A palaeoequatorial ornithischian and new constraints on early dinosaur diversification

Abstract

Current characterizations of early dinosaur evolution are incomplete: existing palaeobiological and phylogenetic scenarios are based on a fossil record dominated by saurischians and the implications of the early ornithischian record are often overlooked. Moreover, the timings of deep phylogenetic divergences within Dinosauria are poorly constrained owing to the absence of a rigorous chronostratigraphical framework for key Late Triassic-Early Jurassic localities. A new dinosaur from the earliest Jurassic of the Venezuelan Andes is the first basal ornithischian recovered from terrestrial deposits directly associated with a precise radioisotopic date and the first-named dinosaur from northern South America. It expands the early palaeogeographical range of Ornithischia to palaeoequatorial regions, an area sometimes thought to be devoid of early dinosaur taxa, and offers insights into early dinosaur growth rates, the evolution of sociality and the rapid tempo of the global dinosaur radiation following the end-Triassic mass extinction, helping to underscore the importance of the ornithischian record in broad-scale discussions of early dinosaur history.

Postcranial anatomy ofLesothosaurus diagnosticus(Dinosauria: Ornithischia) from the Lower Jurassic of southern Africa: implications for basal ornithischian taxonomy and systematics

“…In lateral view the distal end of the scapular blade is dorsoventrally expanded relatively to the shaft, similar to the condition seen in Laquintasaura venezuelae (Barrett et al, 2014). The extent of this distal expansion is much greater in Lesothosaurus than in Heterodontosaurus tucki (SAM-PK-K1332; Santa Luca, 1980) and also differs from the condition in Eocursor parvus in which the distal scapula is only weakly expanded (Butler, 2010).…”

“…The femur of SAM-PK-K1105 is bowed anteriorly, but not medially, as in SAM-PK-K1107 and the head is offset from the shaft to a greater degree than in NHMUK PV RU B17, SAM-PK-K400 and BP/1/4855. Otherwise, the femur of SAM-PK-K1105 is very similar in overall morphology to the specimens of L. diagnosticus, as well as to several other ornithischians such as Laquintasaura venezuelae, Agilisaurus louderbacki, Jeholosaurus shangyuanensis and Hypsilophodon foxii (Galton, 1974;Peng, 1990Peng, , 1992Peng, , 1997Butler, 2005;Han et al, 2012;Barrett et al, 2014). Tibia (Fig.…”

“…Although it is out of life position, the size, morphology and preservation of this ischium all indicate that it belongs to the same block and is probably from the same individual as the rest of the material. This ischium is incomplete, with only the proximal end preserved, but its shape suggests that the anterior margin of the shaft formed a continuous, gentle expansion, rather than a distinct 'tab-shaped' obturator process and so is more similar to the condition seen in Laquintasaura venezuelae (Barrett et al, 2014) than that in Agilisaurus louderbacki, Hexinlusaurus multidens, Jeholosaurus shangyuanensis or Hypsilophodon foxii (Galton, 1974;Peng, 1990Peng, , 1992Peng, , 1997Barrett et al, 2005Barrett et al, , 2014Han et al, 2012).…”

“…In recent years, a variety of new specimens and revisions of previously known material have helped to better constrain the timing of major events in ornithischian evolution, as well as giving insights into their early diversity (e.g. Knoll, 2002;Butler, 2005Butler, , 2010Butler, Smith & Norman, 2007;Irmis et al, 2007;Butler, Porro & Norman, 2008b;Butler, Upchurch & Norman, 2008a;Norman et al, 2011;Pol, Rauhut & Becerra, 2011;Porro et al, 2011;Sereno, 2012;Barrett et al, 2014). Although the early ornithischian record is richer than previously recognized , it is still poor in comparison with that of early saurischians.…”

“…Pisanosaurus, Eocursor, many heterodontosaurids: Bonaparte, 1976;Butler, 2010;Porro et al, 2011;Sereno, 2012) or disarticulated bonebed material (e.g. Laquintasaura, Barrett et al, 2014), although Heterodontosaurus (e.g. Santa Luca, 1980;Butler et al, 2008b;Norman et al, 2011) and Scelidosaurus (Owen, 1861(Owen, , 1863 are also represented by well-preserved articulated specimens and Scutellosaurus (Colbert, 1981) by a well-preserved but disarticulated specimen.…”