Beu: Evolution of Janthina and Recluzia 
179 
Remarks. Janthina chavani resembles J. typica closely, 
but can be distinguished by three characters: (1) the spiral 
folds are weaker, particularly over the sutural ramp, than 
on J. typica. The two uppermost spiral cords, at least, are 
suppressed, so that the ramp is smooth (apart from the 
fine axial ridgelets) in almost all specimens. Many Roe 
Calcarenite specimens have very weak, almost uncountable 
spiral folds, and resemble J. janthina quite closely (Figs 
27A-I, 28C, F, J). Spiral cords are not visible at all on 
spire whorls of most specimens, and only around the outer 
edges of the spire whorls of others. (2) The fine axial 
ridgelets tend to fade out after the spire whorls and, on 
many specimens, particularly the (slightly abraded?) Roe 
Calcarenite population, the axial ridgelets are very weak on 
or absent from the last whorl. (3) Teleoconch spire height 
exhibits a much greater range of variation than in any other 
Janthina species, tending to produce an allometrical change 
with growth from very low-spired juvenile specimens to 
tall-spired adults. Obviously, this third character is visible 
only in large collections, and it is doubtful whether it would 
have been recognized without observing the large number 
of beautifully preserved specimens from Roe Calcarenite in 
southeastern Western Australia. Once it became evident in 
Roe Calcarenite collections, the great shape variability of 
other populations in New Zealand, southern Australia and 
Japan became more comprehensible. Many specimens have 
an obviously convex (cyrtoconoid) spire outline, resulting 
from the change in shape during growth, but others have 
straight outlines. Still others have an unusually rapid whorl 
translation, so they accommodate the change in shape with 
a stepped spire, each succeeding whorl descending below 
the periphery of the preceding one. 
Shape differences are shown in Fig. 29, a scatter diagram 
comparing height with width in Janthina typica, J. chavani 
and J. krejcii sp. nov. The field occupied by J. typica in Fig. 
29 is aligned more nearly along the x = y diagonal axis than 
that of J. chavani. This demonstrates that on average, height is 
almost equal to diameter in J. typica , and there is little change 
in shape with growth, although most small specimens have 
shorter spires than large specimens; height increases regularly 
throughout growth. The field of J. krejcii sp. nov. lies well 
below but parallel to the x = y axis, confirming the very low, 
wide shape of this species. All specimens are wider than they 
are high, and again there is no obvious change of shape with 
growth, although only five specimens were complete enough 
to include in this diagram. However, the field occupied by J. 
chavani is aligned up a steeper axis than x = y. On average, 
small (juvenile) specimens are wider than they are high, large 
adults are markedly taller than they are wide, and there is a 
weak allometrical increase in height as the shell grows in J. 
chavani. Although the allometry is weak, 22 of the 50 plotted 
specimens of J. chavani (44%) are taller than all 29 specimens 
of J. typica plotted in Fig. 29, confirming the visual estimation 
of shape and size differences. 
The sinus in the outer lip in Janthina chavani is basal, as 
in J. typica , but while in many specimens it is semicircular 
(Figs 28C, E), others have a wider, narrowly V-shaped sinus, 
and in still others it is intermediate in shape (Ludbrook, 
1978: pi. 12, figs 3, 5-6,10). Specimens of J. chavani reach 
48 mm in height and 40 mm in diameter, with 8-11 spiral 
folds on the last whorl. Most specimens have 9 or 10 folds, 
as in J. typica. Presumably the allometrical increase in spire 
height in J. chavani produces a taller sutural ramp in adults 
that allows almost the same number of spiral folds to be 
present in both species, bordered above by an unfolded area 
in J. chavani that is absent from J. typica. The spiral folds 
of J. chavani also possibly are slightly narrower than those 
of J. typica , although any difference is not obvious. A high 
proportion of Roe Calcarenite specimens is conspicuously 
large, robust, thick-shelled, weakly sculptured and tall-spired 
for a fossil Janthina species, but this is presumably partly 
because of the large population available to select from. 
The weakly consolidated nature of the formation allows the 
excellent preservation and easy collection of fragile shells. 
Ludbrook (1978: pi. 12, figs 13-14) illustrated a specimen 
with all sculpture abraded off the earliest 1.5 teleoconch 
whorls, which are weakly inflated in this specimen, and 
labelled it as “showing smooth protoconch”, but this is 
misidentified and does not resemble the very small, tall, 
pupiform, planktotrophic Epitonium protoconch of all living 
Janthina species. It is quite similar in appearance to the 
smooth apex of the holotype of Ennaticina abyssalis (Fig. 
25G), although the outer layer is present and the smoothness 
results from surface abrasion rather than corrosion in this 
case. The protoconch of J. chavani has not been observed. 
The younger Japanese specimens are referred to planktonic 
foraminiferal zones N21 and N22 and were identified by 
Japanese authors as Janthina (or Hartungia ) japonica. 
They are relatively small and, consequently, have low spires 
(Tomida & Itoigawa, 1982: pi. 19, figs la-c, holotype of 
Parajanthina japonica ; Tomida & Itoigawa, 1984: pi. 31, 
figs la-2b; 1989: pi. 23, figs la-2d; Noda et al., 1995: figs 
11,7a-d; Tomida & Kitao, 2002: figs 2. la-2c; Tomida et al., 
2013: figs 3E-L). However, they agree with J. chavani in 
having weaker spiral sculpture than older specimens, they 
closely resemble most New Zealand specimens identified 
as J. chavani, and they overlap with the range of variation 
in spire height of New Zealand and southern Australian 
specimens. The apparent difference in spire height results 
from the scarcity of large adults in Japanese samples, and 
their abundance in Roe Calcarenite collections. Like New 
Zealand specimens, many Japanese ones also have been 
distorted and crushed to varying extents by compaction. In 
the writer’s estimation, these youngest Japanese specimens 
fall within the range of variation of J. chavani. 
The overall impression of the characters and range of 
variation of Janthina chavani is that this species is closely 
similar to J. janthina as well as to J. typica. It is feasible that 
it was the immediate ancestor of J. janthina. This closely 
similar appearance provides the main evidence that fossils 
allow for the evolutionary history of the entire group, and so 
must be given strong weight when evaluating the phytogeny 
of Janthina. The very weak spiral folds on some specimens 
of J. chavani from Roe Calcarenite are only slightly more 
obvious than the faint spiral folds and grooves on the base 
of some Recent specimens of J. janthina, and the most 
fundamental differences between them are the narrow basal 
sinus in the outer lip of J. chavani and its wider shape and 
more nearly central apex in J. janthina, the weakly trochiform 
shape of J. janthina compared with an evenly convex shape 
(although with a flattened sutural ramp) in J. chavani, and the 
presence of axial ridgelets over most of the spire whorls in J. 
chavani, whereas they are limited to the first c. 1-1.5 spire 
whorls in J. janthina. Weakening of the axial ridgelets and 
spiral folds and flattening of the sutural ramp in J. chavani 
compared with their state in J. typica are interpreted as 
