Redefining what makes a dinosaur

Ran across this online article (citation below)
summarized: “The once-lengthy list of “definitely a dinosaur” features had already been dwindling over the past few decades thanks to new discoveries of close dino relatives such as Teleocrater. With an April 2017 report of Teleocrater’s skull depression (SN Online: 4/17/17), yet another feature was knocked off the list.”

Evidently the only trait that is still on the list is a perforated acetabulum.

My résponse:
long time readers will recognize this answer:

The dear departed Dr. Larry Martin used to play this game. He’d say ‘tell me a character you think defines a clade and I’ll give you an exception.’ In dinosaur pelves, ankylosaurs are the exception that do not have a perforated acetabulum. The lesson: You can’t define a clade by a single or a dozen character traits. 

You can define a clade using a cladogram. A cladogram uses hundreds of traits to recover relationships including “the last common ancestors and all of its descendants.” On that basis Dinosauria include Herrerasaurus at the base and the first dichotomy splits Theropoda from Phytodinosauria. The proximal outgroup is the Crocodylomorpha, basal members of which were small and bipedal, like early dinosaurs. That means Archosauria includes only dinos and crocs. Teleocrater is in the lineage of stem Archosauria. Unfortunately, prior workers excluded many relevant taxa, which is why they did not recover these relationships. Cladogram, links and more data here: 
The last few items on the dinosaur list:
  1. Until Teleocrater came along, only dinosaurs were known to have a deep depression at the top of the skull, an attachment site for some jaw muscles probably related to bite strength.
  2.  Dinosaurs and some other dinosauromorphs such as Silesaurus opolensis have an enlarged crest on the upper arm bone where muscles attached
  3. Along with dinosaurs, dinosauromorphs S. opolensis and Asilisaurus kongwe may have had epipophyses, bony projections at the back of the neck vertebrae.
  4. An extra (fourth) muscle attachment site, called a trochanter, at the point on the femur that meets the hip is also found in dinosauromorph Marasuchus lilloensis.

Sources: S.J. Nesbitt et al/Nature 2017; S.L. Brusatte et al/Earth-Science Reviews 2010

Taxon exclusion. Phylogenetic analysis. Yada-yada. 


Nesbitt et al. 2017 The earliest bird-line archosaurs and the assemblof the dinosaur body plan. Nature (Teleocrater paper).


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