The lower Colorado River corridor below Grand Canyon developed sometime between ∼5.3 Ma and ∼4.8 Ma, as evidenced by the first appearance of Colorado River-derived sediments of the Bouse Formation in basins along its modern course and the abrupt appearance of distinctive hematite-stained “C-suite” Colorado River sand grains near the base of the Wind Caves member of the Latrania Formation in the Salton Trough. The oldest recognized evidence for a through-flowing Colorado River is the ∼4.5 Ma to ∼3.5 Ma Bullhead Alluvium. Interest and debate focuses on whether a “pre-Bullhead” Colorado River might have first reached the Salton Trough at 5.3 Ma, prior to deposition of some of the Bouse Formation. Fine-grained but undated Colorado River sediments have been recognized on the Yuma Proving Grounds, Arizona, since the 1970s. These sediments have been correlated by some to the Bouse Formation, and thus, would pre-date the Bullhead Alluvium and might provide evidence for the existence of a “pre-Bullhead” Colorado River. In this study, I use the extent of amino acid racemization in ostracode valves and microfossil assemblages to test correlations between the fine-grained sediments on the Yuma Proving Grounds with other independently dated lower Colorado River corridor alluvial packages that were deposited at ∼5 Ma (Bouse Formation) and at ∼75 ka to ∼60 ka (Chemehuevi Formation). The results of this study confidently demonstrate that the fine-grained sediments on the Yuma Proving Grounds are not late Miocene or early Pliocene in age but instead are correlative with the late Pleistocene Chemehuevi Formation. Evidence for a “pre-Bullhead” Colorado River remains elusive.

A distributional and zoogeographical analysis based on published records and approximately 13,000 specimens is provided for the 122 naturally occurring mammalian species in Sonora, Mexico. Twenty-eight species of unverified occurrences which could be present in Sonora are discussed. Domesticated mammals are listed and introduced species are briefly discussed. Descriptions of the topography, climate, vegetation, and hydrological features of Sonora are included. Dichotomous keys provide for the identification of Sonoran mammals to the level of order, family, and species. Species accounts include the following: scientific and common names, additional identification remarks, general distributional discussions, various ecological and biological remarks, records of known occurrence, and distributional range maps.

Distributional patterns are discussed ecologically and geographically. Eighteen physiognomically described community types are defined. Similarities in mammalian composition among community types are compared using a phenogram constructed by the Unweighted Pair-Group Method using Arithmetic averages (UPGMA) clustering process. The similiarity values clustered were Jaccard's Coefficient of Communities which were calculated between each community type. The clustering process suggested three main ecological complexes of mammals in Sonora: a subtropical group, a mid-to-upper elevation group, and a lower elevation group.

A computer-constructed species density contour map indicated the number of species across Sonora varied between 54 and 95 species. Superimposed species range limits and analysis (UPGMA cluster analysis of distance dissimilarity coefficients) of presence-absence data of species in a grid system suggested five different mammalian provinces in Sonora: Sonoran, Trans Sonoran, Sinaloan, Yaquian, and Upper Mapimian. Faunal element analysis suggested at least five faunal units (Chihuahuan, Yuman, Neotropical, Campestrian, and Eastern) contribute species elements to the total fauna of Sonora.

Angling with live baitfish has been a popular fishing method among many anglers. In Arizona, the use of live baitfish encompasses approximately 59% of anglers. Non-native fish introductions have become a serious issue in many states, and live baitfish are a potential vector for these introductions. We used the native desert sucker (Catostomus clarki) and the longfin dace (Agosia chrysogaster) to assess persistence on a hook compared with a non-native species, fathead minnow (Pimephales promelas) when used as baitfish. The desert sucker and the longfin dace are widespread throughout Arizona and fathead minnows are the most widely available baitfish. We compared time to mortality between all three species when hooked and manipulated similar to fishing action. There were no significant differences in mortality times between species, and time to mortality was greater than one hour for most individual fish.