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My daughter, while reading a book about dinosaurs, asked me if prosauropods are ancestors of all sauropods. Are they? From the name ("pro-sauropods"), I guess that they are, but am I right?

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Welcome to Biology.SE! Your daughter asks interesting questions!

The question is debated. It is currently mostly thought that sauropods and pro-sauropods are sister lineages and therefore, one does not contain the other. In other words, the pro-sauropods are no more the ancestors of the sauropods than were the mammals are the dogs are the ancestors to the cats.

One of the most updated source of information for phylogenetic questions of the kind is tolweb.org. You should have a look. Here is the page for the Sauropodomorpha (which contains the sauropods and the pro-sauropods). You have a nice list of reference on this page including some from Sereno, who is the author who first defined the pro-sauropods.

I quickly sought through the literature. It seems indeed that all phylogenetic analysis were based on phenotypic comparisons and not on genetic comparisons. Citing from wikipedia:

Evidence against sauropod ancestry within Prosauropoda comes from the fact that prosauropods had a smaller outer toe on their hind feet than the sauropods. Many maintain that it is easier for digits to be reduced or lost during evolution than the reverse, however there is no evidence for this. The lengthening, or gaining of extra digits is common in marine reptiles, and within the theropods digit lengthening occurred at least once. Therefore, using this as evidence against ancestral prosauropods is questionable.

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  • $\begingroup$ To be fair, genetic analysis of dinosaurs would be very very hard, phenotypic comparisons are probably the best we can ever do. $\endgroup$ – user137 Apr 17 '15 at 14:36

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