Saniwa orsmaelensis Dollo, 1923

Čerňanský, Andrej, Smith, Richard, Smith, Thierry & Folie, Annelise, 2024, Timing of intercontinental faunal migrations: Anguimorph lizards from the earliest Eocene (MP 7) of Dormaal, Belgium, Zoological Journal of the Linnean Society 201 (4), pp. 1-25 : 14-16

Saniwa orsmaelensis Dollo, 1923

( Fig. 9 )

New referred material: IRSNB R 495, left dentary; IRSNB R 496, IRSNB R 497, two presacral vertebrae; IRSNB R 498, distal portion of humerus.

Locality and horizon: Dormaal, Flemish Brabant, eastern Belgium, Dormaal Member, Tienen Formation, Landen Group, earliest Eocene (MP 7).

Description

Dentary: A mid-section of a left dentary, IRSNB R 495, is preserved ( Fig. 9A–C ). It bears six and half tooth positions (only four teeth are mostly preserved). The bone is anteroposteriorly elongated, having a slightly concave appearance. In the posterior preserved section, it widens dorsoventrally, being slightly dorsally elevated. The lateral surface of the bone is smooth, except for three large elliptical labial foramina located more-or-less in the dorsal anterior third of the lateral wall. In medial view, the Meckelian canal is fully open, although narrow. It gradually dorsoventrally widens posteriorly. The subdental shelf is thin and only weakly medially expanded.

Dentition: The dentition is subpleurodont (sensu Hoffstetter 1954, 1955), the jaw parapet is low and the bases of the teeth are attached to a sloping, concave surface, without any angle between two different planes ( Fig. 9A–C ). The tooth bases are mesiodistally broad and bear well-preserved typical basal striae, i.e. plicidentine is present (the infolding of dentine in the pulp cavity; Kearney and Rieppel 2006). It is complex and dense, as many closely spaced basal infoldings are present. All tooth tips are broken off. However, the labiolingual tooth crown compression is visible in broken teeth and partly, to a small extent, in the virtual 3D model of a pulp cavity ( Fig. 9C ).

Remarks

The overall morphology and plicidentine support allocation of the dentary to a varanoid ( Kearney and Rieppel 2006, Georgalis and Scheyer 2019). Unfortunately, the Dormaal dentary is only partly preserved. In fact, the preserved portion of this dentary is very similar to that of Saniwa orsmaelensis described by Augé et al. (2022) from the same locality by having many closely spaced basal infoldings present on tooth bases—in contrast to Palaeovaranus Zittel, 1887 –90, which has more widely spaced infoldings ( Georgalis and Scheyer 2019). The presence of a labiolingual tooth crown compression, with mesial and distal carinae, is considered a synapomorphy of the varanoid genera Lanthanotus Steindachner, 1878 and Varanus ; in Varanus this compression extends to the tooth base and is visible in the pulp cavity ( Georgalis et al. 2023). Although some degree of the labiolingual compression of the pulp cavities is visible in IRSNB R 495 ( Fig. 9C ), this compression is markedly smaller (not highly compressed) in comparison to extant Varanus (see: Georgalis et al. 2023: fig. 3D, F).

Dorsal vertebra: Two varanoid dorsal vertebrae are available in the material ( Fig. 9D–O ). They are medium-sized, with centra that widen anteriorly, being almost triangular in ventral view. The maximum anteroposterior length is 5.7 mm in IRSNB R 496 and 7 mm in IRSNB R 497. The subcentral ridges are more-or-less straight in ventral view. The neural spine is well developed, starting its rise dorsally approximately in the half of the anteroposterior length the neural arch ( Fig. 9F ). It is trapezoidal in shape; however, its distal tip is broken off in both specimens. Its dorsal posterior portion is narrow, being drop shaped in cross-section ( Fig. 9D, J ). This portion occupies the posterior third of the anteroposterior length of the neural arch. Anteriorly, there is a median ridge that continues almost until the anterior end of the neural arch. The neural canal is large, pentagonal in shape. It is vaulted in posterior view ( Fig. 9I, O ). The prezygapophyseal articular facets are large and broad in dorsal view. The postzygapophyseal articular facets are also large. There is a slight constriction between pre- and postzygapophyses. In this region, a small foramen is located on the lateral side near the constriction on each side in dorsal aspect ( Fig. 9G–M ). Overall, the vertebrae are wide in dorsal view. A pseudozygosphene and a pseudozygantrum are present [this would correspond to character state 468: 1 in Gauthier et al. (2012)]. The synapophyses are damaged. The cotyle and the condyle are dorsoventrally depressed. In lateral view, the cotyle is distinctly inclined—its ventral rim is located more posteriorly than the dorsal one. Thus, the dorsal portion of the cotyle is clearly visible in ventral view ( Fig. 9E–K ). The condyle is well exposed from both dorsal and ventral views. The ventral surface of the centrum is slightly ventrally concave in lateral view. Small subcentral foramina are visible on the ventral side of the centrum.

Remarks

The pseudozygosphene–pseudozygantrum are very prominent in these specimens ( Fig. 9H, I, N, O ). This favours their assignment to the genus Saniwa ( Gilmore 1922, Rage and Augé 2003, Augé 2005, Georgalis et al. 2021). The precondylar constriction appears to be practically absent here, but this could be due to bad preservation (the condyles are eroded on these two vertebrae) and (to a degree) variable through ontogeny. Moreover, Saniwa does not have very prominent precondylar constriction in any case (relative to Varanus , e.g. Rieppel 1980, Estes 1983, Smith et al. 2008, Holmes et al. 2010, Čerňanský et al. 2022b). Vertebrae of Saniwa orsmaelensis from Dormaal were previously described by Dollo (1923), Hoffstetter (1969), and Augé et al. (2022: fig. 5A–F).

Humerus: Only the distal portion of the left humerus, IRSNB R 498 is preserved ( Fig. 9P–T ). It is robust and large. The maximum width of the preserved specimen is 8.6 mm. The preserved portion of the diaphysis is elliptical in cross-section ( Fig. 9T ). The entepicondyle is well developed, being set off from the posterior margin of the diaphysis. An entepicondylar foramen is absent. The distal portion of the ectepicondyle is damaged and partly broken off, but the ectepicondylar ridge is well developed. This region is pierced by an ectepicondylar foramen ( Fig. 9Q ). The ulnar and radial condyles are large, proximally accompanied by a fossa.

Remarks

The humerus is also allocated to the genus Saniwa based on the following features (see: Rieppel and Grande 2007, Augé et al. 2022): (i) an entepicondylar foramen is absent and (ii) the entepicondyle is set off from the posterior margin of the diaphysis of the humerus (although not in such degree present in humerus of Saniwa ensidens ).

Figures

Figure 9. Saniwa orsmaelensis from the earliest Eocene of Dormaal. Left dentary IRSNB R 495 in A, lateral; B, medial (with virtually segmented pulp cavities of the anterior teeth); and C, dorsal (with virtually segmented pulp cavities of the anterior teeth) views.Dorsal vertebrae IRSNB R 496 (I–N) and IRSNB R 497 (O–T) in I, O, dorsal; J, P, ventral; K, Q, lateral right; L, R, lateral left; M, S, anterior; and N, T, posterior views. Left humerus IRSNB R 498 in U, ventral; V, dorsal; W, posterior; X, anterior; and Y, medial views (all micro-CT visualizations).