Evaluation of coralline algal diversity from the ...

Carbonates Evaporites DOI 10.1007/s13146-014-0190-9

ORIGINAL ARTICLE

Evaluation of coralline algal diversity from the Serravallian carbonate sediments of Little Andaman Island (Hut Bay), India Suman Sarkar • Amit K. Ghosh

Accepted: 13 February 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract Palaeodiversity of coralline algae has been studied from the Serravallian carbonate sediments outcropping in the Little Andaman Island (Hut Bay). Little Andaman is the southernmost island of the Andaman Group, situated between South Andaman and Car Nicobar. The algae described in the present case study have been recovered from the fossiliferous limestone samples of Hut Bay Limestone Quarry belonging to the Long Formation and examined by means of thin-section microscopic analysis. Shannon–Weaver index has been applied for measuring the degree of algal diversity. A total of nine coralline algal genera, including three geniculate and six non-geniculate algae have been reported with the statistical data indicating a fairly conducive benthic environment. This is supported by the presence of a considerable number of growth-forms characterizing the concerned coralline algal taxa. The limited occurrence of taphonomic signatures observed in the algal thalli further strengthens the interpretation of a conducive environment. Greater diversity and abundance of benthic foraminifera present in close proximity to the coralline algae have resulted in possible failure of the latter in attaining higher levels of diverse assemblages. Factors of mild environmental disturbances, hydrodynamic conditions, and substrate availability also supposedly played a major role in determining the diversity of coralline algae. Keywords Diversity Coralline algae Growth-form Serravallian Little Andaman Hut Bay

S. Sarkar (&) A. K. Ghosh Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow 226 007, India e-mail: [email protected]

Introduction Coralline red algae (Corallinales, Rhodophyta) are one of the critical biogenic components in most Cenozoic shallow-water carbonate successions from the Eocene to the Recent (Braga and Aguirre 2004; Payri and Cabioch 2004; Bassi 2005; Perry 2005; Bassi et al. 2009a; Bassi and Nebelsick 2010). The ubiquitous nature of coralline algae is well reflected in their occurrence across a wide array of facies and biogeographical settings in the fossil record since their first confirmed appearance in the early Cretaceous (Arias et al. 1995; Aguirre et al. 2000; Braga and Bassi 2007). They extend right from the polar regions of the earth to the tropics (Roberts et al. 2002; Irving et al. 2005; Braga et al. 2009) and have long temporal ranges, which reduce their biostratigraphic importance, but increase their applications in palaeoenvironmental reconstructions and palaeobiogeographic analyses (Bosence 1991; Braga and Bassi 2007). Most coralline algal genera are indicative of certain depths and hence they are useful in palaeobathymetric estimations. Morphology of coralline algae is related to the corresponding energy conditions as coralline algae having robust and fused thallus with thick crusts, branches and columns are observed in shallow water, high energy conditions (Bosence 1991). On the other hand, coralline algae growing in deeper water, low-moderate energy conditions possess a delicate thallus with thin branches and crusts (Bosence 1983, 1991; Kundal 2011). Coralline algae also have economic importance in hydrocarbon exploration industry as a potential tool for preparing palaeoenvironmental models and as important building block in carbonate reservoir rocks and reefs (Wray 1977; Kundal 2011). The fabric of carbonate rocks is principally determined by the diversity and skeletal morphology of their

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constituent biogenic components including coralline algae (Bassi et al. 2000; Nebelsick and Bassi 2000). Taxonomic concepts pertaining to living and fossil coralline red algae have undergone numerous modifications in the last few decades (Woelkerling 1988; Harvey et al. 2003; Bassi et al. 2009b). Latest research has shown that diagnostic taxonomic features of corallines used by phycologists at generic and subfamily levels for recent material can also be identified in fossil specimens (Braga et al. 1993; Rasser and Piller 1999; Braga 2003). A major characteristic of all the corallines is an early calcifying thallus which enables these forms to be readily preserved as fossils. In terms of architecture, corallines can be divided into two groups—geniculates and non-geniculates (Woelkerling 1988; Womersley 1996). In geniculate coralline forms, the thallus is composed of rigid, calcified segments termed as intergenicula alternating with more flexible rarely calcified segments termed as genicula. Nongeniculate coralline algae have no genicula. The nongeniculate coralline algae mostly occur in crustose growthform (Woelkerling 1988) while the geniculate corallines present an arborescent (tree-like) form (Bassi et al. 2009a). The taphonomy of coralline red algae is highly dependent on their initial growth-form and environmental setting. Processes like incidence of diseases and shallow grazing are hard to determine in fossil material. Taphonomic processes like abrasion, fragmentation, and disarticulation are detrimental to the preservation potential of coralline algae while encrustation has a positive impact on improving their chances of surviving as intact artifacts (Nebelsick and Bassi 2000; Nebelsick et al. 2011). The composition, growth-forms, and taphonomic aspects of the coralline algal assemblages can represent useful palaeoecological indicators (Aguirre 1992; Bassi 1995, 2005; Perrin et al. 1995; Nebelsick and Bassi 2000; Barattolo et al. 2007; Checconi et al. 2007). To date, Palaeogene and Neogene coralline algae from the Andaman and Nicobar Islands have not been studied in detail except a few taxonomic investigations (Gee 1927; Gururaja 1977; Venkatachalapathy and Gururaja 1984; Badve and Kundal 1987, 1988, 1998; Chandra et al. 1999; Ghosh et al. 2004; Saxena et al. 2005; Ghosh and Sarkar 2013) and geological analyses (Van Bemmelen 1949; Rodolfo 1969; Srinivasan 1969, 1975; Sastry et al. 1973; Srinivasan and Singh 1978). The Neogene stratigraphic sequence of the Little Andaman Island can be divided into seven chronostratigraphic stages based on planktic foraminiferal zones (Srinivasan 1978, 1988; Srinivasan and Singh 1978; Sharma and Srinivasan 2007) viz., Jarawaian (late early Miocene), Inglisian (early middle Miocene), Ongeian (late middle Miocene), Havelockian (early late Miocene), Neillian (late Miocene), Sawaian (early Pliocene) and Taipian (late Pliocene). First

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five of these stages belong to the Archipelago Series while the remaining two are in the Nicobar Series. Long Formation of the Little Andaman Island under study here belongs to the Ongeian Stage (Sharma and Srinivasan 2007). Ongeian Stage is named after the Ongeian tribe of Little Andaman and encompasses rocks deposited during the period extending from the evolutionary first appearance datum of planktic foraminifera Globorotalia fohsi fohsi to the last appearance datum of the entire Fohsella lineage. Biostratigraphically, the Long Formation is of Serravallian age (13.9 to 11.5 Ma) based on presence of Globorotalia fohsi fohsi and Globorotalia fohsi robusta Zones at the base and top of the Ongeian stage, respectively (Srinivasan 1988; Sharma and Srinivasan 2007). Some other common planktic foraminifera of this stage include Globorotalia praemenardii, G. mayeri, G. fohsi lobata, Orbulina universa, Sphaeroidinellopsis seminulina, Protentella bermudezi, and Globigerina druryi (Sharma and Srinivasan 2007). The present study is an attempt in determining algal diversity of the Serravallian (late middle Miocene) sediments from the Little Andaman Island with supportive growth-form and taphonomic analyses. Possible competition for space with benthic foraminifera has also been associated with the eventual diversity of coralline algae as observed during the thin-section analysis. The study also aims at identifying the influence of environmental factors like bathymetry and availability of substrate in determining the diversity of coralline algae.

Geological setting The Andaman and Nicobar Islands are the sub-aerial expressions of a continuous ridge which connects the Arakan-Yoma Range of Western Burma to the drape of islands, south and west of Sumatra. They are divided into two groups—the Andamans in the north and Nicobars in the south with Ten-Degree Channel separating the two. These islands can be separated into two concentric arcs i.e., the western (outer) sedimentary arc comprising major islands of the Andamans and the Nicobars, extending southeastwards to form Indonesian orogenic belt and the eastern (inner) volcanic arc including the conical volcanoes of Narcondam and Barren Islands. A third island arc to the west of the Andaman–Nicobar Islands is in the process of emergence (Eremenko and Sastri 1977; Saxena et al. 2005). The Little Andaman Island, lying between South Andaman and Car Nicobar, is the southernmost island of the Andaman Group. The geographical position of Little Andaman is between 108300 N–108540 N, and 928200 E– 928370 E, which is separated from the South Andaman and


Fig. 1 Location map of Little Andaman Island showing the study area with respect to India and the Andaman–Nicobar Group of Islands

Car Nicobar by the Duncan Passage and the Ten-Degree Channel, respectively (Fig. 1). In terms of area, Little Andaman is nearly 43 km long and 24 km wide. This is situated about 96 km south of Port Blair. The coralline algae reported in the present case study have been

recovered from the carbonate samples belonging to the Long Formation. The limestone unit studied here unconformably overlies the calcareous mudstone unit, and is exposed along the Saw Mill Road and in a channel cutting section, adjacent to the Quarry No. 4.

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Fig. 2 Litholog of the studied section showing location and interval of sample collection

Srinivasan (1975) distinguished two principal litho-units sharing an unconformable contact in the southeastern part of the Little Andaman Island (Hut Bay) i.e., creamish yellow, molluscan rich limestone underlain by dark bluish grey, highly calcareous, soft and fragile mudstone. The limestones of Hut Bay Quarry exhibit features of dark grey to white mottling, algal oncoliths and a high fraction of vuggy and moldic porosity (Srinivasan and Chatterjee 1981).

Materials and methods Modern taxonomic concepts pertaining to neontological systematics (Rasser and Piller 1999; Maneveldt et al. 2007)

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commonly used for diversity determination of coralline red algae have been applied in the present work. The statistical parameter of Shannon–Weaver index has been used for the first time in global perspective for diversity determination of fossil algae in thin-section study. Altogether thirty limestone samples (RHQ 1–30) were collected from the Hut Bay Quarry No. 4 (Fig. 2). One hundred and twenty palaeontological thin sections were studied for the diversity, growth-form, and taphonomic analyses. For algal identification, dimensions of cells and conceptacles were measured according to the latest method proposed by Aguirre and Braga (1998) and Rasser and Piller (1999). Special emphasis was given to cut the rock samples in various orientations and obtain good visualization of tissue organization and morphology of the reproductive organs (conceptacles). Two types of sections have been studied for the present analysis i.e., parallel to the direction of filament growth and perpendicular to the thallus surface. Thin sections were studied with Olympus BX 50 Microscope and photographs were taken with Olympus PM-20 Exposure Control Unit. As a measure of diversity, the H’ index of Shannon and Weaver (1949) has been applied for the present analysis. The formula is H0 = -Rpi log2pi; where H0 is the diversity of a theoretically infinite population and pi is the proportion of the number of individuals of a particular species and total number of all the species present in the community. This statistical method is commonly applied for the living biotic assemblages but it has been applied as a novel approach in the context of algal palaeodiversity. Firstly, uniform dimensions were set for the slides under study (3.5 9 5.0 cm) and a standardized quantitative analysis was carried out. Each slide was carefully scanned to count the number of algal specimens in relation to the total population. To avoid counting the same form more than once (in case of fragmented and disarticulated specimens), cell dimensions were considered under various resolutions to tally the specimens (see Table 1). A degree of uncertainty did prevail in case of very small specimens (\100 lm) but on the overall scale, margin of error was very limited due to rare occurrence of such specimens. In case of larger unconsolidated, encrusting and arborescent (geniculate) algae, there was no discrepancy in differentiating between separate specimens. Therefore, the presently applied concept indicates palaeodiversity of algae and similar micropalaeontological entities quite precisely. The growth-form analysis follows the terminology proposed by Woelkerling et al. (1993) and taphonomic assessment is according to Nebelsick and Bassi (2000) and Basso et al. (2009). Growth-form determination in thinsection analysis is influenced by orientation and sectioning effects. Differences in growth-form have been widely applied to delimit and identify genera, species and

Carbonates Evaporites Table 1 Cell and conceptacle dimensions of coralline algae observed in the present study Genus

Cell length (in lm)

Cell diameter (in lm)

Conceptacle/sporangia size (in lm)

Observed growth-forms

Corallina

22–48

8–20

–

Arborescent

Jania

15–36

7–16

–

Arborescent

Amphiroa

45–85 (L)

8–14 (L)

–

Arborescent

17–28 (S)

6–11 (S) 85–295 (H) 9 170–430 (D)

Unconsolidated, encrusting, warty

255–325 (H) 9 380–460 (D)

Unconsolidated, encrusting, lumpy

Lithothamnion Mesophyllum

9–24 (P)

8–16 (P)

9–21 (PRF)

6–14.5 (PRF)

8–17 (POF)

5–13 (POF)

13–19 (PRF)

8–15 (PRF)

8.5–13 (POF)

6–11 (POF)

Lithophyllum

8–13 (PRF)

5–12 (PRF)

90–137 (H) 9 227–266 (D)

Unconsolidated, layered, lumpy

Titanoderma

7–12.5 (POF) 12–35 (PRF)

4–8.5 (POF) 5–10 (PRF)

70–98 (H) 9 235–320 (D)

Encrusting, layered

8–43 (POF)

4–9.5 (POF)

14–22 (PRF)

12–19 (PRF)

122–146 (H) 9 190–215 (D)

Encrusting, unconsolidated

11–20 (POF)

24–31 (POF) 39–80 (H) 9 20–75 (D)

Warty, lumpy, unconsolidated

Lithoporella Sporolithon

8–18 (PRF)

4–12 (PRF)

10–19 (POF)

6.5–10 (POF)

Common growth-forms associated with these genera are also indicated. No genicula and conceptacles were observed in case of geniculate forms (length and diameter of intergenicula mentioned here) L long cells of Amphiroa, S short cells of Amphiroa; P peripheral region, PRF primigenous filaments, POF postigenous filaments, H height, D diameter

infraspecific taxa of various coralline algae (Braga and Martin 1988; Woelkerling et al. 1993; Geronimo et al. 2002). As the growth-form analysis for smaller algal specimens (\200 lm) was difficult to state with complete precision, they were considered for diversity determination but omitted during the growth-form analysis.

Results and discussion Diversity in coralline algae Coralline algal diversity in the studied samples ranged from 1.1 to 1.9 (H’) with an average of 1.54 (Fig. 3). Diversity of benthic foraminifera (average H’ of 2.6) was higher than that of coralline algae. Benthic foraminiferal assemblages mainly comprised of smaller miliolids and rotaliids with some larger nummulitids (species of Amphistegina, Heterostegina and Operculina). Apart from coralline algae and benthic foraminifera, secondary biogenic constituents observed were molluscs, echinoderms and gastropods. As observed and interpreted both in case of modern and geological environments, several benthic foraminifera and coralline algae share common habitats at equivalent bathymetric levels (Rasser and Nebelsick 2003; Beavington-Penney and Racey 2004; Bassi and Nebelsick 2010; Ghosh and Sarkar 2013). In the present case study,

competition for space between coralline algae and benthic foraminifera is likely to have been dominated by the latter as reflected in their greater diversity. Coralline algae can be hypothesized to have attained greater diversity if benthic foraminifera were less abundant or non-dominating. Nongeniculate coralline algae were recorded in greater abundance throughout the entire section but geniculates outnumbered them in few cases (Fig. 4). This distribution of geniculates and non-geniculates with secondary presence of coral fragments presents a complete reefal palaeoenvironment with back-reef/lagoon, reef and fore-reef settings (Ghose 1977; Jauhri 1994; Ghosh and Sarkar 2013). Three geniculate i.e., Corallina (Fig. 5h), Jania, Amphiroa (Fig. 5f, g) and six non-geniculate coralline algal genera i.e., Sporolithon (Fig. 5a), Lithothamnion (Fig. 5c), Mesophyllum, Titanoderma (Fig. 5b), Lithoporella (Fig. 5d) and Lithophyllum (Fig. 5i) have been reported in the present case study. Titanoderma and Lithophyllum have been treated as synonyms by several workers till date (Campbell and Woelkerling 1990). However, diagnostic palisade cells, persistent presence of longer cells and other characteristics including gene sequence analysis have facilitated the form to be safely placed under Titanoderma (Chamberlain 1991; Bailey 1999). According to the intermediate disturbance model of Connell (1978), formation of an ecosystem is characterized by an increase in diversity when the time period is

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Fig. 3 Variation of diversity (H’) in the collected samples

marine environment which play an important role in the build-up of coralline algae-rich carbonate facies (Bassi 2005; Checconi et al. 2007; Bassi and Nebelsick 2010). Since a moderate amount of diversity has been observed in the studied samples, we can state that the factors of water circulation and quality (salinity and turbidity) in the Little Andaman Island during Serravallian were relatively favorable for algal growth and diversification. Occurrence of muddy substrate (observed in wackestone facies) is also important in determination of algal diversity as suitability of substrate is critical to the settlement and growth of algae (Nebelsick and Bassi 2000; Schneck et al. 2011). This is proven by the higher average range of coralline algal diversity in the mud-supported wackestone facies (RHQ 9 to 20) as compared to grain-supported grainstones and packstones (RHQ 1–8 and RHQ 21–30). Parameters like light and food availability, primary productivity and water temperature are controlled by water depth, which in turn affects coralline algal growth and development (Braga and Aguirre 2004; Kroeger et al. 2006; Benisek et al. 2009). Water depth apparently had an impact on the coralline algal diversity of Little Andaman since both the geniculate as well as non-geniculate forms largely indicative of shallow to relatively deeper bathymetric horizon (10–40 m) have been recorded in the present study. Interpretation of deeper-water deposition is constraining due to presence of deeper-water algae like Lithothamnion, Mesophyllum and Sporolithon present in close vicinity of shallow water-geniculate coralline algae, Lithoporella and Lithophyllum (Ghose 1977; Braga and Aguirre 2004). As algae are basically photoautotrophic, water depth has a critical role in framing the algal community structure (Adey et al. 1982; Minnery 1990; Braga and Aguirre 2004). Growth-form analysis

Fig. 4 Comparison of the abundance of geniculate versus nongeniculate coralline algae

punctuated by episodes of subtle disturbances. Moderate degree of coralline algal diversity in the present analysis shows the possible incidence of mild disturbance events like cyclones, storms, biotic turnovers, and phase shifts at regular time intervals during the period of deposition that contributed to the resulting diversity (Aronson and Precht 1995; Sigala et al. 2012). The climatic cooling to have occurred during the Miocene (Micheels et al. 2009) may be considered a major controlling factor of the diversity turnovers. Impact on diversity may also be attributed to the nutrient regime fluxes and hydrodynamic conditions of the

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Six growth-forms have been reported in the present study i.e., unconsolidated, encrusting, warty, lumpy, layered, and arborescent that represent a wide array of morphological shapes characterizing the concerned taxa (Fig. 6). Arborescent growth-form is estimated for all the geniculate specimens as their modern counterparts uniformly possess this growth-form (Bassi et al. 2009a). However, an intact arborescent growth-form in the geniculate fossil material is rare due to highly fragile nature of genicula (non-articulated) segments that are susceptible to incidences of mildest stress. The variation of growth-forms as observed in the studied coralline algae can be attributed to the differences in hydrodynamic energy and substrate morphology (Johnson 1961; Cabioch 1969; Adey and Adey 1973; Bosence 1976; Dethier 1994; Nebelsick and Bassi 2000; Nebelsick et al. 2013). Variation of coralline algal growth-

Carbonates Evaporites Fig. 5 a Sporolithon sp., lumpy thallus with sporangia (S). Sample RHQ 16. b Titanoderma pustulatum, thallus with sickle-shaped palisade primigenous cells (PL) and uniporate conceptacle (C). Sample RHQ 20. c Thallus of Lithothamnion sp. with multiporate conceptacles (C), observable bioerosion (B) and warty growth-form. Sample RHQ 7. d Encrusting thallus of Lithoporella melobesioides with magnified view of welldefined uniporate conceptacle (C). Sample RHQ 16. e Thallus of unconsolidated alga. Sample RHQ 9. f Amphiroa sp., thallus with distinct intergenicula. Core (CF) and peripheral (PF) filaments observed. Sample RHQ 13. g Amphiroa medians, intergenicula showing taphonomic trait of fragmentation (F). Sample RHQ 20. h Corallina sp. showing disarticulation with genicula (G) and intergenicula (I) both observed. Sample RHQ 5. i Layered thallus of Lithophyllum sp. Sample RHQ 24. j Single protuberance of an undetermined non-geniculate coralline alga. Sample RHQ 17

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Carbonates Evaporites Fig. 6 Incidence of different growth-forms observed in the current study

forms across the hydrodynamic gradients in reefs has also been demonstrated in the past (Martindale 1992; Gherardi and Bosence 1999). Unconsolidated growth-form showing irregular orientation of core and peripheral filaments is observed in case of Lithothamnion and Mesophyllum. At some places intergrading between unconsolidated and encrusting growthforms can be observed (only for melobesioids). Very tiny fragments of non-geniculate algae (\200 lm) superficially denote unconsolidated material but owing to lack of proper visualization of their filaments and orientation, they have

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not been considered for growth-form analysis in the present study. Fundamentally crustose coralline algae lacking protuberances and lamellate branches are the encrusting forms that have been recorded in good abundance. These growthforms have been observed both in the case of mastophoroid as well as melobesioid non-geniculates. It is very likely that they have formed as a result of transported thalli of primary encrusters of soft material which has subsequently decomposed (Nebelsick and Bassi 2000). This is due to the orientation of the thalli in the sediments along with the


general facies interpretation. They are largely or entirely attached ventrally to the substratum by cell adhesion, and thallus shape is often influenced by the nature of the substratum (Nebelsick and Bassi 2000). Common intergrading of encrusting forms can be observed with the warty, lumpy and layered forms. Warty and lumpy forms were observed abundantly in case of genus Sporolithon (Fig. 5a) and Lithothamnion (Fig. 5c). They are common for the non-geniculate coralline algae having compressed to cylindrical outgrowths that are mostly irregular having a radial organization. The commonly termed protuberances (Fig. 5j) include both these growth-forms. The coralline algae having warty growth-form demonstrate verrucose protuberances that are unbranched and less than 3 mm long. This is in contrast to the lumpy forms that have more or less swollen, contiguous protuberances. Intergrades between warty and lumpy forms are very common in the present study. Layered forms demonstrated flattened and curved lamellae i.e., lamellate branches arranged in horizontal sequence. Layered growth-form had the lowest abundance in the analyzed samples for this case study with rare occurrences in case of genus Lithophyllum (Fig. 5i). Common intergrading usually observed between layered, discoid and foliose forms (Woelkerling et al. 1993) was not recorded due to a complete absence of discoid and foliose growth-forms.

Taphonomic assessment Algal taphonomy is highly dependent on the initial growthform of coralline algae and their specific depositional environment (Nebelsick and Bassi 2000; Rasser and Nebelsick 2003; Nebelsick et al. 2011, 2013). Some taphonomic processes like encrustation increase the preservation potential through embedding by other organisms while others like abrasion, disarticulation, and fragmentation are detrimental, making it difficult to recognize the coralline algal taxa (Nebelsick and Bassi 2000; Nebelsick et al. 2011). Incidences of taphonomic processes were found to be low to moderate that indicated a supportive environmental factor contributing to the ultimate moderate scale of coralline algal diversity as observed in the present thinsection analysis. Encrustation, disarticulation, bioerosion, fragmentation, abrasion, and diagenesis have been observed in the present study. Encrustation can be readily observed in the studied samples as commonly found in melobesioid species (Kroeger et al. 2006; Checconi et al. 2007). Complex encrustation sequences can involve not only a variety of coralline algal taxa but also encrusting foraminifera and bryozoans (Nebelsick and Bassi 2000; Berning et al. 2009).

Some protuberances of coralline algal forms have also been preserved within the encrusting sequences. Disarticulation is observed in case of all the geniculate specimens in the present study (Fig. 5h). The soft, uncalcified genicula connecting the harder, calcified intergenicula represents inherent points of weakness in the algal skeleton which subsequently disarticulates/dislodges after death and decay (Nebelsick and Bassi 2000; Kundal 2011). No in situ preservation of articulated or geniculate form with compact arborescent growth-form by virtue of encrustation (only possibility of genicula to get preserved) was observed in the present analysis. Macroborers (e.g., sponges, bivalves), microborers (e.g., cyanobacteria, algae, fungi, micro-invertebrates) and grazers (e.g., molluscs, echinoderms) are the dominant agents of bioerosion (Tribollet and Payri 2001; Checconi and Monaco 2008). Bioerosion has been observed in few cases represented by mud-filled, rounded holes continuing deep into the algal crust. Few large, unconsolidated thalli of Lithothamnion have been found to present traits of bioerosion (Fig. 5c). A possible explanation for this can be herbivory operating in the buried sediments (Adey and MacIntyre 1973; Nebelsick and Bassi 2000; Basso et al. 2009). Fragmentation is observed both in the case of geniculate (Fig. 5g) as well as non-geniculate coralline algae (Nebelsick and Bassi 2000; Checconi and Monaco 2008; Nebelsick et al. 2011). It frequently results in isolated protuberances and fruticose fragments, as well as thallial segments of crusts (Nebelsick and Bassi 2000). Fragmentation is primarily a product of biological activity as well as high levels of water turbulence (Basso et al. 2008; Nebelsick et al. 2011). As the overall rate of fragmentation is not very high, it can be assumed that the unfavorable physical and biological processes acting on the carbonate sediments were not regular in nature or low in magnitude. The determination of both the taxonomic identification and growth-form analysis is dependent on the degree of fragmentation as a loss of characteristic features like the conceptacles can result in lack of identification and hence subsequent problems in further studies of facies and fabric determination (Rasser and Nebelsick 2003; Nebelsick et al. 2011). Abrasion shows a similar ratio to fragmentation in the studied samples and at places evident in the production of relatively rounded components (Basso et al. 2009; Nebelsick et al. 2011). This holds true for the intergenicula of geniculate forms, which represent relatively robust cylindrical structures pertinent to post-disarticulation phase (Nebelsick and Bassi 2000; Checconi and Monaco 2008; Nebelsick et al. 2011). In the present study, some unidentifiable algal debris can be attributed to the process of abrasion caused by transport and sediment agitation that

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has removed diagnostic characters useful for proper taxonomic identification. Diagenesis has affected the maximum specimens but it is rather difficult to investigate this taphonomic feature as muddy matrix is prevalent in most of the facies observed. Therefore, it becomes slightly difficult to differentiate between the actual leached specimens and the ones covered by muddy material. Micritization has been observed which has resulted in masking of characters, especially the cell wall structure (Nebelsick and Bassi 2000). Most of the conceptacles observed are filled with sparite which can be the result of diagenetic effect.

Conclusion Our findings suggest that the coralline algae reported in the present study were deposited in a reefal environment. Shannon–Weaver Index applied for the quantification of diversity in the samples of Little Andaman (Hut Bay) indicates an overall moderate scale of coralline algal diversity. The variety of growth-forms observed and analyzed in the present case study correlate well to the moderate scale of diversity and gives a clear indication that during the Serravallian, palaeoenvironmental conditions were pretty much conducive for the growth of coralline algae amidst other biogenic entities like benthic foraminifera and molluscs. Taphonomic characteristics of fragmentation and abrasion were observed in very few specimens that support this interpretation. Although disarticulation was interpreted for all the geniculate coralline algae, presence of disarticulation cannot be necessarily attributed to unfavorable environmental conditions. Persistent diversity throughout the studied section indicates mild incidences of disturbances. The magnitude of disturbances increased during some phases that gets reflected in decline of H’ in some samples. However, the H’ is consistently greater than one. This shows the absence of any extreme environmental conditions prevailing in the Little Andaman during the late middle Miocene. On basis of the micropalaentological data, it can be presumed that the algal community of the Little Andaman Island thrived in a fairly suitable environment with normal water temperature, salinity, substrate availability and primary production. As benthic foraminifera are more abundant and diversified in the vicinity of algal populations as observed, hypothesis of competition for space can be proposed as a possible reason for failure of algae to have attained greater abundance and diversity in the Little Andaman. Acknowledgments The authors are grateful to the Director, Birbal Sahni Institute of Palaeobotany for his kind permission to communicate this paper. One of us (SS) is thankful to the Council of

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Scientific and Industrial Research, New Delhi, India for the NET fellowship (Grant No. 09/528-2009-EMR-I). We would like to express our sincere gratitude to the anonymous reviewer for all the valuable suggestions that helped a lot in improving this paper.

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