Los roedores más antiguos del Caribe

Ya estamos en enero, el primer mes del nuevo año y con el llega la primera edición del año de la revista científica Journal of Vertebrate Paleontology. En este ejemplar se han publicado muchos trabajos interesantes, incluyendo varios sobre mamíferos marinos, lo cual es un tema recurrente en este blog. Sin embargo, en esta entrada hablaremos de otros fósiles. En esa misma edición ha salidos publicado un trabajo donde mis colegas y yo describimos fósiles de roedores de Puerto Rico (Vélez-Juarbe et al., 2014). Estos fósiles son únicos ya que son la evidencia más antigua de roedores caviomorfos en las Antillas Mayores, y nos ayudan a entender el cuando llegaron los vertebrados terrestres a la región, lo cual ha sido tema de debate durante varias décadas.

Roedores fósiles de Puerto Rico
No es la primera vez que se encuentran fósiles de roedores en Puerto Rico. De hecho, es bien conocido que durante el Pleistoceno* en Puerto Rico, La Española, Cuba, Jamaica, y algunas de las Antillas Menores, habitaban distintos grupos de roedores endémicos, perezosos terrestres, monos, y musarañas (Woods y Sergile, 2001). Muchos de estos fósiles de mamíferos terrestres del Pleistoceno fueron descritos a principios del siglo pasado (e.g. Anthony, 1918), aunque han habido descubrimientos más recientes (Turvey et al., 2006). Algunos de los roedores fósiles que han sido descritos de estas islas eran gigantes, como por ejemplo, Amblyrhiza inundata de la isla de Anguila, la cual tenía una masa corporal similar a la de un oso negro americano (Biknevicious et al., 1993)!
*periodo geológico que comenzó hace 2.6 millones de años y duró hasta hace 10,000 años

Siendo más específico, durante el Pleistoceno, en Puerto Rico habitaban al menos tres especies de ratas espinosas (Heteropsominae), y posiblemente dos especies de hutías gigantes (Heptaxodontidae) (información adicional aquí). Un tercer grupo de roedores endémicos que existió durante el mismo tiempo, en incluso todavía habita algunas de las otras Antillas, eran las hutías capromíidas, pero hasta donde sabemos estas nunca habitaron Puerto Rico. Todos estos grupos de roedores pertenecer a un grupo taxonómico más grande llamados caviomorfos. Los caviomorfos son endémicos de Sur América; sus ancestros llegaron de África alrededor de 54 millones de años atrás (Antoine et al., 2012). Una vez llegan a Sur América, los caviomorfos se diversifican y se dispersan a través de todo el continente y el Caribe. Algo muy similar también ocurrió con los primates suramericanos (platirrinos) (Kay, 2014), los cuales tienen una historia casi tan antigua como los roedores caviomorfos.

Foto tomada a principios de enero cuando visité de nuevo la localidad de las Calizas Lares donde encontré el diente incisor.

Los nuevos fósiles de Puerto Rico consisten de un par de dientes incisores, uno proveniente de la Formación San Sebastián, y el otro de las Calizas Lares (ver foto arriba). Respectivamente, estas formaciones se depositaron durante las épocas geológicas conocidas como Oligoceno temprano (29.78-26.51 Ma) y Oligoceno tardío (26.51-24.73 Ma) (Ortega-Ariza et al., 2015). Cuando encontré los fósiles allá para el 2005, me comuniqué con Ross MacPhee, curador de mamíferos del Museo Americano de Historia Natural en Nueva York, y quien durante años ha tenido interés en los orígenes de la fauna terrestre de las Antillas Mayores. Basado en lo que ya conocíamos sobre el registro fósil de Puerto Rico, Ross y yo sospechábamos que estos dientes incisores pertenecían a un roedor caviomorfo. Sin embargo, teníamos un problema, como pueden observar en la foto abajo a la derecha, los incisores son muy sencillos, ya que carecen de las crestas, cúspides y valles que uno vería en un molar, esto nos hacía difícil el poder identificarlos usando solo sus características externas. Lo siguiente que se nos ocurrió fue tratar de ver la estructura microscópica (microestructura) del esmalte, esta si preserva características que nos podrían ayudar a identificar el diente y saber a que grupo de roedores pertenecía. Estudiar la estructura microscópica de los dientes toma varios pasos: hay que hacer un corte transversal del diente, pulir la superficie y finalmente, utilizando un microscopio electrónico de barrido, ver la microestructura del diente. Para llevar a cabo esta tarea decidimos contactar a Thomas Martin, paleontólogo de la Universidad de Bonn, en Alemania y quien es uno de los expertos en el estudio de microestructura de los dientes. Luego convencer a Thomas, le enviamos los fósiles, y luego de varios meses, nos envió un correo electrónico con detalles de los resultados.

A la izquierda una representación de como son los roedores caviomorfos mostrando la posición del diente incisor. A la derecha el diente incisor fósil de la Formación San Sebastián. Arriba, imagen de microscopio electrónico de barrido mostrando la estructura microscópicas del incisor (píquenle a la imagen para que la vean más grande).  

El correo que nos envió Thomas fue muy alentador. Después de todo, resultó que nuestros fósiles si pertenecían a roedores caviomorfos, confirmando lo que ya sospechábamos. Ahora que sabíamos a que tipo de roedor pertenecían los dientes, y su importancia respecto a los orígenes de la fauna antillana, deseábamos obtener edades más precisas para las localidades. Para esto me comuniqué con mi amiga y colega Diana Ortega-Ariza, candidata doctoral de la Universidad de Kansas. Diana hizo su maestría en el Departamento de Geología en la Universidad de Puerto Rico (mi alma mater) donde estudió las distintas unidades calizas del norte de la isla, incluyendo las Calizas Lares. Como parte de su estudio ella obtuvo edades radiométricas utilizando isótopos de estroncio* preservados en los tubos calcíticos que servían de hogar a el bivalvo Kuphus incrassatus, el cual es uno de los fósiles más comunes en rocas del Oligoceno y Mioceno. Los resultados de ese estudio isotópico revelaron que las Calizas Lares se depositaron entre 26.51-24.73 millones de años atrás durante una era geológica llamada Chattiano la cual es la subdivisión más joven del Oligoceno**. Esa edad la podíamos entonces utilizar para estimar que la Formación San Sebastián, la cual se encuentra debajo de la Lares, tiene una edad mayor de 26.51 millones de años, en otras palabras, que esa formación se depositó durante el Rupeliano, la cual es la subdivisión más antigua del Oligoceno.
*Este tipo de análisis funciona de forma similar al Carbono 14, con la ventaja que nos sirve para fechar fósiles más viejos. Las edades máximas que se pueden obtener con Carbono 14 son de 50,000 años, mientras que el estroncio nos sirve para calcular fechas de hasta cientos de millones de años.
**Todo el periodo Oligoceno duró entre 33.9-23.0 millones de años atrás y tiene dos subdivisiones Chattiano y Rupeliano.

Orígenes de los vertebrados terrestres de las Antillas Mayores
Charles Darwin nunca estuvo en las Antillas. Aún así, la fauna de la región no pasó desapercibida y este incluso escribió en uno de sus libros más populares Voyage of the Beagle que "El caracter suramericano de los mamíferos antillanos parece indicar que este archipiélago estuvo en algún momento unido al continente sureño, y que subsiguientemente ha sido un área de subsistencia." Darwin, al igual que muchos otros antes y después de el, tenía razón respecto al lugar de origen de los mamíferos terrestres de las Antillas. Sin embargo, una de las preguntas más debatidas no ha sido el donde, sino el cuando llegaron los ancestros de los vertebrados terrestres de las Antillas.

Por años, el debate se ha centrado en dos hipótesis: una propone que estos llegaron a la región mediante múltiples eventos de dispersión a lo largo de los últimos 60 millones de años (e.g. Hedges et al., 1992), la otra, que llegaron alrededor del mismo tiempo durante un solo evento de dispersión (e.g. MacPhee e Iturralde-Vinent, 1995). La hipótesis de GAARlandia* propone que entre el Eoceno tardío y Oligoceno temprano las islas de Cuba, Española, y Puerto Rico junto con la Cresta de Aves formaron un puente terrestre casi continuo que los unió con el norte de Sur América (vean la figura abajo). Este puente terrestre, aunque de poca duración - probablemente estuvo formado entre 37.8-28.1 millones de años atrás - sirvió como un corredor para la dispersión de vertebrados terrestres hacia las masas terrestres que más tarde pasarían a ser las Antillas Mayores (para un resumen detallado de la evidencia geológica vean Iturralde-Vinent y MacPhee [1999]). Esto significa que, idealmente, deberíamos encontrar fósiles de mamíferos terrestres en depósitos del Eoceno tardío-Oligoceno temprano que representen a los mismos grupos que conocemos del Pleistoceno.
*Greater Antillean Aves Ridge land (MacPhee e Iturralde-Vinent, 1995)

Figura 1 de nuestra publicación. Aquí mostramos las distintas localidades (A) en Sur América donde se encuentran roedores fósiles del Eoceno y Oligoceno junto con la localidad de Domo de Zaza (DZ) en Cuba donde se han encontrado roedores del Mioceno temprano. En la figura B vemos las localidades de Puerto Rico, Río Guatemala (RG) y Calizas Lares (LL). C muestra la reconstrucción paleogeográfica de la región del Caribe durante el Eoceno tardío-Oligoceno temprano. Durante este periodo Cuba, La Española y Puerto Rico estaban unidos a la Cresta de Aves formando un puente terrestre conectando con el norte de Sur América.

El registro fósil de la región no nos ha fallado y nos ha dado algunos fósiles que apoyan esta segunda hipótesis. Uno de los primero que se descubrieron fue parte de un fémur (hueso del muslo) de un perezoso terrestre (megaloniquido) en la Formación Juana Díaz, al suroeste de Puerto Rico, la cual se depositó alrededor de 31 millones de años atrás (MacPhee e Iturralde-Vinent, 1995). Los roedores que describimos en nuestro trabajo son similarmente antiguos, y juntos representan la evidencia más temprana de la presencia de perezosos terrestres y roedores caviomorfos en la región.

A lo largo del Mioceno se encuentran otros fósiles de vertebrados terrestres en la región de la Antillas Mayores, estos al igual que los del Oligoceno pertenecen a grupos que existieron durante el Pleistoceno. El Mioceno es el periodo geológico que le sigue al Oligoceno y que duró entre 23 a 5.3 millones de años. Estos fósiles del Mioceno incluyen perezosos terrestres, primates platirrinos, y hutías del Mioceno temprano de Cuba (MacPhee et al., 2003), una iguana y una boa del Mioceno medio de Puerto Rico (MacPhee y Wyss, 1990), al igual que varias especies de ranas, guecos y lagartijas del Mioceno medio de La Española (e.g. De Queiroz et al., 1998; Daza y Bauer, 2012). Como podrán notar la lista de fauna del Mioceno es más larga y diversa que la del Oligoceno. La ausencia de algunos de estos en los depósitos más antiguos se podría atribuir al registro fósil, el cual es imperfecto, ó podría ser una ausencia real, implicando que estos organismos llegaron más tarde durante otros eventos de dispersión luego de la fragmentación de GAARlandia (Dávalos, 2004). Por ejemplo, basándose en el registro fósil y relojes moleculares, se ha estimado que los primates platirrinos llegaron a la región durante el Mioceno temprano (Cooke et al., 2011; Kay, 2014). En comparación, el grupo de sapos endémicos de las Antillas Mayores, los cuales se les conoce con el nombre científico de Peltophryne (y es el grupo que incluye el sapo concho), tienen un registro fósil que solo se extiende al Cuatenario (Pregill, 1981). Sin embargo utilizando relojes moleculares se ha estimado que los sapos Peltophryne llegaron a la región caribeña mucho antes, durante el Eoceno tardío-Oligoceno temprano lo cual sería consistente con la hipótesis de GAARlandia (Alonso et al., 2012).

Así que como pueden ver, la evidencia que tenemos hasta ahora apunta a un origen más complicado de los vertebrados terrestres de las Antillas Mayores y parece que ambas hipótesis han jugado un rol en el origen de los mismos. Además, como mencionamos brevemente en nuestro trabajo, no todo tiene que haberse dispersado de Sur América a las Antillas. Por ejemplo, los gaviales gryposuquinos pudieron utilizar ese puente terrestre para dispersarse del Caribe a Sur América. Para lograr entender mejor esta complicada historia se necesita hacer más trabajo y más descubrimientos en la región. Trabajar en los trópicos no es fácil por la extensa vegetación y limitada exposición de las rocas, sin embargo, la recompensa es grande cuando se hacen descubrimientos como los que acabamos de publicar.


Referencias

Ali, J. R. 2012. Colonizing the Caribbean: is the GAARlandia land-bridge hypothesis gaining a foothold? Journal of Biogeography 39:431-433.

Alonso, R., A. J. Crawford, and E. Bermingham. 2012. Molecular phylogeny of an endemic radiation of Cuban toad (Bufonidae: Peltoprhyne based on mitochondrial and nuclear genes. Journal of Biogeography 39:434-451.

Anthony, H. E. 1918. The indigenous land mammals of Porto Rico, living and extinct. American Museum of Natural History, Memoirs 1(2):324-435.

Antoine, P.-O., L. Marivaux, D. A. Croft, G. Billet, M. Ganerod, C. Jaramillo, T. Martin, M. J. Orliac, J. Tejada, A. J. Altamirano, F. Duranthon, G. Fanjat, S. Rousse, and R. Salas Gismondi. 2012. Middle Eocene rodents from Peruvian Amazonia reveal the pattern and timing of caviomorph origins and biogeography. Proceedings of the Royal Society B 279:1319-1326.

Biknevicious, A. R., D. A. McFarlane, and R. D. E. MacPhee. 1993. Body size in Amblyrhiza inundata (Rodentia: Caviomorpha), an extinct megafaunal rodent from the Anguilla Bank, West Indies: estimates and implications. American Museum Novitates 3079:1-25.

Dávalos, L. M. 2004. Phylogeny and biogeography of Caribbean mammals. Biological Journal of the Linnean Society 81:373-394.

Daza, J. D., and A. M. Bauer. 2012. A new amber-embedded sphaerodactyl gecko from Hispaniola, with comments on morphological synapomorphies of the Sphaerodactylidae. Breviora 529:1-29.

De Queiroz, K., Ling-Ru Chu, and J. B. Losos. 1998. A second Anolis in Dominican amber and the systematics and ecological morphology of Dominican amber anoles. American Museum Novitates 3249:1-23.

Hedges, S. B., C. A. Hass, and L. R. Maxson. 1992. Caribbean biogeography: molecular evidence for dispersal in West Indian terrestrial vertebrates. Proceedings of the National Academy of Sciences of the United States of America 89:1909-1913.

Iturralde-Vinent, M. A., and R. D. E. MacPhee. 1999. Paleogeography of the Caribbean region: implications for Cenozoic biogeography. Bulletin of the American Museum of Natural History 238:1-95.

Kay, R. F. 2014. Biogeography in deep time – what do phylogenetics, geology, and paleoclimate tell us about early platyrrhine evolution? Molecular Phylogenetics and Evolution In press.

MacPhee, R. D. E., and M. A. Iturralde-Vinent. 1995. Origins of the Greater Antillean land mammal fauna, 1: new Tertiary fossils from Cuba and Puerto Rico. American Museum Novitates 3141:1-31.

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

Ortega-Ariza, D., E. K. Franseen, H. Santos-Mercado, W. R. Ramírez-Martínez, and E. E. Core-Suárez. 2015. Strontium isotope stratigraphy for Oligocene-Miocene carbonate systems in Puerto Rico and the Dominican Republic: implications for Caribbean processes affecting depositional history. Journal of Geology 123:539-560.

Pregill, G. 1981. Late Pleistocene herpetofaunas from Puerto Rico. University of Kansas Museum of Natural History Miscellaneous Publication 71:1-72.

Turvey, S. T., F. V. Grady, and P. Rye. 2006. A new genus and species of ‘giant hutia’ (Tainotherium valei) from the Quaternary of Puerto Rico: an extinct arboreal quadruped? Journal of Zoology 270:585-594.

Velez-Juarbe, J., T. Martin, R. D. E. MacPhee, and D. Ortega-Ariza. 2014. The earliest Caribbean rodents: Oligocene caviomorphs from Puerto Rico. Journal of Vertebrate Paleontology 34:157-163.

Woods, C. A., and F. E. Sergile (eds.). 2001. Biogeography of the West Indies: Patterns and Perspectives, second edition. CRC Press, Boca Raton, Florida, 608 pp.

The oldest Caribbean rodents

Its January 2014, and with it comes the first issue of the Journal of Vertebrate Paleontology. There are many interesting papers, including several ones on fossil marine mammals, which are a frequent subject in this blog. However this post will be about another paper in that same issue, where several of my colleagues and myself describe fossil rodents from Puerto Rico (Velez-Juarbe et al., 2014). These are not just any fossil rodents, they are the oldest evidence of caviomorph rodents in the Greater Antilles, and help us further understand the timing of arrival of terrestrial vertebrates to the region, an issue which has been the matter of debate for several decades now (keep reading).

An account of fossil rodents from Puerto Rico

It’s not the first time that fossil rodents have been found in Puerto Rico. In fact, it is well known that during the Pleistocene, Puerto Rico, Hispaniola, Cuba, Jamaica, and several of the Lesser Antilles, were home to endemic groups of rodents and other land mammals (Woods and Sergile, 2001). Many of these Pleistocene mammals were described in the early 1900’s (e.g. Anthony, 1918), although there have been some more recent discoveries as well (Turvey et al., 2006). Some of the rodents included giant forms, with some, like Amblyrhiza inundata, had body masses similar to those of the American black bear (Biknevicious et al., 1993)! 

During the Pleistocene Puerto Rico was home to about  two or three species of heteropsomine spiny rats (Echymyidae), and possibly two species of plate-tooth (Heptaxodontids). A third group that is still present in the region, are the Capromyids, but these, apparently, never reached Puerto Rico. All of these groups of rodents, are part of a larger, more inclusive group of rodents known as caviomorphs. caviomorphs are endemic to South America, their ancestors arriving from Africa about 54 million years ago (Antoine et al., 2012). Once in South America caviomorphs underwent an explosive radiation, spreading throughout the continent and as we now know, the Caribbean, early on in their evolutionary history. A similarly fast radiation also happened with South American primates (Platyrhines) (Kay, 2014), which share a comparable history to that of the caviomorphs.

This picture is from last week, when I revisited the Lares limestone site where i found the rodent incisor.

The new fossils from Puerto Rico consist of a couple of isolated incisors, one from the early Oligocene San Sebastian Formation (from this locality), and the other from the late Oligocene Lares Limestone (from this locality; also see picture above). When I found the first fossils back in 2005, I contacted Ross MacPhee, curator of mammals at the American Museum of Natural History, and who has had a long interest in the origins of the Greater Antillean land mammal fauna. Based on what we know about the fossil record of Puerto Rico, Ross and I suspected these incisors were from a caviomorph. However, we had a problem, as you can see in the picture below, isolated incisors are really hard to identify based only on external features, as they are very simple and lack the cusps, valleys and ridges that characterize their molars (this is true for nearly all mammals). We then thought about looking at the enamel microstructure, as it does preserved features that can be extremely useful in identifying isolated finds like ours. Studying the enamel microstructure usually involves making cross-sections of the teeth, the cut surfaces are polished, etched in a light acid, and observed with the aid of scanning electron microscope (SEM's). To do this, we decided to contact Thomas Martin at Universität Bonn, an authority on enamel microstructure. After recruiting Thomas, we sent him the fossils, and within several months, he emailed us back with the description and interpretation of the fossils. 

Lateral (left) and anterior (right) views of the San Sebastian caviomorph incisor. Here the fossil was still partially surrounded by rock and had not been sectioned to study the enamel microstructure. 

Figure 2 from our paper. Here we show the cross section outline and the enamel microstructure of the San Sebastian caviomorph (left column) and Lares caviomorph (right column).

Our fossils did belong to a caviomorph, thus confirming our initial suspicions and excitement about the discovery. Because of the importance of these fossils, we wanted to get as precise dates on the localities as possible, so I contacted my friend and colleague Diana Ortega-Ariza, a PhD candidate at the University of Kansas. For her masters at the University of Puerto Rico, Diana studied several of the limestone units in Puerto Rico, including the Lares Limestone. As part of her study, she obtained radiometric dates using isotopic signals preserved in the calcitic tubes that serve as home to the bivalve Kuphus incrassatus, which is commonly found in Oligocene through Miocene marine deposits. Based on the results she obtained, we are now able to say that the Lares Limestone was deposited between 27-24 million years ago during a geologic age called Chattian which is the youngest sub-division of the Oligocene. We could also used that timeframe to place deposition of the underlying San Sebastian Formation as older than 27 million years ago, or during the age known as Rupelian, the oldest subdivision of the Oligocene (the whole Oligocene period lasted between 33.9-23.0 million years ago).

Origins of the Greater Antillean land vertebrate fauna

“The South American character of the West Indian mammals seems to indicate that this archipelago was formerly united to the southern continent, and that it has subsequently been an area of subsidence.” Charles R. Darwin The Voyage of the Beagle

That is one of my favorite sentences in Voyage of the Beagle. Charles Darwin never visited the Greater Antilles, but he was right about where did the mammals of the region originated. However, one of the long-standing questions regarding the origins of Greater Antillean land vertebrates is not where, but when did they arrived. For years the debate has been whether they arrived to the region at different intervals throughout the Cenozoic (e.g. Hedges et al., 1992) or in tandem during a single dispersal event (e.g. MacPhee and Iturralde-Vinent, 1995). The GAARlandia* hypothesis postulates that during the late Eocene-early Oligocene the islands of Cuba, Hispaniola and Puerto Rico together with the Aves Ridge formed a continuous, or nearly continuous landspan connected to northern South America (see figure below). This landspan, even though short-lived - it probably lasted between about 37.8 to 28.1 million years ago, or less - would have served as a corridor for the dispersal of land vertebrates into what eventually became the Greater Antilles (see Iturralde-Vinent and MacPhee [1999] for a very detailed overview of the geologic evidence). Ideally, we should be finding fossils representing the different groups of Pleistocene and recent Greater Antillean land vertebrates in late Eocene-early Oligocene deposits, and we do, at least in part. 

*Greater Antillean Aves Ridge land (MacPhee and Iturralde-Vinent, 1995)

Figure 1 from our paper. Here we show the various localities (A) in South America where Eocene and Oligocene rodents are known and Domo de Zaza (DZ) in Cuba, where early Miocene rodents are known. In B are the localities in Puerto Rico, Río Guatemala (RG), and Lares Limestone (LL). C is the paleogeographic reconstruction of the Caribbean region during the late Eocene-early Oligocene. During this time Cuba, Hispaniola and Puerto Rico were joined with the Aves Ridge (C) forming a nearly continuous landspan connected to northern South America.

The fossil record of the region has so far provided a few clues supporting this hypothesis as well. One of the first ones found was a femur (leg bone) of a ground sloth (megalonychid) in the early Oligocene Juana Diaz Formation in southwestern Puerto Rico which was deposited about 31 million years ago (MacPhee and Iturralde-Vinent, 1995). The rodents described in our work are just as old, and together they are the earliest evidence of these two groups in the region.

Throughout the Miocene (the period between 23-5.3 million years ago) in the Greater Antilles there are other occurrences of terrestrial vertebrates that were present during the Pleistocene and some which are even still around today. There are early Miocene sloths, rodents and primates from Cuba (MacPhee et al., 2003), a boa and an iguana from the early Miocene of Puerto Rico (MacPhee & Wyss, 1990), as well as a number of frogs, geckos and anoles from the middle Miocene of Hispaniola (e.g. De Queiroz et al., 1998; Daza and Bauer, 2012). The absence of some of these groups in Oligocene rocks in the Greater Antilles could be due to the scarcity of the fossil record, or could indeed be real, implying that some of these groups arrived by random overwater dispersal after fragmentation of GAARlandia (Dávalos, 2004). For example, based on fossils and molecular data, primates have an estimated time of arrival to the region during the early Miocene (Cooke et al., 2011; Kay, 2014). On the other hand, toads of the genus Peltophryne which is endemic to the region, are known only from Pleistocene deposits (Pregill, 1981) but molecular estimates place their split from their closest relatives during the late Eocene-early Oligocene (Alonso et al., 2012).

So, as you can see, the evidence seems to point to a more complex origin of the Greater Antillean land vertebrates and does not seem to favor one over the other. Also, not everything had to have come from South American to the Greater Antilles. As we mentioned briefly in our paper, organisms that were present in the Antilles, such as gryposuchine gavials, could have used that same corridor to disperse to the southern continent. Further unraveling of this complex history can only be achieved by more fieldwork and discoveries in the Greater Antilles. 

References

Ali, J. R. 2012. Colonizing the Caribbean: is the GAARlandia land-bridge hypothesis gaining a foothold? Journal of Biogeography 39:431-433.

Alonso, R., A. J. Crawford, and E. Bermingham. 2012. Molecular phylogeny of an endemic radiation of Cuban toad (Bufonidae: Peltoprhyne based on mitochondrial and nuclear genes. Journal of Biogeography 39:434-451.

Anthony, H. E. 1918. The indigenous land mammals of Porto Rico, living and extinct. American Museum of Natural History, Memoirs 1(2):324-435.

Antoine, P.-O., L. Marivaux, D. A. Croft, G. Billet, M. Ganerod, C. Jaramillo, T. Martin, M. J. Orliac, J. Tejada, A. J. Altamirano, F. Duranthon, G. Fanjat, S. Rousse, and R. Salas Gismondi. 2012. Middle Eocene rodents from Peruvian Amazonia reveal the pattern and timing of caviomorph origins and biogeography. Proceedings of the Royal Society B 279:1319-1326.

Biknevicious, A. R., D. A. McFarlane, and R. D. E. MacPhee. 1993. Body size in Amblyrhiza inundata (Rodentia: Caviomorpha), an extinct megafaunal rodent from the Anguilla Bank, West Indies: estimates and implications. American Museum Novitates 3079:1-25.

Dávalos, L. M. 2004. Phylogeny and biogeography of Caribbean mammals. Biological Journal of the Linnean Society 81:373-394.

Daza, J. D., and A. M. Bauer. 2012. A new amber-embedded sphaerodactyl gecko from Hispaniola, with comments on morphological synapomorphies of the Sphaerodactylidae. Breviora 529:1-29.

De Queiroz, K., Ling-Ru Chu, and J. B. Losos. 1998. A second Anolis in Dominican amber and the systematics and ecological morphology of Dominican amber anoles. American Museum Novitates 3249:1-23.

Hedges, S. B., C. A. Hass, and L. R. Maxson. 1992. Caribbean biogeography: molecular evidence for dispersal in West Indian terrestrial vertebrates. Proceedings of the National Academy of Sciences of the United States of America 89:1909-1913.

Iturralde-Vinent, M. A., and R. D. E. MacPhee. 1999. Paleogeography of the Caribbean region: implications for Cenozoic biogeography. Bulletin of the American Museum of Natural History 238:1-95.

Kay, R. F. 2014. Biogeography in deep time – what do phylogenetics, geology, and paleoclimate tell us about early platyrrhine evolution? Molecular Phylogenetics and Evolution In press.

MacPhee, R. D. E., and M. A. Iturralde-Vinent. 1995. Origins of the Greater Antillean land mammal fauna, 1: new Tertiary fossils from Cuba and Puerto Rico. American Museum Novitates 3141:1-31.

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

Pregill, G. 1981. Late Pleistocene herpetofaunas from Puerto Rico. University of Kansas Museum of Natural History Miscellaneous Publication 71:1-72.

Turvey, S. T., F. V. Grady, and P. Rye. 2006. A new genus and species of ‘giant hutia’ (Tainotherium valei) from the Quaternary of Puerto Rico: an extinct arboreal quadruped? Journal of Zoology 270:585-594.

Velez-Juarbe, J., T. Martin, R. D. E. MacPhee, and D. Ortega-Ariza. 2014. The earliest Caribbean rodents: Oligocene caviomorphs from Puerto Rico. Journal of Vertebrate Paleontology 34:157-163.

Woods, C. A., and F. E. Sergile (eds.). 2001. Biogeography of the West Indies: Patterns and Perspectives, second edition. CRC Press, Boca Raton, Florida, 608 pp.

Back to the western Caribbean, Pt. 3

This will be, for now, the final part of this series. This is going to be a bit of a tease, as I'll go over some of the cetacean find we've made in the last several months won't go into many details as to what each of the fossils are. That will be the subject of future posts, once the fossils are properly described and published. There are a good amount of pictures to make up for it, so enjoy!

Trash road cetacean
As you may have read previously (here, here and here) collecting fossil vertebrates in the Chagres Formation is not an easy task and takes some planning before collecting each specimen. Back in the summer I found what seemed to be the remains of a fossil cetacean partially exposed on on the wave cut platform (see pictures below). To get to this locality we had to walk through a 10 meter stretch of road that is always full of trash, hence the locality name. As for the fossils, not a lot of it was exposed, and the day when we found it, we were on another mission. So we ended going back to the site to collect the fossils two days before Thanksgiving Day, and like most other digs here, we only had about four hours before the high tide took over.

Left: The exposed fossil; right: Chris (Summer 2013 intern) looking at it somewhat in disbelief. 

Finally, in November we collected the fossil. James and I started digging the trench around the fossil, soon after the other interns joined the digging party.

Zach, Sarah, Elena and James (Fall 2013 interns) are happily posing with the jacketed fossil, it only took us about 2 and a half hours. 

And liftoff!! We take the jacket to the truck and off to STRI where it will be properly prepared.

Chagres Norte dolphin
Back in early September, I had the chance to go prospecting in the Chagres Formation with two of my colleagues from invertebrate paleontology at the Florida Museum of Natural History, Roger Portell and Austin Hendy. They've previously done some more work in the marine units here in Panama, so it was good to go out with them and see new localities. One of these, was located further north of the village of Piña than I'm used to. Getting there involved a really muddy road, fortunately, we had all terrain vehicles, and it was actually fun driving through it.

The site, which I dubbed Chagres Norte, was one of the northern-most localities near the village of Piña.

At this site as with others along this whole coast, the Chagres Formation is exposed along the wave-cut platforms and sea cliffs. One of the fossils we knew was there (Roger and Austin had found it some days earlier) was part of the vertebral column of a small cetacean.

Elena (Fall 2014 intern), Roger and Austin collect the cetacean vertebrae. This is one of the reasons I like these guys, even of they specialize in invertebrates, they know not to ignore vertebrate fossils and will collect them.

This was Elena's second day in the field. She got hooked on the Chagres and even "claimed" the fossil saying that she was going to do the prep work, which she did.

The partial vertebral column we collected back in September. This is after Elena's careful and fantastic prep job.

Chagres Sur odontocete
Later that day, we went to another locality, this one south of Piña, so I gave it the obvious name of Chagres Sur. At this site I spotted some cetacean fossils sticking out of the sea cliff (see picture below).

One of the localities south of Piña, where I spotted some cetacean bones (you can see the bones towards the center right). For reasons I can't remember, Elena is staring in the wrong direction.

At that point, I didn't know what this fossil was, and we were also running out of daylight, so we left it there, and hoped to come back another day to collect it. It wasn't until a couple of weeks ago (1st week of December), that I decided to re-visit the locality with the interns. It was then, upon seeing this fossil again, that I had one of those, aha! moments and I realized what it was. So we had to collect it!

Here I am with the Chagres Sur odontocete, just before we removed it.

I won't go into details of what the fossil is at this moment (I warned you this was a post to spark your curiosity). But, if you plan on attending the 10th North American Paleontological Convention in Florida next February, you'll definitely find out more about it!

So stay tuned, you'll hear more about these discoveries in 2014!

Back to the western Caribbean, Pt. 2

The previous post was to give you a brief introduction into the geology of the Chagres Formation, which is where we are focusing part of our collecting efforts. This next post is to give you an idea of what happens when we find fossils of large marine vertebrates in the Chagres.

Often, when we find fossils in the Chagres these are on the rock exposed in the wave-cut platform along the beach (although there are exceptions). The benefit of this is the relatively easy access, the downside, is that we usually have only about 4 hours (spanning between the period before, during and after, the low tide) to collect the fossil. Back in April, during one of our trips to the Caribbean coast, we saw a partial marlin mandible exposed on one of the wave-cut platforms (picture below). That day we were prospecting, that is, looking for new localities and new fossils that were exposed so that we could plan to collect them in the near future. This was one of those finds.

A partial marlin mandible exposed on the wave-cut platform (anterior to the left).

The near future ended up being July. And so it was that together with Carlos De Gracia (a STRI intern whose main interest are fossil fishes), and my summer 2013 interns, Chris, Christina, and Silvia (previously featured here) we set out to the Caribbean to collect that fossil marlin.

Collecting the fossil took us three days. The first day was cut short and we couldn't do much work as the weather turned bad, and we had to leave after about an hour of work. Day two was purely devoted to digging a trench around the fossil so that we could wrap it in a protective plaster jacket for proper removal and transportation back to STRI (see picture below). This is a near obligatory task and method of proper collection of fossil vertebrates and has been used at least  since the late 1800's. While digging around it, we realized that we had more than we thought, the fossil consisted of mandible and skull, which meant we had to dig deeper, and wider around the fossil to be able to remove it completely.

Day 2 of the excavation. From left to right: Carlos De Gracia (STRI intern) and my summer 2013 interns, Christina, Chris and Silvia, make the trench around the marlin skull and mandible.

After we dug a deep enough trench around the fossil, we were ready to put a plaster jacket around it. Except that by then it was late, and the tide was coming back in, meaning we had to jacket the specimen another day. Finally, on our third day at this particular site, were were able to put a jacket around the fossil (see pictures below).

Day 3 of the excavation. Christina and Silvia are finish the jacket protecting the marlin skull and mandible.

This is the locality, if you click on the image you can see Silvia (near the center) and Christina (to the right); behind her is the jacket (the white blob on the ground).

After the plaster dried we undercut the block and flipped it so that we could remove excess rock and make it a bit lighter to carry back to the truck and then drive back to STRI (see pictures below). This specimen, as well as several other marlins that have been collected from the Chagres (including the one mentioned here), will be prepared and studied by Carlos.

After we popped and overturned the jacket,  me, Chris, and Carlos removed some of the excess rock in order to make it lighter.

Liftoff!! Even after trimming some of the rock off, this was still a pretty heavy block.

Other fishes known from the Chagres include a variety of bony fishes (mainly know from otoliths, which I mentioned in Part 1) as well as a several species of sharks (of which cookie-cutter sharks are one of the most common). Marlins are also relatively common and easy to recognize in the field; in fact, this year alone, we have collected several other skulls, mandibles and vertebrae.

Because they are common, the fossil marlins of the Chagres have not gone unnoticed, unlike the marine mammals. In 1978, Harry L. Fierstine described the first fossil marlin known from Panama. The fossil, consisting of a nearly complete skull, represented a new species, which he named Makaira panamensis. Modern relatives of Makaira panamensis include the black and the blue marlins. A second fossil marlin from Panama was described in 1999 (Fierstine, 1999). This one was found in the late Miocene Gatun Formation, which underlies the Chagres Fm. The fossil consisted of a partial rostrum, and was identified as Makaira cf. M. nigricans, the same species as the blue marlin (Fierstine, 1999). This implies that this particular species has been around for several million years, or more likely, that some extant members of this group are morphologically conservative and show very little differences from its fossil relatives.  

I'm sure we'll learn more about the fossil fishes of the Chagres and Gatun formations in the not so distant future. For now, stay tuned as this series is not yet over!

Literature Cited

Fierstine, H. L. 1978. A new marlin, Makaira panamensis, from the Late Miocene of Panama. Copeia 1978:1-11.

Fierstine, H. L. 1999. Makaira sp., cf. M. nigricans Lacépede, 1802 (Teleostei: Perciformes: Istiophoridae) from the Late Miocene, Panama, and its probable use of the Panama Seaway. Journal of Vertebrate Paleontology 19:430-437.

New Title Banner!!

After a few years, I thought it was fair to change the look of the blog a little bit. If you look up I've changed the banner, to one thats better (I think) and less crowded than the previous one. The pictures on the new title banner are all from different localities I've been doing fieldwork over the last couple of years and expect to continue to do so.

As for the old one, here's what was in it.

1) Pleistocene beach deposits in Isabela, Puerto Rico

2) Illustration of the skull of a new dugongid taxon (more on this in 2014)

3) Portunid crab from the San Sebastian Formation

4) Cross-bedded sandstones of the San Sebastian Formation

5) Tooth of Physogaleus contortus from the Lares Limestone

6) Tooth of Hemipristis serra from the Juana Diaz Formation

7) Outcrop of the Lares Limestone along road 111 in San Sebastian, Puerto Rico

8) Illustration of the skull of Dugong dug on showing the muscle attachment sites, modified from Domning (1977)

9) Mandible of Nesophontes edithae, one of several extinct Pleistocene mammals from PR 

Of course, feel free to guess where the pictures in the new banner were taken and leave your comments below. Good luck!!

Back to the western Caribbean, Pt. 1

Its been quite a while since the last post here at Caribbean Paleobiology. Lots of traveling and working hard on publishing parts of my dissertation as well as my current research projects here in Panama (stay tuned for more on this next year) have kept me extremely busy.

As I have mentioned previously, my work during this postdoc requires that I lead a group of interns (you can learn more about the internship here) as we search for terrestrial vertebrates in early Miocene deposits exposed on the Pacific side of the Panama Canal (see previous post). However, every now and then, as you may have seen in previous posts (here and here), we get to go to the Caribbean side of Panama in search of late Miocene marine vertebrates in the Chagres Formation. This is the first part of a series about our recent efforts to collect fossil marine vertebrates and to better understand the geology of the Chagres Fm.

Map of the northern part of the Panama Canal Basin. Here you can see the extension of the Chagres Formation and its members (map from Collins et al., 1996). (Click on the image to see a larger version.)

The Chagres Formation
This formation, exposed on the northern part of the Panama Canal Basin (see map above), generally consist of three distinct members or facies: Toro Member, silty sandstone facies, and Rio Indio facies (Collins et al., 1996). Age estimates for the deposition of the Chagres have been made using Foraminifera (which are extremely good index fossils). As a result, we known that the formation was deposited between 8.6-5.6 million years ago (Collins et al., 1996), during the final part of a geologic period known as the Miocene.

Toro Point, located southwest of the Caribbean exit of the Panama Canal, located within Ft. Sherman, which is a former US military base.

The Toro Member is the basal unit of the formation, and consist of cross-bedded coquinas and medium to coarse sands (see picture below). Coquina, is a term used to describe a sedimentary deposit that consists mostly, if not entirely, of shell fragments. In the case of the Toro Member, it is made up almost entirely of echinoid (sea urchin) and barnacle fragments, together with other less common bivalves and gastropods. Both, the types of invertebrates that make up the coquina, as well as the cross-bedding is indicative of high-energy, shallow marine habitats (Hendy, 2013)

Cross beds of the Toro Member, as exposed in Toro Point.

Disregarding what the geology and macroinvertebrates suggest, the Toro Member has been interpreted as being deposited at much deeper depths (several hundreds of meters), by a high-energy stream flowing from the Pacific Ocean towards the Caribbean sea, and thus representing the final connection between these two oceans (Collins et al., 1996). This interpretation, is based on the occurrence of deep-water fossils of Pacific affinities within the silty sandstone facies.

The silty sandstone facies of the Chagres Formation, as exposed in Playa Tortuguilla, located northeast of Fuerte San Lorenzo, and the mouth of the Chagres River. From left to right: James, Sarah, Elena and Zach (Fall 2013 interns) are studying the trace fossils of the Chagres. 

About 4-5 kilometers southwest of Toro Point, is where the main part of the Chagres Formation, the silty sandstone facies, is exposed. This is the most extensive of the members, going from southwest of Toro Point to about ~6 kilometers southwest of the village of Piña. Foraminifera (or forams for short) are not only used for estimating when marine units were deposited, but can also serve as index fossils for depositional depth, and environment. Forams collected from the silty sandstone facies of the Chagres were used for estimating the depth of this part of the formation, resulting in an estimate of somewhere between 200-500 meters (Collins et al., 1996).

Outcrop of the silty sandstone facies of the Chagres Formation near the village of Piña.

Forams are not the only fossils known from these units. Fierstine (1978) described a fossil marlin which he dubbed Makaira panamensis, an extinct species only known from this place and time. Other fossils found in these facies are fish otoliths. Otoliths are fish ear bones; they can be identified fairly accurately, and, similar to forams, they can be used as index fossils. A preliminary study of the otolith fauna of the silty sandstone facies collected near the village of Piña, suggests that these facies were deposited somewhere between 100-700 meters (De Gracia et al., 2012). This is a broader estimate, than that obtained using the forams, but still consistent with the idea that the silty sandstone facies represent relatively deep marine environments. These units, have so far, proven to be the most productive in terms of vertebrate fossils, thus most of our efforts have been in this area. In fact, back in 2011, I was near Piña, collecting a fossil dolphin* as part of the Pyenson Lab, and, more recently, with the Spring 2013 interns we collected a fossil sperm whale and parts of a marlin skull. (More about even more recent discoveries in the following iterations of this series).
*You can see more of the fossil dolphin collected during the Pyenson Lab 2011 expedition here!

Outcrop of the Río Indio facies of the Chagres Formation, somewhere south of La Boca del Indio. 

Towards the southwest, along the opposite site of the basin, between Palmas Bellas and Rio Gobea, is where we find the Rio Indio facies. These facies are characterized by siltstones and sandstones (Collins et al., 1996). Estimates of the depositional environment of these facies are variable, but generally much shallower than those of the silty sandstone facies (see below). Based on forams, it ranged from 50-80 meters, whereas estimates based on fish otoliths (= fish ear bones) it ranges from 0-100 meters (Collins, 1996; Collins et al., 1996; Aguilera and Aguilera, 1999). Just today was our first time visiting some of the Río Indio localities, and although we didn't find any vertebrates, we did find mollusks which are consistent with the shallower depth interpretations of previous workers.

So, stay tuned for the upcoming installments of this series!


Literature Cited

Aguilera, O., and D. R. de Aguilera. 1999. Bathymetric distribution of Miocene to Pleistocene Caribbean teleostean fishes from the coast of Panama and Costa Rica. Bulletins of American Paleontology 357:251-270.

Collins, L. S. 1996. Environmental changes in Caribbean shallow waters relative to the closing Tropical American Seaway; pp. 130-167, in J. B. Jackson, A. Budd, and A. Coates (eds.), Evolution and Environment in Tropical America. University of Chicago Press, Chicago, Illinois.

Collins, L. S., A. G. Coates, W. A. Berggren, M.-P. Aubry, and J. Zhang. 1996. The late Miocene Panama isthmian strait. Geology 24:687-690.

De Gracia, C., J. Carrillo-Briceño, W. Schwarzhans, and C. Jaramillo. 2012. An exceptional marine fossil fish assemblage reveals a highly productive deep-water environment in the Central American Seaway during the late Miocene. Geological Society of America Abstracts with Programs 44:164.

Fierstine, H. L. 1978. A new marlin, Makaira panamensis, from the late Miocene of Panama. Copeia 1978:1-11.

Hendy, A. J. W. 2013. Spatial and stratigraphic variation of marine paleoenvironments in the middle-upper Miocene Gatun Formation, Isthmus of Panama. Palaios 28:210-227.