A basal sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the early evolution of Sauropodomorpha

Abstract

Background: The earliest dinosaurs are from the early Late Triassic (Carnian) of South America. By the Carnian the main clades Saurischia and Ornithischia were already established, and the presence of the most primitive known sauropodomorph Saturnalia suggests also that Saurischia had already diverged into Theropoda and Sauropodomorpha. Knowledge of Carnian sauropodomorphs has been restricted to this single species.

Methodology/principal findings: We describe a new small sauropodomorph dinosaur from the Ischigualsto Formation (Carnian) in northwest Argentina, Panphagia protos gen. et sp. nov., on the basis of a partial skeleton. The genus and species are characterized by an anteroposteriorly elongated fossa on the base of the anteroventral process of the nasal; wide lateral flange on the quadrate with a large foramen; deep groove on the lateral surface of the lower jaw surrounded by prominent dorsal and ventral ridges; bifurcated posteroventral process of the dentary; long retroarticular process transversally wider than the articular area for the quadrate; oval scars on the lateral surface of the posterior border of the centra of cervical vertebrae; distinct prominences on the neural arc of the anterior cervical vertebra; distal end of the scapular blade nearly three times wider than the neck; scapular blade with an expanded posterodistal corner; and medial lamina of brevis fossa twice as wide as the iliac spine.

Conclusions/significance: We regard Panphagia as the most basal sauropodomorph, which shares the following apomorphies with Saturnalia and more derived sauropodomorphs: basally constricted crowns; lanceolate crowns; teeth of the anterior quarter of the dentary higher than the others; and short posterolateral flange of distal tibia. The presence of Panphagia at the base of the early Carnian Ischigualasto Formation suggests an earlier origin of Sauropodomorpha during the Middle Triassic.

Figure 1. Geologic map of the Ischigualasto–Villa Unión Basin in northwestern Argentina.

The red arrow points to the holotypic site of Panphagia protos, which is located near the base of the Carnian Ischigualasto Formation.

Figure 2. Silhouette reconstruction of the skeleton of Panphagia protos.

Reconstruction shows preserved bones (white) and missing bones (light grey for left side; dark grey for right side). Body length is approximately 1.30 m.

Figure 3. Preserved skull bones of the new basal sauropodomorph Panphagia protos (PVSJ 874).

Right nasal in lateral (A) and dorsal (B) views. (C)-Right prefrontal in lateral view. (D)-Left frontal in dorsal view. (E)-Left parietal in dorsal view. (F)-Left quadrate in posterior view. (G)-Supraoccipital in posterior view. (H)-Right prootic in lateral view. Abbreviations: avp, anteroventral process of nasal; en, external nares; fmb, dorsal border of foramen magnum; ip, internarial process; lf, lateral flange; me, median eminence; nf, lateral fossa on nasal; o, orbit; PFs, prefrontal suture; plp, posterolateral process of nasal; POs, postorbital suture; pptm, M. protractor pterygoideus attachment; PTf, pterygoid flange; Qc, quadrate condyle; Qf, quadrate foramen; stf: supratemporal fossa; stfb, medial border of supratemporal fossa; V, trigeminal notch; vcdf, vena capitis dorsalis fossa. Scale bar equals 1 cm.

Figure 4. Lower jaw of the new basal sauropodomorph Panphagia protos (PVSJ 874).

Right lower jaw in lateral (A) and medial (B) views. (C)-Fragmentary left lower jaw in lateral view. (D)-Posterior end of right lower jaw in dorsal view. Abbreviations: A, angular; AR, articular, aSAf, surangular foramen; C, coronoid; D, dentary; emf, external mandibular fenestra; emg, external mandibular groove; g, groove; IC, intercoronoid; imf, internal mandibular fenestra; PA, prearticular; pvp, posteroventral process of dentary; rap, retroarticular process; SA, surangular; SP, splenial. Scale bar equals 1 cm.

Figure 5. Features of the dentition of the new basal sauropodomorph Panphagia protos (PVSJ 874).

Anterior dentary tooth in labial (A) and lingual (B) views. Abbreviations: lae, labial eminence; lie, lingual eminence. Scale bar equals 1 mm.

Figure 6. Postcranial axial skeleton the new basal sauropodomorph Panphagia protos (PVSJ 874).

(A)-Posterior cervical vertebra (presumptive C8) in ventral view. (B)-Anterior cervical vertebra in lateral view. (C)-Posterior cervical vertebra (presumptive C7) with abnormal outgrowth in dorsal view. Dorsal neural arch in lateral (D) and posterior (E) views. First primordial sacral (S1) in lateral (F) and dorsal (G) views. (H)-Posterior caudal vertebrae in lateral view (reversed). Abbreviations: ao, abnormal outgrowth in prezygapophysis; d, diapophysis; e: eminence; ha, haemal arch; hs, hyposphene; lk, lateral ventral keels; lr, lateral ridge; ns, neural spine; os, oval scars; p, parapophysis; prz, prezygapophysis; poz, postzygapophysis; sr, sacral rib; tp, transverse process; vk, median ventral keel. Scale bar equals 1 cm.

Figure 7. Scapula of the new basal sauropodomorph Panphagia protos (PVSJ 874).

Left scapula in lateral (A), anterior (B), and medial (C) views. Abbreviations: ac, acromion; g, glenoid surface; pdp, posterodistal process. Scale bar equals 2 cm.

Figure 8. Pelvic bones of the new basal sauropodomorph Panphagia protos (PVSJ 874).

Left ilium in lateral (A), posterior (B), and anterior (C) views. Left pubis in anterodorsal (D) and lateral (E) views. Left ischium in lateral (F) and distal (G) views. Abbreviations: de, distal expansion; ib, iliac blade; is, iliac spine; ml: medial lamina of brevis fossa; mvl, medioventral lamina; pap, postacetabular process; pf, pubic facet; pp, pubic process; sac, supraacetabular crest. Scale bar equals 2 cm.

Figure 9. Hind limb and pedal elements of the new basal sauropodomorph Panphagia protos (PVSJ 874).

Right tibia in lateral (A), distal (B), and posterior (C) views.(D)-Right astragalus in proximal view. Right metatarsal 3 in anterior (E) and proximal (F) views. Abbreviations: cc, cnemial crest; db, dorsal border; ep, extensor pit; ff, fibular facet; lc, lateral condyle of proximal tibia; plf, posterolateral fossa on proximal surface; pmc, posteromedial corner; pvp, posteroventral process. Scale bar equals 2 cm.

Figure 10. Consensus tree.

Consensus of three most-parsimonious trees resulting from the present parsimony analysis (tree length 187 steps; consistency index 0.561, retention index 0.568). The jackknife frequency (p = 0.36′ 1000 replications) and Bremer support values for each node are depicted above and below the internal branch leading to that node, respectively.