Plotopteridae Howard, 1969

Kaiser, Gary, Watanabe, Junya & Johns, Marji, 2015, A new member of the family Plotopteridae (Aves) from the late Oligocene of British Columbia, Canada, Palaeontologia Electronica 25 (3), pp. 1-15 : 7-8

Family Plotopteridae Howard, 1969

Remarks. Known genera of the Plotopteridae include Copepteryx Olson and Hasegawa, 1996 ; Hokkaidornis Sakurai et al., 2008 ; Phocavis Goedert, 1988 ; Plotopterum Howard, 1969 ; Tonsala Olson, 1980 . We propose Stemec as a new genus.

The fossil of RBCM.EH2014.032.0001.001 is a nearly complete coracoid ( Figure 1 ) that preserves the acrocoracoid, the facies articularis humeralis (FAHU), the facies articularis clavicularis (FACL), the triosseal canal, and the main body of the shaft, including the crista intermedia and a deep fossa, omal to the sternal articulation. The facies articularis scapularis (FASC) and the extremity of the procoracoid have been lost to erosion.

When compared to examples from extant families, the overall shape of the Sooke fossil exhibits discrete characters described by Olson (1980) as typical of “pelecaniform” birds, including the “large, flat furcular facet” and the absence of a foramen associated with the procoracoid. The omal part of the sulcus m. supracoracoidei is excavated to form a triangular depression whose margin is the medial extension of facies articularis clavicularis and tuberculum brachiale. A comparable triangular depression appears in modern members of the Anhingidae and Phalacrocoracidae , but is absent from Sulidae and many other groups of waterbirds, including the Gaviidae , Procellariidae , Ardeidae , and from wing-propelled divers such as the Spheniscidae and Alcidae .

RBCM.EH2014.032.0001.001 also exhibits derived character states for the Plotopteridae , allowing the referral of the new genus to this family. The most prominent characters include the convex and swollen caudal area of the triosseal canal ( Howard, 1969; Olson, 1980; Smith, 2010: character 176:1), and strong omo-medial development (or eversion) of the FACL, which is evident as an abrupt protrusion from the shaft in dorsal and medial aspects ( Figure 1.2, 1.4 ; see Olson, 1980; Smith, 2010, character 178:1).

In addition, the shaft of the coracoid is shallow dorsoventrally in Plotopteridae ; the ratio of dorsoventral depth to mediolateral width of the shaft, measured just sternal (caudal) to the procoracoid, is less than 1.0 (confirmed in Copepteryx hexeris , Tonsala hildegardae , Plotopterum joaquinensis and in RBCM.EH2014.032.0001.001). This same ratio is greater than 1.0, and typically near 1.2, in members of the Recent Sulidae , Anhingidae , and Phalacrocoracidae (confirmed in 140 individuals from 16 phalacrocoracid species). The only exception occurred in Phalacrocorax melanoleucos where one of the 11 individuals examined showed a ratio of 0.93. The coracoid in the Plotopteridae is also characterized by its extreme slenderness ( Olson, 1980), as is evident from the ratio of mediolateral width of sternal articulation to the overall length (see below).

Figures

FIGURE 1. Stemec suntokum gen. et sp. nov. holotype right coracoid (RBCM.EH2014.032.0001.001) from the Sooke Formation, at the southwestern end of Vancouver Island, British Columbia as re-assembled after preparation. 1-4, dorsal (1), medial (2), lateral (3), ventral (4) aspects. Abbreviations: FACL, facies articularis clavicularis; FAHU, facies articularis humeralis; PRO, procoracoid.