Eozygodactylus americanus, Weidig, 2010

Weidig, Ilka, 2010, New Birds from the Lower Eocene Green River Formation, North America, Records of the Australian Museum 62 (1), pp. 29-44 : 36-38

Eozygodactylus americanus n.gen. and n.sp.

Holotype. USNM 299821 , partial articulated skeleton lacking the pelvis and the legs ( Fig. 9 ).

Type locality. Tynsky Quarry (F2), Kemmerer, Lincoln County, Wyoming, USA.

Type horizon. Tertiary, Eocene, Wasatchian, Green River Formation, Fossil Butte Member.

Paratype. WDC-CGR-014, articulated skeleton lacking the skull—the “skull” is painted by the preparator; from 18-inch-layer ( Fig. 10 ).

Referred specimen. Tentatively referred: UWGM 40705, left wing, scapula, coracoid, parts of the sternum; from split-fish-layer.

Etymology. americanus —after America, the continent in which the Green River Formation is situated.

Diagnosis. As for genus.

Dimensions. See Table 4, 5 and 6.

Description. Eozygodactylus americanus resembles Primozygodactylus danielsi in its size and is thus one of the smaller species of the Zygodactylidae , but still larger than Primoscens minutus . Unless otherwise indicated, the description is mainly based on the USNM 299821 and WDC-CGR-014 specimens, since they are more complete and better preserved.

The skull in USNM 299821 is slightly crushed, but some features may still be recognized. The prefrontal is not visible, it is either absent or hidden by the ripped and dislocated sclerotic ring. The preserved parts of the sclerotic ring do not allow detailed comparison with the sclerotic ring of Primozygodactylus ( Mayr, 1998: fig. 21). With a length of approximately 13 mm, the maxilla is slightly shorter than that of Primozygodactylus danielsi (the two species have the same overall size). The narial opening is long, it measures about three quarters of the length of the maxilla. An ossified nasal septum does not seem to be present which is in accordance with Primozygodactylus , the Passeriformes and most Pici. The processus orbitalis of the quadratum is short in contrast to the elongated process of the Pici and Passeriformes .

As in Primozygodactylus , the symphysis mandibularis measures one fifth of the whole mandible. A groove is situated at the location where a fenestra mandibulae would otherwise be present.

The coracoid is long and slender, but neither the distal nor the proximal end are well enough preserved to show any details. A medium-sized processus procoracoideus is present (UWGM 40705). Primozygodactylus has a rather small processus procoracoideus, whereas cf. Primoscens has a very large processus procoracoideus ( Mayr, 1998: fig. 22).

The scapula is bent more sharply than that of Primozygodactylus . The acromion is only moderately long and thus is similar to Primozygodactylus danielsi , whereas Primozygodactylus major has a very long acromion ( Mayr, 1998: fig. 22).

The sternum resembles that of Primozygodactylus ( Mayr, 1998: fig. 24), it is stocky, relatively short and mediolaterally broad. It is nearly as long as the humerus and, in relation to the wing, thus considerably longer than that of Primozygodactylus danielsi . The margo caudalis shows four incisurae, the incisurae laterales are deeper than the incisurae mediales. In Eozygodactylus , the trabeculae laterales and intermediales widen toward their caudal ends and reach further caudad than the trabecula mediana of the sternum. The trabeculae laterales reach even farther distad than the trabeculae intermediales. The spina externa is not visible in any specimen. The exact number of ribs is not determinable, at least four of them possess a processus uncinatus.

The humerus closely resembles that of Passeriformes . In accordance with WN 88583A and WN 92747 (collection Daniels from the London Clay, shown in Mayr, 1998: 79, pl. 7), the crista bicipitalis is only of moderate size, whereas Primozygodactylus has a pronounced crista bicipitalis. The tuberculum ventrale is prominent. The crista deltopectoralis of Eozygodactylus is low and short as in the other members of Zygodactylidae , it only measures about a fourth of the length of the humerus. The crista deltopectoralis approaches the shaft, whereas in Primozygodactylus , the crista is nearly parallel to the humerus shaft. As in Primozygodactylus , the shaft is only slightly curved. The apparently stronger curvature of the left humerus shaft in the USNM specimen is due to a fracture in the middle of the bone. The distal end of the humerus shows a pronounced processus supracondylaris dorsalis, which is similar in its size and location to that of the Zygodactylus and Passeriformes . Primozygodactylus and WN 87558A (cf. Primoscens ) have a considerably smaller processus supracondylaris dorsalis ( Mayr, 1998: fig. 25), which is oriented less proximad, but is perpendicular to the shaft. The fossa m. brachialis is moderately deep. As in Primozygodactylus and modern Piciformes , the ulna is longer than the humerus. The ratio humerus:ulna is concordant with that of Primozygodactylus danielsi and of Primozygodactylus ballmanni . However, the ulna is shorter than the tarsometatarsus in Eozygodactylus , whereas the two bones have nearly the same length in Primozygodactylus . In contrast to most Pici and many Passeriformes , the ulna lacks papillae remigiales. In Primozygodactylus , papillae remigiales are also absent. Neither the olecranon nor the distal end are discernible in any specimen. The carpometacarpus of Eozygodactylus americanus resembles closely that of Primoscens and of Primozygodactylus . The most conspicuous character is the large processus intermetacarpalis which reaches as far as to the os metacarpale minus, but is not fused to it. The os metacarpale minus is longer than the os metacarpale majus. The difference in length is larger than in Primozygodactylus . The proximal end of the carpometacarpus is only poorly preserved and thus allows few observations. The proximal end of the os metacarpale minus bears a slight tubercle, which probably corresponds to the “transverse ridge extending to the pisiform process” as described by Harrison (1982: 78). In how far this tubercle reaches the processus pisiformis is not visible in Eozygodactylus . A comparable tubercle is not visible in Primozygodactylus , but this might be due to preservation ( Mayr, 1998). In Eozygodactylus , the processus extensorius is turned proximad to a greater extend than in Primozygodactylus ( Mayr, 1998: fig. 26). As in Primozygodactylus , the synostosis metacarpalis distalis is broad. In contrast to Primozygodactylus ( Mayr, 1998: fig. 26), the cranial margin of the os metacarpale majus is nearly straight and shows only a slight concave curvature. The phalanges match those of Primozygodactylus . The digitus alulae has only one phalanx. The distal phalanx of the digitus majoris is long and slender.

In contrast to Primozygodactylus and extant Passeriformes and Pici, the foramen obturatum is open caudally. In accordance with Primozygodactylus , the processus terminalis ischii is narrow and the distal end seems to be widened. The description of the legs is only based on the WDC specimen, since the other two specimens lack the hind limbs. As in Primozygodactylus , the tibiotarsus is by far the longest limb element. The distal condyles are craniocaudally deep and low, even lower than in Primozygodactylus . The tarsometatarsus is long and slender and in that respect resembles that of Passeriformes . Very few details are discernible at the proximal end, the hypotarsus seems to be small. The foramen vasculare distale is large and oval. In distal view, the trochleae are aligned as a curve, not planar like in passerines. The trochlea metatarsi III reaches considerably farther distad than the trochleae metatarsorum II and IV, which are approximately of the same length. The trochlea metatarsi II is only poorly preserved and a plantarly directing process is not visible. The fourth toe is opposed to the second and the third toe. All toes have the usual number of phalanges. The toes are longer than those of Primozygodactylus and have different proportions (tab. 5). The claws are short with a pronounced sulcus neurovascularis.

Figures

Figure 9. Eozygodactylus wyomingensis n.gen. and n.sp., holotype, USNM 299821, scale bar 20 mm.

Figure 10. Eozygodactylus wyomingensis n.gen. and n.sp., paratype, WDC-CGR-014, scale bar 20 mm. The skull is fabricated.