most seagrass beds in the Western Atlantic and Caribbean region consist of several species, with Thalassia testudinum (turtle grass), Halodule beaudettei (shoal grass), and Syringodium filiforme (manatee grass) as the more common ones. However, the seagrass bed that I was looking at was monospecific (= only one species present). As far as I could tell it consisted solely of turtle grass. Monospecific beds of turtle grass are not uncommon, in fact, Thalassia is what is known as a climax species. That means that it is the dominant species of seagrass; other species (like the ones mentioned above) are only present in small, disturbed patches or at the periphery of the beds. The dominance of turtle grass over other seagrass species and its consequences (like mass die-offs) in historical times has been partly blamed on overkill of large marine herbivores, mainly seacows and green turtles (Jackson et al., 2001). However, this could be slightly different when viewed from a deeper historical perspective. I hope to bring more on this sometime in the upcoming months…

Jackson, J. B. C. et al. 2001 Historical overfishing and the recent collapse of coastal

ecosystem. Science 293, 629-638.

The hand of Steller’s sea cow, revisited

It’s been quite a while since my last post. A lot has been going on, mostly research-related, which is good. Just last week I was in San Diego where I participated in the Sixth Triennial Conference of Secondary Adaptation of Tetrapods to Life in Water held at San Diego State University. The meeting was a great opportunity to see colleagues as well as making new acquaintances, hat tip to the host committee: Annalisa Berta, Tom Deméré and Eric Ekdale for such a great meeting! The week before that I spent some days visiting the collection at the Natural History Museum of Los Angeles where I got to look at some interesting sirenian and cetaceans, many thanks to Larry Barnes and Sam McLeod for access to specimens.

Ok, so now to the meaty part of this post. Back in 2008 I wrote about the hand of Steller’s sea cow (Hydrodamalis gigas) or more likely the lack of one. Back then I mentioned that G. W. Steller was one of the few people who saw H. gigas alive, and in his account he mentions the lack of fingers in this species. However, so far no hand bones have been found associated with Hydrodamalis gigas or even H. cuestae (see reconstruction below) (the presumed direct ancestor of the former), this lead to skepticism about his observations. So, how was that Steller’s account about the hand of H. gigas was corroborated?

Hydrodamalis cuestae, in left lateral view (picture of the reconstruction on exhibit at the San Diego Natural History Museum). Notice the reduced forelimbs.

The discovery of Dusisiren dewana in the Late Miocene (10-8 million years ago [mya]) of Japan, provided the key elements to support Steller’s account. Dusisiren is a genus of sea cow that include two other, earlier species: D. reinharti from the Early Miocene (20-16 mya) and D. jordani from the Middle-Late Miocene (12-10 mya) (see picture of D. jordani below). The species of Dusisiren together with Hydrodamalis cuestae (see picture above), H. spissa and H. gigas are members of a subfamily of dugongids (which means they are more closely related to dugongs than to manatees) known as the Hydrodamalinae (Domning, 1994; Domning & Furusawa, 1994). Hydrodamalines, as they are also known, ranged throughout coastal waters of the northern Pacific, from Baja California to Japan, with one species surviving until historical times (Domning & Furusawa, 1994). Some distinctive features of this group include, large body size, reduction or loss of dentition, and reduction of the digits, which brings us back to D. dewana.

Dusisiren jordani, anterior view (specimen at exhibit at the Natural History Museum of Los Angeles). Notice the "normal" hand in this species.

Described by Takahashi et al. in 1986, the remains of Dusisiren dewana were found in Late Miocene deposits in Japan and together with other elements of the skeleton there was also a nearly complete forelimb, including the hand. These were crucial in providing an insight into what the hand of later hydrodamalines was like. Dusisiren dewana shows reduction of the hand bones, in this respect differing from D. jordani, its predecessor, whose hands showed no reduction of elements (i.e. long metacarpals and long fingers composed of multiple elements) (see picture above). As for D. dewana, the carpals (wrist bones) are quite similar to those of other sirenians, however, the metacarpals (the bones between the wrist and fingers) and the phalanges (finger bones) are reduced, actually extremely reduced in the case of the phalanges (see picture below of Hydrodamalis cuestae). D. dewana as the sister taxon (closest relative/ancestor) of Hydrodamalis spp. is best evidence at hand (unintended pun) supporting Steller’s account on the hand of the sea cow that bears his name and allowing for reconstructions like the one seen here for H. cuestae.

Hydrodamalis cuestae, right lateral view (picture of specimen on exhibit at the San Diego Natural History Museum). Notice the reduced hand.

Domning, D. P. 1994. A phylogenetic analysis of the Sirenia. Proceedings of the San Diego Society of Natural History 29:177-189.

Domning, D. P. & H. Furusawa. 1994. Summary of taxa and distribution of Sirenia in the North Pacific Ocean. Island Arc 3:506-512.

Takahashi, D., D. P. Domning & T. Saito. 1986. Dusisiren dewana, n. sp. (Mammalia: Sirenia), a new ancestor of Steller’s sea cow from the upper Miocene of Yamagata Prefecture, northeastern Japan. Transactions and Proceedings of the Paleontological Society of Japan, New Series 141:296-321.

When you're strange, or how different are the teeth of extant sirenians

Living sirenians (sea cows: manatees and dugongs) are but the remnants of a much more diverse group whose history extends back at least to the late early Eocene (~50 million years ago [Ma]). There are only four living species of sirenians: the manatees (grouped in the family Trichechidae) and including three species: the Antillean manatee (Trichechus manatus)*, the Amazonian manatee (T. inunguis) and West African manatee (T. senegalensis). The other extant species is the dugong (Dugong dugon) that, together with the now extinct Steller’s sea cow (Hydrodamalis gigas), represents the family Dugongidae.

*T. manatus includes two subspecies: the West Indian manatee (T. manatus manatus) and the Florida manatee (T. manatus latirostris).

Above, Homo sapiens and Trichechus manatus latirostris. (Pic. courtesy of B. Bonde).

Although overall very similar to their extinct relatives, some characteristics of these extant sirenians are quite different and unique, not really representative of each particular group. One of those characteristics, and the one I will discuss here, is the dentition of Dugong dugon. This species, which lives in the Indo-Pacific region, is characterized by having molars that are best described as nearly enamel-less (a thin enamel cap is quickly worn off), open-rooted, ever-growing pegs (see pictures below), resembling more those of sloths, than those of manatees!!

Lower molars of Dugong dugon, notice that root and crown are continuous. The dark area is the part that was exposed above the gum line, hence the staining.

To understand how they differ let me give you a little background info on sirenian dentition. Primitively, the dental formula of sirenians consisted of three incisors, one canine, five premolars* and three molars (Domning, 1994; Savage et al., 1994). Throughout sirenian evolution, this formula has been reduced, first by loss of the fifth permanent premolar in the earliest dugongids (see figure below), then by subsequent loss of second and third incisors, canine and the other permanent premolars. In terms of general features, the molars of most fossil sirenians can be described as bilophodont, with enameled crowns that are clearly differentiated from the root (these being closed and multiple [with three in upper molars and two in the lowers]). These kind of molars somewhat resemble those of the most primitive proboscideans (Sanders et al., 2010), which, by the way, are the closest relatives of sirenians.

*Presence of a fifth premolar is reminiscent of the primitive eutherian condition and, among Cenozoic placental mammals, a pretty unique condition only seen in sirenians (Rose, 2006).

Partial left maxilla of a late Eocene (37.2-33.9 Ma) dugongid, (A) lateral view (reversed); (B) occlusal view (anterior to the right). Notice that the deciduous premolar is molarized and multi-rooted, whereas the permanent premolars are more conical and single rooted. dP = deciduous premolar; M = molar; P premolar (permanent).

Beginning about the early Oligocene (33.9-28.4 Ma) the dental formula of most dugongids (including Dugong) stayed the same; it consisted of the first upper incisor, upper and lower deciduous premolars 3-5 and permanent molars 1-3 (see A-B in figure below)* (Domning & Pervesler, 2001). Up until the appearance of Dugong dugon the morphology of the molars was, in general, much the same as that described above. The peculiar, somewhat aberrant, dentition of dugong likely evolved quite recently (geologically speaking). Unfortunately there are no fossils of D. dugon to really know when. The best evidence comes from a very Dugong-like sirenian skull from the Pleistocene of Florida (Domning, 2001) (sadly, it is in a private collection). This skull is from sediments that were deposited about two million years ago and is nearly identical to Dugong dugon; one of the few differences, actually the most obvious one, is that its teeth are much like those of earlier dugongids, bilophodont, with enamel and multiple, closed roots. Therefore, if this fossil is taken as the closest ancestor of extant dugong, it would imply that its bizarre dentition is a quite recent (less than 2 million years) evolutionary novelty.

*Or even further reduced from that condition (as was the case of Steller’s sea cow and its closest relatives).

Occusal view of various sirenians (A-C). A. Left upper toothrow of a late Oligocene (28.4-23 Ma) dugongid from Puerto Rico. B. Right lower toothrow of Dugong dugon, notice that the lack of enamel, leaves a nearly featureless surface; the black rim is just stain on the portion of the tooth that was above the gum line. C. Left upper toothrow of Trichechus inunguis.

Manatees (Trichechus spp.) also have unusual dentition when compared to dugongids and some of their ancestors (for starters they lack incisors). If you look at the figure above (C) you’ll notice that there seems to be more molars than there should be. And that is correct; manatees have supernumerary molars that are constantly being replaced throughout their entire life (not to be confused with the delayed eruption that characterizes elephants). The molars in Trichechus are also proportionately small, that way more can fit in the toothrow at a given time (sometimes up to eight! [Domning & Hayek, 1984]). The earliest evidence for supernumerary molars in trichechids is seen in Ribodon limbatus, a fossil manatee from the late Miocene (11.6-5.3 Ma) or early Pliocene (5.3-3.6 Ma) of Argentina (Ameghino, 1883; Pascual, 1953; Domning, 1982). Ribodon does differ in that its molars were not reduced, so there were less per toothrow. Other fossil trichechids have the more “normal” type of molars described above.

Anyways there you have it; extant seacows have quite different dentition from their extinct relatives. Of course, there remains the questions of why evolve ever-growing molars, or ever-replacing supernumerary molars. But I’ll leave those for some other occasion.

Ameghino, F. 1883. Sobre una colección de mamíferos fósiles del piso mesopotámico de la formación patagónica, recogidos en las barrancas del Paraná por el professor Pedro Scalabrini. Boletín de la Academia Nacional de las Ciencias de Córdoba 5:101-116.

Domning, D. P. 1982. Evolution of manatees: a speculative history. Journal of Paleontology 56:599-619.

Domning, D. P. 1994. A phylogenetic analysis of the Sirenia. Proceedings of the San Diego Society of Natural History 29:177-189.

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

Domning, D. P. & L.-A. C. Hayek. 1984. Horizontal tooth replacement in the Amazonian manatee (Trichechus inunguis). Mammalia 48:107-127.

Domning, D. P. & P. Pervesler. 2001. The osteology and relationships of Metaxytherium krahuletzi Depéret, 1895 (Mammalia: Sirenia). Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft 553:1-89.

Pascual, R. 1953. Sobre nuevos restos de sirénidos del mesopotamiense. Revista de la Asociación Geológica de Argentina 8:163-181.

Rose, K. D. 2006. The Beginning of the Age of Mammals. John Hopkins University Press, Baltimore, Maryland, 428 pp.

Sanders, W. J., E. Gheerbrant, J. M. Harris, H. Saegusa & C. Delmer. 2010. Proboscidea; pp. 161-251 in L. Werdelin & W. J. Sanders (eds.), Cenozoic mammals of Africa. University of California Press, Berkeley, California.

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

Dominican Republic, part II

Hello there dear readers. Today I bring you the second installment of this series. Like I mentioned in the previous post we moved from the Albian to the Miocene.

The Miocene of the Dominican Republic is probably best known for the amber deposits. These have not only produced beautiful amber, which is used in jewelry, but also a number of fossil taxa have been described based on remains entombed in the amber. These fossils consist mostly of invertebrates, however, several vertebrates have been described as well, including anoles, frogs, and an insectivore. This gives us a glimpse into the smaller fauna that inhabited the island during the Miocene, in contrast to what is known from the same age in Puerto Rico and Cuba where most of the vertebrate fossils of that age consist of larger animals (sloths, rodents, primates, sirenians, crocodylians and turtles). A good summary of the vertebrates known from the Tertiary of the Greater Antilles can be found in table 1 of MacPhee et al. (2003) (free download here).

One of our first excursions into the Miocene was on a Wednesday afternoon. We spent the morning collecting the last samples from one of the outcrops of the Hatillo Limestone and some rudists from near the entrance of a cave in that same limestone unit. We then headed west towards the Monte Plata Province where we knew there was a new road cut exposing units of Miocene age.

The first outcrop of the Miocene that we visited. This is actually one of the best Miocene outcrops I've seen in a while.

By the time we got there it was near the middle of the afternoon, which meant we didn’t had much sunlight left. We stopped in the first large outcrop we saw (Picture above) and it was worth it! We quickly started finding turtle shell fragments, and occasionally we saw cross sections of turtle shells that went into the outcrop. I decided to collect one of these. It was not big, and as I found out after prep work, not too good (see pictures below). However, it was worth going there, as there is more material to be found and collected!

Top left: me overturning the jacket with the turtle shell remains; top right: the jacket back in the lab; bottom left: view of the jacket when it was first opened; bottom right: the fossil turtle remains. (Click on the image to see the larger version.)

Several days latter we had some free time, so we decided to go back to this area. This time we drove in from the north, driving through Los Haitises where we saw beautiful karst topography. Our time was limited in this occasion as well, but we made the most out of it.

The second outcrop we visited, smaller than the first one, but still interesting. Alvin Bonilla for scale.

We hit a couple of outcrops (Picture above) where we found the usual (at least what usual for the Tertiary of Puerto Rico and Cuba); croc teeth, turtle shells and sirenians (which was what I was really after). The sirenian material was in a concretion, half of it which had fallen off the outcrop so it made it easy to collect. The other part is still in the outcrop and I hope to go back sometime in the spring to collect it. Actually, if it wasn't for Alvin, who saw it, I almost stepped on the part that had fallen trying to get a better view of the part still in the outcrop. After some prep work on the sirenian material it turned out to be mostly ribs, and a few vertebrae. However, even if it is a little frustrating, it will nonetheless be an important contribution to the understating of sirenian distribution during the Miocene of the Caribbean and demonstrates that there is probably more, better material waiting to be collected!

Top left: me collecting the concretion with the sirenian remains; top right: view of the jacket in the lab; bottom left: the jacket, recently opened; bottom right: the concretion, as of now, still need some prep work to be done. Oysters (near the center of the concretion and to the right) used some of the ribs as hardground. (Click on the image to see the larger version.)

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.

Dominican Republic, part I

It’s been quite a while since I posted something here as I’ve been very busy since the last post, which you should check out, if you haven’t! The absence was mostly because I’ve been distracted with university-related stuff, work (I had a real job over the summer), meetings and a month-long fieldtrip to the Dominican Republic, which will be the subject of this and the following entries.

Top: on our way to Bonao. Bottom: some of the beautiful rice field near Bonao. (Click on the picture to see the larger version.)

To begin with, I had never been to the Dominican Republic before; it was actually my first time in another Caribbean island other than my own, Puerto Rico. I did not know what to expect, but I ended up with a very good impression of the island and its people! There were many things I liked, for example, there seems to be a lot of land dedicated to agriculture (which I think is awesome), the food was delicious and of course, the geology is very interesting!!

The focus of the trip to the DR was to collect rock samples and fossils from Early Cretaceous limestones as part of the thesis project of my friend and colleague Alvin Bonilla-Rodriguez from the Geology Department at the University of Kansas. Our collecting efforts were mainly focused in the central part of the country where Albian-age limestones occur. We stayed most of the time near the towns of Bonao in the Monseñor Nouel Province and in Hato Mayor del Rey in Hato Mayor Province. Other people that joined us during our time there were: Luis Gonzalez from the Department of Geology at the University of Kansas (and Alvin’s advisor) and Wilson Ramírez and Hernán Santos from the Department of Geology at the University of Puerto Rico-Mayagüez. The Servicio Geológico Nacional and its director Santiago Muñoz Tapia were also very helpful in terms of logistics and advise.

At first it was hard finding good outcrops, some were heavily overgrown and in other occasions our compact car was obviously ill-fitted for some of the roads we needed to go through. But, by about the middle of our first week we finally found some outcrops, not the best, but it was a start!

Top: our first official stop, near the southeast corner of Presa Hatillo, the outcrop was completely overgrown. You can see Alvin (left), Luis (right) and Wilson (background), and our (very) compact cars. Bottom: one of the road we manage to get our car trough, there were others which were definitely impassable by our vehicle. (Click on the picture to see the larger version.)

Our luck got better when we moved east towards Hato Mayor. There we found a very nice outcrop (with lots of fossils) of the Hatillo Limestone! The fossils were very important, specially the rudist, for determining the age of the rocks. And yes, we were in the Albian!

Top left: our first outcrop, yes there is an outcrop in there! Top right: the nice outcrop of the Hatillo Ls is in the top of that hill. Bottom left: Coalcomana ramosa, a rudist, finding this particular species meant we were in the Albian. Bottom right: Hernán (left) and Alvin (right) collect rock samples from the Hatillo Ls. (Click on the picture to see the larger version.)

Well, this is it for this first post. Remember there are more to come, so stick around! On the next post we’ll move from the Albian, to the Miocene!

Convergencia: el caso de Odobenocetops ó el delfín que parecía morsa

Convergencia; en biología y paleobiología, se utiliza este término para referirse a instancias donde dos (o más) organismos, de grupos no relacionados, poseen rasgos morfológicos similares los cuales sirven un mismo propósito. Estas similitudes se pueden clasificar como homólogas (aquellas que están compuestas por estructuras similares, como las alas de las aves y las de los murciélagos) o análogas (aquellas que han evolucionado de estructuras distintas, como las alas de las aves y las de una mariposa). Otro ejemplo de convergencia se ve en los mamíferos con dientes hipsodontes. Caballos, algunos roedores y los desmostilios* tienen (o tenían) dientes hipsodontes. Este tipo de diente tiene coronas muy altas, por ende tardan más en desgastarse; esta morfología se correlaciona con dietas abrasivas (como comer grama o comer en lugares donde hay mucho sedimento mezclado con el alimento). Si estos organismos no tuvieran este tipo de dientes sería contraproducente continuar con ese modo de alimentación ya que sus dientes se gastarían muy rápido. Así que la selección natural ha favorecido tener dientes hipsodontes en estos organismos que llevan una dieta que incluye material abrasivo.

*Información adicional sobre desmostilios, incluyendo reconstrucciones, aquí, aquí y aquí.

En organismos vivos es relativamente fácil determinar si estructuras parecidas son convergentes o no, ya que se pueden hacer observaciones directas sobre el comportamiento o función de la estructuras o morfología que se consideran convergentes. Sin embargo, con los fósiles es distinto ya que generalmente no podemos hacer observaciones directas sobre el comportamiento de un organismo extinto. Es por esto que nos toca entonces interpretar la funcionalidad de esas estructuras comparando con lo que conocemos de organismos vivos. A continuación les traigo un interesante ejemplo de convergencia entre un fósil y un organismo moderno

Odobenocetops

En el 1993, Christian de Muizon describió un fósil de delfínido (el grupo de ballenas que incluye a los delfines) que representa uno de los mejores ejemplos de convergencia que conozco. El fósil consistía en parte de un cráneo (ver la imágen abajo) colectado en el sur de Perú. Los sedimentos donde se encontró el fósil son parte de una secuencia de rocas sedimentarias conocida como la Formación Pisco, de edad Pliocénico Temprano (5.3-3.6 millones de años). El fósil no solo representaba una nueva especie, sino también un nuevo grupo de delfínidos al cual llamó Odobenocetopsidae.

El primer espécimen descrito lo llamaron Odobenocetops peruvianus Muizon, 1993 (ver imagen abajo). Los rasgos morfológicos más distintivos de esta especie consisten en tener el rostro corto (el área frente a los ojos), procesos alveolares de la premaxilla (los espacios para los dientes) agrandados y orientados hacia abajo; los procesos alveolares era donde tenían unos colmillos también agrandados. La premaxilla también tiene varios forámenes y áreas de inserción de músculos bien marcadas que indican la presencia de labios grandes y quizás hasta bigotes. Otras características del cráneo incluye tener las órbitas de los ojos posicionadas dorsalmente y un paladar profundo. Estas características que tanto distinguen a los odobenocetópsidos son reminiscentes a las morsas* más que a otros delfinoideos.

*Vean las imágenes de un cráneo de morsa y comparen.

Odobenocetops peruvianus, arriba, foto del cráneo en vista lateral (anterior hacia la derecha); abajo, ilustración del cráneo mostrando los diferentes huesos que lo componen y el área de la órbita (cículo azul), modificada de Muizon (1993). Abreviaciones: Al = aliesfenoide; Bo = basioccipital; Fr = frontal; Mx = maxila; Na = nasal; Oc = occipital; Or = órbita; Pa = parietal; Pal = palatino; Pmx = premaxila; Ppo = proceso postorbital; Ppr = proceso preorbital; Pt = terigoide; Z = zigomático.

Otra especie adicional fue descrita unos años más tarde (Muizon et al., 1999). Odobenocetops leptodon también fue encontrado en la Formación Pisco, sin embargo, proviene de un horizonte alrededor de un millón de años más joven que donde se encontró O. peruvianus. Algunas de las características que distinguen a esta nueva especie es un rostro más grande y redondeado, mayor tamaño del paladar y la presencia de fosas (depresiones) para los sacos premaxilares. La ocurrencia de esta última estructura que menciono, se correlaciona con la presencia del órgano del melón (Mead, 1975), la cual es una estructura utilizada para ecolocalización. O. peruvianus aparentemente carecía de esta estructura ya que carece de fosas para los sacos premaxilares, sin embargo, la posición de las órbitas y la forma de los procesos supraorbitales le permitía tener visión binocular; en O. leptodon los procesos supraorbitales eran distintos y sus orbitas estaban posicionadas más hacia el lado, careciendo de visión binocular, pero al menos auxiliado por la presencia del órgano del melón y la capacidad de hacer ecolocalización. Así que al parecer, O. peruvianus dependía de la visión para localizar su alimento, mientras que O. leptodon probablemente dependía mayormente de ecolocalización (como es el caso de muchos otros delfinoideos).

Los odobenocetópsidos están relacionados al único otro grupo de cetáceos que tienen colmillos agrandados, los narvales. Estos también tienen un colmillo agrandado (y en ocasiones los dos), pero estos están orientados hacia adelante. En los odobenocetópsidos la función de estos colmillos parece haber sido social ya que presentan dimorfismo sexual; los machos poseen un colmillo (el de la derecha) más agrandado que el otro (algunos hasta midiendo más de un metro), mientras que en las hembras ambos eran más pequeños y similares en tamaño (Muizon et al., 1999; Muizon & Domning, 2002).

Reconstrucción de Odobenocetops ilustración de Mary Parrish (National Museum of Natural History, Washingon, DC), tomado de Muizon et al. (1999).

¿Que pudo llevar a un delfínido a parecer una morsa?

El parecido de los odobenocetópsidos a las morsas (convergencia) llevó a los paleontólogos que los han estudiado a inferir que su modo de alimentación era muy similar. O sea, que posiblemente, al igual que las morsas, los odobenocetópsidos se alimentaban de bivalvos bentónicos (que viven en el suelo marino), localizándolos con asistencia de los bigotes, aguantando el bivalvo con los labios y utilizando la lengua como un pistón, creando un vacío dentro de la cavidad bucal para lograr succionar el organismo fuera de su concha.

La distribución geográfica de las morsas, tanto vivas como las fósiles, está limitada al hemisferio norte (Deméré et al., 2003). Asumiendo que las especies extintas de morsas tenían las mismas preferencias alimenticias que la especie moderna (y al parecer así era) y al tener una distribución limitada, ese espacio ecológico, ocupado por las morsas en el hemisferio norte, estuvo “vacío” en el sur. Es así que posiblemente surgió este caso de convergencia, donde un delfinoideo termina pareciendo una morsa, ocupando ese espacio ecológico y utilizando los recursos disponibles.

Este es uno de los casos en paleontología donde el presente es la llave del pasado. Si no conocieramos las morsas habría sido muy difícil interpretar la funcionalidad morfológica de estos extraños delfinoideos.

Deméré, T. A., A. Berta & P. J. Adam. 2003. Pinnipedimorph evolutionary biogeography. Bulletin of the American Museum of Natural History 13 (279):32-76.

Mead, J. G. 1975. Anatomy of the external nasal pasajes and facial complex in the Delphinidae (Mammalia, Cetacea). Smithsonian Contributions to Zoology 207:1-72.

Muizon, C. de. 1993. Walrus-like feeding adaptation in a new cetacean from the Pliocene of Peru. Nature 365:745-748.

Muizon, C. de & D. P. Domning. 2002. The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its paleobiological implications. Zoological Journal of the Linnean Society 134:423-452.

Muizon, C. de, D. P. Domning & M. Parrish. 1999. Dimorphic tusas and adaptive strategies in a new species of walrus-like dolphin (Odobenocetopsidae) from the Pliocene of Peru. Comptes-rendus de l’Academie des Sciences, Paris, Sciences de la Terre et des Planetes 329:449-455.

Better late than never: GSA 2009

This has been a particularly long hiatus, and not because I wanted to, but it has been a particularly busy summer.

Anyways, if you missed my poster presentation at the 2009 annual meeting of the Geological Society of America, here is your chance to see it. Below is the abstract (you can also see it here) as well as the poster itself.

Evolution and diversification of Dugonginae (Sirenia: Dugongidae) in the West Atlantic and Caribbean region

Jorge Velez-Juarbe & Daryl P. Domning

Sirenian evolution presumably started in the southern Tethys realm of the Old World. However, the most primitive sirenians have been found in the Caribbean region in rocks that date back to the Early or Middle Eocene, evidence that they took on an aquatic lifestyle early in their history and were able to disperse around the Atlantic. Since then, the West Atlantic and Caribbean (WAC) region has been important for the evolution of this group of mammals, with all four of the known families (Prorastomidae, Protosirenidae, Dugongidae, Trichechidae) occurring in the region. From the Oligocene through Pliocene, the predominant groups in the WAC region were members of the dugongid subfamilies Dugonginae and Halitheriinae, with the former being the more speciose of the two. In some parts of the WAC, dugongines and halitheriines are found in the same deposits, indicating that some form of niche partitioning was occurring among three or even more sympatric species of sirenians, something that is not observed among the living species.

Niche partitioning among sympatric sirenian species is most obviously explained by differences in: (1) tusk morphology; (2) rostral deflection; and (3) body size. The most primitive members of the Dugonginae are found in the WAC region. Their distinctive cranial morphology seems to have evolved as a specialization for harvesting larger seagrass rhizomes that were out of reach of the less specialized halitheriines. New fossils from Puerto Rico, Florida, and Yucatán help to reinforce our ideas about niche partitioning among sympatric sirenian species as well as establish the WAC region as the center of known dugongine diversity.

This was my first poster presentation about one of the topics that I’m working on as part of my thesis. The poster was a success! I was awarded runner-up of the 2009 Paleontological Society Poster Award. Congratulations to the first place and to the other runner-up too! It was a great meeting with very interesting talks as well as getting to spend some time with good friends!!

Enjoy the poster and feel free to ask question or make comments!