By Jean Guex
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Additional resources for Retrograde Evolution During Major Extinction Crises
Primitive-looking forms derived from their immediate ancestors by retrograde evolution (a phenomenon that has been described as proteromorphosis), often associated with a reduction in body size. These organisms are not Lazarus taxa because their absence in sediments older than the ones where they are found is fully reproducible worldwide and the duration of the intervals of time in which they are totally absent can last several millions or several tens of millions of years. In other words, a “Lazarus” explanation of such organisms cannot be reconciled with the geological record.
2006). During the topmost Pliensbachian–Early Toarcian major crisis, these forms give rise to smooth derivatives without the abdominal costae (E. disparile), that evolve to smooth poreless forms (E. unumaense) in the early Middle Jurassic. Abdominal costae gradually reappear in the Bathonian (E. dentatum—E. pustulatum—E. semifactum, E. ptyctum and later E. pyramis). The pores on the abdomen progressively close in this lineage. The drastic simplification of the ornamentation and retrograde evolution of Eucyrtidiellum during the Pliensbachian-Toarcian crisis is clearly related to the major environmental perturbation which occurred during that period (Gorican et al.
From Th. greenhornensis to Th. multiloculata, the chambers evolve progressively from crescentic to globular shape, the raised sutures become more depressed, and the periumbilical flanges disappear progressively on the umbilical side. The junction between the sutures and the periphery that is oblique in Th. greenhornensis, becomes perpendicular in Th. multiloculata. In lateral view, specimens of Th. greenhornensis that are compressed in form gradually change to a globular shape (Desmares et al. 2008).