Preserved placoderm organs inform early jawed vertebrate evolution

Exceptionally well preserved fossilized soft-tissue organs from Devonian-age placoderms reveal new insights into the early evolution of jawed vertebrates, according to a new study. The origin and early diversification of jawed vertebrates involved major changes to skeletal and soft tissue anatomy. Because skeletons are readily preserved in the fossil record, skeletal transformations in stem gnathostomes (early jawed vertebrates) can be directly examined. However, preservation of soft tissue is exceedingly rare. Here, Kate Trinajstic and colleagues present the only known example of three-dimensionally preserved soft tissue organs from Late Devonian (~370 million years ago) arthrodire placoderms – one of the earliest known jawed vertebrates. Recovered from the Late Devonian Gogo Formation in Western Australia, Trinajstic et al. describe and characterize exceptionally preserved fossilized soft tissue organs – a heart, thick-walled stomach, and bilobed liver. Using synchrotron and neutron microtomography, the authors found evidence of a flat S-shaped heart well separated from the liver and other abdominal organs, which is associated with the evolution of the jaws and neck. The findings also suggest the absence of lungs in these ancient fish, refuting a controversial hypothesis that the presence of lungs is ancestral in jawed vertebrates. “Together with previously described musculature, the three-dimensionally preserved organs make the Gogo arthrodires the most fully understood of all jawed stem gnathostomes, help resolve conflicting phylogenies for early fish and can validate evolutionary transition hypotheses generated by extant developmental models,” write the authors.