Largest known madtsoiid snake from warm Eocene period of India suggests intercontinental Gondwana dispersal

Systematic paleontology

Squamata Oppel, 1811

Ophidia Brongniart, 1800

Madtsoiidae (Hoffstetter 1961) McDowell, 1987

Vasuki indicus gen. et sp. nov.

Etymology

Generic name after the well-known Hindu mythical serpent ‘Vāsuki’ around the neck of Lord Shiva; specific name is for the country of origin i.e., India.

Holotype

IITR/VPL/SB 3102-1-21; a partial vertebral column representing the precloacal region (Figs. 2, 3; Supplementary Table 1).

Horizon and locality

Naredi Formation; Panandhro Lignite Mine, district Kutch, Gujarat state, western India.

Diagnosis

Vasuki exhibits a unique combination of the following characters: presence of prominent paracotylar foramina (shared with Madtsoiidae); middle-sized cotyle (shared with Madtsoiidae); median prominence on ventral margin of centrum (shared with Madtsoiidae); prezygapophyseal process absent; high angle of synapophysis with horizontal in anterior view (avg. 71.5°); MTV diapophysis level with dorsoventral midpoint of neural canal (shared with Madtsoia madagascariensis, Madtsoia camposi, Wonambi barriei and Adinophis); prezygapophyseal buttress succeeded posteriorly by elliptical fossa (shared with Madtsoia pisdurensis); deep V-shaped embayment (shared with Gigantophis garstini and Madtsoia pisdurensis); oval precloacal cotyle (shared with Gigantophis garstini and Madtsoia pisdurensis); transversely wide vertebrae (shared with Gigantophis garstini and Madtsoia pisdurensis); neural spine posteriorly canted (shared with Gigantophis garstini and Madtsoia pisdurensis); broad hemal keel with posterior process (shared with Gigantophis garstini and Madtsoia pisdurensis); strongly notched anterior zygosphenal margin; endozygantral foramen present (shared with Madtsoia madagascariensis, Powellophis and Gigantophis garstini).

Autapomorphies: exceptionally large vertebrae [centrum length (cL): 37.5–62.7 mm and prezygapophyseal width (prW): 62.4–111.4 mm]; neural spine cross-section spade-shaped; poorly developed hemal keel which remains dorsal to the parapophyses; chisel-shaped posterior process of the hemal keel.

Description

The collection comprises 27 associated vertebrae which are mostly well-preserved and include a few in articulation (Figs. 2A–T, 3A–W). 22 out of the 27 specimens can be confidently assigned to the precloacal region based on the absence of hemapophyses, pleurapophyses and lymphapophyses, and are further constrained to a position anterior to the posterior trunk region as suggested by a greater mediolateral width of the neural arch compared to centrum length (sensu LaDuke¹; Rio and Mannion²; Supplementary Tables 1, 2; Supplementary Fig. 2). Such vertebral dimensions are usually found in large-bodied madtsoiids such as, Gigantophis²; Yurlunggur¹¹, Madtsoia^1,10,13, and Wonambi²⁵. Moreover, the closure of vertebral sutures suggests these specimens likely reached skeletal maturity, similar for instance to Madtsoia pisdurensis⁸.

Vasuki is characterized by exceptionally large vertebrae where centrum length (cL) and prezygapophyseal width (prW) range between 37.5–62.7 and 62.4–111.4 mm, respectively (Supplementary Table 2). We recognize this as an autapomorphy since these proportions eclipse all large-sized madtsoiids [Madtsoia (cL = 18–25 mm; prW = 35–65 mm; LaDuke et al.¹), Gigantophis (cL = 28–41 mm; prW = 44–66 mm; Rio and Mannion²), Platyspondylophis (cL = 18–21 mm; prW = 26–43 mm; Smith et al.²¹) and Yurlunggur (cL = 15–22 mm; prW = 19–41 mm)]. Some caution, however, is warranted here because of uncertainties as to whether the largest size of these large-bodied madtsoiids has been captured, although, the same is true for Vasuki.

In overall form, the vertebrae of the new Indian taxon are massive (prW >> cL) and comprise a procoelous centrum. Anteriorly, the centrum preserves an anteroventrally inclined cotyle, whereas the posterior condyle is deflected posterodorsally resulting in considerable visibility of the condyle and cotyle in dorsal and ventral views, respectively (Fig. 2C,E). In anterior view, the cotyle is strongly concave with its ventral margin recessed relative to the dorsal. The cotyle is mediolaterally wider than dorsoventrally high (Figs. 2P, 3A,F,K; IITR/VPL/SB 3102-4, coW/coH = 1.2; Supplementary Table 2) as in all madtsoiids [e.g., Gigantophis garstini² (NHMUK R8344, coW/coH = 1.2), Madtsoia madagascariensis (FMNH PR 2551, coW/coH = 1.24) Yurlunggur (NTM P8695-243, coW/coH = 1.22), and Wonambi (QMF23038, coW/coH = 1.4]. Laterally, the cotyle is bordered on each side by a well-developed and moderately deep paracotylar fossa (Figs. 2K,P, 3A,K). The dorsal and ventral margins of the fossa are prominent and defined by bony struts emanating from the dorsolateral and lateral cotylar margins, respectively. The lateral margin of the fossa, however, is flush with the surface. Furthermore, in some specimens the paracotylar fossa is divided into a shallower dorsal and deeper ventral sub-fossa by a weak secondary strut extending laterally from the dorsolateral margin of the cotyle. A tiny paracotylar foramen is present on the dorsal-most part of one or both paracotylar fossae, immediately lateral to the neural canal (Figs. 2F,K,P, 3A,K). While the presence of paracotylar fossae and foramina is a synapomorphy of Madtsoiidae^16,26, the exact morphology of these features is variable across the clade. “Gigantophis sp.” (CPAG-RANKT-V-1), Menarana nosymena and Adinophis fisaka (FMNH PR 2572) differ from Vasuki in the presence of paired paracotylar foramina on each side^1,16,27. In Madtsoia and Eomadtsoia (MPEF-PV 2378) the foramina are deep and comparatively large, whereas in Yurlunggur these occur in clusters^7,8,10,11,13. Eomadtsoia, however, shares with Vasuki the presence of prominent ventral rim of the paracotylar fossa⁷. In Gigantophis garstini the paracotylar fossa lacks a ventral margin and in Platyspondylophis the paracotylar foramen is absent altogether^2,21.

The posterior condyle is transversely wider than high (IITR/VPL/SB 3102–4, cnW/cnH = 1.2; Supplementary Table 2) with the width progressively increasing from ATV (Fig. 2B,G; cnW/cnH = 1.1) to MTV (Fig. 3G, Q; cnW/cnH = 1.2–1.3). Similar proportions of the posterior condyle characterize most madtsoiids [e.g., Nidophis (LPB FGGUB v.547/3, ATV, cnW/cnH = 1.1; LPB FGGUB v.547/1, MTV, cnW/cnH = 1.2); Gigantophis garstini (NHMUK R8344, MTV, cnW/cnH = 1.2 Rio and Mannio²); Madtsoia camposi (DGM 1310b, MTV, cnW/cnH = 1.3] (Fig. 3G,I,Q). Furthermore, in posterior view, two small, distinct fossae are discernible on the lateral surface of the centrum immediately posterior to the left diapophysis (Fig. 3G,I,Q). The fossae are vertically arranged, on top of each other, and separated by a prominent ridge. Whether these unilateral fossae represent an individual condition or a general feature cannot be currently ascertained and will require additional specimens of Vasuki.

The synapophysis is dorsoventrally high and comprises a distinct diapophysis and parapophysis (Figs. 2M,R, 3C,R) unlike in Gigantophis garstini, Madtsoia madagascariensis, and Madtsoia pisdurensis^1,2,8. In anterior view, the orientation of the synapophysis changes from ventrolateral (Fig. 2F,K) to somewhat laterally facing (Fig. 3K,P,U) across the precloacal series. This change is marked by an increase in the synapophyseal angle (α), with the horizontal, from ATV (α = avg. 56.6°) to MTV (α = avg. 71.5°). A narrower synapophyseal angle was observed in most of the comparative madtsoiid taxa including Eomadtsoia [MPEF-PV 2378 (MTV), α = 45°], Gigantophis garstini [NHMUK R8344 (MTV) α = 48], Madtsoia madagascariensis [FMNH PR 2549 (ATV), α = 47°; FMNH PR 2551 (MTV), α = 56°], “Gigantophis sp.” [CPAG-RANKT-V-1 (MTV), α = 56°], Madtsoia camposi [DGM 1310c (MTV), α = 57°], Wonambi [QMF23038 (MTV) α = 58°] and Madtsoia bai [AMNH 3155 (MTV), α = 62°]. In lateral view, the synapophysis is inclined at (β) 20°–27° from the vertical in Vasuki. This is similar to Wonambi [QMF23038, β = 25°], Nanowana [QMF19741, β = ~ 25°], Madtsoia camposi [DGM 1310c, β = 26°] and Yurlunggur [P8695, β = 22°–26°]. In contrast, wider angles characterize Gigantophis garstini [NHMUK R8344, β = 30°²], Platyspondylophis [β = 30°–35°], Madtsoia madagascariensis [FMNH PR 2549, β = 33°] and “Gigantophis sp.” [CPAG-RANKT-V-1, β = ~ 90°], whereas in Patagniophis [β = 7°–9°], Powellophis [PVL 4714–4, β = 18°] and Madtsoia pisdurensis [225/GSI/PAL/CR/10, β = 12°] the angles are narrower.

An arcuate paracotylar notch (sensu LaDuke et al.¹), between the ventral cotylar rim and the parapophysis, is consistently present in all specimens (Fig. 2A,F). The parapophysis comprises a sub-rectangular facet, in lateral view, and extends below the ventral cotylar rim in ATV (Fig. 2F,H,P,R). In MTV it lies dorsal to the ventral cotylar rim (Fig. 3F,P) unlike Madtsoia pisdurensis and Gigantophis garstini where the parapophyseal base is ventral and in level with the ventral cotylar rim, respectively^2,8. The diapophysis is bulbous and extends laterally beyond the prezygapophysis (Figs. 2F,H, 3P,R), contrary to Powellophis³, Patagoniophis australiensis²⁸, Madtsoia pisdurensis⁸, Madtsoia madagascariensis¹ and Nidophis⁹. The dorsal margin of the diapophysis remains ventral to the dorsal cotylar margin in ATV (Fig. 2A,F), but becomes level with the dorsoventral midpoint of the neural canal in MTV (Fig. 3K,P). A similar disposition of the MTV diapophysis is observed in Madtsoia madagascariensis, Madtsoia camposi, Wonambi barriei and Adinophis^1,2,13,27. The dorsal diapophyseal margin lies between the ventral margin of the neural canal and the dorsoventral midpoint of the cotyle in “Gigantophis sp.”¹⁶, Gigantophis garstini², Nidophis⁹, Yurlunggur¹¹ and Powellophis³. In Platyspondylophis the diapophysis extends beyond the ventral margin of the neural canal in all preserved precloacal vertebrae²¹.

The prezygapophyseal buttress is massive, lacks a prezygapophyseal process and bears an oblique, blunt ridge anteriorly (Fig. 2F,K). In lateral view, the buttress is succeeded posteriorly by an elliptical fossa (Fig. 2C,H,R). The fossa occurs immediately ventral to the interzygapophyseal ridge and medial to the diapophysis, similar to Madtsoia pisdurensis (Mohabey et al.⁸). The prezygapophyseal facets are elliptical (5022–4, przL/przW = 1.3) and inclined ventromedially (prα = 20°–28°; Fig. 2A,D,F,I). In dorsal view, these facets diverge at 45° from the sagittal plane, contrary to the transversely oriented facets in Madtsoia bai¹⁰, Madtsoia madagascariensis¹, Platyspondylophis²¹, and Yurlunggur¹¹. Strongly divergent prezygapophyses are also observed in Gigantophis garstini² (~ 70°) and Eomadtsoia⁷ (60°–80°). The postzygapophyseal facets in Vasuki are also elliptical (IITR/VPL/SB 3102-8II, pozL/pozW = 1.2; Supplementary Table 2) and medioventrally oriented (poα = 12°–26°; Figs. 2G,J, 3B,E). The interzygapophyseal ridge is thick and posterodorsally directed, acting as a bridge between the pre- and postzygapophyses. A small lateral foramen is present ventral to the ridge (Fig. 3L,Q) as in Powellophis³. In dorsal view the interzygapophyseal ridges are straight and differ from the arcuate ridges seen in most madtsoiids [e.g., Madtsoia, Gigantophis garstini, Wonambi, Yurlunggur and Platyspondylophis]^{2,8,10,11,13,18,21,28}.

The neural canal is reniform (Figs. 2P,Q, 3F,G) in cross-section and significantly wider than high (ncW/ncH = 3–3.6). It differs from the comparatively narrower and trilobate neural canal in Gigantophis garstini² (NHMUK R8344, ncW/ncH = 2.3), Platyspondylophis (WIF/A 2271, ncW/ncH = 2.1), Madtsoia (ncW/ncH = 1.3–2.3), Yurlunggur (NTM P8695-243, ncW/ncH = 2.3), “Gigantophis sp.” (CPAG-RANKT-V-1, ncW/ncH = 1.8) and Powellophis (PVL 4714–4, ncW/ncH = 1.6), and the sub-elliptical canal in Wonambi (QMF23038, ncW/ncH = 1.3).

The zygosphene is trapezoidal and mediolaterally wider than high (zsW/zsH = 1.4–1.8; Fig. 2A,K), as in Gigantophis garstini (NHMUK R8344, zsW/zsH = 2²), Madtsoia bai (AMNH 3155, zsW/zsH = 1.8) and Madtsoia madagascariensis (FMNH PR 2551, zsW/zsH = 1.9). Transversely much wider zygosphenes characterize Nidophis (LPB FGGUB v.547/1, zsW/zsH = 5), Madtsoia camposi (DGM 1310a, zsW/zsH = 2.8), Eomadtsoia (MPEF-PV 2378, zsW/zsH = 2.6), Platyspondylophis (WIF/A 2269, zsW/zsH = 2.2) and Patagoniophis (QMF 19717, zsW/zsH = 5). In Vasuki, the zygosphene is wider than the cotyle, contrary to Gigantophis garstini, “Gigantophis sp.”, Platyspondylophis and Madtsoia^{1,8,10,13,16,21}. In anterior view, dorsal margin of the zygosphene is straight and the articular facets are steeply inclined (~ 40° form the vertical; Figs. 2F,P, 3A). These facets are oval in lateral view (IITR/VPL/SB 3102–6, zsfL/zsfW = 1.1). The anterior zygosphenal margin is markedly notched in dorsal view (zsα = 118°–128°; Figs. 2I,N, 3N), and differs from the non-notched zygosphene in Madtsoia pisdurensis⁸, Madtsoia camposi¹³, Eomadtsoia⁷ and Platyspondylophis²¹. In “Gigantophis sp.” (zsα = 145°) and Madtsoia madagascariensis (zsα = 145°–147°) the zygosphene is weakly notched.

The zygantrum is mediolaterally wider than high, with steeply inclined facets (50°–60° from the horizontal; Fig. 2B,G,Q). The facets are elliptical in posterior view, but devoid of a median wall present in Gigantophis garstini². An anteroventrally directed fossa is present at the base of each facet, and accommodates an endozygantral foramen (Figs. 2G, 3B). The latter is also present in Madtsoia madagascariensis¹, Powellophis³ and Gigantophis garstini². In Vasuki, the zygantral roof above each facet is medio-dorsally convex and descends as sub-vertical ridges into the zygantrum (Fig. 2Q) as in Madtsoia madagascariensis¹. The roof is ventrally convex in Eomadtsoia and Madtsoia pisdurensis, and straight in Powellophis, Platyspondylophis, Yurlunggur and Gigantophis garstini^3,7,8,11,21. A large, dorsolaterally oriented, elliptical parazygantral fossa flanks the zygantrum laterally on either side and bears a small parazygantral foramina (Fig. 2B,G,Q).

The neural spine is dorsoventrally high (MTV, nsH/tvH = 0.21–0.29, Supplementary Table 2) and buttressed posteriorly by the neural arch laminae (Fig. 3B–D,V,W). The latter extend anterodorsally from the dorsolateral margin of the postzygapophyses up to the dorsal spinal margin, resulting in a deep median embayment. In lateral view, the spine is steeply inclined posterodorsally (12°–19° from the vertical) with a concave anterior and a straight posterior margin. While a high neural spine characterizes most large madtsoiids [Madtsoia camposi (DGM, 1310b, MTV, nsH/tvH = 0.22), Madtsoia madagascariensis (FMNH PR 2551, MTV, nsH/tvH = 0.33), Madtsoia bai (AMNH 3154, MTV, nsH/tvH = 0.22), Wonambi (QMF23038, MTV, nsH/tvH = 0.27)], it is more gently inclined in these large-sized taxa [e.g., Madtsoia madagascariensis (27°–33°), Wonambi (30°), Gigantophis garstini (30°)]. A convex anterior margin in Madtsoia madagascariensis as well as Powellophis and Nanowana further distinguishes them from Vasuki. Furthermore, the presence of a sharp postspinal lamina (sensu Tschopp²⁹) on the posterior spinal surface and a spade-shaped cross-section of the spine differentiates Vasuki from other madtsoiids (Figs. 2D,S, 3D). In dorsal view, the neural spine base is flanked on either side by a prominent fossa (Fig. 2I,S), as in Madtsoia pisdurensis⁸ and Madtsoia madagascariensis¹. The fossae occur immediately posterior to the zygosphene and are bordered ventrally by weak, rounded bony struts emanating from the posterolateral zygosphenal margin. Ventral to these struts, a prominent foramen is present on the dorsal surface of the neural arch posterior to the zygosphene (Fig. 2I), similar to Madtsoia madagascariensis¹.

In ventral view, the centrum is triangular and widest across the parapophyses. Large paired subcentral fossae, more prominent in the anterior trunk vertebrae (ATV), occupy most of the ventral surface of the centrum (Figs. 2J,O, 3E,T). The fossae are bordered laterally by robust subcentral ridges that extend posteromedially from the parapophyses to the dorsoventral midpoint of the condyle. These ridges are straight to weakly convex in ventral view and differ from the concave ridges in Patagoniophis²⁸ and Madtsoia madagascariensis¹. The subcentral fossae are separated by a transversely convex low hemal keel (Figs. 2T, 3E,O,T). The latter is broad, weakly raised and terminates anterior to the precondylar constriction. The hemal keel is not prominent, unlike the narrow/sharp keel in “Gigantophis sp.”, Eomadtsoia, Nidophis, Nanowana and Powellophis^2,3,7,9,16. In Vasuki, this keel remains dorsal to the ventral parapophyseal margin (Figs. 2M,R, 3M,R,V) unlike the hemal keel of other madtsoiids which descends below the parapophysis. Consequently, we identify the disposition of the hemal keel as an autapomorphy of Vasuki.

A small subcentral foramen is present on either side of the ventral shaft in Vasuki (Fig. 2E,O,T), as in Madtsoia madagascariensis¹, Madtsoia camposi¹³, Nidophis⁹, and Patagoniophis²⁸. The hypapophysis is paddle-like with sharp lateral margins and extends up to the level of the ventral condylar rim in ATV (Fig. 2G,H,J,L,M,O). The hypapophysis is directed posteroventrally unlike the ventrally directed hypapophysis in Madtsoia madagascariensis¹ and Patagoniophis²⁸. Across the precloacals, the hypapophysis progressively reduces in prominence and is replaced by a chisel shaped structure with paired protuberances separated from the ventral condylar rim by a short, sharp ridge in the mid-trunk vertebrae (MTV; Fig. 3J,O,T,X). This chisel shaped structure appears autapomorphic for Vasuki as it differs from the condition in other madtsoiids.

Phylogenetic analysis

The position of Vasuki within Madtsoiidae was tested in a modified version of the character-taxon matrix of Zaher et al.³⁰ (Analysis 1; see “Methods” section and Supplementary Note 2). 50 most parsimonious trees were recovered with a tree length of 1610, consistency index (CI) of 0.386 and retention index (RI) of 0.73. The resultant tree topologies are largely consistent with Zaher et al.³⁰ as Madtsoiidae was recovered as a distinct clade within crown Serpentes (Fig. 4, Supplementary Fig. 3). Madtsoiidae, however, was poorly resolved and did not provide insights into the inter-relationship of Vasuki with the other members of the clade. The poor resolution is likely a reflection of the absence of cranial material in majority of madtsoiids and a function of the large matrix where very few vertebral characters could be scored for most madtsoiid taxa. We, therefore, ran a second analysis (Analysis 2) by removing all non-madtsoiid Serpentes and combining the cranial and vertebral characters of Zaher et al.³⁰ and Garberoglio et al.³, respectively (see “Methods” section and Supplementary Note 3). The latter dataset was used because as the study focused on madtsoiid ingroup relationships. Our analysis recovered only two most parsimonious trees with a tree length of 191, CI of 0.634 and RI of 0.62. Both trees (Fig. 5, Supplementary Fig. 4) were mostly well resolved and the resultant topologies largely consistent with recent studies^2,3,7 on madtsoiid inter-relationships. Madtsoiidae shows size-based clustering with the small (< 2 m) and medium–large bodied (> 3 m) taxa recovered as separate clades (Fig. 5). Vasuki is nested within a distinct clade (Bremer support = 3) as a sister taxon to Indian Late Cretaceous Madtsoia pisdurensis + North African Late Eocene Gigantophis garstini.

Estimation of body length

Quantitative estimates of total body length (TBL) of Vasuki were made based on two separate methods which have been used in recent years for size estimation of extinct large-bodied snakes (see “Methods” section and Supplementary Tables 3–5). In these methods TBL was regressed on the postzygapophyseal width (following Head et al.³¹; Rio and Mannion²) and the prezygapophyseal width (= trans-prezygapophyseal width; following McCartney et al.³², Garberoglio et al.³), respectively. In the present study estimates were made from MTV (IITR/VPL/SB 3102-4, 3102-8I–II, 3102-11II–III), the largest specimens in the collection, following Rio and Mannion², McCartney et al.³² and Garberoglio et al.³. Both regression models were statistically significant (p < 0.05) and had a high explanatory power (r² = 0.83–0.96) which asserts their validity. The TBL estimates following Head et al.³¹ ranges between 10.9 and 12.2 m (Fig. 6A,B), whereas those following McCartney et al.³² is between 14.5 and 15.2 m (Fig. 7A). These estimates, however, should be treated with caution as the collection lacks posterior precloacal and cloacal vertebrae, and an understanding of the intracolumnar variation in madtsoiids is currently non-existent.

It is worth noting that the largest body-length estimates of Vasuki appear to exceed that of Titanoboa, even though the vertebral dimensions of the Indian taxon are slightly smaller than those of Titanoboa. We acknowledge that this observation may be a reflection of the different datasets used to formulate the predictive equations. However, we do not disregard the results based on the dataset of MacCartney et al.³², since the equations derived from the dataset of Head et al.³¹ involve measurements of extant boine taxa that are taken from vertebrae 60–65% posteriorly along the column. Caution is warranted here because of the uncertainties surrounding the phylogenetic position of Madtsoiidae relative to crown snakes which make estimations based on a model depicting intracolumnar variation in vertebral morphology of a particular extant family/taxa tentative. Consequently, predictive regression equations following McCartney et al.³², which comprise vertebral data from an array of extant snakes, are also considered in our study.

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