Zygodactylidae, Brodkorb, 1971

Weidig, Ilka, 2010, New Birds from the Lower Eocene Green River Formation, North America, Records of the Australian Museum 62 (1), pp. 29-44 : 39-40

Zygodactylidae indet.

Besides Eozygodactylus americanus , a second species of Zygodactylidae occurs in the Green River Formation. It is represented by three specimens: BHI 1285, a complete articulated skeleton; NAMAL 2000-0217-004, also a complete articulated skeleton ( Fig. 11 ) (the counterslab is in private hands and not available for study); and UWGM 21421, consisting of a right leg. These three specimens represent a new species, which differs from Eozygodactylus americanus in the following characters: (1) smaller overall size (length of tarsometatarsus 14.5 mm (BHI 1285) versus 21.7 mm in Eozygodactylus americanus ) ( Table 4, 6); (2) humerus with small processus supracondylaris dorsalis; (3) relatively longer ulna, which exceeds the tarsometatarsus in length.

In character (3), the undescribed species also differs from Primozygodactylus .

Because neither the BHI 1285 nor the NAMAL 2000- 0217-004 specimen were available for detailed studies, the new species is not named or described in detail. However, a few observations are made. The processus acrocoracoideus of the coracoid is large and hooked distally and resembles cf. Primoscens (WN 87558 collection Daniels). The processus procoracoideus is rather large, it seems to be somewhat relatively larger than that of Eozygodactylus americanus and clearly larger than that of Primozygodactylus danielsi . The extremitas omalis of the furcula is broad like that of Primozygodactylus , but contrary to the very narrow extremitas omalis of cf. Primoscens ( Mayr, 1998: fig. 23). The humerus differs from that of Eozygodactylus americanus , but resembles that of Primozygodactylus in having a large crista bicipitalis. The processus supracondylaris seems to be considerably smaller than in Eozygodactylus americanus and thus is similar to Primozygodactylus and cf. Primoscens ( Mayr, 1998: fig. 25).

Discussion. The fossil Zygodactylidae Brodkorb, 1971 are known from both complete articulated skeletons and three-dimensional preserved bones. Three genera have been described so far: Zygodactylus Ballmann, 1969 from the lower Oligocene to Miocene of Europe, originally known from distal tibiotarsi and tarsometatarsi ( Ballmann, 1969a,b), until Mayr (2008b) described a new species based on a complete skeleton; Primoscens Harrison & Walker, 1977 and the taxon Primoscenidae from the Lower Eocene London Clay of England, was based on and only known from a single incomplete carpometacarpus (Harrison & Walker, 1977) until Mayr (1998) referred additional skeletal elements from the London Clay kept in a private collection; and Primozygodactylus Mayr, 1998 from the German Messel site was based on complete articulated skeletons ( Mayr, 1998). The Primoscenidae were regarded as Passeriformes in the original description (Harrison & Walker, 1977), but their passeriform affinities were later doubted (Olson & Feduccia, 1979). The Messel specimens led Mayr (1998) to tentatively consider the Primoscenidae to be the sister group of a clade including the Miocene Zygodactylus and the Pici, mainly due to the presence of a zygodactyl foot with a trochlea accessoria (a sehnenhalter as described by Steinbacher, 1935). A first cladistic analysis including both the Primoscenidae and Zygodactylus resulted in a sister group relationship between the clade ( Primoscenidae + Zygodactylidae ) and Passeriformes ( Mayr, 2004b) .A new species of Zygodactylus showed the great similarities between Zygodactylus and Primoscenidae ; in consequence, Mayr (2008b) synonymized Primoscenidae with Zygodactylidae . The new species of Zygodactylus also showed that zygodactylids are the sister taxon of Passeriformes ( Mayr, 2008b) .

From the Green River Formation, six specimens, which represent two species, were available for this study and are described above. In their overall appearance, the specimens from the Green River Formation closely resemble the Messel specimens. However, the Green River specimens can be distinguished from the Messel genus Primozygodactylus , mainly by characters of the humerus. Mayr (1998) mentioned several specimens from the London Clay ( England) which are in the private collection of Michael Daniels. According to Mayr (1998), the specimens from the Daniels collection represent three different types of Zygodactylidae : One type probably belongs to the genus Primozygodactylus , one type represents the genus Primoscens and the third type “represents a not yet described genus” ( Mayr, 1998: 51). This third type ( Mayr, 1998: pl. 7) has a humerus that is very similar to that of Eozygodactylus americanus and may belong to the same genus. Both humeri exhibit a pronounced processus supracondylaris dorsalis at the distal end of the humerus, which is also found in some Passeriformes . The smaller species of Zygodactylidae from the Green River Formation differs in several aspects especially of the humerus from Eozygodactylus americanus . The humerus of the small Green River species resembles more closely that of cf. Primoscens , to which it also corresponds in size. However, it differs from cf. Primoscens in the shape of the extremitas omalis of the furcula. A closer study might reveal whether this small species is congeneric with one of the already described genera or if it represents a new genus of Zygodactylidae . Unfortunately, the characters listed by Mayr (2008b) as apomorphies for the clade ( Zygodactylidae and Passeriformes ) are not visible in any of the Green River specimens, with the exception of a well-developed processus acromialis, which is visible in the NAMAL specimen.

?Galbulae

Figures

Figure 11. Zygodactylidae indet., NAMAL 2000-0217-004, scale bar 20 mm.