Wednesday, November 23, 2011

Thoughts on the Origins of the Tobago Herpetofauna

Three species of frogs from 
Tobago with close relatives in 
northern Venezuela. Top to 
bottom: Hylinobatrachium 
orientale, Pristimantis 
charllottvilensis, and 
Mannophryne olmonae.
Currently the H. orientale is
thought to be the same species
present in Venezuela. P. 
charllottvillensis is very
similar to P. Pristimantis 
terraebolivaris  a
species with which it was
long confused. And. M.
olmonae's sister species
is M. riveroi. See Map.
Look at a map of northern South America and most people assume that the coastal Cordillera of Venezuela, Trinidad and Tobago were continuous and that Tobago forms the eastern most end of the mountain range. The flora and fauna of Tobago have some Lesser Antillean species, but most of the fauna is continental. So, the assumption that Tobago is in fact part of South America's Northern Andes is quite reasonable, but one that is proving to be incorrect.

Tobago has Mesozoic rocks of two quite different types: (1) an oceanic-arc rock sequence (about 105 mya) and (2) older North Coast Schist (about 120 mya). The schist layer has been melted and cooled - probably multiple times - its metamorphic. The structural rock overlying Tobago Volcanic Group has also undergone some metamorphosis and has been titled by faulting. The time difference (~15 million years) suggests something dramatic happened in the island's development. Fossils are uncommon in igneous and metamorphic rocks but ammonites and radiolarians recovered from the Spring Gardens quarry and Bacolet in Tobago are of middle and early Albian (112.0 to 99.6 mya) age (Snoke and Nobel, 2001).

Several authors have hypothesized that Mesozoic oceanic-arc rocks scattered along the northern and southern margins of the Caribbean plate were once part of the Great Arc of the Caribbean (see Pindell and Kennan, 2007). This oceanic arc entered the Caribbean region of today from the Pacific and through intra-arc rifting and back arc spreading expanded greatly to form the Caribbean Plate of the present day as well as much of Middle America.

Today, Tobago lies within the southern plate-boundary zone between the South American and Caribbean plates. Its present geographic position is the structural high point at the northeast corner of the South American continental shelf and it reflects a long and complex history of tectonic displacement - in other words Tobago (or at least pieces of it) as well as Little Tobago, have traveled a considerable distance from their point of origin, which was far to the west of its current position.
The Venezuelan distributions of three species of frogs with close ties to frogs on Tobago.

So how did the continental flora and fauna present today come to be there if it's not the eastern end of the Coastal Cordillera of Venezuela? Tobago has some very cool frogs: glass frogs (Hyalinobatrachium), at least two species of endemic Pristimantis, and an endemic Mannophryne. But it also has some widespread frogs, like Leptodactylus fuscus, Rhinella marina, and Trachycephalus typhonius. All the frogs suggest a close relationship with Venezuela. There are also some very interesting endemic squamates: Gonatodes ocellatusErythrolamprus ocellatus, and Mastigodryas dunni. But there are also widespread squamates like Spilotes pullatus and Pseudoboa neuwiedii, and the gecko Thecadactylus rapicauda.

The influences of the Lesser Antilles are really very minimal in the herpetofauna, the large Anolis richardii is the only one that immediately comes to mind. And, that raises the question often asked by people who study Anoles - why are there so few anoles on Tobago? A. richardii  is generally thought to have been introduced from the Grenada Bank. So Tobago (and Little Tobago) have no native Anolis or even widespread Anolis associated with mainland South America - this is very strange. It's very difficult to find any piece of neotropical real estate - no matter how small - that does not have native anoles.

In the 1997 book I placed an emphasis on changing sea levels in the Pleistocene in an attempt to explain how and why Trinidad and Tobago have the species they do. At the time I wrote the book, Haffner's forest refugia hypothesis was being used to explain the distribution of many South American organisms, particularly vertebrates and butterflies. But, 25 years later it is clear that while the forest refugia may have been influential, they are by no means the whole story. In South America, the changing course of the Amazon and Orinoco Rivers combined with marine intrusions into continental South America during the Miocene play an important role in biogeographical theories. Trinidad and Tobago were most certainly impacted by these Miocene events.

Clues as how how Tobago got its mainland fauna (and flora) are going to be found in the DNA of those species living there today as well as their relatives found on mainland South America and Trinidad. In fact this has already started to happen. Camarego et al. (2009)  found that Leptodactylus validus colonized Trinidad a mere 1.025 mya, and only arrived in Tobago 219,000 years ago. Gamble et al. 2008 found Gonatodes ceciliae (a Trinidad endemic) and Gonatodes ocellatus (a Tobago endemic) shared an ancestor about 3.8 mya. So, from these dates it appears that there is not going to be a single event to explain how Tobago got its continental fauna. However, it seems probable that there will be some event to explain the origin of a large number of the species present on the island.

Two years ago I started to take a second, very careful look at the species on Trinidad and Tobago and compare their morphology to relatives on the mainland, the results have been startling. Many of what are believed to be widespread species are not, they are endemic species, often cryptic in appearance, suggesting that, while the islands have been connected to the mainland many times in the Pleistocene as sea level rose and fell with glacial cycling, there was considerable speciation prior to Pleistocene, and the islands were invaded and re-invaded by different lineages of mainland species. So the extant fauna is the result of both dispersal and vicariance, speciation and extinction.

Thus, the stream frogs of the genus Mannophryne on Trinidad and Tobago are not sisters, that is they are not each others closest relatives as might be expected if Tobago was the eastern end of the Venezuela's Coastal Andes. Manzanilla et al. (2009) used partial sequences of mitochondrial 16S and cytochrome oxidase and analysed 1.2 kb from 13 of the 15 described species of Mannophryne at the time and found three deeply differentiated clades that split from each other in a relatively short period of time. The evolution of the frog clades occurred well before the glacial-interglacial periods of the Quaternary. Also they found the Tobago Mannophryne olmonae to be more closely related to the Venezuelan species M. riveroi ( from Cerro Azul on the Península de Paria, Sucre, Venezuela, between 400–1000m ) than to the Trinidad island endemic M. trinitatis which is more closely related to M. venezuelensis (from the slopes of the Peninsula de Paria, Sucre, Venezuela from sea level to 600 m) .

Santos et al. (2009) wrote, "Interestingly, the contemporaneous divergence of the Trinidad and Tobago species (Mannophryne trinitatis and M. olmonae) from Venezuelan relatives at 8.3 MYA suggests a global period of high sea level."

There seems to be a problem here, because trinitatis and olmonae are not sisters and they did not necessarily diverge at the same point in time from their respective sisters in Venezuela.

Pindell and Kennan (2007) describe the Gulf of Paria as a pull-apart basin formed in the late Miocene. Assuming that this was in part responsible for the opening of the the channel between Trinidad's northern Peninsula and the Paria Peninsula, as well as the formation of the Bocas Islands this event would have effectively separated the two sister species of M. trinitatis and M. venezuelensis (or their ancestors). But, the separation of M. olmonae and M. riveroi. How did this happen? One possibility, that no longer seems viable, is that M. riveroi was widespread and ranged from Venezuela, across Trinidad's Northern Range and right out on to Tobago and then became extinct on Trinidad - but Tobago was not in its present location it was moving on the Caribbean plate towards its current position.

Also, sea level has undergone dramatic changes over the past 30 my. Below, is a graph that shows sea level changes, note that in the Oligocene there were times when sea level was ~200 m above present levels and in the Miocene there was a point when it was ~75 m below present levels. Directly above the graph are maps of T&T showing elevations. When sea level rises a substantial part of the islands are covered by seas, greatly restricting the area of land available for terrestrial vertebrates, and when sea level drops the amount of land available of terrestrial organisms expands. I have attempted to color code the graph to the maps. Assuming Tobago did not rise or sink due to tectonics, when sea level rose 150 m there was only 150.7 km2 of exposed land on Tobago, when sea level was up 200 m there was only 73.6 km2 of land surface exposed.

Tobago's movement is shown in the bottom pane of the attached figure taken from Pindell and Kennan (2007) - the red line.  Tobago is moving on the Caribbean plate and 11 mya (towards the end of the middle Miocene) Tobago is located about 50 km north of the Paria Peninsula, at a time when sea level is low - about 75 m lower than it is today. Thus, it is entirely possible that there was a dry land connection between Tobago and the Paria Peninsula, or perhaps Tobago was pushed directly up to the continent and made contact with it directly. In either case the fauna from northern Venezuela could have moved directly onto Tobago and as the island continued its travels towards its present location it may have by-passed Trinidad completely without contact. Sea level was also low at various time during the last 3 my, but by that time it had moved passed the Venezuelan coast well on its way to its present position.

This hypothesis can be tested with DNA.

Camargo A, W. R. Heyer, R. O. de Sá. 2009. Phylogeography of the frog Leptodactylus validus (Amphibia: Anura): patterns and timing of colonization events in the Lesser Antilles. Molecular Phylogenetics and Evolution 53:571-579.

Castroviejo-Fisher, S., J. C. Señaris, J. Ayarzagüena, and C. Vilà . 2009 "2008". Resurrection of Hyalinobatrachium orocostale and notes on the Hyalinobatrachium orientale species complex (Anura: Centrolenidae). Herpetologica 64: 472-484.

Eberli, G. P. 2000. The record of Neogene sea-level chances in the prograding carbonates along the Bahamas Transect - Leg 166 Syntheisis. Pages 167-177 In: Swart, P.K., Eberli, G.P., Malone, M.J., and Sarg, J.F. (Eds.), 2000. Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 166.

Gamble, T., A. M. Bauer, E. Greenbaum, and T.R. Jackman. 2008. Evidence for Gondwanan vicariance in an ancient clade of gecko lizards. Journal of Biogeography 35: 88-104

Manzanilla, J., M. J. Jowers, E. La Marca, and M. García-París . 2007. Taxonomic reassessment of Mannophryne trinitatis (Anura: Dendrobatidae) with a description of a new species from Venezuela. Herpetological Journal, 17: 13-42.

Murphy, J. C. 1997. Amphibians and Reptiles from Trinidad and Tobago. Krieger Publishing Company. Malabar. 245 pp.

Pindell, J. and  Kennan, L. 2007. Cenozoic kinematics and dynamics of oblique collision between two convergent plate margins: the Caribbean-South America collision in eastern Venezuela, Trinidad, and Barbados. In: Kennan, L., Pindell, J. L. & Rosen, N. C. (eds) Transactions of the 27th GCSSEPM Annual Bob F. Perkins Research Conference: The Paleogene of the Gulf of Mexico and Caribbean
Basins: Processes, Events and Petroleum Systems, 458-553.

Pindell, J. and Kennan, L. 2009. Tectonic evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference frame: an update. In: James, K., Lorente, M. A. & Pindell, J. (eds) The geology and evolution of the region between North and South America. Geological Society of London, Special Publication.

Santos J.C., L. A. Coloma, K. Summers, J. P. Caldwell, R. Ree, et al. 2009. Amazonian Amphibian Diversity Is Primarily Derived from Late Miocene Andean Lineages. PLoS Biology 7(3): e1000056. doi:10.1371/journal.pbio.1000056.

Snoke, A. and P. J. Noble. 2001. Ammonite-radiolarian assemblage, Tobago Volcanic Group, Tobago, West Indies—Implications for the evolution of the Great Arc of the Caribbean. GSA Bulletin, 113: 256–264

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