Goodwin and Martin—Sciurid rodent diversity of the Meade Basin
1245
Figure 1. Location of the Meade Basin (gray ellipse).
Ridge ash-fall may have played a part in dental structural and size change within Ictidomys.
Materials and methods
Diagnostic dental specimens were digitally photographed in occlusal view (and usually in other views) along with a scale through a Leica MZ8 stereoscope (Leica Microsystems, Inc., Bannockburn, Illinois). Dental photographs were organized in a FileMaker Pro database (http:://
www.Filemaker.com), along with associated taxonomic and locality data. Specimens examined for this study are listed in the appendix (provided as Supplemental Data 1). Digital photographs were output from the FileMaker
database, calibrated to the scale, and measured digitally using GraphicConverter (
https://www.lemkesoft.de/en/products/ graphicconverter/) to obtain the following variables: greatest mesiodistal length of upper and lower cheek teeth; tooth width from the lingual margin of the protocone to buccal margin of the paracone on P3–M3; and tooth width across the trigonid on p4–m3, and likewise across the talonid on p4 (Fig. 3). Relative trigonid width of p4 was calculated as trigonid width/length of p4. Metric data were employed to graphically and statistically explore taxonomic and stratigraphic patterns in dental size and shape. All statistical
analyses and graphing were done with SPSS (http://www-01.
ibm.com/software/analytics/spss/). Digital photographs were also used in detailed qualitative
comparisons within and across putative taxa, across sites, and with relevant modern taxa using a companion photographic database of modern ground squirrels described elsewhere (Goodwin, 2009). Genus and species assignments were informed by both quantitative and qualitative comparisons; descriptions in the text employ standard dental terminology (Fig. 3).We followed the taxonomy of Helgen et al. (2009) when assigning fossil species to genera. The way that paleontologists recognize and describe species
Basin, C.W. Hibbard and his students attempted to fit their fossil localities in Meade, Seward, and Clark counties into a common
in the fossil record strongly affects subsequent diversity analyses. Our perspective since beginning the MBRP was to make the database as biologically realistic as possible. Consequently, we do not accept anagenesis, the phyletic origin of new species. New species are recognized only as the result of cladogenesis, or lineage-splitting. The criteria for recognizing cladogenesis in the Meade Basin were delimited by Martin and Peláez-Campomanes (2014), and no intra-basin speciation events have been recognized to this point, though the origin of the diminutive gopher Geomys tyrioniMartin, 2016was likely induced by theHuckleberry Ridge ash-fall on the Central Great Plains (Martin, 2016). During the previous generation of work in the Meade
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