Acanthizidae Bonaparte, 1854

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 : 209-210

Family Acanthizidae Bonaparte, 1854

Genus et species indet.

Fig. 8

Material. QM F57928 (AR10832), left carpometacarpus; QM F22796, distal left tibiotarsus.

Measurements (mm). QM F22796: preserved length 4.2, distal width 1.7, depth of condylus lateralis ca 1.6, depth of condylus medialis> 1.6. QM F57928 : preserved length 6.8, proximal width>1.4, distal width>1.5.

Description and comparisons. Carpometacarpus. QM F57928 ( Fig. 8A–B ) is a very small carpometacarpus with damage to the proximal and distal ends, proc. intermetacarpalis and to the dorsal edge of the os metacarpale minus. This fossil is tentatively assigned to Acanthizidae because it exhibits the following suite of features. It is tiny and overall similar in proportions and shape to the carpometacarpi of acanthizids. The fovea lig. ventralis is deep and recessed cranially. The fovea carpalis caudalis is small and shallow. Although the proc. cranialis is broken off, it appears to have been approximately level with this fovea. The proc. dentiformis is well defined and located at about the proximo-distal midpoint of the os metacarpale minus. There is a moderately deep ventral fossa on the distal end of the os metacarpale minus.

QM F57928 is closest in size to the carpometacarpus of the Weebill, Smicrornis brevirostris . It can be distinguished from species of Pycnoptilus , Calamanthus , Crateroscelis , Acanthiza and Sericornis studied in that it is not as dorsoventrally curved. The os metacarpale minus in the fossil is not bowed as in Pycnoptilus and Crateroscelis . The fossil is relatively less robust than the carpometacarpus of Origma , and it has a deeper ventral fossa on the distal end of the os metacarpale minus than in Smicrornis . In the absence of more complete fossil material, it is not possible to resolve the identification of this specimen to genus level.

The fossil carpometacarpus can be excluded from other families of characteristically small passerines found in Australia, including Maluridae , small members of Meliphagidae , Pardalotidae , Rhipiduridae , Petroicidae , Estrildidae and Zosteropidae , by the following character states. QM F57928 is excluded from Maluridae because it is not bowed and the os metacarpale alulare is relatively wider. It is excluded from Meliphagidae because the depression for the M. flexor digiti minoris on the trochlea carpalis ventralis is deeper and the distal margin of the fovea carpalis caudalis is about level with, not distally of, the proc. cranialis. The fossil is excluded from Pardalotidae and Petroicidae because the proc. dentiformis is situated at the midpoint of the os metacarpale minus, not further distally. QM F57928 further differs from Pardalotidae because it has a ventral fossa on the distal end of the os metacarpale minus. It is excluded from Rhipiduridae because it possesses a proc. dentiformis. The fossil is excluded from Estrildidae and Zosteropidae because it has a considerably deeper ventral fossa on the distal end of the os metacarpale minus.

Tibiotarsus. QM F22796 ( Fig. 8E ) is a distal tibiotarsus with damage to the cristae trochleae. This fossil is provisionally referred to Acanthizidae because it possesses the following combination of features. The tibiotarsus is characteristically small and corresponds in shape and proportions to the tibiotarsi of acanthizids. The distal shaft is conspicuously narrow in comparison to the width of the distal end (except in Pycnoptilus and Calamanthus ). The pons supratendineus is long with respect to its width (except in Calamanthus , where its length is about equal to width). The lateral bony ridge for attachment of the retinaculum m. fibularis is well developed. The tuberositas retinaculi extensoris lateralis is elongate and prominent. Although it is slightly abraded in the fossil, the tuberositas retinaculi extensoris medialis appears to have been protuberant.

QM F22796 corresponds in size to the tibiotarsus of Sm. brevirostris . Its overall morphology is very similar to those of the acanthizid species examined here, except that it has a relatively longer pons supratendineus than Pycnoptilus , Calamanthus , Origma and Crateroscelis . The fossil further differs from Pycnoptilus and Calamanthus in that the distal shaft is narrower with respect to the distal end.

The fossil tibiotarsus can be excluded from families of characteristically small passerines in Australia, including Maluridae , Rhipiduridae , Petroicidae , Estrildidae and Zosteropidae , because it has a long pons supratendineus. QM F22796 is morphologically similar to those of meliphagids but can be distinguished by the following combination of features. The fossil is considerably smaller in size than the tibiotarsi of meliphagids examined. The bony ridges of the retinaculum m. fibularis are relatively shorter and more prominent, and the sulcus m. fibularis is deeper. The tuberositas retinaculi extensoris lateralis is more elevated than in the meliphagids studied. This tuberosity is located on the proximo-lateral portion of the pons supratendineus, whereas in meliphagids it is situated further proximally on the pons. The incisura intercondylaris is slightly narrower in the fossil than in meliphagids.

The tibiotarsal morphologies of Acanthizidae and Pardalotidae are also very similar, but they differ in the following features. In pardalotids, the lateral shaft edge is less flared and near parallel with the shaft long axis. The condylus lateralis is greater in distal extent than its medial counterpart, whereas in acanthizids and the fossil they are near equal. The length of the lateral bony ridge for the retinaculum m. fibularis is less than or about equal to that of the pons supratendineus in pardalotids, whereas it is greater in acanthizids as it is in QM F22796.

Remarks. Acanthizidae (thornbills, scrubwrens and allies) is a family of very small to medium-sized, wren-like passerines. It is a large, integral component of the primarily Australasian songbird infraorder Meliphagides , with about 65 species in 13 genera (Dickinson & Christidis, 2014). Acanthizids occupy a wide variety of habitats including rainforest, sclerophyll forests, mangroves, dry woodland, grasslands, heath, shrubland and saltmarsh. These birds forage in all vertical strata, from the ground to the upper canopy, and are found in tropical to arid zones ( Higgins et al., 2002; Gregory, 2007).

The extinct Pycnoptilus fordi Baird, 1993 , as well as material referable to the extant Pilotbird, P. floccosus , were described from the late Pleistocene Pyramids Cave in Victoria ( Baird, 1993). Remains of P. floccosus have also been identified from the late Pleistocene Cloggs Cave and Holocene Mabel Cave in Victoria ( Baird, 1991b). Holocene fossils referred to Acanthiza sp. have been reported from Mabel Cave in Victoria and Madura Cave in Western Australia ( Baird, 1986). Acanthizidae is currently represented in the Riversleigh region by Sm. brevirostris and the Western Gerygone Gerygone fusca .

Figures

Figure 8. Comparison of fossil and extant acanthizid bones. Left carpometacarpi of (A–B) acanthizid gen. et sp. indet. QM F57928 and (C–D) Gerygone fusca AM O.66008 in (A,C) ventral and (B,D) dorsal aspects. Distal left tibiotarsi of (E) acanthizid gen. et sp. indet. QM F22796 and (F) Gerygone fusca AM O.66008 in cranial aspect. Some distinguishing features of Acanthizidae highlighted: 1, fovea lig. ventralis deep and recessed cranially; 2, ventral fossa on distal end of os metacarpale minus deep; 3, proc. dentiformis prominent and situated at proximo-distal midpoint of os metacarpale majus; 4, fovea carpalis caudalis small and shallow; 5, distal shaft narrow with respect to width of distal end; 6, pons supratendineus long; 7, lateral bony ridge for retinaculum m. fibularis well developed; 8, tuberositas retinaculi extensori lateralis prominent. Scale bar = 2 mm.