A primitive ornithischian dinosaur from the Late Triassic of South Africa, and the early evolution and diversification of Ornithischia

Abstract

Although the group played an important role in the evolution of Late Mesozoic terrestrial ecosystems, the early evolutionary history of the ornithischian dinosaurs remains poorly understood. Here, we report on a new primitive ornithischian, Eocursor parvus gen. et sp. nov. from the Late Triassic (?Norian) Lower Elliot Formation of South Africa. Eocursor is known from a single specimen comprising substantial cranial and postcranial material and represents the most complete Triassic member of Ornithischia, providing the earliest evidence for the acquisition of many key ornithischian postcranial characters, including an opisthopubic pelvis. A new phylogenetic analysis positions this taxon near the base of Ornithischia, as the sister taxon to the important and diverse clade Genasauria. The problematic clade Heterodontosauridae is also positioned basal to Genasauria, suggesting that an enlarged grasping manus may represent a plesiomorphic ornithischian condition. This analysis provides additional phylogenetic support for limited ornithischian diversity during the Late Triassic, and suggests that several major ornithischian clades may have originated later than generally believed. There are few morphological differences between Late Triassic and Early Jurassic ornithischians, supporting previous suggestions that the Early Jurassic ornithischian radiation may simply represent the filling of vacant ecological space following Late Triassic terrestrial extinctions.

Figure 1

Skeletal reconstruction of E. parvus gen. et sp. nov. based upon SAM-PK-K8025, demonstrating preserved elements and body proportions. Scale bar, 100 mm. Reconstruction by Scott Hartman.

Figure 2

Anatomy of the holotype of E. parvus (SAM-PK-K8025). (a) Right mandible in lateral view. (b) Right scapula in lateral view. (c) Right humerus in posterior view. (d) Preserved manual phalanges in dorsal view. (e) Left ilium in lateral view (preacetabular process is missing). (f) Preacetabular process of right ilium in lateral view. (g) Prepubic process of left pubis in medial view. (h) Elements of pelvic region and hindlimb, including dorsal and sacral vertebrae, ischia (right ischium in medial view; left ischium in dorsal view), pubes, right tibia and fibula. (i) Right femur in lateral view. (j) Right metatarsals 2 (left) and 3 (right) in medial view. Scale bars, 10 mm. Abbreviations: ac, acromion process; at, anterior trochanter; be, buccal emargination; bs, brevis shelf; dna, neural arch of dorsal vertebra; dpc, deltopectoral crest; dsna, neural arch of dorsosacral vertebra; ex pt, extensor pit on dorsal surface of distal end of manual phalanx; fib, fibula; ft, fourth trochanter; lisch, left ischium; lpub, left pubis; obt f, obturator foramen; posp, pubic shaft; prp, prepubic process; risch, right ischium; rpub, right pubis; saf, supra-acetabular flange; spt, spout-shaped mandibular symphysis; srgrd, weak ridge on lateral surface of surangular; tib, tibia.

Figure 3

Temporally calibrated phylogeny of early ornithischian dinosaurs. The phylogeny is a simplified version of a cladogram produced by an analysis of 27 taxa and 150 characters (electronic supplementary material). Grey lines indicate ghost lineages. Minimum origination dates for major ornithischian clades include: Carnian (Late Triassic), Ornithischia; Hettangian (Early Jurassic), Genasauria, Thyreophora, Neornithischia; Callovian (Middle Jurassic), Cerapoda, Ornithopoda; Oxfordian (Late Jurassic), Marginocephalia. Numbers indicate clades: 1, Ornithischia; 2, Genasauria; 3, Neornithischia; 4, Cerapoda; 5, Marginocephalia.