A new basal sauropod dinosaur from the middle Jurassic of Niger and the early evolution of sauropoda

Abstract

Background: The early evolution of sauropod dinosaurs is poorly understood because of a highly incomplete fossil record. New discoveries of Early and Middle Jurassic sauropods have a great potential to lead to a better understanding of early sauropod evolution and to reevaluate the patterns of sauropod diversification.

Principal findings: A new sauropod from the Middle Jurassic of Niger, Spinophorosaurus nigerensis n. gen. et sp., is the most complete basal sauropod currently known. The taxon shares many anatomical characters with Middle Jurassic East Asian sauropods, while it is strongly dissimilar to Lower and Middle Jurassic South American and Indian forms. A possible explanation for this pattern is a separation of Laurasian and South Gondwanan Middle Jurassic sauropod faunas by geographic barriers. Integration of phylogenetic analyses and paleogeographic data reveals congruence between early sauropod evolution and hypotheses about Jurassic paleoclimate and phytogeography.

Conclusions: Spinophorosaurus demonstrates that many putatively derived characters of Middle Jurassic East Asian sauropods are plesiomorphic for eusauropods, while South Gondwanan eusauropods may represent a specialized line. The anatomy of Spinophorosaurus indicates that key innovations in Jurassic sauropod evolution might have taken place in North Africa, an area close to the equator with summer-wet climate at that time. Jurassic climatic zones and phytogeography possibly controlled early sauropod diversification.

Figure 1. Spinophorosaurus nigerensis, holotype skeleton GCP-CV-4229 in situ during excavation in the region of Aderbissinat, Thirozerine Dept., Agadez Region, Republic of Niger.

Figure 2. Spinophorosaurus nigerensis GCP-CV-4229 (holotype).

(A–C)— Braincase in dorsal (A), caudal (B), and left lateral (C) views. (D, E)— Right quadrate and pterygoid in lateral (D) and medial (E) views. (F, G)— Dorsal end of right quadrate in lateral (F) and caudal (G) views. Scale bars  = 10 cm (A–C), 5 cm (D, E), and 2 cm (F, G).

Figure 3. Spinophorosaurus nigerensis GCP-CV-4229 (holotype; C, E-I) and NMB-1698-R (paratype; A, B, D).

(A, B)— Mid-cervical vertebra in left lateral (A) and ventral (B) views. (C)— Last dorsal and first sacral vertebrae in left lateral view. (D)— Clavicle in cranial view. (E, F)— Proximal caudal neural spines in lateral (E) and cranial (F) views. (G)— Mid-caudal vertebra in lateral view. (H, I)— Distal caudal vertebrae in left lateral (H) and ventral (I) views. Abbreviations: pcdl, posterior centrodiapophyseal lamina; podl, postzygodiapophyseal lamina; spol, spinopostzygapophyseal lamina. Scale bars  = 10 cm.

Figure 4. Spinophorosaurus nigerensis GCP-CV-4229/NMB-1699-R (holotype; A-E, H-N) and NMB-1698-R (paratype; F, G).

(A-C)— Contralateral spike-like osteoderms in dorsolateral (A), ventral (B), and cranial (C) views. (D, E)— Left coracoid (D) and scapula (E) in left lateral views. (F, G)— Right humerus in cranial (F) and distal (G) views. (H)— Left femur in caudal view. (I)— Left pubis in left lateral view. (J)— Left ischium in lateral view. (K, L)— Left tibia in proximal (K) and lateral (L) views. (M)— Left fibula in medial view. (N)— Left astragalus in proximal view. Scale bars  = 10 cm (A–C, N) and 20 cm (D–M).

Figure 5. Skeletal reconstruction of Spinophorosaurus nigerensis.

Dimensions are based on GCP-CV-4229/NMB-1699-R, elements that are not represented are shaded. Scale bar  = 1 m.

Figure 6. Phylogenetic relationships of Spinophorosaurus, based on an analysis of 27 taxa and 235 characters.

(A)— Single most parsimonious tree. Numbers next to nodes indicate bootstrap support values for nodes that show more than 50% support. White numbers in black circles: 1, Sauropoda; 2, Eusauropoda; 3, Neosauropoda; 4, Mamenchisauridae. Dashed line indicates alternative sister-group relationship of Barapasaurus and Patagosaurus as hypothesized in the main text, requiring only a single additional step. (B)— Proposed evolutionary scenario of early sauropods with an endemic South Gondwanan clade during the Lower and Middle Jurassic. White bars indicate insecurities in the dating of the formations in which these taxa were found. Abbreviation: CGD, Central Gondwanan Desert.

Figure 7. Congruence between Middle Jurassic sauropod distribution and paleoclimatic zones.

Although standing close to the origin of Eusauropoda, Spinophorosaurus exhibits strong similarities to East Asian Middle Jurassic sauropods (Shunosaurus, 6), and much less so to South Gondwanan forms, e.g. the late Lower Jurassic Barapasaurus from India and the late Middle Jurassic form Patagosaurus (1). The explanation is a separation of global sauropod faunas during the Lower Jurassic by the Central Gondwanan Desert (CGD), forming two different paleobiogeographical domains. Neosauropods had their origin in the same climatic zone as Spinophorosaurus and gained global distribution in late Middle Jurassic times (Atlasaurus, 4; Bellusaurus, 6; Lapparentosaurus, 2; Tehuelchesaurus, 1). Map redrawn after and . Abbreviations: CT, cold temperate climate; WT, warm temperate climate; SW, summer-wet climate; WW, winter-wet climate.