earth sciences

The Winthrop Formation Flora

The Early Cretaceous fossil flora from the Winthrop Formation in Washington State

The abundant megaflora of the Albian (~105 Ma) Winthrop Formation of north-central Washington state allows unparalleled investigation of Albian floristic phylogeny, diversity, and paleoecology during the rapid diversification of the angiosperm clade and provides independent paleolatitudinal constraints for the tectonic history of the western Cordillera of North America. Taxonomic analysis of the flora shows 145 plant taxa distributed in eight vascular and nonvascular plant clades. Approximately 50% of the species are angiosperms; cycadophytes, conifers, and ferns comprise roughly equal parts of the remaining taxa. As such, the Winthrop flora represents the most diverse Early Cretaceous flora with nearly equal angiosperm and non-angiosperm components. Sixty new species and 20 new genera have been identified, the largest number of which belongs to the Magnoliopsida: 42 new species and 13 new genera. Eight extant magnoliopsid angiosperm orders have been identified, including the earliest definitive occurrences of leaves in many of these groups, corroborating evidence from micro- and mesofossil records and molecular dating that these orders were present in the Albian. Counts of 2,599 fossils from 14 field censuses were keyed to microstratigraphic sections to examine the paleoecology of the flora in stable paleosol and disturbed near-stream environments. Multiple analyses of species abundance and species presence-absence matrices showed that angiosperms were taxonomically diverse in both the stable and frequently disturbed environments by the mid-Albian. However, in the stable environments, angiosperms shared niche space with many other plant clades dominant earlier in the Mesozoic, showing that the floristic composition of Early Cretaceous floras is heterogeneous and varies with depositional environment. The tectonic setting and stratigraphy of the Winthrop Formation links it to the Baja BC block—a large crustal element comprising western Washington state, British Columbia, and southern Alaska postulated to have originated at the latitude of Mexico in the Early Cretaceous. Based on a new prediction relationship, a MAT of 25.1°C ± 2.9°C (95% CI) is estimated for the Winthrop flora. The tropical MAT value from the Winthrop flora is used to estimate a paleolatitude of 32.5°N, indicating ~2,850 km of northward offset for the Baja BC block relative to stable North America. This result is corroborated by low-latitude, relict taxa in the Winthrop Formation, including Neocalamites, which is confined to central Mexico by the mid-Cretaceous.


Ian Miller, PhD

Director of Earth and Space Sciences, Associate Curator of Paleobotany

Back To Top