Robert W. Wilson, University of Kansas Museum, has provided many particularly relevant suggestions in rodent research and has kindly read that portion of the manual. Edwards began field work on a survey of the red clastic rock deposits, with the encouragement and counsel of staff from the Survey and the Instituto de Geologia. The main efforts were focused on the red conglomerate in the Guanajuato district, with the main objectives of learning the depositional environment of the conglomerate and searching for fossil remains.
This search was rewarded with the discovery of some bone fragments in silty beds in the lower part of the conglomerate. The northeastern side of the fault consists of metamorphosed sediments (knife in Figure 2), mainly phyllites, and some intrusive igneous rocks. I23 and lanite, unlike the andesitic volcanoes in the lower part of the formation.
August 1950 in the red conglomerate about 2,000 meters south of the town of Marfil (see fig. 2) in thin beds of poorly consolidated reddish-brown sandstone and bentonitic siltstone dipping about 35°. to the southwest. The remainder of the material was examined in the winter of 1950-51 but was not found. The urgent need to assign an age to the conglomerate stimulated a continued search of the beds for additional fossil material.
Stirton suggested that the remains probably belonged to an artiodactyl of the family Merycoidodontidae, indicating an early Cenozoic age.
DESCRIPTION OF THE FOSSIL-BEARING BEDS
Edwards and Ortiz alerted Fries to the site later that month, and after several hours of further exploration, Fries found some rostral parts of the tiny rodent and a fragment of a jawbone with two tiny cheek teeth. Edwards returned to the site in October 1951 to complete some mapping details and look for more fossils, but found nothing of diagnostic value. Accompanied by Kenneth Segerstrom from the Geological Survey, he was lucky this time in finding other parts of the skulls.
This new material was picked up at a point about 15 meters from the first find in beds perhaps 3 meters higher in the section. Segerstrom and several associates guided junior author Dunkleto to the site in October 1952, on which occasion the lizard specimens described herein and some additional rodent bones were obtained at widespread and undetermined levels in the exposure. Hibbard for study, and about the same time the Instituto de Geologia gave the original Edwards specimen to R.
The establishment of the early Cenozoic age of the beds, within relatively narrow limits, has been based largely on Hibbard's study of rodent remains. Lizard specimens have been studied by Dunkle, but, because of the scant fossil record of Lacertilia, they have proved to be of little use as indicators of time.
8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23
SIGNIFICANCE OF THE FOSSIL DISCOVERY
9the plateau through Oligocene time, just as strong movements in the plateau through Oligocene time, just as strong movements through Oligocene time in the coastal country. The fossil discovery also indicates that most of the volcanic rocks in the central part of the Mexican Plateau are probably of late Oligocene and Miocene age, because these rocks match the upper part of the red conglomerate and are separated by strong faults and a period of erosion of the next younger rocks, which are conglomerates and fine elastics known as middle and. The great masses of tin-bearing rhyolite scattered over the plateau region of Mexico, as for example to the northeast of the city of Guanajuato, are almost certainly of Miocene age rather than of Pliocene age as hitherto generally supposed.
Coronoid projects high above the dorsal margin of the mandible and in lateral aspect with both the anteroventral and posteroventral angles somewhat produced. Dentition pleurodont; individual teeth relatively large and few in number; each with a laterally placed, longitudinal cutting edge which is vertically wrinkled but not cuspidate, and with the crown slightly expanded transversely.
10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127,
Therefore, only the thoroughly fossilized bones of the occiput and the ventral skull base are observable. It can also be assumed that they contact the forelimbs of the squamos posteriorly. The posterior corner of the latter is prominent, dorsoventrally flattened and has a small mesially directed retro-articular process.
Due to anteroposterior compression, the transverse diameter of the teeth is greater than the longitudinal dimension. Currently, Paradipsosaunis is therefore considered an extinct representative of the family Iguanidae. I23 therefore does not provide any clue as to the specific age of the sediments from which it was extracted.
I23 leading edge of the paracone and passes slightly upward to near the inner base of the protocone. The valley is slightly deeper on the lip side and widens between the middle edges of the tips. The metacone is slightly larger than the hypocone and is the size of the protocone.
There is a shallow fossa between the posterior edge of the accessory conule and the posterior cingulum. It extends from the anterolabial edge of the paracone to the anteromedian edge of the protocone. The lingual part of the transverse valley is closed by a large accessory conule, i.e.
Posterior to the small metacone and forming the posteromedial edge of the tooth, a cusp is larger than the metacone. The size and shape of the teeth and their tip development are as the holotype. The upper incisors are as much larger than the upper incisors of the holotype, as the lower incisors in fragmentaryramus.
The dentition of Floresomys is reminiscent of the Heteromyidae, although thediastema and the M^ exclude such a relationship unless. The more generalized characters of the mandibles and rostral region of the skull(?) of Floresomys indicate a greater antiquity than that of Kansasimys.
LITERATURE CITED
If Floresomys had split from the Taxymys tribe, its specialized development could have occurred during the late Eocene. Floresomys is more closely related to the Sciuravus powayensis Wilson line (1940). There is still no morphological evidence that it is younger than the late Eocene. It may be that Floresomys is a descendant of a generalized paramyline tribe and that the tooth pattern is a simplification of the paramyline tooth pattern.
The development of the lower jaw and of the lower and upper dentition indicate that it is no younger than the earliest Oligocene. In the comparative study of these specimens, the outlier Kansasimys (Wood, 1936) from the middle Pliocene of Kansas should be noted because of its probable relationship to the Ischyromyidae. The generalized characters and the small size of the tapiroid foot may be taken to indicate an age no younger than the earliest Oligocene for the deposit.
