Prashant P. Sharma
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Phylogeny of Protobranchia
Protobranchiate bivalves are among the most poorly understood molluscan groups, but among the most successful lineages in deep sea environments. These mollusks possess a protobranch gill that is not modified for filter feeding (in contrast to other bivalve) and a lecithotrophic pericalymma larva. Many lineages also bear sulfur-oxidizing bacterial chemosymbionts. Approximately 750 species are described and classified into five superfamilies.
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Subsequent to several collecting efforts spanning nearly 12 years, we sampled for the first time representatives of all described protobranch bivalve families in a five-gene phylogeny. 
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Sharma et al. (2013) Mol. Phylogenet. Evol. 69: 188-204.
The same four clades, indicated by the colors in the branches, are obtained under a variety of algorithmic approaches.
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Sharma et al. (2013) Mol. Phylogenet. Evol. 69: 188-204.
Molecular estimates of divergence times suggest a crown age in the early Paleozoic, with subsequent diversification of families occurring in the Mesozoic.
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Sharma et al. (2013) Mol. Phylogenet. Evol. 69: 188-204.
The log-lineage through time plot of protobranch bivalves has a marked anti-sigmoidal shape characteristic of a lineage radiating in the wake of a mass extinction. The timing of the upturn in diversification suggests that the phylogeny of protobranch bivalves has captured the signature of the end-Permian mass extinction. Shown in the inset of the figure below is the best fitting three-rate diversification model selected by LASER (Rabosky, 2006).
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Sharma et al. (2013) Mol. Phylogenet. Evol. 69: 188-204.
To test this hypothesis, we simulated 500 phylogenies undergoing a 99% extinction cull at time t = 250 (in yellow) and compared them to the empirical plot (in purple). These simulations demonstrate that the log-lineage through time plot of Protobranchia is indistinguishable from a model of constant net diversification rate, interrupted by the end-Permian event.
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Sharma et al. (2013) Mol. Phylogenet. Evol. 69: 188-204.
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