New toothed Early Cretaceous ornithuromorph bird reveals intraclade diversity in pattern of tooth loss

The earliest record of the Ornithuromorpha, which includes crown birds, is currently known from the Early Cretaceous Jehol Biota in north-eastern China. Here we describe a new ornithuromorph bird, <i>Mengciusornis dentatus</i> gen. et sp. nov., from the Jiufotang Formation of this biota. <i>Mengciusornis</i> preserves a suite of morphological features exclusively observed among ornithuromorphs in the sympatric taxon <i>Schizooura</i>, including a robust and ‘V’-shaped furcula with a short hypocleidium, and a humerus with a large, rectangular deltopectoral crest in which the distodorsal corner is distinctively developed into a sharp, angular point. The results of our phylogenetic analysis recover these two species together in a clade, positioned stemward within the Ornithuromorpha. <i>Mengciusornis</i> has toothed premaxillae, in stark contrast with the edentulous jaws of <i>Schizooura</i>, demonstrating that dentition varied considerably between some closely related species. Furthermore, all previously known Jehol ornithuromorphs with toothed premaxillae have an edentulous rostral portion of the element, suggesting that tooth loss in this clade proceeded from the rostral end caudally, whereas in <i>Mengciusornis</i> the premaxilla lacks this edentulous rostral tip although the maxilla and dentary are toothless, indicating that the pattern of tooth loss, not unexpectedly, varied between lineages of ornithuromorphs. This observation suggests a strong degree of plasticity in the developmental mechanisms regulating tooth and beak formation, which is supported by earlier evidence that indicates teeth have been lost multiple times within Aves and Dinosauria. <i>Mengciusornis</i> preserves a ventrally hooked scapular acromion that is convergently evolved in parallel with some enantiornithines and neornithines, providing further evidence of the homoplasy that characterizes avian evolution. http://zoobank.org/urn:lsid:zoobank.org:pub:311B34B9-1CBF-47A6-B821-650C04560137