The probable trace maker of B. hemispherica is the fact that one encounters different age groups of sea anemone burrows extended at variable depths in soft, unconsolidated, non-fluidized mud to get stability. It occurs as densely packed, vertical to subvertical, usually unornamented, occasionally showing faint, thin, ring-like structures, allochemic sandstone-filled cylinders with hemispherical base extending to variable depths in the shale. The diameter-to-height ratio calculated for 44 burrow specimens shows that the diameters of most of the specimen are smaller than their height. The probable trace maker of B. hemispherica is the fact that one encounters different age groups of sea anemones, which extended their column at variable depth in soft, unconsolidated, non-fluidized mud to get stability. The inclined nature of paired burrows towards each other suggests social aggression while unpaired inclined burrows suggest swaying in search of food. The monodominant occurrence of B. hemispherica as pre-storm colonization of r-selected organisms (sea anemones) suggest sstressed environment and simultaneous vacation of the burrower reflects rapid sedimentation due to high-energy storm events. Further, storm and inter-storm events deposited sandy allochemic limestone and shale series, respectively, but did not form an identical condition for the colonization of the sea anemone. The species B. hemispherica of the Bagh Group revealed physicochemical parameters (energy conditions, turbidity, sedimentation rate, bathymetry, suspended organic matter, substrate consistency, oxygen, and salinity) of the Late Cretaceous transgressive sea.
La trace fossile de Bergaueria hemispherica appartenant à un groupe de terriers en forme de bouchons est préservée sous l’aspect d’hypichnia dans le calcaire sableux allochimique du Crétacé supérieur du groupe de Bagh affleurant près du village de Karvi, district de Chlota Udepur, Inde occidentale. Elle apparaît très tassée, verticale à subverticale, en général non ornementée ; elle présente occasionnellement des structures en forme d’anneaux, minces et décolorés, des cylindres remplis de grès allochimique à base hémisphérique s’étendant jusqu’à des profondeurs variables dans le shale. Le rapport diamètre/hauteur, calculé sur 44 échantillons de terriers, montre que les diamètres de la plupart des échantillons sont plus petits que la hauteur. L’auteur probable des traces, B. hemispherica, appartient à des groupes d’anémones de mer d’âges différents, qui étendent leur colonne à des profondeurs variables dans une boue meuble, non consolidée, non fluidifiée en voie de stabilisation. L’aspect incliné l’un vers l’autre de terriers en paire suggère une agression d’organismes vivant en société, alors que les terriers qui ne se présentent pas par paire suggèrent une oscillation à la recherche de nourriture. L’occurrence monodominante de B. hemispherica sous forme d’une colonisation pré-tempête d’organismes sélectionnés pour la stratégie r (anémones de mer) suggère un environnement de stress, tandis que l’absence simultanée de l’organisme fouisseur reflète une sédimentation rapide due à la forte énergie de l’événement de tempête. Plus tard, les événements de tempête et inter-tempêtes déposent des séries de calcaire sableux allochimique et de shale, respectivement, mais n’entraînent pas de conditions identiques pour la colonisation par des anémones de mer. L’espèce B. hemispherica du groupe de Bagh révèle les paramètres physicochimiques (conditions d’énergie, turbidité, taux de sédimentation, bathymétrie, matière organique en suspension, oxygène et salinité) de la mer transgressive du Crétacé supérieur.
Plug-shaped burrows, R-selected organisms, Social aggression, Physicochemical parameters, Shoreface, TuronianTerriers en forme de bouchons, Organismes sélectionnés r, Agression d’organismes vivant en société, Paramètres physicochimiques, Avant-côte, TuronienpresentedHandled by Annalisa FerrettiIntroduction
Single-entrance plug-shaped burrows are observed since the Precambrian and are divided into five groups by Pemberton et al. (1988) based on overall burrow geometry, surface ornamentation, distal termination, and internal structure of the burrow. In addition, with the above criterion, Pemberton et al. (1988) adopted a numerical taxonomical approach to further classify plug-shaped forms based on consistent results of diameter/height ratios of the ichnogenera and identified four distinct forms:
diameter about 0.25 times the height (Dolopichnus);
diameter about 0.5 times the height (Conichnus);
diameter about twice the height (Conostichus and Bergaueria);
diameter about 7 times the height (Astropolichnus).
Subsequently, Buatois et al. (2017) considered nine valid ichnogenera, typical representatives of plug-shaped burrows that include AmphorichnusMännil, 1966, AstropolichnusCrimes and Anderson, 1985, BergaueriaPrantl, 1946, ConichnusMännil, 1966, and ConostichusLesquereux, 1876, LithoplaisionDiblin et al., 1991, MammillichnisChamberlain, 1971, MetaichnaAnderson, 1975, and SolicyclusQuenstedt, 1879. However, Bergaueria, characterised by a cylindrical to hemispherical base, with or without central depression and radial ridges, differs from Amphorichnus and Mammillichnis in the lack of distinct mammilla peak, Astropolichnus in the lack of radial ridges around central axial cylinder, Conichnus and Conostichus in lacking conical to subconical shape; Solicyclus and Metaichna in the lack of elliptical relief, and Lithoplaision in the lack of large, elongate, conical to subcylindrical burrows and in having a central axial tube.
The ichnogenus BergaueriaPrantl, 1945 shows a wide geological range. It is reported from the Precambrian (Crimes and Germs, 1982, Hofmann and Aitken, 1979 and Kumar et al., 1984) to Miocene (Pokorný et al., 2017 and Solórzano et al., 2017) sediments deposited in the marine environment. Bergaueria is interpreted as being a cubichnion (Arai and McGugan, 1968, Bayet-Goll and De Carvalho, 2017, Fürsich, 1974, Fürsich et al., 2018, Hakes, 1976, Häntzschel, 1965, Häntzschel, 1975 and Narbonne, 1984) or a domichnia (Alpert, 1973, Pickerill, 1989 and Pokorný et al., 2017).
The Late Cretaceous rise in sea level was concomitant with the rift-related episodes in the Indian subcontinent (Biswas, 1999). During this period, the Tethys Sea invaded the Narmada Basin and deposited fossiliferous and bioturbated nonclastic sediments of the Bagh Group, which yield a variety of shallow marine body fossils and trace fossils. Various workers have studied the rocks of the Bagh Group exposed in the lower Narmada valley (Madhya Pradesh and Gujarat states) for ichnological aspects (Badve, 1987, Chiplonkar and Badve, 1969, Chiplonkar and Badve, 1970, Chiplonkar and Badve, 1972, Chiplonkar and Badve, 1978, Chiplonkar and Ghare, 1975a, Chiplonkar and Ghare, 1975b, Chiplonkar and Ghare, 1977, Ghare and Badve, 1980, Kundal and Sanganwar, 1998, Sanganwar and Kundal, 1997, Singh and Dayal, 1979 and Verma, 1971). Recently, Patel et al. (2018) have reported plug-shaped burrows Conichnus conicus, Conostichus broadheadi, and C. stouti from the intercalated sandy allochemic limestone-shale sequence of the Bagh Group in the Narmada district. In addition to these burrows, the present paper describes another plug-shaped burrow, i.e. Bergaueria hemispherica, which occurs in the same stratigraphic level, but from a different locality. The various aspects of burrows such as ethology, preservation, and physicochemical parameters affecting the trace maker are discussed in the present paper.
Location and stratigraphy
The ichnospecies B. hemispherica is reported from the 6-m-thick sequence of intercalated sandy allochemic limestone and shale of the Bagh Group exposed near the village of Karvi in Chhota Udepur District, Gujarat (Fig. 1). The Bagh Group rocks of the lower Narmada valley are severely affected by Late Cretaceous–Early Palaeogene Deccan trap formation and emplacement of alkaline igneous rocks (Ray and Pande, 1999 and Viladkar and Wimmenauer, 1992) and by intense weathering and erosion that have restricted the continuity of the exposures. The isolated occurrences of these outcrops make them difficult to correlate and have been described variably by several workers (Bose, 1884, Guha, 1976, Murty et al., 1963 and Poddar, 1964) using informal units and local geographic names.
Ray (1981) subdivided the Bagh Group of Gujarat into the Lower Songir and Upper Uchad formations. The Songir Formation is subdivided into the Mohanfort and Raisingpur members, while the Uchad Formation is subdivided into the Bilthana and Galesar members.
Recently, Prasad et al. (2017) have revised the stratigraphy of the Late Cretaceous sedimentary succession of Madhya Pradesh and divided it into the Bagh and Lameta groups. The Bagh Group is further subdivided into the Nimar Sandstone, Nodular Limestone, and Coralline Limestone formations (in ascending order). The Songir Formation, equivalent to the Nimar Sandstone Formation of Prasad et al. (2017) is Cenomanian in age and mainly comprises conglomerate, cross-bedded sandstone and rippled micritic sandstone. The Uchad Formation (Ray, 1981) can be correlated with the Turonian Nodular Limestone Formation (Jaitly and Ajane, 2013, Prasad et al., 2017 and Tripathi, 2006) and consists of siliceous sandstone, sandy allochemic limestone–shale intercalation, mudstone, micritic sandstone, ferruginous sandstone, and muddy micrite and sandy micrite. Bergaueria has been observed in the thin-bedded oyster-rich sandy allochemic limestone of the Bilthana Member (Ray, 1981), which can be correlated with the Karondia Member (Nodular Limestone Formation) of Prasad et al. (2017). This unit comprises an intercalated sequence of oyster-rich sandy allochemic limestone and shale. The Coralline Limestone Formation (Coniacian) overlying the Nodular Limestone Formation, which can be correlated with the Galesar Member (Ray, 1981), is comprised of thinly bedded mudstone, sandstones, shales and mixed siliciclastic–carbonate sediments. The Lameta Group was deposited over the Bagh Group and is represented as thin sedimentary units comprising cherty limestone exposed in discontinuous outcrops.
The exposed studied section comprises 1.7-m-thick micritic sandstone at the base followed up by 6-m-thick intercalated series of oyster-rich sandy allochemic limestone, wackestone, and shale succeeded by 2-m-thick mudstone (Nodular Limestone), 3-m-thick shale and 1.8-m-thick wackestone at the top (Fig. 2).
Materials and methods
The sedimentary succession was studied near the village of Karvi on the western side of the Amba Dongar ring complex, where a 6-m-thick succession above the rippled sandstone is comprised of oyster limestone–shale intercalations containing the trace fossils. Rock samples from the 6-m-thick Bergaueria-bearing succession were systematically collected and analysed in the laboratory for texture and mineral composition. Further, the carbonate and mixed siliciclastic-carbonate rocks are classified according to Dunham (1962) and Mount (1985), respectively. More than 300 specimens were observed and photographed in the field and amongst them, 44 have been collected and examined in the laboratory. The identified trace fossils are measured following the morphometric parameters proposed by Pemberton et al. (1988) and are described systematically. These 44 specimens are housed with an accession number GEPBE1-GEPBE44 in the Museum of the Department of Geology, The Maharaja Sayajirao University of Baroda, Gujarat, India.
Sedimentology
The 14.5-m-thick sequence is comprised of mainly mixed siliciclastic–carbonate sediments and shales (Fig. 2). The base of the sequence is marked by micritic sandstone (1.7 m) consisting of ∼ 80% well-sorted fine/medium-grained quartz grains bounded by micritic cement. The micritic sandstone is thinly bedded and shows the presence of wave ripple marks. It is overlain by 6-m-thick oyster-rich sandy allochemic limestone and shale intercalated series. The Bergaueria-bearing shale consists of fine-grained clastics, while the sandy allochemic limestone consists of ∼ 40% fine-grained, well-sorted quartz with ∼ 20% allochems in a micritic matrix. The petrographical analysis of the burrow reveals that it is filled with allochemic sandstone, which is comprised of ∼ 70% fine-grained quartz with subordinate grains of microcline, plagioclase feldspar, and ∼ 17% shell fragments in a micritic cement (Fig. 3A). The sandy allochemic limestone grades into oyster-rich rudstone intercalated with shale. The 6.8-m-thick nodular limestone has a homogeneous micritic composition that grades into wackestone comprised of unidentified fossil fragments..
Taxonomy
Ichnogenus BergaueriaPrantl, 1945
Type ichnospecies Bergaueria perataPrantl, 1945
Diagnosis. Cylindrical to hemispherical, vertical burrows with rounded base, lacking ornamentation, circular to elliptical in cross-section, structureless fill, with or without central depression and radial ridges (Prantl, 1945).
B. hemispherica Crimes, Legg, Marcos and Arboleya, 1977
(Fig. 3B–L)
Diagnosis. Bergauerians lacking a shallow central depression (Crimes et al., 1977).
Description. Usually unornamented, vertical to inclined (35–90), cylindrical burrow with rounded base, preserved as hypichnia on the sole of sandy allochemic limestone. The burrow fill is essentially structureless, some specimens display faint, thin, equally spaced ring-like structures (Fig. 3C). The dimensions of 44 specimens have been measured (Table 1); the diameter (D) of the burrow ranges from 1.5 to 2.9 cm and its height (H) varies from 0.5 to 4.0 cm. The measured specimen displayed variable D/H ratios; 29 specimens show D/H = 0.7–1.5, 11 specimens have D/H = 1.5–2 and, for 3 specimens, D/H exceeds 2.0 (Table 1).
Remarks. Four ichnospecies of Bergaueria, namely B. perata, B. langi, B. hemispherica, and B. radiata, have been identified based on wall lining and ornamentation; B. hemispherica, thin-walled, unornamented differs from thick-walled, unornamented B. langi and thinly-lined, ornamented B. perata and B. radiata (Pemberton et al., 1988). Based on morphological features such as thin wall lining and lack of ornamentation, the present specimens are identified as B. hemispherica (Crimes et al., 1977).
The burrow occurs as hyporelief on the sole of oyster bearing sandy allochemic limestone, within shale and is filled with allochemic sandstone and small bivalves shell fragments (Fig. 3A, F). The general shape is cylindrical to hemispherical, which resembles the ichnospecies B. hemispherica (Crimes et al., 1977). The holotype specimen described by Alpert (1973) shows a diameter of 5.5 and a depth of 1.4 cm, while the three specimens of B. hemispherica described by Hofmann et al. (1994) from the Cambrian of Arctic Canada are 2.5–5.0 cm wide and 1.0–2.7 cm long. The Late Cretaceous Bagh Group specimens show large variation in height, which causes variation in D/H as compared to the type specimen. Pemberton et al. (1988) have noticed overlap in diameter and height of plug-shaped burrows, but they evidenced consistency in the diameter/height (D/H) ratio; the considered Bergaueria has a diameter about twice the height. The numerical data (Table 1) of the 44 measured specimens of B. hemispherica reveals three specimens having diameters twice the height, whereas most of the specimens have either the diameter less than or equal to height. The specimens show large variations in D/H but show a high degree of similarity in morphological features, which have priority to consider as B. hemispherica (Crimes et al., 1977).
Occurrence.B. hemispherica is observed in the sandy allochemic limestone (oyster limestone) of the Bilthana Member of the Uchad Formation (N 21° 59′ 35″ latitude, E 74° 2′ 43″ longitude).
Interpretation.Bergaueria is regarded as a cubichnion or domichnion structure produced by suspension feeding anthozoans (Fürsich, 1975, Hantzschel, 1958, Howell and Hutchinson, 1958, Lessertisseur, 1955, Prantl, 1945 and Prantl, 1946) and more specifically by actinarians (Alpert, 1973, Arai and McGugan, 1968, Häntzschel, 1965, Radwański and Roniewicz, 1963 and Seilacher, 1956). B. hemispherica is interpreted as made by actinarian sea anemones (Mángano et al., 2005 and Pemberton and Magwood, 1990). The cylindrical shape of the burrow and its rounded base (Fig. 3E, F) represent the morphology of burrowing actinarian solitary sea anemones. The large variations in height displayed by B. hemispherica specimens of the Late Cretaceous Bagh Group of Gujarat are purely biological, i.e. with variable penetration of column in cohesive muddy sediments by different age groups of similar sea anemones species.
Discussion
The ichnospecies B. hemispherica is reported from clastic and nonclastic sediments of Proterozoic (Netto, 2012) and Phanerozoic Era (Blissett and Pickerill, 2004 and Han and Pickerill, 1994. Hofmann et al., 1994, Knaust, 2006, Mángano et al., 2005, Pemberton and Magwood, 1990, Raina et al., 1983 and Tunis and Uchman, 1996) and shows a wide range of bathymetry.
Ethology
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).
Preservation
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.
Environmental significance
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.
Conclusions
The conclusions that can be drawn from this study are as follows:
B. hemispherica, abundantly found in Turonian of the Bagh Group, suggests that solitary burrowing sea anemones were the trace makers that flourished in the invaded arm of the Tethys Sea.
The monospecific occurrence of B. hemispherica is limited to the fine-grained soft, unconsolidated, nonfludised clastic sediments, which appears to be favourable for sea anemone colonization.
Uniform fill, prominent relief and lack of slump structures suggest that all the sea anemones simultaneously vacated the burrow, which was subsequently filled by the mixed siliciclastic–carbonate sediments, indicating an increase in energy conditions in the shoreface environment.
The variable depths of B. hemispherica indicate different age group of sea anemones, while the paired burrows inclined towards each other and the inclined, unpaired burrow suggests social aggression and swaying in search of food respectively.
The monodominant occurrence of B. hemispherica in the Bagh Group sediments indicates stressed environmental conditions favourable for opportunistic solitary actinarians.
Acknowledgements
We thank Shrinivas Viladkar for introducing the site. We are grateful for the detailed comments and suggestions by Aram Bayet-Goll, Alfred Uchman and Annalisa Farretti, which have helped to improve the paper. ADS is thankful to Department of Science and Technology INSPIRE Grant (DST/INSPIRE/03/2015/001104).
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Geological map of the study area (after Viladkar, 1996) and locality of the collection site of the specimens of Bergaueria hemispherica (star mark).
Carte géologique de la zone étudiée (d’après Viladkar, 1996) et localisation du site de récolte des échantillons de Bergaueria hemispherica (marqué par une croix).
Measured litholog of the study area, depicting the stratigraphic position of the occurrence of Bergaueria hemispherica.
Log lithologique mesuré de la zone d’étude montrant la position stratigraphique et l’occurrence de Bergaueria hemispherica.
Photographs of burrow fill material and Bergaueria hemispherica. (A) Microphotograph of the burrow in 2.5× showing allochemic sandstone composition and subordinate grains of microcline (M), plagioclase feldspar (F) and oyster shell fragments (O). (B) Field photograph of inverted blocks of sandy allochemic limestone showing densely packed B. hemispherica burrows. (C) Close-up view of B. hemispherica. Note the overall size variation and the circular impressions (arrow) of circumferential muscles on the lower part of the burrow. Morphological variation of B. hemispherica showing (D) vertical burrow with subconical base and (E) slightly inclined burrow with rounded base showing contracted and relaxed state of muscles respectively. (F) Inclined burrow showing infill of abundant oyster shell fragments. (G) Normal and (H) inverted view of vertical to subvertical burrows of different sizes, indicating different age group of sea anemones. (I) Normal and (J) inverted view of burrows inclined away from each other suggesting social aggression. (K) Surface of sandy allochemic limestone crowded with oysters. (L) Side view of the sandy allochemic limestone showing randomly oriented oyster shells. Scale bar represents 1.5 cm. Note that B is a field photograph, while C–L are the photographs of specimens that are preserved in the museum of the Maharaja Sayajirao University of Baroda.
Photographies du matériau de remplissage des terriers et de Bergaueria hemispherica. (A) Microphotographie du terrier (grossissement 2, 5) montrant la composition du grès allochimique et les grains subordonnés de microcline (M), feldspath plagioclase (F) et de fragments de coquilles d’huître (O). (B) Photo de terrain de blocs retournés de calcaire sableux allochimique montrant des terriers de B. hemispherica fortement comblés. (C) Vue de B. hemispherica. À noter la variation globale de taille et les impressions circulaires (flèche) des muscles circonférentiels sur la partie inférieure du terrier. Variation morphologique de B. hemispherica montrant (D) un terrier vertical à base conique et (E) un terrier légèrement incliné à base arrondie présentant un état contracté et relaxé des muscles, respectivement. (F) Terrier incliné montrant un abondant remplissage de fragments d’huître. (G) Vue normale et (H) vue renversée de terriers verticaux à subverticaux de différentes tailles, indiquant un groupe d’anémones de mer d’âges différents. Vues normale (I) et vue retournée (J) de terriers inclinés et éloignés l’un de l’autre, suggérant l’agression d’organismes vivant en société. (K) Surface de calcaire sableux allochimique chargé d’huîtres. (L) Vue de profil du calcaire sableux allochimique montrant des coquilles d’huîtres orientées au hasard. La barre d’échelle représente 1,5 cm. À noter que B est une photographie prise sur le terrain, alors que les photographies de C à L ont été prises sur des spécimens conservés au musée de l’université Maharaja Sayajirao de Baroda.
Reconstructed possible burrowing mechanisms of the sea anemones and their preservation. (A). Different stages of burrowing behaviour of sea anemone. (i) Stranded sea anemone. (ii) Contraction of longitudinal and circumferential muscles during the initial stage of burrowing. (iii) Penetration of almost one-third of the column through the contraction of muscles. (iv) Relaxation of muscles at the reach of desired depth. (B) Diagram depicting the behaviour of sea anemones: (i) stranded, (ii) rested (iii) swaying for food, and (iv) social aggression. (C) The burrow features representing different behaviours of sea anemone; uniformly filled burrows (allochemic sandstone) mark the simultaneous vacation. Note: the top surface of B. hemispherica bearing bed is studded with oyster shells.
Reconstitution de possibles mécanismes de creusement par des anémones de mer et leur conservation. (A) Différentes étapes de creusement par des anémones de mer. (i) Anémone de mer échouée sur l’estran. (ii) Contraction des muscles longitudinaux et circonférentiels pendant le stade initial de creusement. (iii) Pénétration d’environ un tiers de la colonne par contraction des muscles. (iv) Relaxation des muscles quand la profondeur souhaitable est atteinte. (B) Diagramme figurant les comportements des anémones de mer : (i) échouées ; (2) au repos ; (iii) à la recherche de nourriture ; (iv) agression entre organismes sociaux. (C) Aspects de terriers représentant différents comportements d’anémones de mer ; les terriers uniformément remplis (grès allochimique) marquent un abandon simultané. À noter que la surface sommitale du banc renfermant B. hemispherica est constellée de coquilles d’huîtres.
Morphometry of the 44 collected specimens of B. hemispherica. Note: highly variable height compared to diameter.
Morphométrie des 44 échantillons de B. hemispherica récoltés. À noter la hauteur très variable comparée au diamètre.