Fossilised casts of shrimp burrows at Pollen Island

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Fossilised casts of shrimp burrows at Pollen Island
Tane 35: 149 - 159 (1995)
FOSSILISED CASTS OF SHRIMP BURROWS A T P O L L E N ISLAND,
WAITEMATA HARBOUR, AUCKLAND
by Hugh R. Grenfell and Bruce W. Hayward
Auckland Institute and Museum, Private Bag 92018, Auckland
SUMMARY
Three-dimensional fossilised, limonite-cemented casts of network burrows are
present within Pollen Island Marine Reserve in the upper Waitemata Harbour.
They are eroding out of a soft, late Pleistocene intertidal mud deposit and are
inferred to have been made by burrowing callianassid, alpheid or mantid shrimps.
The burrows of nearby living mud crabs (Helice crassa) are short, unbranched
and near vertical, often with a distinctive kink part way down.
Sections of a single network burrow possess the features of three separate
ichnogenera - Ophiomorpha, Thalassinoides and Spongeliomorpha, thereby
calling into question the usefulness of their separate recognition. These casts
show that shape, partial infilling and location of scratch marks on the walls may
all be used to determine facing direction (which way was up) of older
sedimentary rocks in which similar network burrow trace fossils occur.
INTRODUCTION
During a joint Auckland Museum Geology Club and Conchology Section field
trip to Pollen Island in 1994, we investigated an area of hard, limonitised burrow
casts scattered over a mid-tide sandy mud flat. The casts cover approximately
half a hectare and are located 50-100m offshore from the south-east corner of
Pollen Island (Figs. 1,2).
Pollen Island is a low, flat island in the upper reaches of the Waitemata
Harbour off the end of Rosebank Peninsula, adjacent to the Whau River and on
the seaward side of Auckland's North-western Motorway (Fig. 1). The island and
the surrounding mudflats, including these fossilised burrow casts, were declared
a Marine Reserve in 1995.
G E O L O G I C A L SETTING OF P O L L E N ISLAND AND
T H E BURROW CASTS
Pollen Island is a narrow, low, vegetated bank protecting an extensive area
of high tide Sarcocornia salt meadow, salt marsh and low mangrove flats. These
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Fig. 1. The burrow casts occur in an area off the south-east end of Pollen Island, in the upper
reaches of the Waitemata Harbour, Auckland, New Zealand. The casts are eroding out of a
stiff, blue-grey mud that overlies an extensive peat layer that underlies the island.
ecosystems are established in a thin veneer (l-10cm thick) of sandy mud or soil
deposited over a terrace of stiff Pleistocene sediments. The seaward side of the
island is eroding, with 0.3-lm high banks of stiff Pleistocene sediments along
most of its length. The island probably extended much further eastwards a few
thousand years ago and has since been eroded back to its present narrow form.
The area of burrow casts was undoubtedly once beneath part of the island that
has since been eroded away.
The subsurface composition of Pollen Island has been investigated to a depth
of 1.5-2m through the sinking of approximately c. 170 bore holes in the 1950s
(Chapman & Ronaldson 1958). The island is composed of 0.1-lm thick partly
weathered blue-grey mud overlying a peat deposit. Attempts to radiocarbon date
the peat in the 1950s suggested that it is older than 20,000 years. The 0.5-lm
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Fig. 2. Burrow casts, partly mud covered and partly encrusted with oysters and barnacles, form
a lag deposit over an area of mud flats off Pollen Island.
thick peat in turn overlies further weathered mud. In borehole 141 (Chapman and
Ronaldson 1958), on the corner of Pollen Island nearest the burrow casts, there
is l m of weathered mud overlying 0.3m of peat and 0.5m or more of peaty clay.
We infer that the peat accumulated in a freshwater swamp environment at a
time of lower sea level during one of the glacial episodes. The muds above and
below it were possibly deposited in intertidal environments during interglacial
periods when sea levels were close to the present.
Most of the burrow casts appear to be a lag deposit left behind on the surface
after erosion has removed the softer blue-grey mud in which they were originally
formed. On the landward edge of the cast area, some of the casts are still in-situ
partially eroded out of the surrounding mud.
Examination of the stratigraphy between the area of casts and the southern end
of Pollen Island (by digging through the surface mud veneer), shows that the
Pleistocene layers are dipping gently seawards (to the east). The downhole
sequence (Fig. 1) consists of l m plus of blue-grey mud containing the burrow
casts overlying 0.1m of medium-coarse sand overlying 0.2m of leaf-bearing
chocolate mud then l m plus of black, woody peat with occasional carbonaceous
sand lenses.
The blue-grey mud containing the casts was examined for foraminifera and
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any other microfossils without success. The sand fraction in this sediment is
dominated by pumiceous material.
FOSSIL BURROW CASTS
A n area of approximately half a hectare of mudflats offshore from the southeast corner of Pollen Island is covered in a lag deposit of numerous hard burrow
casts, appearing like numerous stag horns partly buried in mud. They have a
density of approximately 0.3/m . They often occur singly with one burrow
system per heap, but sometimes there are several burrow system casts sitting
together. Close examination of their orientation shows that the majority of
burrow cast systems still lie with the longer main burrow concave up and
therefore still in the orientation in which they were made. It appears that they
have recently been exhumed by erosion of the softer surrounding sediment. This
is confirmed by examination of those burrows nearest Pollen Island, which only
have their upper portions exhumed and are still firmly anchored and buried in the
original blue-grey mud substrate.
Portions of the casts that stick out above the mud surface are partly covered
in encrusting Pacific oysters (Saxostrea gigas) and acorn barnacles (Elminius
modestus), sometimes with small anemones (Anthopleura aureoradiata) around
their base.
The burrow casts are branching forms. The total length of the main, subhorizontal central burrow is 15-40cm. Off this there are many 6-15cm long
branches, inclined at all angles from horizontal to vertical (Figs. 3, 4). Each
branch culminates in a non-bulbous rounded end. The burrows have diameters
of 2-3cm. The burrow casts are filled with fine to medium, slightly shelly sand
that has been cemented together. Some of the short branches appear to be filled
with coarser sediment. Limonite staining and cementing is always present in the
outer 0.2-0.3cm of the casts and sometimes permeates throughout, giving them
an orange-brown colour.
Some casts appear to have an additional 0.2-0.3cm thick nodular lining
preserved around the burrow but it is absent from others or even from other parts
of the same burrow system. The surface of most burrow casts is textured, often
with the relief of internal scratch marks on the walls and lower sides of
horizontal portions of burrows and on all sides of more vertical portions.
The upper surface of the more horizontal portions of some burrow casts are
commonly smoother, or less cemented and are flat in profile. This suggests there
was sometimes incomplete filling of the upper few millimetres of the burrow by
sand.
2
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Fig. 3. Photos of Pollen Island burrows: A - C . cement casts of the burrows of the mud crab
(Helice crassa) A K 98680, 98681, 98682 respectively; D-G. fossilised burrow casts, D & E = A K
98677, F & G = A K 98676. (Scale bars = 10cm). A K numbers refer to the collections of the
Auckland Institute & Museum.
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Fig. 4 A. Reconstruction of a typical callianassid burrow system. B. Cross-section of a burrow;
IL = smooth, slimy inner lining; O L - outer lining = matrix; SS = surrounding sediment. C .
A callianassid within its burrow. (Modified after de Vaugelas & Buscail 1990 and Bromley
1990).
T R A C E FOSSILS
Ichnogenera
These burrow casts are excellent three dimensional examples of trace fossils
that are elsewhere used in paleoenvironment assessments (e.g. Bromley 1990).
The artificial nature of the ichnogenera used in the study of trace fossils
(palaeoichnology) is shown by the fact that some of our burrow casts have
separate sections that could be placed in three different ichnogenera. The Pollen
Island specimens can be assigned to the network ichnogenera - Ophiomorpha
Lundgren, 1891 (with a nodular lining around the walls), Thalassinoides
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Ehrenberg, 1944 (with smooth walls) and Spongeliomorpha Saporta, 1887 (with
scratched/ridged walls). This problem has been discussed by Bromley (1990, pp.
159-161), who concluded that the three genera were clearly morphological
developments representing distinctive behavioural attributes within a complex
burrow system. However, Bromley also considered that since they can occur
separately, they must be named separately. The Pollen Island material could be
said to belong to a "Ophiomorpha - Thalassinoides - Spongeliomorpha complex".
Which way was up ?
The Pollen Island burrow casts are good examples of the potential of trace
fossils as geopetal (that is, facing direction -"which way was up") indicators. The
first indicator is the presence of claw scratch marks on the lower surface only of
more horizontal parts of the burrow networks. Inclined or more vertical parts are
scratched on all sides. Therefore, scratches on one surface only, suggest the
lower part of a horizontal burrow.
The second indicator is the apparent partial filling of some horizontal burrows
as seen by a flatter profile and a smoother, less cemented upper surface. The
unfilled part of the burrow was uppermost.
The third indicator is the gentle concave-up curve of the main horizontal
sections of the burrow network (Fig. 3).
M U D C R A B BURROW CASTS
The mudflat in which these burrow casts occur today is pockmarked with the
openings of numerous burrows of the living mud crab (Helice crassa). To see
whether Helice was a possible producer of our fossil network burrows, we
prepared a quick setting cement grouting mix and poured it down a number of
mud crab burrows to make artificial casts for comparison. We returned several
days later and dug up the cement casts.
The mud crab burrows were unbranched with only a single opening. Each was
1.5-2.5cm in diameter and 17-25cm long. They were near vertical in orientation,
often with a sub-horizontal to steeply inclined kink 5-10cm below the surface
(Fig. 3). The kink is possibly an anti-predation strategy or is linked to retaining
ground water in the burrow at low tide. The burrows tapered to a rounded point,
and had numerous claw scratch marks preserved along their length.
Observations of Helice crassa burrows elsewhere show that the burrow
morphology is quite variable. In Hobson Bay, Auckland the burrows, in a similar
substrate to Pollen Island, are shallow and have a simple curved, sub-horizontal
shape (pers. obs.). In the Avon-Heathcote Estuary the sandy, clay banks are so
riddled with burrows that they often intercommunicate (McLay 1988). Burrow
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morphology here also differs from Pollen Island in that there is a narrow neck
which widens below the surface.
These modern New Zealand mud crab burrows are similar in shape to those
of all five genera of crabs studied in the Seychelles by Braithwaite and Talbot
(1972).
WHAT M A D E T H E FOSSILISED BURROWS?
Our artificial cement casts of modern burrows at Pollen Island strongly
suggest that mud crabs did not excavate our fossil burrow casts. Additional
studies overseas indicate that no other sort of crab is likely to have made them
either (e.g. Braithwaite & Talbot 1972).
Worms are also common burrowing organisms in the mudflats of the upper
Waitemata Harbour. Very few make burrows that remain open; none are known
to make branching burrows and there are none that would produce burrows as
large as 2-3cm in diameter.
Fish such as gobies can produce burrows (Bromley 1990, p.89) but the
structure is usually a simple U-shape. It may be extended into a small network
occupied by several individuals.
The most probable culprits are burrowing shrimps, in particular alpheids,
mantids, callianassids, which live in intertidal and shallow subtidal environments
in harbours and sheltered bays around New Zealand today (Morton & Miller
1973). A l l produce burrows that can be of a similar diameter, overall size and
complexity to our fossil burrow casts.
Alpheid shrimps live at mid-tide level and below whereas mantid and
callianassids live at low tide level and below. Although the body of alpheid and
callianassid shrimps is usually 1cm or less in diameter, their burrows are bigger.
This is because of the outstretched span of the legs of alpheids as they move
around in the burrow and because of the large size of one chela of the
callianassids (Fig. 4). Mantid shrimps can produce 2-3cm diameter burrows
because of the larger size of their bodies.
The burrow systems of all these shrimps have two, near vertical burrows
leading to the surface from the more complex, more deeply buried network
section. No long vertical burrow sections appear to have been preserved in these
Pollen Island casts.
These three groups produce a wide variety of burrow networks with
callianassids perhaps producing the greatest diversity of form (e.g. Bromley
1990, pp. 73-99). In callianassids, the network is commonly occupied by several
individuals, for example, a male and a number of females. There may also be a
commensal relationship with other Crustacea, bivalves or fish. Most of the
burrowing shrimps are deposit feeders which utilise the meiofauna of surface
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sediments as food. The New Zealand callianassid, Callianassa filholi, is a deposit
feeder whose food includes pollen grains, dinoflagellates, diatoms, ciliates,
platyhelminthes and nematodes (Devine 1966). In some species the coarse
sediment is stored in lateral branches and sometimes it is ejected to be
"harvested" again later once it has been recolonised by meiofauna. Others are
scavengers and some harvest plant material (e.g. sea grass). These species store
food in the burrow network for later use or to allow decomposition to take place
(Bromley 1990).
FOSSILISATION OF T H E BURROWS
It appears that the burrow construction methods used by these shrimps
enhances their preservation potential since they are commonly seen in the fossil
record. Most shrimp burrows remain open during their occupancy and for some
time after the death of their producers, because the wall is strengthened by a
mucus lining (e.g. de Vaugelas & Buscail 1990).
These Pollen Island burrows are infilled with fine to medium sand which must
have swept in over the intertidal mud flat at some stage soon after the burrows
had been made and abandoned. It is likely that some mechanism which allowed
the movement of oxygenated sea water/ground water through an otherwise
anaerobic environment caused the oxidation of iron sulphides (e.g. pyrite) to iron
oxides (e.g. limonite). We speculate that the cementation process may have
already started while the burrow was still occupied, increasing its ability to
remain open and become progressively filled later.
A second, more likely, scenario is that the concentration of organic material
in the burrow wall, and the relatively coarse nature of the sediment fill, results
in a geochemical environment favourable to the formation of limonite cements.
The burrow wall of a callianassid species studied by de Vaugelas and Buscail
(1990) had an organic carbon and humic matter concentration 11-17 times greater
than the ambient sediment. We speculate that such a high organic content would
provide conditions favourable to sulphide producing bacteria. Although the
movement of oxygenated ground water was impeded by the impervious mud layer
it was possible through the more permeable sand-filled burrows. This caused the
conversion of the sulphides present to iron oxides. The ground water may have
already been enriched in iron oxides from the underlying peat. Whatever the
source of the iron oxides, permeability facilitated the deposition of a limonite
cement in the burrow walls, and in some places throughout the sand-filled
burrows, producing the hard cemented casts which have subsequently been
exhumed.
The lack of any preserved long vertical burrow sections that would have led
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to the surface might be explained in several ways. It is possible that an episode
of erosion removed the top of the original mud layer and the vertical entry and
exit burrows after they had been infilled with sand but before they were cemented
by limonite. It is also possible that ground water conditions were such that only
the lower levels of the network burrows were cemented with limonite and
fossilised as hardened casts.
Limonite-preserved fossil burrow casts are not present around other parts of
Pollen Island where the same stiff, blue-grey mud layer is exposed. In some of
the low eroding banks on the east side of Pollen Island, uncemented mud-filled,
abandoned crustacean burrows are visible. These may be the long vertical
sections of the burrow networks leading to the surface. Because they are softer,
many are being re-excavated and reoccupied by Helice crassa today. Another
explanation for the localised burrow fossilisation may be that sand was swept
over only part of the tidal flat and filled only burrows in this area with a coarser
sediment that was later permeable to the oxygenated ground waters which
facilitated limonite deposition.
CONCLUSIONS
The burrow casts found near Pollen Island are almost certainly produced by
shrimps that lived in the mid to low tide mudflats of the upper Waitemata
Harbour, possibly during the Last Interglacial. Since alpheid, mantid and
callianassid burrowing shrimps can all produce network type burrow systems with
diverse morphologies, it is not possible to determine precisely which of these
shrimp groups is the culprit.
Similar trace fossil network burrow systems are commonly found in exposures
of older, hardened sedimentary rocks, even around Auckland (e.g. Hayward
1976), but at Pollen Island we have an exceptionally rare example of threedimensional fossilised burrow casts eroded out of soft, late Pleistocene sediments.
These Pollen Island casts provide important evidence for determining facing
directions in older rocks, in the form of preserved scratch marks, partially filled
burrows and gently curved, concave-up horizontal burrow sections.
ACKNOWLEDGMENTS
We thank Annette Pullin for assistance in the field and Krzys Pfeiffer for taking the photographs
of the burrow casts. Mark Weatherhead, NIWA, provided advice and material for making the cement
crab burrow casts and Katrin Berkenbusch, Otago University, provided information on previous
studies on crustacean burrows around the world. Murray Gregory, Brett Stephenson and Mark
Weatherhead are thanked for providing literature, reading the draft manuscript and suggesting
improvements.
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REFERENCES
Braithwaite, C.J.R. & Talbot, M.R. 1972: Crustacean burrows in the Seychelles, Indian Ocean.
Paleogeography, Paleoclimatology; Paleoecology 11: 265-285.
Bromley, R.G. 1990: "Trace fossils: Biology and Taphonomy. Special Topics in Palaeontology."
Unwin Hyman, London.
Chapman, V.J. & Ronaldson, J.W. 1958: The mangrove and salt-marsh flats of the Auckland
Isthmus. D.S.l.R. Bulletin 125: 1-79.
de Vaugelas, J. & Buscail, R. 1990: Organic matter in burrows of the thalassinid crustacean
Callichirus laurae, Gulf of Aqaba (Red Sea). Hydrobiologica 207: 269-277'.
Devine, C . E . 1966: Ecology of Callianassa filholi Milne-Edwards 1878 (Crusteacea, Thalassinidea).
Transactions of the Royal of New Zealand, Zoology 8(8): 93-110.
Hayward, B.W. 1976: Lower Miocene bathyal and submarine canyon ichnocoenoses from Northland,
New Zealand. Lethaia 9: 149-162.
M Lay, C . L . 1988: Brachyura and crab-like Anomura of New Zealand. Leigh Laboratory Bulletin
22.
Morton, J.E. & Miller, M . C . 1973: "The New Zealand Sea Shore. 2nd ed." Collins, London. 638pp.
c
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