Stemec suntokum, Kaiser, Gary, Watanabe, Junya & Johns, Marji, 2015

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 : 9-12

Stemec suntokum gen. et sp. nov.

Figures 1-3

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Etymology. Stemec derived from a generic word for long-necked, black waterbird in the Coast Salish language, originally spoken in the type locality ( Mitchell, 1968; Montler, 1991). We consider it neuter. The species name, suntokum , is derived from the family name of Leah and Graham Suntok, who found the specimen and donated it to the Royal BC Museum. Suffx ‘–um’ is used here for a third declension, genitive plural, non-Latin consonant root.

Holotype. RBCM.EH2014.032.0001.001, a nearly complete right coracoid, missing only the procoracoid and small areas of the acrocoracoid. The dorsolateral surface of the shaft has been eroded. Stratigraphic position: Sooke Formation, Carmanah Group. Age : Late Oligocene. Collected on 28 December 2013 by Leah and Graham Suntok and deposited in the Royal BC Museum, 675 Belleville Street, Victoria, BC, Canada V8 W 9W2.

Differential Diagnosis. Stemec differs from most other plotopterids in its small size. Only Plotopterum joaquinensis Howard, 1969 is a plotopterid of similar size. Stemec differs from Plotopterum in the following features: the FAHU is a raised, flat platform, omal to the procoracoid with clearly demarcated lateral and sternal margins that are strongly elevated from the shaft (in Plotopterum , the FAHU is not raised from the shaft; Figure 3.1, 3.2 ); and the sulcus m. supracoracoidei forms a shallow depression pointing omally ( Figure 3.4 ), but does not become the prominent, steepwalled groove seen in Plotopterum ( Figure 3.3 ). Apart from much smaller size, Stemec differs from larger plotopterids in: the FAHU lies omal to the procoracoid (in Copepteryx and Tonsala , the FAHU extends beyond the procoracoid); the shaft widens more gradually towards the sternal articulation ( Figure 1 ) than in Copepteryx and Hokkaidornis (see Olson and Hasegawa, 1996; Sakurai et al., 2008); the facies articularis sternalis faces relatively medially (in Copepteryx and Hokkaidornis , it is almost perpendicular to shaft in dorsal view); and the facies interna is wide omo-sternally with the omo-dorsal lip lying omal to the crista intermedia ( Figures 1.1 , 2.3 ) (in Copepteryx and Hokkaidornis , the facies interna is restricted near the crista intermedia).

Age, Locality and Stratigraphic Units. Late Oligocene near Sooke, at the southern end of Vancouver Island, British Columbia, Canada; Carmanah Group, Sooke Formation.

Comparative Anatomy. The only known coracoid from a plotopterid of comparable size to Stemec is the omal fragment of Plotopterum joaquinensis Howard, 1969 (LACM 8927). Coracoids of Copepteryx hexeris Olson and Hasegawa, 1996 , Hokkaidornis abashiriensis Sakurai et al., 2008 , Tonsala hildegardae Olson, 1980 , and T. buchanani Dyke et al., 2011 are distinctly larger. No coracoid is known for either C. titan Olson and Hasegawa, 1996 or Phocavis maritimus Goedert, 1988 , but other preserved elements indicate that those species were also very large.

Major osteological features tend to vary among the Plotopteridae and Stemec differs from other genera, including Plotopterum . The triangular depression on the medial side of the acrocoracoid, between the FACL and the tuberculum brachiale, is particularly variable. In Plotopterum it extends omally to form a “deep, triangular groove” which reaches the omal end of acrocoracoid (see also Howard, 1969). In Tonsala and Stemec (and also in Phalacrocorax ), the groove is much less developed omally ( Figure 3.2 ), and the omal apex of the depression lies roughly at the center of the medial surface of acrocoracoid. In Copepteryx , the overall structure lies dorsally with the apex of its margin pointing dorsally.

The FACL in both Plotopterum and Stemec is almost circular and unlike the rather oblong structure in Tonsala . In both Plotopterum and Stemec the FAHU is oval, relatively short longitudinally, and its sternal margin is clearly marked and elevated from the shaft but in Tonsala and Copepteryx , the FAHU is elongated, extending sternally over the base of procoracoid, and its caudal portion lies almost flush with the surface of the shaft ( Figure 7.1, 7.2 ).

In Stemec , the facies interna of crista articularis sternalis (sternal facet) lies at a right angle to the shaft and extends omally. Its omo-dorsal margin is marked by an overhanging ridge that is separated from the crista intermedia by a deep fossa. In Copepteryx and Tonsala (?) sp. ( Olson and Hasegawa, 1996), and in Hokkaidornis , the facies interna is restricted sternally. In Copepteryx the crista articularis sternalis forms a right angle with the shaft in dorsal view, differing from that in Stemec where it slopes medially as in most other waterbirds, including sulids, phalacrocoracids, and anhingids.

Description. Total length 74.8 mm; maximum width at the sternal articulation 12.8 mm; length of the head 24.0 mm. The FACL is nearly circular, 5.2 by 6.6 mm. The FAHU is an oval, 10.1 by 7.8 mm. Mediolateral width and dorsoventral depth of shaft measured just sternal to procoracoid, 6.8 and 6.5 mm, respectively. Distance across sternal articulation, from omal to sternal margin, 6.1 mm. The ratio of the sternal articulation to overall length in Stemec is 5.8, exceeding the minimum criterion for “exceptionally elongate,” set by Livezey and Zusi (2006, character 1292), by 45% ( Table 2).

Small areas on the omal surface of the acrocoracoid have been eroded without significant damage, however, erosion has removed the procoracoid, facies articularis scapularis (FASC) ( Figure 1.1 ), and the processus lateralis at the sternal end. The medial margin and the sternal articulation are preserved and appear to be complete.

Overall, the surface of the shaft is smooth, as expected in an adult bird, and there are no rugose areas to suggest the presence of un-ossified cartilaginous structures that might be found in juvenile birds. The length of the coracoid is slightly greater than the largest Recent cormorants (e.g., Phalacrocorax carbo ). All other plotopterids, except Plotopterum joaquinensis , are distinctly larger than Stemec suntokum . The shaft retains convex dorsal and ventral surfaces throughout its length.

The FACL is a clearly marked circular facet in both Stemec and Plotopterum . It is oblong in Tonsala , but its morphological details are not clearly observable in other plotopterids. In Plotopteridae , the FACL is directed medioventrally as in Sulidae , rather than ventrally as in the Phalacrocoracidae . The impressio ligamenti acrocoracohumeralis is well marked in Stemec , Plotopterum , and Copepteryx , with a shallow but clear longitudinal sulcus running between its medial and lateral margins (see also Smith, 2010, character 169:1). It is not observable in Tonsala , but this may be due to poor preservation in the available specimens.

In Plotopterum , as well as in Recent Phalacrocoracidae , the sternodorsal portion of FAHU lies at the base of procoracoid and is strongly elevated from the shaft, whereas the omal portion lies on the laterodorsal surface of shaft omal at the base of procoracoid. The caudal margin of the omal portion is clearly marked and slightly elevated from the shaft. Howard (1969) apparently refers to the sternodorsal portion as the “scapular facet” but it appears to be a part of FAHU as in Recent Phalacrocoracidae . Probably the FASC of Plotopterum was either indistinct (as in Phalacrocoracidae ), or has been lost along with the tip of the procoracoid. As noted above, the FAHU is longitudinally elongated in Copepteryx and Tonsala , where the sternal margin extends over the base of procoracoid. At that point it is indistinct and barely elevated from the shaft. In the latter two genera, FASC is well developed as a cup-like cotyla scapularis lying on the base of the procoracoid (see also Olson, 1980; Olson and Hasegawa, 1996; Dyke et al., 2011). In most aspects of FAHU and FASC, Stemec is similar to Plotopterum , although the sternodorsal portion of FAHU and FASC are likely to have been largely abraded away. In Plotopteridae , the dorsomedial margin of the FAHU forms a thick lip over a longitudinal groove on the dorsal margin of the triosseal canal (see also Howard, 1969; Olson, 1980), which is somewhat less prominent in Copepteryx and Tonsala .

The shaft sternal to the procoracoid is semicircular in cross-section ( Figure 2.1 ), and slightly compressed dorsoventrally as in most other plotopterids, rather than compressed mediolaterally as in their relatives, including sulids, phalacrocoracids, and anhingids (see above). In Copepteryx the shaft is rather strongly compressed dorsoventrally. In Plotopteridae , the medial and lateral margins of the mid-shaft region are rather straight.

On the sternal end of the Stemec coracoid, facies interna of crista articularis sternalis extends omally, and its omo-dorsal margin is prominently marked by an overhanging ridge as in sulids, phalacrocoracids, and anhingids, but the cranial extension is slightly more pronounced. In other plotopterids where the sternal end of the coracoid is well preserved ( Copepteryx hexeris and Hokkaidornis abashiriensis ), the sternal facet is quite narrow omo-sternally, with the omo-dorsal margin lying only slightly omal to the crista intermedia (the sternal margin of facet) ( Figure 2.3 ) (see also Olson and Hasegawa, 1996, figure 1; Sakurai et al., 2008, figure 3). This feature is apparently unique to Stemec among known plotopterids, although it is not certain whether the feature is an apomorphy of the genus within the family or a plesiomorphically retained character.

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.

FIGURE 2. Components of Stemec coracoid (RBCM.EH2014.032.0001.001). 1, cross-section of the shaft immediately caudal of the cranial portion (a-a of Figure 1.1); 2, cross-section cranial to the sternal segment (b-b of Figure 1.1); and 3, structural features near the sternal articulation.

FIGURE 3. Comparison of the acrocoracoid of Plotopterum joaquinensis (LACM 8927) (1 and 3) to Stemec suntokum (RBCM.EH2014.032.0001.001) (2 and 4). 1 and 2, dorsal aspects, 3 and 4, cranial aspects. See Figure 1 for abbreviations.

FIGURE 7. Casts of coracoids in the Smithsonian Institution, Washington DC, referable to Copepteryx, one of the very large plotopterids from the Oligocene Ashiya Group of Kitakyushu, Japan. 1, Dorsal aspect of shaft lacking the omal portion and 2, Isolated omal portion, ventral aspect. Abbreviations: FASC, facies articularis scapularis; see Figure 1 for others.