RESEARCH REPORTS
445
Embedment Cavities in Lacustrine Stromatolites:
Evidence of Animal Interactions from Cenozoic
Carbonates in U.S.A. and Kenya
ROBERT E. LAMOND* and LEIF TAPANILA**
Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112-0111,
Email: [email protected]
PALAIOS, 2003, V. 18, p. 445–453
Embedment structures are formed by the growth of a skeleton-producing host organism around a cavity maintained
by an infesting organism. Through the process of embedding, the infesting animal passively produces a secure
dwelling structure. This embedment ethology is common in
modern and ancient marine settings. Previously unrecognized embedment cavities from ancient lacustrine stromatolites are identified and described here for the first time.
Cenozoic lake deposits from the Washakie Basin in Wyoming and Turkana Basin in Kenya contain stromatolites
with regularly pitted surfaces, which represent openings to
embedment cavities. Stromatolitic laminae deflect downwards adjacent to the cavities. These cavities are circular to
oval in cross-section and terminate with a blind base. A lack
of branching, basal rootstock, and encrustation by stromatolitic laminae suggest that the cavities were not formed by
the overgrowth of plant material, as has been suggested previously. Moreover, the rounded and evenly spaced cavities
indicate that these structures are not the spaces between
abutted columns of stromatolite material.
These findings suggest that some lacustrine stromatolitic
ecosystems are more diverse than the body-fossil record
demonstrates. It is probable that more embedment structures will be recognized in lacustrine and marine stromatolitic settings.
INTRODUCTION
Embedment structures are cavities formed by the
growth of a skeleton-producing organism (host), such as a
coral, around a generally poorly skeletonized animal or
plant (embedded or infesting organism). Growth of the
host skeleton is inhibited adjacent to the embedded organism. After successive layers of host skeleton are deposited,
an elongate cavity perpendicular to the host surface is created. Through the embedding process, the invading organism takes advantage of an open cavity in which to live
without having to generate its own metabolically produced skeletal material, or excavate its own burrow or boring. The deflected growth of host skeletal material that
forms the borders of the embedment cavity can be distinguished easily from borings, which cut through and trun-
cate skeletal material rather than deflect it (Bromley,
1970).
Embedment is a common process in modern marine environments (e.g., serpulid worms and pyrgomatid barnacles embedded in corals), and its fossil record extends back
to the Early Paleozoic. Nearly all accounts of embedment
structures occur in a metazoan host, and they are especially well preserved in host organisms that produce a
laminated or basal skeleton, such as tabulate corals, bryozoans, and stromatoporoids (Sokolov, 1948; Stel, 1976;
Palmer and Wilson, 1988; Darrel and Taylor, 1993; Tapanila and Copper, 2002; Tapanila, 2002).
A recent study of lake El Mojarral East in the desert basin of Quatro Ciénegas, Mexico found oncoids with pitted
surfaces (Winsborough et al., 1994). The elongate pits
(commonly 2 mm diameter; 10 mm deep) extend into the
oncoids perpendicular to the growth surface. These pits
were interpreted as being produced by the inhibition of
stromatolitic growth by the activity of benthic invertebrates. These stromatolites were produced largely
through the layered accumulation of the calcifying cyanobacterium Homeothrix balearica. The abundance of this
species decreases rapidly with depth into the cavity, providing evidence of the inhibition of stromatolitic growth in
close proximity to the holes. Although no mechanism was
proposed for the initiation of these cavities, they appeared
to be maintained open by the movement of benthic organisms (e.g., amphipods) within the cavities (Winsborough et
al., 1994). The resulting pits in these oncoids are embedment structures.
These modern embedment structures found in oncoids
from Mexico suggest that cavities may be produced by
similar processes in other laminated organosedimentary
structures (e.g., stromatolites, thrombolites, and biolithites). This paper describes two such occurrences in Cenozoic lacustrine stromatolites from Wyoming, U.S.A., and
Lake Turkana, Kenya.
GEOLOGICAL SETTING
Wyoming
*Current address: Imperial Oil Resources, Fifth Avenue Place, Room
7007, 237–4th Avenue S.W., Calgary, Alberta, T2P 3M9, Canada.
**Corresponding author.
Carbonate deposits of the Green River Formation from
Eocene Lake Gosiute are well preserved in the Washakie
Basin, near the town of Wamsutter, southern Wyoming
(Fig. 1). Stromatolites are common sedimentary constituents in the Green River Formation, and particularly so in
the laminated carbonate lithofacies of the LaClede Bed,
lower Laney Member (Surdam and Stanley, 1979). Stromatolites are found interbedded with, and overlying, oolit-
Copyright Q 2003, SEPM (Society for Sedimentary Geology)
0883-1351/03/0018-0445/$3.00
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LAMOND & TAPANILA
FIGURE 1—Isopach map of LaClede Bed, Laney Member, Green River Formation (adapted from Roehler, 1993). Tipton Landslide and Tipton
Road localities in the Washakie Basin are marked by the asterisk. Contours represent isopach thickness (in meters) of the Laney Member of
the Green River Formation.
ic–pisolitic grainstones, which in turn overlie mudcracked dolomicrite (Surdam and Stanley, 1979). These
stromatolites have been interpreted as forming in a nearshore, littoral environment in a transgressing lake system
(Bohacs et al., 2000). The shallow, lake-margin waters
where stromatolites formed were likely briny, having been
enriched from highly soluble evaporitic minerals associated with the desiccated dolomicrite beds. As transgression
continued, kerogenous laminated carbonates were deposited in increasingly deeper and fresher waters.
The Eocene stromatolites discussed in this paper are
from the LaClede Bed (dating between 45.5 to 48 Ma;
Mauger, 1977) in two localities on the northern edge of the
Delaney Rim, southwest of Wamsutter, Wyoming. The stromatolite layer forms a resistant ridge on Tipton Road
(41.564858N, 108.273888W), and it occurs 35 m below ridgeline at the Tipton Landslide (41.582508N, 108.182968W)
outcrop. The stromatolites are of apparent algal (Chlorellopsis) origin (Bradley, 1929), and morphologies include
densely laminar, finely digitate, and thrombolitic forms.
Other fossil indications of life include common ostracode
valves, brine-shrimp egg cases (M.A. Loewen, pers. comm.,
2001) and, in other locations, caddisfly larvae cases. These
fossil remains are commonly found incorporated within the
stromatolite laminae, indicating synchronicity with stromatolite growth.
Kenya
FIGURE 2—Map of Lake Turkana area showing NAR1 locality in the
Nariokotome Member of the Nachukui Formation.
Turkana Basin sediments record more than 4 Ma of
large-scale water-level fluctuations in ancient Lake Turkana in northern Kenya. The Nachukui and Koobi Fora
Formations, which outcrop on the west and east margins
of modern Lake Turkana, respectively (Fig. 2), preserve
dozens of stromatolitic horizons dated between 0.7 and 1.9
Ma (Harris et al., 1988). Stromatolites were deposited in a
littoral lacustrine environment, and appear to have
tracked the expanding shoreline as lake waters transgressed over basin-flanking fluvial and alluvial sediments. These stromatolites commonly utilized coarse alluvial clasts as nuclei on which to initiate growth. Stromatolite heads were formed through biochemical precipitation of low-Mg calcite by cyanobacteria (Abell et al.,
1982; Casanova, 1986), which followed a sequential formation from oncolitic to domal stromatolitic modes of
growth. Early-stage stromatolites are free-rolling oncoids
that become larger up-section. With the larger size of the
oncoids limiting movement, concentric growth expands to
EMBEDMENT CAVITIES IN LACUSTRINE STROMATOLITES
lateral amalgamation of the oncoids and eventually to vertical lamination, forming domal stromatolites.
The embedment cavities described here occur in stromatolites recovered at field locality NAR1 (4.117838N,
35.857678E) located up Lagga Kaitio, approximately 8 km
west of the shoreline of Lake Turkana. These stromatolites are in the Nariokotome Member of the Nachukui Formation. Many other stromatolites in the basin were observed to be regularly pitted, and the distribution of the
embedments is not confined to this one locality and time.
EMBEDMENT CAVITIES
The embedment structures from Wyoming and Kenya
are elongate, cylindrical cavities oriented roughly perpendicular to the surface of the stromatolites. The stromatolite laminae thin and deflect downward immediately
around the cylindrical cavity. The shape and size of the
cavities differ between the two localities.
Wyoming
Embedment cavities were found on the surfaces and in
cross-sections of laminated mats, fine digitate stromatolites, and densely laminated stromatolites in the Washakie Basin of Wyoming (Fig. 3). The cavities in the Wyoming
specimens have a circular to sub-circular aperture. They
gently taper with depth to form an elongate cylinder within the stromatolite. The cavities are discontinuously lined
with a thin calcitic layer. This layer is interpreted to be the
result of the convergence and thinning of stromatolite
laminae as they approach and bend downward close to the
cavity. The lining likely is not produced by the embedding
organisms themselves.
The lower termination of the cavity is constricted (1 to 2
mm diameter), rounded at the margins, and flat near the
center. The base is marked by a stromatolitic lamina that
is layered perpendicular to the cavity axis. Many cavities
appear to initiate at the same horizon within a stromatolite. Occasionally, stromatolitic layers grew over the top of
the cavity. Embedment cavities preserved in fine digitate
stromatolites can be distinguished from inter-digit spaces
by their circular to sub-circular cross-sections and regular
sizes. Spaces between stromatolite digits are generally
stellate or polygonal in shape and are highly variable in
size.
Cavity diameter is most commonly 7 mm, but different
cavities may be as narrow as 2 mm or as wide as 16 mm
(s.d. 5 3.6 mm, n 5 32). The upper size range of diameters
may be the result of enhanced weathering. In unweathered samples, the largest diameters observed are roughly
10 mm. Variation in cavity diameter within the same stromatolite head generally is much less than the variation in
diameters observed between different heads. Observed
cavity depth ranges from 1 mm to 200 mm, though some
might reach even greater depths. The maximum depth of
the cavity is limited to the thickness of the stromatolite.
Small-diameter cavities (;5 mm) occur in densities of up
to 33 per 100 cm2. Larger-diameter cavities (;10 mm) are
evenly spaced and occur in densities of up to 15 per 100
cm2. The cavities commonly are filled with ooids and detrital ostracode valves, which comprise the dominant sediment type between the stromatolitic mounds.
447
Kenya
Embedment cavities occur in both oncoids and domal
stromatolites in the Turkana Basin of Kenya. They are especially common along the west side of modern Lake Turkana.
Two types of cavity morphologies occur (Fig. 4). The first
appears to be very similar to those described from the
Green River Formation, with a circular cross-section. It occurs abundantly in massive stromatolitic heads. The second type of cavity has an oval to biconvex outline, which
extends straight down into the stromatolite, perpendicular to the surface (Fig. 4). The cavity is occasionally lined
with calcite, and it terminates at depth with stromatolitic
laminae oriented perpendicular to the cavity axis. Multiple initiations of cavities appear to be synchronous. Stromatolitic layers occasionally cap the cavity.
The oval apertures range from 5 to 8 mm across the minor axis (mean 5 6.9 mm, s.d. 5 0.6 mm, n 5 21) and 9 to
15 mm along the major axis (mean 5 12.3 mm, s.d. 5 1.6
mm, n 5 21). The cavities extend to a depth of as much as
10 cm. Within a stromatolite, cavities are aligned by the
long axes of their apertures, and they generally are spaced
at least one aperture’s distance away from the nearest
cavity, with a density of 18 per 100 cm2.
At the surface, the cavity often is surrounded by a funnel-like depression where the outer lamina has deflected
inwards around the cavity (Fig. 5). On broken samples,
the underside of the stromatolite forms a positive-relief
protrusion around the cavity. The traces are not filled
with any material and are preserved as negative epirelief.
Oysters and gastropods are common in sediments between stromatolites, and occasionally are incorporated
into the stromatolitic layers. These shells have never been
found inside the embedment cavities.
DISCUSSION
In surveying the Green River Formation of Wyoming,
Bradley (1929) was the first to describe pitted stromatolites and speculate on their formation. He recognized two
cavity types in the stromatolites: regular cup-shaped shallow pits and irregular cone-shaped deep pits. In this study,
it is suggested that this distinction represents a range in
the form and depth of one broad type of cavity. In his interpretation on the formation of the cavities, Bradley
(1929) suggested that the stromatolite grew around a
large sedge (e.g., Scirpus lacustris), producing molds of the
stems. However, lacking evidence for interconnectedness
of the cavities (i.e., branching) or a trace of the rootstock,
he conceded that this explanation could not account for
the origin of the cavities.
The pitted stromatolites near Lake Turkana in Kenya
have been recognized previously (Johnson, 1974), though
an embedding origin of the structures was not acknowledged. Ekdale et al. (1989) reported two types of boring
structures (Sertaterebrites and Trypanites) that clearly
truncate laminae in Lake Turkana stromatolites. In addition to the borings, their stromatolite samples also show
cavity structures formed by deflected stromatolitic
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FIGURE 3—Green River Formation embedment structures and stromatolites. (A) Outcrop view of thrombolites and stromatolites at the Tipton
Road outcrop. The core of the structure is composed predominantly of thrombolitic buildups. The upper bed is a more densely laminated
stromatolite. Scale bar 5 10 cm. (B) Bedding-plane view of upper surface of a stromatolite head riddled with embedment structures. The largediameter holes are the surface expression of where the stromatolite laminae deflect into the embedment cavity. Embedment apertures are
slightly enlarged by surface weathering. Scale bar 5 10 cm. (C) Surface view of small stromatolite head (University of Utah Ichnology Collection
(UUIC), UUIC-1182,) from the Tipton Landslide locality. The dashed line indicates the cross-section shown in E. Scale bar 5 1 cm. (D) Closeup of ooid-filled embedment structure shown within dashed box in E. The deflection and convergence of stromatolite laminae surrounding the
embedment produce a dense lining to the structure. Scale bar 5 2 mm. (E) Longitudinal section through small stromatolite shown in C. Note
the strong deflection exhibited by the stromatolite in close proximity to the embedment cavity. Scale bar 5 1 cm.
EMBEDMENT CAVITIES IN LACUSTRINE STROMATOLITES
449
FIGURE 4—Turkana Basin embedment structures and stromatolites. (A) Large stromatolite head containing abundant, regularly spaced embedment cavities. Scale bar 5 6 cm. (B) Close-up of embedments on the stromatolite in A. Scale bar 5 1 cm. (C) Longitudinal section of the
basal portion of an embedment structure. Scale bar 5 1 cm. (D) Small, flattened stromatolite head containing numerous oval-shaped embedments aligned with one another. Weathering appears to have enlarged the embedment apertures. Scale bar 5 6 cm. (E) Slabbed section of
stromatolite (UUIC-257P) showing downward-deflected laminae surrounding embedment cavities. Scale bar 5 1 cm. (F) Close-up of the
embedment structure in E. Scale bar 5 5 mm.
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LAMOND & TAPANILA
FIGURE 5—Stereo-image pair photographs of the upper surface (A, A’) and the lower surface (B, B’) of a thin stromatolite slab (UUIC-1196)
broken off of a domal stromatolite head from the Tipton Road field location, southern Wyoming. Top surface shows strong downward deflection
of laminae into the embedment cavity. Lower surface shows convex protrusion of the stromatolite laminae surrounding the embedments. Note
even spacing and crude alignment of the embedments. Scale bar 5 1 cm.
growth, but they did not recognize those features as embedment structures.
The Wyoming and Lake Turkana cavities that show deflected laminar margins are the result of animal inhibition
of stromatolitic growth. Thus, they are categorized as embedment structures.
FIGURE 6—Schematic illustration of embedment structures (e) in a
domal stromatolite as seen in longitudinal view. Upper laminae commonly break off (top) of the main stromatolite head (bottom). Arrows
indicate downward-deflected laminae adjacent to the cavities, which
can be observed as positive epirelief on the bottom of broken laminae
and negative epirelief on upper surfaces of the laminae. Not to scale.
Recognizing Embedment Structures in Stromatolites
Recognition of embedment structures in stromatolites
requires observations from both longitudinal and plan (or
transverse) views. Figures 6 and 7 illustrate criteria that
characterize embedment structures in stromatolites.
These features in stromatolites are similar to those described for embedment structures produced in other types
of calcareous substrates (e.g., Bromley, 1970; Ekdale et al.,
1984; Palmer and Wilson, 1988; Scoffin and Bradshaw,
2000). Spaces between stromatolitic or thrombolitic digits
and stromatolitic growth around plant material also can
produce elongate cavities within stromatolites. Although
cavities formed in these manners superficially resemble
embedment structures, they can be distinguished readily
based on key criteria (Table 1).
The most important feature of embedments in longitudinal view (Fig. 6) is that the margins of the cavity are
formed by the downward deflection of stromatolitic laminae. Detailed descriptions of plant overgrowth by stromatolites show that laminae characteristically encrust stems
and other plant material, rather than produce cavities by
deflection (Freytet et al., 1996; Pedley et al., 1996). Embedment cavities are invariably unbranched, elongate,
and oriented perpendicular to the stromatolite surface.
Upward branching of a cavity system may indicate overgrown plant material. Cavities that have a simple, blind
base on top of stromatolitic laminae (i.e., no rootstock or
root traces) are most likely formed by the embedding pro-
EMBEDMENT CAVITIES IN LACUSTRINE STROMATOLITES
FIGURE 7—Schematic oblique view of embedment cavities in a digitate stromatolite. Adjacent digits (d) of a stromatolite may abut to form
inter-digit spaces (i) that often are stellate in cross-section and are
interconnected (arrow). Embedment cavities (e) are generally rounded
in cross-section and more evenly distributed over the stromatolitic surface than inter-digit spaces. Not to scale. (1) Cavity margins are
formed by downward deflected stromatolitic laminae. (2) Cavity tends
to be elongate and unbranched. (3) Cavity is oriented perpendicular
to stromatolite surface and terminates with blind base. (4) Cavity has
round cross-section of regular shape. (5) Cavities are distributed evenly over the stromatolite surface.
cess. The walls of embedment cavities are smooth, not ragged as is typical of the spaces between stromatolite digits.
In plan view, embedment structures are easily discerned from inter-digit spaces. Embedment-cavity margins are generally rounded, while inter-digit spaces are
generally stellate or polygonal in cross-section (Fig. 7).
They also show more interconnections between cavities,
unlike embedment structures, which are usually discrete
and evenly spaced. Finally, embedment cavities typically
occur in groups, and, in any one stromatolite head, they
generally exhibit the same shape and size.
Paleoecology of Lacustrine Stromatolitic Facies
Some marginal lake environments with stromatolitic
deposits host a diverse ecosystem of invertebrates and microbes (e.g., Winsborough et al., 1994). This diversity,
which has been documented in modern stromatolitic settings, is unlikely to be expressed fully in the rock record
since many of the organisms in the community do not pro-
duce hard parts. Embedment structures, on the other
hand, provide highly preservable evidence of benthic activity in the same environment as growing stromatolites.
Although it is often unclear what organism(s) produced
embedment structures, they are valuable traces of animal
behavior and contribute to the paleoecological interpretation of an environment. Embedment cavities are most often evenly distributed on the upper surface of domal stromatolites. This suggests interference among the embedded organisms, possibly as a result of competition for
space and resources. Embedment cavities also tend to occur in groups and they are rarely found solitarily within a
stromatolite head. This aggregative distribution of embedments may have included generations of the same species
of organisms living in the same stromatolitic heads. The
clustered, but competitively spaced distribution of the Wyoming and Kenya embedment structures resembles populations of embedment structures described from Ordovician and Silurian corals and stromatoporoids (Tapanila
and Copper, 2002; Tapanila, 2002).
Embedded animals benefited from the microbial production of carbonate by acquiring a resistant domichnion.
The impact of the embedded organism on the stromatoliteforming microbes is less certain. While embedment structures clearly inhibited proximal stromatolitic growth, the
long-term occupation of some embedment structures (indicated by their great depth) suggests that the host tolerated the activities of the embedding animals. Deep embedment structures (up to 200 mm) suggest that some embedded organisms had a long and successful period of occupancy in stromatolites. It is possible that multiple
generations of embedders colonized the cavities held open
by their predecessors. Such embedment structures may be
useful in acting as a timeline, from which rates of stromatolitic growth could be determined.
Embedment cavities found in Wyoming and Kenya are
abundant locally, but they are restricted to stromatolites
deposited in nearshore lacustrine environments. No embedment structures are known in marine or deeper-water
stromatolites. Organisms confined to lake margins (e.g.,
animals embedded in stromatolites) are vulnerable to sudden changes in water chemistry and rapid flooding or exposure as the lake levels fluctuate through time. Embedment structures in the LaClede Bed in Wyoming are found
in shallow-water stromatolite horizons separated by thick
profundal deposits. Similarly, at Lake Turkana, embedment structures recur in several temporally separated
stromatolite horizons, and certain stages of the lake’s history record no stromatolite deposition at the shorelines at
all. If the causative animals were obligate embedders,
they must have migrated and infested new stromatolites
during episodes of lake contraction and dilation. It is more
likely, however, that the causative animals were facultative embedders that would produce burrows or find shel-
TABLE 1—Five common characteristics of embedment cavities in stromatolites.
(1)
(2)
(3)
(4)
(5)
451
Cavity margins are formed by downward deflected stromatolitic laminae.
Cavity tends to be elongate and unbranched.
Cavity is oriented perpendicular to stromatolite surface and terminates with blind base.
Cavity has round cross-section of regular shape.
Cavities are distributed evenly over the stromatolite surface.
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LAMOND & TAPANILA
ter elsewhere when stromatolites where absent. Brine
shrimp observed inhabiting, but not confined to, embedment cavities from Recent oncoids at Cuatros Ciénegas,
Mexico (Winsborough et al., 1994), support a facultative
embedder lifestyle.
The known occurrence of embedment structures in lacustrine stromatolites (Holocene of Mexico, Eocene of
USA, and Plio–Pleistocene of Kenya) suggests that the
process is widespread and has a long history. Embedment
structures are a frequently overlooked type of fossil preservation (Taylor, 1990). It is likely that many examples of
embedment structures exist in lacustrine stromatolites,
but simply have not been recognized. For example, Krylov’s (1982) vertically oriented holes resembling phantom
columns in Pliocene lacustrine stromatolites in the Altai
Mountains, southern Russia may be embedment structures. It is also likely that marine examples of embedment
structures in stromatolites will be recognized, since shallow-marine stromatolitic facies have a long geologic history.
Embedment structures in lacustrine stromatolites provide another mechanism for animals to produce a domicile
in a hard and stable substrate. The process of embedment
may have been one of the earliest methods for soft-bodied
animals incapable of secreting their own skeletal precipitates to occupy a hard domicile.
CONCLUSIONS
Embedment structures are recognized in lacustrine
stromatolites and oncoids from basins in Mexico, the United States, and Kenya. These structures are geographically
widespread and have a geologic history of at least 45 Ma.
Embedment structures preserve a record of benthic activity and suggest that some stromatolite-bearing lacustrine
habitats hosted diverse and complex ecosystems. Further
recognition of embedment structures in stromatolites, in
both lacustrine and marine settings, is anticipated and
will benefit paleoecological studies of these environments.
ACKNOWLEDGEMENTS
We would like to thank the Leakey Foundation,
ExxonMobil, and the College of Mines and Earth Sciences
at the University of Utah for providing funding for this
project. Tony Ekdale provided critical feedback on this paper and collected many of the Kenyan field specimens figured herein. Dr. Frank Brown and Patrick Nduru Gathogo directed one of us (REL) in the field, placed the field localities into stratigraphic context and provided constructive information on other stromatolite horizons not seen
by the authors. Mark Loewen directed us to field localities
in Wyoming. The final manuscript benefited from helpful
reviews by Drs. Paul Taylor and Eric Verrecchia. Thanks
also to Drs. Kevin Bohacs, John Warme and Andrew Cohen for assistance in the field in Wyoming and for finding
the first North American examples of embedment-bearing
stromatolites.
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ACCEPTED APRIL 14, 2003