Without knowing the condition of the pelvic fin in spinacanthids we cannot assess the implications of the
placement of the rudimentary pelvic fin at the posterior end of
the pelvis just in front of the anus in Eospinus and balistids,
versus thoracically placed in all other tetraodontiforms with a
pelvic fin (triacanthoids and eoplectids) and in the extraordinal
outgroups. If one presumes that the loss of the pelvic fin in
ostraciids took place after its migration posteriorly to the end of
the pelvis, as was probably the comparable situation with the
posterior migration of the spiny dorsal fin associated with its
loss, then the position of the pelvic-fin rudiment in Eospinus
and balistids is plesiomorphic for balistoids.
We note that if Eospinus had encasing scales around the
rudimentary pelvic spine we would accept this as important
evidence linking it with the balistids (just as we would if
Eospinus had a locking mechanism between the first and
second dorsal spines). We believe it unlikely that the loss of the
pelvic fin in ostraciids, even if it involved migration to the
posterior end of the pelvis, proceeded through a stage in which
the rudimentary spine became encased in the specialized
series of flexible scale segments found in balistids. This
complex apparatus in balistids serves variously to wedge the
species into crevices and to flare a dewlap of skin to increase
apparent body size in a presumably protective-defensive
behavior, and the development of this apparatus has apparently
been a key factor in the diversification of the group. We doubt
that ostraciids also developed this complex pelvic apparatus
only to subsequently lose it before their probable pre-Eocene
origins (the ostraciin Eolactoria and the aracaniin Proaracana,
both without a pelvis or pelvic fin of any kind, being known
from the early Eocene). The balistid pelvic apparatus (and the
dorsal-spine locking mechanism) is fully developed in all of the
fossil balistids. Balistins are first recorded from the Oligocene
by the three species of Balistomorphus from Switzerland and
the monotypic Oligobalistes from Russia (Tyler, 1980), while
monacanthins are first known from the Pliocene by several
species of Aluterus and Cantherhines reported by Sorbini
(1987, 1991) from Italy.