Biostratigraphy

The arvicoline and sciurid rodents are the only components of the fauna that have been studied sufficiently to allow table 7.1

Percentages of Arvicoline Rodents Identified from the DMNH Velvet Room Excavation

Horizon

Taxon

A

B

C

D

E

F

G

H

I

Phenacomys sp. (not P. gryci)

4

33

Microtus meadensis

54

33

40

32

Lemmiscus (4 triangles)

8

7

4

23

33

Lemmiscus curtatus (sagebrush vole)

83

85

23

13

20

12

33

9

Microtus paroperarius

2

8

7

7

4

33

41

33

Microtus sp. (5+ triangles)

8

20

15

33

7

4

Mictomys kansasensi/meltoni

13

7

16

23

17

Mictomys sp.

7

28

9

17

Total minimum number of individuals

12

51

13

15

14

25

3

23

7

biostratigraphic conclusions to be reached. Relative abundance calculations (table 7.1; see also tables 10.11, 10.12) place horizons C-F in the Microtus 5T Zone. The sample in horizon G is too small to record the pertinent taxa. Horizons H and I clearly fall within the Mictomys Zone, with Mictomys specimens composing 32% and 34% of the samples, respectively, and Microtus meadensis and Microtus 5T composing 0%.

Horizons A-G are characterized by a majority of Lemmiscus specimens with at least five triangles, resembling levels 1 and 2 in the Pit. In fact, the proportion of five-triangle specimens in Velvet Room horizons A-C (treated as a lumped sample) is significantly greater than that in Pit levels 1 and 2 (p < 0.0001 for x2 and Fisher's exact test). Given the trend for populations of Lemmiscus to exhibit more five-triangle forms through time, horizons A-C in the Velvet Room probably are younger than levels 1 and 2 in the Pit. Horizons H and I yield more four-triangle forms, resembling Pit levels 3 and below. More four- than five-triangle forms characterize horizons H and I (p < 0.0001 for both x2 and Fisher's exact test) in comparison with levels A, B, and C in the DMNH excavation and levels 1 and 2 in the Pit, but not in comparison with levels below 2 in the Pit (p = 0.15 for Fisher's exact test; p = 0.08 for x2). This would be consistent with correlating horizons H and I somewhere below Pit level 2.

DMNH Velvet Room horizons D-G contain more five-triangle than four-triangle Lemmiscus. They show statistically more four-triangle forms than horizons A-C, but fewer four-triangle forms than horizons H and I (p < 0.005 for x2 and Fisher's exact test). Individually each of horizons D-G produced a very small sample of Lemmiscus (see tables 10.11, 10.12), but even so most horizons exhibit (horizons E and F) or approach (horizon D) statistically significant differences with the horizon H and I sample, but not with the horizon A-C sample (horizon E, with statistically more four-triangle forms, being the exception). Horizon G, with a single specimen, could not be statistically differentiated from either the A-C or H and I samples. Horizons D-F as a lumped sample could not be statistically distinguished from Pit levels 1-3 (p = 0.77 for X2; p > 0.9999 for Fisher's exact test). The Lemmiscus morphotype percentages thus are consistent with correlating DMNH Velvet Room horizons D-F near Pit levels 1-3. However, consideration of the Mictomys/Microtus (44%/36%) abundances constrains placement of Velvet Room horizon F nearer to Pit levels 3 or 4—the transitional zone between the two relative abundance zones—than to Pit level 1.

Based on the derived condition of Cynomys cf. C. leucurus and Spermophilus cf. S. elegans, Goodwin interpreted horizons A and B as younger than the entire Pit sequence (chapter 17).

These considerations suggest that horizons A-C are younger than levels 1-3 in the Pit (figure 7.4). Horizons F-I probably correlate with Pit levels 4 and below. Interestingly, Baxter (chapter 15) reported one Hypolagus specimen from horizon G in the DMNH Velvet Room excavation, which would imply that horizons G and below extend into the Hypolagus zone, i.e., are at least as old as levels 9 or 10 in the Pit. If so, an unconformity or extremely low depositional rates between horizons F and G would be implied.

However, considerably more study of the complete DMNH Velvet Room fauna is needed before these conclusions should be regarded as firm. For example, the absence of taxa such as Allophaiomys pliocaenicus and Mimomys virginianus presents an inconsistency if DMNH Velvet Room horizons F-I correlate with levels 4 to as old as 10 in the Pit. However, these taxa are always found in low abundance in the Pit, which yielded only 23 specimens of A. pliocaenicus and 17 of M. virginianus in the sample of 1004 arvicolines. The DMNH Velvet Room sample comprises 254 arvicoline specimens, only 118 of which are from horizons where the two "missing" taxa would be expected. Therefore it is possible that the absence of these taxa simply reflects the relatively lower sample size. Both taxa, as well as Phenacomys gryci, are present in DMNH Velvet Room excavation samples that could not be reliably assigned to a stratigraphic horizon (see table 10.13).

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