The species marked by an asterisk are regarded by Doctor Sellards and others as belonging to the Miocene or Pliocene (8th Ann. Rep. Florida Geol. Surv., p. 94). See also Sellards, 1913 (5th Ann. Rep. Florida Geol. Surv., p. 58; 8th Rep., p. 104).
On the basis of the fossil vertebrates it can hardly be denied that the Alachua clays and the phosphate mines at Dunnellon are of the same geological age. According to Sellards, the formation belongs to the upper Miocene or to the lower Pliocene. Merriam (Bull. Dept. Geol. Univ. Cal., vol. X, p. 439) refers it to the Pliocene. Although there is present a strong palæontological element which represents the Pleistocene, the reference of the formation to the late Miocene or early Pliocene has seemed to be required by the presence of Gomphotherium, Procamelus, Teleoceras, and Hipparion. The Pleistocene species are usually accounted for on the supposition that they are intrusions from more recent deposits.
A figure from Sellards (Geol. Surv. Florida, vol. VII, p. 53), only slightly modified is intended to show the relation of the phosphate-bearing formations to those underlying them (fig. 21).
It is worth our while to consider whether or not the reference of the Alachua formation to the Miocene or early Pliocene is required by palæontological evidence. Gomphotherium is characterized by having molar teeth which on abrasion at one or both ends of each crest, present a trefoil pattern of the enamel; also by having a band of enamel on each of the upper tusks. Now, teeth having the same structure are not uncommon in deposits of undoubted Pleistocene age in Kansas and Texas. That the animals possessing these teeth had tusks with enamel bands is not known, but it is quite possible that such enamel bands were present.
Fig. 21.—Diagrammatic sketch of geologic structure of Florida from north to south passing through the hard rock and pebble phosphate fields, showing relation of the phosphate deposits to the underlying formations. After Sellards.
1. Georgia-Florida State line. 2. Suwannee River. 3. Lake City. 4. Santa Fe River. 5. Withlacoochee River. 6. Lakeland. 7. Arcadia. 8. Caloosahatchee River. 9. Gulf Coast. a Upper Oligocene phosphatic marls. b Ocala limestone. c Hard rock phosphate. d Bone Valley formation. e Pleistocene deposits (Pliocene and Pleistocene of Sellards).
The genus Hipparion is not confined to the Tertiary. Teeth have been discovered in the Aftonian of Iowa (Hay, Geol. Surv. Iowa, vol. XXIII, p. 150) and in Missouri (op. cit., p. 149). The writer has described a species of the genus, Hipparion cragini, collected by Professor Cragin in the Sheridan beds in Kansas (Kansas Univ. Sci. Bull., vol. X, p. 42).
One may be justified in suspecting that Procamelus lived on into the Pleistocene. Not only has it been found associated with Pleistocene fossils in five places in Florida—Archer, Williston, Dunnellon, Hernando, and Ocala—but it has been met with in possible Pleistocene deposits (the Idaho formation) in Idaho, which furnishes Equus, Cervus, Castor, and Stegomastodon mirificus (the type of which belongs in the Sheridan beds). Furthermore, the writer has had occasion to describe a collection of fossils, believed to belong to the early Pleistocene, which was obtained at Anita, Coconino County, Arizona. Among these fossils are two species of Procamelus much like those described by Leidy from the Alachua formation (Proc. U. S. Nat. Mus., vol. LIX, pp. 622–626). The writer believes that the genus Procamelus persisted into the early Pleistocene.
Two species of rhinoceros have been collected in the Alachuan formation, Teleoceras proterus Leidy and Aphelops longipes Leidy. Both occurred at Archer, while T. proterus was found near Williston and A. longipes at Dunnellon. A rhinoceros has been discovered in the Idaho formation, with the Pleistocene species named above in connection with Procamelus of these beds. In Oregon Cope made a collection which has been examined by Dr. W. D. Matthew (Bull. Amer. Mus. Nat. Hist., vol. XVI, p. 321). Here again Teleoceras was supposed to have been found with Hipparion, camels belonging to Camelops (or Procamelus), Elephas, and Equus. Matthew thought that there had happened, either before the fossils were collected or afterwards, a mingling of elements of two distinct faunas.