A problem with the word "dinosaur"

Take note of the following problem:

http://www.ucmp.berkeley.edu/diapsids/dinomm.html

One important dinosaurian synapomorphy is the perforate [completely open] acetabulum, simply a "hip bone" (actually three connected bones, together called the pelvis) with a hole in the center where the head of the femur ("thigh bone") sits. This construction of the hip joint makes an erect stance (hindlimbs located directly beneath the body) necessary — like most mammals, but unlike other reptiles which have a less erect and more sprawling posture. Dinosaurs are unique among all tetrapods in having this perforate [completely open] acetabulum.

Notice that I have added the words "completely open" within square brackets.  I add that because as the quote says, it is:
"actually three connected bones, together called the pelvis with a hole in the center where the head of the femur ("thigh bone") sits". 

In other words, it is completely open. 

But birds do not have a completely open acetabulum. So it is misleading to say they both have a "perforate" acetabulum.

One needs to distinguish between actual dinosaurs (eg. tyrannosaurids) on the one hand and flying (and secondarily flightless) feathered Paraves with long bony tail (eg. long tailed Scansoriopterygids) on the other. 

Cup-like Acetabulum

Basal paraves had a partially closed (cup-like) acetabulum that allowed them to abduct (splay, sprawl) their legs.
Pterosaurs had a completely closed (cup-like) acetabulum that allowed them to abduct (splay, sprawl) their legs.
Dinosaurs had a completely open acetabulum that did not allow them to abduct (splay, sprawl) their legs.

Dinosaur to bird theorists focus on whether the acetabulum had a hole in it or not. From that point of view birds and dinosaurs were somewhat similar in that they both had a hole in the acetabulum. (Although one had a partially closed acetabulum, the other a completely open acetabulum).

But that overlooks the important issue which is whether the acetabulum was cup-like or not. From that point of view dinosaurs are not like birds. While pterosaurs in that respect are like birds. 

This is significant because dinosaurs could not splay their legs, but basal paraves and pterosaurs could

For more details see:

https://pterosaurnet.blogspot.com/2014/10/pelvic-girdle.html

Ambopteryx

https://www.nature.com/articles/s41586-019-1137-z (2019)

Yi qi, which has membranous wings—a flight apparatus that was previously unknown among theropods but that is used by both the pterosaur and bat lineages6. This observation was not universally accepted7. Here we describe a newly identified scansoriopterygid —which we name Ambopteryx longibrachium, gen. et sp. nov.—from the Upper Jurassic period. This specimen provides support for the widespread existence of membranous wings and the styliform element in the Scansoriopterygidae

The scansoriopterygids (including Ambopteryx) are members of basal Paraves. With their wing skin membrane and longest outermost finger, they are a candidate transitional between pterosaur and later Paraves. They fit right into the pterosaur to bird theory.

On the other hand, there is zero evidence of membrane wings in any claimed dinosaur ancestor. And also, scansoriopterygids have the outermost digit as the longest, which is not found in any claimed dinosaur ancestor. 

For details about Yi qi see here:
http://pterosaurnet.blogspot.com/2015/05/another-scansoriopterygid.html

For details about Scansoriopteryx see here:
https://pterosaurnet.blogspot.com/2014/07/scansoriopteryx.html

https://m.phys.org/news/2019-05-jurassic-non-avian-theropod-dinosaur-flight.html

Convergence

It is sometimes said that coelurosaur dinosaurs and the ancestors of pterosaurs convergently evolved characteristics related to flying.
It is said that pterosaur characteristics related to flying were analogous (convergent) to that of birds, rather than homologous (ancestral).

Here is an example:
https://en.wikipedia.org/wiki/Pterosaur

A 2009 study showed that pterosaurs had a lung-air sac system and a precisely controlled skeletal breathing pump, which supports a flow-through pulmonary ventilation model in pterosaurs, analogous to that of birds.

But the parsimonious conclusion is that basal Paraves evolved FROM pterosaurs.