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Records of the Australian Museum (2017) Vol. 69 
Age 
m.yr. 
Janthina 
phylogeny 
7 J---- 
Figure 23. Suggested phylogeny of Janthina species. Bold lines = recorded time ranges; thin lines = suggested phylogeny. Evolutionary 
changes at nodes: (1) loss of spiral folds; retention of low-spired juvenile shape in adult, apparently heterochrony; (2) loss of axial riblets; 
increase in spire height; sinus migrated up outer lip; (3) retention of axial riblets; decrease in size; increase in spire height; sinus migrated 
up outer lip; (4) weakening of spiral folds; heightening and flattening of sutural ramp; increase in spire height allometry; (5) loss of spiral 
folds; adoption of trochiform shape; sinus migrated up outer lip; adoption of brooding?; (6) expansion of aperture into evenly oval, flared 
lip; shallowing of outer lip sinus; (7) greatly decreased prominence of and spacing between axial riblets (drawings not to scale; uppermost 
five drawings from Laursen, 1953: figs 14, 22, 26, 30, 36). 
it should be noted that similar spiral grooves also occur in 
some specimens of J. globosa (Fig. 2G). The sutural ramp 
of J. chavani also is taller and less strongly convex than 
in J. typica, i.e., with a straighter outline, possibly to be 
interpreted as the beginning of development of the trochiform 
shape characteristic of J. janthina. These similarities 
indicate that the most likely ancestor of J. janthina is J. 
chavani. The evolution of J. janthina would have involved 
further weakening of the spiral folds, reduction of the axial 
ridgelets to only the earliest 1-1.5 spire whorls, widening 
of the outer lip sinus and progression of its apex higher up 
the lip, further weakening of the convexity of upper and 
lower whorl surfaces to generate the trochiform shape, and 
some reduction of the variability in spire height and its 
allometric increase with growth. Presumably it also involved 
the evolution of egg brooding, although of course it is not 
possible to know when brooding evolved. 
The second key phylogenetic indicator is the retention 
of prominent axial sculpture by the Janthina exigua-J. 
umbilicata species pair and its loss from the J. globosa- 
J. pallida species pair. The presence of prominent axial 
sculpture—essentially fine, closely spaced Epitonium 
sculpture—over the entire teleoconch exterior indicates that 
the J. exigua-J. umbilicata species pair originated from one 
of the extinct Janthina species before the genus lost axial 
sculpture, but after it lost prominent spiral folds. Identical 
axial sculpture is unlikely to have evolved again after it was 
lost. Janthina exigua apparently originated during Zanclean- 
early Gelasian time, presumably from J. krejcii , the one fossil 
species with axial ridgelets covering the exterior, but with 
no spiral folds. It is assumed to have been the parent species 
in this pair because its coarse axial sculpture is identical to 
that of J. typica and J. krejcii , and it seems more likely that 
the finer sculpture of J. umbilicata was derived from that 
of J. exigua than the other way around. The main character 
changes in the evolution of J. exigua would have been a 
reduction in size, a very marked increase in spire height, 
and the development of a much wider, deeper sinus with its 
apex in the centre of the outer lip. 
The Janthina globosa-J. pallida species pair also lacks 
spiral folds, but differs further from all earlier Janthina 
species in also lacking axial ridgelets. As J. globosa had 
