A new series of tropical islands emerged during the Triassic period, formed by the merging of the state's older volcanic chains. The only plant fossil from Oregon's Triassic formations is Diplopora oregonensis. As in the Paleozoic, shallow water invertebrates made up Oregon's Triassic fauna. These include sponges, ammonites, radiolarians, brachiopods, and the belemnite Aulacoceras. The trace fossil Chondrites, a species of fodinichnid, has been found in the same formations. Although corals have also been found in these assemblages, paleontologists debate the presence of true coral reefs in the region during the Triassic. The oldest vertebrate fossils in Oregon's fossil record appear in Triassic-aged limestones in the Wallowa Mountains. These fossils, including vertebrae, ribs, and a partial skull, have been assigned to the early ichthyosaur genus Shastasaurus. The breakup of the Pangaean supercontinent during the transition to the Jurassic period created a subduction zone in Oregon's ancient seaway, burying older rock formations and giving way to new volcanic island chains. Oregon's Jurassic invertebrates, such as the reef-building clam Lithiolus problematicus and the mussel-like Buchia piochii, indicate shallow sea environments similar to those of the state's late Triassic. Among Oregon's Jurassic-aged vertebrate fossils are the remains of marine crocodiles, tentatively identified in the genus Metriorhynchus. These crocodiles likely shared Oregon's Jurassic seaways with ichthyosaurs. Plant fossils from Oregon's Jurassic period show that the terrestrial environment became warmer and wetter, creating swampy conditions. These fossils, from the Coon Hollow Formation and others of similar age, include ferns, quillworts, cycads, and conifers.