A new study quantifies rates of morphological and molecular evolution for arthropods during the critical Cambrian explosion. Both morphological and molecular evolution are accelerated--but not so much to break any speed limits.

The Arachnomorpha or Arachnata concepts have resolved Trilobita as most closely related to Chelicerata amongst extant Arthropoda. An alternative position of trilobites in the stem lineage of Mandibulata is suggested by their pattern of head tagmosis. The antennae of trilobites and Mandibulata are considered non-homologous with the antennae of Onychophora and stem lineage Euarthropoda: they repr...

Two characters are stratigraphically compatible if some phylogenies indicate that their combinations (state-pairs) evolved without homoplasy and in an order consistent with the fossil record. Simulations assuming independent character change indicate that we expect approximately 95% of compatible character pairs to also be stratigraphically compatible over a wide range of sampling regimes and g...

BACKGROUND Sphaerexochinae is a speciose and widely distributed group of cheirurid trilobites. Their temporal range extends from the earliest Ordovician through the Silurian, and they survived the end Ordovician mass extinction event (the second largest mass extinction in Earth history). Prior to this study, the individual evolutionary relationships within the group had yet to be determined uti...

—Ontogenetic stages of trilobites have traditionally been recognized on the basis of the development of exoskeletal segmentation. The established protaspid, meraspid, and holaspid phases relate specifically to the development of articulated joints between exoskeletal elements. Transitions between these phases were marked by the first and last appearances of new trunk segment articulations. Here...

La localidad de Murero (provincia Zaragoza) es conocida internacionalmente como punto referencia para el estudio del límite Cámbrico Inferior-Medio, debido a su continuidad estratigráfica (Pisos Bibiliense al Caesaraugustiense), carácter monofacial mixto (secuencia marina plataforma terrígeno-carbonatada), riqueza paleontológica (60 taxones trilobites) y posibilidad correlación intercontinental...