GERASTOS (PROETIDAE) FROM LOWER AND MIDDLE DEVONIAN STRATA OF SOUTHERN MOROCCO

At least twelve species level taxa of Gerastos, mostly new species, are identified on the basis of exceptionally preserved, articulated specimens from Lower and Middle Devonian strata of southern Morocco. Specimens of this genus referred to as izine (flies) by the Berber trilobite collectors of southern Morocco, because of their abundance in many levels. The specimens of Gerastos were collected from numerous horizons in ten different sections, primarily from the Ma'der, Tafilalt and Tindouf basins. These sections can occasionally be correlated with one another on the basis of continuous exposure, but also through the preservation of other faunas, such as: conodonts, ammonoids, brachiopods, trilobites and other invertebrates. The distribution of Gerastos species in these sections' horizons suggests that several of the subspecies and species are constrained to limited time intervals, and may be excellent regional zone fossils. Examination of the features of these species (in relation to their stratigraphic positions) suggests that some broad morphological trends may be recognized, such as the disappearance of tubercles on the anterior portion of the glabella, and reduction of the genal spines in many of the younger species. Details of the morphology of the species, and the frequency of their articulation and enrolment in various facies help us to speculate on the life modes of these small trilobites, suggesting reasons why members of this family of small generalists outlived other more spectacular, but perhaps more specialized, trilobite taxa. A cladistic analysis of these Gerastos species, together with other well preserved species of that genus from elsewhere (primarily Europe), and the type species of closely related genera, supports the suggestion of others that Longiproetus and Devonoproetus are junior synonyms of Gerastos. It also illuminates the relationships of those species considered.