Megalurus 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 : 218-221

Megalurus sp.

Figs 15–16

Material. QM F30820, QM F30852, right carpometacarpi; QM F57934 ( AR 19823), left tarsometatarsus; QM F30360, proximal right tarsometatarsus; QM F57935 ( AR 21602), QM F57936 , QM F57937 , distal left tarsometatarsi.

Measurements (mm). QM F30360: preserved length 11.5, proximal width ca 3.2, proximal depth (lateral aspect)>2.1. QM F30820: preserved length 13.9, proximal width 3.4, length of os metacarpale alulare 1.9, distal width 2.7. QM F30852: preserved length 11.5, proximal width 3.1, length of os metacarpale alulare 1.5, distal width 2.4. QM F57934 : preserved length 24.8, proximal width 2.8, proximal depth (cotyla lateralis to hypotarsus) 2.4, distal width ca 2.3, depth of tr II 0.9, depth of tr III 1.2, depth of tr IV 0.9. QM F57935 : preserved length 7.8, distal width 2.3, depth of tr II 1.0, depth of tr III 1.2, depth of tr IV 1.0. QM F57936 : preserved length 11.7, distal width 2.2, depth of tr II 0.9, depth of tr III 1.1, depth of tr IV 0.9. QM F57937 : preserved length 3.4, distal width 2.5, depth of tr II 1.0, depth of tr III 1.4, depth of tr IV>0.8.

Description and comparisons. Carpometacarpus. QM F30820 ( Fig. 15A,D ) has damage to the trochlea carpalis and the distal end, and most of the os metacarpale minus is broken off. QM F30852 ( Fig. 15B,E ) is near complete with minor damage to the proc. alularis and the distal end of the os metacarpale majus. These fossil carpometacarpi are referred to Megalurus sensu lato ( Alström et al., 2011; Gill & Donsker, 2016) because they exhibit the following suite of features. The cranial extent of the proc. extensorius is far greater than that of the proc. alularis. The trochlea carpalis ventralis only slightly extends caudally beyond the os metacarpale minus. The proc. cranialis is long. The depression for the origin of M. flexor digiti minoris terminates distally of the proximal edge of the proc. pisiformis. The proc. intermetacarpalis does not protrude or only slightly protrudes beyond the caudal edge of the os metacarpalis minus. In dorsal view, the spatium intermetacarpale proximally of the proc. intermetacarpalis is visible and moderately wide. The proc. dentiformis is situated distally of the proximo-distal midpoint of the os metacarpale majus. In caudal aspect, the dorsal edge of the os metacarpale minus is slightly undulated. The sulcus interosseus is shallow.

QM F30820 falls within the size range observed for the carpometacarpi of M. cruralis and M. mathewsi ( Fig. 15C,F ), whereas QM F30852 is slightly larger than the corresponding bone of M. timoriensis . These fossils differ from M. carteri in being larger in size and proportionately longer. They further differ from M. carteri in that the os metacarpale minus is straight, not slightly curved cranio-caudally, and the fovea carpalis caudalis is deeper. In the fossils, the proc. dentiformis is more pointed and situated further distally on the os metacarpale minus than in M. carteri . QM F30820 differs from M. timoriensis in that the fossa for the M. flexor digiti minoris and the ventral fossa on the distal end of the os metacarpale minus are deeper. Both fossils differ from M. gramineus in that the distal end is squarer and less pointed. The fossil carpometacarpi differ from M. mathewsi and M. cruralis in that the trochlea carpalis dorsalis does not extend as far proximally. QM F30852 further differs from M. mathewsi and M. cruralis in having a shallower ventral fossa on the distal end of the os metacarpale minus. It differs from M. cruralis in having a shallower fovea carpalis caudalis. QM F30852 differs from QM F 30820 in having a slightly more prominent proc. dentiformis, a notched distal end and a shallower ventral fossa on the distal end of the os metacarpale minus.

Tarsometatarsus. QM F57934 ( Fig. 16B ) is a nearcomplete tarsometatarsus, with the hypotarsus and proximal part of the crista plantaris lateralis broken off and the trochleae metatarsorum II and IV abraded. QM F30360 ( Fig. 16C ) preserves the proximal tarsometatarsus, whereas QM F57935 ( Fig. 16G ), QM F57936 ( Fig. 16D ) and QM F57937 ( Fig. 16E ) preserve the distal end. QM F 57936 in particular bears shallow punctures, a deep scratch and some breakage of the shaft, as well as damage to the trochleae metatarsorum. These fossil tarsometatarsi are assigned to Megalurus sensu lato ( Alström et al., 2011; Gill & Donsker, 2016) because they possess the following combination of character states. On the proximal end, the tuberositas m. tibialis cranialis is distally adjacent to the arcus extensorius and located medially of the shaft midpoint. The lateral foramen vasculare proximale is large. The crista plantaris lateralis is deep. In medial aspect, the shaft is very shallow and forms a sharp crest. The impressio lig. collateralis medialis is oval-shaped and at about level with or proximally overlaps with the arcus extensorius. The sulcus flexorius on the proximal half of the tarsometatarsus is very shallow.

On the distal tarsometatarsus, the foramen vasculare distale is small and situated far proximally of the incisura intertrochlearis lateralis by a distance about equal to or greater than twice the length of the trochlea metatarsi IV. The trochlea metatarsi II is about similar in width to the trochlea metatarsi III. There is a moderately deep furrow circumscribed along the entire trochlea metatarsi II that becomes deeper plantarly. The distal profile of the trochlea metatarsi II is notched, and the medial rim of this trochlea extends far distally of its lateral counterpart. The medial portion of the trochlea metatarsi II is greater in dorsal extent than the lateral portion. The incisura intertrochlearis medialis is narrow and relatively shorter than the lateral incisura. In dorsal aspect, the lateral margin of the trochlea metatarsi IV is situated medially of that of the shaft. The distal edge of the trochlea metatarsi IV is at a disto-lateral angle to the long axis of the shaft.

The fossil tarsometatarsi are similar in size to the corresponding bone of the Tawny Grassbird M. timoriensis ( Fig. 16A ). They also fall within the size range observed for songlarks M. mathewsi and M. cruralis ( Fig. 16H ), both of which exhibit sexual size dimorphism ( Higgins et al., 2006). In QM F57935 and QM F57937 the medial edge of the trochlea metatarsi II is a pronounced bulge that abruptly protrudes from the shaft margin ( Fig. 16E,G ), which is characteristic of M. mathewsi and M. cruralis . In QM F57934 and QM F57936 , however, the trochlea metatarsi II does not protrude as far medially ( Fig. 16B,D ). The medial edge of this trochlea gradually merges with that of the shaft, as in M. gramineus , M. timoriensis and M. carteri .

Apart from the medial projection of the trochlea metatarsi II, the tarsometatarsal morphology is very similar among the species of Megalurus examined. The fossils differ from the extant species in a few features. They differ from M. carteri in that they are larger in size and proportionately more elongate. The fossils that preserve the proximal tarsometatarsus (QM F30360 and QM F57934 ) differ from M. carteri in that the impressio lig. collateralis medialis is more developed. The fossils differ from M. carteri and M. gramineus because the crista plantaris lateralis terminates at about level with the fossa metatarsi I, not proximally of the fossa. They further differ from M. carteri and M. gramineus , and from M. timoriensis , in that the medial rim of the trochlea metatarsi II extends slightly further distally relative to the lateral rim. QM F30360 and QM F57934 differ from M. timoriensis in having a slightly more elevated tuberositas m. tibialis cranialis. The fossil distal tarsometatarsi (except for QM F57937 ) differ from M. mathewsi in that the foramen vasculare distale is situated proximally of the incisura intertrochlearis lateralis by a distance of about twice the length of the trochlea metatarsi IV. In QM F57937 and M. mathewsi , however, this distance is greater. The fossils differ from M. cruralis in having a slightly shallower sulcus extensorius on the distal tarsometatarsus.

QM F57934 and QM F57936 differ from QM F 57935 in having a more prominent crista plantaris lateralis on the distal shaft. QM F30360 differs from QM F 57934 in having a deeper sulcus extensorius. Owing to the condition of the fossils and the overall similarity in tarsometatarsal morphology across species of Megalurus , the fossils cannot be confidently determined to species level.

Genus et species indet.

Figs 15G , 16F

Material. QM F57941 , distal left tibiotarsus; QM F30375, distal right tarsometatarsus.

Measurements (mm). QM F30375: preserved length 2.0, distal width>1.7, depth of tr II 0.9, depth of tr III ca 1.2. QM F57941 : preserved length 9.4, distal width 2.9, depth of condylus lateralis 2.6, depth of condylus medialis 2.9.

Description and comparisons. Tibiotarsus. QM F57941 ( Fig. 15G ) is tentatively referred to Locustellidae because of the following suite of features. In cranial view, the condylus lateralis is wider than the condylus medialis. The condylus medialis is moderately displaced medially relative to the shaft edge. The incisura intercondylaris is wide and its distal profile is asymmetrical, with the medial portion being more proximally excavated than the lateral portion. The tuberositas retinaculi extensori lateralis occupies most of the lateral half of the pons supratendineus and does not protrude beyond the proximal edge of the pons. The bony ridges for attachment of the retinaculum m. fibularis are long and prominent flanges. The tuberositas retinaculi extensori medialis is mostly broken off in the fossil but part of its base is preserved. The base of this tuberosity indicates that its distal edge is at about the level of the bony ridges for the retinaculum m. fibularis, as in extant locustellids studied. The trochlea cartilaginis tibialis is moderately wide.

The fossil tibiotarsus corresponds in size to that of M. mathewsi ( Fig. 15H ). A tuberositas retinaculi extensori lateralis that occupies the lateral half of the pons supratendineus was also observed in members of Dasyornithidae , Orthonychidae , Cinclosomatidae , Corvidae , Corcoracidae and Turdidae . QM F57941 can be excluded from Corvidae and Turdidae because the condylus medialis is narrower than the condylus lateralis, not equal in craniocaudal width. The fossil is excluded from Orthonychidae and Cinclosomatidae because the shaft is narrow compared to the width of the distal end, not considerably broad. QM F57941 is excluded from Corcoracidae because the bony ridges for the retinaculum m. fibularis are well developed, not low. It is excluded from Dasyornithidae because the shaft is narrower with respect to the distal end and the lateral bony ridge for the retinaculum m. fibularis is a prominent flange.

Tarsometatarsus. QM F30375 ( Fig. 16F ) preserves only the trochleae metatarsorum II and III. This fossil specimen, however, is distinctive enough to allow its assignment to Locustellidae because it possesses the following features. The widths of the trochleae metatarsorum II et III are near equal. There is a deep furrow on the trochlea metatarsi II, resulting in a notched distal profile. The medial rim of the trochlea metatarsi II is greater in distal extent than the lateral rim.Although it is damaged, the preserved medial rim of this trochlea is dorsally elevated from the lateral rim.

The tarsometatarsi of locustellids are similar to that of Acrocephalus australis in that they share a distinctly deep furrow on the trochlea metatarsi II. However, QM F30375 can be excluded from Acrocephalus because it has a trochlea metatarsi II that is about equal in width to the trochlea metatarsi III, not narrower. In the fossil the medial and lateral rims of the trochlea metatarsi III are of equal distal extent, whereas in Acrocephalus the medial rim is greater in distal extent. It further differs from Acrocephalus because the notch in the distal profile of the trochlea metatarsi II is deeper. In dorsal aspect, the incisura intertrochlearis medialis is narrow but the trochleae metatarsorum II et III are clearly separated. In Acrocephalus , however, this incisura is much narrower and the trochleae metatarsorum II et III are in contact.

Remarks. Locustellidae is a family of small insectivorous passerines that are represented in Africa, Eurasia and Australasia ( Alström et al., 2011). In Australia, this family is represented by five species in the genus Megalurus (as recognised by Gill & Donsker, 2016) which were formerly assigned to Megaluridae (Christidis & Boles, 2008) . Songlarks, M. cruralis and M. mathewsi (sometimes placed in Cincloramphus), are endemic to Australia and are commonly found in open eucalypt woodlands and forests with scattered trees, grasslands, shrublands and savanna. Grassbirds, M. timoriensis and M. gramineus , occur in reedbeds, grasslands, swamps, marshlands and wet coastal heathlands in Australo-Papua and Southeast Asia. The Spinifexbird, M. carteri (previously in Eremiornis ), is endemic to Australia and inhabits spinifex ( Triodia ) grass near ranges and along watercourses ( Pringle, 1982; Bairlein et al., 2006; Higgins et al., 2006).All of these species forage for small invertebrates and occasionally seeds on or near the ground. The Rufous Songlark, M. mathewsi, Brown Songlark , M. cruralis and M. timoriensis , occur in the Riversleigh region today.

In Victoria, late Quaternary fossils of M. timoriensis and M. cruralis have been recorded from Mabel Cave; those of M. mathewsi were reported from Harman’s Cave; and remains of M. cruralis , M. mathewsi and Megalurus (Cincloramphus) sp. were identified from Cloggs Cave ( Baird, 1986, 1991a). Late Pleistocene material referred to M. cruralis and cf. Megalurus (Cincloramphus) were found in Koonalda Cave in the Nullarbor region, Western Australia ( Baird, 1991a). The specimens described here, which are at least early Pleistocene in age, represent the geologically oldest fossils of Locustellidae found in Australia. Fossils of Locustella spp. were reported from late Miocene and Pliocene deposits in Hungary ( Kessler, 2013).

The fossil tarsometatarsi QM F57935 and QM F57937 share with songlarksa characteristically protuberant trochlea metatarsi II. In QM F57934 and QM F57936 , however, the trochlea metatarsi II is not protuberant but gradually merges with the medial shaft edge, as in grassbirds and spinifexbirds. The differences in morphological features among these specimens indicate that at least two locustellid species were present in the Rackham’s Roost avifauna.

Figures

Figure 15. Comparison of (A–B, D–E, G) fossil bones of Megalurus sp. with those of (C,F,H) extant M. mathewsi AM O.59292. (A,D) QM F30820, right carpometacarpus. (B,E) QM F30852, right carpometacarpus. (G) QM F57941 distal left tibiotarsus. Some distinguishing features of Megalurus labelled: 1, cranial extent of proc. extensorius far greater than that of proc. alularis; 2, depression for attachment of M. flexor digiti minoris terminates distally of proximal edge of proc. pisiformis; 3, proc. dentiformis situated distally of midpoint of os metacarpale majus; 4, proc. cranialis long; 5, sulcus interosseus shallow; 6, distal edge of tuberositas retinaculi extensoris medialis at about level with bony ridges for retinaculum m.fibularis; 7, bony ridges for attachment of the retinaculum m. fibularis long and prominent flanges; 8, tuberositas retinaculi extensori lateralis occupies most of lateral half of pons supratendineus and does not protrude beyond its proximal edge; 9, condylus lateralis wider than condylus medialis; 10, incisura intercondylaris wide and its distal profile asymmetrical.Scale = 2 mm.

Figure 16. Tarsometatarsi of (B–G) fossil Megalurus sp. compared with the corresponding bones of (A) M. timoriensis AM O.65209 and (H) M. cruralisAM O.59220 in dorsal view.(B) QM F57934, left tarsometatarsus;(C) QM F30360, proximal left;(D) QM F57936, distal left; (E) QM F57937,distal left;(F) QM F30375, distal right;(G) QM F57935, distal left.Some distinguishing features labelledas follows: 1, impressio lig. collateralis medialis at about level with arcus extensorius; 2, lateral foramen vasculare proximale large; 3, tuberositas m. tibialis cranialis located distally adjacent of arcus extensorius and medially of shaft midpoint; 4, foramen vasculare distale situated proximally of incisura intertrochlearis lateralis by distance of ≥ 2× length of tr IV; 5, tr II similar width to tr III; 6, incisura intertrochlearis medialis narrow; 7, medial rim of tr II greater in distal and dorsal extents than lateral rim; 8, distal edge of tr IV at disto-lateral angle to shaft long axis. Arrows indicate different extent of medial protrusion of trochlea metatarsi II. Scale = 2 mm.