Hoeckosaurus mongoliensis, Čerňanský, 2019

HOECKOSAURUS MONGOLIENSIS SP. NOV.

( FIGS 2–4 View Figure 2 View Figure 3 View Figure 4 )

2007 Squamata indet. – Böhme: p. 46, text figures 1, 2a, b urn:lsid:zoobank.org:act:8A581C67-C0F7-4106-84AC-91406471872A

Holotype: NHMW 2007 View Materials z0054/0001, an almost complete right dentary (section TGL-A/2 , Fig. 2A–C View Figure 2 ; figured by Böhme, 2007: text figures 1, 2a, b).

Paratypes: Right dentary NHMW 2016 View Materials / 0198/0001 (s e c t i o n T G R - Z O / 2); r i g h t d e n t a r y N H M W 2007z0054/0003 (section TGL-A/2 ); and left dentary NHMW 2007 View Materials z0054/0004 (section TGL-A/2 ) .

R e f e r r e d m a t e r i a l: L e f t m a x i l l a N H M W 2007z0054/0002 (section TGL-A/2).

Localities and horizons: (1) Mongolia, Taatsiin Gol left river side, section TGL-A/2, biozone A, lower Oligocene (early Rupelian); and (2) Mongolia, Taatsiin Gol right river side, section TGR-ZO/2, biozone B, lower Oligocene (late Rupelian).

Diagnosis: A dibamid that differs from other extant forms in the following combination of features: (1) open Meckel’s groove; (2) three labial foramina; and (3) lingual side of tooth crowns bearing fine striations. Besides these features, this taxon is characterized by the combination of the following characters: (4) dentary is long and slender (if its dorsoventral size is compared with its anteroposterior length); (5) subpleurodont implantation; (6) number of teeth in dentary is ten; (7) pointed teeth that more-or-less increase in size anteriorly; (8) presence of the triangular medioventral projection on the ventral section of the symphyseal region; (9) symphyseal region is slightly angled; (10) a deep subdental shelf, forming a raised flange lingual to the dentition; and (11) an intramandibular septum along the entire tooth row.

Description

Dentary: The description is based on one left and three right dentaries. The almost complete dentary ( Fig. 2A–C View Figure 2 ) from Taatsiin Gol left ( NHMW 2007 View Materials z0054/0001) is designated as the holotype. Its length is 3.8 mm. A specimen from Taatsiin Gol right ( NHMW 2016 View Materials /0198/0001) lacks the anterior region ( Fig. 2D–G View Figure 2 ) but preserves more of the postdental process. The dentary is slender. The holotype specimen has preserved an entire tooth row bearing ten tooth positions. Unfortunately, only one tooth is still attached, which is the seventh, counting from anterior (in Böhme, 2007: p. 46, figs 1, 2a, b, two teeth were preserved). The incomplete specimen NHMW 2016 View Materials /0198/0001 bears seven tooth positions, but three teeth are still attached ( Fig. 2D–G View Figure 2 ) .

In dorsal aspect, the dentary is straight, with only its anterior region slightly curved medially. In medial aspect, the dentary is slender, becoming slightly taller posteriorly. The symphyseal region is angled and dorsally elevated.The angle between the long axis of the symphysis and the posteriorly located subdental shelf is 144°. In its ventral region, the symphysial region bears a small triangular projection that is directed ventromedially. Meckel’s groove is fully open, but narrow. The alveolar foramen is large. It is separated from Meckel’s groove by a vertical intramandibular septum, the end of which is fused with the bone. The septum itself extends until the posterior end of tooth row (slightly beyond the last tooth position). Ventral to it, there is a wedge-shaped facet that most probably represents the facet for the splenial. This facet reaches anteriorly to the level of the fourth tooth position (counted from posterior). Below this facet, there is a second facet that reaches to the level of the second tooth position (counted from posterior) and was probably for articulation with the angular (or compound bone). This facet forms a narrow but welldeveloped groove. The subdental shelf (sensu Rage & Augé, 2010) is straight; its medial portion forms a low, dorsally convex (rounded) edge (or flange, especially in its mid-region). The presence of this flange is also evident posteriorly in the paratype dentaries ( Figs 2G View Figure 2 , 3B, F View Figure 3 ), although the anterior sections are broken. The posterior region of the holotype dentary (NHMW 2007z0054/0001) is broken off, but this region is partly preserved in NHMW 2016/0198/0001 ( Fig. 2D–G View Figure 2 ). The posteroventral process (sensu Gans & Montero, 2008) is broad and well expanded posteriorly, with no indication of a bifurcation. The posterodorsal process (sensu Gans & Montero, 2008; = coronoid process) is broken here, but is preserved in specimen NHMW 2007z0054/0004. This process is small and slightly elevated dorsally ( Fig. 3D, E View Figure 3 ), but it appears not to be preserved completely. Dorsally, the preserved portion reaches the level of the apex of the third tooth (counted from posterior).

The external surface of the holotype dentary is pierced by three large labial (neurovascular) foramina. The posteriormost one is oval (anteroposteriorly elongated). Only two foramina are preserved in NHMW 2016 View Materials /0198/0001, but the anterior region is broken away and probably had the same number of foramina .

Maxilla: Only a small fragment of the left maxilla is preserved ( Fig. 4 View Figure 4 ). It bears two teeth, the anterior one being larger. The tooth morphology is identical to that of the dentary. The preserved lateral portion of the bone is smooth.

Dentition: The implantation is subpleurodont (sensu Hoffstetter, 1954, 1955), with the tooth bases ankylosed to their sockets and surrounded by a ring of bony tissue. The teeth are unicuspid and pointed, with the apex being slightly curved distally. The tooth crowns have fine mesial and distal cutting edges. A concavity at the base of the crown is present. The lingual side of the tooth crowns bears fine, almost indistinct striations (visible with scanning electron microscopy, see Fig. 2E View Figure 2 ). Although only the tooth bases are preserved for most teeth, it appears that tooth size decreases posteriorly. Based on preserved teeth and on the size of the tooth bases in dorsal aspect, the largest tooth was the second from the symphysis. Small circular resorption pits are present in some tooth bases.

Remarks

The dentary NHMW 2007z0054/0001 (TGL-A/12) was originally described by Böhme (2007: text figures 1, 2a, b) as Squamata indet. The sixth tooth, which is present in Böhme’s figure, is now broken off and missing.

PHYLOGENETIC ANALYSIS

All phylogenetic trees presented here are based on limited fossil material, and more complete fossil specimens of this taxon are needed to draw more robust conclusions. However, in all the analyses, despite the different topologies in Squamata, Hoeckosaurus is consistently recovered as the sister taxon to Dibamus (or Dibamus + Anelytropsis ), forming a clade ( Fig. 5 View Figure 5 ).

1. In analysis 1, both heuristic and NT searches in TNT produced a single tree ( Fig. 5A View Figure 5 ). This analysis groups Gekkota, Dibamidae and Amphisbaenia (Bremer value 5, relative Bremer 71). Hoeckosaurus mongolensis and D. leucurus form a clade (Bremer value 1, relative Bremer 50; see Fig. 5B View Figure 5 ) that is sister to the Amphisbaenia (Bremer value 5, relative Bremer 71). Gekkota is not recovered as monophyletic, but the two species included in the analysis are placed at the base of the limbless group. The analysis finds a monophyletic Scincomorpha (Bremer value 1, relative Bremer 20), placing it as sister to Anguimorpha (Bremer value 2, relative Bremer 23; this clade is called Autarchoglossa sensu Estes et al., 1988). Iguania are sister to all other squamate clades used in the analysis (Bremer value 6, relative Bremer 40; this clade is called Scleroglossa sensu Estes et al., 1988). 2. In this analysis, the molecular phylogeny of Pyron et al. (2013) is used to constrain the ingroup relationships. The constraint tree places Dibamus as sister to other squamates, with Scincoidea and Lacertoidea as separate clades and Lacertidae as sister to Amphisbaenia (and Rhineura as sister to all other amphisbaenians). The analysis produces a single tree, in which Dibamus and Hoeckosaurus form a clade sister to all other squamates ( Fig. 5C View Figure 5 ).

3. In this analysis, the full Gauthier et al. (2012) matrix is used to test the relationship of the Mongolian Oligocene taxon to Squamata. The heuristic analysis in TNT supports the results of the previous analyses. Hoeckosaurus is found to be sister to Dibamus + Anelytropsis ( Fig. 5D View Figure 5 ). This analysis groups limbless squamates, such as amphisbaenians, dibamids, snakes and Anniella , in a single clade, as also found by Gauthier et al. (2012). It should be noted that this analysis expands on the previous results, because both extant dibamid genera are included. Hoeckosaurus is placed on the stem of two extant dibamid taxa here.