A new fossil kangaroo from the Oligocene-Miocene Etadunna Formation of Ngama Quarry, Lake Palankarinna, South Australia

The Etadunna Formation deposits of Ngama Quarry in the Lake Eyre Basin of central Australia (Pledge 1984; Woodburne et al. 1993) have produced some twenty discernable taxa of higher vertebrates, mostly represented by jaw fragments and/or isolated teeth. Only three specimens have been recovered that preserve associated or articulated bones: a crocodilian hind limb referred to Australosuchus clarkeae, the lower tibiotarsus and pes of an indeterminate water bird (possibly a flamingo), and the partial postcranium of a small macropodoid (SAM P23821); this specimen is significant because it is the only articulated macropodoid skeleton thus far described from the Oligocene-Miocene deposits of the Lake Eyre Basin.

SAM P23821 was found in July 1981 by a field party from the South Australian Museum, in the saddle section of Ngama Quarry at Mammalon Hill, Lake Palankarinna, north-eastern South Australia (see Pledge 1984, 347, Fig. 1 for locality map). It came from the upper fossiliferous sandy layer of the upper Etadunna Formation (Pledge 1984) and occurred at the same level (and spatially within a few metres) as a calcaneum (SAM P23637, which articulates with the cuboid of SAM P23821), and the jaw of a primitive macropodoid (SAM P23626), here attributed to a new genus and species Ngamaroo archeri gen. et sp. nov. These specimens (SAM P23821, P23637, P23626) probably represent a single individual, as evidenced by their close spatial/stratigraphic proximity, comparable size/morphology, and immature ontogenetic stage: epiphyses not fully fused in SAM P23821, P23637, and P₃ not erupted in SAM P23626.

This paper describes remains attributable to Ngamaroo archeri gen. et sp. nov. and provides a preliminary assessment of its potential phylogenetic relationships. Postcranial skeletal and myological terminology follows Murray (1995), Bishop (1997) and Kear et al. (2007). Dental homology follows Flower (1867) and Luckett (1993). Institutional abbreviation SAM refers to the South Australian Museum, Adelaide.

Type and only known species: Ngamaroo archeri, sp. nov.

Fossil macropodoid differentiated from balbarids and hypsiprymnodontids on the basis of an elongate P₃ (>1.5 M¹/₁) with horizontal occlusal margin, transversely broad trigonid basin on the M₁, and absence of a discrete M₁ protostylid. Differs from advanced macropodids (e.g. derived bulungamayines, macropodines and sthenurines) by the retention of bunolophodont molars and a lower molar hypolophid formed by a buccally directed crest from the entoconid. Differs from primitive bunolophodont macropodids (i.e. the bulungamayines Nowidgee and Gumardee) by the P₃ bearing a high number (12–13) of fine (shallowly incised) cuspids/transcristids. Differs from potoroines sensu stricto in having the P₃ base expanded into distinct labial and lingual cingulids, M₄ not markedly smaller than anterior molars, sub-equally sized capitellum and trochlea on the distal end of the humerus, humeral entepicondyle closely abutting the trochlea, and ulna shaft with sinuous ventral edge. Differs from the incertae sedis taxa Purtia in P₃ cuspid/transcristid number (12–13 v. 10), and in the molars having closely aligned longitudinal axes (i.e. molar row straight) and a rectangular (length at least 0.3 > width) occlusal outline; Wakiewakie in P₃ cuspid/transcristid number (12–13 v. 20) plus the rectangular molar occlusal outline and M₄ being not markedly smaller than the anterior molars; and Palaeopotorous by the absence of a discrete M₁ protostylid.

To test the relationships of Ngamaroo archeri, a cladistic analysis was conducted using a revised version of the most comprehensive published phylogenetic dataset of Tertiary Macropodoidea – Kear et al. (2007). This character matrix (see NEXUS format in Appendix) has been recoded to correct various minor errors and three new taxa added (Bettongia penicillata, Purtia mosaicus, Wanburoo hilarus) to help resolve placement among basal potoroids/macropodids. Three modified and four novel characters have also been added; these are listed and described below.

1. P³/₃ – (0) short (<1.5 M¹/₁); (1) elongate (>1.5 M¹/₁). Original description included nature of the transcristids; now distinguished as separate characters.

2. 1. P₃ – (0) cingulids absent; (1) with distinct labial and/or lingual cingulids. Modified to accommodate additional presence of a labial cingulid.

   2. 1. Hypolophid formation – (0) hypolophid and buccal crest from entoconid absent; (1) formed by buccal crest from entoconid with posthypocristid low and posteriorly positioned; (2) formed by elevated posthypocristid, buccal crest from entoconid reduced or lost. Original character included information on other crests that needed to be more clearly conceptualised. Ordered 0–1-2.

         1. Ventral margin of mandible – (0) convex (anterior mandible steeply inclined); (1) straight (anterior mandible shallowly inclined).

         2. P₃ transcristids – (0) absent; (1) coarse, deeply incised; (2) fine, shallowly incised. Character ordered 0–1-2 because state 1 is expressed in basal Macropodoidea.

         3. P₃ transcristid number – (0) transcristae absent; (1); 5 or less; (2) between 5 and 10; (3) 10 or more. Character is unordered because polarity of derived states is unclear.

         4. Lower molar occlusal outline – (0) rectangular (length at least 0.3 > width); (1) square (sub-equal length/width).

Phylogenetic analyses (incorporating a total of 108 characters and 29 taxa) returned identical trees whether all multistate characters were left unordered or were ordered where possible into morphoclines. Maximum parsimony trees, bootstrap frequencies (1000 replicates) and Bremer support were calculated using heuristic searches in PAUP* ver. 4.0b10 (Swofford 2002) with 200 random-addition replicates. Analyses with characters ordered yielded 160 most parsimonious trees (Length = 282; Consistency Index = 0.461, Retention Index = 0.716); relationships between major macropodoid clades are resolved sensu Kear et al. (2007) but show low bootstrap/Bremer support (<50/1 influenced by large amounts of missing data) at nodes other than Balbaridae (60/1), Potoroidae+Macropodidae (90/3), Macropodidae (50/1, including bulungamayines), Macropodinae+Sthenurinae (100/3), and Sthenurinae sensu stricto (100/4). A 50% majority rule consensus tree is shown in Fig. 3; strict consensus trees failed to resolve the basal bulungamayine polytomy within Macropodoidea.

The present phylogeny nests Ngamaroo in a polytomy with potoroine taxa and a discrete macropodid clade (Fig. 3). Affinity with these lineages (and differentiation from balbarids/hypsiprymnodontids) is supported by several strong synapomorphies (subscript numbering refers to characters listed above and those in the matrix of Kear et al. 2007): elongate P₃ [>1.5 M¹/₁] with horizontal occlusal margin₂₃, M₁ with transversely broad trigonid basin₃₀, absence of a discrete M₁ protostylid₃₁. Ngamaroo lacks the derived states characterising advanced bulungamayines and macropodines/sthenurines (lophodont molars₂₄, hypolophid formed by elevated posthypocristid, buccal crest from entoconid reduced or lost₃₃). However, it does share key synapomorphies (P₃ with fine and shallowly incised transcristids₁₀₆, P₃ transcristid number >10₁₀₇) with the controversial genera Purtia and Wakiewakie, which have been variously allied with either basal bulungamayine macropodids (e.g. Nowidgee; Flannery 1989; Cooke 1997; Kear and Cooke 2001), or more traditionally, potoroines (Case 1984; Woodburne 1984). The principal character states common to these taxa (bunolophodont molars₂₄, hypolophid formed by a buccal crest from the entoconid₃₃) are symplesiomorphic; however, Purtia, Wakiewakie and Nowidgee do display a derived square (sub-equal length/width) lower molar occlusal outline₁₀₈, which may be homologous (see Cooke 1997). Ngamaroo manifests rectangular molars (length at least 0.3 > width) – the primitive condition among macropodoids. Nevertheless, the presence of an M₄ that is not markedly smaller than the anterior molars₂₅ (an unexpected apomorphy considered unique to advanced macropodines and sthenurines; Kear et al. 2007), together with a P₃ that bears distinct labial and lingual cingulids₂₇ constitute robust dental synapomorphies that might support a potential macropodid relationship.

The postcranial skeleton of Ngamaroo is rather generalised but includes some novel derived traits also suggestive of macropodid affinity: humerus with entepicondyle closely abutting the trochlea ₉₆ – this feature occurs elsewhere in Hypsiprymnodon and the macropodine Dorcopsis but differs from potoroines and most other macropodids, in which the entepicondyle and trochlea are typically separated by a broad groove (Kear et al. 2001, 2007); humerus with capitellum and trochlea of sub-equal size₉₈; and ulna shaft with distinctly sinuous ventral edge₉₉. The latter two characters are typical of advanced macropodids (Kear et al. 2001, 2007).

Both potoroids (comprising potoroines only sensu Cooke and Kear 1999; Kear and Cooke 2001) and basal macropodids (vis-à-vis bulungamayines) are morphologically conservative and their monophyly/inter-relationships are poorly resolved in most recent analyses (see Cooke 1997, 1999; Burk and Springer 2000; Cardillo et al. 2004; Kear et al. 2007). Accordingly, bootstrap frequencies in the present phylogeny are relatively weak (influenced by >50% missing data for most fossil taxa in these groups) and examination of the 160 best trees shows that Ngamaroo, and the dentally comparable Purtia and Wakiewakie, form alternate clades with either potoroids (54 trees) or macropodids (24 trees) respectively. Given these results, definitive placement of Ngamaroo (together with Purtia and Wakiewakie) is deferred with a recommendation that Potoroidae and Bulungamayinae be treated as incertae sedis within Macropodoidea (see also Burk and Springer 2000; Cooke and Kear 1999; Kear and Cooke 2001; Kear et al. 2007), pending discovery of more complete material and/or the re-evaluation of existing taxa using expanded morphological/molecular datasets.