Australlus gagensis, Worthy, 2011

Worthy, Trevor H., 2011, Australlus, a New Genus for Gallinula disneyi (Aves: Rallidae) and a Description of a New Species from Oligo-Miocene Deposits at Riversleigh, Northwestern Queensland, Australia, Records of the Australian Museum 63 (1), pp. 61-77 : 69-72

Australlus gagensis n.sp.

Figs 4–6 ; Table 2

Holotype. QM F54511 , complete right humerus ( Fig. 4 ).

Diagnosis. A flightless rail having the diagnostic characters of the genus Australlus that differs from A. disneyi by its considerably larger size (humeral proximal width and distal width are 117.5% and 116.5%, respectively, of the largest values for A. disneyi ) and by having the humeral facies bicipitalis in the area overlying the corpus depressed into a distinct depression relative to the intumescentia humeri and caput humeri; a near obsolete sulcus lig. transversus that is separated from the incisura capitis by a caudocranially thicker ridge; a less prominent proc. supracondylaris dorsalis; and proc. flexorius extending markedly further distally than the condylus ventralis.

with AM O.60402. %L is percentage length of the comparable vertebra in AM O.60402 ( Table 1).

Measurements (mm) of holotype. Length, 54.3; proximal width, 13.4; shaft width at mid-length, 4.4; maximum distal width, 11.3; depth of condylus dorsalis, 6.1.

Type locality. AL90 Site, Gag Plateau, Faunal Zone C, Riversleigh World Heritage Property, Boodjamulla (Lawn Hill) National Park, northwestern Queensland ( Archer et al., 1989, 1994; Creaser, 1997; Travouillon et al., 2006). Details of the site locality are available from the University of New South Wales or Queensland Museum on request.

Stratigraphy, age, fauna and depositional environment. AL90 Site is a fossil cave pitfall deposit formed within the freshwater limestone at the Riversleigh World Heritage Property. The site is considered part of Riversleigh’s Faunal Zone C deposits, which, based on stage of evolution of contained mammal taxa and faunal correlation, is Middle Miocene in age ( Archer et al., 1997, 2006; Creaser, 1997; Travouillon et al., 2006).

Paratype. QM F54536 , R coracoid lacking sternal end, AL90 Site ( Fig. 5 ) .

Measurements (mm) of paratype. Preserved length 25.0; length facies artic. humeralis from cotyla scapularis 5.3; maximum width proc. procoracoideus—lateral side cotyla scapularis 7.8; maximum length cotyla scapularis to proc. acrocoracoideus 7.4; width cotyla scapularis 4.5.

Referred material. QM F31479, distal L femur, Ringtail Site

(previously referred to A. disneyi ), referred to A. gagensis on basis of larger size than A. disneyi (midshaft width 5.6 mm, distal width> 13 mm, depth lateral condyle 11.5 mm); QM F54512 , distal L tarsometatarsus (missing trochleae metatarsi II and IV), Jim’s Jaw Site. This fragment is of a significantly larger rallid tarsometatarsus than those referred to A. disneyi , and for this reason it is tentatively referred to the new species. QM F54514 , vertebra number 6, from AL90 Site is referred to A. gagensis , because while it is of similar size (length) to those from, for example, White Hunter Site or G. tenebrosa AM O. 60402 ( Table 2), the centrum is proportionally wider and it is also much deeper, with the facies articularis caudalis 2.7 mm deep compared to 2.1 mm in G. tenebrosa AM O. 60402. The single rail bone from Golden Steph Site, QM F36354, vertebra number 22, also has a stouter centrum and a larger neural canal than G. tenebrosa AM O. 60402, though it does not differ much in length ( Table 2), so is also tentatively referred to A. gagensis .

Etymology. The species is named after the Gag Plateau, where fossil sites containing it were located.

Description

This species differs from A. disneyi by its larger size and diagnostic humeral characters listed above ( Fig. 4 ). The marked depression of the facies bicipitalis is lacking in A. disneyi and is likely associated with A. gagensis having evolved in flightless state for a longer period. Whereas in most Gallinula and Gallirallus species, there is no similar depression, in the flightless taxa that are the most divergent from the volant condition (such as Gallirallus australis and Gallinula mortierii ), a similar depression is seen, but in both of these species the depression is shallower. Similarly, the fossa pneumotricipitalis is shallower, and the crista deltopectoralis more thickened than in A. disneyi . The brachial fossa is relatively shallower than in A. disneyi and does not undercut the tuber. supracondylare ventrale. All these features likely are associated with this taxon having been flightless longer than A. disneyi . As in A. disneyi , ventrodorsal overlap of the condyli dorsalis and ventralis is apparent.

In addition, some features are further exaggerated from the condition seen in A. disneyi . For example, the crus dorsale fossae is relatively stouter and the tuber. ventrale is more massive and clearly shows the generic autapomorphic triangular arrangement of the three ligamental scars that are of near equal size: that for M. coracobrachialis caudalis is positioned dorsad of that for M. subcoracoideus, which is oriented somewhat ventrally, and that for M. subscapularis is distinctly craniad of the other two scars, not on the facies lining the incisura capitis, but rather is oriented caudoproximally. We do not consider that this structure of the tuber. ventrale is related to the flightless condition: in the humerus of all Gallirallus and Gallinula species, no matter how altered by their flightless condition, the tuberculum is caudocranially elongate, with the elongate scars for lig. m. subscapularis and lig. m. subcoracoideus positioned adjacent to each other and markedly cranial to a much larger ovate scar for lig. m. coracobrachialis caudalis on the caudal tip of the tuberculum. The arrangement described for Australlus was not observed in any other gruiform taxon examined.

The paratype coracoid ( Fig. 5 ) is larger and more robust than coracoids of A. disneyi with a shallower cotyla scapularis. It has a large and elongate foramen nervi supracoracoidei with foramina penetrating the corpus. The sharp-edged crista procoracoidei extends c. 14 mm from the cotyla scapularis sternally and merges with the shaft about 1.0 mm cranially and slightly ventrally of the crista medialis: if both crista were extended, they would overlap ventrodorsally. The proc. acrocoracoideus overhangs the sulcus m. supracoracoidei and, as in A. disneyi , is aligned at about 45 degrees in a dorsocranial–ventrosternal plane. However, it is less protuberant over the sulcus than A. disneyi , extending less medially than its height cranial to the sulcus (in A. disneyi , medial extent is similar to cranial extent). Unlike in A. disneyi , the impressio lig. acrocoracohumeralis is flat, lacking both a shallow sulcus and transverse groove. Also, in contrast to A. disneyi , the ventral facies of the shaft is broadly convex so that the adjacent lateral facies is rounded and lacks the distinct flattened surface seen in A. disneyi . As in A. disneyi , the proc. procoracoideus extends cranially of the cotyla scapularis. The preserved part of the impressio m. sternocoracoidei is deeper than in A. disneyi , and it is bound medially by a strongly overhanging crest.

The new species is also represented by a distal femur QM F31479, previously referred to A. disneyi , from which it is distinguished by much larger size ( Fig. 6 ). It has a distinct pit for the fibular stop and shows that the tuber. gastrocnemialis lateralis abuts the impressio m. ansa iliofibularis caudalis on the caudolateral border of the trochlea fibularis, as it does in Gallinula ventralis . This tuberculum is distinctly separated from the ansa impression in G. tenebrosa , G. mortierii and Porphyrio porphryio . The pit for the fibular stop is distinct and deep in G. ventralis , but shallow and indistinct in G. tenebrosa and G. mortierii . In species of Amaurornis and Porphyrio , it is distinct.

Figures

Fig. 4. Left humerus of Australlus gagensis (holotype) QM F54511 in caudal (A,B), dorsal (C), cranial (D), and ventral (E) views. Abbreviations: cb, crista bicipitalis; cd, condylus dorsale; cdf, crus dorsale fossae; cdp, crista deltopectoralis; cv, condylus ventrale; fp, fossa pneumotricipitalis; mcc, M. coracobrachialis caudalis; msc, M. subcoracoideus; mss, M. subscapularis; pf, processus flexorius; st, sulcus lig. transversus; td, tuberculum dorsale; tsv, tuberculum supracondylare ventrale; tv, tuberculum ventrale. Scale bar left of A is 10 mm (for A, C, D, E); scale bar left of B is 5 mm.

Fig.5. Right coracoid of Australlus gagensis (paratype) QM F54536 in ventral view (A) and dorsal view (B). Scale bar is 10 mm.Abbreviations: cp, crista procoracoidei; cs, cotyla scapularis; fah, facies artic. humeralis; fns, foramen nervi supracoracoidei; ia, impressio lig. acrocoracohumeralis; is, impressio m. sternocoracoidei; pa, processus acrocoracoideus; pp, processus procoracoideus; ss, sulcus m. supracoracoidei.

Fig. 6. Distal left femur, QM F31479, referred to A. gagensis in anterior (A), caudal (B), medial (C) and lateral (D) views.Abbreviations: cm, condylus medialis; ia, impressio M. ansa iliofibularis caudalis; pit, pit for the fibular stop; tgl, tuberculum gastrocnemialis lateralis; tf, trochlea fibularis. Scale bar is 10 mm.