Pomatostomus sp.

Nguyen, Jacqueline M. T., Hand, Suzanne J. & Archer, Michael, 2016, The Late Cenozoic Passerine Avifauna from Rackham’s Roost Site, Riversleigh, Australia, Records of the Australian Museum 68 (5), pp. 201-230 : 210-213

Pomatostomus sp.

Figs 9–11

Material. QM F30358, distal left humerus; QM F57939 , right ulna; QM F36368, distal left tibiotarsus; QM F36670, distal right tibiotarsus; QM F30357 and QM F57900 (AR19820), proximal right tarsometatarsi.

Measurements (mm). QM F30357: preserved length 8.6, proximal width ca 2.9, proximal depth (cotyla lateralis to hypotarsus) ca 3.0. QM F30358: preserved length 5.8, distal width 4.6, depth of condylus dorsalis 2.2. QM F36368: preserved length 14.6, distal width 2.9, depth of condylus lateralis 2.7, depth of condylus medialis 3.0. QM F36670: preserved length 16.1, distal width 2.8, depth of condylus lateralis 2.8, depth of condylus medialis 3.0. QM F57900 : preserved length 15.3, proximal width 3.3, proximal depth (cotyla lateralis to hypotarsus) 3.3. QM F57939 : preserved length 19.7, proximal width 3.1, length of proc. cotyla dorsalis 1.3, distal width ca 2.3, depth of condylus dorsalis ca 2.0.

Description and comparisons. Humerus. QM F30358 ( Fig. 9A–B ) is identified as Pomatostomus because of the following combination of features. The distal humerus is very wide relative to its depth and is well expanded ventrally from the shaft edge. The sulcus humerotricipitalis is very shallow, such that it is near planar with the caudal surface of the distal humerus. The sulcus scapulotricipitalis is shallow and wide. The distal extent of the proc. flexorius is much greater than that of the condylus dorsalis. On the epicondylus ventralis, the scars for attachment of M. pronator profundus and M. flexor digiti superficialis are marked. The proc. supracondylaris dorsalis is proximo-distally long and bifurcated. The condylus ventralis is long; its length is greater than 80% of that of the condylus dorsalis. The incisura intercondylaris is very wide and moderately deep.

The fossil humerus differs from P. superciliosus , P. halli and P. ruficeps in having a deeper bifurcation of the proc. supracondylaris dorsalis. The fossil further differs from P. halli and P. ruficeps , and from P. temporalis ( Fig. 9C–D ) in that the proc. supracondylaris dorsalis protrudes slightly further dorsally. QM F30358 differs from Garritornis isidorei (sometimes subsumed in Pomatostomus , e.g., Clements et al., 2015) in that the sulcus humerotricipitalis is not ventrally demarcated by a low ridge; the proc. flexorius is slightly longer; the fossa m. brachialis is deeper; and the proc. supracondylaris dorsalis protrudes slightly further dorsally.

Ulna. QM F57939 ( Fig. 9E–F ) is a near-complete ulna with breakage to the ventral side of the proximal end and the condyles. The fossil is referred to Pomatostomus because it possesses the following suite of features. The olecranon is short; its length is about half of that of the cotyla ventralis. The distal edge of the proc. cotyla dorsalis is about level with that of the cotyla ventralis. The cotyla ventralis is shallow. The impressio m. brachialis is very shallow and near planar with the shaft surface. In caudal view, there is a shallow depression distally of the attachment site of the trochlea humeroulnaris. The papillae remigales caudales are very low markings on the shaft. On the distal end, the incisura tendinosa is wide and deep.

The fossil ulna differs from P. temporalis in having a shallower depressio radialis and a less developed protuberance for attachment of the trochlea humeroulnaris. It differs from P. ruficeps ( Fig. 9G–H ) in having a comparatively shorter olecranon. QM F57939 differs from Garritornis in having a narrower, pointed olecranon and a shallower impressio m. brachialis and sulcus intercondylaris.

Tibiotarsus. QM F36368 ( Fig. 10A–B ) and QM F36670 ( Fig. 10C–D ) are referred to Pomatostomus because they share the following combination of character states. The distal tibiotarsus is robust. The length of the pons supratendineus is about equal to its width. The tuberositas retinaculi extensoris lateralis is prominent. This tuberosity is situated on the proximo-lateral part of the pons supratendineus and protrudes beyond the proximal edge of the pons. The lateral bony ridge for attachment of the retinaculum m. fibularis is a well-developed flange, but its medial counterpart is low. The trochlea cartilaginis tibialis is very wide and its cristae are near parallel.

The fossil tibiotarsi differ from P. halli and P. temporalis ( Fig. 10E–F ) in that the tuberositas retinaculi extensori lateralis is more elongate and the epicondylaris lateralis is less developed. The fossils differ from P. halli and P. ruficeps in that the lateral bony ridge for the retinaculum m. fibularis is longer than, not equal to, its medial counterpart. QM F36368 and QM F36670 differ from P. ruficeps and P. superciliosus in that the bony ridges for the retinaculum m. fibularis are situated at about level with the tuberositas retinaculi extensoris lateralis, not slightly further proximally. They differ from Garritornis in that the condylus medialis is not medially displaced from the shaft edge, and the scar for attachment of the lig. tibiometatarsale intercondylare is much shallower.

QM F36368 and QM F36670 are similar in size and in overall morphology, except that the tuberositas retinaculi extensoris medialis is less developed in the latter. QM F36670 also differs from QM F 36368 in that the condylus lateralis is greater in proximal extent than the condylus medialis. This second character state was present in all specimens of P. ruficeps and Garritornis studied, but was variable among individuals of P. halli and P. superciliosus examined.

Tarsometatarsus. QM F57900 ( Fig. 11A–C ) and QM F30357 ( Fig. 11D–F ) are referred to Pomatostomus because they exhibit the following suite of features. The arcus extensorius is longer than its width. Although the arcus extensorius is broken off in QM F30357, the remaining bony ridges indicate that it was relatively long. The sulcus extensorius is deep. The medial shaft depth is very shallow and the impressio lig. collateralis medialis is elongate. The tuberositas m. tibialis cranialis is low and situated close to the arcus extensorius. The tendinal bridge between the hypotarsus and crista plantaris lateralis is ossified and plantarly bounds a small foramen (‘peroneal foramen’ in Orenstein, 1977). In QM F30357, part of the crista plantaris lateralis is broken off, but what remains of the crista indicates that the tendinal bridge was ossified and that the foramen was small. In QM F57900 , there is damage around the peroneal foramen but the ossified tendinal bridge is preserved. On the lateral surface of the tarsometatarsus there is a distinct groove for the cranial branch tendon of M. fibularis longus ( Raikow, 1993), which joins to the peroneal foramen. Other features that are shared among the fossils and extant pomatostomids include a deep fossa infracotylaris dorsalis and a proximally situated cotyla medialis relative to the cotyla lateralis.

The fossil tarsometatarsi differ from those of extant species of Pomatostomus in that the plantar part of the hypotarsus does not project as far proximally. The fossils differ from P. halli and P. ruficeps ( Fig. 11G–I ) in having a more elevated impressio lig. collateralis medialis and a deeper sulcus ligamentosus. The fossils further differ from P. ruficeps in having a shallower groove for the cranial branch tendon of M. fibularis longus. This groove, however, is deeper in the fossils than in P. temporalis . QM F30357 also differs from P. ruficeps in having a less prominent impressio lig. collateralis lateralis (it is broken in AR19820). The fossils differ from P. superciliosus in having a slightly longer arcus extensorius with respect to width, whereas it is relatively shorter than in P. temporalis . QM F30357 and QM F57900 further differ from P. temporalis in possessing a shallower fossa infracotylaris dorsalis. They differ from Garritornis in that the impressio lig. collateralis lateralis is shorter and far less developed, and the peroneal foramen is circular rather than oval-shaped. The tuberositas m. tibialis cranialis is distally adjacent to or closely located to the arcus extensorius in the fossils, whereas in Garritornis the distance between the tuberosity and arcus is greater. QM F30357 differs from QM F 57900 in having a shallower fossa infracotylaris dorsalis and medial shaft depth.

Remarks. Australo-Papuan babblers ( Pomatostomidae ) are medium-sized passerines that only superficially resemble their Eurasian, unrelated namesakes in appearance, sociability and foraging behaviour (Schodde & Mason, 1999). They typically glean and probe for insects among leaf litter, grasses and fallen trees. Some babblers, particularly the Grey-crowned Babbler P. temporalis , also forage in low shrubs and on trunks and branches of trees near the ground (Schodde & Mason, 1999; Higgins et al., 2002). In Australia, these birds inhabit open eucalypt forests, woodlands, shrublands and semi-arid scrub ( Boles, 1988; Higgins et al., 2002). The Papuan Babbler Garritornis isidorei occurs in rainforest, tall secondary growth and lowland gallery forest ( Matthew, 2007).

There are no significant morphological differences between the pomatostomid fossils and any of the extant taxa that would enable confident species determination. Nevertheless, the fossils indicate the presence of a pomatostomid smaller than the White-browed Babbler P. superciliosus in the Rackham’s Roost Local Fauna. With a minimum estimated age of early Pleistocene, these fossils represent the oldest known record of Pomatostomidae . Holocene fossils of P. superciliosus have been reported from Madura Cave in Western Australia ( Baird, 1991a). Today, this family is represented in the Riversleigh region by P. temporalis .

Figures

Figure 9. Comparison of fossil and extant pomatostomid wing bones. (A–B) distal left humerus of Pomatostomus sp. QM F30358 and (C–D) distal right humerus of P. temporalis AM O.65103 (mirrored). Right ulnae of (E–F) Pomatostomus sp. QM F57939 and (G–H) P. ruficeps AM O.60045. (A,C,F,H) caudal aspect; (B,D,E,G) cranial aspect. Some characteristic features of Pomatostomus shown: 1, proc. supracondylaris dorsalis long and bifurcated; 2, sulcus humerotricipitalis very shallow; 3, distal end of humerus well expanded ventrally; 4, distal extent of proc. flexorius much greater than that of condylus ventralis; 5, condylus ventralis long with respect to length of condylus lateralis; 6, olecranon short; 7, distal edge of proc. cotyla dorsalis about level with that of cotyla ventralis; 8, cotyla ventralis shallow; 9, impressio m. brachialis very shallow; 10, shallow depression situated distally of attachment scar for trochlea humeroulnaris. Scale bar = 2mm.

Figure 10. Tibiotarsi of (A–D) fossil Pomatostomus sp. compared with (E–F) a left tibiotarsus of P. temporalis AM O. 68479. (A–B) QM F36368, distal left tibiotarsus. (C–D) QM F36670, distal right tibiotarsus.(A,C,E) cranial view,(B,D,F) caudal view. Some characteristic features are highlighted: 1, tuberositas retinaculi extensoris lateralis protuberant and at about level with the bony ridges for attachment of retinaculum m. fibularis; 2, lateral bony ridge for retinaculum m. fibularis a moderate flange, whereas medial bony ridge low; 3, trochlea cartilaginis tibialis very wide; 4, cristae near parallel. Scale bar = 2 mm.

Figure 11. Proximal right tarsometatarsi of (A–F) fossil Pomatostomus sp. compared to the corresponding bone of (G–I) P. ruficeps AM O.60045. (A–C) QM F57900. (D–F) QM F30357. (A,D,G) dorsal view, (B,E,H) plantar view, (C,F,I) lateral view. Some characteristic features of Pomatostomus labelled: 1, arcus extensorius longer than wide; 2, tuberositas m. tibialis cranialis low and located close to arcus extensorius;3,groove forM. fibularis longus marked; 4, ossified tendinal bridge between hypotarsus and crista plantaris lateralis, enclosing a small foramen in crista. Scale bar = 2 mm.