Abstract
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.
Highlights
The hearts of ray-finned fishes are presently described as a succession of four muscular chambers that perform inflow and outflow roles, followed by the bulbus arteriosus, a terminal, non-chambered, elastic cardiac segment (Simoes-Costa et al, 2005; Grimes et al, 2006; Duran et al, 2008).Maldanis et al eLife 2016;5:e14698
In the course of a wider search for fossil hearts, we fortuitously found evidence for a long and gradual evolutionary reduction of the conus arteriosus and of its multiple fibrous valve rows in teleosts
The relevant fossils are from the extinct pachyrhizodontid fish Rhacolepis buccalis (Agassiz, 1841), known from fossils of remarkable three-dimensional (3D) preservation (Maisey, 1994)
Summary
The hearts of ray-finned fishes (actinopterygians) are presently described as a succession of four muscular chambers that perform inflow (sinus venosus and atrium) and outflow (ventricle and conus arteriosus) roles, followed by the bulbus arteriosus, a terminal, non-chambered, elastic cardiac segment (Simoes-Costa et al, 2005; Grimes et al, 2006; Duran et al, 2008). Ecology Genomics and evolutionary biology eLife digest Modern research has majorly advanced our understanding of how the heart works, and has led to new therapies for cardiac diseases. Little is known about how the heart has evolved throughout the history of animals with backbones – a group that is collectively referred to as vertebrates. This is partly because the heart is made from soft muscle tissue, which does not fossilize as often as harder tissues such as bones
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