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Themes of land plant evolution, a celebration of the contributions of Leo J. Hickey

Miller, Ian [1].

The Taxonomy, Paleoecology and Paleolatitude of the Early Cretaceous (Albian) Winthrop Formation Flora, Washington State, USA.

The abundant megaflora of the late Albian (~105 Ma) Winthrop Formation of north-central Washington State allows for the investigation of plant phylogeny, diversity, and paleoecology during the rapid diversification of the angiosperm clade, and offers independent paleolatitudinal constraints for the tectonic history of the western Cordillera of North America. With an estimated diversity of more than 150 species distributed in eight plant classes, the Winthrop is the most diverse North American flora yet described from the Early Cretaceous. Approximately 50% of the species are angiosperms; cycadophytes, conifers and ferns comprise nearly equal parts of the remaining taxa. Seventy-three new species and 22 new genera have been identified, about half of which belong to the Magnoliopsida (42 new species and 13 new genera). Eight extant magnoliopsid orders have been identified, some by the earliest definitive occurrences of leaf-species in clades previously known only from micro- and mesofossil records or indicated by molecular clocks. The Winthrop flora was deposited in a braided-stream setting, with the richest plant sites occurring above incipient paleosols within crevasse splays. 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. Analyses of species abundance and species presence-absence matrices showed that angiosperms were taxonomically diverse in both stable and frequently disturbed environments. However, in the stable environments, angiosperms shared niche space with other plant clades that were dominant earlier in the Mesozoic, showing that the floristic composition of the flora was heterogeneous and varied with depositional environment. Additionally, consistently low leaf mass per area of Winthrop dicot leaves reinforce the hypothesis that Early Cretaceous landscapes were colonized by fast-growing, weedy angiosperms. 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, which is postulated to have originated at the latitude of Mexico in the Early Cretaceous. Using dicot leaf-species from the Winthrop flora, we estimate an MAT of ~25°C ± ~3°C. This tropical MAT value is used to estimate a paleolatitude of 32.5°N for the Winthrop Formation section of the Baja BC block, indicating ~2,850 km of northward offset 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.

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1 - Denver Museum of Nature and Science, Earth Sciences, 2001 Colorado Blvd, Denver, CO, 80205, USA

Keywords:
paleofloristics
paleoecology
Early Cretaceous.

Presentation Type: Symposium or Colloquium Presentation
Session: C6
Location: Belle-Chasse/Riverside Hilton
Date: Tuesday, July 30th, 2013
Time: 2:00 PM
Number: C6002
Abstract ID:481
Candidate for Awards:None


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