Cinclosoma elachum, Nguyen & Archer & Hand, 2018

Nguyen, Jacqueline, Archer, Michael & Hand, Suzanne, 2018, Quail-thrush birds from the Miocene of northern Australia, Acta Palaeontologica Polonica 63 (3), pp. 493-502 : 495-498

Cinclosoma elachum sp. nov.

Fig. 1 .

Etymology: From Latinised Greek elachum , little; neuter in gender. It is suggested that this new species be informally known as the Little Quail-Thrush.

Type material: Holotype: QM F57949 , right carpometacarpus with distal end broken off . Paratype: QM F57951 , distal left tibiotarsus from Neville’s Garden Site , Riversleigh World Heritage Area, Queensland, Australia .

Type locality: Wayne’s Wok Site , Riversleigh World Heritage Area, Queensland, Australia .

Type horizon: Wayne’s Wok Site is located on Hal’s Hill in the central section of the D Site Plateau at Riversleigh. Neville’s Garden Site occurs on the northern edge of this plateau ( Creaser 1997). Wayne’s Wok and Neville’s Garden Local Faunas are part of Riversleigh’s Faunal Zone B and are interpreted to be early Miocene in age based on biocorrelation, stratigraphy, geochronology, and multivariate analyses ( Archer et al. 1989; Creaser 1997; Travouillon et al. 2006; Arena et al. 2015, Woodhead et al. 2016). The two assemblages are interpreted as samples of the same early Miocene Riversleigh palaeocommunity by Myers et al. (2017). The early Miocene age estimate was confirmed by U/Pb radiometric dating of fossil speleothems at Neville’s Garden Site (18.24 ± 0.29 Ma and 17.85 ± 0.13 Ma; Woodhead et al. 2016).

Material.— Type material and tentatively referred specimen ( QM F57948 ), a proximal left humerus from Neville’s Riches Site, Riversleigh World Heritage Area , Queensland, Australia. Neville’s Riches Site is tentatively assigned to Faunal Zone C (middle Miocene) until more taxa from this site are recovered and studied to allow confident biocorrelation ( Archer et al. 1989) .

Diagnosis.—Slightly smaller in size than Cinclosoma cinnamomeum , the smallest living species of this genus ( Higgins and Peter 2002; Boles 2007). The proximal length and width of the carpometacarpus of Cinclosoma elachum sp. nov. is about 15% and 10% smaller, respectively, than the corresponding bone of C. cinnamomeum . The distal width of the tibiotarsus of the new species is about 18% narrower than that of C. cinnamomeum . The new species is distinguished from extant species of Cinclosoma that were examined in this study in the following features. Carpometacarpus: dorsal portion of trochlea carpalis proportionately shorter; fovea carpalis caudalis markedly deeper; sulcus tendinosus terminates further proximally. Tibiotarsus: shaft more cranio-caudally compressed; condyli slightly narrower; distal edge of pons supratendineus at about level with proximal edges of condyles, rather than being slightly proximally located.

Differs from C. punctatum in having a more proximally located proc. cranialis; a larger proc. dentiformis; a relatively shorter fovea lig. ventralis; a shallower sulcus extensoris; a narrower sulcus m. fibularis; and a more distinct ridge on trochlea cartilaginis tibialis.

Differs from C. castanotum in having a deeper fossa infratrochlearis; a deeper fovea lig. ventralis; a larger opening located proximally of proc. intermetacarpalis; longer and more prominent bony ridges for the retinaculum m. fibularis; a tuberositas retinaculi extensori lateralis that extends further proximally beyond the edge of the pons supratendineus; and a shallower impressio lig. intercondylaris.

Differs from C. castaneothorax in having a relatively shorter fovea lig. ventralis; a more developed proc. cranialis; a longer proc. intermetacarpalis; a larger proc. dentiformis; a relatively longer tuberositas retinaculi extensori medialis; longer bony ridges for retinaculum m. fibularis; and a deeper impressio lig. intercondylaris.

Differs from C. cinnamomeum in having a more proximally located proc. cranialis; a larger opening situated proximally of proc. intermetacarpalis; a more prominent proc. dentiformis that is situated slightly more proximally on the os metacarpale majus; more prominent bony ridges for the retinaculum m. fibularis; and a tuberositas retinaculi extensori lateralis that extends further proximally beyond the edge of the pons supratendineus.

Although specimens of C. ajax were not available for study, it is specifically distinct from C. elachum sp. nov. in its larger size ( Boles 2007).

Measurements (in mm).—QM F57949, carpometacarpus: preserved length 9.2; proximal width 3.8; proximal length 2.7; length of os metacarpale alulare 2.1. QM F57951, tibiotarsus: preserved length 14.4; distal width c. 3.3; depth of condylus lateralis>2.9; depth of condylus medialis> 3.1.

Description. —QM F57949 ( Fig. 1A ) is a proximal right carpometacarpus with abrasion to the ventral portion of the trochlea carpalis and the proc. pisiformis and breakage of the distal end. QM F57951 ( Fig. 1C ) preserves the distal end and shaft of a left tibiotarsus that is partly covered with black mineral dendrites and has minor breakage to the cristae trochleae. The fossils appear to represent adult individuals because they do not show signs of incomplete ossification, such as pitted appearance of the bone surfaces and visible fusion lines, as observed in immature birds ( Campbell 1979; Watanabe and Matsuoka 2013). In addition to the traits that characterise cinclosomatids and those used to diagnose the new species, Cinclosoma elachum sp. nov. exhibits the following features.

The proximal end of the carpometacarpus (QM F57949) is broad and bears a moderately deep fossa infratrochlearis. Viewed cranially, the trochlea groove is very shallow. The dorsal portion of the trochlea carpalis is long. Although the apex of the proc. pisiformis is broken, its base indicates that it is broad like in extant species of Cinclosoma examined. The depression on the caudal surface of the os metacarpale minus and caudo-ventral side of the trochlea carpalis is distinctly deep and terminates proximally of the proc. pisiformis. The proc. cranialis at the proximal end of the sulcus tendinosus is low. As in other cinclosomatids, the proc. intermetacarpalis is completely fused to the os metacarpale minus and does not protrude beyond its caudal edge. At about level with the proc. intermetacarpalis is a low protuberance on the os metacarpale minus. The proc. dentiformis is triangular in cranial profile. The os metacarpale minus is incompletely preserved but, based on its proportions and those of the proximal end, the proc. dentiformis appears to be situated well distally of its midpoint.

The cranial and caudal shaft surfaces of the tibiotarsus (QM F57951) are planar. The sulcus extensorius is very shallow. The tuberositas retinaculi extensori medialis is well developed and slightly overhangs the sulcus extensorius. On the cranio-lateral surface of the shaft there are two long ridges that serve as attachment points for the retinaculum m. fibularis. These ridges distally terminate at about level with the pons supratendineus. The lateral bony ridge for this retinaculum is longer than its medial companion. The pons supratendineus is slightly angled disto-laterally. The tuberositas retinaculi extensori lateralis is situated on the lateral part of the pons and extends well proximally beyond the edge of the pons. There is a large foramen immediately proximally of the condylus lateralis. The condyles of the tibiotarsus are long in cranial view. There is a pronounced bump on the condylus medialis, located proximally of the low epicondylus medialis. Viewed laterally, the condylus lateralis is circular in shape and the epidondylus lateralis is indistinct. There is a low but distinct proximo-distal ridge that is centred on the trochlea cartilaginis tibialis. This ridge extends along the caudal and distal surfaces of the trochlea. The presence of this ridge is intraspecifically variable in the extant species of Cinclosoma examined. The cristae trochleae are damaged in the fossil but, from what is preserved, the trochlea appears to have been wide as in extant cinclosomatids. Viewed distally, the condyles converge caudally. The condylus medialis protrudes further cranially than its lateral counterpart.

QM F57948 ( Fig. 1E ) is a proximal left humerus with damage to the tub. ventrale, crus dorsale fossae, and incisura capitis region. The pneumotricipital fossa area is encrusted with black mineral dendrites. The fossil humerus is 8.7 mm long and 7.0 mm wide, and its crista deltopectoralis is 4.0 mm long proximo-distally. QM F57948 shares the following features with species of Cinclosoma : caput humeri broad and dome-like; tub. dorsale large and directed caudo-dorsally; crista deltopectoralis reduced; crista bicipitalis moderately short; ventral pneumotricipital fossa pneumatic; scar for insertion of M. scapulohumeralis cranialis shallow; and shallow fossa distal to the caput humeri and ventrally bound by margo caudalis. In extant species of Cinclosoma studied, the crista bicipitalis extends far ventrally of the tub. ventrale. In the fossil, the tub. ventrale is broken off but the preserved edge of the crista bicipitalis protrudes well ventrally of this region. The proximal profile of the ventral part of the caput humeri is flat in the fossil, but this feature was observed to be intraspecifically variable in specimens of extant species of Cinclosoma .

QM F57948 differs from species of Ptilorrhoa examined in having a less developed margo caudalis; a shallower sulcus lig. transversus; in caudal view, a shallow excavation beneath the caput humeri; and the distal edge of the crista bicipitalis forming a less obtuse angle from the shaft. In QM F57948 the crista deltopectoralis terminates slightly distally of the distal-most point of the crista bicipitalis, whereas in species of Ptilorrhoa it terminates further distally from the crista bicipitalis.

The fossil humerus differs from species of Psophodes in having a better developed crista deltopectoralis; a caudo-dorsally directed tub. dorsale, rather than cranio-dorsally; a proportionately longer crista bicipitalis; a round and shallow scar for M. scapulohumeralis cranialis, rather than an elongate and deep fossa; and a fossa situated ventrally of the margo caudalis and distally to the caput humeri.

Owing to its fragmentary condition, QM F57948 is tentatively assigned to Cinclosoma elachum sp. nov. because of its small size and based on the relative sizes of the fossil carpometacarpus QM F57949 and tibiotarsus QM F57951. It differs from extant species of Cinclosoma studied in having a relatively shorter margo caudalis and a more distinct linea m. latissimi dorsalis. The fossil differs from C. punctatum , C. castaneothorax , and C. cinnamomeum in that the crista deltopectoralis terminates slightly distally, instead of at about level, of the distal-most point of the crista bicipitalis. It differs additionally from C. castanotum in having a shallower fossa ventrally of the margo caudalis.

Stratigraphic and geographic range. —Early and middle Miocene; Wayne’s Wok Site, Neville’s Garden Site, Neville’s Riches Site, Riversleigh World Heritage Area, north-western Queensland, Australia.

Figures

Fig. 1. Quail-thrush bird Cinclosoma elachum sp. nov. (A, C, E) from the Riversleigh World Heritage Area, Australia, compared with the Recent Cinclosoma punctatum (Shaw, 1794) (B, D, F). A. QM F57949 (holotype), early Miocene Wayne’s Wok Site, right carpometacarpus in ventral (A1) and dorsal A2) views. B. AM O.67887, left carpometacarpus (mirrored) in ventral (B1) and dorsal (B2) views. C. QM F57951 (paratype), early Miocene Neville’s Garden Site, distal left tibiotarsus in cranial view. D. AM O.67887, distal left tibiotarsus in cranial view. E. QM F57948, middle Miocene Neville’s Riches Site, proximal left humerus in caudal view. F. NMV B.12648, proximal left humerus in caudal view. Scale bars 2 mm.