J. Paleont., 80(4), 2006, pp. 768–774
Copyright q 2006, The Paleontological Society
0022-3360/06/0080-768$03.00
A NEW SOCIAL INSECT NEST FROM THE UPPER CRETACEOUS
KAIPAROWITS FORMATION OF SOUTHERN UTAH
ERIC M. ROBERTS1
1
AND
LEIF TAPANILA2
School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa, ,[email protected]., and
2
Department of Geosciences, Idaho State University, Pocatello 83209-8072, ,[email protected].
ABSTRACT—A fossilized social insect nest, Socialites tumulus new ichnogenus and ichnospecies, is described from continental strata
of the Upper Cretaceous Kaiparowits Formation in southern Utah. Nine discrete nest structures are preserved along a single bedding
plane, within an area of about 25 m2. Fine-grained, ripple-laminated sandstone, interpreted as a fluvial crevasse splay deposit, drapes
the nest structures and is thought to have rapidly buried and preserved much of the aboveground architectural morphology of the nest
structures. Such social insects as ants and termites are considered as possible tracemakers. Comparison to fossil and modern nests
suggests that Socialites tumulus is more similar to nests produced by ants than by termites. This trace fossil preserves rare, tangible
evidence of nest construction by social insects during the Late Cretaceous, adding new information on insect diversity and habitat
utilization in Mesozoic terrestrial ecosystems. Detailed study of these complex nest structures also aids in paleoenvironmental reconstruction, revealing multiple phases of nest construction, burial, and reestablishment.
INTRODUCTION
fossils are valuable sources of information for reconstructing ancient ecosystems, and aid in paleoenvironmental and paleoecological interpretation (Hasiotis and
Bown, 1992; Genise and Bown, 1994; Laza, 1995; Martin and
West, 1995; Genise, 1997; Hasiotis, 2002, 2004). Specifically, information pertaining to water table, soil moisture, and climate
may be deduced through ichnological investigations. Hampering
these efforts, however, is a relatively poor record of insect body
and trace fossils (Hasiotis and Dubiel, 1995). Recent interest in
continental ichnology has greatly improved understanding and
recognition of trace fossils in terrestrial ecosystems (Bown and
Laza, 1990; Bown et al., 1997; Genise, 1997; Hasiotis, 2000,
2002, 2003; Bordy et al., 2004). Hasiotis (2003) recently presented a comprehensive review of complex ichnofossils produced
by soil-dwelling insects, suggesting a new behavior category, Polychresichnia, for trace fossils representing many simultaneous
uses and behaviors. Additionally, Genise et al. (2000) proposed
the Coprinisphaera ichnofacies, a distinct continental trace fossil
assemblage produced by bees, wasps, ants, beetles, and termites
in paleosols associated with terrestrial herbaceous communities.
This study describes a new social insect nest attributed to an unknown taxon of ant, or possibly termite, aiding in the interpretation of a Late Cretaceous terrestrial ecosystem along the margin
of the Western Interior Seaway.
C
ONTINENTAL TRACE
from the formation, including dinosaurs, mammals, turtles, crocodiles, lizards, bivalves, gastropods, ostracods (Hutchison et al.,
1997; McCord, 1997; Sampson et al., 2002; Smith et al., 2003),
and now trace fossils produced by social insects. The most common vertebrate fossils in the formation are dinosaurs, dominated
by herbivorous hadrosaurs, ceratopsians, ornithipods, and ankylosaurs, and a rarer, but diverse assemblage of carnivorous theropods, including a single taxon of large tyrannosaurid (Roberts
et al., 2003). An important and diverse Judithian ‘‘Age’’ mammal
assemblage is also known from the formation (Eaton and Cifelli,
1988; Cifelli, 1990a, 1990b; Eaton, 2002). The most common
invertebrates include freshwater unionid bivalves, prosobranch
gastropods, and freshwater and terrestrial pulmonate gastropods
(McCord, 1997).
MATERIALS AND METHODS
Samples described here were analyzed as part of a larger study
on the stratigraphy and depositional environments of the Kaiparowits Formation (E. Roberts, personal observation). The nestbearing layer was discovered in the middle unit of the Kaiparowits
Formation along Utah Highway 12 in the region called the Blues
(Fig. 1).
GEOLOGICAL SETTING
The Kaiparowits Formation is well exposed in the Grand Staircase–Escalante National Monument of southern Utah (Fig. 1), and
is recognized by its distinctive badland-forming, dark gray-blue
sandstones and mudstones. Strata of the Kaiparowits Formation
were deposited as part of a prograding clastic wedge, derived
from a source area located 300–650 km to the southwest in the
Sevier fold and thrust belt in southeastern Nevada and eastern
California (Eaton, 1991; Goldstrand, 1992). The 860 m thick formation is subdivided into three informal units (lower, middle, upper) based on distinct changes in alluvial architecture (Roberts et
al., 2003) (Fig. 2). The Kaiparowits Formation was deposited by
fluvial and overbank processes (Eaton, 1991; Little, 1995). Large
channels, thick paludal and floodplain deposits, and poorly developed paleosols suggest a highly aggradational, wet alluvial system (Roberts et al., 2003). The preponderance of fine-grained deposits and continental fauna demonstrate deposition within a
low-relief, inland alluvial-plain setting (Eaton, 1991).
Diverse vertebrate and invertebrate faunas have been recovered
FIGURE 1—Locality map with inset map showing the spatial distribution
of nests T1–T9 along a single bedding plane.
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ROBERTS AND TAPANILA—CRETACEOUS ANT NESTS FROM UTAH
769
FIGURE 2—Stratigraphic section for the Kaiparowits Formation in the present study area. 1, Overall stratigraphic relationships within the formation;
2, detailed stratigraphic relations associated specifically with strata bounding the nest-bearing unit; 3, 4, nest T8 draped by overlying sandstone,
scale 5 10 cm; 3, outcrop view; 4, close up of 3. Ms 5 mudstone; Ss 5 sandstone; Cg 5 conglomerate.
Field investigations included detailed mapping of the nest locality, measurements on nest morphometry (diameters and
heights) and spatial distribution, and gallery and chamber characterization. Nine nests were documented and are named informally T1–T9. A stratigraphic section was measured in the vicinity
of the nest locality and detailed lateral and vertical facies relationships were described (Fig. 2). Three loose nests were collected
and reposited in the Utah Museum of Natural History (UMNH
IP–2301; holotype) and in the University of Utah Ichnology Collection (UUIC–04052.01 and UUIC–04052.02; paratypes). One
of the paratype nests (UUIC–04052.02) was sectioned and polished to produce horizontal and vertical surfaces, in order to observe and describe internal features of the nest (Fig. 3).
SYSTMEMATIC ICHNOLOGY
Ichnogenus SOCIALITES new ichnogenus
Type ichnospecies.Socialites tumulus n. isp.
Diagnosis.Complex burrow network of unlined, cylindrical
galleries and chambers. Dense arrangement of branching and
anastomosing galleries dominant, forming irregular boxwork
throughout the burrow complex. Chambers connect to galleries,
and are most common below and near perimeter of nest. Chamber
diameter two to three times wider than galleries, ovate or J-shaped
with elongation and compression in horizontal plane, with blind
termination.
Etymology.Socialis (Latin), associate, ally, sociable. Refers
to the presumed social nature of insects that constructed this nest.
Discussion.Socialites is readily distinguished from other
ichnogenera that describe complex burrow networks from continental sedimentary deposits. Tacuruichnus Genise, 1997 has welldefined walls that separate a central cavity from peripheral galleries and chambers. No such wall structure is evident in
Socialites. The burrow networks of Termitichnus Bown, 1982 and
Vondrichnus Genise and Bown, 1994 have dominant spherical
subterranean nest morphologies. Archeoentomichnus Hasiotis and
Dubiel, 1995 has a distinct arrangement of stacked floors and
ramps in the central column. Krausichnus Genise and Bown, 1994
and Fleaglellius Genise and Bown, 1994 are typified by upwardly
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FIGURE 3—Sedimentary architecture of Socialites tumulus n. isp. demonstrated by paratype UUIC-04052.02, Kaiparowits Formation, Utah. 1, Oblique
upper bedding-plane view of cone-shaped nest structure, A-A9 transect denotes vertical slab section of 2, B-B9 transect denotes horizontal slab
section of 3; 2, vertical slab section bisecting cone-shaped nest structure; 3, horizontal slab section taken 8 cm below top of cone-shaped structure;
4, diagrammatic view of horizontal and vertical slabs in 2 and 3, grey values indicate sedimentary texture from very fine sand (light grey) to
coarse sand (dark grey). All scale bars 5 5 cm.
stacked arrangements of shelves or hemispheric chambers. Parowanichnus Bown et al., 1997 and Attaichnus Laza, 1982, the
only burrow networks assigned to activity of ants, resemble Socialites by their lack of burrow linings. In contrast to Socialites,
both Parowanichnus and Attaichnus burrows penetrate beyond
depths of 1 m, their galleries are dominantly vertical and diffusely
arranged, and their chamber diameters are 2–10 times larger.
Syntermesichnus Bown and Laza, 1990 differs from Socialites
in being more than twice the size, having lined burrows, and two
distinct size-classes of galleries.
SOCIALITES
new ichnospecies
Figures 2, 3, 5
Diagnosis.Members of this ichnospecies possess an aboveground conical structure with concave flanks. Galleries and chambers shallow in depth and concentrated in and around flanks of
cone.
Description.The complex burrow network forms a general
conical shape, having a basal diameter of 10–50 cm (mean 5 26
cm) that tapers upward to a diameter of 4–15 cm (mean 5 8 cm)
at the top. The height of the positive relief cone ranges from 5 to
TUMULUS
15 cm (mean 5 10 cm) above the top of the bedding-plane surface (Fig. 4). The flanks of the cone are typically concave, although the plan view geometry of the cone is rarely symmetrical
and may include lateral bulges on the outermost flank. A complex
network of branched and anastomosing galleries occurs within,
around, and below the conical structure. Galleries are cylindrical
and vary slightly in diameter, between 3 and 5 mm with a mean
diameter of 3.9 mm. Galleries are the most abundant type of burrow in the structure and they connect to the larger, but less abundant chambers. The chambers are ovate to J-shaped and they are
elongated and flattened primarily in the horizontal plane. They
have diameters between 8 and 18 mm and lengths between 14
and 65 mm. The trace fossils are exposed along a single beddingplane surface, weathering out in positive epirelief (Fig. 5).
Etymology.Tumulus (Latin), a mound of earth, hillock, hill.
Refers to the conical, moundlike morphology of the nest, interpreted to be the surface entrance.
Type.Holotype, UMNH IP–2301.
Other material examined.Two paratypes: UUIC–04052.01
and UUIC–04052.02.
Occurrence.Holotype and paratypes collected near the top of
ROBERTS AND TAPANILA—CRETACEOUS ANT NESTS FROM UTAH
771
FIGURE 4—Range of conical morphology of Socialites tumulus n. isp. Idealized, symmetrical forms based on basal diameter, top cone diameter, and
maximum relief above bedding-plane surface. The dashed line represents the true shape for the top of each nest.
the middle unit of the Kaiparowits Formation (;475 m from base
of formation) in southern Utah (378389100N, 1118509300W). Nine
examples of the trace fossil are located in the type area, all within
an area of about 25 m2 and along a single bedding plane.
Discussion.The most common traces observed with Socialites tumulus are large meniscate back-filled burrows (8–10 mm in
diameter) that typically are around the nest structures. These burrows likely were produced by another insect of larger body size,
such as beetles; however, they may also represent backfilling of
chambers and passages by the actual nestbuilders (Bordy et al.,
2004). The close proximity of nine nests within a single bedding
plane suggests that they may be modules of a single large colony;
however, no unequivocal evidence of connectivity between nests
was observed. Also, it is possible that even more nests existed
prior to construction of the road that cuts across the nest-bearing
layer. Draping of the nest structures by ripple-laminated sandstone
suggests that they are the subaerial to shallow subterranean portions of the nest.
laterally for about 20 m. A variety of different trace fossils also
are observed in this unit, including Skolithos Haldeman, 1840,
meniscate back-filled burrows, and large 5–8 cm diameter subvertical burrows that may be of vertebrate origin, based on their
large size. The most distinctive traces observed, however, are
those assigned herein as Socialites tumulus n. isp.
The sedimentary unit below the nests consists of either thin,
wavy (30–50 cm), olive green, fissile mudstone or thicker (.1
m) fine- to medium-grained sandstone. Few identifiable trace fossils or sedimentary structures are preserved in these underlying
deposits.
Above the nest-bearing unit, tabular fine- to medium-grained,
ripple-laminated sandstone (50–75 cm) drapes the conical nest
structures (Figs. 2, 3, 6). An exposed view of nest T8 clearly
shows the positive relief of the nest structure in relation to the
overlying sandstone unit (Fig. 2). Although most galleries and
chambers are infilled with fine-grained sediment, many also are
passively infilled by coarser sand derived from the overlying
sandstone unit (Figs. 3, 5, 6).
SEDIMENTOLOGY OF NEST SITE
Sedimentologic investigation of the nest-bearing layer and immediately overlying units provides insight into the development
and preservation of these complex trace fossils. The nest locality
is located within a fluvial and overbank sequence. Large channel
sandstones, relatively thick packages of floodplain mudstones, and
thin crevasse-splay sandstones dominate the nest site stratigraphy.
Weakly developed paleosols (entisols and inceptisols) are also
common, characterized by drab gray colors, poorly developed horizons, occasional small carbonate nodules (4–10 mm), and moderate to minor bioturbation (e.g., root traces, burrows, and insect
nests). The nests are located within a lenticular siltstone that
coarsens up into planar and ripple-laminated, very fine-grained
sandstone. The portion of each nest located below the conical
mound structures is typically observed to be in siltstone that
grades upwards into very fine-grained sandstone (Fig. 3). However, the cone-shaped portion of each nest is typically within the
very fine-grained sandstone. The nest-bearing unit can be traced
PALEOENVIRONMENTAL INTERPRETATION
The sandstone unit that drapes the fossil insect nests is interpreted as a fluvial crevasse-splay deposit, consistent with rapid
burial and preservation of the subaerial portion of the nest structures during an overbank flooding event. Coarser sand infilling of
many galleries and chambers occurred passively during this rapid
burial event. Fine-grained filling of many galleries (e.g., T1–T7)
may have been caused by: active sealing-off of galleries by the
tracemaker to prevent inundation by floodwater (Hasiotis, 2000,
2002), redistribution of sediment as the nest was reestablished, or,
alternatively, it may be due to passive infilling of abandoned nests
prior to the flooding event (Fig. 6). Nests T8 and T9 do not show
evidence of fine-grained backfill, suggesting that certain portions
of the colony were most likely abandoned following the flooding
event.
The shallow nature of these nests, with dense concentrations
of galleries and rarer chambers near the surface, is consistent with
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FIGURE 5—Socialites tumulus n. isp., Kaiparowits Formation, Utah. 1, Sandstone bedding plane with series of conical nest structures in positive
relief with arrow pointing to nest T1; 2, T7, UUIC–04052.01, oblique plan view, note typical chamber (white arrow) and gallery (black arrow);
3, 4, T6, UMNH IP–2301; 3, oblique plan view; 4, vertical view, note arrow showing the coarser (dark) sandstone surrounded by finer (light)
sandstone. Scales, 1–3, 10 cm; 4, 5 cm.
shallow ant or termite nests from moist environments or areas
with a high water table (Bown et al., 1997; Hasiotis, 2002, 2003).
The location of the nests in a proximal floodplain setting with
abundant floodbasin pond deposits and weakly developed, hydromorphic paleosols support this hypothesis (Roberts et al., 2003).
Additionally, the identification of these nests demonstrates that
there were periods of soil development and relative floodplain
stability in this highly aggradational alluvial setting.
DISCUSSION OF PROBABLE TRACEMAKER AND PALEOECOLOGY
The distribution and consistent size and shape of galleries and
chambers observed in Socialites tumulus n. isp. are the hallmarks
of nest structures produced by such social insects as termites and
ants (e.g., Wilson, 1971; Hölldobler and Wilson, 1990; Genise,
1997; Hasiotis, 2002, 2003). Analysis of the architecture (size,
morphology, distribution, and complexity) of fossil insect nests is
important for determining the identity of the tracemaker. Termite
nests typically are more complex than ant nests, and are constructed of mud paste formed by the mixing of saliva or excrement and soil (Genise, 1997; Hasiotis, 2003). Termite nests commonly have walls and an inner chamber (paraecie), and may
extend for several meters above the surface. By contrast, most
modern ants simply excavate their nests without lining the walls
with saliva or excrement (Wilson, 1971). Subterranean ant nests
may be composed of shallow mazes of horizontal galleries and
chambers or of deeper vertical galleries connecting discrete chambers (Tschinkel, 2003). Many species of ants also produce conical
surficial mounds as entrance and exits directly above their nests
(Sudd, 1967; Wilson, 1971). Both ants and termites are also
known to infest and build nests in shallow-rooted tree buttresses
or trunks, during or after death of the tree. The Kaiparowits nest
trace fossils were closely examined to determine if they may actually be the remains of colonized tree trunks. Polished cross sections of one of the nests (UUIC–04052.02) reveal a uniform veneer of coarse sand covering the entire cone structure, with
galleries along the top external surface of the cone (Fig. 3), suggesting that these are not the remnants of tree buttresses.
Socialites tumulus appears to be more similar to nest structures
produced by ants, based on reviews of both modern and fossil ant
and termite nests (Sudd, 1967; Wilson, 1971; Laza, 1982, 1997;
Levan and Stone, 1983; Bown and Laza, 1990; Hölldobler and
Wilson, 1990; Hasiotis and Dubiel, 1995; Wang et al., 1995;
ROBERTS AND TAPANILA—CRETACEOUS ANT NESTS FROM UTAH
773
FIGURE 6—Diagrammatic interpretation of sequential formation of Socialites tumulus n. isp. 1, Floodplain deposition of fining-upward (arrow) sand
and silt; 2, initial colonization by social insects producing conical nest in floodplain deposit; 3, burial of nest structure by later crevasse-splay
deposit; note passive infilling of upper chambers and galleries by overlying coarse sediments (e.g., T8); 4, reestablishment of aboveground portion
of the nest, infilling of fine-grained sediment from below into overlying coarse sediment (e.g., T1–T7).
Bown et al., 1997; Genise, 1997; Smith and Mason, 1998; Genise
et al., 2000; Hasiotis, 2002, 2003; Tschinkel, 2003; Bordy et al.,
2004). Ichnological evidence supporting this interpretation includes apparently unlined, horizontal and vertical galleries and
flattened subhorizontal chambers; lack of central, columnar walls
or paraecie; and distinctive, cone-shaped nest architecture. Additionally, the presence of at least nine conical nest structures
within an area of approximately 25 m2 is consistent with modern
ant supercolonies, which are composed of multiple, interconnected nests extending for 10–1,000 m2 (Wilson, 1971; Hölldobler
and Wilson, 1990; Hasiotis, 2002, 2003). However, due to the
many similarities between ant and termite nests and a paucity of
unequivocal features in Socialites tumulus, termites cannot be
ruled out as possible tracemakers.
Fossil ant nests are among the least common and least understood of all insect nest trace fossils (Hasiotis, 2003), and they
have been described previously from only a few Mesozoic localities in the western United States, including the Triassic Chinle
Formation, the Jurassic Morrison Formation, and the Lower Cretaceous Cedar Mountain Formation. Body fossils of Mesozoic
ants also are rare, extending back to the early Late Cretaceous of
New Jersey and Siberia (Hölldobler and Wilson, 1990; Agosti et
al., 1997; Grimaldi et al., 1997; Grimaldi, 1999).
Discovery of Socialites tumulus from the Late Cretaceous of
southern Utah provides an important new datum for reconstructing the evolutionary and ecological history of social insects
(ants?). Since nests commonly have multiple functions, including
shelter, food and garbage storage, and egg and young rearing, they
can provide rare insight into the behavior and social structure of
the tracemaker. Socialites tumulus is placed in the behavioral category Polychresichnia of Hasiotis (2003), constructed to recognize trace fossils that represent multiple uses and behaviors. Such
fossil insect nests as Socialites tumulus provide direct evidence
tracing back the antiquity of social behavior in the fossil record.
CONCLUSIONS
Social insects are important components of many modern terrestrial ecosystems, but the low preservation potential of insect
body fossils has limited their recognition in ancient ecosystems.
The trace fossil record of social insects provides alternative means
of assessing their relative diversity and importance in the geologic
record. Trace fossils provide proxy evidence of the presence and
diversity of social insects, and also yield evidence pertaining to
behavior and evolution. Additionally, valuable clues concerning
such paleoenvironmental data as soil moisture, water table, and
climate may be gleaned through such ichnological investigations.
In this study, Socialites tumulus n. isp. is interpreted as the
shallow subterranean and aboveground portions of a social insect
nest. These ichnofossil nests provide evidence for the existence
of ants (or termites) in the Late Cretaceous Western Interior Basin
of North America, and, if produced by ants, it is the first record
of Late Cretaceous nest-building behavior by ants. Recognition
of these trace fossils also aids in our understanding of soil development and floodplain stability in this dynamic alluvial system.
Additionally, this is a report of the rare insect trace fossils in the
Kaiparowits Formation.
ACKNOWLEDGMENTS
Field work and funding for this project was supported by the
BLM–Grand Staircase–Escalante National Monument and a grant
(JSA 035007) to EMR and M. Chan. We thank D. Powell for
facilitating this research and for his help in obtaining collection
and research permits. We thank S. Hasiotis and T. Demko for
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constructive and thought-provoking reviews, and A. Ekdale for
helpful suggestions in an earlier review of this paper.
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ACCEPTED 25 MAY 2005