Aves Linneaus 1758
Parapasseres Mayr 2015
Zygodactylidae Brodkorb 1971
Zygodactylus Ballmann 1969
Zygodactylus ochlurus sp. nov.
YPM VPPU 17053, partially articulated skeleton preserved on main slab and counterpart, including two smaller flakes detached from the counterpart (Fig. 1a-e). Feather impressions surround the skeleton. Most elements are preserved as mouldic impressions and fragments of bone.
From ὀχληρός (ochlhros), Classical Greek for troublesome, in reference to “Trouble,” an orphaned magpie chick that Becker hand-reared in the Fossil Basin camp during his 1959 field season .
Type locality and horizon
Becker locality #2 in Fossil Basin, Upper Ruby Valley, Madison County [15, 20, 21] (Fig. 1f). Plant fossils from this locality are assigned to the Ruby Basin Flora, lithostratigraphically positioned at the boundary between the Climbing Arrow and Dunbar Creek Members of the Renova Formation . The Dunbar Creek Member is locally capped by the Williams Creek basalt, K-Ar dated to 32.2 ± 0.4 Ma , providing a hard upper bound for the age range. Paleoclimate studies place the Ruby Flora in the earliest Oligocene, ca. 33 Ma .
Zygodactylus ochlurus shares the combination of a large intermetacarpal process, a dentiform process of the carpometacarpus, a zygodactyl foot, and a greatly elongate tarsometatarsus (longer than or subequal to the humerus) unique to Zygodactylidae [5, 6, 22]. The new taxon is assigned to Zygodactylus based on the distal projection of metacarpal III past metacarpal II, and on the presence of a convexity proximal to the trochlea of metatarsal IV [2, 6]. The latter character is automorphic for Zygodactylus.
Z. ochlurus is differentiated from all other Zygodactylus spp. by (1) anterior projection of cranial cnemial crest greater than anteroposterior width of tarsometatarsus, (2) hallucal digit proximal phalanx longer than pedal digit III proximal phalanx, and (3) diminutive size: most other limb elements are approximately two-thirds the size of corresponding elements in congenerics Zygodactylus grivensis, Z. ignotus, Z. luberonensis, and Z. grandei (Table 1) [2, 6, 7, 11]. Complete fusion of the synsacral vertebrae in YPM VPPU 17053 are taken here as an indicator of skeletal maturity, and thus adult size, for this specimen.
Z. ochlurus is differentiated from Eozygodactylus americanus by (1) shorter lateral sternal trabeculae, (2) femur shorter than humerus, (3) ulna approximately equal to tarsometatarsus in length, (4) presence of a dentiform process on the cranial margin of the carpometacarpus.
Z. ochlurus is differentiated from all Primozygodactylus spp. by (1) a large dorsal supracondylar process of humerus, (2) femur shorter than humerus, (3) metacarpal III longer than metacarpal II, and (4) obturator foramen continuous with ischiopubic fenestra. The new taxon is differentiated from all Primozygodactylus spp. except P. danielsi by a relatively elongate proximal phalanx of pedal digit II.
See Table 1.
The skull is badly crushed, and is split between main slab (Fig. 1a) and counterpart (Fig. 1b) on a largely parasagittal plane through the right orbit. A short maxillary rostrum relative to skull length is more similar to Eozygodactylus americanus than Z. luberonensis [3, 9]. The bodies and otic processes of both left and right quadrate are visible on the part. Dark-stained matrix in the middle of the skull rests in the right orbit, and obscures the orbital process of the right quadrate. The occipital region remains on the counterpart, including the outline of the left paroccipital process and the middle ear cavity (cavum tympanicum). The outline of the tympanic crest is similar to E. americanus. The mandibular rami are distorted, but appear to display a ventral curvature similar to E. americanus. Crushed remnants of the left palatine are visible rostral to the middle ear cavity on the counterpart.
Faint remnants of the cervical column are split between main slab and counterpart, without substantial exposed surfaces. The body of the sacrum is split between counterpart and first intermediate flake (Fig. 1c). It appears to contain ~ 11 vertebrae; nine are visible, but the series is cranially incomplete. Costal processes of the acetabular vertebra are visible on both slabs, while the adjacent body of the sacrum is largely crushed onto the counterpart. Foramina intertransversaria look to be enclosed for at least two of the caudal sacral series.
Pectoral girdle and limb
The sternal keel is preserved in the main slab. The body of the sternum forms a mouldic impression around the fragmented keel. Spina externa may be fragmentarily preserved alongside the left coracoid on the part, but its outline cannot be determined with any certainty. Right cranial portion of sternum, showing the pila coracoideus, is preserved on the counterpart. There are four sternal incisions caudally, with medial and lateral trabeculae ending in approximately the same plane as the broad caudal midline extremity.
Left scapula is preserved on the counterpart, with matching impression in first intermediate flake. The body of the scapula is less curved than in E. americanus.
An impression of the right coracoid is exposed in dorsal view on the part, missing medial and proximal parts of the flange. The impression shows a weakly developed procoracoid process, as in Z. luberonensis. An incompletely exposed bony remnant adjacent to the right coracoid is tentatively identified as the triangular omal extremity of the furcula. Crushed portions of the left coracoid, without the flange, are preserved in dorsal view on the part. The cranial end projects beneath the impression of the sternum and ends in close proximity to the cranial end of the left scapula.
The left humerus is preserved as an impression of the cranial surface of the shaft with crushed portions of the head on the main slab, and as fragmentary remains on the counterpart. The humerus is stocky and slightly shorter than the ulna. The deltopectoral crest morphology is poorly defined as a faint impression as the element lacks most of its proximodorsal margin. The humeral head is more dorsoventrally narrow than in Z. luberonensis. An impression of a prominent dorsal supracondylar process is apparent proximal to the impression of the dorsal condyle. A narrow flexor process protrudes distal to the ventral condyle, and a portion of this structure is also visible in the first intermediate flake. Humerotricipital and scapulotricipital sulci are present.
The left ulna is preserved as an impression on the main slab, with fragmentary remnants of the body on the counterpart. The distal end and body of the right ulna are preserved on the main slab, with a small fragment of the proximal end preserved on the counterpart. Papillae remigales are not apparent, and the ulnar shaft is straight. The olecranon process is poorly preserved, with a moderate point, similar to Z. luberonensis but less pronounced than in extant Passeriformes.
Left radius is preserved as a thin impression on the main slab and as fragments on counterpart, positioned slightly out of articulation with the left ulna. Right radius is preserved as collapsed fragments in the main slab, fully disarticulated from the right ulna. The left ulnare is a poorly-exposed and crushed body on the main slab. The radiale is not visible.
Left carpometacarpus is preserved as a crushed body and impression on the main slab. Metacarpal I is broken off but the area of breakage on metacarpal II suggests it would have been quite short. The ulnocarpal trochlea is preserved as a faint impression and does not appear to be large or significantly projected caudoventrally. Metacarpal III extends distal to metacarpal II. A prominent intermetacarpal process is visible within the narrow intermetacarpal space (Fig. 2b). A dentiform process is visible on the cranial margin of major metacarpal. Left major digit is preserved in articulation with carpometacarpus on the main slab. All digits are preserved as impressions in the main slab. Digit III:1 appears to be displaced to lie along side distal II:1. Phalanx II:1 flares distally and is partially overlain by digit II:2. As preserved, the impression of digit II:1 appears to possibly bear a distinct caudodistal processus not dissimilar to that seem in some Galbulae. However, the impression shows a proximodistal furrow that is interpreted as more consistent with an underlying, separate, element such as a carpal or phalanx.
Pelvic girdle and limb
Left and right pelvic elements are disarticulated from the sacrum and preserved on the main slab, with an additional impression of left pelvic elements on the second intermediate flake (Fig. 1d). The ilium is short and narrow with its preacetabular portion broadly recurved dorsally and longer than its postacetabular portion. The pubis is thin and angles away from the ischium rather than being subparallel to its ventral margin. The obturator foramen is continuous with the ischiopubic fenestra. The right femur is preserved as an impression on main slab, with the distal end passing ventral to sternal impression. Left femur crosses under the impression of the left humerus on the same slab. The femora (Fig. 1e) are elongate and straight. The trochanteric crests are not well projected proximally.
The body and medial cnemial crest of the right tibiotarsus are preserved as an impression with collapsed fragments on the main slab. The large cranial cnemial crest, visible as an impression on the counterpart, is strongly projected cranially and dorsally. Distal end of the left tibiotarsus is visible on the main slab; body may continue beneath the sternum. Both tarsometatarsi are partially crushed. The right tarsometatarsus shows a prominent lateral plantar crest of the hypotarsus, as well as a slight convexity proximal to trochlea IV. Left metatarsal I visible as a separate element alongside the left tarsometatarsus. Pedal phalanges of left digits I, II, and III, right digit III preserved in articulation on the main slab. Digit I:1 is proportionally more elongate in the new taxon than in Z. luberonensis and P. danielsi (Table 1), but comparable to the proportional lengths of digit I:1 in basal Psittacopasseres Avolatavis tenens  and Messelastur gratulator .
Capital and cervical tract feathers preserved as a darker halo of filamentous structures. Distal primary remiges (IV-IX?) are preserved as impressions on the main slab and second intermediate flake.