The complexity of biozonations, with Devonian ammonoid examples

Biostratigraphy explores the distribution of fossils in time and space to identify intervals that can be correlated regionally/globally. The time resolution of Devonian ammonoid biozones varies between 200 Ka (upper Givetian) and >2 Ma (middle Eifelian). Precision for zone bases and tops is shorter. Due to complex environments, the nature of biozones is also complex, which is not visible in common biozone terminology. Marine animal ranges are in general affected by sea-level, condensations, facies shifts, and sedimentary gaps. Despite their open shelf, pelagic lifestyle, even ammonoids were highly facies-sensitive. Different zone types are proposed:

Global Phylozones are defined by speciations and FADs (first appearances) within non-endemic, branching lineages, suggesting changes in pantropical populations (middle Frasnian Beloceratidae). Endemozones are defined by the FADs of endemic taxa (regionally different Famennian Prolobitidae). Migrozones reflect sudden immigrations of lineages that have a longer evolutionary history elsewhere (Triainoceratidae in Europe-North Africa-North America). Cryptozones are defined by the sudden spread of taxa with unknown ancestry (Frasnian Devonopronoritidae in the Altai Mts. and Iran). Ecozones include epiboles (population bursts) in wide-spread event beds (“Archoceras” in Upper Kellwasser Beds). Desaster zones can be recognized after major extinctions by blooms of opportunistic survivors (e.g. Postclymenia Zone after the Hangenberg Extinction). LAD zones follow small-scale disappearances of index taxa (e.g. upper Anarcestes Zone in the upper Emsian).

Internationally, the best correlation is achieved using genozones, with FADs of index genera (or genus groups) but different oldest species in different regions. This implies a mixture of phylogeny and migration.