Acrocephalus 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 : 221-223

Acrocephalus sp.

Fig. 17

Material. QM F30356, right carpometacarpus; QM F57902 (AR 10838), left carpometacarpus.

Measurements (mm). QM F30356: preserved length ca 11.75, proximal width>2.26, distal width>2.23. QM F57902 : preserved length 11.93, proximal width ca 2.49, proximal length>1.91, length of os metacarpale alulare>1.27, distal width 2.42.

Description and comparisons. QM F57902 ( Fig. 17A–B ) and QM F30356 ( Fig. 17C–D ) are referred to Acrocephalus because they possess the following combination of features. The carpometacarpus is straight and elongate. The trochlea carpalis ventralis moderately protrudes caudally from the edge of the os metacarpale minus (damaged in QM F30356). The depression for the origin of M. flexor digiti minoris is deep and terminates distally of the proximal edge of the proc. pisiformis. In dorsal view, the spatium intermetacarpale proximally of the proc. intermetacarpalis is visible. The proc. intermetacarpalis is moderately short and does not protrude beyond the caudal edge of the os metacarpale minus. The fovea carpalis caudalis is small and deep. In QM F30356 and A. australis ( Fig. 17E–F ), the proc. dentiformis is long, low and situated well distally of the proximo-distal midpoint of the os metacarpale minus. Although the tip of the proc. dentiformis is broken in QM F57902 , its base indicates that it was located well distally of this midpoint. The distal end of the os metacarpale minus is square and bears on its ventral surface a large, deep fossa.

The fossil carpometacarpi differ from that of Acrocephalus australis in only a few features. In the fossils, less of the spatium intermetacarpale is visible proximally of the proc. intermetacarpalis. The ventral fossa on the distal end of the os metacarpale minus is larger and deeper in the fossils, especially in QM F57902 , than in A. australis . QM F57902 differs from QM F30356 and A. australis in having a slightly shorter proc. intermetacarpalis.

QM F30356 and QM F57902 are morphologically similar and are likely to represent one species. These fossils resemble A. australis in size and overall morphology but they cannot be confidently assigned to this species, owing to their fragmentary nature and unavailability of comparative material for other species of Acrocephalus .

Remarks. Reed warblers ( Acrocephalus spp. ) are small passerines that are currently distributed in the Old World: in Africa, Eurasia, Australasia and the west Pacific islands ( Bairlein et al., 2006). They inhabit aquatic or riparian areas with a dense cover of reeds, rushes, sedges, grasses and other rank vegetation ( Pringle, 1982; Higgins et al., 2006). These areas include reed beds, swamps, marshes and occasionally mangroves ( Courtney-Haines, 1991; Higgins et al., 2006). As well as these habitats, reed warblers have been recorded foraging in shrublands and woodlands close to wetland nesting sites ( Gynther, 1994). These birds forage through dense growth for mainly insects and spiders (Courtney- Haines, 1991; Higgins et al., 2006).

The Rackham’s Roost material described here represents the only known fossil record of Acrocephalidae in Australia. Fossil species of Acrocephalus and Hippolais have been described from pre-Pleistocene sites in Hungary ( Jánossy, 1991; Kessler, 2013). Acrocephalidae is currently represented in Australia by the Oriental Reed Warbler A. orientalis and the Australian Reed Warbler A. australis , the latter of which occurs in the Riversleigh region.

Family, genus et species indet.

Material. Passeriformes indet.: AR21856, distal right humerus; AR21857, distal left ulna; QM F30853, left carpometacarpus; QM F29628, right carpometacarpus; QM F36371, QM F36646, QM F39851 , proximal right carpometacarpi; AR21603, distal left carpometacarpus; AR21858, AR21859, proximal left femora; AR21860, distal left femur; QM F30362, proximal right tibiotarsus; AR21861, distal left tibiotarsus; AR21862, proximal right tarsometatarsus; AR17405, AR21863, AR21864, AR21865, distal left tarsometatarsi; QM F30822, QM F36370,AR21866, AR21867, distal right tarsometatarsi. Passeri indet.: QM F30369, proximal left humerus; AR19822, right ulna; AR21868, AR21869, proximal left ulnae; AR21870, proximal right ulna; AR16067, AR19824, right carpometacarpi; AR10837, AR21871, distal left carpometacarpus; AR21872, AR21873, AR21874, distal right carpometacarpi. Corvides indet.: QM F36672, left carpometacarpus.

Remarks. Thirty-five Rackham’s Roost postcranial fossils are identifiable as passerines but, owing to their fragmentary condition, they cannot be assigned to a family. Differences in morphology and size of these fossils suggest that they represent a minimum of 5 taxa. Of these fossils, QM F36646 represents the largest bird and is comparable in size to the carpometacarpus of Meliphaga lewinii . AR21863 represents the smallest individual and is similar in size to the corresponding bone of a White-browed Scrubwren, Sericornis frontalis . Five of the indeterminate passerine fossils (AR16067, AR19824, QM F30822, QM F36672, AR21862) represent immature birds because the bone surfaces have a pitted appearance and the articular facets are incompletely ossified ( Campbell, 1979). Four of the fossils (QM F30369, QM F30822, AR21858, AR21860) bear tooth punctures.

Among the indeterminate passerine fossils, 14 can be assigned to the suborder Passeri (oscines or songbirds). A proximal humerus, QM F30369, is identified as Passeri because it has a deep fossa pneumotricipitalis II that merges with the fossa pneumotricipitalis I and a crus dorsale fossae that is reduced to a small ridge. Together, these features are present in Pardalotidae and are characteristic of several families within Passerida sensu stricto ( Johansson et al., 2008) ( Bock, 1962; Jánossy, 1983), a major songbird radiation that is primarily distributed in the Northern Hemisphere. Its fragmentary condition precludes the fossil from being excluded from Pardalotidae and referred to Passerida sensu stricto. Four ulnae are identified as Passeri because they have a reduced tub. lig. collateralis ventralis, which is characteristic of oscine passerines ( Ballmann, 1969). Eight carpometacarpi are identified as Passeri because the disto-caudal portion of the os metacarpale majus is planar, and because they lack a distally protruding process on the disto-cranial edge of the os metacarpale minus. In Suboscines and Acanthisittidae , the disto-caudal portion of the os metacarpale majus protrudes dorsally ( Manegold, 2008). Suboscines are further differentiated from other passerines by a distinct process on the distal end of the os metacarpale minus ( Mourer-Chauviré et al., 1989; Mayr & Manegold, 2006; Manegold, 2008), which is absent in all of the fossil carpometacarpi in this study.

One fossil is further identified as a member of the infraorder Corvides (formerly “core Corvoidea”), which includes the “crow-like” songbirds and kin. The carpometacarpus QM F36672 is referred to Corvides because it lacks a proc. dentiformis. This process is present in Acanthisittidae and suboscines but is absent in several oscine taxa within Corvides , including Campephagidae , Oriolidae , Artamidae , Rhipiduridae , Laniidae , Corvidae and Corcoracidae ( Pocock, 1966; Harrison, 1969; JMTN, pers obs.). The proc. dentiformis is also absent in some ptilonorhynchids (in other species it is extremely reduced, JMTN, pers obs.). However, QM F36672 can be excluded from Ptilonorhynchidae because it is considerably smaller in size and the trochlea carpalis dorsalis is relatively shorter. In the fossil, the distal edge of the trochlea carpalis ventralis smoothly merges with the caudal edge of the os metacarpale minus, whereas in ptilonorhynchids it is angular and abruptly joins the os metacarpale minus.

Figures

Figure 17. Carpometacarpi of(A–D) fossilAcrocephalus sp. compared with the corresponding element of(E–F) A.australisAM O.58015. (A–B) QM F57902, left carpometacarpus. (C–D) QM F30356, right carpometacarpus. Some characteristic features of Acrocephalus shown: 1, depression for origin of M. flexor digiti minoris deep, terminates distally of proc. pisiformis; 2, ventral fossa on distal end of os metacarpale minus large and deep; 3, fovea carpalis caudalis small and deep; 4, proc. intermetacarpalis moderately short and does not protrude beyond caudal edge of os metacarpale minus; 5, proc. dentiformis long, low and situated well distally of midpoint of os metacarpale minus. Scale = 2 mm.