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Re: pelvs/sacrum question
Sacral ribs in
dinosaurs appear to fuse earlier in ontogeny than they do in other
am not sure the data has been published on the juvenile pterosaurs that
known, though I know Peters rejects these as juveniles, and thus may not
their use as benchmarks for ontogeny in pterosaurs.
Clades of tiny pterosaurs have a fused sacrum. Clades of large to giant
ornithocheirids do not. So, sacral fusion, for now at least (until
'mothers' and 'sons' are shown to be united) is a phylogenetic phenom,
not an ontogenetic one. I have no doubt that juveniles of fused sacrum
adults will have at least looser sacrums. Unfortunately in the one test
case, Pterodaustro, the sacrum is not found in the adult specimens I
know of. A sister taxon, Ctenochasma, gives no clue.
But getting back to mammals, perhaps the answer is in the size squeeze
that David M. wrote about, because Thrinaxodon and Probelesodon have the
full lineup of sacrals, but Morganucodon has the mammalian pattern in
which the posterior ones are disarticulated from the ilium. I was
wondering if frogs and their bounding behaviour can offer any sort of
analogy. Or perhaps if egg passage might have loosened those last few to
some benefit that was carried on.