Procerophis sahnii, Rage & Folie & Rana & Singh & Rose & Smith, 2008
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publication ID |
https://doi.org/ 10.4202/app.2008.0303 |
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persistent identifier |
https://treatment.plazi.org/id/03C0B830-5E5A-FFDB-8AED-6DE8FA44FA3D |
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treatment provided by |
Felipe |
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scientific name |
Procerophis sahnii |
| status |
sp. nov. |
Procerophis sahnii sp. nov.
Fig. 4 View Fig .
Etymology: After Ashok Sahni (Panjab University, India), for his contributions to the vertebrate paleontology of India.
Type material: Holotype: One posterior trunk vertebra (VAS 1014). Paratypes: Seven vertebrae: one anterior trunk (VAS 1015), two mid−trunk (VAS 1016, 1046), one posterior trunk (VAS 1057), one posteriormost trunk (VAS 1047), and two caudal (VAS 1048, 1058) vertebrae. All from the continental beds of the early Eocene Cambay Formation .
Type locality: Vastan Lignite Mine, northeast of Surat, Gujarat, India.
Type horizon: Early Eocene (middle to late Ypresian) continental beds of Cambay Formation.
Diagnosis.—Differs from all other snakes, except Nigerophis , in having prezygapophyseal buttresses compressed, forming a vertical ridge, prolonged by anteriorly oriented prezygapophyseal processes. Differs from Nigerophis in having very lightly built vertebrae, a wide and thin zygosphene, a blade−like and long neural spine, well−marked subcentral ridges, and paracotylar foramina (irregularly occurring).
Description of the holotype.—The holotype is a well−preserved vertebra; only its neural spine is damaged. It comes from the posterior trunk region, but it is not a posteriormost trunk vertebra. Usually, the vertebra selected as the holotype is from the mid−trunk. But, in the present case, only two mid−trunk vertebrae are available; one (VAS 1016) is poorly preserved, whereas the other (VAS 1046) is clearly larger (perhaps overgrown) than the other specimens and seemed inappropriate to be selected as the holotype.
The holotype is small, elongate, and very lightly built. Its measurements are as follows (in mm): total length from prezygapophysis to postzygapophysis (prezygapophyseal process excluded): 3.8; width through prezygapophyses: 3.2; width of zygosphene: 1.8; length of centrum from edge of cotyle to tip of condyle: 3.5; width of interzygapophyseal constriction at smallest circumference: 1.9.
In anterior view, the vertebra is wide and slightly depressed. The section of the neural canal is large and its lateral walls are thin. The zygosphene is wide and thin, its roof being slightly convex dorsally. The prezygapophyseal articular facets are nearly horizontal; the prezygapophyseal plane lies clearly above the floor of the neural canal. No part of each prezygapophyseal buttress projects laterally beyond the articular facet, but a strong prezygapophyseal process projects anteriorly. The cotyle is somewhat depressed and it is narrower than the neural canal. A large paracotylar foramen is present on the right side only.
In dorsal view, the vertebra is markedly elongate. The interzygapophyseal constriction is shallow and the apex of its curvature is shifted anteriorly. The zygosphene is wide, it forms three lobes; the lateral lobes are acute whereas the median lobe is wide. The articular facets of the prezygapophyses are elongate and narrow, their major axis being oblique; on either side, a blunt prezygapophyseal process projects anteriorly beyond the facet. The neural spine is thin and long; anteriorly it reaches the roof of the zygosphene. The posterior median notch is shallow.
In lateral aspect, the neural spine is rather high. The zygosphenal articular facets are elongate anteroposteriorly. The prezygapophyseal buttress is compressed, forming a blunt keel that originates just dorsal to the diapophysis. The prezygapophyseal process appears as an anterior projection of the latter keel; it is compressed laterally and its lateral face is slightly concave. The interzygapophyseal ridge is strongly marked and it juts out as a crest in its posterior part. A small, incipient epizygapophyseal spine hardly protrudes posteriorly. The paradiapophyses are small; their articular surfaces are eroded. The condyle is small; it is borne by an elongate neck. The lateral foramina are present.
In posterior view, the neural arch is depressed. The zygosphenal roof is thin. Some foramina open lateral to the zygantrum, but they are not located in fossae.
In ventral view, the centrum is elongate and well−limited laterally by sharp subcentral ridges. The latter ridges are almost parallel. The haemal keel is wide, spatulate and well−marked off from the centrum. The surface of the centrum is slightly concave. The subcentral foramina are present.
Intracolumnar variation.—The only available anterior trunk vertebra displays the differences that are usually observed between vertebrae from the anterior and mid− or posterior trunk regions. Compared with the holotype, the anterior trunk vertebra is shorter, its neural arch is more vaulted (but it remains weakly vaulted), the paradiapophyses are more distant from the centrum, the ventral face of the centrum is poorly limited laterally and it widens anteriorly, and a hypapophysis is present.
Vertebrae from the mid−trunk region are more elongate than the anterior trunk vertebra but less elongate than the posterior trunk vertebrae. Their neural arch is almost as vaulted as in the anterior trunk vertebra. Their centrum is elongate but it slightly widens anteriorly. The haemal keel is markedly thinner than that of the holotype and it is not spatulate, its lateral borders being subparallel.
Posterior, but not posteriormost, trunk vertebrae are illustrated by the holotype. One posteriormost trunk vertebra is available. It differs from the holotype in having paradiapophyses more distant from the centrum and cotyle, articular surfaces of paradiapophyses facing more ventrally, deep subcentral grooves, and a deeper and narrower haemal keel. The caudal vertebrae are very elongate and narrow. They are provided with haemapophyses and pleura− or lymphapophyses.
Intraspecific variation.—The presence of paracotylar foramina is confirmed in only two vertebrae; lateral and subcentral foramina also occur irregularly. Epizygapophyseal spines are present only in the holotype and in the anterior trunk vertebra (not verifiable on VAS 1016). The articular surface of the paradiapophyses is preserved in a single vertebra (VAS 1047): the dia− and parapophyseal areas appear to be poorly differentiated from each other and their sizes are similar.
Discussion.—The light build and marked elongation of the vertebrae as well as the presence of prezygapophyseal processes and paracotylar foramina (although the latter occur irregularly) clearly point to the Colubroidea . However, in extant colubroids, the prezygapophyseal process arises from the non−compressed prezygapophyseal buttress, just below the articular facet, and it projects laterally or slightly anterolaterally. In the extinct families assigned to the Colubroidea , the Anomalophiidae and Russellophiidae , the prezygapophyseal buttress is compressed and it forms a vertical ridge, which is reminiscent of Procerophis . In these two families, however, the ridge extends from the diapophysis to the tip of the articular facet and there is no prezygapophyseal process; the ridge does not project beyond the articular facet. In Procerophis , the ridge does not reach the tip of the facet as a result of the presence of the prezygapophyseal process, the latter appearing as an outgrowth of the ridge. A similar condition is known in Nigerophis , the only nigerophiid in which the morphology of the prezygapophysis is well−known. Nigerophiids do not appear to be colubroid snakes; they are referred to the Acrochordoidea ( Rage 1984; McDowell 1987). It is of interest to note that the prezygapophyseal buttress of the extant Acrochordidae displays a morphology that is intermediate between that of Anomalophiidae and Russellophiidae on one hand, and that of Procerophis and Nigerophis on the other hand. In the Acrochordidae , the buttress is compressed and forms a vertical ridge, as in anomalophiids and russellophiids, but the dorsal part of the ridge protrudes beyond the facet; however, it does not form a process.
Based on the morphology of the prezygapophyses, Procerophis appears to be close to the nigerophiids, i.e. the Acrochordoidea. Nevertheless, the very light build, marked elongation that recalls extant arboreal colubrids, and blade−like, long neural spine are colubroid features. Apart from the prezygapophyses, vertebrae of Procerophis are similar to those of various modern colubrids. Unfortunately, available vertebral characters are not sufficiently numerous to allow reliable phylogenetic analysis. Moreover, some of them are not discrete (lightening, elongation) and others are affected by polymorphism (paracotylar foramina, epizygapophyseal spines). Thus, despite the morphology of the prezygapophyses, Procerophis is referred to the Colubroidea ; the association of its very light build and strong vertebral elongation is inconsistent with acrochordoids. Because of the morphology of the prezygapophyses and the reduced paradiapophyses in which the two articular areas are hardly distinct from each other, Procerophis cannot be referred to an extant colubroid family. Moreover, in having paracotylar foramina and prezygapophyseal processes, Procerophis is more advanced than both the Anomalophiidae and Russellophiidae . Therefore, like various other fossils, Procerophis is a colubroid that cannot be referred to a known family. Among these fossils, Procerophis is reminiscent of Headonophis harrisoni Holman, 1993 from the latest middle Eocene (formerly regarded as late Eocene) of England. However, Headonophis lacks prezygapophyseal processes and its neural spine is short anteroposteriorly ( Holman 1993). Other Colubroidea incertae sedis include some unnamed forms and Vectophis wardi Rage and Ford, 1980 . The oldest colubroid is an unnamed snake from the Cenomanian of Sudan ( Rage and Werner 1999); unfortunately, its prezygapophyses are unknown. This fossil differs from Procerophis sahnii mainly in having less depressed vertebrae and clearly shorter posterior trunk and caudal vertebrae. Augé et al. (1997) reported a Colubroidea incertae sedis from the early Eocene of France. This snake differs from Procerophis in having less elongate vertebrae, clearly shorter prezygapophyseal processes, and a neural spine shorter anteroposteriorly. The species of Colubroidea previously reported from the early Eocene of India ( Rage et al. 2003) has vertebrae more massively built and shorter than those of Procerophis , and parazygosphenal foramina are present. Based on the presence of parazygosphenal and multiple paracotylar foramina, Head et al. (2005) suggested that this snake may belong to acrochordoids. The vertebra from the late middle Eocene of Myanmar assigned to the Colubroidea by Head et al. (2005) differs markedly from Procerophis in being more blocky and shorter than those from Vastan Mine. Vectophis , from the same beds as Headonophis , was tentatively regarded as a colubroid by Rage et al. (2003). Its vertebrae mainly differ from those of Procerophis in being clearly shorter and higher. Whatever its precise relationships, Procerophis is a very distinctive snake.
Caenophidia incertae sedis
Genus Thaumastophis nov.
Etymology: Greek Thaumastos, astonishing, in reference to the prezygapophyseal morphology.
Type species: Thaumastophis missiaeni sp. nov. from Vastan Lignite Mine, northeast of Surat, Gujarat, India; monotypic.
Diagnosis.—As for the type and only known species.
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