Gondwana
Chilean acavid
02-12-2009 20:13
The family Acavidae is
a rare find in the Neotropics. This family may be
found in the ‘Old World’ in Australia, Seychelles,
Sri Lanka and Madagascar. And in Chile, where the
genus Macrocyclis
occurs with its sole
species M.
peruvianus (Lamarck, 1822).
This large species is up to 60 mm diameter. Although it is commonly known as the largest snail in the Chilean land snail fauna, its anatomy was hitherto unknown. Recently, da Silva & Thomé (2009) unveiled more details on the ecology and anatomy.
It was founds after a hard day of searching, under fallen logs near the vicinity of Lago Azul.
This is another case of Gondwanan distribution at the family level. It would be interesting to see if the affinities are confirmed by molecular evidence.
Reference:
da Silva, L.F. & Thomé, J.W., 2009. Macrocyclis peruvianus (Gastropoda, Acavidae), an endemic land snail from Chile. - Iheringa, Zoologia 99(2): 125-128.
This large species is up to 60 mm diameter. Although it is commonly known as the largest snail in the Chilean land snail fauna, its anatomy was hitherto unknown. Recently, da Silva & Thomé (2009) unveiled more details on the ecology and anatomy.
It was founds after a hard day of searching, under fallen logs near the vicinity of Lago Azul.
This is another case of Gondwanan distribution at the family level. It would be interesting to see if the affinities are confirmed by molecular evidence.
Reference:
da Silva, L.F. & Thomé, J.W., 2009. Macrocyclis peruvianus (Gastropoda, Acavidae), an endemic land snail from Chile. - Iheringa, Zoologia 99(2): 125-128.
Gondwana: the debate continues
06-04-2008 21:20
Much has been said already (and undoubtedly more will
still be said) about the biogeography of the southern
hemisphere. In a recent review, Paul Upchurch
summarizes hypotheses about Gondwanan biogeography
into four models [here a broad summary of a summary]:
a) A 'Samafrica model' where Gondwana was divided into South America and Africa on one hand and East Gondwana (Antarctica, India, Madagascar and Australia) on the other hand.
b) The 'Africa-first' model' assumes that Africa was separated in an early stage, while South America remained longer in contact with East Gondwana until they finally broke up.
c) The 'Pan-Gondwana model' suggesting that all areas remained connected and then separated almost simultaneously.
d) 'Trans-oceanic dispersal model' following investigations of divergence times for species living on different continents and comparing them with the ages of the oceanic barriers between them.
From a theoretical point of view two phenomena have largely been ignored in biogeographical analyses: distributional noise (created by sampling errors) and the effect of geodispersal producing multiple signals. Geodispersal is the expansion of the ranges of species in response to the removal of a geographical barrier (e.g. erosion of a mountain range). Recent insights are that vicariance and geodispersal are alternating by the creation (upheaval) and destruction (erosion) of geographical barriers. The result is a network of interactions between biotas, rather than a simple branching pattern produced by vicariance alone (also called 'reticulate model'). This pattern may be group specific, depending on their specific reactions to physical events and their ecological requirements. Thus multiple biogeographical patterns may occur and recently some patterns-spotting techniques have been developed to untangle reticulate histories.
Upchurch continues by giving some implications for palaeobiogeography and molecular biology. E.g. he criticizes the lack of uncertainty in calibration of ages of fossil taxa (divergence times should be given with a confidence interval) and the calibration of molecular clocks using the age of geographical barriers. Different groups produce different patterns because of genuine biological processes (e.g. their difference in dispersal mechanism), but it is likely that partly the debate is flawed by ambiguities caused by distributional noise and multiple signals. His conclusion is that Gondwanan biogeographical history is proving to be even more complex than previously realized. He strongly advocates the use of pattern-spotting methods to distinguish between genuine patterns and artefacts produced by sampling and other errors.
I think that Upchurch has a point in arguing that the biogeographical history of Gondwana is even more complex than realized so far. Ranges in time expressing uncertainty and different scenarios could be more explicit, especially when dealing with older geological times. Possibly too often nuances are lost (or condensed) in the editing process, given the maximum number of pages a paper is allowed in many journals. And I wonder if the dispersal mechanism of the particular group under consideration is always given sufficient weight. It may be a dilemma to balance between generalizations and the specifics of the group.
There are several points in this useful review that are worthwhile to consider when we look into the relationships of the Orthalicidae (sensu lato). I will undoubtedly come back to this issue later.
Reference:
Upchurch, P. (2008). Gondwanan break-up: legacies of a lost world? Trends in Ecology & Evolution, 23, 229-236.
a) A 'Samafrica model' where Gondwana was divided into South America and Africa on one hand and East Gondwana (Antarctica, India, Madagascar and Australia) on the other hand.
b) The 'Africa-first' model' assumes that Africa was separated in an early stage, while South America remained longer in contact with East Gondwana until they finally broke up.
c) The 'Pan-Gondwana model' suggesting that all areas remained connected and then separated almost simultaneously.
d) 'Trans-oceanic dispersal model' following investigations of divergence times for species living on different continents and comparing them with the ages of the oceanic barriers between them.
From a theoretical point of view two phenomena have largely been ignored in biogeographical analyses: distributional noise (created by sampling errors) and the effect of geodispersal producing multiple signals. Geodispersal is the expansion of the ranges of species in response to the removal of a geographical barrier (e.g. erosion of a mountain range). Recent insights are that vicariance and geodispersal are alternating by the creation (upheaval) and destruction (erosion) of geographical barriers. The result is a network of interactions between biotas, rather than a simple branching pattern produced by vicariance alone (also called 'reticulate model'). This pattern may be group specific, depending on their specific reactions to physical events and their ecological requirements. Thus multiple biogeographical patterns may occur and recently some patterns-spotting techniques have been developed to untangle reticulate histories.
Upchurch continues by giving some implications for palaeobiogeography and molecular biology. E.g. he criticizes the lack of uncertainty in calibration of ages of fossil taxa (divergence times should be given with a confidence interval) and the calibration of molecular clocks using the age of geographical barriers. Different groups produce different patterns because of genuine biological processes (e.g. their difference in dispersal mechanism), but it is likely that partly the debate is flawed by ambiguities caused by distributional noise and multiple signals. His conclusion is that Gondwanan biogeographical history is proving to be even more complex than previously realized. He strongly advocates the use of pattern-spotting methods to distinguish between genuine patterns and artefacts produced by sampling and other errors.
I think that Upchurch has a point in arguing that the biogeographical history of Gondwana is even more complex than realized so far. Ranges in time expressing uncertainty and different scenarios could be more explicit, especially when dealing with older geological times. Possibly too often nuances are lost (or condensed) in the editing process, given the maximum number of pages a paper is allowed in many journals. And I wonder if the dispersal mechanism of the particular group under consideration is always given sufficient weight. It may be a dilemma to balance between generalizations and the specifics of the group.
There are several points in this useful review that are worthwhile to consider when we look into the relationships of the Orthalicidae (sensu lato). I will undoubtedly come back to this issue later.
Reference:
Upchurch, P. (2008). Gondwanan break-up: legacies of a lost world? Trends in Ecology & Evolution, 23, 229-236.