Fossils of the oldest diplodocoid dinosaur suggest India was a major centre for neosauropod radiation

The Early Jurassic and Cretaceous deposits of India are known for their diverse sauropod fauna, while little is known from the Middle and Late Jurassic. Here we report the first ever remains of a dicraeosaurid sauropod from India, Tharosaurus indicus gen. et sp. nov., from the Middle Jurassic (early–middle Bathonian) strata of Jaisalmer Basin, western India. Known from elements of the axial skeleton, the new taxon is phylogenetically among the earlier-diverging dicraeosaurids, and its stratigraphic age makes it the earliest known diplodocoid globally. Palaeobiogeographic considerations of Tharosaurus, seen in conjunction with the other Indian Jurassic sauropods, suggest that the new Indian taxon is a relic of a lineage that originated in India and underwent rapid dispersal across the rest of Pangaea. Here we emphasize the importance of Gondwanan India in tracing the origin and early evolutionary history of neosauropod dinosaurs.

Etymology.Generic name is a combination of Tharo, referring to the 'Thar desert' of western India where the type specimen was found, and saurus, which is derived from the Greek word 'sauros' meaning lizard; specific name is for the country of origin i.e., India.
Horizon and locality.Fort Member of the Jaisalmer Formation (early-middle Bathonian), Jethwai village, District Jaisalmer, Rajasthan state, western India.
The centrum shows strong anterior constriction with the lateral surfaces excavated by large fossae, similar to many flagellicaudatans 8,13,24,26 (Fig. 2B-E).The fossa is partially preserved with much of the dorsal rim and medial wall broken off.The preserved portion suggests the fossa was elliptical, being elongated and dorsoventrally compressed (Fig. 2B-C).The dorsal margin of the fossa is rod-like and extends laterally beyond the ventral margin.The latter is robust, rounded and thickest near the posterior end of the centrum, accentuating the depth of the fossa.Anteriorly it becomes flush with the lateral surface of the centrum (Fig. 2B-C).Beneath the ventral margin of the fossa, the lateral surface of the centrum is markedly concave and flares posteriorly.The fossa is restricted to the posterior half of the centrum and includes two distinct halves: a short and shallow anterior half and a longer and deeper posterior half.The two halves are demarcated by a low U-shaped ridge on the ventral surface of the fossa, medial to the ventral margin and extending slightly onto the medial wall of the fossa.This contrasts with the dicraeosaurids Amargasaurus, Dicraeosaurus, Bajadasaurus, and Pilmatueia where the fossa is undivided and shallow 5,7,8 .In the middle cervicals of Lingwulong and Suuwassea, however, the lateral fossa is deep and extends nearly along the entire length of the centrum 13,24 .Furthermore, the lateral fossa in Tharosaurus  is succeeded by a posteroventral fossa (Fig. 2D,E), as seen in the mid-cervicals of Amargasaurus, Apatosaurus, Bajadasaurus, and Pilmatueia 26,28 .
The ventral surface of the centrum (Fig. 2F-G) accommodates paired longitudinal depressions/fossae.Such depressions are common in diplodocids and most dicraeosaurids, barring Bajadasaurus which only bears a longitudinal keel 7,8,13,26 .The fossae are asymmetrical, with the right fossa mediolaterally wider, and extend up to the posterior margin of the centrum where they flare out.However, in Dicraeosaurus, Lingwulong, and Pilmatueia the fossae are symmetrical and strongly expressed anteriorly, but do not reach the posterior margin of the centrum 8,13,26 .The fossae in Tharosaurus are flanked laterally by prominent lateroventral flanges and separated by a sharp midline keel (Fig. 2F,G).The keel shows left lateral convexity and remains prominent throughout its preserved length.It extends nearly up to the posterior margin of the fossae and bifurcates into two short ridges.The latter, however, do not reach up to the lateroventral flanges.An accessory ridge, present only on the right ventral surface, extends posterolaterally from the midline keel.The presence of a ventral keel is shared with a diverse array of flagellicaudatans including most dicraeosaurids (Lingwulong, Dicraeosaurus, Pilmatueia, Bajadasaurus and Brachytrachelopan) [7][8][9]13 and some later-diverging diplodocids (Barosaurus lentus, Diplodocus carnegii, Dinheirosaurus) 23,26 . Itis, however, absent in some putative plesiomorphic dicraeosaurids (Suuwassea and Smitanosaurus) 24,29 , barring Kaatedocus 27 which is the earliest-diverging dicraeosaurid.The disposition and morphology of this midline keel differentiate Tharosaurus from Lingwulong, Dicraeosaurus, Brachytrachelopan and Barosaurus where the keel is prominent anteriorly 4,8,9,26 .In Dinheirosaurus the keel is restricted to the posterior part of the centrum and does not extend anteriorly as in Tharosaurus, whereas in Pilmatueia the keel forks anteriorly and posteriorly with the bifurcation point placed slightly anterior to the mid-length of the centrum.
The neural arch is partially preserved and includes the right prezygapophysis and the region surrounding the neural canal (Figs.2B-K, 3, Supplementary Table 1).The anterior exit of the canal, although partial, appears oval, but the posterior exit is complete and elliptical, being transversely wider than dorsoventrally high.In Pilmatueia this exit is triangular, whereas in Suuwassea and Lingwulong it is circular-subcircular 8,13,24 .www.nature.com/scientificreports/ The prezygapophysis is anterodorsally directed with the articular facet strongly convex transversely and longitudinally (Fig. 3A-G).The latter feature is shared with middle and posterior cervicals of the flagellicaudatans Diplodocus carnegii (CV8) and Kaatedocus (CV10) [26][27][28][29] .The prezygapophyseal articular surface is oblong with the posterior border offset from the surrounding dorsal surface of the prezygapophysis by a transverse sulcus (Fig. 3C,D,G), similar to the posterior cervicals of Kaatedocus 27 .Immediately ventral to the articular surface, the lateral surface of the prezygapophysis bears an anterodorsally-posteroventrally oriented pre-epipophysis (sensu Tschopp et al. 26 ; Fig. 3A,B).The latter is dorsoventrally compressed and succeeded ventrally by a shallow fossa.Such a ridge is also reported in the middle and posterior cervicals of Kaatedocus 27 (CV7-10), Apatosaurus louisae 26 (CV11) and Suuwassea 24,26 .The prezygapophysis is supported ventrally by a dorsoventrally high and robust centroprezygapophyseal lamina (cprl, Fig. 3C-F).The lamina is divided into a larger and medially concave lateral branch connecting with the prezygapophysis (lcprl) and a smaller medial branch joining the intraprezygapophyseal lamina (mcprl).The two halves of the cprl accommodate a triangular and deep centroprezygapophyseal fossa (Fig. 3C,D) which borders the neural canal dorsolaterally and is roofed by a sheet-like intraprezygapophyseal lamina.While a divided cprl is common in Flagellicaudata 1 , in dicraeosaurids the medial branch of the cprl connects with the intraprezygapophyseal lamina and is listed as a synapomorphy of Dicraeosauridae (sensu Whitlock and Wilson Mantilla 29 ).Unlike in Tharosaurus, however, both branches of the cprl join the prezygapophysis in diplodocids 1,26,29,31 .Nonetheless, the anterior surface of the lcprl immediately ventral to the prezygapophyseal articular surface bears a depression giving the impression of a split cprl, where both branches connect to the prezygapophysis, akin to diplodocids.Further scrutiny shows this depression to be almost indistinct and unlike the morphology of the bifurcated crpl of diplodocids 26,32 .Furthermore, in diplodocids the apex of this bifurcation is ventrally directed in contrast to the incipient depression in Tharosaurus which is broad and dorsally convex.Therefore, the expression of the divided cprl in the new Indian taxon is closer to that of dicraeosaurids and supports its taxonomic allocation.
The preserved neural arch above the neural canal shows deep bifurcation which descends to the roof of the canal and encloses an anteroposteriorly extensive passage of uniform width (Fig. 2H-K; Supplementary Fig. 3).The broken ends of the neural arch dorsal to the cpof(e) suggest the presence of strongly divergent postzygapophyses.These features are reminiscent of the deeply bifurcated neural arch-spine complex of middle to cervicodorsal vertebrae of Amargasaurus and Pilmatueia 5,8,28 .Moreover, the surface of the neural arch within this passage is finished (as seen in dorsal view, Fig. 2J,K), and bears rugose scars along the mid-line which possibly represent ligament scars.The scars largely occupy the posterior half of the passage and bordered laterally by sub-parallel striations.Rugose tuberosity on the floor of bifurcated neural arch-spine complexes of Apatosaurus ajax and Dicraeosaurus have been interpreted as ligament scars 33 .Although Tharosaurus lacks the tuberosity, a feature shared with Amargasaurus 8 , these scars attest to the presence of bifid neural spines in Tharosaurus.Similar bifid neural spines are listed as a synapomorphy of Flagellicaudata in previous studies (e.g., Wilson 1 ).
Three isolated specimens including a partial neural arch and two partial neural arch-spine complexes are referable to the dorsal vertebral series (Fig. 4A-L, Supplementary Table 1).The neural arch comprises a stout, sub-horizontal transverse process with an anteroposteriorly broad and sigmoidal diapophysis (Fig. 4A-F).Anteriorly, the transverse process bears a broad and moderately deep prezygapophyseal centrodiapophyseal fossa (Fig. 4A,B), whereas its posterior surface bears a thin, mediodorsally-lateroventrally directed lamina, possibly representing the postzygodiapophyseal lamina (Fig. 4C,D).A deep centrodiapophyseal fossa (cdf) positioned ventral to the transverse process is bordered by an anteroventrally directed anterior centrodiapophyseal lamina and a near-vertical posterior centrodiapophyseal lamina (Fig. 4E,F).The specimen is tentatively assigned to the anterior dorsals based on similarity in overall morphology including laminae and fossae configuration with those of the flagellicaudatans Apatosaurus louisae, Dinheirosaurus, Lingwulong and Amargasaurus cazaui 5,13,23,25,26 .
The neural arch-spine complex is strongly compressed anteroposteriorly and expanded transversely, suggesting a more posterior position in the dorsal vertebral series (sensu McPhee et al. 34 ).The arch comprises a transversely broad and subtriangular spinoprezygapophyseal fossa (Fig. 4G,H).The transverse process is laterally directed, with a vertically oriented distal end, similar to all diplodocids 23,35 .This laterally oriented transverse process can be considered a local autapomorphy of Tharosaurus unlike a dorsolaterally oriented transverse process in other dicraeosaurids 5,9,13,36 .This feature may alternatively represent a symplesiomorphy, although Mannion et al. 23 listed the dorsolaterally projecting diapophysis in the dorsal vertebrae of the diplodocid Dinheirosaurus as a local autapomorphy.The neural spine is transversely expanded and non-bifid and the disposition of the lateral spinal margins suggests dorsal flaring in anterior/posterior views (Fig. 4G-J).This suggests affinity with the middle/posterior dorsal vertebrae because non-bifurcated neural spines appear from the 6th/7th dorsal of dicraeosaurids, as seen in Lingwulong and Brachytrachelopan 9,13 .Such flaring of the lateral spinal margins is also known from the middle and posterior dorsals of dicraeosaurids 4,13,35 .In lateral view, the anterior and posterior borders of the neural spine remain subparallel (Fig. 4K,L), unlike the sub-triangular spinal lateral profile characteristic of many titanosauriforms 34,37 .The neural spine bears a sharp prespinal lamina placed along the longitudinal midline on the anterior surface (Fig. 4G,H).The prespinal lamina does not extend up to the base of the spine and terminates dorsal to the spinoprezygapophyseal fossa.This lamina, however, extends down to the www.nature.com/scientificreports/base of the neural spine in the posterior dorsals of Amargasaurus and Dicraeosaurus 5,36 .Only a small part of the spinoprezygapophyseal lamina is preserved along the lateral margin of the anterior spinal surface (Fig. 4G,H).The spinodiapophyseal lamina is robust and broad, descending gently from the neural spine to the transverse process (Fig. 4G,H,K,L).The posterior surface of the spine preserves only the right spinopostzygapophyseal lamina (Fig. 4I,J).It is robust and well-rounded, extending medially from the lateral margin of the posterior spinal surface towards the midline.In lateral view, the anterior surface of the spinopostzygapophyseal lamina unites with the posterior surface of the spinodiapophyseal lamina as seen in the posterior dorsals of Brachytrachelopan 9 (Fig. 4K,L), and these laminae enclose the postzygapophyseal spinodiapophyseal fossa.Thus, these vertebral specimens likely belong to the middle/posterior dorsals.An isolated, nearly complete rib (Fig. 4M,N) is identified as the right anterior dorsal rib, based on similarity with that of Galeamopus pabsti 30 .The specimen is Y-shaped, flares out proximally, and becomes dorsoventrally skewed distally.The rib lacks pneumatic openings as in diplodocoids but contrasts with those of titanosauriforms 38 .The capitulum is elliptical, anteroposteriorly compressed, and directed anterodorsally.It is longer than the tuberculum and bears prominent striations on the anterior surface (Fig. 4M).The tuberculum is short and curved towards the capitulum.The tuberculum and capitulum enclose a broad U-shaped space.A robust and rounded ridge occupies the anterior surface of the rib distal to the capitulum and tuberculum (Fig. 4M).The ridge extends distally and gradually becomes flush with the surface.The ventral margin is sigmoidal with the portion immediately distal to the tuberculum strongly convex.The dorsal margin is concave, and its posterior surface is largely flat and featureless, except for the strongly rugose tuberculum.
The second caudal (RWR-241-K) vertebra is a nearly complete centrum but without the neural arch (Fig. 5D-G).The centrum is anteroposteriorly elongated, suggesting a more posterior position in the caudal series compared to RWR-241-I (sensu Coria et al. 8 ).The centrum is 1.6 times as long as high, with a deep fossa extending along the entire length of the centrum, identifying it to be a middle caudal (sensu McPhee et al. 34 ).This is corroborated by similarities with middle caudal centra of Suuwassea 24 (cL/cH = 1.5) and Amargatitanis 11 (cL/cH = 1.3).A prominent lateral fossa is also reported in the dicraeosaurid Amargatitanis 11 .Furthermore, judging from the broken dorsal surface of the centrum, the neural arch was possibly more anteriorly placed in RWR-241-J as in the afore-mentioned dicraeosaurids.The centrum is platyceolous with the anterior and posterior cotyles dorsoventrally taller than mediolaterally wide (Fig. 5D,E), although there is some evidence of deformation.The ventral surface is strongly concave in lateral view with the ventral margin of the posterior cotyle descending below the level of the anterior cotyle.The fossa on the ventral surface is deep, constricted at midlength and bordered by robust ventrolateral ridges (Fig. 5G), similar to the derived diplodocids such as Diplodocus 44 , Seismosaurus 45 , and Barosaurus 46 .

Phylogenetic analysis.
The inter-relationship of Tharosaurus within Sauropodomorpha was tested in a reduced version of the data matrix used by Gallina et al. 7 (see "Methods" and Supplementary Note 2).The analysis recovered 20 most parsimonious trees with a tree length of 760, consistency index (CI) of 0.483 and retention index (RI) of 0.647.The topology of the strict consensus tree (Supplementary Fig. 4) shows a well resolved clade Sauropoda with diplodocoids and macronarians showing distinct clustering within Neosauropoda, consistent with previous studies 7,13 .Tharosaurus is recovered as a dicraeosaurid flagellicaudatan, although the clade Dicraeosauridae is poorly resolved.In the 50% majority rule tree (Fig. 6), Dicraeosauridae is better resolved with Tharosaurus being a sister taxon to ((Pilmatueia + Amargatitanis) + (Brachytrachelopan + (Dicraeosaurus + Amargasaurus))).

Discussion
Phylogenetic implications.Tharosaurus shares five synapomorphies with Flagellicaudata [divided lateral pleuroceols on cervical centra (ch.115); bifurcated presacral neural spines (ch.376); lateroventral flanges on middle/posterior cervical centra (ch.382); heart-shaped anterior caudal centra cross-section (389); pre-epipophysis on middle/posterior cervicals (ch.393)] and one unambiguous synapomorphy supports its recovery as a dicraeosaurid [divided cprl in cervicals with the medial lamina connecting with the intraprezygapophyseal lamina ( 127 To further test the position of Tharosaurus within Sauropodomorpha, two additional analyses were conducted (A1 and A2; Supplementary Notes 3 and 4) using an expanded version of the data matrix of Gallina et al. 7 which comprises a spatiotemporally and phylogenetically diverse array of sauropodomorphs, thereby allowing Tharosaurus to be placed anywhere within Sauropodomorpha.Furthermore, in analysis A2, characters involving cervical spines were scored as '?' , as these are not preserved in Tharosaurus, although the neural arch morphology strongly suggests the presence of bifurcated spines.Both analyses produce the same results where Tharosaurus is recovered as a dicraeosaurid (Supplementary Figs.5-8).Furthermore, the phylogenetic bracketing of Tharosaurus by Bajadasaurus and ((Pilmatueia + Amargatitanis) + (Brachytrachelopan + (Dicraeosaurus + Amargasaurus))) in the majority rule tree of analysis A2 (Fig. 8), supports our original inference suggesting bifurcated cervical neural spines in Tharosaurus, as the bracketing taxa also possess the same feature.
Palaeobiogeography.Sauropods are considered to have originated in the Late Triassic/Early Jurassic 38,47 but the origin and radiation of Neosauropoda and its major clades (Diplodocoidea and Macronaria) are still amongst the most contentious issues 48 .Non-neosauropods were restricted to eastern Gondwana (India and Zimbabwe) and parts of Laurasia (Thailand, Germany and China) during Late Triassic-Early Jurassic, suggesting possible physiological constraints to their dispersal to the Americas and Australia 47 , although sampling biases cannot be ruled out.Neosauropods possibly appeared and radiated during the late Early/ early Middle www.nature.com/scientificreports/Jurassic, with Asia and North-South America being some of the areas occupied by their most recent common ancestors (MRCAs) 13,49,50 .Palaeogeographic reconstructions support this hypothesis since Gondwana and Laurasia remained united for much of the early Mesozoic as the supercontinent Pangaea 51 .Although the Tethys Ocean was a barrier between Europe + Asia and Gondwana, land connections between North America and South America + Africa during the Triassic and Early Jurassic would have allowed sauropod dispersal 51,52 .Similarly, dispersal between North America and Europe 53 possibly occurred during the Middle Jurassic, although these continents were likely separated by a narrow epicontinental sea (the Viking Corridor) during the Early Jurassic 54 .However, by the early Bajocian (~ 175 Ma), sea-floor spreading started in the western Tethys, the Central Atlantic region and the Gulf of Mexico [55][56][57] , and global transgressions flooded most continental shelves during the Middle and Late Jurassic, separating North America from South America and Africa 56,57 .Consequently, major neosauropod clades in these Laurasian and Gondwanan continents must have originated and radiated, prior to the Bajocian rifting 57 .
Until recently, the East Asian Jurassic dinosaurs were regarded as endemic fauna, characterized by mamenchisaurids and tetanurans 13,58 .Dispersal of neosauropods into East Asia apparently occurred only during the Early Cretaceous with the appearance of titanosauriforms, whereas the absence of diplodocoids was thought to be a result of reduced diversity and geographical range due to an end-Jurassic extinction event 13 .Faunal differences between East Asia and the rest of Laurasia were explained by the presence of an epicontinental seaway isolating Central and East Asia from the rest of Laurasia during the Middle and Late Jurassic 49,59 .Nonetheless, discoveries of the Chinese Middle and Late Jurassic neosauropods (Dashanpusaurus, Lingwulong, Bellusaurus) tend to weaken the isolation hypothesis 13,50 and support a circum-Pangaean distribution of the major neosauropod clades by Middle Jurassic 13,50 .The age of the Chinese macronarian Dashanpusaurus is controversial but possibly Bajocian 50 , whereas the age and stratigraphic horizon in which the dicraeosaurid Lingwulong was found has recently been revised from the late Toarcian-Bajocian Yanan Formation to the Bathonian-Callovian Zhiluo Formation 50,60 .In their palaeobiogeographic analysis of sauropods, Ren et al. 50(p.6, Fig. 3) assigned a Callovian age to Lingwulong.Thus, the discovery of Tharosaurus assumes considerable significance owing to its older, Early-Middle Bathonian age.
To assess the palaeobiogeographic significance of Tharosaurus, a time-calibrated phylogenetic tree was constructed (Figs. 7, 8).The resultant topology is consistent with the current consensus on rebbachisaurid origins 29,61,62 as it suggests a largely Gondwanan origin since most of the taxa are from South America and Africa.Although the oldest rebbachisaurid in this study, Histriasaurus, is from Croatia, it is regarded as a Gondwanan taxon because Croatia was part of the Adriatic-Dinaric Carbonate Platform during the Early Cretaceous, sharing biotic affinities with Africa 61,63 .Based on the Late Jurassic Maraapunisaurus 64 , a North American origin of rebbachisaurids and their dispersal into South America through Europe and Africa during the latest Jurassic-earliest Cretaceous has been suggested.However, the rebbachisaurid affinity of this sauropod is questionable 29 .Early Cretaceous rebbachisaurids from England including Xenoposeidon, claimed to be the oldest member of this clade 29,62 , also add uncertainty to the Gondwanan origin of Rebbachisauridae.The lower-level phylogenetic affinity of these sauropods remains unresolved and further work is necessary to better understand rebbachisaurid origins.Our study recovers a long rebbachisaurid ghost lineage extending into the Middle Jurassic, corroborating recent work on the timing of origin and dispersal of this clade 13,50,65 .This work also suggests that differentiation within Rebbachisauridae started during the Late Jurassic, consistent with previous studies 53,64 .
The diplodocids included in our time-calibrated tree are from the Upper Jurassic horizons of North America (Figs. 7, 8) and suggest a North American origin for this clade.Current consensus on diplodocid taxonomy supports this inference since most of the other valid taxa (Brontosaurus, Galeamopus, Supersaurus and Amphicoelias) are also from North America 29,30,37 .Furthermore, the tree topology supports a pre-Bajocian divergence and dispersal of diplodocids from dicraeosaurids, consistent with most previous studies on flagellicaudatan dispersal and palaeobiogeography 29,57,65 .Diplodocids were, however, not restricted to North America, but are also known from the Late Jurassic of Europe (Dinheirosaurus 66 ) and Africa (Tornieria 56 ), and the Early Cretaceous of South America (Leinkupal 57 ).
The clade Dicraeosauridae is represented by nine taxa ranging from Middle Jurassic to Early Cretaceous (Figs. 7, 8).The tree topology shows a late-diverging clade with a preponderance of South American dicraeosaurids (Pilmatueia, Amargatitanis, Brachytrachelopan and Amargasaurus) along with the African Dicraeosaurus, and argues for a Gondwanan origin for this clade.However, recovery of the Late Jurassic Suuwassea as the earliestdiverging dicraeosaurid raises the possibility of a North American origin for Dicraeosauridae and its subsequent migrations to South America and Africa, as initially proposed by Whitlock and Wilson Mantilla 29 .Recent work on flagellicaudatan palaeobiogeography based on the dicraeosaurid Lingwulong identified Asia + South America as some of the areas for MRCAs of Dicraeosauridae 13 .
Although later-diverging than Suuwassea and Lingwulong, the discovery of Tharosaurus in a much older Middle Jurassic horizon calls into question the above two hypotheses.The early-middle Bathonian age of Tharosaurus (see Supplementary Note 1) makes it the oldest diplodocoid globally.Even if Lingwulong is reinstated to its original estimated age of 174 Ma (sensu Xu et al. 13 ), Tharosaurus remains the oldest Gondwanan diplodocoid as the African and South American diplodocoids appear from the Kimmeridgian (157 Ma) 13,64 (Fig. 7).The sister taxon relationship between the new Indian taxon and the later-diverging African and South American dicraeosaurids indicates ease of faunal exchanges between India and western Gondwana.Based on the age of Tharosaurus and its phylogenetic position near the base of the clade Dicraeosauridae (Fig. 7), India (or a geographically proximate region of eastern Gondwana) is hypothesized here as a potential centre for the radiation and perhaps origin of dicraeosaurids/diplodocoids.Current palaeogeographic reconstructions 67 lend support to this hypothesis since plausible dispersal routes from India to western Gondwana-Laurasia, through Madagascar, still remained in place during the Middle Jurassic.Furthermore, the estimated ancestral ranges depicted by the time-calibrated tree show the origin of Neosauropoda and its major clades-Macronaria + Diplodocoidea-to straddle the late Early Jurassic-early Middle Jurassic interval, consistent with recent studies on neosauropod origins 13,50,65 .
Our proposal favouring possible diplodocoid radiation/origin in India still needs to be reconciled with the phylogenetically more basal Asian taxon Lingwulong, the only other Middle Jurassic dicraeosaurid apart from Tharosaurs.An explanation for these two geographically disparate occurrences is perhaps a circum-Pangaean dispersal event.A direct dispersal to or from Asia is precluded by the Tethys Ocean which acted as a major barrier to terrestrial fauna during the Mesozoic.Furthermore, there is little support for the introduction of diplodocoids into India from Asia via North America and western Gondwana, since Tharosaurus is geologically older and phylogenetically early-branching relative to nearly all African and South American diplodocoids (Figs. 7, 8).The only exception is the South American Bajadasaurus, which is phylogenetically bracketed by Suuwassea + Lingwulong and Tharosaurus, but known from a much younger stratigraphic horizon (Fig. 7).The phylogenetic position of Bajadasaurus may possibly be explained by a radiation event from North America or from India predating Tharosaurus.However, additional sampling leading to greater anatomical coverage of Tharosaurus in the future may change its phylogenetic position.In any case, the long ghost lineage leading to Bajadasaurus presents the possibility of finding earlier-diverging taxa in western Gondwana.
Thus, a more plausible hypothesis, based on the older stratigraphic age of Tharosaurus, is that migrations from India to Asia could have taken place through western Gondwana and North America via Europe (Fig. 8).

Figure 1 .
Figure 1.Geological map of Jaisalmer Basin showing (a) the fossil locality; (b) stratigraphic column showing the position of the dinosaur fossil yielding horizon; (c) photograph of the fossil site.The map and stratigraphic column were drawn by K.K. using CorelDRAW 2019 (Version number: 21.0.0.593,URL link: http:// www.corel.com/ en/).

Figure 2 .
Figure 2. Cervical vertebrae (CV6/8) of Tharosaurus indicus.(a) RWR-241-A, anterior cotyle in anterior view.(b-k) RWR-241-B, partial vertebra, photographs and line drawings in (b,c) right lateral view, red line indicates U-shaped ridge demarcating anterior and posterior halves of lateral pneumatic fossa; (d,e) left lateral view; (f,g) ventral view, red line indicates posteriorly bifurcated midline keel and arrow indicates accessory ridge; (h,i) posterior view, arrows and red arrowheads indicate deep bifurcation of neural arch and triangular facets below cotyle, respectively.(j,k) dorsal view, arrowhead indicates passage enclosed by bifid neural arch and ligament scars and striations marked in red and purple, respectively.Broken areas and artefacts in grey and pink, respectively.c centrum, cpof centropostzygapophyseal fossa, cpol centropostzygapophyseal lamina, lf lateral fossa, lvf lateroventral flange, mk midline keel, na neural arch, nc neural canal, pvf posteroventral fossa, tpol intrapostzygapophyseal lamina.Scale bars represent 50 mm.

Figure 7 .
Figure 7. Time-calibrated phylogenetic tree, based on the 50% majority-rule tree of Supplementary Fig. 6.Macronarians have been combined into a single lineage to enhance clarity.Red star indicates position of Tharosaurus indicus.