Retallack Et Al. 2013 Horodyskia

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Precambrian Research 226 (2013) 125–142

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Precambrian Research

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Problematic Mesoproterozoic fossil Horodyskia from Glacier National Park,

Montana, USA

a,∗ b b

Gregory J. Retallack , Kimberley L. Dunn , Jennifer Saxby

Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA

Ocean and Earth Science, University of Southampton, Waterfront Campus, Southampton SO14 3ZH, United Kingdom

a r t i c l e i n f o

a b s t r a c t

Article history: String-of-beads fossils (Horodyskia moniliformis and Horodyskia williamsii) from the 1.48 Ga lower

Received 31 July 2012

Appekunny Argillite of Glacier National Park have been re-examined, and collected from both scree,

Received in revised form

which yielded most prior specimens, as well as outcrops. The fossils come from laminated silty shales

20 November 2012

and carbonaceous-swirl shales, with local sandstone paleochannels, interpreted as a very shallow lake

Accepted 19 December 2012

margin. Very weakly developed paleosols also are present, but do not contain Horodyskia, which lived in

Available online 31 December 2012

very shallow water, seldom exposed and rilled. Chemical index of alteration at horizons with Horodyskia

are evidence of a warm temperate to subtropical humid paleoclimate, unlike arid and cool paleoclimates

Keywords:

Horodyskia at other stratigraphic levels in the Belt Supergroup. Thin section examination reveals that the beads are

Montana associated with a system of tubes, including connecting strings, and other tubes radiating outward from

Mesoproterozoic each bead. Partial burial and branching of these tubes may be evidence of a benthic sessile life style.

Appekunny Argillite A variety of explanations for Horodyskia are falsiﬁed by our new observations: including pseudofossil,

String of beads fossil dubiofossil, prokaryotic colony, foraminifera, slime mold, puffball fungus, brown alga, sponge, hydrozoan

or bryozoan colony, or metazoan fecal string. Our remaining working hypothesis is that Horodyskia beads

were endolichen bladders, comparable with living Geosiphon pyriformis (Archaeosporales, Glomeromy-

cota, Fungi), which has heterocystic cyanobacterial photosymbionts (Nostoc punctiforme). This hypothesis

is not without problems, because bladders of Geosiphon are mostly erect and clavate, but beadlike only in

early growth stages, form clusters or close strings rather than elongate strings, and are terrestrial rather

than aquatic. Nevertheless this new hypothesis for Horodyskia is compatible with what little is known

about fungal evolution, and testable by additional studies of its paleoenvironments and associated fossils.

1. Introduction et al., 2007; Dong et al., 2008), and 0.51–0.42 Ga in northern India

(Mathur and Srivastava, 2004; Kaufman et al., 2006), A part of the

The Mesoproterozoic string-of-beads fossil Horodyskia remains problem is a deceptively simple inferred morphology of spherical

an enigma, as can be appreciated by the diversity of afﬁnities or ovoid structures (beads) on a slender tubular structure (string:

proposed: pseudofossil (Hofmann, 1992), dubiofossil (Horodyski, Fedonkin and Yochelson, 2002; Fedonkin, 2003). Another problem

1982, 1993), prokaryotic colony (Knoll et al., 2006), foraminifer has been lack of detailed information about their sedimentary envi-

(Dong et al., 2008), brown alga (Grey and Williams, 1990), sponge ronment (Martin, 2004). This ﬁeld and petrographic study has been

(Hofmann, 2001), hydrozoan or bryozoan colony (Fedonkin and designed to gather additional details of their micromorphology

Yochelson, 2002), or metazoan fecal string (Yang and Zheng, and sedimentary context relevant to understanding the biological

1985). This enigma is compounded by its wide distribution in afﬁnities and ecological role of Horodyskia.

space and time: 1.48 Ga in Montana (Evans et al., 2000; Fedonkin

and Yochelson, 2002; age model herein), 1.47–1.07 Ga in West-

ern Australia (Grey et al., 2010), 1.3–0.8 Ga in Tasmania (Calver

2. Materials and methods

et al., 2010), 0.65–0.51 Ga in three separate terranes of China (Shen

This investigation includes observations from three separate

localities in Glacier National Park (Fig. 1), and new collections

∗ (curated by Dierdre Shaw in the national park headquarters at West

Corresponding author. Tel.: +1 541 346 4558; fax: +1 541 346 4692.

Glacier).

E-mail address: [email protected] (G.J. Retallack).

126 G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142

Fig. 1. Localities in Glacier National Park, Montana, USA, examined for this study (red open circles): (A) Rising Sun; (B) Appekunny Falls; (C) Two Medicine. (For interpretation

of the references to color in this ﬁgure legend, the reader is referred to the web version of the article.)

2.1. Appekunny Falls outcrops of greenish gray to brown Appekunny Argillite are not

more than 5 m stratigraphically above the Altyn Limestone. Very

This locality of Horodyski (1982) is in scree and a low bench of poorly preserved fragments comparable with H. williamsii were

◦

rock immediately above the scree 800 m N13 W from that point on seen here, but not collected for this study.

the hiking track when it reaches a level near the top of Appekunny

◦ ◦

Falls (N48.81884 W113.64379 ). Numerous fossils of Horodyskia 2.3. Two Medicine

were found in place within a measured section only 2 m thick

(Figs. 2 and 3A,B). Many specimens were found in scree below Small rock benches of argillite on a low ridge 200 m on the

this stratigraphic level, but none above it. The 2 m section shown south ﬂanks of Rising Wolf Mountain southwest of the bridge across

◦

in Fig. 2 is the only known source of Horodyskia in this area, and Two Medicine Creek at Two Medicine campground (N48.94040

◦

includes the thin white sandstone marker of Whipple et al. (1984), W113.66849 ) also yielded specimens of Horodyskia (Fedonkin and

53 m stratigraphically above the top of the Altyn Limestone. All Yochelson, 2002). Here the fossils are in black argillite very differ-

specimens illustrated in this paper are from this locality, including ent from the Rising Sun and Appekunny Falls locality. Stratigraphic

Horodyskia moniliformis (Figs. 4D,E, 5A–E, 6 and 7E–H), Horodyskia position is difﬁcult to estimate on this forested spur, but judg-

williamsii (Figs. 4A–C and 7A–D), large concentric structures (sand ing from comparable laterally impersistent black argillites seen

stromatolite: Fig. 4F), carbonaceous fragments (Lanceoforma stri- in surrounding cliffs this locality is some 20 m above the top of

ata: Figs. 4F and 5A,C), and “old elephant-skin” crust (Rivularites the Altyn Limestone. The holotype of H. moniliformis (Fedonkin

repertus: Fig. 5F). The two species of Horodyskia are distinguished and Yochelson, 2002, Fig. 1) was collected here, as well as another

largely on the basis of bead size: 1–3 mm for H. williamsii and specimen regarded here as H. williamsii (Fedonkin and Yochelson,

2–10 mm for H. moniliformis following Grey et al. (2010) and Calver 2002, Fig. 13a). Reference specimens of each species were collected

et al. (2010). from this locality as part of this work (GLAC24701 and GLAC24702,

respectively).

2.2. Rising Sun

2.4. Laboratory methods

Fedonkin and Yochelson (2002) collected specimens from road

cuts 1 km west of Rising Sun Campground on Going-to-the- Specimens of Horodyskia were measured with digital calipers,

◦ ◦

Sun Highway (N48.81463 W113.64104 ). These small overgrown and also studied in petrographic thin sections. These specimens

G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142 127

Retallack, 2001) with constants for marine shale gives compaction

to 42% of former thickness and with constants for marine clayey

sand gives 52%. These can be taken as extremes for Horodyskia beads

measured in thin sections of clayey siltstone at up to 1.4 mm thick,

which were thus some 2.6–3.4 mm thick originally.

Fracture cleavage, chlorite and illitized clay of the Appekunny

Formation is evidence of lower greenschist facies metamorphism

(González-Álvarez and Kerrich, 2012). Metamorphism has severely

degraded the quality of organic matter preservation in Horodyskia,

as for other organic remains in the Belt Supergroup (Walter et al.,

1976; Kidder and Awramik, 1990). This regional metamorphism

was mainly Grenvillian in age (Stenian or late Mesoproterozoic:

Gradstein et al., 2004), judging from U–Pb ages from metamor-

phic titanites of 1090–1030 Ma in tholeiitic Moyie sill intruding

the Aldridge Formation, low in the Purcell (=Belt) Supergroup in

Canada (Anderson and Davis, 1995). In the region of highest ther-

mal alteration of Belt Supergroup to amphibolite metamorphic

facies in central Idaho, Lu–Hf dating of garnets reveals metamor-

phic episodes at 1463 ± 24, 1379 ± 8, and 1151 ± 41 Ma, as well

as Grenvillian episodes at 1081± 20 to 1064 ± 10 Ma (Zirakparvar

et al., 2010).

3.2. Geological age

Horodyskia is early Mesoproterozoic (Calymmian: Gradstein

et al., 2004), as suspected from stromatolite biozones of Collenia

in the Altyn Limestone and Collenia and Conophyton in the Helena

Formation (=“Siyeh Limestone” of Fenton and Fenton, 1937; Rezak,

1957; Ross, 1959). Carbonaceous algal compressions and acritarchs

in the Belt Supergroup do not yet have temporal signiﬁcance

(Walter et al., 1976, 1990; Kidder and Awramik, 1990).

The age of the Belt Supergoup is more precisely constrained

by SHRIMP zircon U–Pb ages of 1454 ± 9 Ma for a tuff in the

upper Helena Formation, 1443 ± 7 Ma for rhyolite in the Snowslip-

Shepard Formation, and 1401 ± 6 Ma for tuff in the uppermost

Bonner Quartzite (Evans et al., 2000). Taking correlative levels for

these three ages in Glacier National Park as 2450, 2750, and 4400 m

respectively, and a general section as in Table 1, gives an linear

model of age (y) versus stratigraphic level (x) according to the for-

− 2

mula: y = 0.0266x + 1517.8 (r = 0.99). This equation gives an age

Fig. 2. Detailed stratigraphic section of strata yielding Horodyskia at the Appekunny

Falls locality, Glacier National Park, Montana. of about 1480 Ma for the Horodyskia horizons at all three locali-

ties: Appekunny Falls at 1453 m, Rising Sun at 1405 m, and Two

Medicine at 1420 m in the general section (Table 1). These ages

have little organic matter or opaque coating to contrast with their

assume uniform sediment accumulation, and are compatible with

matrix, so are not suitable for X-ray microtomography, as used by

U–Pb zircon ages of 1469 ± 2 Ma for basaltic Moyie sills intruding

Huldtgren et al. (2011). One billet was selected for serial grinding in

the Aldridge Formation in the lower Purcell Supergroup in Canada

order to obtain a three dimensional understanding of the beads, an

(Anderson and Davis, 1995), and of 1469 ± 2.5 and 1457 ± 2 Ma

approach comparable with that of Watters and Grotzinger (2001).

for basaltic Plains and Paradise sills, respectively, intruding the

The matrix proved soft, so was ground in 120 grit for only 10 min

Prichard Formation in the lowest Belt Supergroup in western Mon-

before polishing and photographing. Ten successive images were

tana (Sears et al., 1998).

obtained by grinding and polishing through 4.76 mm thickness of

billet for equal durations of time. Selected beads in the images were

3.3. Sedimentology and depositional environment

outlined and these outlines were stacked in the computer program

ImageJ (freeware from U.S. National Institutes of Health) in order

At all localities, Horodyskia is found in laminated shales in the

to obtain a three dimensional rendition.

lower part of the Appekunny Argillite, a thick shaley unit between

very shallow water carbonates (Altyn and Helena formations) with

3. Geological setting stromatolites (Table 1). Stromatolites and shales of the Belt Super-

group have been considered marine (Fenton and Fenton, 1937;

3.1. Stratigraphy and metamorphism Fedonkin and Yochelson, 2002), but there is now growing evidence

that much of the Belt-Purcell Basin was an intracontinental rift

Horodyskia is widespread in the lower part of the Appekunny basin with large lakes and braided streams (Winston et al., 1984).

Argillite, a thick sequence of shallow-dipping, green and gray meta- This is compatible with the very sparse distribution of pyrite around

morphosed shales, overlying stromatolitic Altyn Limestone, and Horodyskia (Fig. 3D,E), and lack of glauconite in thin section (also

underlying red beds of the Grinnell Formation. The Appekunny noted by Fedonkin and Yochelson, 2002).

Argillite was buried by at least 3.4 km of sedimentary rocks Sedimentary rocks of Horodyskia are its likely life environ-

(Table 1), so that using standard compaction curves (Sheldon and ment because of a variety of evidence that these organisms were

128 G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142

Table 1

Stratigraphic section of Belt Supergroup in Glacier National Park.

Glacier National Park Equivalents in Big and Little Belt Mountains Description Level (m)

(Eroded) Pilcher Quartzite Cross bedded arenite with white/purple cross laminae (Eroded)

Garnet Range Formation Garnet Range Formation Dark greenish gray, micaceous arenite and argillite (Eroded)

McNamara Formation McNamara Formation Cherty red and green argillite 4800

Bonner Formation Bonner Formation Pink to purple, cross-bedded arenite, and red argillite 4450

Mount Shields Formation Mt Shields Formation Red, green, black argillite, pink arenite, tan carbonate 4200

Shepard Formation Shepard Formation Dark argillite and tan-weathering laminated carbonate 3300

Purcell Lava (No equivalents) Dark gray pillow basalt, minor argillite, carbonate 2950

Snowslip Formkation Snowslip Formation Red and green argillite and red arenite 2750

Helena Formation Helena Formation Light to dark gray stromatolitic limestone 2500

Empire Formation Empire Formation Green wavy bedded rippled argillite with gray arenite 2000

Grinnell Formation Spokane Formation Red quartzite and argillite 1850

Appenkunny Argillite Greyson Shale Dark shale and wavy laminated silty shale 1400

Altyn Limestone Newland Limestone Light to dark gray stromatolitic limestone 450

(Covered) Chamberlain Shale Black shale with lenticular sand lenses 0

(Covered) Neihart Quartzite Coarse-grained cross-bedded orthoquartzite (Covered)

Note: Data from Ross (1959) and Winston and Link (1993).

sessile, rather than redeposited: ﬁlamentous structures radiat- seldom exposed. The Two Medicine locality is in silty shale facies

ing from beads cutting down through laminae, non-overlapping of Schieber (1989, 1990), but dark gray and silty to shaley, rather

strings, and even spacing of beads (Fedonkin and Yochelson, 2002; than sandy greenish gray and red. The Two Medicine locality may

Grey et al., 2010). This study found Horodyskia near Appekunny Falls represent a deeper lacustrine paleoenvironment. Such variation in

on carbonaceous laminae of the carbonaceous-swirl shale facies, water depth is comparable with the subtidal to intertidal paleoen-

and also within laminae of the silty shale facies of Schieber (1989, vironments envisaged for Horodyskia in Western Australia, where

1990). As noted by Fedonkin and Yochelson (2002), who found the sedimentary environments are considered marine rather than

low amounts of iron and manganese in these rocks, the ubiquitous lacustrine (Grey et al., 2001, 2010; Martin, 2004; Calver et al., 2010).

opaque laminae are carbonaceous, with scattered opaque miner- A humid tropical paleoclimate has been interpreted for the

als including illmenite and magnetite, but no hematite or goethite. lower 60 m of the Appekunny Argillite from a chemical index of

There is one red and gray sandstone body, 45 cm thick, with trough alteration of 77–86, but a swing back to an index of 62 and thus

cross bedding, in the middle of the measured section (Fig. 2). drier and cooler climate is found above 60 m (González-Álvarez and

Comparable red sandstones and mud-cracked red siltstones of the Kerrich, 2012), where Horodyskia disappears. The lack of calcareous

Appekunny Formation are found elsewhere in Glacier National Park nodules or evaporate crystal casts associated with Horodyksia, but

(Fenton and Fenton, 1937). There also are small-scale (2–3 cm) cut- their abundance at higher stratigraphic horizons, such as the Mt

and-ﬁll intervals (Fig. 8D,E), and some intervals of climbing ripples Fields Formation (Fenton and Fenton, 1937; Ross, 1959; Winston

(Fig. 2). Another distinctive structure is thin stringers of silt-sized and Link, 1993), is additional evidence for a paleoclimatic change

quartz and feldspar (Fig. 8B,C), each one to two grains thick, and from humid to arid. Hot humid paleoclimate is compatible with

overall dominance of silt (Fig. 9F). paleomagnetic evidence for Belt-Purcell deposition at a paleolati-

◦ ◦

These facies of the Appekunny Formation are very similar to oil tude of 10 to 35 (Chandler, 2000; Leach et al., 2010).

shales of the middle Eocene Green River Formation at fossil insect

and plant localities of the Piceance Basin, Colorado (Surdam and 4. New observations

Stanley, 1979; Grande, 1984). Notably absent in both Appekunny

and Green River Formations are sedimentary structures, such as In addition to new observations of the geological occurrence

ﬂaser and lenticular bedding, typical for muddy marine tidal ﬂats of Horodyskia (Figs. 2 and 3), our new collections allow the

(Reineck and Singh, 1973). following new petrographic and structural observations, not only

The silty shale and carbonaceous-swirl shale facies are the shal- on Horodyskia, but also on associated fossils.

lowest aquatic facies of Schieber (1989, 1990), passing laterally

into red, cross-bedded sandstones, and red, mud-cracked siltstones 4.1. Bead shapes

interpreted as ﬂuvial paleochannels and ﬂoodplains, respectively

(Fenton and Fenton, 1937; González-Álvarez and Kerrich, 2012). Beads are broadly circular and arranged at equal intervals

The locally iron stained and trough cross bedded channel in the along the strings as originally deﬁned for the genus Horodyskia

measured section (Fig. 2) may have been a shallow paleochannel by Fedonkin and Yochelson (2002), but ﬁve distinct bead variants

in temporarily exposed mudﬂats. Local iron stain of some sand- can now be recognized, sometimes on the same string (Fig. 6): (1)

stone beds can be attributed to short episodes of emergence and lozenge, (2) dimple-mound, (3) plush-curl, (4) lobate, and (5) pear-

oxidation, but without mixing of bedding seen in other Proterozoic shaped. Small beads are generally lozenges, shaped like small round

paleosols (Retallack, 2012a). If paleosols, these reddish units were cushions (Figs. 4A–C and 7A,B). Many large beads are lozenges,

exposed for only a small part of the year, extremely weakly devel- but a few are dimple mounds, with a conical central depression

oped, and did not support Horodyskia. Dominantly silt grain size (Fig. 7B,C), as also illustrated by Fedonkin and Yochelson (2002, Figs.

(Fig. 9F), and local thin stringers of silt (Fig. 8A–E), were attributed 3a,b, 10b and 12a,b) and Grey et al. (2010, Figs. 2b, 3b and 4b,c). The

to loess deposition by Fedonkin and Yochelson (2002). They envis- plush-curl variant has beads like a boldface-“C”, and intergrades

aged stringers of silt sticking to exposed mat surfaces as the bulk of with dimple mounds by means of a radial groove connecting the

silt was carried further on by wind. Small scale erosional features central conical pit to the outer rim. It is plush like a stuffed toy

ﬁlled with ripple marks (Fig. 8D,E) may be attributed to rills and because both ends of the C are rounded (Figs. 4D, 5A and 7C,F),

wind ripples from temporary exposure (Winston et al., 1984). Thus as also illustrated by Fedonkin and Yochelson (2002, Figs. 3a,

at Appekunny Falls and in similar facies at Rising Sun, Horodyskia 8b and 11b). Lobate beads are generally found on the larger

is envisaged as a sessile organism of shallow muddy lake bottoms, specimens, where the outline of the cushion is deformed by

G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142 129

Fig. 3. Appekunny Falls locality overview (A), and detail of green brown bed (B) below the marker sandstone 53 m above Altyn Limestone (Whipple et al., 1984), and bedding

plane with Horodyskia williamsii (C) and polished slabs (D–F) cut vertical to bedding from selected stratigraphic levels in Fig. 2: 65 cm (C,D), 45 cm (E) and 118 cm (F). Specimen

numbers archived with Glacier National Park repository in West Glacier, include GLAC24697 (C,D), GLAC24698 (E), and GLAC24703 (F). Arrows indicate Horodyskia beads.

(For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of the article.)

irregular, but mostly radial lobes (Fig. 4D,E, 5B,C and 7E,G,H: see common along most of a string, with the long axis of the pears

also Fedonkin and Yochelson, 2001, Figs. 8a,b, 11b, 16a and 17b,c). oriented along the string so that the bulging end of one bead

Lobate, plush-curl, and dimple-mound variants are uncommon, onlaps a pit at the narrow end of the next bead (Figs. 4F and 7D)

with generally only one or two along an individual string of other as also seen in Western Australian specimens (Grey et al., 2010,

variants, mainly lozenges. In contrast, pear-shaped beads may be Fig. 6a–d; Seilacher, 2007, pl. 61). Consistently pear-shaped beads

130 G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142

Fig. 4. Specimens of Horodyskia williamsii (A–C,F), H. moniliformis (D,E) and concentric structure (F) all from Mesoproterozoic (1.4 Ga) Appekunny Argillite at Appekunny

Falls locality. Specimen numbers archived with Glacier National Park repository in West Glacier, include GLAC24689 (A), GLAC24682 (B), GLAC24694 (C), GLAC24680 (D),

GLAC24696 (E), and GLAC24693 (F).

are rare in Montana: 1 out of 24 in our collection. None were beads in thin section were made by Fedonkin and Yochelson (2002,

illustrated by Horodyski (1982, 1993) or Fedonkin and Yochelson Fig. 9), and their silty clay internal ﬁll was conﬁrmed by Rule and

(2002). Consistently pear-shaped string-of-beads may prove to Pratt (2012). In general the beads have a thick organic pellicle on

be a separate species, when more specimens become available top, but a thinner organic pellicle on the bottom, and hollows in

for study. between ﬁlled with silt (Fig. 9A,B,E). They are not inverted pyriform

structures above a basal string (as reconstructed by Fedonkin and

Yochelson, 2002 and Fedonkin, 2003), nor spheres linked through

4.2. Bead interiors

their centers by strings (as reconstructed by Dong et al., 2008). Some

of these differences can be attributed to burial compaction, but sev-

A surprising observation from thin sections is the internal

eral hollow tubes curl around the base, form different levels within

complexity of Horodyskia beads. The expectation from specimens

the beads, and curl up from below. Three dimensional restoration

exposed on slabs that they were hollow structures with a lighter

from serial grinding (Fig. 10) demonstrated that the beads have

colored silty ﬁll connected by threads on their lower sides was con-

hollows above a tangle of tubes at various levels and orientations.

ﬁrmed by discovery in thin sections of hollow vesicles and tubes

There is no hint of radial, bilateral, or dorsoventral symmetry in

surrounded by organic ﬁlms of the same size and arrangement as

thin section.

the beads and threads on the slabs. Comparable observations of

G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142 131

Fig. 5. Specimens of Horodyskia moniliformis (A–E), carbonaceous fragments (A: Lanceoforma striata), and “old elephant skin” (F: Rivularites repertus) all from Mesoprotero-

zoic (1.4 Ga) Appekunny Argillite at Appekunny Falls locality. Specimen numbers archived with Glacier National Park repository in West Glacier, include GLAC24684 (A),

GLAC24686 (B), GLAC24691 (C), GLAC24685 (D), GLAC24681 (E), and GLAC24679 (F).

4.3. String morphology remarkable specimen (Fig. 5A) shows a wide carbonaceous zone

between beads. This is regarded not as a string, but as a juxtaposed

In many specimens the string connecting the beads is only partly fragment of carbonaceous compression like others found isolated

visible or covered by sediment, because natural fracture exposes at Appekunny Mountain (Section 4.7). Strings are seen in oblique

beads at a higher level than the one containing the string. One sections within thin sections and have rounded ends, silt-ﬁlling,

132 G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142

(along direction of the string). Our measurement of spacing is also

different from Calver et al. (2010) who measured spacing from the

edge of one bead to the next. We measured from the central dimple

or midpoint of the beads, because we observed in thin section that

strings and radiating ﬁlaments converge near the center of the base

of the beads (Fig. 8B–E). Our normal distributions for comparison

with frequency data were computed from the mean and standard

distribution of all the data (Fig. 11).

Our histograms of bead width (Fig. 11A), length (Fig. 11B) and

spacing (Fig. 11C) are bimodal, but not so strongly bimodal as found

by Calver et al. (2010). A normal distribution is a poor ﬁt to these

data (Fig. 11A–C). Our data also shows strong correlations between

bead length and width (Fig. 11D), with a continuous distribution.

There is a strong correlation between bead width and bead spacing

Fig. 6. Horodyskia moniliformis from the Appekunny Falls locality illustrated by

Horodyski (1993, Fig. 7A) showing a variety of bead types on a single string. (Fig. 11E) and between bead width and average length of radial

structures (Fig. 11F), though large specimens are separated from

the small ones.

opaque walls, sparse branches, and more or less even thickness

(Figs. 8C–E and 9C,D). One tube shows a branch directly down and

then curling back (Fig. 9D). Another string is compounded from

4.7. Carbonaceous fragments

several small tubular features (Fig. 9C).

One specimen (Fig. 5A) has a large carbonaceous area between

4.4. Radiating ﬁlaments several Horodyskia beads. This is a separate carbonaceous com-

pression lodged between the beads, because comparable isolated

Almost all beads in our collection have dark ﬁlaments radiat- carbonaceous fragments were found (specimen GLAC24700). Com-

ing from the base of the bead outward and slightly downward parable striated and acutely pointed carbonaceous sheets from the

into the matrix. The ﬁlaments do not extend directly down Greyson Shale of the Little Belt Mountains have been identiﬁed as

beneath the beads in thin section (Fig. 7B–E), but rather slope Lanceoforma striata, and regarded as remains of megascopic eukary-

outward obliquely downward from beneath the beads like an Eliz- otic algae (Walter et al., 1976).

abethan collar. They are not superﬁcial stains of rotted beads as

argued by Fedonkin and Yochelson (2002). Radiating ﬁlaments are

4.8. Concentric structure

mostly in a lower bedding plane than the one exposing the beads

(Figs. 4A–C and 9A,B,E), but some slabs show that the ﬁlaments

One specimen (Fig. 4F) has strongly concentric ridges, which are

radiate outward (Figs. 4D,E and 6). The ﬁlaments ﬁll space as they

edges of erosion of domed laminations. Specimens of Horodyskia

radiate outwards, and they do not seem to do so by branching,

with distinctive pear-shaped beads are on the same slab. Compa-

but rather by additional ﬁlaments coming from above or below.

rable structures are illustrated in association with Horodyskia in

Observations in thin sections suggest that these ﬁlaments are sim-

Western Australia, and interpreted as a “fairy ring” bacterial mat,

ilar tubular forms compounded in the main string (Fig. 9C), and

which overgrew Horodyskia (Grey et al., 2010). This explanation is

mostly branch from it and each other under the dome of the bead

not compelling because in both our, and Western Australian, mate-

(Fig. 10A–D).

rial Horodyskia is not deformed or cut by the concentric ridges. This

independence of ridges and beads is evidence that Horodyskia col-

4.5. Stellate structures

onized an erosional or overlying surface on the concentric ridges.

On our specimen, domed laminations incline down into the matrix

Several specimens found in naturally weathered scree at the

similar to sand stromatolites found within submerged sand bars

Appekunny Falls locality (Fig. 5D–E) show indistinct bead-like cen-

of Great Sippewisset Salt Marsh, Massachusetts (Nicholson et al.,

ters with radial disruptions of argillite by tubes ﬁlled with white

1987; Pierson et al., 1987).

silt. The centers stand up from the slab, and the radial tubes slope

downward slightly, like strata-transgressive oblique radiating ﬁl-

aments. Unlike radiating ﬁlaments, which are black with organic

4.9. Old elephant skin

carbon, these stellate structures are light greenish gray to white.

Nevertheless, they could represent decayed and exhumed exam-

Many bedding planes of the Appekunny Formation are ferrug-

ples of large Horodyskia, because they form statistically continuous

inised, pustulose and cracked (Fig. 5F; Fedonkin and Yochelson,

distributions with the other material (Fig. 11).

2002, Fig. 9b–d), a microbially induced sedimentary structure

(Noffke, 2010) widely called “old elephant skin” (Runnegar and

4.6. Growth series Fedonkin, 1992) or assigned to the ichnospecies Rivularites repertus

(Retallack, 2009, 2012a). Although the best specimens of Rivular-

As noted by Fedonkin and Yochelson (2002) and Calver et al. ites were found loose in scree, their red color and comparable

(2010) the spacing between beads of Horodyskia shows a high surfaces in outcrop narrow their provenance to red beds within

degree of correlation with bead length as if these were growth the measured section (Fig. 2). These red units are regarded here

variants. Grey et al. (2010) recognize separate species for strings as weakly developed paleosols (Section 3.2). Rivularites in other

with large beads (H. moniliformis) and small beads (H. williamsii), parts of the world also has been found in paleosol surface horizons,

so there may be two, rather than one growth series. Both questions and interpreted as a cast of biological soil crusts (Retallack, 2009,

are addressed by measurements of our Appekunny Falls collec- 2012a,b). Horodyskia on a Rivularites surface ﬁgured by Fedonkin

tion (Fig. 11). Although most beads are spherical, one variant is and Yochelson (2002) was severely degraded by pustular over-

pear-shaped, and our data includes separate measurements of bead growth, and no Horodyskia was found on the Rivularites surfaces

width (perpendicular to the direction of the string) and bead length examined during this work. This may be evidence conﬁrming

G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142 133

Fig. 7. Individual beads of Horodyskia moniliformis (E–H) and H. williamsi (A–D). Specimen numbers archived with Glacier National Park repository in West Glacier, include

GLAC24682 (A–C), GLAC24693 (D), and GLAC24684 (E–H).

interpretation of sedimentary facies (Section 3.2), that Horodyskia Australia (Jackson et al., 1987). Rule and Pratt (2012) also argue that

was aquatic, and did not survive prolonged exposure on dry land. Horodyskia is a pseudofossil formed of accidently aligned, rounded

claystone clasts. By both arguments the so-called “beads” are so

5. Comparable fossils abundant and varied in distinctness that the eye is drawn to ﬁnd

illusory alignments among better deﬁned examples. Variation in

Horodyskia may have been an extinct life form without modern distinctness is a feature of Horodyskia (Figs. 6 and 7), which can be

counterpart, as argued by Fedonkin and Yochelson (2002). If so, explained by varying degree of decay of organic structures (Grey

other comparable fossils would be expected, as evaluated in the et al., 2010), but is not a feature of evaporate or claystone breccia

following sections. beds. We are unaware of any evaporites in the Appekunny Argillite,

which has a chemical index of alteration of 85, indicative of pale-

5.1. Pseudofossil oclimate too humid for evaporites (González-Álvarez and Kerrich,

2012). Nor did we ﬁnd other claystone breccias or claystone pellet

Hofmann (1992) suggested that “string of beads” bedding plane beds within the source strata (Fig. 2). Furthermore, our thin section

markings described by Horodyski (1982) might have been casts examination of these structures showed a scattering of spherical

of evaporites comparable with “millet seed” gypsum casts from structures in transverse sections comparable with size and abun-

the Balbirini Dolomite of the MacArthur Basin, Northern Territory, dance on bedding plane surfaces (Figs. 3D–F and 8). The beads

134 G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142

Fig. 8. Petrographic thin sections of Horodyskia-bearing layers, cut vertical to bedding. Specimen numbers archived with Glacier National Park repository in West Glacier,

include GLAC24697 (A), GLAC24682 (B,E), GLAC24688 (C), and GLAC24680 (D). Arrows indicate Horodyskia beads.

are large (2–7 mm) yet scattered in laminated shale, and are more lithology is much less varied than for dropstones (Gostin et al.,

silty, not more clayey, than their matrix. As clasts they would have 2010). The beads are organically bound, varied in shape and dis-

to be glacial dropstones, because their deposition requires more tinctness (Figs. 6 and 7), sediment-ﬁlled (Fig. 8B–D), and resisted

energetic currents than their matrix, yet their range in size and erosion, then ﬁlling of small rills (Fig. 8E). As argued by Fedonkin

G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142 135

Fig. 9. High magniﬁcation photomicrographs of beads (A,B,E) and strings of (C,D) of Horodyskia moniliformis (all in plane light) and loessic siltstone matrix (E, under crossed

nicols) from the Appekunny Falls locality. Specimen numbers archived with Glacier National Park repository in West Glacier, include GLAC24688 (A,C), GLAC24680 (B,D,F),

and GLAC24682 (E).

and Yochelson (2002), consistent arrangement of beads into chains simplicity of form and novelty. Horodyskia is no longer novel:

connected by a string readily distinguishes Horodyskia from true thousands of specimens are now known from Montana (Fedonkin

evaporite sand crystals and claystone breccias (Retallack, 2012b), and Yochelson, 2002), Western Australia (Grey and Williams, 1990;

carbonate or iron nodules (Retallack, 2008), raindrop impressions Grey et al., 2002, 2010), Tasmania (Calver et al., 2010), China (Shen

(Metz, 1981, 1982), or lichen encrustation of the modern outcrop et al., 2007; Dong et al., 2008), and India (Mathur and Srivastava,

(Brodo et al., 2001). Rain drop impressions, evaporite crystal casts, 2004). Our observations of radial ﬁlaments (Figs. 6 and 7) and

claystone breccias and calcareous nodules are found in the Mount internal tubules (Fig. 9A,B) suggest it was not the simple string

Fields Formation (Table 1) in Glacier National Park (Ross, 1959; of beads envisaged by Horodyski (1982), Fedonkin and Yochelson

Winston and Link, 1993), but none of these are arranged in strings (2002) and Fedonkin (2003), either. Grey et al. (2010) used the six

or hollow and silt-ﬁlled. criteria of biogenicity established by Hofmann (2004) as evidence

that Horodyskia from Western Australia was a genuine fossil. Com-

5.2. Dubiofossil parable arguments can be made from our material, which is (1)

from rocks of known provenance (good outcrops; Fig. 3A,B), (2)

Horodyski (1982, 1993) and Runnegar and Fedonkin (1992) indigenous to the rock (broken out in ﬁeld and observed in thin

regarded Horodyskia as a dubiofossil, largely because of its section: Figs. 7, 8A–E and 9A–E), (3) formed at the same time as the

136 G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142

helanensis and Shaanxilithes cf. S. ningqiangensis from early Edi-

acaran (635–551 Ma) slate of the upper Zhengmuguan Formation

near Suyukou, and sandstone of the Zhoujieshan Formation near

Quanjishan, in North China (Shen et al., 2007). Shaanxilithes is

a comparable segmented form with irregular annulations, and

Helanoichnus is a lobate tube, like an empty sheath or halo of the

other fossils. The simple geometric form of segments and durable

halo of the Chinese fossils is quite different in form from the

Montana type material of Horodyskia (Fedonkin and Yochelson,

2002), and in our opinion none of these Chinese fossils should

be referred to the genus Horodyskia. Dong et al. (2008) consid-

ered both “Horodyskia” minor and Palaeopascichnus foraminifera.

Antcliffe et al. (2011) rule out foraminiferal, brown algal, prokar-

yotic and metazoan afﬁnities for South Australian Palaeopascichnus,

and consider it another problematic Ediacaran fossil. Despite differ-

ences, the possibility that Chinese and South Australian Ediacaran

“Horodyskia” or Palaeopascichnus were descendents of Mesopro-

terozoic Horodyskia remains open.

5.5. Trace fossil

Yang and Zheng (1985) and Hofmann (1992) compared speci-

mens later identiﬁed as Horodyskia by Shen et al. (2007) and Dong

et al. (2008) with trace fossil burrows or fecal strings such as

Neonereites uniserialis (Brasier and McIlroy, 1998; Seilacher, 2007)

and Torrowangea rosei (Webby, 1970). Strings of ellipsoidal feces

are plausible for opaque serial spheres and discs of “Horodyskia”

minor and P. jiumenensis (Dong et al., 2008), but not for the more

complex interiors of Horodyskia (Fig. 8B–D). Origin as a segmented

Fig. 10. Three dimensional structure of Horodyskia moniliformis beads (A–D) from

burrow is also falsiﬁed by the radial ﬁlaments of Horodyskia, which

serial grinding of specimen GLAC24680, with polished section (E) from mid-range

form a collar below each bead (Figs. 4D,E and 5A–E).

of grinding (orange hues of topography in A–D). (For interpretation of the references

to color in this ﬁgure legend, the reader is referred to the web version of the article.)

5.6. Segmented sponge

rock (aligned in bedding planes with radiating delicate ﬁlaments:

Hofmann (2001) compared Horodyskia with segmented sponges

Figs. 5A–E and 9A,B), (4) in paleoenvironments suitable for life (with

such as Armstrongia oryx (Hexactinellida, Porifera), from Late Devo-

ripple marks and sand stromatolites: Figs. 2 and 4F), (5) shows

nian marine rocks of northeastern Pennsylvania (Finks et al., 2004).

evidence of varied preservation (due to growth and decay:

Sponges have mineralized spicules not seen in microscopic exami-

Figs. 4A–F and 7A–H), and (6) has biofabric (distinctly differentiated

nation of Horodyskia by us (Fig. 9), nor by Fedonkin and Yochelson

organic layers and connecting strings: Figs. 8A–E and 9A,B).

(2002). Extinct segmented sponges branched and rose erect above

a basal attachment, whereas Horodyskia has never been demon-

5.3. Franceville fossils

strated to branch or overlap, and no holdfasts are found at the end

of strings (Fedonkin and Yochelson, 2002; Mathur and Srivastava,

Un-named pyritized fossils from lagoonal black shales of the

2004; Grey et al., 2002, 2010; Calver et al., 2010).

Paleoproterozoic (2.1 Ga) FB2 formation, near Franceville, Gabon

(El Albani et al., 2010) have clavate to nodular structures centered

on radially lobed plates, and internal tubular features, compa- 6. Comparable modern analogs

rable with the plush-curl bead morphotype of H. moniliformis

(Figs. 6 and 7C,F). Differences are also apparent, because individual Several past opinions on the biological afﬁnities of Horodyskia

Franceville fossils are at least twice the size of Horodyskia beads can be falsiﬁed from our observations. These opinions are arranged

and radial rims. Although Franceville fossils are arranged in chains in order of biological complexity. Alternatives 1–5 would not be

end to end, no connecting string has been noticed. Nevertheless, it out of place in the Mesoproterozoic, but alternative 6 represents

is difﬁcult to rule out the Franceville fossils as possible ancestors of organisms not generally considered to have evolved as long ago as

Horodyskia. 1.48 Ga.

5.4. “Horodyskia” minor 6.1. Prokaryotic colony

Dong et al. (2008) described siliciﬁed specimens from cherts Knoll et al. (2006) suggested that Horodyskia might be a micro-

of the upper Ediacaran (551–542 Ma) Liuchapo Formation near bial colony, comparable with “strings-of-pearls”, found in found in

◦

Jiumen, South China, as “Horodyskia” minor and Palaeopascich- cold (10 C) sulfur springs of Sippenauer Moor, Bavaria, Germany

nus jiumenensis. These have opaque, cleanly deﬁned spheres (Rudolph et al., 2001). “Strings-of-pearls” have walls and strings

and dishes, respectively, within a bleached and lobed elongate of Thiothrix (Proteobacteria, Bacteria) and interiors with un-named

halo, and a very weakly carbonized string oriented through the microbial strain SM1 (Euryarchea: Moissl et al., 2003). “Strings-of-

middle of the spheres or dishes. Comparably weak strings and pearls” are irregularly spaced and of different sizes, ranging from

haloes are also seen in compression fossils of “Horodyskia monil- 0.5 to 8 mm, on variable thickness strings suspended in marsh

iformis?,” Palaeopascichnus meniscatus, P. minimus, Helanoichnus water between blades of grass. These are all differences from

G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142 137

Fig. 11. Size distributions and allometry of Horodyskia moniliformis and H. williamsii from Mesoproterozoic (1.4 Ga) Appekunny Argillite at Appekunny Falls locality.

Horodyskia, which is notable for uniformly sized beads with propor- anything like a holdfast (Fedonkin and Yochelson, 2002; Mathur

tional and equal spacing on strings of consistent diameter (Fig. 11). and Srivastava, 2004; Grey et al., 2002, 2010; Calver et al., 2010).

A gelatinous, solid ball of microbes suspended in water would Large stellate structures (Fig. 5D,E) could be interpreted as hold-

not be preserved like Horodyskia, which has compaction-resistant fasts, but these form a continuous distribution of size and spacing

beads and tubules, with radiating ﬁlaments within sediment with those of smaller beads (Fig. 11). Finally, brown algae are

(Figs. 8B–D and 9A,B,E). Finally, pyrite framboids are rare and scat- marine, and although the Belt Supergroup has been considered

tered (Fig. 3D–F) in the matrix to Horodyskia, which would not be marine (Fenton and Fenton, 1937; Fedonkin and Yochelson, 2002),

the case if groundwater at the time of deposition were unusually there is now growing consensus that the Belt-Purcell Basin was

sulfurous. an intracontinental rift basin with large lakes and braided streams

(Section 3.3).

6.2. Brown alga

6.3. Agglutinating foraminifera

Grey and Williams (1990) and Grey et al. (2002) suggested

that Horodyskia might have been a brown algae (Phaeophyta), like Shen et al. (2007) and Dong et al. (2008) considered “Horodyskia”

common Australian and New Zealand species Hormosira banksii minor to have been foraminifera, like the allogrommid Resigella

(Osborn, 1948) and Scaberia agardhii (Nizamuddin, 1962). These moniliforme (Gooday et al., 2004). By this interpretation, the

species have hollow beads and ﬂoats with elastic narrow con- haloes of “Horodyskia” minor and radiating ﬁlaments of H. monil-

nections, easily broken to send the bead as a projectile, as long iformis and H. williamsii could be rhizopods for sediment feeding.

appreciated by Australasian children. Both modern species branch Allogrommid foraminifera have tests with segments separated by

and ﬂoat erect above a holdfast on the seaﬂoor. Branching and narrow waists and individual segments of different size, becom-

overlapping Horodyskia are unknown, and no end of the string has ing smaller in the direction of growth. Horodyskia in contrast

138 G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142

perlatum (Agaricales, Basidiomycota: Gregory, 1949; Bowerman,

1961). This explanation fails for the same reasons as aethalia of plas-

modial slime molds: puffballs are hollow, not arranged in strings,

and found on terrestrial wood or leaf litter substrates.

6.6. Stoloniferous colonial eukaryotes

Fedonkin and Yochelson (2002) regarded Horodyskia as “mul-

ticellular, tissue grade, colonial eucaryotes” showing “growth of

stolon length and bead size”. Fedonkin and Yochelson (2002) and

Fedonkin (2003) make comparisons with hydrozoans, but had in

mind a chemosynthetic sessile autotroph, and an organism more

primitive than any living metazoan. They also regarded radiating

ﬁlaments around Horodyskia beads as decay products of rotting

specimens, which from the extent of the rot would indicate orga-

nisms quite ﬂeshy in life. This aspect of their interpretation is

falsiﬁable from our observation of radiating ﬁlamentous structures

angling down across lamination (Figs. 5D,E and 9A,B). Comparison

with stoloniferous hydrozoans (such as Obelia longissima: Cnidaria)

or bryozoans (Plumatella repens: Phylactolaemata), also is falsiﬁed

because these stolons are branched, have erect as well as bottom-

hugging portions, and are festooned with stalked zooids (Riisgird

et al., 2004; Genzano et al., 2008). Horodyskia beads in contrast are

unbranched, sessile, unstalked, and the string has no speciﬁc end

attachment. The radiating ﬁlaments are unlikely to have been zooid

tentacles, because they incline downward on oriented specimens,

and are irregular in shape.

7. New hypothesis: Horodyskia as endocyanotic fungus

bladders

Fig. 12. Geosiphon pyriformis from Bieber, Germany (A) and its chlamydospores (B).

Images are reprinted courtesy of Arthur Schüßler.

“Best of ﬁeld” is a logical fallacy in which one hypothesis is

preferred from a ﬁeld of unpalatable alternatives, when the cor-

has beads well separated along strings and of uniform size from

rect hypothesis may be an unconsidered alternative. Endocyanotic

one end of the string to the other. There is also the problem

fungus is thus suggested here as a previously unconsidered alterna-

that foraminifera are marine, whereas the Belt-Purcell Basin may

tive, when so many other alternatives are falsiﬁed or unappealing,

have been a non-marine intracontinental rift basin (Winston et al.,

but recognizing that there may be yet other alternatives worth

1984; González-Álvarez and Kerrich, 2012). Finally, there is doubt consideration.

that “Horodyskia” minor from China is congeneric with Horodyskia

impressions from Montana. “Horodyskia” minor is preserved as 7.1. Geosiphon analog

fully inﬂated permineralized specimens with a three dimensional

bleached halo around uniformly dark sediment beads (Shen et al., The remaining working hypothesis for the biological afﬁni-

2007; Dong et al., 2008), unlike the planar radial ﬁlaments sur- ties of Horodyskia is as bladders of an “endolichen” (fungus

rounding organic shrouded beads with complex sediment ﬁll in H. with intracellular photobiont) like living Geosiphon pyriformis

moniliformis and H. williamsii (Figs. 7 and 8). (Archaeosporales, Glomeromycota, Fungi: Schüßler, 2012), which

has much expanded cells (“bladders”) containing heterocys-

6.4. Slime mold sporangia tic cyanobacterial photosymbionts, Nostoc punctiforme (Fig. 12).

Endolichens differ from ordinary lichens which have haustorial

A novel hypothesis worthy of exploration for the beads of capture of intact photobionts (Brodo et al., 2001), and some

Horodyskia is that they were reproductive structures comparable authorities deﬁne lichens to exclude endosymbiotic organisms like

with aethalia (such as those of Lycogala epidendrum: Liu et al., Geosiphon (Hawksworth and Honegger, 1994).

2006), or capillitia (such as those of Stemonaria fuscipes: Bosselaers, Although basal radiating ﬁlaments, connecting threads and

2004) of plasmodial slime molds (Myxogastria, Mycetozoa). These bladders of Geosiphon are generally comparable in size and arrange-

are hollow structures protruding from the substrate to which they ment with Horodyskia, there are important differences, because

are attached by hyphae. However, aethalia are hollow spheres and bladders of Geosiphon are mostly erect and clavate, and beadlike

capillitia are ﬁlled with complex axial and lateral branches unlike only in early growth stages. Geosiphon bladders also are arranged in

the tubular structures of Horodyskia (Figs. 8B–D and 9C,D), and fur- clusters and close strings rather than elongate strings, and are ter-

thermore are arranged in local clusters, rather than long strings. restrial in humid nutrient-poor soils, rather than aquatic (Schüßler,

Mycetozoan slime molds generally have wood or leaves as a sub- 2012). Nevertheless, implications of the working hypothesis of an

strate and are terrestrial, unlike aquatic habitats envisaged for endocyanotic interpretation of Horodyskia are worthy of further

Horodyskia (Section 3.3). consideration.

6.5. Puffball fungus 7.2. Inferred mode of life

Another novel hypothesis for beads of Horodyskia is that they Fedonkin and Yochelson (2002) proposed that Horodyskia was

were basidiocarps, like those of puffballs such as Lycoperdon autotrophic because it was sessile in nutrient-poor substrate,

G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142 139

and these arguments also support interpretation of Horodyskia as levels both above and below the Appekunny Argillite (Horodyski,

an endocyanotic lichen like Geosiphon. However, Fedonkin and 1980; Kidder and Awramik, 1990), but not yet from the Appekunny

Yochelson (2002) objected to photosynthetic Horodyskia because Argillite. Although such large acritarchs have been interpreted

of presumed muddy substrate, turbid and anoxic water, and active as fungal by Pirozynski (1976), Hermann (1979), Locquin (1983),

sedimentation. These features do not tally with our microbial- Brunel et al. (1984), Burzin (1993), and Butterﬁeld (2005), they

mat-stabilized mudﬂat interpretation of sedimentary facies of have also been interpreted as algal microplankton (Grey, 2005;

Horodyskia (Section 3.3). Silt dominance and silty stringers in the Moczydłowska et al., 2011), and as animal resting cysts (Cohen

matrix of Horodyskia (Fig. 7) are evidence that there was little turbid et al., 2009). Additional ultrastructural and geochemical studies are

clay delivered by wind, as also noted by Fedonkin and Yochelson needed to tease apart these alternatives.

(2002). In addition pyrite and organic matter are sparse, unlike

anoxic black shales found in other parts of the Belt Supergroup

7.5. Evolutionary implications

(Schieber, 1989, 1990).

Fedonkin and Yochelson (2002) advocated chemosymbiosis, but

Molecular clocks now place the antiquity of fungi at about

there is no mineralization of Horodyskia by gypsum, pyrite or iron-

1000 Ma, but that estimate is based on Devonian appearance of

manganese. Chemosymbionts associated with modern bivalves

Ascomycota at 452 Ma, and a poorly constrained split between

produce much oxidized sulfur, and upon burial this is bacterially

plants and animals-fungi at 1700 ± 300 Ma (Berbee and Taylor,

reduced to abundant pyrite (Duplessis et al., 2004). Chemosymbi-

2010). Nevertheless, such an evolutionary timetable is supported

otic invertebrates of deep-sea black smokers are also associated

by suggestions that there is a Proterozoic record of fungi among

with massive sulﬁde chimneys (Rona et al., 1986).

the enigmatic microfossil palynomorphs known as acritarchs

(Pirozynski, 1976; Butterﬁeld, 2005). For example, Tappania from

7.3. Growth model

the 1.47 Ga Roper River Group of Northern Territory has the

following fungal features: irregular polyhedral–spherical shape,

The growth model of Fedonkin and Yochelson (2002) and

large size (up to 160 ␮m), spherical cell wall protrusions, hyphal

Fedonkin (2003) had closely spaced beads of small Horodyskia suc-

attachment, and branching hyphae (Javaux et al., 2001). Tap-

cessively decimated with colonial growth to well spaced beads of

pania from the 820 Ma Wyniatt Formation of Nunavut has in

large Horodyskia. This model rules out stoloniferous hydrozoans

addition elongate and lobate shapes, rhizine-like attachments,

or bryozoans, which do not show such self-thinning, and is dif-

fused hyphae, and sizes up to 300 ␮m long (Butterﬁeld, 2005).

ﬁcult to envisage in any sessile organisms other than fungi. The

Fusion of sparsely septate hyphae is not evidence of higher

scope of such self-thinning is reduced if there are two species in

fungi (Dicarya = Ascomycota + Basidomycota), as Butterﬁeld (2005)

our collection: large H. moniliformis and small H. williamsii. Our

thought, because hyphal fusion and septae are now known in

size distributions show two modes (Fig. 11A–C), but they are not

Glomeromycota (Bever and Wang, 2005), as well as Oomycota and

as marked as in the data of Calver et al. (2010). Size distributions

Plantae (Berbee and Taylor, 2010). Moczydłowska et al. (2011) con-

of Horodyskia are log normal as in creatures with indeterminate

sider the collar-like extensions of the wall of Tappania to be algal

growth, such as fungi and plants, as opposed to normal distribution

excystment structures, rather than fungal hyphae or rhizines. True

of metazoans (Retallack, 2007). Mycelium of fungi continue to grow

algal excystment structures are not collars, but slits, often deﬁning

in length and complexity, and can put up bladders, basidiocarps

an operculum, as demonstrated for fossil (Yuan et al., 2001) and

(mushrooms), ascocarps (morels), or other megascopic structures

living algae (Bowers and Korn, 1969). Tappania may have been the

at various intervals, commonly in arrangements known as fairy

hypothetical free-living glomeromycotan living in association with

rings (Ingold, 2000).

cyanobacterial mats predicted by Sherwood-Pike (1991). Within

Grey et al. (2010) acknowledged variation in shape of individ-

the fungal classiﬁcation of Hibbett et al. (2007), Tappania is best

ual beads that we have illustrated (Figs. 6 and 7), but interpreted

accommodated within the pre-mycorrhizal Order Archaeosporales

this as different preservational variants, of more or less uniform

(Glomeromycota), like Geosiphon (Schüßler et al., 2001). Another

circular or ovoid beads of an eukaryotic organism, perhaps as

un-named fossil from the Ediacaran (599 Ma) Doushantuo Forma-

complex as metazoans with zooids, as reconstructed by Fedonkin

tion at Weng’an has phosphate-permineralized branching hyphae

and Yochelson (2002) and Fedonkin (2003). Interpretation of

with terminal saccules or spores, intimately associated with coc-

Horodyskia as endocyanotic bladders of a fungus opens the pos-

coid cells, and has been interpreted as a cyanoglomerolichen

sibility that some of this variation in bead shape was natural

by Yuan et al. (2005). Thus interpretation of Horodyskia as an

variation of a population, because Geosiphon bladders start as small

endocyanotic glomerolichen does not conﬂict with evidence from

spheres, then elongate to lobate or curved sausage-shaped struc-

molecular clocks or the fossil record of fungi.

tures (Schüßler, 2012), comparable with lozenge, plush curl and

lobate variants (Fig. 6).

8. Conclusions

7.4. Interpreted reproduction

A new age model based on radiometric dates of Evans et al.

Geosiphon like other glomeromycotan lichens may have repro- (2000), constrains the geological age of string-of-beads fossils (H.

duced asexually from fragments (isidia, soredia) as well as sexually moniliformis and H. williamsii) from the lower Appekunny Argillite

by large chlamydospores on hyphae in the soil (Schüßler, 2002, of Glacier National Park to 1.48 Ga, which is early Mesoproterozoic

2012). Scattered lobate extensions of some Horodyskia beads or Calymmian in the time scale of Gradstein et al. (2004). Most

(Fig. 7), are superﬁcially similar to lichen isidia (Brodo et al., specimens collected previously from this locality by Horodyski

2001). Unfortunately these lobes are not preserved in sufﬁ- (1982) and Fedonkin and Yochelson (2002) were from scree, but

cient organic detail to be certain of their biological function. our study traced them back to a 2 m ledge of bedrock. Horodyskia

␮

Spores of Geosiphon are large (250 m diameter) chitinous spheres was found in laminated silty shales and carbonaceous-swirl shales,

with stalks (Schüßler et al., 1993), comparable with Proterozoic with local sandstone paleochannels (Figs. 2 and 3A,B). Following

acritarchs, such as Ceratosphaeridium mirabile (Grey, 2005) and Ger- facies interpretations of such lithologies in other parts of the Belt

minosphaera sp. (Strother et al., 2011, Fig. S7). Similar large smooth Supergroup (Schieber, 1989, 1990), and evidence that the Belt-

acritarchs are also known from the Belt Supergroup at stratigraphic Purcell Basin was an intracontinental rift (Winston et al., 1984),

140 G.J. Retallack et al. / Precambrian Research 226 (2013) 125–142

an endocyanotic glomerolichen is testable by additional studies of

its paleoenvironments, and associated fossils.

Acknowledgements

Tara Carolin of Glacier National Park expedited a collecting

permit, Deidre Shaw provided catalog numbers for curation of spec-

imens, and Liz Makarowski aided with a hiking permit. Jessica and

Jeremy Ditto assisted with photomicroscopy. Dima Grazhdankin,

Stefan Bengtson, and Bob Horodyksi offered useful discussion.

Appendix A. Supplementary data

Supplementary data associated with this article can be

found, in the online version, at http://dx.doi.org/10.1016/ j.precamres.2012.12.005.

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