ESPECIALLY MYCTOPHOIDS 239 
Tertiary, is that the early evolution of these groups could have occurred in fresh- 
water, and fresh-water deposits are rare. This proposition has already been 
tentatively suggested by Greenwood, et al. (1966 : 347). The Ostariophysi and 
Gonorhynchiformes are predominantly fresh-water forms, while many of the present 
day Salmonoidei spend the breeding part of their lives in fresh-waters. Possibly 
then the ancestral stock of the Protacanthopterygii as a whole was to be found in 
fresh-water. This hypothesis would also help to explain the divergence between the 
Elopiformes and the Leptolepidae on the one hand and the Salmoniformes on the 
other, the initial evolution of the two groups occurring in marine and fresh waters 
respectively. 
No mention in the discussion so far has been given of the Osteoglossomorpha of 
Greenwood, et al. (1966). This division they tentatively considered to have a 
separate origin in the Pholidophoridae. Patterson (1967c : 107) indicates that they 
could have been derived with the Ichthyodectidae from such Jurassic forms as 
Allothrissops. Bardack (1965), however, postulated the derivation of the Chirocen- 
tridae from forms such as Thvissops and Allothrissops. Both Cavender (1966) and 
Patterson (1967c) refute this, the latter suggesting that these two genera are conver- 
gent with the clupeoids and were derived from a leptolepid and not a pholidophorid 
as Bardack (1965) suggested. Patterson (1967c) thus thinks that the Osteoglosso- 
morpha can be placed with the Elopomorpha, Clupeomorpha and the Leptolepidae 
as a related group of lineages. The osteoglossomorphs however are predominantly 
fresh-water and may have arisen in this medium with some closer affinity with the 
basal protacanthopterygians. 
There is apparently a ‘ time-lag ’ between the origin of the teleosts as a whole and 
their explosive radiation in the Cretaceous. Both the Pholidophoridae and Lepto- 
lepidae occur in the Triassic and the elopiforms are represented in the Jurassic. 
This lag in the radiation of the teleosts during the Jurassic occurred at the time of 
the maximum holostean diversification. Schaeffer (1965) has pointed out that the 
ecological situation at this time is difficult to assess, so that the role that competition 
might have played is impossible to determine. It is possible, as Schaeffer (1965) 
indicated, that the superior halecostome locomotor mechanism plus the plastic 
pholidophoroid jaw design was gradually perfected throughout this period and the 
teleost grade began to exhibit its tremendous potential at the end of the Jurassic. 
It would appear that the elopoids and leptolepids (which arose in the Triassic 
and Lower Jurassic) did not represent a significant advance over the Holostei and 
merely existed as contemporaries. However if the halecostome ancestors of the 
salmoniforms had moved into fresh-water in the late Triassic and Jurassic, and 
gradually evolved during that period in the absence of competition from the holo- 
steans, then this time-lag would not be so apparent. Finally at the close of the 
Jurassic sufficient evolutionary advance had been accomplished by the protacantho- 
pterygians to provide overwhelming competition for the holosteans and the group 
re-entered the seas. Once back in a marine environment in the Cretaceous, the 
Protacanthopterygii rapidly diversified giving the appearance of an ‘ explosive 
radiation ’ at the sub-ordinal and familial levels. 
