7303

• UCREFRP Catalog Number: 7303
• Author: Tyus, H. M.
• Title: Life Strategies in the Evolution of the Colorado Squawfish ‎(
• USFW Year: 1986
• Copyright Material: YES

October 1986

TYUS: COLORADO SQU A WFISH

657

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Fig. 1. Upper Colorado River Basin and Green River study area (shaded).

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occurred in Ptychocheilu8 by the mid-
Pliocene (Miller 1961). Nonetheless, the
largest Ptychocheilu8 reported in the fossil
record lived in Pliocene Lake Idaho (Smith
1975), indicating that Ptychocheilu8 success-
fully utilized both riverine and lacustrine sys-
tems.
The Southwestern United States is more
arid today than in the Late Cenozoic, and this
increasing aridity no doubt resulted in the loss
or reduction of lacustrine habitats and the
extinction oflake dwelling salmonids and cen-

trarchids from the Colorado River system.
This change was progressive from the
Pliocene, when a system of lakes covered the
lower and upper Colorado River Basins, and
persisted during pluvial periods until the
Pleistocene. During this epoch the life history
of fishes was remarkably impacted by such
long pluvial periods interrupted by short peri-
ods of desert conditions (G. Smith 1981).
An evaluation of the fish fauna of the Colo-
rado River in recent times (before introduc-
tions by man) might lead one to conclude that