LENGTH OF DORSAL-FIN SPINES.—Eospinus has exceptionally
long and robust dorsal spines, the first being about 73% SL
and the second 63% SL. The plesiomorphic condition for
tetraodontiforms is a moderate dorsal-spine length of 17%-
43% SL as found in the Triacanthoidei (for specimens over 60
mm SL; smaller specimens with somewhat longer spines,
Tyler, 1968). In the only group of Tetraodontoidea with a
relatively well-developed spiny dorsal fin, the primitive Eocene
Eoplectus (Tyler, 1973b), the spines are incompletely preserved,
but from the shape and size of the parts that remain the
spines were obviously of only moderate length and probably
somewhat shorter than in triacanthoids. In the Balistidae the
longest (first) dorsal spine is plesiomorphically moderate in
length, usually between 20% and 30% SL, and always less than
40% SL. In the Spinacanthidae the dorsal spines are exceptionally
long and strong in both of the monotypic genera, 54% and
92% SL in Protobalistum imperiale (Massalongo) and
Spinacanthus cuneiformis (de Blainville), respectively. The
greatly increased length of the dorsal spines is a synapomorphy of the two genera of Spinacanthidae, as is their origin far
forward on the head over the small and highly placed eye.
A hypothesis of moderate dorsal-spine length as found in
triacanthoids and eoplectids being ancestral for balistoids and
increased length acquired by spinacanthids and Eospinus (one
step if spinacanthids and Eospinus are sister groups or two
steps if balistids and Eospinus are sister groups) is more
parsimonious than increased length being ancestral for balistoids
and there having been reduction in the length of the spine
in balistoids and ostraciids (three steps). Within the balistoids,
however, it is equally parsimonious to assume that the
spinacanthid-balistid ancestor had increased spine length that
was reversed in balistids or that this ancestor had moderate
length and that spinacanthids and Eospinus independently
acquired increased length if, as evidenced by the number of
dorsal spines, Eospinus and balistids are sister groups (two
steps in both scenarios).
LENGTH OF DORSAL-FIN SPINES.—Eospinus has exceptionallylong and robust dorsal spines, the first being about 73% SLand the second 63% SL. The plesiomorphic condition fortetraodontiforms is a moderate dorsal-spine length of 17%-43% SL as found in the Triacanthoidei (for specimens over 60mm SL; smaller specimens with somewhat longer spines,Tyler, 1968). In the only group of Tetraodontoidea with arelatively well-developed spiny dorsal fin, the primitive EoceneEoplectus (Tyler, 1973b), the spines are incompletely preserved,but from the shape and size of the parts that remain thespines were obviously of only moderate length and probablysomewhat shorter than in triacanthoids. In the Balistidae thelongest (first) dorsal spine is plesiomorphically moderate inlength, usually between 20% and 30% SL, and always less than40% SL. In the Spinacanthidae the dorsal spines are exceptionallylong and strong in both of the monotypic genera, 54% and92% SL in Protobalistum imperiale (Massalongo) andSpinacanthus cuneiformis (de Blainville), respectively. Thegreatly increased length of the dorsal spines is a synapomorphy of the two genera of Spinacanthidae, as is their origin farforward on the head over the small and highly placed eye.A hypothesis of moderate dorsal-spine length as found intriacanthoids and eoplectids being ancestral for balistoids andincreased length acquired by spinacanthids and Eospinus (onestep if spinacanthids and Eospinus are sister groups or two
steps if balistids and Eospinus are sister groups) is more
parsimonious than increased length being ancestral for balistoids
and there having been reduction in the length of the spine
in balistoids and ostraciids (three steps). Within the balistoids,
however, it is equally parsimonious to assume that the
spinacanthid-balistid ancestor had increased spine length that
was reversed in balistids or that this ancestor had moderate
length and that spinacanthids and Eospinus independently
acquired increased length if, as evidenced by the number of
dorsal spines, Eospinus and balistids are sister groups (two
steps in both scenarios).
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