heart fossilization is possible and informs the evolution of cardiac outflow tract in vertebrates. lara maldanis et al (2016), elife http://dx.doi.org/10.7554/elife.14698
elucidating cardiac evolution has been frustrated by lack of fossils. one celebrated enigma in cardiac evolution involves the transition from a cardiac outflow tract dominated by a multi-valved conus arteriosus in basal actinopterygians, to an outflow tract commanded by the non-valved, elastic, bulbus arteriosus in higher actinopterygians. we demonstrate that cardiac preservation is possible in the extinct fish rhacolepis buccalis from the brazilian cretaceous. using x-ray synchrotron microtomography, we show that rhacolepis fossils display hearts with a conus arteriosus containing at least five valve rows. this represents a transitional morphology between the primitive, multivalvar, conal condition and the derived, monovalvar, bulbar state of the outflow tract in modern actinopterygians. our data rescue a long-lost cardiac phenotype (119-113 ma) and suggest that outflow tract simplification in actinopterygians is compatible with a gradual, rather than a drastic saltation event. overall, our results demonstrate the feasibility of studying cardiac evolution in fossils.
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