48
Y. Kumazawa et al.
seems to lack positively supportive evidence. At present we have little evidence
with which to deduce what the labroid ancestor(s) of cichlids were. Phylogenetic
relationships among labroid families should be investigated further by using more
taxa from each family.
Although the molecular data collected in this study do not specify one of the
three models, they can constrain conditions under which the models hold. Figure
3C suggested that the dispersal or origination of cichlids could not have happened
in the early Cenozoic. The gap in the fossil record may not become strong evidence
of a Cenozoic dispersal, because the incompleteness of fossil records for long periods is now likely for perciforms in general (Table 3).
4.2 Apparent Discrepancies Between Molecular and
Fossil Chronologies
Macroevolutionary history of organisms through time has been traditionally reconstructed on the basis of fossil records. In principle, the divergence time for two
lineages can be deduced from the date of the earliest fossil records clearly attributable to either of the two lineages (Marshall 1990; Benton 1993). However, in general, fossil evidence is incomplete in various aspects. It may be lacking or fragmentary for some taxa with a small population size or with less preservable components, biased by various environmental factors for fossilization, or obscured by the
arbitrariness of researchers' morphological interpretations (Marshall 1990; Carroll
1997; Benton 1998). It thus seems significant to evaluate the fossil-based history
using independent approaches such as molecular evolutionary methods.
The present study shows the possibility of a long-term lack of fossil records for
not only cichlids but also many other families of the Perciformes. If this is really the
case, how can it have happened? We provide two explanations that may stand simultaneously. First, fossils of fishes are not necessarily considered to be preserved
very well. A number of perciform families are completely lacking in their fossil
record and many of the first occurrences for the other families having recognizable
fossil records are based only on otoliths (Benton 1993; Patterson 1993). Moreover,
due to the less preservable nature of fish fossils in general, interpretation of occurrence and disappearance of fish lineages tends to be influenced by a limited number
of fossil localities of exceptional preservation (the Lagerstatten effect; see, e.g.,
Goody 1969; Patterson 1993; Benton 1998). These problems may cause strong bias
in inferring evolutionary history of fishes, leading to considerable underestimation
of origination or divergence dates, and apparently rapid cladogenesis or mass extinction (Benton 1998).
Second, fossil records in general tend to be influenced by population size of the
corresponding species and various taphonomical conditions for fossilization (Carroll
1997; Benton 1998). It may therefore be possible to deduce that perciforms had a
much smaller population size in the Mesozoic than in the Cenozoic and/or they
occupied much more confined ecological niches than today. It is noteworthy that
perciforms are not recognized even in the Santana Formation that produces ample
Y. Kumazawa et al.
seems to lack positively supportive evidence. At present we have little evidence
with which to deduce what the labroid ancestor(s) of cichlids were. Phylogenetic
relationships among labroid families should be investigated further by using more
taxa from each family.
Although the molecular data collected in this study do not specify one of the
three models, they can constrain conditions under which the models hold. Figure
3C suggested that the dispersal or origination of cichlids could not have happened
in the early Cenozoic. The gap in the fossil record may not become strong evidence
of a Cenozoic dispersal, because the incompleteness of fossil records for long periods is now likely for perciforms in general (Table 3).
4.2 Apparent Discrepancies Between Molecular and
Fossil Chronologies
Macroevolutionary history of organisms through time has been traditionally reconstructed on the basis of fossil records. In principle, the divergence time for two
lineages can be deduced from the date of the earliest fossil records clearly attributable to either of the two lineages (Marshall 1990; Benton 1993). However, in general, fossil evidence is incomplete in various aspects. It may be lacking or fragmentary for some taxa with a small population size or with less preservable components, biased by various environmental factors for fossilization, or obscured by the
arbitrariness of researchers' morphological interpretations (Marshall 1990; Carroll
1997; Benton 1998). It thus seems significant to evaluate the fossil-based history
using independent approaches such as molecular evolutionary methods.
The present study shows the possibility of a long-term lack of fossil records for
not only cichlids but also many other families of the Perciformes. If this is really the
case, how can it have happened? We provide two explanations that may stand simultaneously. First, fossils of fishes are not necessarily considered to be preserved
very well. A number of perciform families are completely lacking in their fossil
record and many of the first occurrences for the other families having recognizable
fossil records are based only on otoliths (Benton 1993; Patterson 1993). Moreover,
due to the less preservable nature of fish fossils in general, interpretation of occurrence and disappearance of fish lineages tends to be influenced by a limited number
of fossil localities of exceptional preservation (the Lagerstatten effect; see, e.g.,
Goody 1969; Patterson 1993; Benton 1998). These problems may cause strong bias
in inferring evolutionary history of fishes, leading to considerable underestimation
of origination or divergence dates, and apparently rapid cladogenesis or mass extinction (Benton 1998).
Second, fossil records in general tend to be influenced by population size of the
corresponding species and various taphonomical conditions for fossilization (Carroll
1997; Benton 1998). It may therefore be possible to deduce that perciforms had a
much smaller population size in the Mesozoic than in the Cenozoic and/or they
occupied much more confined ecological niches than today. It is noteworthy that
perciforms are not recognized even in the Santana Formation that produces ample