Above: Extinctions at the Triassic-Jurassic boundary.  A large number of reptile and amphibian groups go extinct at the Triassic-Jurassic boundary. Click on the image to see a larger version. From the website related to the Pole-to-Pole Pangean Coring Workshop.
Left: Triassic-Jurassic boundary section in the Newark basin.  These are among the rocks that were sampled to produce the graph at below. Tr, Triassic rocks. Ju, Jurassic rock. fs, coaly layer in which fern spike is present. Photo by Paul Olsen.  Below: Triassic-Jurassic boundary in the Newark basin.  This boundary section from the Jacksonwald syncline, Exeter, Pennsylvania, is marked by a large increase in the percentage of fern spores in pollen-and-spore assemblages (fern spike) and an increase in the amount of iridium (Ir) in parts per thousand (ppt). Fern spikes typically indicate that an ecologically community has been severely disrupted, with ferns being the first plants to colonize the devastated area.  Iridium is a chemical elements not found in abundance in crustal rocks, but is commonly found in meteorites and asteroids. Olsen et al. (2002) hypothesized that the extinction and iridium spike were causeed by an asteroid impact, with the fern spike marking the initial ecological recovery following the mass extinction.