A highly diverse ichnofauna in Late Triassic deep-sea fan deposits of Oman
ABSTRACT 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.
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ABSTRACT: The most extensive ichnofauna yet recorded from a deep water Lower Palaeozoic sequence occurs within the distal turbidites of the Lower Silurian Aberystwyth Grits Formation of Central Wales.The strata contain an abundant assemblage comprising 25 ichnogenera: Asteriacites, Bergaueria, Chondrites, Cochlichnus, Cosmorhaphe, Glockerichnus, Gordia, Helicolithus, Helminthopsis, Helminthoida, Hormosiroidea, Lorenzinia, Megagrapton, Monomorphichnus, Neonereites, Nereites, Palaeophycus, Paleodictyon, Planolites, Protopaleodictyon, Spirorhaphe, Spirophycus, Squamodictyon, Subphyllochorda, Taphrhelminthopsis; 36 ichnospecies are described, three of which (Asteriacites aberensis, Helminthopsis regularis, Cosmorhaphe elongata) are new.The inorganic sedimentary structures and trace fossils of some 418 sandstone beds were examined in detail; 16 per cent of the beds commence with Divisions A or B and 84 per cent with Division C of the turbidite sequence. This indicates a relatively distal environment, mainly receiving low velocity turbidity currents, and favouring trace fossil preservation. The most common traces were Helminthopsis, Paleodictyon, and Squamodictyon which were found on 46 per cent, 34 per cent, and 19 per cent of the beds examined.Data from this, and other recently described sequences, confirms that there was a gradual increase in trace fossil diversity in the deep oceans throughout the Lower Palaeozoic, in contrast to the situation in shallow water shelf seas where a peak was reached as early as the Lower Cambrian.Geological Journal 12/2006; 26(1):27 - 64. · 1.66 Impact Factor
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ABSTRACT: The Permian Chert Event (PCE) was a 30 Ma long episode of unusual chert accumulation along the northwest margin of Pangea, and possibly worldwide. The onset of the PCE occurred at about the Sakmarian–Artinskian boundary in the Sverdrup Basin, Canadian Arctic, where it coincides with a maximum flooding event, the ending of high-frequency/high-amplitude shelf cyclicity, the onset of massive biogenic chert deposition in deep-water distal areas, and a long-term shift from warm- to cool-water carbonate sedimentation in shallow-water proximal areas. A similar and coeval shift is observed from the Barents Sea to the northwestern USA. A landward and southward expansion of silica factories occurred during the Middle and Late Permian at which time warm-water carbonate producers disappeared completely from the northwest margin of Pangea. Biotically impoverished and increasingly narrow cold-water carbonate factories (characterised by non-cemented bioclasts of sponges, bryozoans, echinoderms and brachiopods) were then progressively replaced by silica factories. By Late Permian time, little carbonate sediments accumulated in the Barents Sea and in the Sverdrup Basin, where the deep- to shallow-water sedimentary spectrum was occupied by siliceous sponge spicules. By that time, biogenic silica sedimentation was common throughout the world. Silica factories collapsed in the Late Permian, abruptly bringing the PCE to an end. In northwest Pangea, the end-Permian collapse of the PCE was associated with a major transgression and with a return to much warmer oceanic and continental climatic conditions. Chert deposition resumed in the distal oceanic areas during the early Middle Triassic (Anisian) after a 8–10 Ma interruption (Early Triassic Chert Gap). The conditions necessary for the onset, expansion and zenith of the PCE were provided by the thermohaline circulation of nutrient-rich cold waters along the northwestern and western margin of Pangea, and possibly throughout the world oceans. These conditions provided an efficient transportation mechanism that constantly replenished the supply of silica in the area, created a nutrient- and oxygen-rich environment favouring siliceous biogenic productivity, established cold sea-floor conditions, hindering silica dissolution, while increasing calcium carbonate solubility, and provided conditions adverse to organic and inorganic carbonate production. The northwest margin of Pangea was, for nearly 30 Ma, bathed by cold waters presumably derived from the seasonal melting of northern sea ice, the assumed engine for thermohaline circulation. This process started near the Sakmarian–Artinskian boundary, intensified throughout Middle and Late Permian time and ceased suddenly in latest Permian time. It led to oceanic conditions much colder than normally expected from the palaeolatitudes, and the influence of cold northerly-derived water was felt as far south southern Nevada. The demise of silica factories was caused by the rapid breakdown of these conditions and the establishment of a much warmer marine environment accompanied by sluggish circulation and perhaps a reduced input of dissolved silica to the ocean. Complete thawing of northern sea ice would have ended thermohaline circulation and led to warm and sluggish oceanic conditions inimical to the production, accumulation and preservation of biogenic silica.
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ABSTRACT: The Paleocene to Middle Eocene Tarcau Sandstone at Buzau Valley, eastern Carpathians, Romania, records sedimentation in a turbidite system. These strata contain a diverse and abundant pre‐ and postdepositional ichnofauna consisting of 35 ichnogenera and 54 ichnospecies. The predepositional assemblage is rich in graphoglyptids and ornate grazing trails; simple grazing trails, resting traces, and feeding structures also occur. The predepositional assemblage includes Acan‐thorhaphe, Belorhaphe, Cardioichnus, Circulichnus, Coch‐lichnus, Cosmorhaphe, Desmograpton, Fustiglyphus, Gordia, Helicolithus, Helminthopsis, Helminthorhaphe, Lorenzinia, Megagrapton, Paleodictyon, Paleomeandron, Protopaleodictyon, Scolicia (S. strozzii), Spirorhaphe, Spirophycus, Treptichnus, and Urohelminthoida. The ich‐nodiversity, composition, ethology, and morphologic complexity of the predepositional association are indicative of the Nereites ichnofacies. The postdepositional association essentially consists of dwelling, feeding, and grazing traces, and is represented by Chondrites, Glockerichnus, Halopoa, Nereites, Ophiomorpha, Phycodes, Planolites, Polykampton, Scolicia(S. prisca. S. striata), Taenidium, Thalassinoides, and Zoophycos. Palaeophycus occurs in both assemblages. Allochthonous Teredolites is present in wood fragments, The postdepositional association includes elements of the Skolithos ichnofacies and facies‐crossing forms that are commonly present in deep‐marine deposits, Elements of the Skolithos ichnofacies are present not only in the most proximal parts of the turbidite system, but also in distal parts. The number of predepositional forms greatly exceeds postdepositional ones, reflecting a dominance of K‐selected over r‐selected population strategies in a stable environment. High levels of ichnodiversity in the Tarcau Sandstone are comparable with deep‐sea ichnofaunas from the Polish Carpathians and with other flysch trace‐fossil assemblages of similar age. This abundant and diverse Eocene ichnofauna supports the idea of extremely rich deep‐sea ichnofaunas in the Cenozoic.Ichnos-an International Journal for Plant and Animal Traces - ICHNOS. 01/2001; 8(1):23-62.
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