Palaeophis colossaeus Rage, 1983

Mccartney, Jacob A., Roberts, Eric M., Tapanila, Leif & O’Leary, Maureen A., 2018, Large palaeophiid and nigerophiid snakes from Paleogene Trans-Saharan Seaway deposits of Mali, Acta Palaeontologica Polonica 63 (2), pp. 207-220 : 211-214

publication ID	https://doi.org/ 10.4202/app.00442.2017
persistent identifier	https://treatment.plazi.org/id/03878781-FF98-0F40-A4D0-CED4FD908FFD
treatment provided by	Felipe
scientific name	Palaeophis colossaeus Rage, 1983
status

Treatment

Figs. 4 View Fig , 5 View Fig .

1983 Palaeophis colossaeus sp. nov.; Rage 1983b: 1029. 2003 Palaeophis colossaeus Rage, 1983 ; Rage et al. 2003: 698.

Holotype: MNHN.F. TGE 614 , a mid-trunk vertebra ( Rage 1983b).

Type locality: Tamaguélelt, Mali ( Rage 1983b).

Type horizon: Early Eocene.

Material.— CNRST-SUNY 306, 308, 309, 323, anterior trunk vertebrae; CNRST-SUNY 297, 299, 300, 301, 313, 316, 321, anterior or mid-trunk vertebrae; CNRST-SUNY 261, 288, 291, 292, 294, 295, 304, 305, 310, 315, 325, mid-trunk vertebrae; CNRST-SUNY 289, 296, 302, 311, 318, 322, mid-trunk or posterior trunk vertebrae; CNRST-SUNY 293, 320, posterior trunk vertebrae; CNRST-SUNY 290, 298, 303, 307, 312, 314, 317, 319, trunk vertebrae. From the Eocene Tamaguilelt Formation, locality Mali-20, near Tamaguélelt, Mali; specimens were recovered from thick, phosphatic conglomerate beds ( Tapanila et al. 2008). CNRST-SUNY 288, 289, 290, 291, 292, 293, 294, 295, 307, 308, 309, 310 from Unit 7 ( Mali 20c in Tapanila et al. 2008); the remaining specimens were recovered from Unit 2 ( Mali 20a in Tapanila et al. 2008).

Emended diagnosis.—Distinct from all other snakes by the following combination of characters: robust, large vertebrae lacking prezygapophyseal accessory processes, with a parazygosphenal foramen in the mid- and posterior trunk. Differentiated from all other palaeophiids by its greater size and the presence of parazygosphenal foramina in the mid- and posterior trunk. Further distinct from Pterosphenus and all Palaeophis except Palaeophis maghrebianus and Palaeophis virginianus by lacking any lateral compression in the vertebrae. Separated from Palaeophis maghrebianus by having a greater width of the zygosphene relative to that of the cotyle, and from Palaeophis virginianus by having a neural spine that arises from immediately posterior to the zygosphene.

Description.— Palaeophis colossaeus is known only from isolated vertebrae. There is substantial variation in the size of the new material, with the smallest vertebra ( CNRST-SUNY 306) measuring 13.6 mm in neural arch width, and the largest ( CNRST-SUNY 289) 47.3 mm across the same. To facilitate comparisons with other species, the mid-trunk vertebrae are described first, and variation in morphology of the subsequent regions is provided thereafter.

Mid-trunk and probable mid-trunk vertebrae make up the bulk of the collection. Throughout the collection, there are no complete neural spines, pterapophyses, or synapophyses. The best-preserved specimens are CNRST-SUNY 294, 310, and 325, and the description is primarily based on these specimens, with variations noted.

The base of the neural spine is triangular in cross-section, and extends from the posterior border of the neural arch to the posterior portion of the zygosphene. The zygosphene is massive, and is transversely wider than the cotyle. The dorsal surface of the zygosphene is highly variable, ranging from weakly concave (e.g., CNRST-SUNY 325), to flattened or even slightly convex dorsally (e.g., CNRST-SUNY 294). The anterior surface is deeply concave in all well-preserved specimens, and bears a weak arched ridge extending from the anterior edge of the neural arch pedicle on each side. An indistinct, thick ridge extends ventrally from the inferoposterior edge of the zygosphenal facet to the interzygapophyseal ridge. Lying between the zygosphenal facet and the prezygapophysis there is a single, large parazygosphenal foramen. On the posterior surface, the zygantrum is broad and tall. The anterior wall is smooth and vertical, and contains a variable number of foramina in a varied arrangement. Most frequently there are three foramina, one set low and close to each zygantral facet, and a median foramen that may be set in line with the other two, or superiorly.

The neural canal is trifoliate in appearance, with low ridges extending along the length of the dorsolateral sides of the canal. The walls of the neural canal are thick, but tapered anteriorly into a sharp crest continuous superiorly with the arched ridge of the zygosphene.

The pre- and postzygapophyses project strongly laterally. The facets are oval in outline, and their long axis is oriented at about 50–60° from the main vertebral axis; based on differences from the anterior trunk vertebrae (see below), higher values suggest a more posterior position in the column. The prezygapophyses are inclined superolaterally at about 20–25° from horizontal. There is no prezygapophyseal accessory process; instead, the thick prezygapophyseal buttress tapers to a ridge that extends ventrally toward the synapophyses. The superior portion of the buttress is convex laterally, such that it projects slightly beyond the lateral extent of the prezygapophyseal facet. The inferior portion of the buttress is weakly concave laterally.Throughout the collection, the height and lateral projection of the superior portion is variable, either narrower and more strongly projecting (exemplified by CNRST-SUNY 310), or deeper and flatter (exemplified by CNRST-SUNY 325). Comparison with anterior trunk vertebrae (see below) suggests the buttress transitions from projecting to flattened. Extending posteriorly from the prezygapophysis is a weakly projecting, thick interzygapophyseal ridge, which expands posteriorly toward the postzygapophysis and upward to the base of the pterapophysis. The lateral surface dorsal to the postzygapophyseal facet is gently concave. Nothing beyond the bases of the pterapophyses are preserved in any specimen, thus their height is unknown.

The cotyle is slightly wider than tall, and its thick lip is occasionally incomplete just inferior to the neural canal, causing its upper edge to appear flat or even slightly concave. The paracotylar fossa is divided into superior and inferior portions by a thickened ridge situated about one quarter of the distance from the top of the cotylar height. In several specimens this ridge bears a single foramen. Another single large foramen sits in the superior part of the paracotylar fossa, (doubled in CNRST-SUNY 288). The inferior half of the paracotylar fossa is occupied by a series of minute foramina surrounding the cotyle. The condyle is best preserved in CNRST-SUNY 310, and it shares the same general shape as the cotyle and is not upturned.

The synapophyses are damaged in all the specimens, but preservation on some (including the holotype, MNHN TGE 614 ) indicates the diapophyseal portion faces laterally, and the parapophyseal portion ventrally ( Rage 1983b). Posterodorsal to the remnants of the synapophysis is a rugose patch, probably a site of muscular attachment. The scar consists of an anteroventrally oriented ridge offset by depressions above and below, with a variable number of other bumps also present. The superior depression bears the lateral foramen; a variable number of small foramina occur in the inferior depression as well. In CNRST-SUNY 302 , the morphology of the rugosity is interrupted by an extremely deep depression .

Mid-trunk vertebrae bear a single, vertically directed hypapophysis. It is most completely preserved in CNRST-SUNY 290 , and partly present in CNRST-SUNY 292 . In all specimens retaining traces of the hypapophysis, the base is thick. In CNRST-SUNY 290 , it is dorsoventrally short and extends anteriorly for approximately two-thirds of the centrum. As in the holotype, the hypapophysis tapers distinctly near its tip. Extending anteriorly from the hypapophysis to the inferior cotylar lip is a moderately thick ridge. The ridge is strongly arched dorsally in CNRST-SUNY 310 and CNRST-SUNY 325 , and weakly arched in CNRST-SUNY 292 . A shallow fossa lies between the ridge and the synapophysis. At the posterior edge of the synapophysis there are one or two foramina .

Anterior trunk vertebrae preserved in the collection include only four specimens, CNRST-SUNY 306 , 308 , 309 , and 323. As in the mid-trunk, there are no complete neural spines or synapophyses, and the hypapophyses are incomplete. The anterior trunk vertebrae are similar to the mid-trunk ones in many respects; the anterior trunk vertebrae are more elongated, and have a smaller second, anterior hypapophysis. Additional differences in morphology are noted below .

The base of the neural spine extends over only approximately two-thirds of the length of the neural arch in all but CNRST-SUNY 323. In that specimen, the spine extends to the posterior edge of the zygosphene, probably an indication of a more posterior position in the vertebral column (see above). In CNRST-SUNY 308, the zygosphene has a notable midline notch, but this is absent in the other specimens. To either side of the notch, the superior edge of the zygosphene is convex dorsally, giving it bilobed appearance in anterior view. There is no parazygosphenal foramen in any of the anterior trunk vertebrae in the collection. The pre- and postzygapophyses do not project strongly laterally as compared to the mid-trunk, with the long axis oriented at about 32° from the main vertebral axis, and they are not as strongly inclined superolaterally (15° from horizontal). The facets are also narrower than they are in most of the mid-trunk vertebrae. The prezygapophyseal buttress has a morphology similar to that of CNRST-SUNY 325, with a dorsoventrally taller, but less projecting superior portion.

On the centrum, the rugose area posterodorsal to the prezygapophysis is most highly developed in the largest specimen ( CNRST-SUNY 323) and only weakly developed in the smaller specimens. On the ventral surface of the centrum in the anterior trunk, palaeophiids bear two median hypapophyses in series. The anterior of these is broken off on each specimen, but a specimen in the British Museum ( NHMUK PV R 9531) shows it to be short, narrow, and anteriorly projecting. The degree to which the size of the anterior hypapophysis changes through the anterior trunk is unclear. Lying between the anterior and posterior hypapophysis is a ridge of variable thickness, highest and narrowest in CNRST-SUNY 306, possibly indicating a more anterior position in the trunk by comparison with the British Museum specimen. In the other vertebrae, the ridge is low and thick. The subcentral foramina are largely similar to those in the mid-trunk, but in CNRST-SUNY 309, there is a single paramedian foramen on the left side of the subcentral ridge.

The posterior trunk is poorly represented in the collection. Only CNRST-SUNY 293 and CNRST-SUNY 320 can be definitively assigned to that region. Both are poorly preserved, with very few complete features. They are recognized as belonging to the posterior trunk by the complete absence of a subcentral keel anterior to the hypapophysis. All other features preserved are similar to the mid-trunk vertebrae.

Remarks.—Intracolumnar variation in palaeophiid snakes is relatively poorly understood, with the only detailed discussions provided for Palaeophis maghrebianus (seeHoussayeet al. 2013), Palaeophis nessovi (see Snetkov 2011), Palaeophis vastaniensis (see Bajpai and Head 2007), Pterosphenus kutchensis (see Rage et al. 2003), and Pterosphenus schweinfurthi (see Janensch 1906a; McCartney and Seiffert 2016). Anterior vertebrae of Palaeophis colossaeus are characterized by several features. The zygapophyses are small, and not laterally projecting; the zygosphenes are relatively wide; and for at least part of the anterior trunk, the neural spines are anteroposteriorly restricted in length. These characters appear to be common in many major clades of snakes ( LaDuke 1991), although their evolutionary polarity is currently unknown. Anterior trunk vertebrae are further characterized in palaeophiids by the presence of an anterior hypapophysis, posited to be pleurocentrally derived ( Hoffstetter 1955; Rage 1983b, 1984), and by vertebrae that are more anteroposteriorly elongate ( Houssaye et al. 2013). In addition, in Palaeophis colossaeus there is a shape change in the prezygapophyseal buttress that appears to occur gradually from the anterior trunk through part of the mid-trunk, resulting in an increasingly elongate and projecting prezygapophyseal process. In the mid-trunk region, there is also a trend toward decreasing height of the keel extending anteriorly from the hypapophysis, resulting in its absence in the posterior trunk (also noted to occur in Pterosphenus ; McCartney and Seiffert 2016). Of note is the absence of caudal vertebrae in this and other collections of Palaeophis colossaeus . Caudals are rare for palaeophiids generally, but those that are known are typical for snakes ( Rage 1983a; Parmley and Case 1988), so it is unlikely that they are simply misidentified for this species.

Palaeophis colossaeus is morphologically distinguished from other palaeophiids by several features. It is the largest known species, exceeding slightly the greatest reported size for Palaeophis maghrebianus from Morocco ( Arambourg 1952). Most of its vertebrae also bear bilateral parazygosphenal foramina, a feature found only in a few distantly related snakes, including two species of Acrochordus View in CoL (see Hoffstetter and Gayrard 1964; Head 2005), and the colubroid snakes Synophis View in CoL (see Bogert 1964), Renenutet (see McCartney and Seiffert 2016), and an unnamed species from the Eocene of India ( Rage et al. 2003). Palaeophis colossaeus also differs from most other palaeophiids by having relatively broad, robust vertebrae with laterally-projecting zygapophyses, features shared with Palaeophis maghrebianus from Morocco and Palaeophis virginianus from North America ( Rage 1983b). Nevertheless, the latter two species have vertebrae that are slightly narrower and more elongate than vertebrae from similar regions of the trunk in Palaeophis colossaeus ; the zygosphene is additionally transversely broader in Palaeophis colossaeus , in some cases exceeding the cotylar width.