Sri Lanka’s Aruwakkalu fossil deposit dates to the Burdigalian Age

Abstract

Aruwakkalu fossil bed is a part of Sri Lanka's Jaffna limestone, which underlies the whole of Jaffna Peninsula and extends southwards mostly along the west coast. Previous authors have suggested that Aruwakkalu contains a rich assemblage of vertebrate and invertebrate fossils. We sought to confirm the Burdigalian age of this northwestern Miocene deposit at Aruwakkalu on the basis of the foraminifer Pseudotaberina malabarica, an index fossil of the Burdigalian stage. General and timeline collections were made at seven selected sites and the fossils collected were identified. The study sites contained six sedimentary layers of which, third and sixth from top were fossiliferous. The sixth (deepest) layer was dominated by gastropod fossils while the third was dominated by fossils of giant oysters. Fossils of P. malabarica were recovered both from timeline and general collections. In the timeline collection, samples of this index fossil were recovered only from the gastropod layer, suggesting that P. malabarica existed during the time the gastropod layer was being laid down, thus confirming a Burdigalian age for the latter, and helping to date a substantial portion of the Sri Lankan fossil fauna with confidence.

Full text

Ceylon Journal of Science (Bio. Sci.) 40 (2): 163-174, 2011
Sri Lanka’s Aruwakkalu fossil deposit dates to the Burdigalian Age
Ranjeev Epa1, Nilmani Perera1, Kelum Manamendra-Arachchi2 and Madhava Meegaskumbura1*
1 Department of Zoology, Faculty of Science, University of Peradeniya, Sri Lanka
2 Postgraduate Institute of Archaeology, 407 Bauddhaloka Mawatha, Colombo 07, Sri Lanka
Accepted 08 November 2011
ABSTRACT
Aruwakkalu fossil bed is a part of Sri Lanka’s Jaffna limestone, which underlies the whole of Jaffna
Peninsula and extends southwards mostly along the west coast. Previous authors have suggested that
Aruwakkalu contains a rich assemblage of vertebrate and invertebrate fossils. We sought to confirm the
Burdigalian age of this northwestern Miocene deposit at Aruwakkalu on the basis of the foraminifer
Pseudotaberina malabarica, an index fossil of the Burdigalian stage. General and timeline collections
were made at seven selected sites and the fossils collected were identified. The study sites contained six
sedimentary layers of which, third and sixth from top were fossiliferous. The sixth (deepest) layer was
dominated by gastropod fossils while the third was dominated by fossils of giant oysters. Fossils of P.
malabarica were recovered both from timeline and general collections. In the timeline collection, samples
of this index fossil were recovered only from the gastropod layer, suggesting that P. malabarica existed
during the time the gastropod layer was being laid down, thus confirming a Burdigalian age for the latter,
and helping to date a substantial portion of the Sri Lankan fossil fauna with confidence.
Key words: Pseudotaberina malabarica, foraminifera, index fossil, Miocene, gastropod layer, oyster layer
INTRODUCTION
Cenozoic era, also known as Age of Mammals
(Deraniyagala, 1969a) comprises two periods,
the Tertiary (65.5-2.6 mya) and the Quaternary
(2.6 mya to the present). The Miocene epoch
(23.0-5.3 mya), a subdivision of the Tertiary,
was an important phase in Sri Lanka’s
geological and faunal history. The island was a
part of the Indian peninsula until the Miocene,
when a belt of sea inundated the southeastern
part of the Indian plate (Madduma Bandara,
1989), separating Sri Lanka from the mainland
for the first time.
Sri Lanka contains two Tertiary deposits
(Fig. 1) dated to the Miocene by Wayland (see
Deraniyagala, 1969a). These marine deposits
occur in two well-separated localities, one
towards the northwest and the other towards the
southeast extremity of the island. They are
indications of a Tertiary marine transgression, of
which the former resulted in the first separation
of Sri Lanka from the Indian mainland. Sri
Lanka’s Miocene deposits classified as Jaffna
limestone are formed of two facies: the
calcareous and the areno-argillaceous
(Deraniyagala, 1969a). The former is found as a
broad exposure throughout the Jaffna peninsula
and adjacent islands. It extends south along the
west coast, gradually thinning out in width. The
southernmost major exposure of the Jaffna bed
is seen at Karativu, but the eastern border cannot
be determined with accuracy due to lack of
outcrops (Cooray, 1984). Lithologically, Jaffna
limestone typically consists of hard, partly
crystalline, compact, indistinctly bedded, cream-
coloured rock and is generally at surface level
with cliffs in areas such as Kirimalai,
Kolanakanatta and Moderagam Aaru (Cooray,
1984).
While a number of Miocene vertebrates and
invertebrates have been described from
Aruwakkalu, a part of the Jaffna limestone
located 25 km north of Puttalam, these have not
been dated so far. This study was conducted
within the Quarry of Holcim (Lanka) Ltd., a
company extracting limestone for the production
of cement. Aruwakkalu has been subjected to
strong elevational changes, probably due to
block faulting, which is evident in the
positioning of the islands along the Kalpitiya
shoreline and the ridges along the eastern shore
of Dutch Bay. Although the Jaffna bed is
considered to be a marine deposit, fossils of
terrestrial reptiles and mammals, often
associated with estuarine habitats, have also
been discovered from the “Malu member”,
located along the eastern shoreline of Dutch
Bay. This suggests that, at least in some areas,
__________________________________________
*Corresponding author’s email: madhava_m@mac.com

Epa et al. 164
fossil accumulation took place under fluvial and
estuarine conditions (Deraniyagala, 1969a).
Index fossils are fossil taxa that have a wide
geographic distribution and show changes in
characters, which can be recognized as a part of
the temporal scale. Taxa belonging to
foraminifera have been present since Cambrian.
Planktonic foraminifera have a wide geographic
distribution and a rapid evolutionary rate.
Therefore they possess a short vertical
stratigraphic time range, enabling them to be
used as index fossils for dating (Boudagher-
Fadel, 2008). The larger benthic foraminifera,
which possess a more complex internal test
(shell) structure, are also used as index fossils.
These are as successful as planktonic
foraminifera and are more abundant in modern
seas (Boudagher-Fadel, 2008). The large
benthic fossil foraminifer Pseudotaberina
malabarica, an index fossil of the Burdigalian
stage (Kulkarni et al., 2010), was used to date
the Jaffna limestone to this stage of the Miocene,
which gives it a temporal range of 20.43-15.97
Ma.
Fossilized tests of the benthic foraminifer
Pseudotaberina malabarica are dimorphic
owing to alternation of generations in their
reproductive life cycle (Banner and Highton,
1989). This is a characteristic feature of large
and medium sized benthic foraminifera.
However, planktonic foraminifera lack evidence
of dimorphism and seem to reproduce sexually
(Hottinger, 2006). Dimorphic forms have two
test types, the megalospheric and the
microspheric. Foraminiferal gamonts and
schizonts produced by asexual reproduction are
megalospheric with a large proloculus (initial
chamber) called the megalosphere, but the
overall test diameter is relatively small.
Agamonts produced by sexual reproduction,
however, are microspheric with a small
proloculus and a relatively large overall test
diameter (Hottinger, 2006).
The objective of this study was to explore for
the presence of Pseudotaberina malabarica in
Jaffna limestone at Aruwakkalu so that this
deposit can be confidently dated, while also
building a reference collection with which future
collections could easily be compared.
Figure 1. Miocene deposits of Sri Lanka (modified from Cooray, 1984).

Sri Lanka’s Aruwakkalu fossil deposit 165
MATERIALS AND METHODS
Study area and sites
This study was conducted within the boundaries
of the limestone Quarry at Aruwakkalu situated
25 km north of Puttalum. Study sites were
selected based on the exposure of fossiliferous
layers and priority was given to areas where the
entire soil profile was available. Seven sites
(Fig. 2) were surveyed and the Quarry Office
(8°14'37.60"N 79°49'25.52"E) was used as the
base point. The sites were named as follows,
Peella (8°14'31.26"N 79°49'7.59"E), Alahakoon
(8°14'53.34"N 79°49'2.89"E), Water hole
(8°15'12.95"N 79°48'58.13"E), Coral
(8°14'17.10"N 79°48'55.50"E), Beach
(8°15'6.30"N 79°48'46.68"E), Lasantha
(8°14'46.06"N 79°49'12.52"E), and Working
(8°15'19.23"N 79°49'9.16"E).
Figure 2. Distribution of study sites within the study area at Aruwakkalu. (Scale bar 1 km)

Epa et al. 166
Peella site, located 0.60 km southwest of the
base, contained an excavated wall with a clear
view of the sedimentary profile. This wall
extends throughout the site but in some areas it
is obscured by dumped limestone and red earth.
The Alahakoon site, located 0.85 km northwest
of the base, contained an extensive wall but the
sedimentary profile was not very clear. The
Water hole site, located 1.37 km northwest of
the base, was a filled up quarry site; thus it did
not contain a clear sedimentary profile.
However, there were fossiliferous rocks which
had fallen from the wall during limestone
extraction. The Coral site, located 1.10 km
southwest of the base, was a very small exposure
and did not contain a clear sedimentary profile.
The Beach site, located 1.50 km northwest of the
base, included mainly fossiliferous intertidal
rocks, along the eastern coast of Dutch Bay,
which were exposed during low tide. The
Lasantha site, located 0.47 km northwest of the
base, was represented by an extensive wall with
a clear sedimentary profile. The Working site,
located 1.37 km northwest of the base, is the one
from which limestone is currently being
extracted.
Collection and identification
The study area was surveyed over a period of
eight days. Fossils were both handpicked and
unearthed by digging through the limestone
already excavated. Two collections were made, a
general collection and a timeline collection. In
the general collection, fossils found scattered on
the ground were collected from each site. In the
timeline collection, fossils found on the walls of
the study sites were collected from the
uppermost layer down to the lowest fossiliferous
layer in a vertical profile.
Fossils collected were carefully wrapped to
prevent abrasion and chipping while in transport.
Fossils of the timeline collection were placed in
separate zip-lock bags based on the layer from
which they were extracted.
Each fossil was catalogued with a reference
number and a photograph. Other information
included date of collection, site of collection,
names of collectors and reference number of the
relevant photograph. The collection was
identified using paleontological literature
(Carter, 1853; Banner and Highton, 1989;
Renema, 2008; and Hottinger, 2006).
Morphological and morphometric data were
obtained using the software program ImageJ.
RESULTS
Stratigraphy
The soil profile at the seven surveyed sites
comprised of six sedimentary layers. Of these,
only two layers contained fossils, the sixth (the
deepest layer, containing predominantly
gastropods, referred to as the gastropod layer,
Fig. 3); and the third (dominated by giant
oysters, referred to as the giant oyster layer, Fig.
4).
Fossils of foraminifera
Fossils of P. malabarica were included in both
the timeline and the general collections (see
Table 1). Microspheric forms were recovered
from the timeline collection at Alahakoon site,
while megalospheric forms were recovered from
general collection at Peella site.
Systematic paleontology
Family-Soritidae Ehrenberg, 1839
Genus- Pseudotaberina Eames, in Davies 1971,
emend. Banner and Highton, 1989
Species- P. malabarica
Repository- Material collected in this study are
deposited in the Zoological Collection of the
Department of Zoology (University of
Peradeniya) (Table 2).
Microspheric form: AR0098, AR0104
Megalospheric form: AR0237, AR0065,
AR0047
Locations- See Table 1
Description
Microspheric form Specimens (fossil reference
number AR0098, Fig. 5 and fossil reference
number AR0104, Fig. 6 ) represented by
equatorial sections; shell initially planispiral,
becoming annular distally. Chamberlets of
annular chambers visible to the periphery of the
mould and stolon axes or axes of intercameral
foramina Y-shaped, not parallel to the radius of
the test (Fig. 5). Test flat and discoid, diameter
reaching up to 16.010 mm (Fig. 5).

Sri Lanka’s Aruwakkalu fossil deposit 167
Figure 3. Sixth layer of the sedimentary profile at Aruwakkalu: layer dominated by fossils of gastropods.
Figure 4. Third layer of the sedimentary profile at Aruwakkalu (Indicated by arrow).

Epa et al. 168
Megalospheric form: Specimens represented
by transverse, axial and oblique sections (fossil
reference number AR0237, Figures 7, 8 and 9).
In the axial section, the large megalosphere at
center; planispiral involute growth observed
where initial chambers are embraced by
successive chambers. Megalosphere and initial
chamber arrangement also observed in oblique
section. Specimens (fossil reference number
AR0065, Fig. 10 and fossil reference number
AR0047, Fig. 11) represent moulds of the shell
cavity occupied by the system of intercameral
foramina and chamberlet lumina indicating the
Y-shaped arrangement of stolon axes. Chamber
walls eroded.
Measurements – Test diameter, 4.841 mm
(AR0237: Fig. 7).
Other material examined- See Table 2
Remarks: The collection comprised approx. 40
individuals, collected from general and timeline
collections at Peella and Alahakoon sites. Of the
40, two large clusters were recovered
comprising of 12-15 and 13 individuals,
respectively.
Fossils of Pseudotaberina malabarica
recorded from Sri Lanka thus far include
specimens from Kirimalai, Puttalum,
Pomaparippu and Pallai (Banner and Highton,
1989).
Table 1. Number of Pseudotaberina malabarica collected from each site within the study area.
Fossil reference No.
Number of fossils
Location
Layer/Collection method
AR0047
12-15
Peella site
General Collection
AR0065
4
Peella site
General Collection
AR0098
2
Alahakoon site
Gastropod layer
AR0104
4
Alahakoon site
Gastropod layer
AR0123
1
Peella site
Gastropod layer
AR0236
1
Unknown
Unknown
AR0237
13
Unknown
Unknown
Table 2. Life stages and dimensions of Pseudotaberina malabarica collected from the study area.
Fossil Reference No.
Figure No.
Generation
Test diameter (mm)
AR0098
5
Microspheric
16.01
AR0104
6
Microspheric
17.14
AR0237
7
Megalospheric
4.84
AR0237
8
Megalospheric
3.13
AR0237
9
Megalospheric
-
AR0065
10
Megalospheric
-
AR0047
11
Megalospheric
-

Sri Lanka’s Aruwakkalu fossil deposit 169
Figure 5. (A) Fossil Reference No. AR0098. Equatorial section of a microspheric form (see Section on
Systematic Paleontology). (B) Magnified portion of the image in Fig. 5A, focused on chamberlets (scale
bar 2 mm).
Figure 6. Fossil Reference No. AR0104 (X6). Equatorial section of a microspheric form (scale bar 2 mm).

Epa et al. 170
Figure 7. Fossil Reference No. AR0237. Transverse section of a megalospheric form. Specimen indicated
by arrow (scale bar 2 mm).
Figure 8. Fossil Reference No. AR0237. Axial section of megalospheric generation showing the initial
involute growth (scale bar 2 mm).
Figure 9. Fossil Reference No. AR0237. Oblique central section of a megalospheric shell (scale bar 2
mm).

Sri Lanka’s Aruwakkalu fossil deposit 171
Figure 10. Fossil Reference No. AR0065. Internal mould of a planispiral megalospheric shell, upper part
not preserved (scale bar 2 mm).
Figure 11. Fossil Reference No. AR0047. Equatorial section of an internal mould of a planispiral
megalospheric shell (scale bar 2 mm).

Epa et al. 172
DISCUSSION
Sri Lanka’s northern Tertiary deposit is vast,
composed of fossiliferous limestone that hosts
fossils dating to the Miocene and late
Pleistocene. Surveys by early explorers
recovered fossils of both invertebrates and
vertebrates, of which the latter are considered
important because the earliest vertebrate fossils
found in Sri Lanka date to the Miocene
(Deraniyagala, (1969b). Being a marine deposit,
Aruwakkalu contains a wide variety of marine
fossil fauna ranging from foraminifera to
mammals. However this study focuses on the
index fossil Pseudotaberina malabarica, an
extinct foraminifer based on which Sri Lanka’s
Miocene was dated.
In this study, the index fossil foraminifer
Pseudotaberina malabarica was discovered
from both timeline and general collections. In
the timeline collection, fossils of the index fossil
were recovered only from the gastropod layer.
Two additional unidentified fossil foraminifera
were found in the gastropod layer at the
Lasantha site. Since general collections do not
provide information on the stratigraphical
occurrence of fossils, it is likely that P.
malabarica only existed during the time at
which the gastropod layer was being laid down.
Therefore, the gastropod layer can be confirmed
as a part of the Burdigalian. Hence the
superficial giant oyster layer could be of a more
recent age than the gastropod layer, since the
two fossiliferous layers flank two other
sedimentary layers between them. However, the
Burdigalian occupies the time period of about
four million years, hence the layers immediately
above the gastropod layer could have formed
during the Burdigalian if the sedimentation rates
were rapid. The exact time frame of the giant
oyster layer cannot be determined with certainty
at present. According to Deraniyagala (1969a),
deposits belonging to Pliocene and Pleistocene
are present above the Miocene at Aruwakkalu.
Deraniyagala, without listing the entire
composition of this late Pleistocene deposit,
stated that it contained recent taxa such as
Anadara granosa. However, fossils of Anadara
granosa could not be discovered from the oyster
layer in the present study. Further, in another
communication (Deraniyagala, 1969b), a
relatively young superficial estuarine deposit is
mentioned, which is probably the same late
Pleistocene deposit mentioned above. This
deposit is said to occupy 240 feet (73 m) above
sea level and was mentioned as being composed
of unweathered molluscan shells. However, the
oyster layers at both the Peella and the Lasantha
sites were situated at heights less than 30 m
above sea level, and fossils recovered were
weathered. It therefore appears that the oyster
layer is not a part of the late Pleistocene.
Information on sedimentation rate could help in
placing this layer within or outside the Miocene
(23.0-5.3 mya). Further the presence of the late
Pleistocene (Quaternary) deposit above the
Miocene should be recognized.
In paleontology, an ideal index fossil should
possess several characters such as being
abundant in the stratigraphic record, easily
distinguishable from other related species,
geographically widespread and having a narrow
stratigraphical range (Stanley, 2005). The fossil
foraminifer P. malabarica possesses all of the
above, making it an excellent fossil for dating a
sedimentary layer
Relative dating of the Tertiary limestone in
Sri Lanka initially placed the deposit at the
Cretaceous and Eocene (see Deraniyagala,
1969a). Then Wayland classified it as Miocene
based on faunal composition (see Deraniyagala,
1969a). Wayland and Davies examined the
Jaffna limestone, which contained fossils of
Taberina malabarica (Pseudotaberina
malabarica), but they overlooked the
foraminifer and classified it as a part of the
Vindobonian (Kulkarni et al., 2010; Mohan and
Chatterji, 1956). Cotter, in 1938 considered the
Jaffna limestone to be of upper Vindobonian age
(Mohan and Chatterji, 1956 ). Eames (1950) re-
examined Davies’ collection and admitted the
importance of Archaias malabaricus
(Pseudotaberina malabarica), which led to the
current classification of the Jaffna limestone as
Burdigalian.
Pseudotaberina malabarica is considered an
important taxon in environmental reconstruction.
All larger benthic foraminifera are marine and
neritic, living largely in oligotrophic reef and
carbonate shoal environments (Boudagher-
Fadel, 2008). However, these foraminifera are
today confined mainly to lower latitudes
(Boudagher-Fadel, 2008). Some large
foraminifera posses symbionts within their
chambers. Thus, their distribution depends on
factors such as temperature, nutrient levels, light
and water depth. Larger benthic foraminifera are
biofacies bound and have biotopes closely
associated with carbonate environments
(Boudagher-Fadel, 2008). They are also
considered as excellent indicators of paleo-
environments. Ecological studies carried out on
larger extant foraminifera suggest that the
minimum water temperature tolerated is 18 oC,
while the maximum water depth habitable is 35
m (Boudagher-Fadel, 2008). Further,

Sri Lanka’s Aruwakkalu fossil deposit 173
Pseudotaberina malabarica has an epiphytic
mode of life: in large aggregations they are
considered as indicators of sea-grass vegetation
(Reuter et al., 2010), which has hitherto not been
recorded from the Miocene of Sri Lanka.
Therefore, both Peella and Alahakoon sites can
be assumed to have possessed sea grass beds
during the Burdigalian.
The dimorphic forms of fossilized
Pseudotaberina malabarica were identified
considering their test diameter and internal and
external shell architecture. In the microspheric
form, planispiral involute growth starts closely
around the small proloculus and become cyclical
and evolute in the latest growth stage resulting in
a flat or even biconcave discoid test. In the
megalospheric form, the test is initially
biconcave with the first chamber initially
developing into a protoconch, followed by a
second reniform growth stage forming a
deutoroconch, and these are followed by
planispiral chambers (Banner and Highton,
1989; Hottinger, 2005). Specimens of
megalospheric forms have been reported to
reach a diameter of up to 6.5 mm and a thickness
of 0.7 mm while microspheric specimens can
reach a diameter of up to 15 mm (Renema,
2008). However, larger specimens with
maximum diameters reaching up to 21 mm have
been recorded by Wayland and Davies (Renema,
2008).
CONCLUSION
The Tertiary and Quaternary exposures along the
northwestern of part of Sri Lanka is composed of
both Miocene and late Pleistocene deposits.
Miocene fossils belonging to the Burdigalian
were recovered only from the gastropod layer,
which is overlaid by a deposit of giant oysters,
possibly older than the late Pleistocene but of
unconfirmed age. Thus, the northwestern
exposures of Sri Lanka probably comprise of
three deposits belonging to three time frames, a
Burdigalian gastropod layer, a late Pleistocene
estuarine deposit and an intermediate giant-
oyster layer.
Fossils of the index foraminifer
Pseudotaberina malabarica recovered from the
gastropod layer confirm the Burdigalian age of
this deposit. However, our results show that the
entire Tertiary deposit of northwestern Sri Lanka
cannot, at present, be referred to the Burdigalian
with certainty.
ACKNOWLEDGEMENTS
We are grateful to Mr. Athula Janz, Mr. Chalaka
Fernando and Holcim Lanka Pvt. Ltd. for giving
us access to study fossils within the Aruwakkalu
quarry site. We thank Prof. Ranjith Dissanayake
for support with initiating this study. We greatly
value the comments by Dr. G. L. Badam and Mr.
Rohan Pethiyagoda, reviewers of the paper. We
acknowledge Mr. Gayan Bowatte for his kind
contribution towards fieldwork and preparation
of the figures and to Prof. Nimal Gunatilleke and
Prof. Savitri Gunatilleke for support with
logistics. This study was entirely self-funded by
the authors.
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