Spinosity in Dalmanitid trilobites: a Devonian record

RANDOLFE, ENRIQUE A.; BIGNON, ARNAUD; RUSTÁN, JUAN JOSÉ

Congreso; Reunión Virtual de Comunicaciones de la Asociación Paleontológica Argentina; 2020

Asociación Paleontológica Argentina

Dalmanitid trilobites are classically considered as having a conservative morphology with few morphological changes from the Ordovician to the Devonian taxa. Ordovician and Silurian representatives lack spines on their exoskeletons, while several Devonian taxa exhibit well-developed dorsal (defensive) spinosity. The analysis of the spines position on the pygidia has allowed us to define four patterns of spinosity for this family, each recognized for different regions: A first pattern characterized by narrow-based spines with stochastic distribution on axial rings and pleurae; and three additional patterns characterized by robust spines arranged in longitudinal rows on the axis, diagnostic at the generic level. These three patterns differ in the number of spines on each axial ring and the distribution of spines on pleural bands. In turn, new taxa recently described from the Talacasto formation of Precordillera Argentina (Lochkovian-Pragian), along with the revision of those previously reported, indicate nearly 40% of spinose genera in high-latitude Devonian basins of southwestern Gondwana (the Malvinokaffric Realm), where all the spinosity patterns are present, with the sole exception of the first one. This evidence, together with the striking record of predation marks, is significant in this austral nearly circumpolar region. The usual postulate is that predation gradients are higher at low latitudes. This is supported by evidence of other fossil groups where spinose taxa or predation marks are rare at higher latitudes. We are exploring the evolutionary meaning of this evidence in the context of the sudden rise of predation pressure postulated for mid-Paleozoic times.