Trilobites ordovícicos del macizo de Aragoncillo (Geoparque Mundial de la UNESCO Molina-Alto Tajo, Guadalajara, España)

Abstract

El Macizo de Aragoncillo, fue testigo del descubrimiento, hace más de 250 años, de los primeros trilobites encontrados en la península Ibérica. Pero más allá de algunas citas posteriores de especies ordovícicas en los siglos XIX y XX, el grupo nunca ha sido objeto de estudios o descripciones paleontológicas locales. En este trabajo se revisan los datos precedentes y se señalan once yacimientos paleontológicos con trilobites, localizados en la parte alta de la Formación Villar del Saz. La asociación estudiada comprende 14 especies, pertenecientes a 13 géneros. Entre los resultados taxonómicos, se presentan los primeros hallazgos ibéricos del facópido Zeliszkella lapeyrei (Bureau), hasta ahora restringido al Macizo Armoricano francés, en tanto que el calimenáceo Colpocoryphe grandis (Šnajdr) es detectado por vez primera en niveles españoles del Dobrotiviense.

Desde el punto de vista bioestratigráfico, la asociación de trilobites se asigna a la parte alta de la Biozona de Placoparia borni del Dobrotiviense superior, que en este caso equivale al Sandbiense inferior por su coetaneidad con graptolitos de la Biozona de Nemagraptus gracilis. Es por ello que la presencia de Neseuretus tristani (Brongniart) en el Ordovícico Superior constituye una de las contadas evidencias mundiales del género Neseuretus previo a su extinción.

The Aragoncillo Paleozoic inlier, located to the east of the Guadalajara province near Molina de Aragón, was the place of discovery, more than 250 years ago, of the first trilobites found in the Iberian Peninsula. But beyond some later cites of Ordovician species in the 19th and 20th centuries, the group has never been the subject of local paleontological studies or descriptions. In this work, all the preceding data are reviewed and eleven Ordovician trilobite localities were studied in the upper part of the Villar del Saz Formation of the Aragoncillo inlier, being placed approximately at the same stratigraphic horizon of fossiliferous nodules included in dark shales. The trilobite assemblage comprises 14 species belonging to 13 different genera of 9 families and 4 orders. Among the taxonomic results, the first Iberian representatives of the phacopid Zeliszkella lapeyrei (Bureau) are described and illustrated for the first time outside the French Armorican Massif. The calymenacean Colpocoryphe grandis (Šnajdr) is also detected for the first time in Spanish Dobrotivian strata. The excellent preservation of some specimens of the phacopids Eodalmanitina destombesi (Henry) and Phacopidina micheli (Tromelin), allows to complete the knowledge of the delicate ornamentation of the cephalon of both species. From a biostratigraphic perspective, the entire trilobite assemblage is assigned to the upper part of the Placoparia borni trilobite Biozone of the upper Dobrotivian, which is correlated with the lower Sandbian due to the concurrence with graptolites of the Nemagraptus gracilis Biozone The late record of Neseuretus tristani (Brongniart) on the same Upper Ordovician strata constitutes one of the very few evidences of the last occurrence of the important Gondwanan genus Neseuretus prior to its extinction. The study of the trilobites and their localities in the Aragoncillo inlier constitutes a contribution to knowledge of the geological heritage in an area that belongs to the Molina-Alto Tajo UNESCO Global Geopark.