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. 2011 Jan 26;6(1):e14572.
doi: 10.1371/journal.pone.0014572.

A New Sauropodomorph Dinosaur From the Early Jurassic of Patagonia and the Origin and Evolution of the Sauropod-Type Sacrum

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A New Sauropodomorph Dinosaur From the Early Jurassic of Patagonia and the Origin and Evolution of the Sauropod-Type Sacrum

Diego Pol et al. PLoS One. .
Free PMC article

Abstract

Background: The origin of sauropod dinosaurs is one of the major landmarks of dinosaur evolution but is still poorly understood. This drastic transformation involved major skeletal modifications, including a shift from the small and gracile condition of primitive sauropodomorphs to the gigantic and quadrupedal condition of sauropods. Recent findings in the Late Triassic-Early Jurassic of Gondwana provide critical evidence to understand the origin and early evolution of sauropods.

Methodology/principal findings: A new sauropodomorph dinosaur, Leonerasaurus taquetrensis gen. et sp. nov., is described from the Las Leoneras Formation of Central Patagonia (Argentina). The new taxon is diagnosed by the presence of anterior unserrated teeth with a low spoon-shaped crown, amphicoelous and acamerate vertebral centra, four sacral vertebrae, and humeral deltopectoral crest low and medially deflected along its distal half. The phylogenetic analysis depicts Leonerasaurus as one of the closest outgroups of Sauropoda, being the sister taxon of a clade of large bodied taxa composed of Melanorosaurus and Sauropoda.

Conclusions/significance: The dental and postcranial anatomy of Leonerasaurus supports its close affinities with basal sauropods. Despite the small size and plesiomorphic skeletal anatomy of Leonerasaurus, the four vertebrae that compose its sacrum resemble that of the large-bodied primitive sauropods. This shows that the appearance of the sauropod-type of sacrum predated the marked increase in body size that characterizes the origins of sauropods, rejecting a causal explanation and evolutionary linkage between this sacral configuration and body size. Alternative phylogenetic placements of Leonerasaurus as a basal anchisaurian imply a convergent acquisition of the sauropod-type sacrum in the new small-bodied taxon, also rejecting an evolutionary dependence of sacral configuration and body size in sauropodomorphs. This and other recent discoveries are showing that the characteristic sauropod body plan evolved gradually, with a step-wise pattern of character appearance.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Geological map of the locality where Leonerasaurus taquetrensis was found (indicated by asterisk and silhouette).
Figure 2
Figure 2. Geological section of Las Leoneras Formation.
A detailed section of the three members recognized here for the Las Leoneras Formation is given, starting from the base (left of the figure) to the top of the unit (right of the figure).
Figure 3
Figure 3. Dentary of Leonerasaurus taquetrensis (MPEF-PV 1663).
A-B, lateral view. C–D, medial view. Scale bar represents 5 mm. Abbreviations: de, dentary; mg, meckelian groove; nv, neurovascular foramina.
Figure 4
Figure 4. Teeth of Leonerasaurus taquetrensis (MPEF-PV 1663).
A–D, SEM image of anterior tooth in A, labial; B, lingual; and C, mesial views. D, detail of unserrated apical region of mesial margin. E, posterior replacement tooth with denticles in lingual view. Scale bars represent 500 µm (A–C, E) and100 µm (D).
Figure 5
Figure 5. Cervical vertebrae 3–5 of Leonerasaurus taquetrensis (MPEF-PV 1663).
A–B, lateral view. C–D, dorsal views. Scale bars represent 10 mm. Hatched pattern represents broken surfaces and dotted pattern represents sediment. Abbreviations: c3-c5, cervical vertebrae 3 through 5; di, diapophysis; pa, parapophysis; pri, prezygapophyseal ridge; epi, epipophysis; psf, postspinal fossa; sk, sagittal keel.
Figure 6
Figure 6. Cervical vertebrae 6–8 of Leonerasaurus taquetrensis (MPEF-PV 1663).
A–B, lateral view. C–D, dorsal views. Scale bars represent 10 mm. Hatched pattern represents broken surfaces and dotted pattern represents sediment. Abbreviations: c6-c8, cervical vertebrae 6 through 8; di, diapophysis; pcdl, posterior centrodiapophyseal lamina; prdl, prezygodiapophyseal lamina; psf, postspinal fossa.
Figure 7
Figure 7. Dorsal vertebrae of Leonerasaurus taquetrensis (MPEF-PV 1663) in lateral view.
A–B, first four dorsals. C–D, mid-posterior dorsals. Scale bar represents 10 mm. Hatched pattern represents broken surfaces and dotted pattern represents sediment. Abbreviations: cpr, centroprezygapophyseal ridge; di, diapophysis; hyp, hyposphene; pa, parapophysis; pcdl, posterior centrodiapophyseal lamina; podl, postzygodiapophyseal lamina; ppdl, parapodiapophyseal lamina; prdl, prezygoodiapophyseal lamina; spol, spinopostzygapophyseal lamina.
Figure 8
Figure 8. Sacral vertebrae of Leonerasaurus taquetrensis (MPEF-PV 1663).
A–B, dorsal view. C–D, ventral view. E–F, lateral view (inverted right side). Hatched pattern represents broken surfaces and dotted pattern represents sediment. Gray areas represent the iliac attachment surface of the sacral ribs. Scale bar represents 10 mm. Abbreviations: cs, caudosacral; csr, caudosacral rib; ds, dorsosacral; dsr, dorsosacral rib; il, ilium; ip, ischial peduncle; s1, first primordial sacral; s1r, first primordial sacral rib; s2r, second primordial sacral rib; s2r, second primordial sacral rib; pap, preacetabular process; pop, postacetabular process; pp, pubic peduncle.
Figure 9
Figure 9. Right scapula of Leonerasaurus taquetrensis (MPEF-PV 1663) in lateral view.
A, photograph; B, interpretive line drawing. Scale bar represents 10 mm. Hatched pattern represents broken surfaces. Abbreviations: ap, acromion process.
Figure 10
Figure 10. Right humerus of Leonerasaurus taquetrensis (MPEF-PV 1663).
A–B, anterior view. C–D, posterior view. E–F, lateral view. Scale bar represents 10 mm. Hatched pattern represents broken surfaces. Abbreviations: cf, cuboid fossa; dc, deltopectoral crest; it, internal tuberosity.
Figure 11
Figure 11. Right ilium of Leonerasaurus taquetrensis (MPEF-PV 1663) in lateral view.
A, photograph; B, interpretive line drawing. Scale bar represents 10 mm. Hatched pattern represents broken surfaces. Abbreviations: ip, ischial peduncle; pap, preacetabular process; pop, postacetabular process; pp, pubic peduncle; sac, supracetabular crest.
Figure 12
Figure 12. Right ischium of Leonerasaurus taquetrensis (MPEF-PV 1663).
A–B, lateral view. C–D, posterodorsal view. Scale bar represents 10 mm. Hatched pattern represents broken surfaces. Abbreviations: ig, ischial groove; is, ischial shaft; op, obturator process.
Figure 13
Figure 13. Pedal remains of Leonerasaurus taquetrensis (MPEF-PV 1663).
A–D, metatarsal I and II in A–B, dorsal; C–D, plantar; and E–F proximal views. G–L, pedal phalanx in G–H, lateral; I–J, dorsal; and K–L, plantar views. Scale bars represent 10 mm. Hatched pattern represents broken surfaces and dotted pattern represents sediment. Abbreviations: mtt I-II, metatarsal I-II; ft, flexor tubercle; lp, ligament pit; vlf, ventrolateral flange; vmf, ventromedial flange.
Figure 14
Figure 14. Reduced strict consensus of the phylogenetic analysis.
Four unstable taxa (Jingshanosaurus, Blikanasaurus, Camelotia, and Ferganasaurus) were excluded from the consensus a posteriori of the heuristic tree searches. Only sauropodomorph taxa are shown (for a complete consensus tree including all outgroup taxa see Appendix S1). Numbers at the nodes represent Bremer support values.
Figure 15
Figure 15. Histological section from a dorsal rib of Leonerasaurus taquetrensis (MPEF-PV 1663).
A, cortical bone composed of fibrolamellar bone tissue with distinct zones and annuli (arrows). Secondarily enlarged erosion cavities are visible in the perimedullar region. B, Close up of the cortex as indicated in white rectangle in (A) showing the fine structure of zones and annuli. Some secondary osteons are scattered in the inner cortex. Scale bars represent 0.5 (A) and 0.2 mm (B). Abbreviations: an, annulus; so, secondary osteon; zo, zone.
Figure 16
Figure 16. Evolutionary history of the acquisition of sacral vertebrae (above) and body size (below) in basal Sauropodomorpha.
Colored boxes and lines on the terminal taxa and branches represent the optimization of the type of sacrum among basal sauropodomorphs. Autapomorphic additions of sacral elements are marked with asterisks (see text for explanation). The curves plotted below the cladogram represent the range of estimated body size (y-axis) in sauropodomorph nodes leading to eusauropods (x-axis). Ancestral reconstructions of body mass are based on femoral lateromedial width (FML; see and Appendix S1 for further data and methods). The terminal ‘Eusauropods’ represents forms more derived than the basal eusauropod Shunosaurus, some of which have further increased the sacral count , .

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