Australlus disneyi (Boles, 2005)

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 : 64-69

Australlus disneyi (Boles, 2005)

Gallinula disneyi Boles, 2005: 182 .

Enhanced description

In the interval since Boles (2005a) described Australlus disneyi , more material has become available that enables a more comprehensive description of this taxon and facilitates comparison with other rallids.

Cranium ( newly referred specimen QM F23806 , Camel Sputum Site) .

A partial cranium ( Fig. 1 ) preserving in one fragment areas caudad of the interorbital zone with loss of the rostrum basisphenoidale and the frontoparietal areas on the left side, but relatively complete on the right side with just the loss of the tip of the proc. postorbitalis and tip of the proc. zygomaticus. Associated fragments include the right side of the nasofrontal hinge and the part of the os mesethmoidale that lies ventral to it ( Fig. 1 ). Measurements: width at temporal fossae, 19.7 mm; cranial height, 20.4 mm; estimated squamosal width, 21.6 mm; foramen magnum width, 6.0 mm, height 5.8 mm.

This cranium is very similar to, though slightly larger than, that of Gallinula tenebrosa . It comes from the same locality as two of the paratypes for Australlus disneyi , and it is of expected size, given typical rallid proportions, for that species. The os lacrimale is unfused, and the os nasale fused with the os frontale dorsal to the lacrimal facet. The nasofrontal hinge has a flexible joint and the same structure as in Gallinula . The fonticuli orbitalocraniales are of a similar relatively small size located in the dorsal half of the orbits. The caudal orbital margins are drawn out farther than in Gallinula , with sharper, more compressed margins, which more broadly overhang the orbits. The fossa temporalis is as shallow as in Gallinula and the crista temporalis is minimally separated from the crista nuchalis transversus by 3 mm. While the proc. postorbitalis is mostly lost, its remaining base suggests that it was relatively small and, likewise, the proc. zygomaticus could only have been small. The crista nuchalis transversus is distinct from a point dorsal to the proc. paroccipitalis to dorsal to the foramen magnum, and there is slight inflation of the prominentia cerebellaris to form hollows either side of it. The recessus tympanicus is relatively longer than in Gallinula ; the proc. paroccipitalis forms a prominent caudal wall strongly linked to the squamosal margin of the recess, enclosing a broad sulcus lateral to the fenestra vestibule, which forms the caudolateral buttress for the cotylae quadratica otici. This area is relatively smaller and much more fenestrated in Gallinula species. The basisphenoid structure does not differ significantly from the conformation seen in Gallinula ventralis , except that the anterior parts of the lamina parasphenoidalis are a little more inflated.

Mandible ( newly referred specimen QM F54503 , White Hunter Site) .

Most of the paratypic material for A. disneyi was recovered from White Hunter Site, so it is possible that this mandible derives from one of the same individuals. This specimen is a single mandible missing its tip ( Fig. 2 ). During preparation, the os dentale fragments were disassociated from the more posterior parts of the mandible, which are preserved in their original depositional relationship to each other by a bridge that remains from the resin used to join fragments of the specimen together when it was in limestone. Measurements (mm): greatest width across the cotyla lateralis, 22.0; width between the tip of the proc. mandibulae medialis and proc. retroarticularis, 7.6; maximum depth, left side of the mandible, 6.5.

The overall form and proportions of the mandible are similar to those of Gallinula tenebrosa and it has a similar ventral depression of the anterior two-thirds of the os dentale. The fenestra caudalis mandibulae is small, but it is located relatively rostrally compared to all rallids examined, such that the distance to the cotyla lateralis exceeds the length of the cotyla lateralis; in other rallids, the cotyla lateralis is longer. The proc. mandibulae medialis houses a foramen pneumaticum dorsally near the tip, and it is larger and relatively more robust toward its tip than that of G. tenebrosa . The cotyla medialis is larger and protrudes over the medial profile in dorsal view to make a distinct angular prominence, much more so than in species of Gallinula . The cotyla medialis also extends farther laterad, such that it extends past the base of the surangular and is wider than the cotyla lateralis, whereas it and the cotyla lateralis have approximately similar widths in Gallinula species, with the base of the surangular at their junction. The proc. retroarticularis is more robust, but shorter, unlike the thin blade-like structure in Gallinula species. The dorsal margin of the crista transversus fossa is nearly flat with only a very shallow notch at mid-width, whereas it is more deeply notched in all compared rallids. A further significant difference from Gallinula species is seen in the profile of the lateral facies ventral to the cotyla lateralis: in QM F54503 , it is concave below the lip of the cotyla, whereas in species of Gallinula , it is convex.

Premaxilla ( newly referred specimens: QM F30864, Wayne’s Wok Site; QM F54505 , Camel Sputum Site) .

These premaxillae are both deeply vaulted ventrally between sharp tomial edges. Laterally, they have several vascular foramina; and the tips over their last 5 mm are slightly depressed relative to the more posterior tomial margin. These fossils are thus rather similar to the premaxilla of Gallinula tenebrosa . The referred cranium of G. disneyi was found in the samples from Camel Sputum Site. Measurements (mm) taken at anterior side of nasal aperture: QM F30864, length to tip, 11.3; width, 4.8; height, 4.2; QM F54505 , length to tip, 11.5; width, 4.6; height, 4.7.

Vertebrae (newly referred specimens: QM F54506 , cervical vertebra number 15 [where number 1 is the atlas vertebra], White Hunter Site; QM F54504 , cervical vertebra number 12, White Hunter Site; QM F54507 , cervical vertebrae number 11, White Hunter Site; QM F54508 , cervical vertebrae number 14, White Hunter Site; QM F54515 , cranialmost 4 fused vertebrae of synsacrum, White Hunter Site; QM F54509 , cervical vertebra number 10, Dirk’s Towers Site; QM F54510 , anterior part synsacrum, Dirk’s Towers Site). These vertebrae were compared with, and are very similar to, those of Gallinula tenebrosa ( Tables 1, 2), G. ventralis and Gallirallus philippensis , and they were easily assigned to position. All came from sites where long bones of A. disneyi were also recovered, so they are referred to that species.

Humerus ( QM F20906, proximal R, holotype, White Hunter Site, QM F31471, proximal R; QM F31472, distal L; newly referred proximal and shaft L, QM F45457 White Hunter Site) .

In addition to the features described by Boles (2005a) for these specimens, we note the following:

1. The tuber. ventrale in A. disneyi is relatively robust and dorsoventrally wide compared to the condition in all rallids examined, in which it is typically dorsoventrally compressed and caudocranially elongated. Associated with this robust tuberculum is an autapomorphic arrangement of the insertion scars for the three ligaments inserting thereon. Typically in rallids, a large ovate scar for the insertion of M. coracobrachialis caudalis occupies the caudal tip of the tuberculum, with an elongate scar for lig. m. subscapularis positioned distinctly cranial to it on the facies bounding the incisura capitis, with a smaller ovate scar for the M. subcoracoideus adjacent to this but on the ventral facies of the tuberculum (following the terminology of Ashley, 1941). In A. disneyi , the holotypic specimen QM F20906 has damage to the cranial part of the tuber. ventrale and QM F31471 is more damaged, but QM F20906 reveals that the scar for the M. subcoracoideus is centred slightly farther caudocranially, but is essentially beside that for the M. coracobrachialis caudalis and is of similar size, rather than being entirely and distinctly cranial to it and smaller. This arrangement of ligament scars was not found in any Recent gruiform examined, but it is seen in the new species described below.

2. The caudal facies of the shaft distal to the crus dorsale fossae is angular.

3. The crus dorsale fossae is more robust than in species of Gallinula and Amaurornis .

4. The tuber. dorsale is relatively small, but prominent.

5. The crista bicipitalis is relatively small in A. disneyi with its distal junction with the shaft positioned slightly distal to the distal end of the crus dorsale fossae, resulting in its largely being occluded in caudal view.

6. The fossa pneumotricipitalis is shallow, and it is not penetrated by any pneumatic foramina.

7. The sulcus lig. transversus is much shallower in A. disneyi than in species of Gallinula , including G. (Tribonyx) mortierii , Amaurornis and most Gallirallus species, and is separated from the incisura capitis by a caudocranially thicker ridge. A similar condition is seen in Gallirallus australis .

8. The ventral side of the crista deltopectoralis has a distinct tubercle, which is variably present in Gallinula . A tubercle is present in Gallinula (Tribonyx) mortierii but not in G. (Tribonyx) ventralis . A tubercle is not present in the flightless and more modified members of the Gallirallus clade, such as Gallirallus australis and Gallirallis sylvestris , so its presence is not contingent on flightlessness.

9. The angulus cristata of the crista deltopectoralis is located distal to the crista bicipitalis and the part of the crista deltopectoralis distal to the angulus is less than the length proximad of the angulus. In contrast, the angulus lies proximal to the junction of the crista bicipitalis and the shaft, and the distal part of the crista deltopectoralis (i.e., that past the angulus) is longer than the proximal section in Gallinula and Gallirallus , including in the flightless taxa compared.

10. The crista deltopectoralis terminates distally on the cranial facies as opposed to on the dorsocranial margin as in most Gallinula and Gallirallus species. In flightless taxa, it is located more ventrally.

11. The crista deltopectoralis is markedly concave caudodorsally, rather than flat to convex.

12. The tuber. supracondylare ventrale is relatively small, not extending proximad so far as to be level with proximal margin of the condylus dorsalis, is wider than high, is relatively close to the condylus ventralis, and has the dorsal part of the articular facet angled dorsodistally, forming a wide angle with the ventral part of the facet. In A. disneyi , the fossa m. brachialis undercuts the tuberculum. In species of Gallinula and Amaurornis , the tuber. supracondylare ventrale is about as wide as long, has a similar proximal extent as the condylus dorsalis, has the dorsal part of the facet angled dorsodistally, and is distinctly separated from the condylus ventralis. In some Gallirallus species, such as G. philippensis , the tuber. supracondylare ventrale is narrower ventrodorsally, with its dorsal side directed significantly dorsally, so that it is nearly at right angles to the ventral side of the tuberculum. In the flightless G. australis , however, the tuberculum is wide and short and relatively small, so its reduced size in the Riversleigh fossil taxa could be related to flightlessness.

13. The proc. flexorius has relatively greater ventral projection than in Gallinula species, and is more robust. Viewed in cranial aspect, with both the proc. flexorius and condylus dorsalis lying on a planar surface, the dorsoventral width of the proc. flexorius exceeds half the dorsoventral width of the condylus ventralis (much thinner in Gallinula ). The proc. flexorius extends slightly farther distad than the condylus ventralis in A. disneyi , as it does in Gallinula species.

14. The relative overlap medially of the condylus ventralis by the condylus dorsalis on its proximal side (in cranial view), used by Brodkorb (1967) to distinguish Gallinula from Fulica and discussed by Boles (2005a), was reassessed. When taxa were compared in the same orientation, where both the proc. flexorius and condylus dorsalis lay on a planar surface, we found no overlap of the condyles in Heliornis and Porphyrio species, but in all Gallinula species, Amaurornis species, Gallirallus species, Rallus limicola , Fulica atra and F. americana , the condyli overlapped on a plane parallel to the axis of the bone. In the material of Australlus disneyi , ventrodorsal overlap of the condyles is apparent.

Ulna ( QM F30693, pL, Dirk’s Towers) .

There is no evidence for a second rallid in Faunal Zone A and B sites, with only Ringtail Site (Faunal Zone?C) containing evidence of multiple species, including one fossil of a much smaller species than those described herein. There is no reason to doubt the referral of this ulna to A. disneyi , based on its appropriate size. The impressio brachialis in A. disneyi has a prominent crista brachialis (sensu Livezey, 1998: char 224), unlike in Gallinula . Otherwise, the ulna is little different from that of rallids.

Carpometacarpus ( QM F30908, L, Dirk’s Towers Site; QM F31478, L, Camel Sputum Site) .

The labrum dorsalis (sensu Livezey, 1998, char 237) is relatively elongate, extending adjacent to the fovea carpalis caudalis, which is deep. In Gallinula species, the dorsal rim typically ends proximad of the fovea, as in G. ventralis and G. tenebrosa . The sulcus trochlearis is shallow and rounded. Otherwise, the differences between A. disneyi and Gallinula species are related to shortening of the element associated with loss of flight.

Coracoid ( QM F30692, cranial part R, Dirk’s Towers; QM F31469, cranial part L, White Hunter Site; QM F31470, cranial part R, White Hunter Site; QM F31477, sternal part and shaft L, Camel Sputum. Newly referred specimens: QM F39864 , cranial part L, Judith’s Horizontalis Site; QM F54513 , cranial part and shaft L, White Hunter Site) .

In addition to the features cited by Boles (2005a), we note the following in A. disneyi :

1 The foramen nervi supracoracoidei is large in A. disneyi , and it has large foramina penetrating the corpus, as in G. (Tribonyx) mortierii , but it is unlike that of G. (Tribonyx) ventralis and G. tenebrosa , in which the foramina opening into the corpus are very small ( Fig. 3 ).

2 The coracoid of A. disneyi is characterized by a prominent, sharp crista procoracoidei ( Livezey, 1998: char. 189), which extends from the proc. procoracoideus to a point close to the crista medialis, as in Psophia ( Olson, 1973b: fig 2). The interpretation that the proc. procoracoideus extends about half way along the shaft, joining it gradually ( Boles, 2005a) might have been inferred from broken specimen QM F31470. Specimen QM F31477 shows this to be incorrect. In this fragment, the proc. procoracoideus preserves its cranial margin and, although the crista is broken lateral to the foramen nervi supracoracoidei, it is clearly preserved sternal to this point. The crista procoracoidei is elongate, broad, dorsoventrally-thin, and it extends at least half the length of the impressio sternocoracoidei. Above the angulus medialis, the crista medialis is broken in QM F31477, so its connection to, or separation from, the crista procoracoidei is unknown. However, the preserved cristae are very similar to the state in Porphyrio and also to that seen in the coracoid attributed to the new species described below, in which the crista procoracoidei extends close to, but does not connect to, the crista medialis ( Fig. 3 ). Mayr (2004) considered the crista procoracoidei to be present in rallids, heliornithids, and messelornithids, and to support the monophyly of these taxa. We follow Livezey’s definition and do not consider that the more derived rallids have a crista procoracoidei because in these birds the proc. procoracoideus merges into the shaft closer to the cranial end, and it is always separated by a broad distinct gap from the enlarged crista medialis, which extends from the angulus medialis. In Heliornis , there is a distinct crista connecting the proc. procoracoideus and angulus medialis, as recorded by Livezey (1998), although it is not as produced from the shaft as it is in Psophia .

3 The proc. acrocoracoideus overhangs the shaft ventrally somewhat more than in all Gallinula species, but it is more notable for the extreme rotation mediad over the sulcus m. supracoracoidei, such that its cranial margin is at a near right angle to the facies articularis humeralis.

4 The impressio lig. acrocoracohumeralis forms a shallow sulcus extending from the facies artic. humeralis to the tip of the proc. acrocoracoideus. This hollow is bisected by a laterodorsal—medioventrally aligned groove.

5 The ventral facies of the shaft is quite compressed in its cranial half (not broadly convex) making the adjacent lateral facies flattened and the resultant ridge is directed somewhat laterad, especially near the facies artic. humeralis. This causes the proc. acrocoracoideus to greatly overhang the ventromedial facies at an angle approaching 45 degrees to the sagittal plane. In all Gallinula species, the plane of the surface in the sulcus supracoracoideus and adjacent to the facies artic. clavicularis is essentially aligned dorsoventrally.

6 The proc. procoracoideus extends craniad beyond the cotyla scapularis.

Femur ( QM F36452, pL, LSO Site) .

The femur of Australlus disneyi is only represented by the relatively uninformative proximal fragment QM F36452. The preserved length of 39.8 mm extends just distal to the nutrient foramen, and its proximal width of 10.0 mm is similar in size and proportions to the femur of G. ventralis AM O. 71392. Given similar total proportions, estimated total length for the fossil femur is about 54 mm. It is generally similar to those of G. ventralis and G. tenebrosa , with the most significant difference being that the insertion for the M. obturator externus is larger, causing a prominence on the caudal margin just distal to the facies articularis antitrochanterica, when viewed laterally, which is absent in extant Gallinula species.

Tibiotarsus ( QM F31473, proximal R, White Hunter Site; QM F31474, distal R, White Hunter Site; QM F31475, distal R, White Hunter Site; QM F24130, proximal L, Camel Sputum Site; QM F31480, proximal R, Dirk’s Towers Site; newly referred specimen QM F30696, distal+shaft L, Camel Sputum Site) .

In addition to the points described by Boles (2005a), we note the following for A. disneyi . The sulcus extensorius grades into a distinct flat groove, which extends farther proximad than in Gallinula species. Specimen QM F30696, which we refer to Australlus disneyi , is instructive as it has a preserved length of 77.6 mm for a midshaft width of 4.9 mm and distal width of 9.2 mm, but the proximal and distal ends of the fibular synostosis are 70 mm and 44.5 mm from distal end, respectively. In contrast, a tibiotarsus of G. tenebrosa (AM O.60402) has the comparable values: midshaft width 4.9 mm, distal width 8.9 mm, and the fibular synostosis extends from 54.5 to 42.5 mm from the distal end. Both observations indicate a tibiotarsus proportionally longer than any Gallinula species.

QM F30696 was identified by Boles (2005a) as the tibiotarsus of a rail, but because of its more robust nature and morphological differences, it was not placed with A. disneyi and left unnamed. It is considered here to fall within the range of variation of that species, and it is included with it.

We take this opportunity to correct a misidentification in Boles (2005a). A distal tibiotarsal fragment (QM F24605) from the Pleistocene Floraville Local Fauna (Site 5c, Floraville Stn, Leichhardt River, North Queensland) was incorrectly regarded as Gallinula mortierii . It is here recognized as Anatidae ,?Dendrocygnine. As this record would have been the most northwestern one for G. mortierii , its reidentification considerably reduces the known mainland distribution of that species.

Tarsometatarsus ( QM F20799, proximal L, Ringtail Site; QM F23723 , proximal R, White Hunter Site; QM F30720, proximal R, Creaser’s Ramparts Site; QM F31476, distal R fragment lacking trochlea metatarsi II., White Hunter Site)

In addition to those characters described by Boles (2005a), we note the following two significant features.

1. The crista medialis hypotarsi is relatively much longer than in Gallinula species, extending to greater than half the length of the hypotarsus, which contributes to the strong enclosure of two hypotarsal canals, as it does in Heliornis . This crista is very short in most rallids (e.g., Gallinula , Gallirallus and Porphyrio ), always much less than half of the hypotarsus length. The enclosure of the two canals is a feature approached in Gallinula , in which two canals are enclosed plantarly, but that for tendinal canal 2 (sensu Mayr, 2004: fig. 5) for the tendon of M. flexor perforatus digiti II (equals canal 2 of Strauch, 1978: fig. 29) is only partly or not closed. In both Fulica atra and F. americana , tendinal canal 1 (for the tendon for M. flexor digitorum longus) is fully enclosed and tendinal canal 2 is nearly closed. Amaurornis species variably have tendinal canal 1 closed or open, with tendinal canal 2 open. Porzana species have tendinal canal 1 closed, or tending so, and tendinal canal 2 open, but Poliolimnas species have both open.

However, closure of any tendinal canals clearly distinguishes A. disneyi from Gallirallus and its flightless relatives and Rallus , in which both canals are open plantarly. In A. disneyi , the greater extent of the crista medialis hypotarsi results in the more plantar of the two canals having carried two tendons, those for M. flexor perforatus digiti II and M. flexor perforans et perforatus digiti II (sensu Mayr, 2004). This arrangement is also seen in Heliornis . In all Gallinula species, the tendon for M. flexor perforans et perforatus digiti II lies in a groove that is open plantarly.

2. The tarsometatarsus is proportionally far longer than in any Gallinula species. The groove passing from the sulcus extensorius mediad around the shaft for the M. extensor hallicus longus ( Owre, 1967) is located half way down the shaft in rallids. In the more complete QM F20799, the midpoint of this groove is 43 mm from the proximal end, suggesting a total length of about 86 mm. Such an elongate shaft is also supported by the observation that the groove traverses the medial surface at a shallower angle than in Gallinula and that the cristae plantares medialis et lateralis remain parallel over the entire preserved length of the fossil. The tarsometatarsus of A. disneyi is thus about 1.6 times as long as the femur, which was estimated above at about 54 mm (based on QM F36542). This is also in accordance with the elongate tibiotarsus mentioned above. In Gallinula species, the tarsometatarsus is only slightly longer than the femur.

Figures

Fig. 1. Crania of Gallinula tenebrosa AM O.67057 (A,C,F) and Australlus disneyi QM F23806 (B,D,E,G) in right lateral (A,B,F), ventral (C,D), caudal (E), and dorsolateral (G) views. Abbreviations: cnt, crista nuchalis transversus; fo, fonticuli orbitocranialis; fol, facet os lacrimale; fm, foramen magnum; ft, fossa temporalis; nh, nasofrontal hinge; pp, processus paroccipitalis; rt, recessus tympanicus. Scale bar for (A–E) is 20 mm and (F, G) is 5 mm.

Fig. 2. Mandible fragments of Australlus disneyi QM F54503 (A) and rallid species QM F54524 (B). The rami of QM F54503 are held together as fossilized by a resin bridge, but the os dentales have separated and so are shown medial facies up.Abbreviations: cl, cotyla lateralis; cm, cotyla medialis; ctf, crista transversus fossae; fcm, fenestra caudalis mandibulae; pmm, processus mandibulae medialis; pr, processus retroarticularis. Scale bars = 10 mm.

Fig. 3. Coracoids (A–C) of Australlus disneyi, left QM F31477 in dorsal (A) and ventral (C), cranial part right QM F31470 in dorsal (B) views; scale bar = 10 mm. The crista medialis leading from the angulus medialis to the crista procoracoidei is broken in this specimen, as indicated by the dotted lines. Left proximal and part shaft tarsometatarsus of Australlus disneyi QM F20799 in medial (D), anterior (E), plantar (F), proximal (G) views (scale bar = 20 mm), and medial view enlarged (H). Abbreviations: cmh, crista medialis hypotarsi; cp, crista procoracoidei; cs, cotyla scapularis; gr, groove for extensor hallicus longus; fns, foramen nervi supracoracoidei; pa, processus acrocoracoideus; pp, processus procoracoideus; se, sulcus extensorius; 1, tendinal canal 1 for M. flexor digitorum longus; 2, tendinal canal 2 for M.flexor perforates digiti II & M. flexor perforans et perforatus digiti II.