Biostratigraphic Zones

Biostratigraphic changes within the stratified Pit sequence may prove useful for correlation of deposits within Porcupine Cave. Two zones based on first or last known stratigraphic appearance data are recognized (figures 7.1-7.3). These zones

FIGURE 7.1 Stratigraphic ranges of Amphibia, Reptilia, ochotonids, leporids, sciurids, and geomyids at the Pit. These ranges and those in figures 7.2 and 7.3 are used to define the biostratigraphic zones noted at the top and explained in the text. Levels 13 and 14 lacked fossils; thus the diagram extends back only to level 13. Abbreviations: G, glacial; IG, interglacial; LSDk, lowest known stratigraphic datum; HSDk, highest known stratigraphic datum; M. 5T Z., Microtus 5T Zone.

FIGURE 7.1 Stratigraphic ranges of Amphibia, Reptilia, ochotonids, leporids, sciurids, and geomyids at the Pit. These ranges and those in figures 7.2 and 7.3 are used to define the biostratigraphic zones noted at the top and explained in the text. Levels 13 and 14 lacked fossils; thus the diagram extends back only to level 13. Abbreviations: G, glacial; IG, interglacial; LSDk, lowest known stratigraphic datum; HSDk, highest known stratigraphic datum; M. 5T Z., Microtus 5T Zone.

conceptually resemble the lowest known stratigraphie datum (LSDk) and highest known stratigraphic datum (HSDk), respectively, as described by Walsh (1998). The Hypolagus Zone is defined by the HSDk for Hypolagus. The overlying Marmota Zone is defined by the LSDk for Marmota. In practice, the approximate boundary between the zones is placed at the boundary between levels 8A and 9. That placement splits the difference between the last appearance of Hypolagus and the first appearance of Marmota in the section, and it takes into account the fact that fossils are rare enough in levels 8A and 9 that it is impossible to know if further sampling would produce either taxon. For correlation purposes, other localities

FIGURE 7.2 Stratigraphie ranges of arvicolines and other rodents at the Pit. See figure 7.1 for further explanation.

in the cave that contain Hypolagus most likely correlate with Pit levels below 8A; those that contain Marmota have a high probability of correlating with levels above 9.

One assemblage zone is recognized: the Allophaiomys Zone, which is defined on the co-occurrence of Allophaiomys plio-caenicus and Mimomys virginianus. A characterizing taxon is Phenacomys gryci (figure 7.2). Other localities within the cave that produce these taxa have a high probability of correlating with levels 4-10 if both of the definitive taxa are present.

Two relative abundance zones are also apparent, based on arvicoline rodent percentages detailed in chapter 23 (see figure 23.7). The Mictomys Zone (figure 7.2) is defined by Mic-tomys exhibiting a higher percentage of individuals (>25%) within the arvicoline rodent component of the fauna than the combined total of Microtus 5T (i.e., Microtus characterized by five or more triangles on the first lower molar) and Micro-tus meadensis. The Microtus 5T Zone has relatively low num bers of Mictomys specimens (<25%), but high percentages of Microtus 5T and/or M. meadensis specimens (>25% combined). Ambiguity in differentiating the zones arises when the percentages of Mictomys versus combined Microtus 5T-M. meadensis are about equal. This situation occurs in level 3 and to a lesser extent in level 4. However, above and below those levels, the percentages provide a reasonable basis for biostratigraphic differentiation. The boundary between these relative abundance zones is thus recognized to be somewhat fuzzy and to encompass the level 4/3 transition. In general, the Mictomys Relative Abundance Zone overlaps broadly with the Allophaiomys Assemblage Zone, running from level 12 to level 5, before becoming hard to differentiate in level 4. The Microtus 5T Zone spans levels 1 and 2 before becoming hard to differentiate in level 3 (figure 7.4).

Goodwin (chapter 17) also recognized conceptual equivalents to relative abundance zones based on the sciurid

HS[>k-LS[)k Zones

Msrmota

ZOfíQ

Hypofcfgus

Zone

Assemblage Zones

Attophaiomys

Zonê

Relative Abundance Zones

If. 57 Z,

?

?

Micíoiuys

Zone

Climatic Interpretation

IC

G

LG

*

IG

F st ¡mated Age (Ka)

>euu

fu laso

to -iSOÛ

>BOO fe <1Q0Q

Stratigraphie Level

L 1

i

4

s

6

7

S

SA

9

11

12

ÎJ

MusK-ll'Inc

.UjiTvfa^svircalVi (Ijgoj-Miled Wen*El]

.IjVj y.viV,' l'rfljriïvhj < Enuimt j

Mmit-la iuanfic.1 ( Black-foottd F«rcr)

!

Mvsitto spcôcs A (Extind Mustelid)

rli.'ji'. i lK. [j. .-¡chinxjrri {^fUrfr^f Wnlvcrini;)

;

Luira ciWdiiii (AnttrlCM Klvtf OtlÉfJ

Tuihti-a ;axtts ( Ba<5gcrJ

Spilr-fit!? patisrw (Spdhei Stunlit

Srai-kypratnmn fshlumtu i shafl-hud.-:k_iil1:'i

CulÉH

Groifs la/mm (Coyotei

(nÎHwrJvj ( TvjLu"jrnTr Walfj

t'ulpax vvlpx flSwiftorKil

Umtj/on rtmnnfiwiïf.'w.uî [Gray Fou)

Tuvalu idnt

riuiygitmtx jp. (PuLLVirt I

t"frvld»t

cf. fttaenrip»1.! sp |.fJ«i|

AnHlKiprtdU

Aatikiaipm iPianjfrun)« Tesmoerys (Aiiclopc)

AnfilmnpridaîL jjcmit and spwin in4crenninjti

:

hvftlK

(htt Jp. llJiulxini Sheepi

Omi iwitui Aimûtgtoii iEminfc M^nrainCioilj

m

I\ijiti2.\ (ilcminawj sa.

Eifiiux sp. lloJ^tl iHOis^h

Eqwvs sp. i'snall.h |Huim)

!

_

FIGURE 7.3 Stratigraphie ranges of carnivores, artiodactyls, and equids at the Pit. See figure 7.1 for further explanation.

FIGURE 7.3 Stratigraphie ranges of carnivores, artiodactyls, and equids at the Pit. See figure 7.1 for further explanation.

component of the fauna. Levels 6 and deeper contain ?Cyno-mys andersoni as the only prairie dog species; levels 4 and 5 yield ?C. andersoni and C. cf. C. leucurus in about equal abundances; and levels 1-3 produce C. cf. leucurus almost exclusively. Wood and Barnosky (1994), Barnosky et al. (1996), and Bell and Barnosky (2000) noted dramatic increases in the relative abundance of Cynomys and Spermophilus relative to Marmota at the level 3/4 transition.

Lemmiscus exhibits a population-level change that may be useful in correlation. Lower first molars with only four closed triangles are equal or greater in abundance than those with five closed triangles in levels 3-10 (see figure 23.8). The first six-triangle forms appear in level 1. Thus other Porcupine Cave localities in which Lemmiscus populations are character ized by a predominance of five-triangle forms correlate more likely with levels 1 and 2 than with lower levels of the Pit, and populations with six triangles probably are coeval with or younger than the uppermost Pit level.

0 0

Post a comment