With the discovery of Orellan apatemyids the geochronological range of
the family in North America is shown to extend from the Torrejonian
through the Orellan land-mammal ages. The discoveries reported here
enlarge the Oligocene record of apatemyids to include not only the type
specimen of _Sinclairella dakotensis_, a skull and associated mandible
from South Dakota, but also seven isolated teeth, representing at least
two individuals, from a Chadronian fossil locality in Nebraska and one
specimen from each of two Orellan fossil localities in northeastern
Colorado. Simpson (1944:73, and 1953:127) presented tabulations of the
published records of American apatemyids and suggested the data
indicated the populations of these mammals were of small size
throughout the history of the family. The few pre-Oligocene occurrences
of apatemyids described subsequently (note McKenna, 1960, figs. 3-10,
and p. 48) and occurrences described here tend to reinforce Simpson's
interpretation. This interpretation may have to be modified to some
degree, however, when current studies of collections of pre-Oligocene
apatemyids are completed (McKenna, personal communication).
Although information concerning the evolutionary trends of American
apatemyids has been published, no data on the morphological variation
in a population are available in the literature. An adequate basis for
evaluating the significance of the morphological differences between
the P4's of Princeton no. 13585 and KU no. 12110 coupled with the
similarities of their M1-2's is lacking. In the evolution of American
apatemyids the P4 underwent reduction in size and, apparently,
curtailment of function. This history suggests the range of
morphological variation of P4 in populations of _Sinclairella
dakotensis_ could be expected to be greater than that of the molars and
encompass the morphological differences between the P4's of Princeton
no. 13585 and KU no. 12110. The difference in age of the Chadronian and
Orellan fossils does not constitute proof that they pertain to
different species. Although the identification is admittedly
provisional until more fossils including other parts of the skeleton
are discovered, the Orellan fossils described here are referred to
_Sinclairella dakotensis_.
TABLE 1.--MEASUREMENTS (IN MILLIMETERS) OF TEETH OF SINCLAIRELLA
DAKOTENSIS JEPSEN.
==========================================================================
| P4 | M1 | M2
-----------------------+------------+------------------+------------------
|length|width|length[1]|width[1]|length[1]|width[1]
-----------------------+------+-----+---------+--------+---------+--------
Princeton no. 13585[2] | 2.1 | 1.1 | 4.0 | 3.7 | 3.4 | 4.7
RAM no. 381 | | | 4.1 | 3.5 | |
RAM no. 1674 | | | | | 3.4 | 4.2
KU no. 11210 | 2.4 | 1.6 | 3.9 | 3.5 | 3.8 | 4.1+
UCM no. 21073 | | | | | 3.6 | 4.1
-----------------------+------+-----+---------+--------+---------+--------
| m1 | m2
+---------+--------+---------+--------
| length | width | length | width
+---------+--------+---------+--------
Princeton no. 13585[3] | 3.5 | 2.4 | 3.7 | 2.8
RAM no. 1000 | 3.5 | 2.2 | |
RAM no. 598 | | | 3.8 | 2.6
RAM no. 1001 | | | 3.6+ | 2.6
RAM no. 1079 | | | 4.0 | 2.8
RAM no. 3013 | | | 3.6 | 2.8
------------------------------------+---------+--------+---------+--------
[Footnote 1: Length defined as maximum dimension of the labial half of
the crown measured parallel to a line drawn through the apices of
paracone and metacone. Width defined as maximum coronal dimension
measured along line perpendicular to line defined by apices of paracone
and metacone.]
[Footnote 2: Dimensions provided by Dr. Glenn L. Jepsen.]
[Footnote 3: Dimensions taken from Jepsen (1934:300).]