Abstract
BackgroundHeterodontosaurids are an important but enigmatic and poorly understood early radiation of ornithischian dinosaurs. The late-surviving heterodontosaurid Fruitadens haagarorum from the Late Jurassic (early Tithonian) Morrison Formation of the western USA is represented by remains of several small (<1 metre total body length, <1 kg body mass) individuals that include well-preserved but incomplete cranial and postcranial material. Fruitadens is hypothesized to represent one of the smallest known ornithischian dinosaurs.Methodology/Principal FindingsWe describe the cranial and postcranial anatomy of Fruitadens in detail, providing comparisons to all other known heterodontosaurid taxa. High resolution micro-CT data provides new insights into tooth replacement and the internal anatomy of the tooth-bearing bones. Moreover, we provide a preliminary functional analysis of the skull of late-surviving heterodontosaurids, discuss the implications of Fruitadens for current understanding of heterodontosaurid monophyly, and briefly review the evolution and biogeography of heterodontosaurids.Conclusions/SignificanceThe validity of Fruitadens is supported by multiple unique characters of the dentition and hindlimb as well as a distinct character combination. Fruitadens shares highly distinctive appendicular characters with other heterodontosaurids, strengthening monophyly of the clade on the basis of the postcranium. Mandibular morphology and muscle moment arms suggest that the jaws of late-surviving heterodontosaurids, including Fruitadens, were adapted for rapid biting at large gape angles, contrasting with the jaws of the stratigraphically older Heterodontosaurus, which were better suited for strong jaw adduction at small gapes. The lack of wear facets and plesiomorphic dentition suggest that Fruitadens used orthal jaw movements and employed simple puncture-crushing to process food. In combination with its small body size, these results suggest that Fruitadens was an ecological generalist, consuming select plant material and possibly insects or other invertebrates.
Highlights
Ornithischian dinosaurs were one of the most important groups of Mesozoic archosaurs, dominating the herbivorous macrofauna of the northern hemisphere during the Cretaceous [1,2,3]
Africa and Lesotho, with approximately 20 specimens known from these strata [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. These specimens form the basis for five monospecific genera, of which three (Heterodontosaurus tucki, Abrictosaurus consors, Lycorhinus angustidens) are considered valid in recent reviews [12,13,14], a fourth taxon appears to be present, and ‘‘Lanasaurus scalpridens’’ may or may not be valid [14,29]
Specimens previously assigned to Heterodontosauridae from the Late Triassic of Switzerland [43], the Early Jurassic of China [44], and the Early Cretaceous of Spain [45] have been recently removed from the clade [46,47,48]
Summary
Ornithischian dinosaurs were one of the most important groups of Mesozoic archosaurs, dominating the herbivorous macrofauna of the northern hemisphere during the Cretaceous [1,2,3]. Africa and Lesotho, with approximately 20 specimens (many of which remain incompletely studied) known from these strata [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] These specimens form the basis for five monospecific genera, of which three (Heterodontosaurus tucki, Abrictosaurus consors, Lycorhinus angustidens) are considered valid in recent reviews [12,13,14], a fourth taxon appears to be present, and ‘‘Lanasaurus scalpridens’’ (double quotation marks indicate that a species may not be diagnostic, or that a proposed taxonomic grouping is probably non-monophyletic) may or may not be valid [14,29]. Fruitadens is hypothesized to represent one of the smallest known ornithischian dinosaurs
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