Fossils of the oldest diplodocoid dinosaur suggest India was a major centre for neosauropod radiation. https://rdcu.be/diyrR

Abstract

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.

Full text

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Fossils of the oldest diplodocoid
dinosaur suggest India was a major
centre for neosauropod radiation
Sunil Bajpai 1*, Debajit Datta 1*, Pragya Pandey
2, Triparna Ghosh
1,3, Krishna Kumar 3 &
Debasish Bhattacharya
4
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 rst 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.
Abbreviations
ANS Academy of Natural Sciences, Philadelphia, USA
MACN Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina
YPM Yale Peabody Museum, New Haven, USA
Sauropods, a speciose group of saurischian dinosaurs that dominated the terrestrial landscapes until the end-
Cretaceous1,2, are characterized by a small skull, elongated neck and tail, columnar limbs, and a quadrupedal
gait3. Within Sauropoda, Dicraeosauridae represents a clade of small-bodied diplodocoids that are known for
their distinctive vertebral morphology with long paired neural spines4–7. Dicraeosaurids range in age from the
Middle Jurassic–Early Cretaceous and are mostly known from the Gondwanan landmasses of Africa and South
America, besides a few Laurasian occurrences in the USA and China3,8. Amongst the well-known dicraeosau-
rids are Dicraeosaurus from East Africa and Brachytrachelopan, Amargasaurus, Bajadasaurus, Pilmatueia and
Amargatitanis from Argentina4,5,8–11, Suuwassea, Kaatedocus and Smitanosaurs from USA12, and Lingwulong
from China13.
In India, early-diverging sauropods Barapasaurus and Kotasaurus are known from the Early Jurassic Kota
Formation of Pranhita–Godavari Basin, whereas putative Middle Jurassic camarasauromorph remains occur in
Kutch2,14–18. No diplodocoid sauropods are yet known from India18,19. Here we report on the discovery of a new
dicraeosaurid from the Jaisalmer Formation, Rajasthan, western India (Fig.1, Supplementary Note 1). Fossils
were collected from a shale unit situated at the base of the early–middle Bathonian Fort Member20,21 and include
disarticulated, but associated, specimens of the axial skeleton spread over an area of ~ 25 m2 (L1–L3, Fig.1). is
discovery provides new insights into sauropod diversity of the Indian Gondwana, with important implications
for the origin and dispersal of Neosauropoda.
Results
Systematic palaeontology. Sauropoda Marsh, 1878
Neosauropoda Bonaparte, 1986
Diplodocoidea Marsh, 1884
Dicraeosauridae Janensch, 1929
arosaurus indicus gen. et sp. nov.
OPEN
1Department of Earth Sciences, Indian Institute of Technology, Roorkee, Uttarakhand 247667, India. 2Geological
Survey of India, Raipur, Chhattisgarh 492010, India. 3Geological Survey of India, Jaipur, Rajasthan 302004,
India. 4Central Head Quarters, Geological Survey of India, Kolkata, West Bengal 700091, India. *email:
sunil.bajpai@es.iitr.ac.in; debajitdatta.pd@es.iitr.ac.in; debajitdatta9@gmail.com
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Etymology. Generic name is a combination of aro, referring to the ‘ar desert’ of western India where the
type specimen was found, and saurus, which is derived from the Greek word ‘sauros’ meaning lizard; specic
name is for the country of origin i.e., India.
Holotype. RWR-241A–K (Palaeontology Division, Geological Survey of India, Western Region, Jaipur,
Rajasthan, India); partial middle/posterior cervical vertebra; middle/posterior cervical anterior condyle and
right prezygapophyses; partial anterior dorsal neural arch; middle/posterior neural spines; anterior dorsal rib;
partial anterior and middle caudal vertebrae (Figs.2, 3, 4, 5; Supplementary Table1).
Horizon and locality. Fort Member of the Jaisalmer Formation (early–middle Bathonian), Jethwai village, Dis-
trict Jaisalmer, Rajasthan state, western India.
Diagnosis. arosaurus exhibits a unique combination of the following characters: middle/posterior cervi-
cal centroprezygapophyseal lamina divided into lateral and medial branches, the latter connecting with the
intraprezygapophyseal lamina (shared with all dicraeosaurids); middle/posterior cervical centropostzygapophy-
seal fossa elliptical and bordered laterally by pillar-like centropostzygapophyseal lamina (shared with the dicrae-
osaurids Lingwulong, Brachytrachelopan, and Pilmatueia); deep bifurcation of cervical neural arch extending up
to the dorsal margin of the neural canal (shared with the dicraeosaurids Amargasaurus and Pilmatueia); paired
fossae on the ventral surface of the middle/posterior cervicalcentrum separated by a mid-line keel (shared
with all dicraeosaurids, barring Smitanosaurus, Kaatedocus and Suuwassea, and the diplodocids—Barosaurus
and Dinheirosaurus); paired ventral fossae extending up to the posterior margin of centrum; anterior condyle
of middle/posterior cervical more rugose compared to the rest of the centrum (shared with the dicraeosaurid
Kaatedocus); bid middle/posterior cervical neural spine (shared with Flagellicaudata); divided lateral fossa/
pleuroceol on cervical centra (shared with Flagellicaudata); lateral fossa divided into deep posterior and shallow
anterior half by weak ridge on the ventral surface of the fossa; prominent lateroventral anges on middle/pos-
terior cervical centrum (shared with Flagellicaudata); middle/posterior cervical vertebrae with pre-epipophysis
(shared with Flagellicaudata); cordiform cross-section of anterior caudal centra (shared with Flagellicaudata);
middle/posterior dorsal transverse process laterally directed (shared with Diplodocidae); middle caudal centra
articular surface with at ventral margin (shared with Diplodocoidea); ventral surface of mid-caudal centrum
with deep median fossa.
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. e 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/).
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Figure2. Cervical vertebrae (CV6/8) of arosaurus 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) le 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 bid 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 ange, mk midline keel, na neural arch, nc neural canal, pvf posteroventral fossa, tpol
intrapostzygapophyseal lamina. Scale bars represent 50mm.
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Autapomorphies: triangular ventrolateral projections lying below posterior cotyle of middle/posterior cervi-
cal centrum visible in posterior view; ventral mid-line keel on middle/posterior cervical centrum bifurcating
posteriorly but not meeting the lateroventral anges; lateral pleuroceol on anterior caudal centra.
Description. e cervical vertebrae are represented by ve partial specimens found in association (Supple-
mentary Table1), including the anterior condyle, posterior half of a vertebra and three right prezygapophyses
(Figs.2, 3). e anterior condyle is U-shaped with an opisthocoelous condition as in other eusauropods22,23
(Fig.2A), and is mediolaterally wider than dorsoventrally tall [RWR-241-A, centrum articular surface height
(cH)/centrum articular surface width (cW) = 0.7; Supplementary Table2, Supplementary Fig.2]. In this respect,
arosaurus is similar to the middle/posterior cervicals of several agellicaudatans: Suuwassea24 (cH/cW = 0.7),
Lingwulong13 (cH/cW = 0.7), Pilmatueia8 (CV7/8, cH/cW = 0.9), Amargasaurus5 (CV8, cH/cW = 0.8), Apatosau-
rus louisae25 (CV9, cH/cW = 0.7) and Diplodocus carnegii26 (CV11, cH/cW = 0.9). e anterior condyle is mark-
edly more rugose than the other vertebral elements, as in Kaatedocus27 (CV14).
e centrum shows strong anterior constriction with the lateral surfaces excavated by large fossae, similar
to many agellicaudatans8,13,24,26 (Fig.2B–E). e fossa is partially preserved with much of the dorsal rim and
medial wall broken o. e preserved portion suggests the fossa was elliptical, being elongated and dorsoven-
trally compressed (Fig.2B–C). e dorsal margin of the fossa is rod-like and extends laterally beyond the ventral
margin. e latter is robust, rounded and thickest near the posterior end of the centrum, accentuating the depth
of the fossa. Anteriorly it becomes ush 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 ares posteriorly. e 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. e 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. is
contrasts with the dicraeosaurids Amargasaurus, Dicraeosaurus, Bajadasaurus, and Pilmatueia where the fossa
is undivided and shallow5,7,8. In the middle cervicals of Lingwulong and Suuwassea, however, the lateral fossa is
deep and extends nearly along the entire length of the centrum13,24. Furthermore, the lateral fossa in arosaurus
Figure3. Right cervical prezygapophysis (CV6/8) of arosaurus indicus. RWR-241-C, photographs and
line drawings in (a,b) lateral view; (c,d); medial view; (e,f) anterior view; (g) dorsal view. Arrowheads
indicate transverse sulcus beneath dorsal articular surface of prezygapophysis. cprf centroprezygapophyseal
fossa, epi pre-epipophysis, lcprl lateral branch of centroprezygapophyseal lamina, mcprl medial branch of
centroprezygapophyseal lamina, nc neural canal, tprl intraprezygapophyseal lamina. Scale bars represent 50mm.
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Figure4. Dorsal vertebrae of arosaurus indicus. RWR-241-F, partial anterior dorsal neural arch, photographs
and line drawings in (a,b) anterior view; (c,d) posterior view; (e,f) anterior view. RWR-241-G, partial middle/
posterior dorsal neural arch-spine complex, photographs and line drawings in (g,h) anterior view; (i,j) posterior
view; (k,l) lateral view. RWR-241-I, nearly complete anterior dorsal rib in (m) anterior view; (n) posterior view.
acdl anterior centrodiapophyseal lamina, ca capitulum, cdf centrodiapophyseal fossa, da diapophysis, ns neural
spine, pcdl posterior centrodiapophyseal lamina, podl postzygodiapophyseal lamina, posdf postzygapophyseal
spinodiapophyseal fossa, prcdf prezygapophyseal centrodiapophyseal fossa, prsl prespinal lamina, spdl
spinodiapophyseal lamina, spol spinopostzygapophyseal lamina, sprl spinoprezygapophyseal lamina, sprf
spinoprezygaposphyseal fossa, tp transverse process, tu tuberculum. Scale bars represent 50mm.
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is succeeded by a posteroventral fossa (Fig.2D,E), as seen in the mid-cervicals of Amargasaurus, Apatosaurus,
Bajadasaurus, and Pilmatueia26,28.
e 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 keel7,8,13,26. e fossae are asymmetrical, with the right fossa mediolaterally wider, and extend up to
the posterior margin of the centrum where they are out. However, in Dicraeosaurus, Lingwulong, and Pilmat-
ueia the fossae are symmetrical and strongly expressed anteriorly, but do not reach the posterior margin of the
centrum8,13,26. e fossae in arosaurus are anked laterally by prominent lateroventral anges and separated
by a sharp midline keel (Fig.2F,G). e keel shows le 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. e latter, however, do not reach up to the lateroventral anges. An accessory ridge, present only on the
right ventral surface, extends posterolaterally from the midline keel. e presence of a ventral keel is shared
with a diverse array of agellicaudatans including most dicraeosaurids (Lingwulong, Dicraeosaurus, Pilmatueia,
Bajadasaurus and Brachytrachelopan)7–9,13 and some later-diverging diplodocids (Barosaurus lentus, Diplodocus
carnegii, Dinheirosaurus)23,26. It is, however, absent in some putative plesiomorphic dicraeosaurids (Suuwassea
and Smitanosaurus)24,29, barring Kaatedocus27 which is the earliest-diverging dicraeosaurid. e disposition and
morphology of this midline keel dierentiate arosaurus from Lingwulong, Dicraeosaurus, Brachytrachelopan
and Barosaurus where the keel is prominent anteriorly4,8,9,26. In Dinheirosaurus the keel is restricted to the pos-
terior part of the centrum and does not extend anteriorly as in arosaurus, whereas in Pilmatueia the keel forks
anteriorly and posteriorly with the bifurcation point placed slightly anterior to the mid-length of the centrum.
e posterior cotyle of the centrum (RWR-241-B, cH/cW = 0.6, Fig.2H,I, Supplementary Table2) is strongly
concave and mediolaterally wider than dorsoventrally tall, a feature seen in the middle cervicals of many sauro-
pods including Lingwulong13 (cH/cW = 0.7), Apatosaurus louisae25 (cH/cW = 0.7) and Gleamopus30 (cH/cW = 0.9),
and the posterior cervicals of Pilmatueia8 (cH/cW = 0.6) and Suuwassea24 (cH/cW = 0.7). e posterior cotyle
is cordiform in outline with a weakly concave dorsal margin immediately ventral to the neural canal as in Ling-
wulong and Pilmatueia8,13. arosaurus, however, can be distinguished by the presence of posteriorly facing
triangular facets along the ventrolateral margin of the posterior articular surface.
e neural arch is partially preserved and includes the right prezygapophysis and the region surrounding the
neural canal (Figs.2B–K, 3, Supplementary Table1). e 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–subcircular8,13,24.
Figure5. Caudal vertebrae of arosaurus indicus. RWR-241-J, partial anterior caudal vertebra in (a) anterior
view; (b) right lateral view; (c) ventral view. RWR-241-K, middle caudal centrum in (d) anterior view; (e)
posterior view; (f) le lateral view; (g) ventral view. c centrum, chf chevron facet, lf lateral fossa, lpfo lateral
pneumatic foramen, vf ventral fossa, vr ventrolateral ridge.
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e prezygapophysis is anterodorsally directed with the articular facet strongly convex transversely and lon-
gitudinally (Fig.3A–G). e latter feature is shared with middle and posterior cervicals of the agellicaudatans
Diplodocus carnegii (CV8) and Kaatedocus (CV10)26–29. e prezygapophyseal articular surface is oblong with
the posterior border oset from the surrounding dorsal surface of the prezygapophysis by a transverse sulcus
(Fig.3C,D,G), similar to theposterior cervicals of Kaatedocus27. 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). e latter is dorsoventrally compressed and succeeded ventrally by a shallow fossa.
Such a ridge is also reported in the middle and posterior cervicals of Kaatedocus27 (CV7–10), Apatosaurus loui-
sae26 (CV11) and Suuwassea24,26. e prezygapophysis is supported ventrally by a dorsoventrally high and robust
centroprezygapophyseal lamina (cprl, Fig.3C–F). e lamina is divided into a larger and medially concave lateral
branch connecting with the prezygapophysis (lcprl) and a smaller medial branch joining the intraprezygapophy-
seal lamina (mcprl). e 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 Flagellicaudata1, in dicraeosaurids the medial branch of the cprl con-
nects with the intraprezygapophyseal lamina and is listed as a synapomorphy of Dicraeosauridae (sensu Whitlock
and Wilson Mantilla29). Unlike in arosaurus, however, both branches of the cprl join the prezygapophysis in
diplodocids1,26,29,31. 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 diplodocids26,32. Furthermore, in diplodocids the apex of this bifurca-
tion is ventrally directed in contrast to the incipient depression in arosaurus which is broad and dorsally
convex. erefore, the expression of the divided cprl in the new Indian taxon is closer to that of dicraeosaurids
and supports its taxonomic allocation.
e posterior exit of the neural canal is anked dorsolaterally by two elliptical centropostzygapophyseal fossae
[cpof(e)] which are bordered laterally by robust pillar-like centropostzygapophyseal laminae [cpol(e), Fig.2H,I].
e cpof(e) are roofed by thin intrapostzygapophyseal laminae. e arrangement of the vertebral laminae and
fossae in arosaurus is comparable with the condition in Lingwulong, Brachytrachelopan, Dicraeosaurus, Amar-
gasaurus and Pilmatueia4,5,8,9,13. However, in the latter two taxa, the cpof(e) are triangular. Furthermore, as in
Amargasaurus5,8, the neural arch in arosaurus is devoid of a median tubercle.
e 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).
e broken ends of the neural arch dorsal to the cpof(e) suggest the presence of strongly divergent postzyga-
pophyses. ese features are reminiscent of the deeply bifurcated neural arch-spine complex of middle to cer-
vicodorsal vertebrae of Amargasaurus and Pilmatueia5,8,28. Moreover, the surface of the neural arch within this
passage is nished (as seen in dorsal view, Fig.2J,K), and bears rugose scars along the mid-line which possibly
represent ligament scars. e scars largely occupy the posterior half of the passage and bordered laterally by
sub-parallel striations. Rugose tuberosity on the oor of bifurcated neural arch-spine complexes of Apatosaurus
ajax and Dicraeosaurus have been interpreted as ligament scars33. Although arosaurus lacks the tuberosity,
a feature shared with Amargasaurus8, these scars attest to the presence of bid neural spines in arosaurus.
Similar bid neural spines are listed as a synapomorphy of Flagellicaudata in previous studies (e.g., Wilson1).
e specimens (RWR-241-A–E) likely represent the 6th/8th cervical based on comparisons with the middle
cervicals of Lingwulong and Bajadasaurus, and the middle–posterior cervicals of Dicraeosaurus, Suuwassea,
Amargasaurus, Pilmatueia and Kaatedocus5,8,13,24,27.
ree 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). e neural arch comprises a stout,
sub-horizontal transverse process with an anteroposteriorly broad and sigmoidal diapophysis (Fig.4A–F). Ante-
riorly, 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). e specimen is tentatively assigned to the
anterior dorsals based on similarity in overall morphology including laminae and fossae conguration with those
of the agellicaudatans Apatosaurus louisae, Dinheirosaurus, Lingwulong and Amargasaurus cazaui5,13,23,25,26.
e neural arch-spine complex is strongly compressed anteroposteriorly and expanded transversely, sug-
gesting a more posterior position in the dorsal vertebral series (sensu McPhee etal.34). e arch comprises a
transversely broad and subtriangular spinoprezygapophyseal fossa (Fig.4G,H). e transverse process is laterally
directed, with a vertically oriented distal end, similar to all diplodocids23,35. is laterally oriented transverse pro-
cess can be considered a local autapomorphy of arosaurus unlike a dorsolaterally oriented transverse process
in other dicraeosaurids5,9,13,36. is 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. e neural spine is transversely expanded and non-bid and the disposition of the
lateral spinal margins suggests dorsal aring in anterior/posterior views (Fig.4G–J). is suggests anity with
the middle/posterior dorsal vertebrae because non-bifurcated neural spines appear from the 6th/7th dorsal of
dicraeosaurids, as seen in Lingwulong and Brachytrachelopan9,13. Such aring of the lateral spinal margins is
also known from the middle and posterior dorsals of dicraeosaurids4,13,35. In lateral view, the anterior and pos-
terior borders of the neural spine remain subparallel (Fig.4K,L), unlike the sub-triangular spinal lateral prole
characteristic of many titanosauriforms34,37. e neural spine bears a sharp prespinal lamina placed along the
longitudinal midline on the anterior surface (Fig.4G,H). e prespinal lamina does not extend up to the base of
the spine and terminates dorsal to the spinoprezygapophyseal fossa. is lamina, however, extends down to the
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base of the neural spine in the posterior dorsals of Amargasaurus and Dicraeosaurus5,36. Only a small part of the
spinoprezygapophyseal lamina is preserved along the lateral margin of the anterior spinal surface (Fig.4G,H).
e spinodiapophyseal lamina is robust and broad, descending gently from the neural spine to the transverse
process (Fig.4G,H,K,L). e 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 Brachytra-
chelopan9 (Fig.4K,L), and these laminae enclose the postzygapophyseal spinodiapophyseal fossa. us, these
vertebral specimens likely belong to the middle/posterior dorsals.
An isolated, nearly complete rib (Fig.4M,N) is identied as the right anterior dorsal rib, based on simi-
larity with that of Galeamopus pabsti30. e specimen is Y-shaped, ares out proximally, and becomes dors-
oventrally skewed distally. e rib lacks pneumatic openings as in diplodocoids but contrasts with those of
titanosauriforms38. e capitulum is elliptical, anteroposteriorly compressed, and directed anterodorsally. It is
longer than the tuberculum and bears prominent striations on the anterior surface (Fig.4M). e tuberculum
is short and curved towards the capitulum. e 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). e ridge extends distally and gradually becomes ush with the surface. e ventral margin is sigmoi-
dal with the portion immediately distal to the tuberculum strongly convex. e dorsal margin is concave, and its
posterior surface is largely at and featureless, except for the strongly rugose tuberculum.
e collection includes two caudal vertebrae (Supplementary Table1). One of these, a partial centrum of an
anterior caudal vertebra, preserves the anterior cotyle and the proximal-most part of the neural arch (Fig.5A–C).
e centrum is robust with a moderately concave anterior cotyle. e latter is dorsoventrally taller than mediolat-
erally wide (RWR-241-J, cH/cW = 1.2, Supplementary Table2) and comparable in proportion to the anterior cau-
dals of other agellicaudatans such as Dicraeosaurus39,40 (cH/cW = c. 1), Suuwassea24 (ANS 21122, cH/cW = 1.1),
Lingwulong13 (cH/cW = 1.1) and Amazonsaurus41 (cH/cW = 1.1). It is, however, dorsoventrally taller compared
to Diplodocus40 (cH/cW = 0.9), Amargatitanis11 (MACN PV N53, cH/cW = 0.7) and Brontosaurus excelsus26,42
(YPM 1980, cH/cW = 0.9). e ventral margin of the anterior cotyle, although partial, preserves a small chevron
facet (Fig.5A). e preserved lateral surface of the centrum is convex and bears a lateral foramen immediately
ventral to the neural arch, a feature present in anterior caudals of most diplodocids23,40,43 (Fig.5B). e lateral
surface, however, does not bear any ridges, similar to Amargatitanis11. In lateral view, the ventral margin of the
centrum is strongly concave, akin to Lingwulong13. Moreover, the curvature of the ventral margin suggests the
ventral rim of the posterior central articular surface to be at a lower level than the anterior, a feature shared with
Dicraeosaurus39 and Suuwassea24. e neural arch is poorly preserved (Fig.5A,B) and placed anteriorly on the
centrum as in Dicraeosaurus39, Lingwulong13 and Suuwassea24. e ventral surface is weakly convex transversely
with an incipient midline keel (Fig.5C). is ventral keel is also present in other dicraeosaurids, including
Suuwassea and Dicraeosaurus8,24,39.
The second caudal (RWR-241-K) vertebra is a nearly complete centrum but without the neural arch
(Fig.5D–G). e centrum is anteroposteriorly elongated, suggesting a more posterior position in the caudal
series compared to RWR-241-I (sensu Coria etal.8). e 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).
is is corroborated by similarities with middle caudal centra of Suuwassea24 (cL/cH = 1.5) and Amargatitanis11
(cL/cH = 1.3). A prominent lateral fossa is also reported in the dicraeosaurid Amargatitanis11. 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. e centrum is platyceolous with the anterior and
posterior cotyles dorsoventrally taller than mediolaterally wide (Fig.5D,E), although there is some evidence
of deformation. e ventral surface is strongly concave in lateral view with the ventral margin of the posterior
cotyle descending below the level of the anterior cotyle. e 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
Diplodocus44, Seismosaurus45, and Barosaurus46.
Phylogenetic analysis. e inter-relationship of arosaurus within Sauropodomorpha was tested in a
reduced version of the data matrix used by Gallina etal.7 (see “Methods” and Supplementary Note 2). e 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. e topology of the strict consensus tree (Supplementary Fig.4) shows a well resolved clade Sau-
ropoda with diplodocoids and macronarians showing distinct clustering within Neosauropoda, consistent with
previous studies7,13. arosaurus is recovered as a dicraeosaurid agellicaudatan, although the clade Dicraeosau-
ridae is poorly resolved. In the 50% majority rule tree (Fig.6), Dicraeosauridae is better resolved with arosaurus
being a sister taxon to ((Pilmatueia + Amargatitanis) + (Brachytrachelopan + (Dicraeosaurus + Amargasaurus))).
Discussion
Phylogenetic implications. arosaurus shares ve synapomorphies with Flagellicaudata [divided lateral
pleuroceols on cervical centra (ch. 115); bifurcated presacral neural spines (ch. 376); lateroventral anges on
middle/posterior cervical centra (ch. 382); heart-shaped anterior caudal centra cross-section (389); pre-epipo-
physis 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)]. arosaurus also shares two synapomorphies with other diplodocoids [transversely concave ven-
tral surface of cervical centra (ch. 112); quadrangular middle caudal centra (ch. 208)]. Furthermore, three auta-
pomorphies characterize arosaurus—smooth and narrow prespinal lamina on non-bid dorsal neural spine
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(ch 143), pleuroceol on anterior caudal (ch 194), ventral mid-line keel in middle/posterior cervicals bifurcated
posteriorly but not meeting the lateroventral anges (ch 380). Barring the last autapomorphy, the remaining are
local, being also present in a few diplodocids.
To further test the position of arosaurus 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 a-
rosaurus to be placed anywhere within Sauropodomorpha. Furthermore, in analysis A2, characters involving
cervical spines were scored as ‘?’, as these are not preserved in arosaurus, although the neural arch morphology
strongly suggests the presence of bifurcated spines. Both analyses produce the same results where arosaurus is
recovered as a dicraeosaurid (Supplementary Figs.5–8). Furthermore, the phylogenetic bracketing of arosaurus
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 arosaurus, as the bracketing taxa also possess the same feature.
Palaeobiogeography. Sauropods are considered to have originated in the Late Triassic/Early Jurassic38,47
but the origin and radiation of Neosauropoda and its major clades (Diplodocoidea and Macronaria) are still
amongst the most contentious issues48. Non-neosauropods were restricted to eastern Gondwana (India and
Zimbabwe) and parts of Laurasia (ailand, Germany and China) during Late Triassic–Early Jurassic, suggest-
ing possible physiological constraints to their dispersal to the Americas and Australia47, although sampling
biases cannot be ruled out. Neosauropods possibly appeared and radiated during the late Early/ early Middle
Figure6. Phylogenetic position of arosaurus indicus gen. et sp. nov. (RWR-241) in 50% majority-rule tree.
Clade Dicraeosauridae shaded in pink. Numbers above nodes indicate Bremer support values.
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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 Lau-
rasia remained united for much of the early Mesozoic as the supercontinent Pangaea51. 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 dispersal51,52. Simi-
larly, dispersal between North America and Europe53 possibly occurred during the Middle Jurassic, although
these continents were likely separated by a narrow epicontinental sea (the Viking Corridor) during the Early
Jurassic54. However, by the early Bajocian (~ 175Ma), sea-oor spreading started in the western Tethys, the Cen-
tral Atlantic region and the Gulf of Mexico55–57, and global transgressions ooded most continental shelves dur-
ing the Middle and Late Jurassic, separating North America from South America and Africa56,57. Consequently,
major neosauropod clades in these Laurasian and Gondwanan continents must have originated and radiated,
prior to the Bajocian riing57.
Until recently, the East Asian Jurassic dinosaurs were regarded as endemic fauna, characterized by mamen-
chisaurids and tetanurans13,58. 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 event13. Faunal dif-
ferences 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 Jurassic49,59. Nonetheless,
discoveries of the Chinese Middle and Late Jurassic neosauropods (Dashanpusaurus, Lingwulong, Bellusaurus)
tend to weaken the isolation hypothesis13,50 and support a circum-Pangaean distribution of the major neo-
sauropod clades by Middle Jurassic13,50. e age of the Chinese macronarian Dashanpusaurus is controversial
but possibly Bajocian50, 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 Formation50,60. In their palaeobiogeographic analysis of sauropods, Ren etal.50 (p. 6, Fig.3) assigned a
Callovian age to Lingwulong. us, the discovery of arosaurus assumes considerable signicance owing to its
older, Early–Middle Bathonian age.
To assess the palaeobiogeographic signicance of arosaurus, a time-calibrated phylogenetic tree was
constructed (Figs.7, 8). e resultant topology is consistent with the current consensus on rebbachisaurid
origins29,61,62 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 anities with Africa61,63. Based on the Late Jurassic Maraapunisaurus64, a North American origin of reb-
bachisaurids and their dispersal into South America through Europe and Africa during the latest Jurassic–earliest
Cretaceous has been suggested. However, the rebbachisaurid anity of this sauropod is questionable29. Early
Cretaceous rebbachisaurids from England including Xenoposeidon, claimed to be the oldest member of this
clade29,62, also add uncertainty to the Gondwanan origin of Rebbachisauridae. e lower-level phylogenetic
anity 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 clade13,50,65. is work also suggests that dierentiation
within Rebbachisauridae started during the Late Jurassic, consistent with previous studies53,64.
e 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 sup-
ports this inference since most of the other valid taxa (Brontosaurus, Galeamopus, Supersaurus and Amphicoelias)
are also from North America29,30,37. Furthermore, the tree topology supports a pre-Bajocian divergence and
dispersal of diplodocids from dicraeosaurids, consistent with most previous studies on agellicaudatan dispersal
and palaeobiogeography29,57,65. Diplodocids were, however, not restricted to North America, but are also known
from the Late Jurassic of Europe (Dinheirosaurus66) and Africa (Tornieria56), and the Early Cretaceous of South
America (Leinkupal57).
e clade Dicraeosauridae is represented by nine taxa ranging from Middle Jurassic to Early Cretaceous
(Figs.7, 8). e tree topology shows a late-diverging clade with a preponderance of South American dicraeosau-
rids (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 earliest-
diverging 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 Mantilla29. Recent work
on agellicaudatan palaeobiogeography based on the dicraeosaurid Lingwulong identied Asia + South America
as some of the areas for MRCAs of Dicraeosauridae13.
Although later-diverging than Suuwassea and Lingwulong, the discovery of arosaurus in a much older Mid-
dle Jurassic horizon calls into question the above two hypotheses. e early–middle Bathonian age of arosaurus
(see Supplementary Note 1) makes it the oldest diplodocoid globally. Even if Lingwulong is reinstated to its origi-
nal estimated age of 174Ma (sensu Xu etal.13), arosaurus remains the oldest Gondwanan diplodocoid as the
African and South American diplodocoids appear from the Kimmeridgian (157Ma)13,64 (Fig.7). e 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 arosaurus 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 reconstructions67 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
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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 origins13,50,65.
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
arosaurs. 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 arosaurus is geologically older and
phylogenetically early-branching relative to nearly all African and South American diplodocoids (Figs.7, 8). e
only exception is the South American Bajadasaurus, which is phylogenetically bracketed by Suuwassea + Lingwu-
long and arosaurus, but known from a much younger stratigraphic horizon (Fig.7). e phylogenetic position
of Bajadasaurus may possibly be explained by a radiation event from North America or from India predating
arosaurus. However, additional sampling leading to greater anatomical coverage of arosaurus in the future
may change its phylogenetic position. In any case, the long ghost lineage leading to Bajadasaurus presents the
possibility of nding earlier-diverging taxa in western Gondwana.
us, a more plausible hypothesis, based on the older stratigraphic age of arosaurus, is that migrations
from India to Asia could have taken place through western Gondwana and North America via Europe (Fig.8).
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
arosaurus indicus.
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However, Lingwulong still poses a biogeographic problem, being older than all western Gondwanan and Laura-
sian diplodocoids. e biogeographic puzzle arising from the two Middle Jurassic dicraeosaurids in India and
China indicates a pre-Middle Jurassic origin of neosauropods followed by widespread dispersal, as inferred in
the present work and previous studies13,50,65. e lack of corroborative fossils representing temporally and phy-
logenetically intermediate taxa appears to be a consequence of sampling and geological biases13, and points to
the need for more rigorous collecting eorts in these regions.
Despite the fragmentary material currently available and the possibility that the taxonomic attribution within
Flagellicaudata may potentially change with the recovery of additional material/character sampling, arosaurus
remains the oldest known diplodocoid. Together with the putative Bajocian camarasauromorph from Kutch17, the
discovery of arosaurus makes India a major centre for not only diplodocoid but neosauropod radiation. While
previous authors13,50 have considered Asia and the Americas as regions inhabited by the MRCAs of these clades,
this study highlights the importance of pre-Bathonian Indian sauropodomorph record in tracing the origins
of Neosauropoda. is suggestion is strongly supported by the well-known early-diverging non-neosauropods,
Barapasaurus and Kotasaurus, from the Early Jurassic (Sinemurian–Pliensbachian) Kota Formation of India18,19.
To conclude, the discovery of arosaurus emphasizes the need for increased sampling of Bathonian and older
Jurassic horizons of India in search of ancestral taxa intermediate between the neosauropods and Barapasaurus-
like eusauropods.
Methods
Osteological description. e osteological description of the skeletal specimens was carried out follow-
ing the nomenclature of Bandyopadhyay etal.2, Coria etal.8 and Xu etal.13. Dierent parameters of the fossil
specimens were measured (Supplementary Fig.2) using Mitutoyo digital callipers with a precision of 0.01mm.
Explanatory line drawings are used wherever necessary. e terminology for vertebral laminae and fossae fol-
lows Wilson25 and Wilson etal.32.
Phylogenetic analysis. e phylogenetic anity of arosaurus was determined through an analysis
based ona combination of characters from the datasets of Xu etal.13 and Gallina etal.7 with 19 additional
Figure8. Palaeogeographic distribution of diplodocoids with taxa of dierent ages plotted together in a
simplied Middle Jurassic (170Ma) map to show their spatio-temporal distribution across Pangea. Silhouettes
indicate the type of diplodocoid and fossil occurrences. Numbers adjoining sauropod silhouettes indicate age of
the fossils as follows: 1—Middle Jurassic (early–middle Bathonian); 2—Late Jurassic; 3—Cretaceous; 4—Middle
Jurassic (Callovian). Palaeogeographic map aer Scotese67 and sourced from https:// www. earth byte. org/ paleo
map- paleo atlas- for- gplat es/ [is work is licensed under the Creative Commons Attribution 4.0 International
License. http:// creat iveco mmons. org/ licen ses/ by/4. 0/]. Source of information on sauropod distribution from the
Paleobiology database (https:// www. paleo biodb. org/) and Ren etal.50.
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characters from Tschopp etal.26 (Supplementary Data 1). e taxa-character matrix included 37 taxa and 394
characters. e phylogenetic analysis was performed in TNT version 1.668 where the soware memory was set
to retain 10,000 trees with a display buer of 10Mb (sensu Coria etal.8). Following Gallina etal.7 the Traditional
Search option was used to analyse the dataset. e constraints for the analysis included 5000 replications of
Wagner trees, where the swapping algorithm was bisection reconnection with 10 trees saved per replication.
To determine the robustness of the nodes, Bremer support values were calculated using the script bremer.run
where only trees suboptimal by 20 steps were retained. e outgroup taxon in this analysis was Shunosaurus lii.
Data availability
All data associated with the manuscript are provided in the supplementary le.
Code availability
Nomenclatural acts. is published work and the nomenclatural acts it contains have been registered in Zoo-
Bank, the proposed online registration system for the International Code of Zoological Nomenclature (ICZN).
e LSIDs for this publication are urn:lsid:zoobank.org:act:EB6CB405-6FD6-4EDA-9E49-ED08105AFBD8
(arosaurus) and urn:lsid:zoobank.org:act:C43C0866-88BB-475B-9439-86C8837AB833 (T. indicus).
Received: 26 April 2023; Accepted: 30 July 2023
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Acknowledgements
We acknowledge with thanks the helpful comments, suggestions and a constructive critique of the manuscript
by the two reviewers and Editor Scientic Reports. We thank Matthew Carrano, National Museum of Natural
History, Smithsonian Institution (Washington DC, USA) for going through the initial dra of the manuscript
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and oering his valuable comments and suggestions. We also thank Pablo A. Gallina for access to the phyloge-
netic dataset used in this analysis. Authors from the Geological Survey of India (GSI) thank Director General,
and the following ocers from the GSI, Western Region, Jaipur for providing departmental facilities, constant
support and permission to publish this study: Additional Director General & HOD, Deputy Director General
& RMH-IV and Director, Palaeontology Division. Some of the material described in this paper is part of the
ongoing doctoral work of one of the co-authors (TG) at IIT Roorkee. SB would like to acknowledge support
obtainedfrom IIT Roorkee as part of his Institute Chair Professorship. e Science and Engineering Research
Board (SERB), Government of India (Grant no. PDF/2021/00468) and Indian Institute of TechnologyRoorkee
are acknowledged for providing nancial and infrastructural facilities to DD.
Author contributions
K.K., P.P., S.B., and D.B. conceived the problem. K.K., P.P. and T.G. collected the data. D.D. and S.B. analysed and
interpreted the data and wrote the manuscript. S.B. and D.D. were involved in further revisions and modications
of the manuscript with contributions from K.K., P.P. and T.G.
Competing interests
e authors declare no competing interests.
Additional information
Supplementary Information e online version contains supplementary material available at https:// doi. org/
10. 1038/ s41598- 023- 39759-2.
Correspondence and requests for materials should be addressed to S.B.orD.D.
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