It is clear that the position of the spiny dorsal fin was
anterior, toward the rear of the skull, in the ancestral balistoid.
One could hypothesize that the spines were lost in ostraciids
while still in this position and that only then did the basal
pterygiophores, having lost their original function of spine
support, migrate posteriorly to the origin of the soft dorsal fin
to become the supraneural. However, in triodontids, which are
the only tetraodontiforms with a rudimentary spiny dorsal fin,
the spiny dorsal fin and its pterygiophores have migrated
posteriorly to just in front of the soft dorsal fin. Moreover,
when the rudimentary spiny dorsal fin is absent (Indian Ocean
populations) in the only Recent species, Triodon macropterus
Lesson, the basal pterygiophores are entirely comparable to the
posteriorly placed supraneural of ostraciids. This analogous
positioning of the pterygial supports of the absent spiny dorsal
fin in triodontids and ostraciids is in conformity with
Winterbottom's (1974) contention of the sister group relationship
of spinacanthids and balistids, both of which have anterior
migration of the spiny dorsal fin, and the sister group
relationship of that clade with the ostraciids, which have
posterior migration of the pterygiophores and loss of the spines.
This scenario also is consistent with Rosen's (1984) hypothesis
of ostraciid relationships.
Eospinus lacks a mechanism by which the second dorsal
spine locks the first spine in an erect position. Such a complex
specialized locking mechanism is present among tetraodontiforms
(and all other fishes) only in the Balistidae (present in all
species except for a few highly specialized monacanthins that
have lost the second spine). If Eospinus is more closely related
to balistids than to other balistoids this would indicate that the
reduction in the number of dorsal spines to three in Eospinus
and balistids took place in that lineage prior to the development
of the locking mechanism in balistids alone, in which case
Eospinus would be the sister group of balistids.
It is clear that the position of the spiny dorsal fin wasanterior, toward the rear of the skull, in the ancestral balistoid.One could hypothesize that the spines were lost in ostraciidswhile still in this position and that only then did the basalpterygiophores, having lost their original function of spinesupport, migrate posteriorly to the origin of the soft dorsal finto become the supraneural. However, in triodontids, which arethe only tetraodontiforms with a rudimentary spiny dorsal fin,the spiny dorsal fin and its pterygiophores have migratedposteriorly to just in front of the soft dorsal fin. Moreover,when the rudimentary spiny dorsal fin is absent (Indian Oceanpopulations) in the only Recent species, Triodon macropterusLesson, the basal pterygiophores are entirely comparable to theposteriorly placed supraneural of ostraciids. This analogouspositioning of the pterygial supports of the absent spiny dorsalfin in triodontids and ostraciids is in conformity withWinterbottom's (1974) contention of the sister group relationshipof spinacanthids and balistids, both of which have anteriormigration of the spiny dorsal fin, and the sister grouprelationship of that clade with the ostraciids, which haveposterior migration of the pterygiophores and loss of the spines.This scenario also is consistent with Rosen's (1984) hypothesisof ostraciid relationships.Eospinus lacks a mechanism by which the second dorsalspine locks the first spine in an erect position. Such a complexspecialized locking mechanism is present among tetraodontiforms(and all other fishes) only in the Balistidae (present in allspecies except for a few highly specialized monacanthins thathave lost the second spine). If Eospinus is more closely relatedto balistids than to other balistoids this would indicate that thereduction in the number of dorsal spines to three in Eospinusand balistids took place in that lineage prior to the developmentof the locking mechanism in balistids alone, in which caseEospinus would be the sister group of balistids.
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