Pollen, plant macrofossils and microvertebrates from mid-Holocene alluvium in east-central Iowa, USA: Comparative taphonomy and paleoecology

Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 204 – 221 www.elsevier.com/locate/palaeo

Pollen, plant macrofossils and microvertebrates from mid-Holocene alluvium in east-central Iowa, USA: Comparative taphonomy and paleoecology Paula T. Worka,T, Holmes A. Semkenb, Richard G. Bakerb a b

Maine State Museum, 83 State House Station, Augusta, ME 04333, USA Department of Geoscience, University of Iowa, Iowa City, IA 52242 USA

Received 6 August 2004; received in revised form 10 March 2005; accepted 5 April 2005

Abstract Pollen, plant macrofossils, and micromammals are commonly used in paleoecological interpretations, but rarely are found associated because of distinctly separate taphonomic pathways into the fossil record. The Lilienthal local biota, dating between 5920 F 60 and 6300 F 80 14C B.P., is a rare exception as the site contains all three proxies juxtaposed in alluvium, impounded upstream from a fossil log near the headwaters of Mud Creek, east-central Iowa, U.S.A. A variety of taphonomic processes can strongly influence the composition of fossil assemblages. This paper compares the taphonomic framework for each fossil group, interprets the paleoecologic signal from each, and then compares the three interpretations. Pollen and plant macrofossils are found in organic silts, indicating deposition in a quiet water environment. Microvertebrates ordinarily occur in sand-sized sediments and show evidence of fluvial transport. Despite these different pathways, all Lilienthal assemblages indicate that a largely closed mesic deciduous forest grew along Mud Creek in midHolocene time. Therefore, a narrow north–south ecotone must have existed between the forest and tall-grass prairie biomes in eastern Iowa. This robust data set provides a detailed picture of the paleoecology and paleogeography of this past environment. D 2005 Elsevier B.V. All rights reserved. Keywords: Holocene; Taphonomy; Paleoecology; Iowa; Plant macrofossils; Pollen; Vertebrates

1. Introduction Pollen data from conventional sites (lakes, bogs, and marshes) are used widely to determine regional shifts in vegetation (McAndrews, 1966; Davis, 1967; T Corresponding author. E-mail address: [email protected] (P.T. Work). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.04.005

Webb, 1974; Webb and McAndrews, 1976; Overpeck et al., 1985). Although plant macrofossils are used less commonly, they are typically more effective in detailed paleoecological reconstructions of Holocene species migration (Baker et al., 1996, 1998, 2002; Jackson et al., 1997, 2000). Micromammals (insectivores, rodents, and lagomorphs) are also important in paleoecological reconstructions and

P.T. Work et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 204–221

in determining species migration (Semken, 1983; Graham, 1993). However, the paleoecological value of alluvial sites, such as Mud Creek, has been challenged because of presumed taphonomic modification of fossil materials (Brush and Brush, 1972; Behrensmeyer, 1982; Fall, 1987; Burnham, 1993). Excavation of the Lilienthal Site, a creek bank exposure representing a single depositional episode along Mud Creek, revealed that all three fossil groups were present. Additionally, the Lilienthal locality offers one of the few Holocene vertebrate local faunas in North America that is not directly associated with an archaeological site. The purposes of this paper are: (1) to evaluate the taphonomic patterns, including possible reworking, related to each fossil type; (2) to compare paleoecological interpretations derived from pollen, plant macrofossils, and vertebrates; and (3) to evaluate the reliability of paleoecological data obtained from alluvial deposits. An intensive examination of Mud Creek stratigraphy (Bettis et al., 1992; Bettis and Autin, 1997) also permitted detailed analysis of the effect of depositional sorting associated with headwater alluvial deposits. Although conventional pollen sites suitable for Holocene paleoecological studies are rare south of the Wisconsinan glacial margin in the upper Midwest, organic-rich sediments from stream cutbanks have proved to be a valuable source of paleoecologic information (Chumbley, 1989; Baker et al., 1990, 1996, 1998, 2002; Miller et al., 1994). Kramer (1972) worked on vertebrate, mollusk, and plant macrofossils from sediments associated with Mud Creek, near the Lilienthal locality, and concluded that this region was forested circa 6000 yr B.P. (Fig. 1). This conclusion was then at odds with the conventional view of a midHolocene expansion of the prairie (Winter, 1962; Wright et al., 1963; McAndrews, 1966; Van Zant, 1979; Webb et al., 1983). The bprairie peninsula,Q an eastward extension of prairie stretching from Iowa into Indiana and Ohio, was proposed by Transeau (1935) to explain remnant patches of prairie vegetation found in these more eastern states. This peninsula was later proposed to have advanced further east during a warmer, drier climatic interval in the middle Holocene (Sears, 1942). Pollen evidence from sites in Minnesota and western to central Iowa supported

205

this picture, and it was assumed that the prairie peninsula expanded through Iowa and Illinois ~8000 yr B.P., was at its maximum extent 7000–5000 yr B.P., and was replaced by savanna ~3000 yr B.P. (Winter, 1962; Wright et al., 1963; McAndrews, 1966; Van Zant, 1979; Webb et al., 1983). Subsequent analyses of pollen and plant macrofossils from cutbanks on Mud Creek in east-central Iowa (Baker et al., 1990), and new investigations on Roberts Creek in northeastern Iowa (Chumbley et al., 1990; Baker et al., 1992, 1996, 1998) and in southeastern Minnesota (Baker et al., 1992, 2001, 2002), have shown that the mesic forest remained in the central Midwest until ~6000 yr. B.P. and that a regional shift from forest to prairie did not occur until ~5400 yr. B.P. (Dorale et al., 1992; Baker et al., 1992, 1996, 1998, 2001, 2002).

2. The Mud Creek basin and the Lilienthal site Present day Mud Creek is a slow moving, permanent meandering stream with intermittent pools and riffles. The creek is approximately 1–3 m wide and 0.1–2 m deep during normal flow periods. At present, plant debris settles out in small pools and behind large pieces of driftwood. Recent point bars are composed chiefly of gravel, with few organic remains. No modern vertebrates were noted. The Mud Creek basin (~62 km2; Fig. 1) lies along the boundary between the Southern Iowa Drift Plain and the Iowan Surface (Prior, 1991; Autin and Bettis, 1991; Bettis et al., 1992). Elevations range from 200 to 260 m. Upland slopes are nearly level in the central and southern portion of the basin and slope moderately (b14%) in the north. Four to 7 m of Peoria Loess blanket the region and cover complex erosional surfaces of late Wisconsinan age (Autin and Bettis, 1991; Bettis et al., 1992). Stratigraphic and chronologic investigations in the Mud Creek basin show that the exposed alluvial units are part of the late Quaternary DeForest Formation (Autin and Bettis, 1991, Bettis et al., 1992). Four regionally recognized members have been identified: the preGunder (11,000 yr B.P.), Gunder (11,000– 4,000 yr B.P.), Roberts Creek (4000–400 yr B.P.), and Camp Creek (400 yr B.P. to present) Members. Within the basin, members of the DeForest Formation

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P.T. Work et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 204–221 Wa p

on n ic

sip i

0

5

41 0 45'

10

410 45'

Ri v er

Kilometers

N

90 0 50'

90 0 35' 410 40'

Mud

ek Cre

410 40'

Hickory Creek

Lilienthal site 90 0 50'

90 0 35'

Minnesota

C Roberts Creek ICNC

Iowa Missouri

90 0 45'

Wisconsin

90 0 40'

Mud Creek

B

A

Fig. 1. Location of (A) State of Iowa; (B) Mud Creek, Indian Creek Nature Center (ICNC), and Roberts Creek sites; and (C) Lilienthal site in Mud Creek Basin.

produce both locally conformable sediment sequences and cut-and-fill sequences associated with facies patterns produced by lateral channel migration and vertical aggradation (Autin and Bettis, 1991; Bettis et al., 1992). The Camp Creek, Roberts Creek, and Gunder Members are exposed at the Lilienthal site, but only the Gunder Member contains abundant fossils. The Lilienthal local biota (Fig. 1), recovered from the Gunder Member of the DeForest Formation, was exposed in the bottom half of a 3.3-m-thick cutbank on the southwest side of Mud Creek (41840V9W N 90854V25W E, NE1/4, NE1/4 Sec 12, T79 R1W) in Cedar County, Iowa. The Gunder Member at this location (1.6 m., Fig. 2) consists of exposed point

bar deposits trapped behind a log that obstructed the paleo-channel and consists of both oxidized and reduced channel belt facies as described by Bettis et al. (1992). Gunder sediments grade from silty-clay at the top of the excavation to a sterile till-like cobble– gravel below a basal Gunder unconformity. Intervening beds consist of crossbedded silt, sand, and gravel lenses (Fig. 2). A randomly oriented, 3-cm-thick leaf-mat at the base of the log (level 14) indicates near-quiescent water conditions. Above this level, finer-grained silts and clays are interbedded with coarser-grained sands and gravels, indicating periods of low level current, punctuated by periods of increased flow. Fossil remains were incorporated into pool sediments during pulses of differing

P.T. Work et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 204–221

207

Level

m1agt

1

Level

2

1 3

w current flo

5t

4

2 3

5

4 4+5

6

1t

1a t

5

7 6,240 +/- 50 yr B.P.

9 10

4

5t 5b

13

4

7 8

3

log

2t

mg

4t

7mg

3 4t

6

7s 3g

3bl + 1bl

4t

1g

2g 8

8

9

ll t Wa Wes 1 1a 2 3 4 5 6 7 7s 8 9

3g

9

3g 8

6 1g

4g

1g

t g bl b m

krotovina tan gray blue gray brown mottled leaf mat

12

14 15

North Wa ll

clayey silt silty clay sandy silt clay quartz sand silt gravel sand with tuffa-like nodules sandy silt with tuffa-like nodules slump gravel with cobbles and silty lenses

11

13 6t

2g

9 10

1g 4

2g

4g

water level

4t + 5gb 4

2 3g

1bl

5,920 +/- 60 yr B.P.

16

2

2t

11 12

6

4

16 location of bulk-pollen sample 6,300 +/- 80 yr B.P.

location of deer scapula

0

20 cm

wood organically laminated fine silty clay

Fig. 2. Stratigraphic cross section of the Gunder Member of the DeForest Formation at the Lilienthal site excavation. Arrows designate position of conventional radiocarbon-dated wood samples. The Roberts and Camp Creek Members of the DeForest Formation overlie the section to the west.

intensity. This resulted in an interbedded mix of sediments of varying grain size and fossil composition. Organically rich silts and clays contain leaf mats, twigs, budscales, charcoal, and seeds in association with low concentrations of tiny gastropods, finger clams, and lighter, flatter bone elements (e.g., fish cranials and amphibian long-bones). At the other extreme, coarser 4–5 mm gravels and sands contain denser, more-rounded microvertebrate material (e.g., micromammal teeth and snake vertebrae) and larger branches. The largest skeletal element recovered from the site, a deer scapula, was located along the basal–

cobble interface below the basal Gunder unconformity (Fig. 2) and was emplaced prior to the log’s influence on the site. The upper surface of the Gunder Member is overlain unconformably by the Roberts Creek Member of the DeForest Formation; Camp Creek deposits cap the section.

3. Methods A total of 1588 kg of Gunder matrix was collected between 170–330 cm below the surface of the

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terrace, in sixteen 10-cm levels (Fig. 2). During excavation the floor of each level was mapped to record sediment and plant macrofossil positions. A 3.5 l bulk-pollen sample, taken prior to the excavation (270 cm below surface), was processed in the laboratory following a procedure modified from Faegri et al. (1989). Sediment was water screened through 1.5 mm mesh either at the site or in the laboratory. In addition, the bulk-pollen matrix was processed through 0.5 and 0.1 mm sieves to retrieve finer size-fraction macrofossils. Residue was dried and picked for plant macrofossils, vertebrates, and invertebrates. Approximately 1500 plant macrofossils and 1165 skeletal fragments are reposited at the University of Iowa Paleontology Repository.

All identifiable mammalian osteological elements (e.g., femur, rib) were determined to species for paleoecological comparisons (Table 1) using nomenclature of Hall (1981). All other bone elements were attributed to taxonomic class (e.g., amphibia, reptilia), and were recorded as single counts in an bidentifiable element category.Q Number of Identified Specimens (NISP), a count of identified fragments for each taxonomic category, was used to quantify all skeletal elements (Fig. 3). Minimum Number of Individuals (MNI) also was used to quantify the mammalian osteological elements by counting the most frequently occurring element per taxon. When calculating MNI, intra-species variation, based on age, body size, and sex were considered.

Table 1 Taxon, habitat preferences, relative abundance, and center of distribution for Lilienthal site mammals Center of distribution Taxon

Common name

Habitat

NISP MNI

Insectivora B D D

Sorex cinereus (SUI 101063) Masked shrew Blarina brevicauda (SUI 101064-66) Short-tailed shrew Scalopus aquaticus (SUI 101067-68) Eastern mole

Moist grassy areas, forest, woodland Moist woodland, moist meadows Moist woodland, moist grassy areas

1 13 13

1 3 2

Lagomorpha W

Sylvilagus/Lepus (SUI 101069)

Rabbit/hare

Meadows, grasslands, thickets, woods

1

1

Tamias striatus (SUI 101070) Sciurus sp. (SUI 101071-72) Sciurus/Tamiasciurus Tamiasciurus hudsonicus (SUI 101073) Peromyscus sp. Peromyscus cf. leucopus Peromyscus leucopus (SUI 101074) Microtus pennsylvanicus (SUI 101075-77) M. ocrhogaster/pinetorum Microtus ochrogaster (SUI 101078) Microtus pinetorum (SUI 101079-82) Ondatra zibethicus (SUI 101083) Synaptomys cooperi (SUI 101084-87) Zapus sp. (SUI 101088)

Eastern chipmunk Tree squirrel Squirrel Red squirrel

Wooded areas Wooded areas – Hardwood forest, wooded river banks

3 4 5 3

1 2

Deer/white footed mouse White footed mouse White footed mouse Meadow vole

– Timbered areas, wooded river banks Timbered areas, wooded river banks Low damp areas along streams

12 1 2 9

Prarie/woodland vole Prarie vole Woodland vole Muskrat Southern bog lemming Jumping mouse

– 3 Dry grassland, brush 1 Oak–hickory, mixed hardwoods, pines 5 Still or slowly running water 6 Bluegrass in low moist places, bogs 25 Grassy meadows, wood edges 4 near streams

1 4 1 4 1

Odocoileus sp. (SUI 101089-90)

Deer

Forest edge

2

Rodentia D D D/B B W W W B S/D S D W B B

1

1 3

Artiodactyla 3

B, Boreal; D, Deciduous Forest; W, Widespread; S, Steppe; All specimens are housed at the University of Iowa (SUI); SUI numbers are assigned to MNI elements.

P.T. Work et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 204–221

209

120

80

mamma l

bird 40

reptile

amphib crania gill arches premaxillae/maxillae mandibles cheek teeth incisors vertebrae ribs spines scapulae/coracoids humeri radii ulnae carpals/tarsals pelvic bones femura tibiae fibulae calcanea/astagali phalanges long-bone frags. scales miscellanea

0

ian

fish

Fig. 3. NISP (number of individual specimen pieces) for all five vertebrate classes recovered from the Lilienthal site. Fish cranial elements and amphibian bones were recovered frequently in the silts, reptile vertebrate and mammal cheek teeth were common in the gravels.

Percent recovery (Fig. 4) was calculated for mammals by charting the number of expected elements, based on MNI for each taxon, against the Minimum Number of Elements (MNE) recovered (Wolff, 1973). For example, an MNI of 3 Microtus individuals (determined from the presence of three right M1 molars) should correspond, under ideal circumstances, to 6 Microtus femurs (3 left and 3 right). If only 1 of the 6 possible Microtus femurs were actually recovered, then the percent recovery is 17%. Sedimentary particles larger than 1.5 mm were weighed, and the percentage of coarse particles was calculated for each excavation level in order to determine the frequency of various grain sizes and overall sediment volume. Sediment volume for each level then was used to calculate concentration per liter for vertebrate and plant macrofossils (Table 2).

Approximately 1500 plant specimens were categorized as seeds, fruits, twigs, leaves, bud-scales, or miscellaneous, and charcoal fragments were noted. Seeds were counted as whole if two-thirds or more of the specimen were present, or if enough fragments were recovered to produce a bwhole seed.Q Plant materials were assigned to the following categories: aquatic, wetland, trees and shrubs, forest herbs, weeds, and grass. These were then organized into forest, shade-tolerant understory, forest edge, marsh/ streamside, and disturbed ground habitat types (Table 3). Plant taxonomic nomenclature follows Gleason and Cronquist (1991).

4. Age of the deposit A piece of wood recovered from the bulk-pollen matrix near the middle of the section provided a

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80 Percent Recovery

70 60 50 40 30 20 ?

10

Wolff (1973)

bacula

pelves

scapulae

phalanges

radii

metapodials

femora

ulnae

humeri

calcaneaastragali

mandibles

tibiae

cheek teeth

incisors

0

Lilienthal site

Barn Owll (Andrews,1990) Fig. 4. Comparison of percent recovery signatures for micromammal (rodents, hares, and insectivores) skeletal elements averaged from five Pleistocene sites in California (1889 specimens–137 individuals from Wolff, 1973), all mid-Holocene Lilienthal levels (313 specimens–26 individuals), and elements recovered from regurgitated barn owl pellets in England (414 individuals from Andrews, 1990). Note differences in taphonomic signatures between fluvial sites and owl pellet accumulations.

of the Lilienthal site excavation, were 14C dated and bracket the section between 6240 F 50 [B-46211] and 5920 F 60 [B-46212] yr B.P., respectively. These two

conventional radiocarbon date of 6300 F 80 yr B.P. [B-41170; Beta Analytic Inc.]. Two additional in situ wood fragments, recovered from the top and bottom

Table 2 Bone and seed densities (per liter) for total sediment volume recovered from 16 arbitrary levels at the Lilienthal site Levels

Bones

Above the fallen log 1 0 2 0 3 0 4 9 5 23 6 69 7 25 8 28 Total 154 Below the fallen log 9 107 10 92 11 168 12 84 13 204 14 70 15 203 16 90 Total 1018

Seeds 14T 17T 0 0 0 0 0 16 24

115 247 175 269 99 55 103 0 1063

Matrix (l)

Density (bones/l)

Density (seeds/l)

Coarsest % (N1.5 mm)

2.5 8.5 25.5 25.5 51 77 102 128 420

0 0 0 35.3 45.1 89.6 24.5 21.8 36.7

T T 0 0 0 0 0 12.5 5.7

0 0 0 0 0 0 0 0.2

154 154 154 154 154 51 154 76.5 1051.5

69.5 59.7 109.1 54.6 132.5 137.3 132.8 117.7 96.8

74.7 160.0 113.6 174.7 64.3 107.8 66.9 0 101.1

0.2 3.8 10.1 1.4 1.2 2.4 1.6 16.6

T Includes seeds recovered from krotovina sediments—not included in total.

P.T. Work et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 204–221 Table 3 Taxon, habitat preference, catalogue number, and relative abundance for Lilienthal site plant macrofossils Forest taxa Count Acer saccharum (sugar maple) SUI 101098-105 173 s Carpinus caroliniana (hornbeam) SUI 101106-112 21 s Ostrya virginiana (hop–hornbeam) SUI 101113-117 6s Carpinus/Ostrya/cf. Ostrya SUI 101118-121 6s (hornbeam) (hop–hornbeam) Carya cordiformis SUI 101122-126 11 s (bitternut hickory) Juglans cinerea (butternut) SUI 101127-131 6s Juglans nigra (black walnut) SUI 101132-134 5s Juglans cf. nigra (black walnut) SUI 101135 1s Tilia americana SUI 101136-143 473 s (American basswood) Ulmus sp. (elm) SUI 101144-149 56 b Quercus sp. (oak) SUI 101150-154 14 a,s Quercus cf. alba (white oak) SUI 101155-156 2s Shade-tolerant understory herb taxa Bidens vulgata (beggar ticks) SUI 101157-158 2s Eupatorium rugosum SUI 101159 1s (white snakeroot) Laportea canadensis (wood nettle) SUI 101160 1s Menispermum canadense SUI 101161-163 3s (moonseed) Polygonum scandens SUI 101164 1s (false buckwheat) Thalictrum dioicum (meadow rue) SUI 101165 1s Upland shrubs (shady to forest edge habitats) Cornus sp. (dogwood) SUI 101166-173 61 s Cornus stolonifera SUI 101174 1s (red-osier dogwood) Cornus cf. stolonifera SUI 101175 1s Corylus americana SUI 101176-180 15 s (American hazel) Crategeus sp. (hawthorn) SUI 101181-185 30 s Prunus sp. (cherry) SUI 101186 1s Rubus sp. (raspberry) SUI 101187 2* s Staphylea trifolia (bladdernut) SUI 101188-192 7s Zanthoxylum americanum SUI 101193-194 3s (prickly ash) Marsh or stream-side taxa Carex spp. (sedge) SUI 101195-198 18 s Cicuta maculata (water hemlock) SUI 101199 1s Echinocystis lobata SUI 101200 1s (wild cucumber) Glyceria grandis SUI 101201 3s (American mannagrass) Glyceria striata (fowl manna-grass) SUI 101202 20 s Helenium autumnale (sneezeweed) SUI 101203 3s Heracleum lanatum (cow parsnip) SUI 101204 1s Impatiens pallida (touch-me-not) SUI 101205 1s Pilea pumila (clearweed) SUI 101205 6s Polygonum lapathifolium SUI 101207 1s (smartweed)

211

Table 3 (continued) Marsh or stream-side taxa Ranunculus pensylvanicus (crowfoot) Scutellaria sp. (skullcap) Sparganium sp. (bur-reed) Aquatic taxa Ceratophyllum demersum (hornwort) Potamogeton sp. (pondweed) Potamogeton cf. richardsonii (pondweed) Riparian trees Fraxinus pennsylvanica (green ash) Populus sp. (cottonwood/poplar) Salix sp. (willow) Disturbed ground taxa Ambrosia sp. (ragweed) Bidens cernua (nodding beggartick) Chenopodium sp. (goosefoot) Echinochloa crusgalli (barnyard grass) Echinochloa crusgalli (barnyard grass) Setaria glauca (yellow foxtail) Urtica sp. (nettles) Verbena hastata (common vervain) Verbena urticifolia (white vervain) Xanthium sp. (cocklebur) Habitat indeterminate Asclepius sp. (milkweed) Bidens sp. (beggarticks) Circium sp. (thistle) Compositae (ray flowers) Echinacea sp. (coneflower) Fragaria sp. (strawberry) Helianthus sp. (sunflower) Impatiens sp. (impatiens) Polygonum sp. (smartweed) Ranunculus sp. (buttercup) Viola sp. (violets) Vitis sp. (grape)

SUI 101208-210

Count 6s

SUI 101211 SUI 101212

1s 2s

SUI 101213-214

2s

SUI 101215 SUI 101216-218

1s 11 s

SUI 101219 SUI 101220-223 SUI 101224

1s 4b 2t

SUI SUI SUI SUI

39 s 1s 4s 1* s

101225-232 101233 101234 101235

SUI 101236

11 s

SUI SUI SUI SUI SUI

101237 101238 101239 101240 101241-244

11* s 2s 1s 1s 8s

SUI SUI SUI SUI SUI SUI SUI SUI SUI SUI SUI SUI

101245-247 101248-249 101250 101251 101252 101253 101254 101255-260 101261-263 101264-265 101266 101267-274

19 5 1 1 1 1 1 14 13 2 1 1

s s s s s s s s s s s s

a, aborted seed; b, budscale: s, seed; t, twig; *recovered from krotovina sediments; all specimens housed at the University of Iowa (SUI).

dates were inverted, possibly because arbitrary excavation levels bisected cross-bedded sediments or the trees were of different age. Neither sample was degraded by surface exposure. Although the sediments probably filled the depression behind the log in a decade or less, the dates restrict the age of the environment sampled to within a 300-year interval in the middle Holocene.

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tration probably results, in part, from sediment oxidation above level 8. Pollen extracted from the bulk-pollen matrix (6300 F 80 yrs B.P., [B-41170]) indicates a predominance of deciduous tree taxa, with arboreal pollen taxa (AP) accounting for 68.4% of the sample (Fig. 5). Non-aboreal pollen (NAP) accounted for 31.6% of the sample with Ambrosia 12.5% and Chenopodiineae 7.3% as the most common taxa. Plant macrofossils (Table 3) reflect forest, shadetolerant understory, forest edge, and marsh or streamside habitats; disturbed ground taxa were also noted. No taxa indicative of prairie vegetation were present. When the identified tree taxa are plotted on maps using their modern geographic distributions, the area of tree–taxa sympatry (where all species co-occur, Fig. 6A) encompasses portions of the oak savanna, maple–basswood, beech–maple, mixed oak–hickory, and bluestem prairie vegetational areas of Bailey (1981). The northern and northeastern boundary is delimited by Juglans nigra, the southern and southeastern by Tilia americana and Juglans cinerea, and the western by Carpinus caroliniana and Acer saccharum.

5. Lilienthal biota, accumulation, and interpretation The entire Gunder section (levels 1–16) was analyzed collectively because of cross cutting relationships between component sedimentary packages. Below level 8 (Fig. 2), sediments were reduced, contained a mix of stratified sands, pebbly sands, loam, and silt loam, with prevalent organics, and were emplaced around a log obstructing the paleo-channel. Above level 8, the log’s influence was minimized and sediments were oxidized, loamy, unstratified, and contained few organics. Bone fragments were heavily concentrated in thin sandy-gravel lenses behind the log (97 bones/l; Table 2) and were definitely influenced by its presence. Plant macrofossils (101 seeds/l) from this interval were restricted to silty-clay or silty-clay loam lenses interbedded with the coarser bone-bearing sediments. Silts dominated the sedimentary sequence above the log (levels 1–8) and bone and seed concentration per liter dropped to 37 bones/l and 5.7 seeds/l. The decrease in bone concentration directly relates to the change in grain size, whereas lower plant concen-

Pollen percentages

Pollen size

Pinus Ostrya/Carpinus Quercus Ulmus Juglans cinerea Juglans nigra Carya Tilia

Poaceae Chenopodiineae Ambrosia Artemisia Tubuliflorae

Total AP Total NAP Other NAP 0

20

40

60

0

20

40

60

80

Fig. 5. Pollen percentages and pollen grain sizes (Am) for tree, shrub, and herd taxa at the Lilienthal site. AP=arboreal pollen, NAP=non-arboreal pollen. Other NAP includes Apiaceae. Liliaceae, Thalictrum sp., and Vitis sp.

P.T. Work et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (2005) 204–221

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Fig. 6. Areas of sympatry (co-occurrence) for (A) tree taxa, (B), mammal taxa, and (C) combined tree and mammal taxa, for the Lilienthal site. Site location noted in black. The tree sympatry is defined by black walnut on the north and northeast, by American basswood and butternut on the south and southeast, and hornbeam and sugar maple on the west. The mammal sympatry is defined by pine vole on the north and northwest, red squirrel on the south and southwest, and prairie vole on the east (see Tables 1 and 3 for binomial nomenclature).

Vertebrate remains are disarticulated, fragmentary, and clearly have undergone fluvial sorting, despite being near the creek’s headwater region. Evidence for this includes rounding, as well as correlation of bone shape and size with sediment size. Identifiable elements from all five vertebrate classes in descending rank are: mammal (27%), fish (21%), reptile (11%), amphibian (8%), and bird (b 1%). Approximately onethird of the recovered bone could not be determined to element, and, hence, to class (Work, 1998). NISP values show distinctive element selection (Fig. 3). Fish elements are dominated by minnow (cyprinid) cranials, operculars, and vertebrae. Reptile remains are largely snake vertebrae. Amphibians are represented by a variety of frog and toad vertebrae, urostyles, and radio–ulnas. Bird elements are primarily songbird-sized (passerine), although the proximal half of a grouse-sized femur was recovered. Mammal elements are dominated by cheek teeth and vertebrae. Excluding the presence of the deer scapula, which was deposited prior to the placement of the log, the grouse-sized femur (3.2 cm) was the largest single bone in the sample. Forty-three additional bone fragments were recovered with long-axes measuring between 9 and 19 mm. Overall, 96.2% of all bone was below 8 mm in size. Thirteen micromammal species (insectivores and rodents) were recovered along with the proximal end of a Leporidae (rabbit/hare) humerus and an Odocoileus sp. (deer) scapula (Table 1). Representatives of deciduous forest, boreal, widespread, and prairie

centers of distribution are present (Bowles, 1975; Table 1). Twelve of the 15 mammals recovered primarily inhabit wooded areas, especially in association with streamside vegetation. Only the prairie vole, Microtus ochrogaster, known from a single ingested molar indicative of predator activity, inhabits dry grassland or brush situations. The mammal sympatry (Fig. 6B) ranges from east-central Iowa to western Ohio, and from the southeastern edge of Minnesota and southwestern Wisconsin to the southeastern edge of Illinois. The mammal species that delimit the boundaries are Microtus pinetorum on the north and northwest, Tamiasciurus hudsonicus on the south and southwest, and M. ochrogaster on the east. If the prairie vole is omitted, the eastern edge of the sympatry more closely mimics the tree sympatry, with a second area of sympatry occurring in the Appalachians.

6. Taphonomic considerations 6.1. Pollen The area of surface runoff, stream discharge, and level of pollen production influence the quantity of pollen in a stream system, and bias pollen taxa in favor of understory and ground-cover plants (Peck, 1973; Crowder and Cuddy, 1973). This pattern is especially true in forests where reduced wind velocities result in understory pollen settling directly to the

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ground (Bonny, 1978) where it is later subjected to surface runoff and moved into a stream (Andersen, 1970). Flow velocities produced by surface runoff are strong enough to sort the pollen grains hydraulically by size, shape, and weight (Brush and Brush, 1972; Catto, 1985; Chen, 1987; Fisk, 1986; Fall, 1987). Size is the most important factor involved in grain sorting, but grain shape and/or density are involved (Brush and Brush, 1972; Holmes, 1990). Thus, two pollen morphotypes of the same size may be deposited in different sedimentary facies. For this reason, pollen analyses of alluvial deposits can be misleading. Despite these experimental caveats, no sorting problems seem to have influenced the Lilienthal pollen assemblage. Although 6 species of understory herb taxa were recovered as plant macrofossils (Table 3), only 1 genus, Thalictrum, was present in the pollen sample at 0.52% (Fig. 5, Other NAP). In addition, regionally consistent and replicable information has been derived from pollen sequences in the headwater regions of several other Midwest drainage basins including Roberts Creek, and others in southeastern Minnesota and eastern Nebraska (Chumbley et al., 1990, Baker, 2000, Baker et al., 1996, 1998, 2000, 2001, 2002). The Lilienthal site is characterized by a high percentage of arboreal taxa (Fig. 5). The high percentage of Quercus (oak) (47%, 180 out of 383 grains) accounts for the high percentage of arboreal pollen (68.4%). Although flume simulations by Brush and Brush (1972) indicate that oak pollen is incorporated more readily into fluvial sediment than most, oak pollen percentages at Lilienthal are well within the range of mid-Holocene oak pollen percentages from lakes and bogs in Wisconsin and Minnesota (Winter, 1962; Maher, 1982; Winkler, 1988) 6300 years ago. This suggests that oak pollen percentages in these Midwestern fluvial systems are representative. Preferential sorting does not appear to be a factor at the Lilienthal site, as the bulk-pollen matrix included a mix of clay, silt, sand, and fine gravel which contained a range of AP and NAP pollen grain sizes (Fig. 4). In addition, Chumbley et al. (1990) documented a decrease in arboreal pollen percentages at Roberts Creek and at Money Creek in southeastern Minnesota at ~5400 yr. B.P., a time which was independently determined by Dorale et al. (1992) to represent a regional shift from forest to prairie, based on cave

isotopic evidence. During this interval, there was no noticeable change in the sediment load at either Roberts Creek or Money Creek, as would be expected if grain size was the primary force dictating the preferential preservation of pollen grain types. This suggests that the headwater deposits of Midwestern creek sites accurately reflect changes in regional vegetation. It may be that the short distance to the head of the drainage basin is not sufficient for significant sorting to occur, or rapid sediment deposition insures minimal sorting. 6.2. Plant macrofossils Plant macrofossils often accumulate upstream from coarse debris. The type of debris entering a fluvial system directly relates to the local vegetation (Keller and Swanson, 1979; Bilby and Likens, 1980). The relative abundance of debris input versus that preserved in alluvium is controlled by the size of the stream and the vegetation, and it differs throughout the system. Larger debris is carried more easily out of the system in a wider stream (Keller and Swanson, 1979). Small, low gradient stream channels in forests (e.g., Mud Creek) accumulate large branches, tree limbs, and logs, primarily through bank collapse, mass wasting, blow-down, and ice loading (Sigafoos, 1964; Keller and Swanson, 1979). The presence of large debris influences channel morphology and sediment transport because small streams have insufficient discharge to redistribute this material (Keller and Swanson, 1979; Trimble, 1997). Thus, debris concentrations are generally high in small headwater streams and decrease downstream. Streambed topography also plays a role because erosion in shallow forest-lined streams exposes tree roots and other large obstacles that trap transported vegetation. Bilby and Likens (1980) and Bilby (1981) present a depositional model that directly applies to plant macrofossil accumulations at the Lilienthal site. When a large limb falls into a stream, it eventually lodges in the channel. Smaller sticks and branches collect against this obstacle and, in turn, provide a framework on which leaves and other organic material accumulates. Ultimately the structure becomes a nearly watertight debris dam resulting in development of an upstream pool where the capacity of a stream to

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transport sediments is dissipated (Heede, 1972; Fisher and Likens, 1973). At the Lilienthal site, a debris dam of branches and small inter-meshed twigs, seeds, and leaf mats was exposed during excavation around the log. The variety and delicate nature of plant material preserved suggests minimal sorting or abrasion by the current. As plant remains entered the creek, floating material was emplaced against the obstruction, while waterlogged debris settled to the bottom of the eddy or pool. Pulses of sediment covered and preserved trapped organic debris. Although debris dams are especially effective traps for plant macrofossils, plant debris also accumulates in other alluvial settings (e.g., Baker et al., 1996, 1998, 2000, 2001, 2002; Strickland, 1998). Plants are fundamentally different from most other organisms because they may shed material at any time (Spicer, 1989; Burnham, 1993). At the Lilienthal site, plant remains reflect the local biota because it is close to the headwaters of the stream and heavier twigs, nuts, seeds, leaves, and other debris cannot have traveled far from the source vegetation (Bilby, 1981; Bilby and Likens, 1980; Ferguson, 1985; Burnham et al., 1992). Burnham (1993) suggests that delicate in situ materials indicate minimal reworking. At Lilienthal, the microstratigraphy and interbedding of delicate plant materials (e.g., Acer samaras with wings) indicates minimal reworking, and also suggests that the Lilienthal deposits result from material shed nearby. The bark, log, and larger branches could have been eroded from an upstream bank and redeposited, but none of this material is abraded or corroded significantly. Thus, all evidence indicates that the plant material was local in origin, minimally transported, and deposited shortly after entering the stream. 6.3. Vertebrates Microvertebrate bones were recovered from both silt and gravel lenses interbedded with botanically rich silty-clay units. Amphibian and fish bones were observed more frequently in silts, whereas rounder, denser elements, such as mammal teeth and reptile vertebrae, were concentrated in the sands and gravels. This pattern suggests that skeletal elements were

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carried both in suspension and as bed-load. Dobson (1973) suggested that waterlogged amphibian and mammal bones travel with the bed-load while buoyant, desiccated bone floats. In laboratory tests, dry mammal bones became saturated in several days and sank, but the amphibian radius–ulna, femur, and tibia remained partially buoyant after a month (Dobson, 1973). Therefore, buoyancy could protect finer elements from destruction, increase their dispersal range, and result in their inclusion in finer-sized (lower-energy) alluvial sediments. At Lilienthal, all recovered skeletal elements, with the exception of a deer scapula located on the basal unconformity, were smaller than 3.2 cm. Moreover, 96.2% of the elements were below 8 mm in size, and were hydrodynamically equivalent in size and shape to the sediments in which they were deposited. Although fossil bone recovered from creek alluvium is often observed to follow Voorhies (1969) dispersal groups, as expanded by Korth (1979); Behrensmeyer (1975), and Dobson (1973), these experiments were conducted on complete bones, and their hydrodynamic properties do not apply to the fragmented Lilienthal sample. Depositional sorting (Wolff, 1973) would be the dominant taphonomic process at this site, because the small rounded skeletal elements behave as rounded sedimentary particles. The taphonomic signature created by plotting percent recovery of the Lilienthal micromammal elements compares well to those from other fluvial bone-bearing deposits, regardless of geographic location (Wolff, 1973), and is distinct from signatures created from other taphonomic pathways, such as owl pellet accumulations (Andrews, 1990). Fig. 4 compares the taphonomic signature of the Lilienthal site, five alluvial sites from Rodeo California (Wolff, 1973), and a raptor (owl) from England (Andrews, 1990). For taphonomic signatures created by processes, geographic locations are of minimal concern. Both the percent recovery (Fig. 4) and the fragmentary condition of individual skeletal elements from Lilienthal (10%), with the exception of incisors, are similar to those reported from Wolff’s Late Pleistocene alluvial sites (12%). In both sites, teeth are disproportionately well represented while fragile or small elements, such as phalanges and bacula, are under represented. Entire limb-bones are rare. For example, only proximal or distal ends of tibae and

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humeri were preserved and mandibles were usually fragmentary. At Wolff’s Rodeo sites, skull bones were rare, either because of sorting or an inability to recognize fragmentary elements. Cranial elements at Lilienthal were recovered but were highly fragmented. Wolff’s (1973) Rodeo micromammal bone elements were concentrated in interbedded fine gravel (4 mm average diameter) and silt facies interpreted as deltaic distributory channel deposits. The Lilienthal micromammal skeletal elements were concentrated in interbedded gravel (4–5 mm diameter) and silt facies derived from the channel’s headwater region. Although the overall geomorphologic and geographic settings are quite different, the fragmentary condition and percent recovery between the two systems are quite similar, indicating that these elements respond similarly in fluvial settings. Wolff’s (1973) Rodeo sites contained more coarse material (6– 7%) between the 1–4 mm grain size, and exhibited a slightly higher percent recovery for nearly all identified skeletal elements. The lower number of identified skeletal elements at Lilienthal may result from either the lower percentages of coarse material (2.4% by weight) or from a smaller source area (e.g., headwater vs. a much larger lowland catchment area). The larger catchment area that yielded Wolff’s (1973) Rodeo sample also may be responsible for the larger number of bone fragments overall, 3400 vertebrate elements recovered from ~1300 kg of sediment (2.6 fragments per kg) versus 1165 bone fragments recovered from 1588 kg of sediment (0.7 fragments per kg) at Lilienthal. The similarity in grain size as well as the condition of the bone at both localities, however, suggests that rounded skeletal elements were transported like any other clast of similar size and shape. Blob and Fiorillo (1996), however, noted that microvertebrate fossils located in similar sedimentary facies could still exhibit strikingly different taphonomic profiles if a specific grain-size unit was not sampled. To achieve maximum species concentration in a vertebrate assemblage (micro or mega) one should collect from contemporaneous sediments that vary in grain size. In addition, comparisons of paleofaunal abundances should be restricted to sites with both similar sedimentary facies and similar profiles of fossil sizes and shapes (Blob and Fiorillo, 1996). The similarity in percent recovery, as well as the fragmentary character of the micromammal skeletal

elements recovered from both the Lilienthal and Rodeo sites also provides information about their durability and transportability. Korth (1979) quantified the action of sedimentary particles on micromammal bone through the use of a rotary tumbler. He tumbled bones from two mouse-sized mammals for 80 h in a matrix of 2–4 mm diameter quartz grains. During this process the skull bones became disarticulated and teeth were lost from the jaws, skull bone elements and scapulae became perforated or eroded along the edges, pelvic elements were broken apart, and the ischium was eventually eliminated. Longbone shafts were worn through, but only after the limb-bone ends had been rounded extensively. Korth (1979) also found that the major tarsal elements (i.e., calcanea and astragali) showed little evidence of abrasion, while the phalanges and metapodials were almost impossible to recover. Korth’s (1979) study included the full spectrum of fractured elements recovered from both the Rodeo (Wolff, 1973) and Lilienthal sites. Subsequent experiments by Andrews (1990) using larger size clasts increased the rate of breakage and wear for small mammal bones, but both he and Korth (1979) concluded that the nature of the breakage is controlled by the structural properties of the skeletal elements themselves. Preservation is related directly to an element’s durability, as well as to the grain size of the enclosing matrix. Vertebrate elements, unlike delicate plant macrofossils, can be transported great distances with little evidence of wear (Korth, 1979; Behrensmeyer, 1982; Graham, 1993). In addition, once bones are fragmented it is difficult to determine how far they were transported. Thus, a temporally and spatially mixed assemblage could go undetected, particularly when amalgamated into a concentrated deposit. The small catchment area above the Lilienthal site (~30 km2) limits the potential extent of transport and reworking of the vertebrate assemblage by the creek system. A small catchment area does not, however, prevent the introduction of non-local elements by far-ranging predators which can affect species richness and skew interpretations of the local paleoenvironment. Small mammal bones frequently accumulate through the activities of raptors and other carnivores (Mellet, 1974; Andrews and Evans, 1983; Andrews, 1990) that can range up to 35 km, or more, from their home site.

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Nevertheless, at the Lilienthal site only 9 of 1165 bone fragments (0.8%) show digestive etching, green bone fracture, or other clear evidence of ingestion by a carnivore. These include 1 fish vertebrae [SUI 101091], 2 teeth and 1 tooth fragment from Microtus sp. [Rm1, SUI 101092; LM2, SUI 101093; and misc., SUI 101094], and 5 teeth from 4 microtines: Microtus ochrogaster [Lm1, SUI 101078], Microtus pennsylvanicus [RM2, SUI 101095], M. pinetorum [Lm1, SUI 101077], and Synaptomys cooperi [LM2, SUI 101096; and RM3, SUI101097]. Distinct evidence for predator activity in a fluvial setting is difficult to discern, however, because digestion may not modify a bone, or it could do so in a similar manner to stream abrasion. Teeth and foot bones are the most likely elements to survive either ingestion or fluvial transport, while thin or fragile elements and parts become broken, rounded, or destroyed (Fig. 4) in either process. As noted by Korth (1979), the original source of microvertebrate fossil accumulations may be through fecal material or regurgitated owl pellets (Fig. 4), but subaerial action, such as rainfall or stream flow even over a very short distance, will mask this signal. Some of the breakage and rounding in the Lilienthal material undoubtedly results from the actions of predators, but only 9 bone fragments show clear evidence for this type of activity. Temporal and spatial mixing of vertebrate material within a deposit may be equally difficult to detect, but precise stratigraphic control as well as species compatibility can indicate the presence or absence of mixing. Temporal mixing through redeposition of older materials is apparent when fossils of very different geological ages or environments co-occur. Cretaceous shark teeth in the Lilienthal mid-Holocene gravels obviously were reworked. Also, the vertebrate material has been sorted extensively by size and density (e.g., small dense rounded fragments were recovered from gravel, thin flat elements were recovered from silts). However, the headwater landscape reduces the spatial extent of the area sampled, and the Paleozoic and shark-tooth-bearing Pleistocene till substrata distinctively limits the temporal sources from which materials could be reworked into preGunder and Gunder sediments. The interbedding of the vertebrate fossil material with the fragile plant macrofossils (e.g., Acer samaras) indicates that the sediments containing the vertebrate elements were

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emplaced almost simultaneously with the sediments containing these plant macrofossils. Moreover, the presence of delicate plant macrofossils indicates that sediment reworking did not occur after their emplacement at Lilienthal. Skeletal elements could have been reworked from the earlier Holocene Gunder and/or late-glacial preGunder members of the DeForest Formation prior to their redeposition with the plant macrofossils. If preGunder sediment was reworked into the Gunder fossil assemblage, any dborealT vertebrate recovered from the Lilienthal site would be suspect. Many mammals with a boreal center of distribution today, however, range into northern deciduous forest habitats to varying degrees and vice-versa. Moreover, the southern boundaries of these species have not been static during the Holocene (Semken, 1983). For example, the red-backed vole (Clethrionomys gapperi), which now has a southernmost range in Minnesota, was known historically from extreme north-central Iowa and was recovered as a fossil from ~4300 yr B.P. deposits at the Rock Run rock shelter in Cedar County, Iowa (Semken, 1983). There is, however, no evidence for the reworking of abundant and resistant boreal plant macrofossils, such as spruce needles, or pollen in Lilienthal sediments, which are known to be prevalent in the preGunder Member. In addition, both the mammal and plant assemblages document identical communities and habitats: forest, forest understory, and marsh/stream-edge. Therefore, The Lilienthal biota appears representative of the mid-Holocene local fauna deposited circa 6000 yrs. B.P.

7. Paleoecologic overview The Holocene Lilienthal biota is paleoecologically important for two reasons. First, the biota was not associated with any archaeological material, a rarity for Holocene vertebrate collections. This removes the human taphonomic factor from the interpretation of Late Quaternary climatic change, paleoecology, and development of modern community structure. Second, the site contains pollen, plant macrofossils, and micromammals, which are rarely recovered from the same deposit, and thus permits evaluation of paleoecological interpretations from three lines of fossil

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evidence. Each line of evidence suggests the presence of a mid-Holocene mature mesic deciduous forest in the headwater region of Mud Creek 6000–6300 radiocarbon years ago. This was a period when the prairie was best developed and water tables were at their lowest in northwest Iowa (Van Zant, 1979). A marsh or stream-edge community paralleled the creek banks. Wetland or stream-edge taxa included bur-reed, sedge, crowfoot, fowl manna-grass, touch-me-not, water hemlock, clearweed, and smartweed. Plant macrofossils and pollen data indicate that a variety of trees including oak, bitternut hickory, hornbeam, butternut, black walnut, hop–hornbeam, basswood, elm, and sugar maple grew in the surrounding woodland. Characteristic shade-tolerant understory plants included prickly ash, meadow rue, bladdernut, white snakeroot, wood nettle, and beggar ticks. These tree canopy and understory species are typical of oak– hickory communities of the eastern deciduous forest. The abundance of oak pollen, along with the fairly high percentages of non-arboreal pollen, suggests that savanna or open woodland also was present in the area. Lastly, disturbed ground and wetland taxa are typical components of creek floodplains, where the soil is moist and disturbance is common. Twelve of the 15 mammals recovered at the Lilienthal site are characteristic of forest habitat, especially where marsh or stream-edge vegetation is present (Table 1). The masked shrew, short-tailed shrew, eastern mole, eastern chipmunk, tree squirrel, red squirrel, woodland vole, and deer prefer forest, forest edge, or woodland; the meadow vole, whitefooted mouse, southern bog lemming, and jumping mouse prefer forest or woodland settings near streams banks, meadows, or other low moist places. The muskrat prefers still or slowly running water while the rabbit/hare is a wide-ranging taxon found in a variety of habitats including meadows, grasslands, thickets, or woods. Only the prairie vole indicates that either prairie or parkland was present in the area. This vole is known at the site by a single molar which, based on external etchings, passed through the digestive track of a predator. It’s presence in the region is significant because prairie is recorded at 5800 yr B.P. at an upland location within the Indian Creek Nature Center preserve (Fig. 1), approximately 70 km northeast of the Lilienthal site. This upland prairie would have been within reach of a variety of Mud Creek basin

avian predators (home range up to 90 km2) and reaffirms that prairie or parkland habitat was available to Mud Creek predators at 6300 yr. B.P. However, small patches of prairie may have been present on dry upland sites nearby. The mammalian-and-tree sympatry diagrams are resolvable, resulting in a slightly reduced overall area (Fig. 6C) with respect to that for each group. While the vegetational groups of Bailey (1981) encompassed by this sympatry include oak savanna, maple–basswood, beech–maple forest, mixed areas of oak–hickory, and bluestem prairie, closed forest species predominate in the mid-Holocene deposits at Lilienthal.

8. Conclusions A) The vertebrates of the Lilienthal biota are one of very few Holocene micromammal associations that has not been associated with an archaeological site. It adds substantially to a meager vertebrate paleoecological data set suitable for comparison with glacial/post-glacial climatic change, paleoecology, and development of modern community structure. B) Plant macrofossils, mammals, and pollen recovered from the Lilienthal cutbank site along Mud Creek yielded similar ecological interpretations and a resolvable area of sympatry, which suggests that each data set produces compatible and, therefore, reliable results. Because of taphonomic processes, micromammals rarely have been reported in association with wellpreserved plant remains. The combined treeand-mammal sympatry ranges from eastern Iowa to western Ohio, and from the southeastern edge of Minnesota and southwestern Wisconsin to the southeastern edge of Illinois (Fig. 6C), and includes portions of oak savanna, maple– basswood, beech–maple, mixed oak hickory, and bluestem prairie vegetational units as defined by Bailey (1981). C) Between 5920 and 6300 yr B.P., the time of presumed maximum prairie expression in western and central Iowa, the upper Mud Creek basin was dominated by a mature, mesic deciduous forest in conjunction with a marsh or stream-edge community.

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D) Although pollen are known to be sorted hydrodynamically, this process was apparently not important at the Lilienthal site as pollen percentages are well within the range of other regional pollen values at ~6000 yr. B.P. from more frequently sampled non-fluvial deposits. E) In the Lilienthal deposit, fossil plants and animals traveled different taphonomic pathways. The plant macrofossil material (1) was transported either as floating or suspended material, (2) underwent minimal hydrodynamic sorting, (3) was locally derived, and (4) was minimally reworked. The vertebrate material (1) was transported as bed-load and suspended elements, (2) was hydrodynamically sorted, and (3) could contain reworked elements but not from a great distance in either time or space. Both plant macrofossils and vertebrates provide equally good paleoecologic information. Here, plant macrofossils are less prone to the fluvial processes of temporal mixing, spatial mixing, and depositional sorting.

Acknowledgements This paper benefited from constructive reviews by B. van Geel and an anonymous reviewer.

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