BODY SIZE TRENDS IN AMMONITES ACROSS THE PLIENSBACHIAN-TOARCIAN EXTINCTION

In the past, mass extinctions have often caused reductions in the body size of many, but not all, groups. The Toarcian extinction, a minor extinction in the Early Jurassic, was a result of anoxia and warming, but very little is known about how it may have affected body size. Using published literature, we have measured over 1,000 ammonite specimens, including nearly 300 species, ranging from the Pliensbachian to the Toarcian stage. There is no evidence for size reduction in ammonites following the Toarcian extinction. On the contrary, body size actually became larger. There are new, larger clades that appear, but families that also span the time period tend to be larger in the Toarcian. The size increase could suggest that the effects of the Toarcian extinction may have favored larger ammonites.