B. hemispherica of the Late Cretaceous Bagh Group occurred in shale and is observed to be unlined and passively filled allochemic sandstone burrows (Fig. 3A) on the sole of the sandy allochemic limestone. The Phanerozoic evidence suggests that sea anemones preferred sandy substrates (Chamberlain, 1971, Hagadorn and Bottjer, 1999 and Orłowski and Radwański, 1986) or muddy substrate (Pacześna, 2010). The recent study suggests the proliferation of sea anemones in siliciclastic shelves of Early Palaeozoic palaeocontinents as compared to carbonate sediments (Hoşgör and Yilmaz, 2018). Recently, Patel et al. (2018) reported plug-shaped burrows Conichnus and Conostichus from the shale of the same stratigraphic level, the Late Cretaceous Bagh Group sediments exposed around the villages of Bhekhadiya and Uchad in the Narmada district. This study suggests the Late Cretaceous sea anemones probably preferred fine-grained, soft, unconsolidated, non-fluidized clastic substrate for burrowing and withdrew themselves from the burrow with the onset of mixed siliciclastic-carbonate sedimentation (Fig. 4).

The analysis of the B. hemispherica specimens shows smooth or equally spaced, thin, ring-like structures on the wall exterior, which suggests that the sea anemones initially remain in an upright position by contracting circular and longitudinal muscles of the column (Fig. 4A, ii). Batham and Pantin (1950) have also observed the contracted state of circular and longitudinal muscles in the recent sea anemones. The sea anemone later relaxes its muscles for stability (Fig. 4A, iv) in the fine-grained clastic sediments and repeats the cycle till the column reaches the desired depth and holdfast sediments (Fig. 4A). This prevents uproot of sea anemones by normal waves and currents and the part of the column remains above the sediment–water interface.

The present specimens of B. hemispherica are vertical to inclined and show a wide range in their dimensions (Table 1). The variable inclinations observed in the present specimens with respect to depth (penetration) suggest the burrows to be the resultant of the different behaviours of sea anemones. The vertical burrows suggest that the column of sea anemones moved downward and remained in an upright position (Fig. 3 and Fig. 4), while the burrows with a highly variable inclination of 35–90 (Fig. 3E–J) suggest that the sea anemones were tangential to the surface. The sea anemones behave aggressively when their tentacles touch a genetically different species; however, it is known as social aggression when they behave aggressively with similar species (Francis, 1988). During social aggression, the aggressor stretches and bends the column in attack while the prey, as a defence, contracts and penetrates its column deeper within the sediment (Francis, 1988). A few of the paired burrows, inclined towards each other and showing variable penetration depths and inclinations (Fig. 3I, J), reflect the social aggression behaviour. However, a few of the unpaired subvertical burrows (Fig. 3G) are considered to be the result of swaying of the organism in search of food, a behaviour analogous to that of the recent sea anemones (Batham and Pantin, 1950).

Alpert (1973) considered the three ichnospecies of Bergaueria, i.e. B. hemispherica, B. radiata, and B. perata as the resultant of the final state of contraction of muscles of the organisms. Pemberton and Magwood (1990) suggested the abundant occurrence of same species as well as different ichnospecies within the same substrate as a result of taxonomic differences in the tracemakers rather than muscle state, the preservational aspect, and the substrate conditions. The low ichnotaxonomic and ethological diversity associated with a high density of trace fossils in shales indicates a stressful environment favourable for an opportunistic trace maker (Bromley, 1996, Ekdale, 1985, Pacześna, 2010 and Uchman, 1992).

The important features observed in specimens of B. hemispherica are its prominent relief and contrasting lithology with host sediments that have enhanced the preservation potential of burrows at the bed junction of sandy allochemic limestone and shale. According to Savrda (2007), the trace fossil taphonomy is the result of two factors, namely ichnological fidelity and trace fossil visibility. Processes such as scavenging, reworking, mechanical fragmentation, microbial decomposition, and dissolution negatively affect the ichnological fidelity. The surface mixed layer, which is a result of active bioturbation, has a poor preservation potential. Owing in part to the lack of a surface mixed layer, the ichnologic fidelity of firmground ichnofabrics is considered to be typically high (Savrda, 2007), which is also supported by the monodominant occurrence of B. hemispherica.

The overlying sandy allochemic limestone shows the presence of unaltered, least abraded, disarticulated, unoriented bivalve oyster shells showing faint growth lines. The plan view of the bivalves shows the convex position upwards (Fig. 3K), while the side view shows the dominance of straight orientation with few gentle inclinations (Fig. 3L) The lack of breakage of shells indicates non-transportation by currents; however, the convex up position and horizontal (side view) orientation of the bivalve shells indicates deposition due to traction currents (Fürsich and Oschmann, 1993). The random orientation of the bivalves also indicates a rapid deposition, episodic toppling and reworking of the sediments (Sanders et al., 2007). Therefore, the overall taphonomy of the shell beds indicates a final deposition caused by short-term high-energy events such as a storm flow; the host rock sandy allochemic limestone represents event-bed sediments.

According to Savrda (2007), B. hemispherica is interpreted as having been produced prior to the deposition of event-beds, i.e. they are pre-depositional traces. In the present study, the plug-shaped open burrows extend down in the fine-grained clastic sediments (shale) and are filled with sharp contrasting lithology, mixed siliciclastic-carbonate sediments, i.e. event-bed sediments.

The trace makers of Bergaueria occur mainly in shallow water deposits (Benyoucef et al., 2017, Crimes and Anderson, 1985 and Narbonne, 1984), flysch deposits (Crimes and Crossley, 1991, Książkiewicz, 1977, Prantl, 1945 and Uchman, 1995), and wave-dominated prograding deltaic deposits (Bhatt and Patel, 2017). Specimens of B. hemispherica are reported from a wide range of environments such as flysch deposits of Eocene of Italy and western Slovenia (Tunis and Uchman, 1996); prograding shallow, shelf environment of Lower Cambrian of Gog Group, Alberta in Canada (Pemberton and Magwood, 1990); deltaic to shoreface deposits of Late Triassic of Nayband Formation of Iran basin (Bayet-Goll and De Carvalho, 2017); shallow marine deposits of Neoproterozoic of Camaquã Basin of Brazil, Poleta Formation, California and Middle Triassic of Germanic Muschelkalk Basin (Knaust, 2006 and Netto, 2012); shallow to deeper subtidal mud flats of the Late Precambrian of the Lolab Formation, India (Raina et al., 1983); offshore environment of the Lower Palaeozoic of the Santa Rosita Formation of northwestern Argentina (Mángano et al., 2005); deep-water facies of the Early to Early–Middle Cambrian of Yukon and Ellesmere Island (Hofmann et al., 1994), and Lower–Middle Eocene of the Montpelier Formation, Jamaica, West Indies (Blissett and Pickerill, 2004); deep sea fans of the Lower Devonian of Wapske Formation, Eastern Canada (Han and Pickerill, 1994). Bayet-Goll et al., 2015 and Bayet-Goll et al., 2016 have potentially used ichnogenus Bergaueria to interpret the physiochemical parameters of the middle, lower shoreface and upper offshore environment with different trace fossil suites of the Late Cretaceous, Iran.

The environmental conditions have a direct effect on the behaviour of an organism and to a large extent affect its preservation too, which is then manifested in the resulting biogenic structure (Savrda, 2007). The trace maker of Bergaueria is considered to be a shallow and mid-tier bioturbator, rarely preferring deep tier (Buatois et al., 2017), and resides generally in an environment that has availability of prey and lack of irritating stimuli such as wave swash or excessive light (Batham and Pantin, 1950). In addition to the above-mentioned factors, the abundant and undeformed nature of the studied burrows B. hemispherica (Fig. 3B) indicates soft, unconsolidated, but non-fluidized substrate. High population density indicates the flourishing of suspension feeders like sea anemones in oxygenated and nutrient-rich water conditions in which abundant organic matter was available in suspended mode.

Sea anemones are indicative of fully marine conditions (Gingras et al., 2011) and, hence, B. hemispherica, whose probable tracemaker is actinarian sea anemones (Alpert, 1973, Arai and McGugan, 1968, Häntzschel, 1965, Radwański and Roniewicz, 1963 and Seilacher, 1956) indicates euryhaline conditions. The lack of slump structure in the burrows, generally caused by the degradation of the body (Alpert, 1973) and a uniform filling of overlying sediments indicates simultaneous departure of trace makers and immediate filling of the vacated burrows (Fig. 4C).

Bergaueria is observed in the lower shale unit of the 6-m-thick intercalated series of sandy allochemic limestone and shale. The shale unit indicates calm to slightly agitated conditions, with a slow rate of sedimentation, probably during inter-storm phases represented by sandy allochemic limestone deposits. Further, the sharp contact between shale and sandy allochemic limestone where B. hemispherica occurs is marked by the erosional surface, indicating a change in the rate and pattern of sedimentation in increasing energy conditions. These cycles of thin sandy allochemic limestone intercalation with the shale indicate a shallow, but basinal marine environment. The sandy allochemic limestone-shale intercalation is overlain by the mudstone and underlain by micritic sandstone beds, which indicates a slightly upward-deepening succession from the shoreface to offshore (above SWWB) in an open shallow marine environment. Such an environment shift is also reported earlier in areas surrounding Margarita Island in Venezuela (Abbott, 1997) and Chachao Formation of Mendoza shelf (Doyle et al., 2005).

The repeated storm events and the fair-weather sedimentation in a low-energy environment produced contrasting hydrodynamic energy affecting the trace maker of Bergaueria. The colonization of sea anemones in shale indicates calm and moderate turbidity conditions with the availability of food in suspension mode since the sea anemones are characterized by filter feeding apparatus and would prefer environment having turbid water rich in suspended load (Pacześna, 2010). The high amount of suspended load near the sediment–water interface is believed to clog the filter feeding apparatus of suspension feeders (Perkins, 1974). The vacation and quick burial of the burrows indicate the slight increase in mixed siliciclastic–carbonate sediment flux, which clogged the filter feeding apparatus of sea anemones and forced them to depart.

The sharp contact of the underlying Bergaueria-bearing shale, a burrow of euryhaline trace maker, with overlying sandy allochemic limestone rich in oysters has palaeoecological implications. Moreover, the present study represents the oyster canopy that starved the sea anemone colonization in changing hydrodynamic condition and did not allow them to settle and caused complete displacement. Though the oysters are considered to be associated with the restricted environment, their low diversity and high density are a result of a reduced competition by the other organisms for space and nutrients (Doyle et al., 2005). In the present study, the occurrence of B. hemispherica as monospecific and high density underneath the oyster-rich sandy allochemic limestone also indicates similar environmental conditions. The monodominant occurrence of B. hemispherica in the Late Cretaceous Bagh Group suggests that the actinarian sea anemones (r-selected) had the ability to physiologically adapt the stressed environmental conditions.