Abstract
The plotopterids (Aves, Plotopteridae) were a group of extinct wing-propelled marine birds that are known from Paleogene-aged sediments (Eocene to Miocene), mostly around the Pacific Rim (especially Japan and the northwest coast of North America). While these birds exhibit a strikingly similar wing morphology to penguins (Spheniscidae), they also share derived characters with pelecaniform birds that are absent in penguins and exhibit apparently superficial similarities with auks (Alcidae: Charadriiformes). Despite quite an abundant fossil record, these birds have been little studied, and in particular their functional morphology remains little understood. Here we present osteological overviews of specimens from the northwest coast of Washington state (USA). We give an amended diagnosis for the well-represented North American genus, Tonsala Olson, 1980, describe a new large species, and examine the functional morphology of plotopterids showing that the ratio of humeral strength to femoral strength is quite low in one well-represented species Tonsala buchanani sp.nov., relative to both extant penguins and alcids. While the femoral strength of Tonsala buchanani is ‘penguin-grade’, its humeral strength is more ‘alcid-grade’. These results have implications for understanding the mode-of-locomotion of these extinct marine birds. Although not related to Spheniscidae, our descriptions and functional results suggest that Tonsala buchanani sustained similar loads in walking, but slightly lower humeral loads during swimming, than a modern penguin. This suggests a swimming mode that is more similar to living alcids, than to the highly-specialised locomotor strategy of living and fossil penguins.
Highlights
The plotopterids (Aves, Plotopteridae) were a group of extinct marine birds that are known from Paleogene sediments spanning the northern Pacific rim [1,2,3,4,5,6,7,8,9,10]
It is true that the fossil record of Paleogene birds from the Pacific Northwest remains little studied, in spite of the numbers of specimens housed in regional museum collections
We present anatomical descriptions of North American plotopterid remains based on five incomplete specimens collected from the mid-1980s onwards from the northern coast of Washington State
Summary
The plotopterids (Aves, Plotopteridae) were a group of extinct marine birds that are known from Paleogene sediments spanning the northern Pacific rim [1,2,3,4,5,6,7,8,9,10]. We present anatomical descriptions of North American plotopterid remains based on five incomplete (but associated) specimens collected from the mid-1980s onwards from the northern coast of Washington State (the Olympic Peninsula). These fossil birds come from sediments that border the Strait of Juan de Fuca (the Coast Range terrane of the Cascadian accretionary wedge) – the Eocene-Oligocene Makah Formation and the overlying Oligocene Pysht Formation [7] – and are among the oldest known records of plotopterids. Institutional Abbreviations UWBM, Thomas Burke Memorial Washington State Museum (Burke Museum), University of Washington, Seattle, USA; RBCM, Royal British Columbia Museum, Victoria, BC Canada; USNM, United States National Museum (Smithsonian Institution), Washington DC, USA
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