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Re: Segregated vs age-mixed sauropod herds

 "Prototheria" is the name for it (or "pan-Monotremata").

Prototheria has AFAIK never been defined, and hasn't been used at all since phylogenetic nomenclature (and, basically, cladistics) got going in the field Mesozoic mammal phylogeny.

 Isn't Mammalia (crown group) usually thought to have originated in
 the Triassic?

No, the oldest fossils (such as the stem-australosphenidans *Asfaltomylos* and *Ambondro*) are Middle Jurassic in age. All Early Jurassic and Triassic synapsids -- *Sinoconodon*, *Morganucodon*, *Megazostrodon*, *Hadrocodium*, to mention the most famous ones -- are on the stem. In terms of continuity with the paleontological literature, the crown-group definition for Mammalia (1988) was a considerably worse idea than the *Sinoconodon*-+-crown-group definition which was published in 2002 or 2004.