Evolution of the Tribe Electronini (Myctophifonnes, Myctophidae)
83
migrating fishes, or to provide a refuge in the austral winter. As has been
shown above, the distribution of modem representatives of these species
(e.g. E. antarctica) appears to be closely correlated with the NADW. The
onset of Northern Hemisphere ice-sheet formation in the Late Pliocene
has been related to intensification of the Gulf Stream system due to the
final closure of the Middle American Seaway [19]. The effect was to
increase precipitation in high northern latitudes. When coupled with
increased glaciation, this would have resulted in an intensification of the
thermo-haline circulation and the increased production ofNADW.
Two data sets lend support to the model. Firstly, fossil otoliths of P. (H.)
thompsoni have been found only in Late Pliocene and Pleistocene deposits
of California. The species is absent from the Miocene deposits [20]. This
is in accordance with Stage 3, which postulates a northern dispersion and
speciation event dating from after the Terminal Miocene Event. Secondly,
Schwarzhans [10,21] has reported fossil otoliths of E. risso from the
Lower Pliocene of Europe, and Fitch [20] has recorded the species from
the Pleistocene of California. These data again concur with an Early
Pliocene distribution for the species as proposed in Stage 3.
Finally, the model predicts that in the subgenus Gymnoscopelus, the
only Antarctic Pattern species (G. opisthopterus) should be the most
derived species, while the two Broadly Antarctic species (G. braueri, G.
nicholsi) should represent the modem derivatives of the most ancestral
species. The testing of these predications must await further investigation.
Unfortunately, Gymnoscopelus has no fossil record because G. fitchi from
the Lower Pliocene of Italy is now considered to belong to the genus
Myctophum [10]. However, the otoliths of extant species of
Gymnoscopelus closely resemble those of Lampanyctodes [10]. On
osteological grounds, Paxton [22] considered the latter to be the most
primitive genus of the Tribe Gymnoscopelini. The monotypic L. hectoris
is a pseudoceanic species found only on the continental shelves and slopes
of Chile, South Africa and southern Australia [6,17]. The suggestion
therefore is that while recent members of Electronini may have arisen
from a pelagic ancestral form living in high southern latitudes, recent
species of the genus Gymnoscopelus may represent derivatives of a high
southern latitude epibenthic ancestor subjected to the same vicariance
events.
Acknowledgments
I am indebted to the captains and crew of the "Walther Herwig" and "Polarstern";
to my ship-board colleagues; and to Dr. B. Cook (South African Museum, Cape
83
migrating fishes, or to provide a refuge in the austral winter. As has been
shown above, the distribution of modem representatives of these species
(e.g. E. antarctica) appears to be closely correlated with the NADW. The
onset of Northern Hemisphere ice-sheet formation in the Late Pliocene
has been related to intensification of the Gulf Stream system due to the
final closure of the Middle American Seaway [19]. The effect was to
increase precipitation in high northern latitudes. When coupled with
increased glaciation, this would have resulted in an intensification of the
thermo-haline circulation and the increased production ofNADW.
Two data sets lend support to the model. Firstly, fossil otoliths of P. (H.)
thompsoni have been found only in Late Pliocene and Pleistocene deposits
of California. The species is absent from the Miocene deposits [20]. This
is in accordance with Stage 3, which postulates a northern dispersion and
speciation event dating from after the Terminal Miocene Event. Secondly,
Schwarzhans [10,21] has reported fossil otoliths of E. risso from the
Lower Pliocene of Europe, and Fitch [20] has recorded the species from
the Pleistocene of California. These data again concur with an Early
Pliocene distribution for the species as proposed in Stage 3.
Finally, the model predicts that in the subgenus Gymnoscopelus, the
only Antarctic Pattern species (G. opisthopterus) should be the most
derived species, while the two Broadly Antarctic species (G. braueri, G.
nicholsi) should represent the modem derivatives of the most ancestral
species. The testing of these predications must await further investigation.
Unfortunately, Gymnoscopelus has no fossil record because G. fitchi from
the Lower Pliocene of Italy is now considered to belong to the genus
Myctophum [10]. However, the otoliths of extant species of
Gymnoscopelus closely resemble those of Lampanyctodes [10]. On
osteological grounds, Paxton [22] considered the latter to be the most
primitive genus of the Tribe Gymnoscopelini. The monotypic L. hectoris
is a pseudoceanic species found only on the continental shelves and slopes
of Chile, South Africa and southern Australia [6,17]. The suggestion
therefore is that while recent members of Electronini may have arisen
from a pelagic ancestral form living in high southern latitudes, recent
species of the genus Gymnoscopelus may represent derivatives of a high
southern latitude epibenthic ancestor subjected to the same vicariance
events.
Acknowledgments
I am indebted to the captains and crew of the "Walther Herwig" and "Polarstern";
to my ship-board colleagues; and to Dr. B. Cook (South African Museum, Cape