Cormobates sp.

Nguyen, Jacqueline M. T., 2016, Australo-Papuan treecreepers (Passeriformes: Climacteridae) and a new species of sittella (Neosittidae: Daphoenositta) from the Miocene of Australia, Palaeontologia Electronica 11 (1), pp. 1-13 : 5-6

Cormobates sp. indet.

Figure 2.1, 2.3, 2.5

Material. QM F57898, a distal left tibiotarsus.

Measurements (mm). Preserved length 9.0; distal width ca 2.8; length of condylus lateralis ca 1.9; length of condylus medialis ca 1.9; depth of condylus lateralis ca 2.2; depth of condylus medialis ca 2.3.

Locality and Horizon. Wayne’s Wok Site, Riversleigh World Heritage Area, northwestern Queensland, Australia; Faunal Zone B (early Miocene).

Description. The fossil tibiotarsus QM F57898 ( Figure 2.1, 2.3, 2.5 ) exhibits the above described features that are characteristic for passerines. It possesses the following suite of features that allow its referral to Climacteridae (Australo-Papuan treecreepers): (1) distal end of tibiotarsus broad relative to shaft, and distal width about equal to or greater than 1.5 times distal length; (2) condyles relatively short and widely set apart; (3) condyles about equal in medio-lateral width; (4) viewed laterally, condylus lateralis oblong in shape with a distinctly thick rim along its disto-cranial edge; (5) pons supratendineus located far proximally of condyles by a distance of at least half the length of condylus medialis; (6) tuberositates retinaculorum extensorium elongate; (7) tuberositas retinaculi extensoris lateralis located on proximo-lateral part of the pons supratendineus; (8) tuberositas retinaculi extensoris lateralis distally separated from its medial companion by a distance about equal to length of latter; (9) bony ridges for attachment of retinaculum m. fibularis are prominent flanges; and (10) sulcus m. fibularis wide.

QM F57898 can be excluded from families of non-scansorial passerines by the presence of a well-proximally located pons supratendineus relative to the condyles. Climacterids can also be differentiated from families of non-scansorial passerines by the laterally-displaced bony ridges for the retinaculum m. fibularis. Climacterids differ from other unrelated scansorial passerines examined in the following features. Differ from Daphoenositta and Mniotilta by the presence of features 1, 2, 4, 5, 9, and 10. Climacterids differ from Certhia in having 3, 6–8, and 10, and differ from Lepidocolaptes in 3 and 5–9. Differ from Sitta by the presence of 1, 2, 4, and 6–10. Differ from Margarornis in having 6–8, and differ from Pseudocolaptes in 5–7. Climacterids differ from Dendroscansor in 1, 2, 5, 7, 9, and 10.

This fossil is conservatively assigned to Cormobates because it shares the following suite of features: tibiotarsus gracile and smaller in size; bony ridges for attachment of retinaculum m. fibularis disto-laterally adjacent to tuberositas retinaculi extensoris lateralis; medial bony ridge for attachment of retinaculum m. fibularis is proximo-distally short (although it is also short in Cl. erythrops ), whereas lateral bony ridge is long; sulcus extensorius proximally narrower; and pons supratendineus is situated farther proximally from condyles by a distance of about equal to or greater than 75% of length of condylus medialis. Although the cristae trochleae are damaged in the fossil, species of Cormobates can also be differentiated from those of Climacteris in having shorter cristae trochleae relative to the width of the distal end. The fossil tibiotarsus assigned to Cormobates is distinguished from that assigned to Climacteris in that it is gracile, the pons supratendineus is situated relatively further proximally, and the bony ridges for the retinaculum m. fibularis are distinctly shorter and located further distally on the shaft relative to the tuberositas retinaculi extensoris lateralis. The fossil differs from Co. leucophaea in that the lateral bony ridge for attachment of the retinaculum m. fibularis is slightly shorter, and the epicondylus medialis and the tuberositas retinaculi extensoris lateralis are less prominent. Although it is distinctive enough to be assigned to Cormobates , the fossil tibiotarsus does not preserve enough features to allow assignment at the species level.

QM F57898 ( Figure 2.1, 2.3, 2.5 ) represents a young individual because the condyles are not fully ossified. This fossil is comparable in size to the tibiotarsus of Cormobates leucophaea . As in all scansorial passerines examined, the epicondylus medialis is protuberant and clearly discernible in cranial aspect. The incisura intercondylaris is markedly wide and deep, and the impressio lig. intercondylaris is deeply excavated. The width of the pons supratendineus is about equal to its length. The tuberositas retinaculi extensoris lateralis is situated on the proximo-lateral part of the pons supratendineus and extends beyond its proximal margin. The sulcus extensorius is centred on the cranial shaft surface. The bony ridges for attachment of the retinaculum m. fibularis are proximo-distally level and bound the wide and deep sulcus m. fibularis. The sulcus m. fibularis is situated slightly more cranially than in Cormobates leucophaea . The trochlea cartilaginis tibialis is very wide relative to its length. There is a very low ridge that is centred on the trochlea cartilaginis tibialis. This ridge is also faintly visible in Cormobates leucophaea and in most species of Climacteris examined.

Figures

FIGURE 2. Distal left tibiotarsus of Cormobates sp. QM F57898 (1, 3, 5) compared with that of Cormobates leucophaea AM.O.60025 (2, 4, 6). 1-2, cranial aspect; 3-4, caudal aspect; 5-6, lateral aspect. Abbreviations: cl, condylus lateralis; em, epicondylus medialis; lrf, lateral bony ridge of retinaculum m. fibularis; mrf, medial bony ridge of retinaculum m. fibularis; ps, pons supratendineus; rct, ridge on trochlea cartilaginis tibialis; rim, thickened rim of the condylus lateralis; trel, tuberositas retinaculi extensoris lateralis; trem, tuberositas retinaculi extensoris medialis. Scale bar equals 2 mm.