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, 279 (1741), 3170-5

A Middle Jurassic Abelisaurid From Patagonia and the Early Diversification of Theropod Dinosaurs

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A Middle Jurassic Abelisaurid From Patagonia and the Early Diversification of Theropod Dinosaurs

Diego Pol et al. Proc Biol Sci.

Abstract

Abelisaurids are a clade of large, bizarre predatory dinosaurs, most notable for their high, short skulls and extremely reduced forelimbs. They were common in Gondwana during the Cretaceous, but exceedingly rare in the Northern Hemisphere. The oldest definitive abelisaurids so far come from the late Early Cretaceous of South America and Africa, and the early evolutionary history of the clade is still poorly known. Here, we report a new abelisaurid from the Middle Jurassic of Patagonia, Eoabelisaurus mefi gen. et sp. nov., which predates the so far oldest known secure member of this lineage by more than 40 Myr. The almost complete skeleton reveals the earliest evolutionary stages of the distinctive features of abelisaurids, such as the modification of the forelimb, which started with a reduction of the distal elements. The find underlines the explosive radiation of theropod dinosaurs in the Middle Jurassic and indicates an unexpected diversity of ceratosaurs at that time. The apparent endemism of abelisauroids to southern Gondwana during Pangean times might be due to the presence of a large, central Gondwanan desert. This indicates that, apart from continent-scale geography, aspects such as regional geography and climate are important to reconstruct the biogeographical history of Mesozoic vertebrates.

Figures

Figure 1.
Figure 1.
Middle Jurassic abelisaurid Eoabelisaurus mefi. (a) Skeletal reconstruction, showing preserved parts. Skull in left lateral (b) and posterior (c) views. Middle cervical vertebra in left lateral (d) and dorsal (e) views. (f) Anterior mid-dorsal vertebra in left lateral view. (g) Posterior dorsal vertebra in left lateral view. (h) Anterior caudal vertebra in left lateral view. (i) Right ulna and radius in medial view. (j) Right manus in plantar view. (k) Pelvic girdle in left lateral view. (l) Left femur in lateral view. Pedal ungual in lateral (m) and ventral (n) views. Abbreviations: al, additional lamina; ap, ambiens process; at, antitrochanter; boc, basioccipital condyle; bt, basal tubera; d, depression; dc, distal carpal; di, diapophysis; epi, epipophysis; f, frontal; fm, foramen magnum; g, groove; il, ilium; ios, interorbital septum; is, ischium; j, jugal; l, lacrimal; lt, lesser trochanter; Mc, metacarpal; n, notch; ns, neural spine; of, obturator foramen; ol, olecranon process; pa, parietal; pap, paroccipital process; ph, phalanges; pl, pleurocoel; po, postorbital; poz, postzygapophysis; pp, parapophysis; prel, prezygoepipophyseal lamina; ps, parasphenoid; psf, prespinal fossa; pt, pterygoid; ptw, pterygoid wing of the quadrate; pu, pubis; q, quadrate; qj, quadratojugal; r, radius; soc, supraoccipital crest; sq, squamosal; ts, trochanteric shelf; u, ulna; vf, ventral fossa. Scale bars: (a) 500 mm; (bl) 50 mm; (m,n) 10 mm.
Figure 2.
Figure 2.
Calibrated phylogeny of ceratosaurian theropods. Thick grey lines indicate major ghost lineages in noasaurids and abelisaurids, resulting from the phylogenetic relationships of Eoabelisaurus. Dashed line in Spinostropheus indicates stratigraphic uncertainty for this taxon [16]. For abelisaurids, outlines of continents indicate geographical distribution.
Figure 3.
Figure 3.
Limb proportions in several theropods, showing reduction of distal elements of the forelimb. Femur and forelimb of the basal theropod Dilophosaurus (left), basal abelisaurid Eoabelisaurus (middle) and the derived abelisaurid Carnotaurus (right). Drawn to same femoral length for comparison.

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