Varanus sp.

Villa, Andrea & Delfino, Massimo, 2022, First fossil of Varanus Merrem, 1820 (Squamata: Varanidae) from the Miocene Siwaliks of Pakistan, Geodiversitas 44 (7), pp. 229-235 : 230-232

Varanus sp.

DESCRIPTION

SNSB-BSPG 1956 II 2010 ( Fig. 3 ) is a large and elongated trunk vertebra, with a procoelous centrum. It is moderately well preserved, but the anterior part and most of its left lateral surface are covered by a hard sandstone matrix that was not removed in order not to damage the fossil. In ventral view ( Fig. 3A ), the centrum is subtriangular, with posteriorly converging and straight lateral margins. The ventral surface is flat, without keel. In spite of the matrix covering, the ventral margin of the anterior cotyle is clearly located posteriorly compared to the dorsal margin ( Fig. 3A ); thus, the anterior cotyle was facing anteroventrally. In posterior view ( Fig. 3B ), the centrum is dorsoventrally compressed. The posterior condyle is strongly eroded, but a distinct precondylar constriction is still recognisable in ventral view ( Fig. 3A ). The width of the centrum at the constriction is 5 mm, whereas the anterior cotyle can be estimated as about 8 mm wide. This would indicate a constriction/cotyle width ratio of 0.63. On the right lateral surface ( Fig. 3C ), the base of the synapophysis is visible, even though the rest of the latter is not preserved anymore. Nevertheless, it was dorsoventrally elongated. The vertebra has no postzygoprezygapophyseal lamina, but a very poorly distinct posterior centrosynapophyseal lamina is present ( Fig. 3C ). In dorsal view ( Fig. 3D ), a distinct interzygapophyseal constriction is visible. Being covered by sediment, the prezygapophyses are not clearly visible. The postzygapophyses are better exposed. Zygapophyses are subcircular to suboval and dorsally inclined of about 45 °. Each prezygapophysis is separated by the pars tectiformis of the neural arch by a deep groove ( Fig. 3D ). The pars tectiformis does not bear a zygosphene, and no zygantrum is visible on the posterior edge of the neural arch. The posterior half of the latter is raised compared to the pars tectiformis ( Fig. 3E, 3F ). The neural spine is well developed, even though broken along its entire length. It starts at the anterior margin of the neural arch and is laminar for most of its length (originating the prespinal lamina sensu Tschopp 2016), but it thickens slightly by the posterior end of the neural arch where it is broken off ( Fig. 3D ). The dorsal surface of the neural arch displays a distinct striation ( Fig. 3D ), composed of fine and dense striae that run posterodorsally from the sides of the arch towards the neural spine. On each side of the same surface, a robust and dorsally-rounded lamina connects the posterior corner of each prezygapophysis with the base of the neural spine around midlength of the vertebra ( Fig. 3C, D, F ). We named this structure mediospinal lamina. It marks the anterior margin of the raised posterior half of the neural arch. Posteriorly, the spinopostzygapophyseal laminae are robust but not sharp ( Fig. 3B ). Due to the covering sediment, it is not possible to describe the centropostzygapophyseal laminae. Further, the broken neural spine hinders the description of a postspinal lamina, in case it was present (as is usually the case at least in Varanus exanthematicus ; Tschopp 2016). The preserved minimum length of the vertebral centrum (measured at midline starting from the ventral margin of the cotyle) is 11 mm.

IDENTIFICATION

The following combination of features clearly indicates that the Pakistani trunk vertebra found in the SNSB-BSPG collections pertains to Varanus ( Hoffstetter 1943; Estes 1983; Smith et al. 2008; Holmes et al. 2010): anterior cotyle facing ventrally due to posteriorly-located ventral margin; presence of precondylar constriction; complete absence of a more-or-less developed zygosphene/zygantrum complex; and presence of dorsal striation. The newly-named structure, the mediospinal lamina, also supports the identification of the vertebra as a varanid.This structure has been also recognised, but not named, by Smith et al. (2008), who reported its sharp development in Varanus and Saniwa , but not in Heloderma . In contrast, it appears to be absent in other lizard groups, such as lacertids ( Tschopp 2016), scincids ( Camaiti et al. 2019), and anguids ( Čerňanský et al. 2019). A more extensive survey of the distribution and development of this structure in lizards could help in the future to better understand its taxonomic significance.

Isolated vertebrae of Varanus are not diagnostic at the species level (see literature in Villa et al. 2018). Thus, SNSB-BSPG 1956 II 2010 cannot be further identified. The development of the precondylar constriction compared to the posterior condyle is considered useful to discriminate between different Varanus subclades ( Conrad et al. 2011; Delfino et al. 2013), but this character is not evaluable in SNSB-BSPG 1956 II 2010 because of its complete erosion. Nevertheless, if this vertebra represents an adult (or subadult) individual, the minimum length of the vertebral centrum suggests that it probably belonged to a small-bodied monitor species (see measurements in e.g., Conrad et al. 2012, and Vasilyan & Bukhsianidze 2020), with a snout-vent length possibly higher than 300 mm and not exceeding 500 mm. A confident estimate of the size is not advisable due to uncertainties regarding the real position of the vertebra in the column, as well as the unknown intracolumnar variation.

Figures

FIG. 3. — Varanus sp. from the Miocene of Pakistan: trunk vertebra (SNSB-BSPG 1956 II 2010) in ventral (A), posterior (B), right lateral (C), dorsal (D), anterior (E), and left lateral (F) views. Abbreviations: ml, mediospinal lamina; pcl, posterior centrosynapophyseal lamina; pl, prespinal lamina; pt, pars tectiformis; s, synapophysis; spl, spinopostzygapophyseal lamina. Scale bar: 5 mm.