The origin of the loop in articulate brachiopods
Williams Alwyn; Wright, A.D.
Palaeontology 4, Part 2: 149-176
ISSN/ISBN: 0031-0239 Accession: 020406865
"Since the earliest known loop, that of the spiriferoid Protozyga, was fully developed in sexually mature shells of that size, it probably appeared coenogenetically in a neotenous stock. The modal size of the earliest terebratuloid loops compares with that of the primary coils posterior of the jugum in contemporary spiriferoids, so that the terebratuloids could have been derived neotenously out of the spiriferoids. The loop of the pentameroid Enantiosphen is only superficially comparable with the centronellaceid one, but that found in the dalmanellaceid Tropidoleptus is a remarkable homeomorph of the terebratellaceid loop and probably differed only in the style of lophophore it supported.".