Dynamics of the Great Ordovician Biodiversification Event in Baltica

The Great Ordovician Biodiversification Event (GOBE) represents a remarkable global biological radiation during the Ordovician. During this interval, marine biodiversity increased dramatically at the order, family, and genus levels and underwent notable faunal succession, e.g., the transition from the Cambrian Evolutionary Fauna to the Paleozoic Evolutionary Fauna, exerting a profound impact on marine communities and ecosystems. Although the global signal of the GOBE has been established, its regional expression remains poorly understood due to substantial data gaps and limited temporal resolution. Baltica, with its long research history, well-constrained geological framework, and abundant fossil data, is an ideal region for investigating the regional biodiversity patterns of the GOBE. Here, we compiled a dataset of 505 sections from Baltica using the OneStratigraphy platform (https://onestratigraphy.ddeworld.org/). We employed the Constrained Optimization (CONOP) approach to construct a composite standard sequence for major fossil groups across Baltica. Shareholder Quorum Subsampling (SQS), rarefaction, and bootstrap resampling methods were used to reduce sampling biases and statistical uncertainty. A species richness curve for the Ordovician of Baltica with a temporal resolution of 43.1 kyr was thus generated. We found that: (1) Species richness in Baltica underwent a long-term, stepwise radiation during the GOBE, with the main radiation phase spanning 458–452 Ma, during which two distinct origination rate peaks were identified. After a fluctuating interval (452–443 Ma), the Late Ordovician Mass Extinction (LOME) caused a ~70% loss of richness within 1 Myr, marked by an extremely high extinction rate at approximately 443 Ma. (2) Six dominant fossil groups (chitinozoans, conodonts, graptolites, trilobites, brachiopods, and ostracods), which account for >95% of our dataset, exhibited different radiation patterns during the GOBE. Graptolites diversified first at 485 Ma, followed by conodonts, chitinozoans, trilobites, and brachiopods, all peaking near 450 Ma, whereas ostracods peaked later at 445 Ma. This study provides key, high-resolution data for analyzing regional patterns of the GOBE and its potential drivers and mechanisms, and offers an important perspective for understanding marine ecosystem transformation during the early Paleozoic.