Grouping the dinosaurs

The group of dinosaurs which contains some 1000 species should be classified somehow. According to the systematics accepted since the end of the 1880s, dinosaurs are grouped based on their pelvis structure. The lizard-hipped dinosaurs separated from the bird-hipped during the late Triassic period. Their pelvis structure is three-pronged with the pubis pointing forward. A new pelvis structure developed for the bird-hipped dinosaurs. In their case, the pubis is parallel to the ischium, and it creates a four-pronged structure.

 According to a study in 2017, theropod dinosaurs, apart from their pelvis, share many common anatomical features with the bird-hipped dinosaurs. Based on this, the two groups should be merged into a new clade, with the name of Ornithoscelida.

Fig 1.
Lizard hip
Source: AdmiralHood, 2012

Fig 2.
Bird hip
Source: AdmiralHood, 2012

lizard hip: pelvis with three tongues, pubis pointing forward
bird hip: pelvis with four tongues, pubis and ischium parallel