Copyright ? 2007, SEPM (Society for Sedimentary Geology)0883-1351/07/0022-0567/$3.00
PALAIOS, 2007, v. 22, p. 567–576
A HIGHLY DIVERSE ICHNOFAUNA IN LATE TRIASSIC DEEP-SEA FAN DEPOSITS OF OMAN
ANDREAS WETZEL,1* INGO BLECHSCHMIDT,2,3ALFRED UCHMAN,4and ALBERT MATTER2
1Geologisch-Pala ¨ontologisches Institut, Universita ¨t Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland;2Institut fu ¨r Geologie, Universita ¨t Bern,
Baltzerstrasse 1-3, CH-3012 Bern, Switzerland;3present address: NAGRA (Nationale Genossenschaft fu ¨r die Lagerung radioaktiver Abfa ¨lle),
Hardstrasse 73, CH-5430 Wettingen, Switzerland;4Institute of Geological Sciences, Jagiellonian University, Oleandry 2a, PL-30-063 Krako ´w, Poland
We encountered a highly diverse ichnofauna within the deep-sea fan
deposits of the Upper Triassic Al Ayn Formation in Oman. It com-
prises 32 ichnogenera: 18 ichnogenera represent predepositional gra-
phoglyptids and other trace fossils that are preserved as casts on
turbidite soles, and 14 ichnogenera represent postdepositional trace
fossils that penetrate turbidite beds. The relatively large size of the
area studied certainly favors encountering a high number of ichno-
genera. The diversity we found approximately doubles the value that
has often been stated in the literature and contradicts the paradigm
that the Triassic represents a time of low ichnodiversity in the deep
sea. Although the data are limited, in general the recovery of deep-
sea tracemakers has been very slow owing to environmental distur-
bances that resulted from cold-bottom-water circulation after the
Carboniferous–Permian glaciation. The high ichnodiversity in the Al
Ayn Formation is explained by its paleogeographic position and lo-
cally formed warm bottom waters. The Al Ayn deposits accumulated
adjacent to wide evaporitic and carbonate shelves, indicating contin-
uous warm conditions. The Al Ayn clastic system was likely influ-
enced by dense, salt-rich, warm water flowing back to the ocean from
the carbonate and evaporitic shelf area. The downwelling water may
have reduced the effects of cold water that formed during the Late
Paleozoic glaciation and the Permian–Triassic anoxia, and, thus, it
may have provided a refuge habitat. Despite the global trend of low-
diversity deep-sea ichnocoenoses, refuge habitats may have been es-
tablished in areas less affected by the otherwise harsh conditions.
The purpose of this paper is to describe and analyze a trace-fossil
community preserved in turbiditic deep-sea deposits of the Upper Triassic
Al Ayn Formation in Oman (Fig. 1), which accumulated along the south-
ern passive continental margin of the Tethys. The trace-fossil taxa have
been compared with ichnocoenoses described in the literature for the
Permian–Middle Jurassic. We discuss the implications on the long-term
diversity trend found in deep-sea trace fossil associations.
The trace-fossil record for the deep sea is affected by a considerable
gap in knowledge from the Permian until the Middle–Late Jurassic (e.g.,
Crimes, 1974; Seilacher, 1974; McCann, 1990; Uchman, 2003, 2004).
Most of deep-marine ichnocoenoses are assigned to the Nereites ichno-
facies (Seilacher, 1967). The high diversity within the Nereites ichnofa-
cies during the late Mesozoic and early Cenozoic is largely attributed to
the occurrence of complex-patterned, surface, and near-surface trace fos-
sils commonly referred to as graphoglyptids (Fuchs, 1895).
The diversity trend of deep-marine trace fossils through time is still a
matter of contention. Crimes (1974) counts the total number of ichnotaxa
for a geologic period and detects a slight increase in diversity through
most of the Phanerozoic, with a strong increase in diversity from the
* Corresponding author.
Cretaceous to the Paleogene. He realizes, however, that data for some of
the periods within the Phanerozoic are sparse. Simultaneously, Seilacher
(1974) describes a steady increase during the Phanerozoic, accentuated
by a peak in diversity during the Late Cretaceous. This analysis was based
on the diversity of individual assemblages. McCann (1990) tests these
observations and finds that the increase in diversity was neither as dra-
matic nor as consistent during the Phanerozoic as originally suggested.
Buatois et al. (2001) reviews those works, suggesting that the former
reflects global diversification patterns and that the latter provides infor-
mation about the structure of individual communities. Uchman (2004)
provides a detailed evaluation of deep-sea ichnodiversity and suggested
a relative maximum during the Early Carboniferous, a marked decrease
at the end of the Carboniferous, a minimum during the Permian and Early
Triassic, and then a stepwise increase—interrupted by a short decline in
the Albian—toward the Eocene maximum, followed by a decline in the
Oligocene. Uchman (2004) invokes oceanographic events that interrupted
the long-term evolutionary trend toward increasing diversity.
In spite of many differences, several studies of diversity trends of deep-
sea ichnocoenoses through time share some similarities. Major crises as
interpreted from the body-fossil record are not so clearly reflected by
deep-sea trace fossils; however, data from trace-fossil associations within
the Nereites ichnofacies are sparse for those times. The strong decrease
in ichnodiversity during the Late Paleozoic occurred before the Permian–
Triassic mass extinction. Uchman (2004), therefore, suggests that the for-
mation of oxygenated cold deep water during the Permo–Carboniferous
glaciation (e.g., Frakes et al., 1992) led to increased oxygenation of bot-
tom waters. This strongly affected the deposition of organic matter in the
deep sea, which, in turn, had severe consequences for deep-sea biota. It
is possible, however, that the causal factors of the biotic crisis were ini-
tiated earlier than previously thought and did not spread to shallow-water
successions until the Late Permian (J.-P. Zonneveld, personal communi-
The very low ichnodiversity within the Nereites ichnofacies during the
Permian and the slight increase during the Triassic and Early and Middle
Jurassic described by all authors have not been analyzed in detail because
few turbiditic deep-sea deposits from this time span have been studied.
Only nine deep-sea ichnofossil occurrences have been analyzed for a time
interval covering more than 120 Ma.
MATERIAL AND METHODS
We logged four sections in detail in the central and eastern Oman
Mountains (see Fig. 1), and we give locations for the individual sections
in Universal Transverse Mercator (UTM) grid data using the World Geo-
detic System 84 (WGS; see Table 1). The biostratigraphy of the sections
we studied is based on radiolarians, sponge spiculae, foraminifers, and
corals (Blechschmidt et al., 2004).
Trace fossils have been classified by comparing them with previously
described material to ascertain accurate identification (e.g., Uchman,
1995a, 1998). To minimize taxonomic artifacts owing to many unresolved
WETZEL ET AL.
FIGURE 1—Study area. Inset shows location of the Sultanate of Oman. Numbers
refer to sections analyzed in detail; their exact positions are listed in Table 1.
FIGURE 2—Paleogeographic map for the Middle Triassic (240 Ma; Scotese, 1997);
arrow points to the study area. Continental areas are shaded; marine realms in white.
TABLE 1—Studied outcrops.
(UTM/WGS 84)Depositional environment
Distance to outcrop no. (km)
depositional lobe to fan fringe
depositional lobe to fan fringe
upper midfan to depositional lobe
midfan to depositional lobe
problems in ichnotaxonomy, we restricted long-term diversity analysis to
the ichnogenus level (e.g., Uchman, 2004).
During the Early Permian, a rift system developed between Gondwana,
including the Arabian Plate, and the Cimmerian blocks to the east (Ziegler
and Stampfli, 2001). Subsequent spreading starting in the Late Permian
led to the formation of the Neo-Tethyan Hawasina Basin along the Ara-
bian Platform (Fig. 2). The passive continental margin deposits comprise
the Hamrat Duru Group (Fig. 3; Blechschmidt et al., 2004). Continuous
Middle–Late Triassic extension led to a subdivision of the Hawasina Ba-
sin by the formation of the Misfah Platform, which separated the proxi-
mal Hamrat Duru Basin from the Umar Basin (Be ´chennec et al., 1990).
During the Late Cretaceous, ocean floor and Hawasina strata were dis-
sected and thrust onto the Arabian Platform (Allemann and Peters, 1972;
Glennie et al., 1973; Watts and Blome, 1990).
The Mesozoic Hamrat Duru Group is subdivided into six formations
based on the proportion of carbonates and siliciclastics and two distinct
phases of high radiolarian content (Blechschmidt et al., 2004; Fig. 3).
Sea-level fluctuations, particularly second and third order, affected de-
position within the basinal settings. In this basin, Blechschmidt et al.
(2004) interpret carbonate dominance as indicative of high sea level,
whereas siliciclastics indicate falling or low sea level. During the Late
Triassic, a siliciclastic-dominated succession accumulated on a deep-sea
fan. These deposits make up the Al Ayn Formation (Fig. 4). The sedi-
mentology is described in detail by Blechschmidt (2002) and Blech-
schmidt et al. (2004), and we briefly summarize that description here.
The Al Ayn Formation exhibits two major progradation phases, which
correspond to third-order fluctuations of relative sea level. The Al Ayn
Formation consists primarily of quartz arenites and locally variable pro-
portions of carbonate-calcareous sandstone, quartz-bearing calcarenite,
and limestone conglomerate. Terrigenous clastics eroded from cratonic
crystalline basement and local sediment cover were transported across
the Arabian Platform carbonate shelf to the Hamrat Duru Basin. Paleo-
flow directions imply that sediment was transported from sources in the
south and southwest to the north in the proximal areas and toward north-
west or east to southeast in the distal regions of the Hamrat Duru Basin.
The paleocurrent pattern and facies relationships suggest the development
of several radial fans along the Arabian passive continental margin. Sed-
iments of the Al Ayn Formation were deposited predominantly by sub-
marine sediment-gravity flows in base-of-slope and abyssal-plain settings.
The architecture of the lithofacies associations provides insights into
the development of the Al Ayn deep-sea fan system (Fig. 5). It can be
adequately described by a sand-prone, radial submarine fan model (e.g.,
Mutti and Ricci Lucchi, 1972; Mutti 1992; Stow et al., 1996). In the
proximal fan areas, channeled and amalgamated thickly bedded sandstone
and conglomerate strata are organized predominantly into fining- and
thinning-upward successions representing submarine channels. Cyclically
stacked, coarsening-upward sandstone lobes grade into sandy-silty distal
lobes of a middle fan and finally into a lower fan to basin-plain distal
facies, indicating a retreat of the fan. We discuss the ichnology of the
mainly centimeter-to-decimeter-thick, middle-to-lower-fan sandstonebeds
RESULTS AND INTERPRETATION
The Upper Triassic Al Ayn Formation contains 32 ichnogenera. Eigh-
teen ichnogenera, mostly graphoglyptid trace fossils, are predepositional
and cast by turbidites (Fig. 6; Table 2). Fourteen ichnogenera are post-
depositional with respect to turbidite deposition and penetrate them (Fig.
HIGHLY DIVERSE TRIASSIC DEEP-SEA ICHNOFAUNA
FIGURE 3—Compound lithologic log of the Hamrat Duru Group (from Blech-
schmidt et al., 2004). The studied interval of the Al Ayn Formation is dominated
FIGURE 4—Schematic lithologic logs of the outcrops studied (from Blechschmidt,
2002). Facies associations represent areas shown in Figure 5. Small-scale and large-
scale trends in bedding indicate coarsening- and thickening-upward and fining- and
thinning-upward of beds, respectively. FA ? facies associations; A ? distal turbi-
dites and hemipelagites in an outer-fan-to-fan fringe setting; B ? medium-to-high
concentrated turbidity current deposits on a sandy lobe with minor channels in a
middle-to-outer fan setting; C ? high-density turbidity current and debris-flow de-
posits in a channelized upper-to-middle fan setting.
7; Table 3). The very high ichnodiversity within the Al Ayn Formation
becomes evident in comparison to other Permian–Middle Jurassic trace
fossil assemblages belonging to the Nereites ichnofacies (see Supple-
mentary Data1). The contribution of graphoglyptid traces to ichnodivers-
ity is 56%, and hence is relatively high when compared with other ichno-
coenoses within the Nereites ichnofacies for the time interval from the
Permian to the Jurassic (15%–66%; Uchman, 2004) or for the Phanero-
zoic (10%–70%; Uchman, 2004). The ichnocoenoses of the Al Ayn For-
WETZEL ET AL.
FIGURE 5—Schematic representation of the Al Ayn depositional system during the
Late Triassic (from Blechschmidt et al., 2004). Letters refer to depositional environ-
ments: A ? fan fringe; B ? depositional lobe; C ? mid-fan; D ? upper fan; E ?
continental slope with slumps.
FIGURE 6—Schematic drawings and photographs of predepositional burrows encountered in the Al Ayn Formation. Short descriptions are given in Table 2.
mation are more diverse than previously described Paleozoic ichnocoe-
noses (a maximum of 27 ichnogenera; Uchman, 2004) but less diverse
than those described from the Late Cretaceous to Paleogene (a maximum
of 50 ichnogenera; Uchman, 2004). Among the trace fossils encountered
in the Al Ayn Formation, we distinguish those that (1) are known to
occur continuously in the fossil record, (2) are known to occur only in
rocks younger than Triassic, (3) are known to occur only in Paleozoic
rocks, or (4) are restricted to the Al Ayn Formation (new ichnotaxa).
Predepositional Trace Fossils
Long-Term Occurrence in Deep-Sea Deposits.—Thirteengraphoglyptid
ichnogenera have been reported from Late Triassic to Middle Jurassic
deep-sea strata (see Supplementary Data1; Uchman, 2003): Belorhaphe,
Buthotrephis, Circulichnis, Cochlichnus, Desmograpton, Glockerichnus,
Gordia, Helminthopsis, Lorenzinia, Megagrapton, Paleodictyon, Strobi-
lorhaphe, and Treptichnus. With the exception of Buthotrephis and Coch-
lichnus, all these ichnogenera are in the Al Ayn Formation.
Glockerichnus, Helminthopsis, Megagrapton, Paleodictyon, and Stro-
bilorhaphe are known from both late Paleozoic and early Mesozoic suc-
cessions. Lorenzinia may have occurred continuously through the Early
Mesozoic but thus far has only been reported from Lower Triassic and
Lower–Middle Jurassic deposits (see Supplementary Data1). Desmograp-
ton pamiricus, found in Triassic–Jurassic deposits, has a typical geometry
and size (for details, see Uchman, 2003). The specimens reported from
the Eocene by Buatois et al. (2001) are smaller and more sharply bent
up than the Mesoczoic ones and, hence, can be clearly distinguished.
Belorhaphe and Gordia are known from the Paleozoic and Late Perm-
ian, respectively, and then from Early and Late Jurassic deep-seadeposits;
however, they have not yet been recorded in the Late Permian–Triassic.
Specimens from Oman either fill an observational gap or they document
a recovery of deep-sea faunas represented by these animal behaviors. The
observational gap appears to be even longer for Agrichnium, Circulichnis,
and Helminthoidichnites. Agrichnium is known from Upper Paleozoicand
Permian flysch (Uchman, 2004) and has been described from Paleogene
flysch (Plic ˇka, 1984); the observational gap equals this time interval.
Circulichnis is known from Ordovician, Early and Middle Jurassic, and
younger deep-sea deposits (see Uchman, 2004). It has been reported,
however, in Carboniferous marginal-marine deposits (e.g., Buatois et al.,
1998). Helminthoidichnites occurs for the first time in the Early–Middle
Ordovician in Norway (Uchman et al., 2005) and is absent apparently
until the Early Cretaceous in Bulgaria (Uchman and Tchoumatchenco,
2003). Helminthoidichnites, however, is a simple tube and abundant that
the most likely explanation for the stratigraphic gap is a lack of obser-
vations. Treptichnus is known from the Paleozoic as well as the late
HIGHLY DIVERSE TRIASSIC DEEP-SEA ICHNOFAUNA
TABLE 2—Short description of predepositional ichnogenera (shown on Fig. 6).
Ichnogenus Short description
Agrichnium Pfeiffer, 1968Small, closely spaced furrows of unequal
length arranged in groups, initiating either
from a branching point or by repeated
Wide 1storder meanders formed by zigzag 2nd
order meanders, sometimes with protru-
sions. A piece was classified as cf. Belorha-
Radial dichotomously branching strings having
knoblike structures at most of the bifurca-
tions. C. bifida has been observed.
Ring-shaped, smooth trace, almost circular or
slightly ellipsoidal. C. montanus has been
Double row of U- or J-shaped, inwardly bent,
in alternating modules; opposite modules
joined by short bar. D. pamiricus was
Usually dichotomously branched strings radi-
ate from a central point or hollow central
Unbranched, horizontal, winding, or irregular-
ly meandering trace fossil, predominantly
horizontal, tending to form loops.
Horizontal, thin, unbranched, straight or
curved to circular burrow. H. tenuis was
Unbranched, cylindrical tube with curves,
windings, or irregular open meanders. H.
hieroglyphica and H. tenuis were found (see
Wetzel and Bromley, 1996).
Simple short, smooth, hypichnial ridges regu-
larly arranged in a circular row, radiating
from a round central area; L. apenninica
has been found.
Irregular net with meshes bordered by slightly
winding strings that branch at nearly right
angles. M. irregulare has been found.
Tubular central axis and regular, lateral, simple
branches. Branches turn nearly rectangular
after a short distance. O. virgatus was
Horizontal net composed of hexagonal meshes
having vertical outlets. P. goetzingeri, P.
maximum, P. arvense were identified.
Simple spiral tube having one or a few wind-
ings. The found part of a spiral shows an
increasing curvature, thus classified as S.
Horizontal string bent at one end in a spiral.
Spirophycus involutissimus has been found.
Horizontal central tube and numerous lateral
short, blunt, clavate branches. S. clavata has
Simple or zigzag, straight or curved segments
associated with vertical or oblique tubes.
Treptichnus isp. has been found.
Overall V-like structure composed of short
tubes arranged in tresslike pattern; both
open to the same side. V. nizwaensis was
Belorhaphe Fuchs, 1895
Chondrorhaphe Seilacher, 1977
Circulichnis Vialov, 1971
Desmograpton Fuchs, 1895
Glockerichnus Pickerill, 1982
Gordia Emmons, 1844
Helminthoidichnites Fitch, 1850
Helminthopsis Heer, 1877
Lorenzinia Gabelli, 1900
Megagrapton Ksia ˛z ˙kiewicz, 1968
Omanichnus Wetzel et al., 2007
Paleodictyon Meneghini, 1850
Spirodesmos Andree, 1920
Spirophycus Ha ¨ntzschel, 1962
Strobilorhaphe Ksia ˛z ˙kiewicz,1968
Treptichnus Miller, 1889
Vitichnus Wetzel et al., 2007
Mesozoic (Uchman, 2004). Also in this case an observational gap is
Occurrence in Post-Triassic Deep-Sea Deposits Only.—Prior to dis-
covery in Oman the earliest occurrence of Chondrorhaphe is Late Cre-
taceous (Uchman, 2003). The Oman specimens may represent the activity
of ancestors with the same animal behavior or other organisms that had
a similar ethology early in the Mesozoic.
Occurrence in Paleozoic Deep-Sea Deposits Only.—Spirodesmos and
Spirophycus are common in Paleozoic flysch; both have been reported
from the Permian and, therefore, represent a continuation of the record.
Spirodesmos is only known from Upper Paleozoic and Lower Triassic
deposits (Uchman, 2003).
New Ichnogenera.—Two trace-fossil forms observed in the Oman ma-
terial share some characteristics with Saerichnites and Agnodipodas. They
exhibit diagnostic features distinguishing them from these and other
ichnogenera, however. These new forms represent new ichnotaxa, Oman-
ichnus and Vitichnus (Wetzel et al., 2007).
Postdepositional Trace Fossils
Several ichnogenera are thought to occur continuously in deep-sea set-
tings since the Paleozoic. We describe the less-common trace fossils in
some detail but only list the other traces (see Supplementary Data1).
Because of their environmental significance, we deal with Arenicolites
and Protovirgularia briefly here. In addition, we discuss Ophiomorpha,
Thalassinoides, and Zoophycos as they represent the earliest deep-sea
occurrences of these ichnogenera so far.
Arenicolites.—Arenicolites is found within a distal channel on a deep-
sea fan. Larval or adult animals might have been carried within the ne-
pheloid layer or by turbidity currents down canyon, and they could have
colonized sediment similar to that which they inhabited in the source area
(e.g., Crimes, 1977; Wetzel, 1984; Grimm and Fo ¨llmi, 1994). Arenicol-
ites, therefore, may indicate down-canyon current flow, even in times
without turbidity current activity.
Protovirgularia.—As the main habitats of the Protovirgularia produc-
ers are shallow-marine environments (e.g., Seilacher and Seilacher,1994),
it is very likely that adults or larvae of the Protovirgularia producers
have been transported to deep sea in a similar way as those of Arenicolites
by suspension currents or within a nepheloid layer. It is also possible that
Protovirgularia producers inhabited deep-sea environs more or less con-
tinuously while being repeatedly imported from shallow-water areas. The
first abyssal Protovirgularia was reported from Silurian flysch (e.g.,
Crimes and Crossley, 1991).
Thalassinoides and Ophiomorpha.—The stratigraphic record of these
ichnogenera has been analyzed by Carmona et al. (2004). Thalassinoides
and Ophiomorpha can belong to the same burrow system in deep-sea
deposits found intercalated with muddy and sandy sediments (Kern and
Warme, 1974); therefore, they are considered together here. The earliest
shallow-marine Ophiomorpha were reported from Middle Pennsylvanian
strata in Utah (Driese and Dott, 1984). The first deep-marine Ophio-
morpha was described from Upper Jurassic flysch (Tchoumatchenco and
Uchman, 2001). The finds in Oman, therefore, indicate an earlier occur-
rence in such an environment. The occurrence of Ophiomorpha in Tri-
assic deep-sea fan deposits is not unexpected because the corresponding
ichnogenus Thalassinoides has been reported from Lower to Middle Tri-
assic flysch (Zhang and Li, 1998). Consequently, the occurrence of
Ophiomorpha and Thalassinoides in the Upper Triassic Al Ayn Forma-
tion suggests that the colonization of the deep-sea by Thalassinoides- and
Ophiomorpha-producing animals started earlier than previously thought,
possibly during the Early Triassic (see Supplementary Data1).
Zoophycos.—The specimens found in the Al Ayn Formation represent
the oldest so far described occurrences from deep-sea fan deposits char-
acterized by the Nereites ichnofacies. Note that the Paleozoic deep-water
Zoophycos of Bottjer et al. (1988) do not match the criteria of the Nereites
ichnofacies (e.g., Frey and Pemberton, 1984). Similarly, the deep-water
Zoophycos reported by Pek and Zapletal (1990) from the Early Carbon-
iferous is not associated with graphoglyptids and, hence, was later placed
by these authors in the Zoophycos ichnofacies (Zapletal and Pek, 1999).
The Zoophycos producers were assumed to have initially colonized
deep-sea turbiditic environments during the late Mesozoic (e.g., Seilacher,
1986; Chamberlain, 2000). The Oman specimens, however, documentthat
colonization of the deep sea started at least by the Late Triassic. The
WETZEL ET AL.
FIGURE 7—Schematic drawings, photographs and short descriptions of postdepositional burrows encountered in the Al Ayn Formation. Short descriptions are given in
simple, rooster-tail-like Zoophycos are penetrated by simple tubes like
Planolites and Paleophycus and hence, were probably produced at a shal-
low depth within the sediment (Fig. 8).
Zoophycos moved from shallow-water to deeper-water habitats through
time (e.g., Seilacher, 1986; Bottjer et al., 1988; Olivero, 2003). In addi-
tion, Zoophycos changed from a simple, rooster-tail-like form (also with
incomplete coiling; see Knaust, 2004) in Paleozic shelf deposits to geo-
metrically complex, coiled, and lobate forms in Upper Cretaceous–
Eocene flysch deposits (e.g., Seilacher, 1986; Chamberlain, 2000). From
the Toarcian to Cenomanian, Olivero (2003) found that simple forms in
shallow-water deposits evolved into increasingly advanced forms when
the Zoophycos producers migrated into deep basinal deposits. At this
time, coiling became complicated, and the spreite-production program
increasingly variable, resulting in lobate forms. The Oman Zoophycos
demonstrates that its morphology did not become increasingly advanced
when the Zoophycos producers moved from bathyal to abyssal environ-
ments and does not confirm the linkage of behavioral change and colo-
nization of a new environment proposed by Seilacher (1986) and Olivero
(2003). Instead, the Oman Zoophycos suggests that behavioral evolution
of simple forms was delayed after colonization of a new environment.
The ichnodiversity of the Al Ayn Formation is the highest reported so
far from Permian–Triassic Nereites ichnofacies deposits. It is similar to
or slightly higher than the Paleozoic maximum but below that for Late
Cretaceous deep-sea deposits (Fig. 9). The extraordinary high diversity
has important implications for understanding Late Paleozoic–Early Me-
sozoic deep-sea trace-fossil communities. Two aspects are of major im-
portance for evaluating the significance of the ichnodiversity of the Al
Ayn Formation: (1) area-size effects—because of the patchiness of the
fauna, the number of ichnogenera observed should increase with the size
of the area investigated (Fu ¨rsich, 1975)—and (2) paleogeographic posi-
tion of the investigation area—in this case, we infer continuous warm
conditions adjacent to wide evaporitic and carbonate shelves.
To some extent, there is a positive correlation between the amount of
section available for examination and ichnodiversity encountered (Fu ¨r-
sich, 1975). This effect is not as important within the context of the
present study because a wide area was investigated. For comparison with
other ichnocoenoses and analysis of the ichnodiversity, there are severe
consequences when exposures of varying sizes have been compared; in
particular, the smaller areas are subject to underestimated real ichnodi-
versity. With respect to the Al Ayn Formation, the size effect may influ-
ence the ichnodiversity in two ways. First, the size effect can be more
pronounced in the deep-sea (compared to shallow-water) ichnocoenoses
that were analyzed by Fu ¨rsich (1975) because of the high patchiness of
the deep-sea fauna (e.g., Gage and Tyler, 1991). Second, the larger the
area investigated within a deep-sea fan setting, the higher the probability
of sampling an increasing number of subenvironments with varying ich-
The studied outcrops of the Al Ayn Formation were up to 45 km away
from each other (Fig. 1). In the outcrop with the highest ichnodiversity,
we found 22 ichnogenera, representing 71% of all ichnotaxa. Within 40
km, we encountered the total ichnodiversity (Table 4). The number of
postdepositional ichnogenera increases by 20%, and that of predeposi-
tional ichnogenera by 35%, when the size of the study area is increased.
We did not observe identical trace-fossil assemblages at any two locali-
ties. These findings in the Al Ayn Formation are similar to the sparse
data in literature, in particular, the study of the Lower Eocene Gurnigel
Flysch in Switzerland by Crimes et al. (1981). In that study, 42 ichno-
genera were observed in one outcrop. The number of postdepositional
ichnogenera increased by increasing the size of the area investigated by
about 20%; the number of predepositional trace fossils increased by 72%
An increase in diversity with increasing area sampled is more pro-
nounced for the predepositional trace fossils and, hence, implies a stron-
ger patchiness of their producers than for postdepositional trace fossils.
This is not unexpected because the postdepositional fauna partially con-
sists of trace-fossil producers living permanently within the habitat, es-
pecially multilayer colonizers (Uchman, 1995b), that can survive turbidite
erosion and deposition. Examples of this ability include the producers of
Ophiomorpha, Thalassinoides, Teichichnus, and Zoophycos. In addition,
some postdepositional trace fossils were produced by animals that recol-
HIGHLY DIVERSE TRIASSIC DEEP-SEA ICHNOFAUNA
TABLE 3—Short description of postdepositional ichnogenera (shown on Fig. 7).
Ichnogenus Short description
Arenicolites Salter, 1857Vertical U-tube without a spreiten, tube diame-
ter 2 and 3 mm, limbs 2–6 cm apart.
From a master shaft, tubes regularly ramify at
depth to form a dendritic network; C. intri-
catus, C. targionii, and C. isp. have been en-
Invaginated, regularly spaced cones form ridges
on a slightly winding burrow. I. torquendus
is nearly straight.
Winding to meandering, central back-filled tun-
nel enveloped by even to lobate zone of re-
worked sediment. N. irregularis has been
Simple to complex burrow systems lined at
least partially with pelletoidal sediment. O.
recta has been found.
Branched or unbranched, lined, cylindrical bur-
row, fill structureless, of the same lithology
as host rock. P. striatus and P. isp. have
Palmately to laterally branched, flabellate bur-
row system; partially preserved burrows
could not be assigned to an ichnospecies.
Repeated narrow, U-shaped tubes enclose sprei-
ten at mm to cm scale, branching from an
axial spreiten. P. incertum has been found.
Unlined, rarely branched, straight to tortuous,
tubular burrow; structureless fill differs from
host rock. P. beverleyensis was found.
Tube, almond-shaped or triangular in cross sec-
tion, internally having successive pads that
form ribs on the exterior. P. isp. has been
Two parallel rows of round, more or less regu-
larly distributed pits or pustules on bedding
plane. Saerichnites isp. has been found.
Long, wall-shaped burrow consisting of a pile
of gutter-shaped laminae. T. rectus is
straight, unbranched with a retrusive spreite.
Three-dimensional burrow system composed of
smooth, cylindrical tubes branching at Y- to
T-shaped, enlarged points. T. suevicus has
U- or J-shaped protrusive elements of variable
length and orientation form a spreiten struc-
ture. Simple rooster-tail–like specimens have
Chondrites Sternberg, 1833
Imponoglyphus Vialov, 1971
Nereites MacLeay, 1839
Ophiomorpha Lundgren, 1891
Palaeophycus Hall, 1847
Phycodes Richter, 1850
Phycosiphon Fischer-Ooster, 1858
Planolites Nicholson, 1873
Protovirgularia McCoy, 1850
Saerichnites Billings, 1866
Teichichnus Seilacher, 1955
Thalassinoides Ehrenberg, 1944
Zoophycos Massalongo, 1855
FIGURE 8—Zoophycos (spreite structure indicated by stippled lines) cross-cut by
Planolites and Palaeophycus tubes implying shallow burial depth of the Zoophycos
producer; scale with cm marks; outcrop Tawi Shu’ah. Pl ? Planolites; Pa ? Pa-
onized the sediment after an event, indicated by sequential cross-cutting
relationships and sediment fill (e.g., Wetzel and Uchman, 2001); among
them are the producers of Chondrites, Nereites or Phycosiphon.
In contrast, the producers of graphoglyptids belong to the equilibrium
fauna (Ekdale, 1985) and, hence, need time to reestablish within an event
bed. Within a given environment, therefore, the patchiness of the fauna
is likely to be inversely related to the recurrence time of turbidity cur-
rents. In the case of prolonged intervals between events, patchiness is
balanced to some degree by the long interval between events during
which time animals may migrate through an area and produce traces.
Short-time intervals between events favor the preservation of patchiness
and do not allow the full reestablishment of a highly diverse fauna. The
local or regional conditions may be important, however; the benthic food
content, currents, substrate consistency, and other factors affect the di-
versity as in modern deep-sea settings (e.g., Gray, 2002).
The evaluation of area size leads to the question of whether ichnodi-
versity depends only on area size or if it also depends on sampling a
potentially larger number of subenvironments. It is important that most
graphoglyptids are preserved in low-erosive turbidites depositedprimarily
in distal settings (e.g., Seilacher, 1977). Consequently, in good outcrops
exposing ?100-m-thick sections, several subenvironments of a middle-
to-lower deep-sea fan can be identified, as each one is represented by bed
packages up to some tens of meters thick (e.g., Stow et al., 1996; Blech-
schmidt et al., 2004). For instance, in the outcrop with the highest ich-
nodiversity of the Lower Eocene Gurnigel Flysch (Zollhaus,Switzerland),
Crimes et al. (1981) recognizes three subenvironments within proximal-
to-distal outer fan settings (channel, interchannel, and depositional lobe).
Similarly, Blechschmidt et al. (2004) identifies the same three subenvi-
ronments for the Al Ayn Formation in the Tawi Shannah section. Con-
sequently, as the thickness of exposed section exceeds several tens of
meters, several subenvironments are likely to be represented.
The size of a sampling area has a major effect on the ichnodiversity
encountered, even if the exposed section is sufficiently long and covers
a variety of subenvironments. These findings imply a patchiness of the
deep-sea fauna; with respect to the available data, the faunal patches may
be in the order of some square kilometers, or more.
Another aspect of late Paleozoic–middle Mesozoic deep-sea ichnodi-
versity is that, at the end of the Permian and the beginning of the Triassic,
the marine fauna heavily suffered from widespread, if not worldwide,
anoxia because oceans became stratified (e.g., Wignall and Twichett,
2002). The strongest decrease in ichnodiversity within the Nereites ich-
nofacies during the Late Paleozoic, however, occurred before the severe
end-Permian environmental crisis. Uchman (2004), therefore, suggests
that the decrease in deep-sea ichnodiversity was related to the onset of
the Permo–Carboniferous glaciation. This glaciation oceanographically is
believed to have led to initiation or intensification of the thermohaline
circulation in the world’s oceans by formation of cold, oxygenated water
in polar regions mainly in the southern hemisphere (e.g., Frakes et al.,
1992). Coincident with the onset of the Paleogene glaciation, deep-sea
ichnodiversity decreased from the Eocene to the Oligocene because of
similar processes (Uchman, 2004). During the Late Paleozoic, such cold-
water masses affected Panthalassa and the southeastern parts of the Pa-
leotethys. In contrast, warm-to-temperate climates surrounded the western
parts of the Paleotethys (e.g., Scotese, 1997). The prevailing cold, deep
water led to an increased formation of deep-marine cherts, which declined
in adundance during the Early–Middle Triassic (e.g., Beauchamp and
Baud, 2002). At this time, around the Ladinian-Carnian boundary, the
climate returned to a so-called warm mode, which persisted until the
Middle Jurassic (Frakes et al., 1992). The reasons for the low ichnodi-
versity within the Nereites ichnofacies are not really known, however, as
the database for this time interval is very small.
If Uchman’s (2004) assumption is correct, then the post-Permian re-
covery of trace-fossil-producing fauna could have started from shelf and
WETZEL ET AL.
FIGURE 9—Variation of ichnodiversity in the Nereites ichnofacies on the ichnogenus level versus time. Line represents the long-term variation found by Uchman (2004)
based on 103 studies; star marks the extraordinary high ichnodiversity of the Al Ayn Formation.
TABLE 4—Ichnodiversity in relation to size of study area.
Outcrop1. Tawi Shannah2. Al Jil 3. Tawi Shu’ah4. Wadi Sal
Al Ayn Formation (Triassic, Oman)
Distance to outcrop 1 (km)035 40
not found in
not found in
not found in
5 17 1217
Outcrop 1. Zollhaus 2. Fa ¨lli Ho ¨lli3. Ho ¨llbach4. Fayaux
Gurnigel Flysch (Eocene, Switzerland)
Distance to outcrop 1 (km)08 10 40
not found in
not found in
not found in
continental slope areas or other refuge habitats located in warm climates.
Within this context, investigations in the recent show that shallow-water
fauna attempting to colonize deep-water settings appear to be strongly
affected by water temperature (e.g., Tyler and Young, 1998).
Potential refuge habitats might have been located on the shelves and
continental margins of the Neotethyan Realm having warm water, for
instance, along the equator or low latitudes during the Triassic (e.g., Sco-
tese, 1997). Such conditions are met at the Triassic continental margin
of the Arabian Peninsula, which could have acted as a nearly ideal refuge
habitat. As a passive continental margin, the Arabian Peninsula has had
a wide, open-marine carbonate shelf (e.g., Ziegler, 2001; Blechschmidt
et al., 2004) and an extensive continental slope because of its gentle
inclination (compared to active continental margins). Furthermore, the Al
Ayn deep-marine clastic system within the Hamrat Duru Basin was partly
incised into the carbonate shelf (e.g., Ziegler, 2001), and oceanward some
ridges might have affected the deep-marine circulation (Blechschmidt et
al., 2004). During the Late Paleozoic, the Cimmerian blocks to the east
(e.g., Ziegler and Stampfli, 2001) might have restricted the influence of
cold deep-marine circulation. In addition, evaporites accumulated on a
wide shelf to the north of the study area from the Late Permian to the
Late Triassic (e.g., Ziegler, 2001), producing warm saline shelf waters
that could have flowed back to the ocean, affecting at least the local
oceanographic conditions and replacing anoxic (e.g., Kump et al., 2005)
or cold deep water. If the assumption holds true that organisms can re-
HIGHLY DIVERSE TRIASSIC DEEP-SEA ICHNOFAUNA
colonize the habitats of their ancestors from refuge habitats, the general
but interrupted evolutionary trend toward increasing ichnodiversity could
have continued during the Triassic at the same or a somewhat lower level
than before the Late Paleozoic crisis in the deep sea.
Thirty-two ichnogenera found within the turbiditic deposits of the Al
Ayn Formation in Oman represent the most diverse deep-sea ichnofauna
observed thus far from the Cambrian until the Early Cretaceous. Post-
depositional burrows provide about half of the diversity, graphoglyptids
the other half. The richness in graphoglyptids is very high compared to
other Paleozoic and Early Mesozoic deep-sea ichnocoenoses.
The analysis of the ichnofauna data from the Al Ayn Formation and
from literature clearly shows that the size of the investigated area affects
the ichnodiversity found. The area size certainly affects the comparison
with other trace-fossil assemblages if the size of the area studied differs.
Within an area of a few kilometers, about two-thirds to three-quarters of
all ichnogenera were encountered; however, additional ichnotaxa often
are found in outcrops tens of kilometers away. The increase in ichnodi-
versity with size of area studied may reflect the patchiness of trace-
producing fauna rather than the number of subenvironments investigated.
As graphoglyptids are preserved best in low-erosive, distal turbidite sub-
environments that form bed packages a few meters to a few tens of meters
thick, in good outcrops numerous subenvironments are consequently en-
The highly diverse ichnofauna in the Al Ayn Formation contradicts
the post-Paleozoic diversity minimum from the Late Paleozoic–Late Ju-
rassic. The high ichnodiversity of the Al Ayn Formation is explained by
its paleogeographic position and the understanding that times of high
ichnodiversity in Nereites ichnofacies coincide with periods characterized
by warm bottom water in the oceans. The Al Ayn sands accumulated
adjacent to wide evaporitic and carbonate shelves since the Late Paleo-
zoic, indicating continuous warm conditions. So the Al Ayn clastic sys-
tem may have been influenced by dense, salt-rich warm water flowing
back to the ocean from the carbonate and evaporitic shelf area. The down-
welling water may have reduced the effects of cold water formed during
the Late Paleozoic glaciation and the Permian–Triassic anoxia, and, thus,
provided a refuge habitat. In spite of a global trend to low-diversity, deep-
sea ichnocoenoses within refuge habitats could have been less affected
by the otherwise harsh conditions.
The present study is part of IB’s Ph.D. research project, financially
supported by the Swiss National Science Foundation (project 2000-
050681). The authorities of the Sultanate of Oman, especially Dr. Hilal
Al Azri, Director General of Minerals, Ministry of Commerce and In-
dustry, Sultanate of Oman, provided logistical help and support. L. Bua-
tois, G. Ma ´ngano, and J.-P. Zonneveld made helpful suggestions. S. Lauer
and A. Reisdorf helped with figure preparations.
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ACCEPTED DECEMBER 23, 2006