Tuesday, January 14, 2014

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 are the localities in Puerto Rico, Río Guatemala (RG), and Lares Limestone (LL). 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. 


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