Pterosphenus muruntau Averianov, 1997

Pterosphenus muruntau Averianov, 1997

( Figs 1–4)

Palaeophis [sp.]: Nesov, 1995: 8.

Palaeopheidae [indet.]: Nesov, 1995: 8.

Pterosphenus muruntau Averianov, 1997: 138 , fig. 7. Pterosphenus muruntau : Alifanov, 2012: 145.

Type material examined. Holotype. ZIN PC 1 View Materials /34 (anterior trunk vertebra), Uzbekistan, Navoiy Prov. ( Navoiy Viloyati), Central Kyzylkum Desert, Dzheroi 2. Middle Eocene ( Bartonian ).

Paratype. ZIN PC 2 View Materials /34 (middle trunk vertebra), same locality .

Other material examined. Uzbekistan, Navoiy Prov. ( Navoiy Viloyati), Central Kyzylkum Desert: Dzheroi 2, ZIN PH 2/287 (neural arch anterior fragment), ZIN PH 3/287 (neural arch posterior fragment), ZIN PH 4/287 (anterior trunk vertebra), ZIN 5/287 (posterior trunk vertebra); Dzheroi 1, ZIN PH 1/287 (neural arch posterior fragment).

Referred specimens. In addition to the holotype and the paratype, Averianov (1997) referred to P. muruntau five unnumbered vertebral fragments, three from the type locality and two from Dzheroi 1 locality. In this work, only two of these specimens from the type locality are referred to P. muruntau that receive numbers ZIN PH 2/287 and ZIN PH 3/287. Only one specimen from Dzheroi 1 locality ( ZIN PH 1/287) is referred here to P. muruntau . The more recently collected specimens are ZIN PH 4/287 and 5/287 from the type locality.

Specimens ZIN PH 1/287, 2/287 and 3/287 are fragmentary and were not used in the redescription, but ZIN PH 2/287 was included in the measurements ( Table 1).

Comparison. The species is assigned to the genus Pterosphenus because of prezygapophyses markedly reduced, zygosphene convex dorsally, and anterior border of the neural spine originating

A.O. Averianov. Pterosphenus muruntau – valid species of sea snakes from the top of the anterior border of the zygosphene ( Rage et al., 2003). Pterosphenus muruntau differs from other species of Pterosphenus in the less laterally compressed vertebrae and lower pterapophyses. Additionally it differs from P. kutchensis Rage et al., 2003 from the early Eocene of India ( Rage et al., 2003) in the left and right synapophyses not originating from a common base, anterior hypapophysis present, much smaller neural canal, oblique prezygapophyseal articular facets, the zygosphene narrower than the cotyle, concave anterior surface of the zygosphene, prominent interzygapophyseal ridge, and thin base of the neural spine above the zygosphene. Pterosphenus muruntau additionally differs from P. biswasi Rage et al., 2003 from the early Eocene of India ( Rage et al., 2003) in the much smaller neural canal, the zygosphene narrower than the cotyle, and anterior surface of the zygosphene more deeply concave.

Redescription. For description of holotype ( Fig. 1) and paratype ( Fig. 2), see Averianov (1997).

ZIN PH 4/287 ( Fig. 3): Almost complete anterior trunk vertebra, missing dorsal part of neural spine and left pterapophysis. Cotyle cordiiform in shape, with concave dorsal margin below neural canal and ventral margin narrowing towards anterior hypapophysis. Condyle narrower than cotyle, subtriangular in shape, with convex dorsal margin. Axis of condyle horizontal. Centrum

A.O. Averianov. Pterosphenus muruntau – valid species of sea snakes constricted close to condyle. Only basal parts of synapophyses preserved. They positioned close to cotyle and below to mid-height on centrum, but not projecting ventrally to centrum. Anteri- or surface of left synapophysis with tiny foramen similar to that in P. kutchensis ( Rage et al., 2003: fig. 2a). Subcentral ridge poorly defined. Two hypapophyses developed and connected by a ventral keel: smaller anterior and larger, thicker posterior. Anterior hypapophysis hook-like, pointed anteriorly. Posterior hypapophysis about twice as high as anterior one. Small subcentral foramen present on right side. Neural canal small, about one-third of cotyle width. Its anterior opening D-shaped, with dorsal margin more convex than ventral margin. Posterior opening of neural canal nearly round. Neural arch similar in length to centrum, with prezygapophyses projecting anteriorly beyond cotyle and postzygapophysis not projecting posteriorly beyond condyle. Zygapophyses greatly reduced. Transverse width of prezygapophysis about one-third of cotyle width. Zygapophyseal articular surfaces oval and horizontal, located slightly above level of neural canal floor. Their long axis oblique in prezygapophyses and transverse in postzygapophyses. Prezygapophyses connected to synapophyses by sharp vertical ridges. Interzygapophyseal ridge stout. Transverse width of zygosphene about 85% of cotyle width. Anteri- or margin of zygosphene convex. Dorsal margin of zygosphene triangular in anterior view, pointing dorsally and continuing into neural spine whose anterior border coinciding with that of zygosphene. Articular surfaces of zygosphene oval, facing ventrolaterally. Zygantrum consisting of two depressions housing articular surfaces and separated by elevated midline surface. Zygantral articular surfaces oval, facing dorsomedially. Irregular tiny neurovascular foramina present dorsal to these surfaces. A large zygantral foramen placed

A.O. Averianov. Pterosphenus muruntau – valid species of sea snakes on midline within zygantrum as in some other palaeophiids ( Rage et al., 2003; Georgalis et al., 2021). Neural spine extending for entire length of neural spine. Its base triangular in cross-section, wide posteriorly and tapering anteriorly where neural spine as a thin vertical plate. Pterapophyses low, only slightly higher than dorsal margin of zygosphene. Pterapophyseal ridge directed towards zygosphene parallel to interzygapophyseal ridge.

ZIN PH 5/287 ( Fig. 4): Unusual posterior trunk vertebra with nearly absent zygapophyses and reduced zygosphene and zygantrum. Neural spine absent. Specimen referred to Pterosphenus because of reduction of zygapophyses and zygosphene. It is not clear if it is an aberrant specimen or it represents a previously unknown positional variation for Pterosphenus .

Measurements. For measurements, see Table 1.