Phylogeny of the Later Tertiary Equidae in the Light of New Pliocene Horses from Chihuahua, Mexico. Origin of the Pioneer Pyrophyllite Deposit, San Diego County, California

Citation

Lance, John Franklin (1949) Phylogeny of the Later Tertiary Equidae in the Light of New Pliocene Horses from Chihuahua, Mexico. Origin of the Pioneer Pyrophyllite Deposit, San Diego County, California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HFAA-MY41. https://resolver.caltech.edu/CaltechTHESIS:06182018-132202171

Abstract

[Major Thesis]. Phylogeny of the Later Tertiary Equidae in the Light of New Pliocene Horses from Chihuahua, Mexico:

Phylogenetic charts currently published showing the evolution of North American Equidae indicate that species of the subgenus Pliohippus (Astrohippus) of the Hemphillian stage are in the line of ancestry of modem horses. Different opinions exist as to the ultimate fate of the typical subgenus, and as to the origin of Old World zebras.

A study of fossil horses from the Yepomera fauna of western Chihuahua, Mexico, demonstrates the presence of four species. Three of these are described, and two are regarded as new to science, namely, Pliohippus (Astrohippus) stockii n. sp. and Pliohippus (Pliohippus) mexicanus, n. sp. It is concluded from a comparison of the horses and of associated elements in the fauna with those found in related assemblages, that the Yepomera fauna is of late occurrence in the Hemphillian stage (middle Pliocene) of North America. The faunas from the Hemphill horizon of Texas, from the Alachua-Bone Valley beds of Florida, and from the Mt. Eden beds of California are believed to be older.

The two new species throw light on the phylogenetic relationships of the genus Pliohippus sl. The subgenus P. (Astrohippus) is removed from direct ancestry to Blancan and later horses, and P. (P.) mexicanus n. sp. is proposed as the most likely known ancestor of living horses, including the zebra. These new interpretations as to the phylogeny of the later Equidae are presented in a new chart.

[Minor Thesis]. Origin of the Pioneer Pyrophyllite Deposit, San Diego County, California:

Pyrophyllite, a hydrous aluminum silicate with properties and uses similar to those of talc, is abundant at the Pioneer deposit near Escondido, San Diego County, California. The mineral occurs in lenticular bodies in the Santiago Peak volcanics of approximate Jurassic age. The deposit is approximately on a Tertiary(?) erosion surface that has probably been exhumed.

A study of field, petrographic, and chemical data indicates that the pyrophyllite was developed by replacement of volcanic flows and breccias of original andesitic and latitic composition. These volcanic rocks had been previously folded and sheared, and subjected to mild regional metamorphism and accompanying propylitic alteration. Silicification in part preceded the formation of pyrophyllite. Development of pyrophyllite is known to be complete or nearly so in only one body, which is being quarried at the present time.

The solutions forming the deposit appear to have followed shear zones, and lithologic control evidently was not important in localizing the mineralization. The solutions were of hydrothermal origin, and possibly were associated with the emplacement of the Peninsular batholith of southern California. They were of intermediate temperature and acid in nature. Silica and possibly alumina were added to the rocks by these solutions, and other elements were removed. Moderate stresses produced a schistosity in pyrophyllite, probably during mineralization.

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