How can we be sure that fossil amniotes, such as the captorhinomorphs, laid amniotic eggs? How can we be sure that at least most of the non-amniote reptiliomorphs (below the level of diadectomorphs) did not?
What will be an ideal response?
Captorhinomorphs are within the extant phylogenetic bracket of modern amniotes; that is, if synapsids laid amniotic eggs (as do monotremes today) and sauropsids laid amniotic eggs (as do most members of all extant groups), then any extinct group in this phylogenetic bracket must have done so, too.
Some seymouriamorph larvae have been found, with evidence of lateral line canals. If these reptiliomorphs had larval forms, they could not have yet evolved the amniotic egg, in which the larval form is bypassed and the lateral line is lost completely.
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