Abstract
Mesozoic bird fossils from the Pacific Coast of North America are rare, but small numbers are known from the Late Cretaceous aged sediments of Hornby Island, British Columbia. Most are unassociated fragments that offer little information, but additional preparation of a large coracoid has revealed more details of its structure, as well as three associated wing bones. Phylogenetic analysis suggests that Maaqwi cascadensis, gen. et sp. nov. represents a derived crown or near-crown member of Ornithurae, and specifically suggests affinities with Vegaviidae. M. cascadensis is characterized by large size, and regressions based on dimensions of the coracoid suggest a large bird, with an estimated body mass of approximately 1.5 kilograms. The bones are robust, with thick walls, suggesting that M. cascadensis was a bird adapted for diving, similar to modern loons and grebes. The wings are short, while the coracoid is unusually short and broad, similar to modern loons. Along with the Ichthyornithes and Hesperornithes, M. cascadensis and Vegaviidae appear to represent a third clade of bird that evolved to exploit marine habitats in the Late Cretaceous, one specialized for foot-propelled diving and rapid cruising flight over water.
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| Fig 2. Photographs and schematic illustrations of Maaqwi cascadensis holotype RBCM.EH2008.011.01120 depicting wing bone orientation. (A, B) dorsal face of right coracoid and partial humerus.
acrocoracoid, partial humerus, ulna and radius. Shading denotes preserved cortical bone (white), exposed trabecular bone (light grey), and matrix (dark grey). c = coracoid. h = humerus. u = ulna. r = radius. Scale bar = 1 cm.
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Systematic paleontology
Avialae Gauthier 1986
Ornithothoraces Chiappe and Calvo 1994
Ornithuromorpha Chiappe and Walker 2002
Ornithurae Haeckel 1866 sensu Chiappe
Vegaviidae Agnolín et al. 2017
Maaqwi cascadensis gen. et sp. nov.
Etymology: The generic name, Maaqwi, is derived from “ma’aqwi”, the Coast Salish word meaning “water bird”. The specific name, cascadensis, reflects provenance in the Cascadia region of western North America.
Holotype: RBCM.EH2008.011.01120 consists of a concretionary mudstone nodule containing a right coracoid, as previously described by Dyke et al. [18]. However, at the time of initial description, the specimen had not been prepared and only the dorsal face of the coracoid was visible [18, Fig 2A]. The acrocoracoid appeared to be missing and only the broken ends of the three associated long bones were visible. Subsequent mechanical preparation of the coracoid revealed that its head was everted ventrally and had been buried within the matrix. Further preparation revealed central portions of three wing elements; a humerus, ulna and radius (Fig 2). The specimen is housed within the RBCM.
Locality: RBCM.EH2008.011.01120 was recovered from a coastal outcrop of the upper Campanian Northumberland Formation exposed on the northwestern shore of Hornby Island, British Columbia.
Diagnosis: Coracoid compact, polygonal in profile, with the omal portion approximately one third of the medial length. Coracoid shaft a stout, flat bar. Coracoid and humerus robust, highly pachyostotic.
Conclusions
Maaqwi cascadensis appears to represent a lineage of Cretaceous marine birds distinct from either Ichthyornithes or Hesperornithiformes. Instead, it appears to be closely allied with—or perhaps part of—crown Aves. The wings are reduced, inconsistent with soaring, and instead suggest a bird specialized for fast cruising flight over water. The thickness of the walls of the bones suggest that it was a diver but the wings are not modified for underwater propulsion. Instead, it was most likely a foot-propelled diver, although it may have made occasional use of its wings for steering underwater. Phylogenetic analysis suggests affinities with Vegavis iaai, which has recently been reinterpreted as a foot-propelled diver, taking its place along side other advanced ornithurines specialized for foot-propelled diving within the Vegaviidae including Australornis lovei, Neogaeornis wetzeli, and Polarornis gregrorii. Clearly, additional fossil material is needed to better understand the affinities and ecology of these Late Cretaceous–early Paleogene marine birds.
Sandy M. S. McLachlan, Gary W. Kaiser and Nicholas R. Longrich. 2017. Maaqwi cascadensis: A Large, Marine Diving Bird (Avialae: Ornithurae) from the Upper Cretaceous of British Columbia, Canada. PLoS ONE. 12(12); e0189473. DOI: 10.1371/journal.pone.0189473
