CONICET | Buscador de Institutos y Recursos Humanos

Spines, particularly those dorsally projected, are usually considered defensive. Studies on invertebrates, as snails, crinoids, brachiopods, and bivalves, indicate that evolutionary bursts of spinosity usually coincide with patterns of rising predation pressure, particularly tested during the mid-Paleozoic and the Jurassic. However, despite of a significant number of contributions on Paleozoic faunas and predation, mentions about trilobite spinosity usually refer to some striking Devonian taxa. The variation of the spinosity overtime on each family has never been tested. We analyzed the number of spinose trilobite genera from the Ordovician to the Devonian, taking into account their proportion on each family, sampling data from the Paleobiology Database. The number of dorsally spinose genera is nearly constant from the Ordovician throughout the Silurian. Fluctuation of this trend is influenced only by diversity changes of typically spinose families, such as Odontopleuridae, Cheiruridae, or Encrinuridae. In contrast, during the Devonian, we recognized a sudden increase of the number of spinose genera. This pattern is significantly enhanced when typically spinose families are disregarded from the analysis. Thus, the Devonian dorsal spinosity peak is mainly defined by families which previously lacked spines, including Acastidae, Dalmanitidae, Homalonotidae, Phacopidae, Proetidae, Stigyinidae, and Tropidocoryphidae. After the Late-Devonian extinction crises, trilobites become represented only by Proetida and the number of spinose genera decreases notably. These preliminary results indicate a polyphyletic evolutionary burst of defensive spines in trilobites in a similar way to other main prey-groups but mainly restricted to the Early-Middle Devonian. Our preliminary results are in accordance with a context of a sudden rise of predation pressure during the Devonian as proposed by the hypothesis of the Mid Paleozoic Marine Revolution.