3. Origin of Euteleostean Biodiversity
45
3.4 Divergence Date Estimates Among Teleosts
The calibrated rate was used to estimate divergence times among teleost orders and
among perciform families (Table 3). Estimated divergence times among five teleost orders dated back to the Paleozoic (mostly Permian). A somewhat broad range
for the possible rates (the dark-shaded area in Fig. 3C) may reduce accuracy of the
estimation, and we are aware that the sequence data should be further expanded in
the numbers of both sites and taxa in order to obtain very accurate phylogeny and
divergence times. We thus consider the time estimates in Table 3 to be tentative
approximation. However, it seems obvious that the range of estimated times is considerably older than the estimates deducible from fossil records. Fossils for these
euteleost orders are not known before the Cretaceous (Benton 1993), whereas the
molecular time estimates for the interordinal divergences even predate the first
occurrence record of Teleostei in the early Late Triassic.
Estimated divergence times among seven perciform families were broadly distributed in the Mesozoic (mostly from the Jurassic to the early Cretaceous; Table
3). They are also considerably older than the fossil-based estimates which supported the notion of rapid radiation of various perciform families after the KiT
boundary (Benton 1993). Although many of the 148 perciform families were not
represented in the present study, four out of six existing families in the suborder
Labroidei were sampled and all the divergence dates among these families predated
that between African and South American cichlids (see Table 3). The apparent
discrepancy between molecular and fossil-based estimates becomes most conspicuous by the fact that no fossils clearly assignable to acanthomorphs (an advanced
euteleost group including the Perciforms) are known before the Cenomanian stage
(90-97 MYA) (Patterson 1993). Taken together, there is a considerable gap between divergence-date estimates for teleosts from molecular evidence and those
based on the first occurrence evidence of fossil records.
4 Discussion
4.1 Intercontinental Distribution of Cichlids
Some previous researchers (see, e.g., Banarescu 1990; Stiassny 1991) considered
that cichlids inhabited Gondwanaland and were separated into African and
neotropicallineages upon its breakup (the vicariance model in Fig. 4A). However,
fossil records of the Cichlidae are known only from the Cenozoic (Oligocene, Benton
1993) while the breakup of Gondwanaland occurred much earlier in the early or
middle Cretaceous. This gap led other researchers (see, e.g., Lundberg 1993; Briggs
1995) to consider an alternative explanation that euryhaline cichlids could disperse
across the paleo-Atlantic ocean over a modest saltwater gap during the late Cretaceous or possibly the early Cenozoic because some (but a very limited number of)
extant cichlid species are adapted to brackish water (Nelson 1994). Maisey (1993)
referred to possible tectonic settings for the long-distance dispersal.
45
3.4 Divergence Date Estimates Among Teleosts
The calibrated rate was used to estimate divergence times among teleost orders and
among perciform families (Table 3). Estimated divergence times among five teleost orders dated back to the Paleozoic (mostly Permian). A somewhat broad range
for the possible rates (the dark-shaded area in Fig. 3C) may reduce accuracy of the
estimation, and we are aware that the sequence data should be further expanded in
the numbers of both sites and taxa in order to obtain very accurate phylogeny and
divergence times. We thus consider the time estimates in Table 3 to be tentative
approximation. However, it seems obvious that the range of estimated times is considerably older than the estimates deducible from fossil records. Fossils for these
euteleost orders are not known before the Cretaceous (Benton 1993), whereas the
molecular time estimates for the interordinal divergences even predate the first
occurrence record of Teleostei in the early Late Triassic.
Estimated divergence times among seven perciform families were broadly distributed in the Mesozoic (mostly from the Jurassic to the early Cretaceous; Table
3). They are also considerably older than the fossil-based estimates which supported the notion of rapid radiation of various perciform families after the KiT
boundary (Benton 1993). Although many of the 148 perciform families were not
represented in the present study, four out of six existing families in the suborder
Labroidei were sampled and all the divergence dates among these families predated
that between African and South American cichlids (see Table 3). The apparent
discrepancy between molecular and fossil-based estimates becomes most conspicuous by the fact that no fossils clearly assignable to acanthomorphs (an advanced
euteleost group including the Perciforms) are known before the Cenomanian stage
(90-97 MYA) (Patterson 1993). Taken together, there is a considerable gap between divergence-date estimates for teleosts from molecular evidence and those
based on the first occurrence evidence of fossil records.
4 Discussion
4.1 Intercontinental Distribution of Cichlids
Some previous researchers (see, e.g., Banarescu 1990; Stiassny 1991) considered
that cichlids inhabited Gondwanaland and were separated into African and
neotropicallineages upon its breakup (the vicariance model in Fig. 4A). However,
fossil records of the Cichlidae are known only from the Cenozoic (Oligocene, Benton
1993) while the breakup of Gondwanaland occurred much earlier in the early or
middle Cretaceous. This gap led other researchers (see, e.g., Lundberg 1993; Briggs
1995) to consider an alternative explanation that euryhaline cichlids could disperse
across the paleo-Atlantic ocean over a modest saltwater gap during the late Cretaceous or possibly the early Cenozoic because some (but a very limited number of)
extant cichlid species are adapted to brackish water (Nelson 1994). Maisey (1993)
referred to possible tectonic settings for the long-distance dispersal.