Wonambi Smith, 1976

Scanlon, John D., 2005, Cranial morphology of the Plio-Pleistocene giant madtsoiid snake Wonambi naracoortensis, Acta Palaeontologica Polonica 50 (1), pp. 139-180 : 142

Wonambi Smith, 1976

Type species: Wonambi naracoortensis Smith, 1976 .

Referred species: Wonambi barriei Scanlon in Scanlon and Lee, 2000.

Diagnosis.—Neural spines of vertebrae high, sloping posterodorsally, with sharp−edged anterior lamina extending to near anterior edge of zygosphene; transverse processes extending laterally beyond zygaphophyses in most trunk vertebrae, diapophyses with concave dorsal edge in lateral view; zygosphene relatively narrow, with steep facets (20–30° from vertical); zygapophyses inclined 20° or more above horizontal; haemal keel in middle and posterior trunk region narrow and weakly defined laterally, but often distinctly bifid or trifid on the posterior third of the centrum. Pterygoid tooth row near middle of bone, away from medial edge, and basipterygoid facet narrow and facing medially as much as dorsally; ectopterygoid process of pterygoid triangular in palatal view. Maxilla and dentary relatively elongate and depressed; maxilla with deep, anterolaterally directed trough on suborbital surface ( Scanlon and Lee 2000).

Remarks.—All characters in the generic diagnosis are observed directly in both species except for the shape of the ectopterygoid process in W. naracoortensis , which is inferred below to be similar to that of W. barriei (and unlike the other madtsoiids where this structure is known). Diagnoses of both species are given in Scanlon and Lee (2000). All Wonambi material from the Pliocene and Pleistocene (southern Western Australia and South Australia, and central New South Wales) is referred to W. naracoortensis ; the smaller W. barriei is known only from Early Miocene and possibly Late Oligocene deposits at Riversleigh, Qld.

Postcranial variation will not be described fully here, but I add some remarks on two points: (A) the relative indistinctness of parazygantral foramina in W. naracoortensis , and (B) the nature and preservation of haemal arches on caudal vertebrae in madtsoiids.

(A) When Wonambi was represented by just a handful of vertebrae, it was referred to Madtsoiidae provisionally or doubtfully, because its parazygantral foramina (not in all vertebrae, but in some including the holotype; Smith 1976: fig. 2E) were smaller or less distinct than in the few giant species of Madtsoia and Gigantophis then known ( Smith 1976; Rage 1984). According to Rieppel et al. (2002: 812), “the association of Wonambi with madtsoiids was considered tentative by Rage (1984; see also Smith 1976), and it still is.” This ambiguous phrasing either attributes to J.C. Rage a view which he does not actually hold (e.g., Rage 1998), or asserts that the grounds for earlier doubt are still valid. In fact we now know rather more about variation of parazygantral foramina in Australian members of this group, and three independent components of variation can be identified. Many W. naracoortensis vertebrae have one or more of the following features: (1) subdivsion of the large, single foramen on each side usual in other madtsoiids; (2) shallowness of the depression (“fossette”, Hoffstetter 1961) containing the foramina; and (3) presence of numerous smaller additional foramina or pits in the parazygantral area, beyond the depression. These features (1–3) have a restricted distribution within Australian Madtsoiidae and are potentially useful as taxonomic characters, as they vary independently within and between some species (particularly those of Wonambi and Yurlunggur ), while others (including species of Alamitophis , Patagoniophis , and Nanowana ) have bilaterally single, relatively large, usually distinctly recessed foramina and no additional pits ( Scanlon 1992, 1993a, 1995, 1996, 1997, in press).

(B) Extant snakes have the caudal haemal arches (haemapophyses, corresponding to chevron bones in most other amniotes), if present, fused to the centra proximally, and forming laterally paired projections which are unfused and usually separated distally ( Hoffstetter and Gasc 1969; Lee and Scanlon 2002, characters 204–206). Paired ventral projections of caudal centra in madtsoiids were thus initially identified as “haemapophyses”, despite having flat or concave facets rather than finished bone distally (e.g., Scanlon 1993a; Rage 1998). However, one caudal vertebra of W. naracoortensis is known (part of SAM P30178, HQ) that retains a complete chevron (V−shaped element, open proximally and fused distally) partially fused to the distal facets of the “haemapophyses”, indicating that the latter are actually pedicels for the chevron like those in varanoid lizards ( Scanlon and Lee 2000: fig. 2g). This lizard−like condition in Wonambi unambiguously supports a phylogenetic position outside the clade of modern snakes characterized by typical haemapophyses. Rieppel et al. (2002: 813, 824) raise several objections to this interpretation: (1) Barrie (1990: fig. 11) illustrated a considerably larger caudal vertebra of P30178 which lacks a chevron; (2) the vertebra with attached chevron was not among the material examined by Rieppel and Kluge; (3) the suggestion was made (attributed to M. Hutchinson, SAM Herpetology) that the specimen illustrated by Scanlon and Lee was from a different locality and age; and (4) “it seems that chevrons are a plesiomorphic trait at the level of Serpentes, and hence cannot be used in an argument supporting the basal position of Wonambi .” These are answered briefly below. (1) The large caudal figured by Barrie is from immediately posterior to the cloacal region (it is almost the same size as the cloacal vertebra figured adjacent to it) and lacks any sign of an ossified haemal arch or pedicels; the presence of at least one such “pygal” vertebra is typical of squamates and has also been demonstrated in W. barriei ( Scanlon 1996; Scanlon and Lee 2000). More posterior vertebrae of the same caudal series are, not surprisingly, smaller; but it is also possible that the caudal with chevron comes from the smaller of the two skeletons comprising SAM P30178. (2, 3) The vertebra in question was not in the display cabinet (donated in 1989) that Rieppel et al. (2002) examined; it remained with the collector and donor (D.J. Barrie) until positively identified as Wonambi in 1998. The final objection (4, repeated by Rieppel et al. 2003: 538) represents a basic misunderstanding of the principles of phylogenetic inference: synapomorphy is evidence for monophyly, while symplesiomorphy is not ( Hennig 1966). All modern snakes either have fixed, paired haemapophyses in place of chevrons (Macrostomata, with few exceptions), fixed median “caudal hypapophyses” (Uropeltinae), or lack any such structures (Scolecophidia, Anilius , Cylindrophis ) ( Hoffstetter and Gasc 1969; Lee and Scanlon 2002, characters 204–206; Szyndlar and Rage 2003). Whether these represent separate derivations from the plesiomorphic state or (as it may be more parsimonious to assume) true chevrons were lost only once in an ancestor of modern snakes, these synapomorphies are indeed evidence for grouping modern snakes to the exclusion of Wonambi , and thus for the relatively basal phylogenetic position of the latter (and of Eupodophis, Rage and Escuillié 2000 ).