3. Origin of Euteleostean Biodiversity
49
marine fish fossils of exceptional preservation for the Aptian-Albian Age (-11 0
MY A) (Maisey 1993), when perciforms may have already radiated according to the
molecular evidence presented in this study (Table 3).
4.3 Implications
The present study suggested that euteleosts have had a much longer history than
previously thought. This notion seems so different from the previous view on this
issue that it should be further evaluated in the future by collecting longer sequence
data for more perciform taxa and/or by using more calibration points for the molecular clock. Possible other candidates for the rate calibration may include
characiforms (Ortf 1997), cyprinodontiforms (Murphy and Collier 1997), and geminate species pairs that most likely speciated by the rise of the Panamanian Isthmus
(Bermingham et al. 1997). At present, the molecular evolutionary rates reported in
the latter two papers may not be directly comparable to ours due to, e.g., the differences in sequenced genes and/or order of time scale for the calibration.
It has been pondered why perciforms appear to have evolved various morphotypes
rapidly in the early Cenozoic, while they have been largely conservative since then
(Carroll 1988). Our results may partly explain this question by relaxing the time
period during which the original morphological changes could occur. According to
molecular evidence presented herein, radiation of perciforms may not have been
related to global changes in climate and ecosystems that led to the extinction of
most archaic actinopterygian groups that flourished throughout the Mesozoic. However, as discussed above, it is still possible that their population size was dramatically increased and/or that they occupicd various niches upon the global changes
after the KIT boundary The long-term lack of fossil records, as well as the apparent
uncoupling between lineage radiation of new groups and extinction of old ones
have also been implicated for terrestrial vertebrates such as mammals and birds by
some molecular studies (Janke et al. 1994; Hedges et al. 1996; Cooper and Penny
1997; Kumar and Hedges 1998).
Occurrence of a similar phenomenon for organisms living in different environments (on land and in the sea) may imply its Ubiquitousness for more diverse groups
of organisms that lived in different geological ages. Vertebrates are considered to
have relatively well-documented fossil records and there have been no substantial
changes in the accepted broad-scale pattern of vertebrate evolution since the 19th
century (Benton 1998). However, this pattern may not be unchangeable or, at least,
it may be worth questioning and reexamining with newer data sets from modern
methods such as molecular evolutionary ones.
Acknowledgments
We thank N. Takezaki for sending us an earlier version of her programs now contained in the Lintre package, and instructions on the use of these programs. We
also thank M. Miya and an anonymous reviewer for many valuable comments on
49
marine fish fossils of exceptional preservation for the Aptian-Albian Age (-11 0
MY A) (Maisey 1993), when perciforms may have already radiated according to the
molecular evidence presented in this study (Table 3).
4.3 Implications
The present study suggested that euteleosts have had a much longer history than
previously thought. This notion seems so different from the previous view on this
issue that it should be further evaluated in the future by collecting longer sequence
data for more perciform taxa and/or by using more calibration points for the molecular clock. Possible other candidates for the rate calibration may include
characiforms (Ortf 1997), cyprinodontiforms (Murphy and Collier 1997), and geminate species pairs that most likely speciated by the rise of the Panamanian Isthmus
(Bermingham et al. 1997). At present, the molecular evolutionary rates reported in
the latter two papers may not be directly comparable to ours due to, e.g., the differences in sequenced genes and/or order of time scale for the calibration.
It has been pondered why perciforms appear to have evolved various morphotypes
rapidly in the early Cenozoic, while they have been largely conservative since then
(Carroll 1988). Our results may partly explain this question by relaxing the time
period during which the original morphological changes could occur. According to
molecular evidence presented herein, radiation of perciforms may not have been
related to global changes in climate and ecosystems that led to the extinction of
most archaic actinopterygian groups that flourished throughout the Mesozoic. However, as discussed above, it is still possible that their population size was dramatically increased and/or that they occupicd various niches upon the global changes
after the KIT boundary The long-term lack of fossil records, as well as the apparent
uncoupling between lineage radiation of new groups and extinction of old ones
have also been implicated for terrestrial vertebrates such as mammals and birds by
some molecular studies (Janke et al. 1994; Hedges et al. 1996; Cooper and Penny
1997; Kumar and Hedges 1998).
Occurrence of a similar phenomenon for organisms living in different environments (on land and in the sea) may imply its Ubiquitousness for more diverse groups
of organisms that lived in different geological ages. Vertebrates are considered to
have relatively well-documented fossil records and there have been no substantial
changes in the accepted broad-scale pattern of vertebrate evolution since the 19th
century (Benton 1998). However, this pattern may not be unchangeable or, at least,
it may be worth questioning and reexamining with newer data sets from modern
methods such as molecular evolutionary ones.
Acknowledgments
We thank N. Takezaki for sending us an earlier version of her programs now contained in the Lintre package, and instructions on the use of these programs. We
also thank M. Miya and an anonymous reviewer for many valuable comments on