Reassessing a cryptic history of early trilobite evolution

A reassessed early-arthropod morphology matrix supports a relatively short cryptic interval before the approximately 521 Ma appearance of widespread trilobites; topology and origin time are analytical outputs rather than a Precambrian absence test or a direct fossil date.

The Trilobita root display retains a rounded 521–252 Ma sampled envelope: approximately 521 Ma marks the widespread early record, and revised Changhsingian material documents survival into the latest Permian; neither endpoint is an exact global FAD or LAD.

The Arthropoda root display replaces the unsupported 540 Ma value with a 521–0 Ma sampled navigation envelope anchored by widespread trilobites, a contained arthropod clade, and living arthropods; it does not date the origin of Arthropoda.

The trilobite evidence-samples entity is a navigation group linking the early-trilobite morphology matrix and approximately 521 Ma widespread sampled record to specimen dossiers. It is not a taxon, direct ancestor route, Precambrian absence test, global FAD or complete Trilobita range.

The 521–443 Ma trilobite evidence route is a navigation envelope between separate early-trilobite and Beecher's Bed samples, not a lineage duration or continuous occurrence series.