Proc. 14th INA Conf., Reston VA, USA (Guest Ed. J. Self-Trail)
J. Nannoplankton Res. 35 (2), 2015, pp.177-184 © 2015 International Nannoplankton Association
ISSN 1210-8049 Printed by The Sheridan Press, USA

177

Morphological variation of pike structures in Corbisema(Silicoflagellata) from the Early and Middle Eocene
Kevin McCartney*

Department of Environmental Studies & Sustainability, University of Maine at Presque Isle, Presque Isle, ME 04769, USA; *kevin.mccartney@
maine.edu

Kenta Abe

Graduate School of Science & Engineering, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan

Richard W. Jordan

Department of Earth & Environmental Sciences, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan

Jakub Witkowski

Geology & Palaeogeography Unit, Faculty of Geosciences, University of Szczecin, ul. Mickiewicza 18, 70-383 Szczecin, Poland
Manuscript received 27th March, 2015; revised manuscript accepted 3rd September, 2015

Abstract Early to Middle Eocene silicoflagellates from three localities (Denmark, Russia and the Arctic Ocean)

exhibit a wide range of morphological diversity in their pikes, including unusual distal terminations that have not beenpreviously reported. Some of the Corbisema possess pikes that are rotated consistently in the same direction. Other
specimens show a sharp distal change of direction, generally away from the centre of the cell, or a distal termination
similar to a sabaton. In some cases, the skeletal morphology and presence or lack of pikes provides information tosuggest the double skeleton configuration of the species.

Keywords silicoflagellates, morphology, Corbisema, Eocene, Denmark, Russia, Arctic Ocean
1. Introduction
Corbisema is of special importance among silicoflagellates as the only genus to survive the Cretaceous-Palaeogene transition, and thus is the ancestor of all Cenozoic
silicoflagellates (McCartney et al., 2010a). Corbisema has
a skeletal morphology comprising a basal ring of threesides and corners, with a simple apical structure madeup of three struts that meet at an apex (Figure 1). Despite
a generally simple skeletal design, there is considerable
morphological diversity. For some species, the basal sides
are more-or-less linear and meet at sharp corners, wherea basal spine completes a triple junction. Other skeletons
have rounded corners that lack spines. There is also considerable diversity in skeleton size, or presence/absence ofbasal pikes or apical plate/ring.
Silicoflagellates are most commonly photographed inapical view (Figure 1a), which obscures the pikes that are
best observed in abapical view (Figure 1b). Pikes are an
important and understudied aspect of Corbisema, and silicoflagellate skeletal design in general. Terms such as ‘ancillary spines’ (Martini, 1977), ‘accessory spines’ (Mandra, 1960) and ‘support spines’ (Gleser, 1959) have beenused in older literature. The term ‘pike’ was first used by
Bukry (1976), and is here restricted to basal structures.

Spines and pikes are quite distinct. Spines, which often
occur at the corners of the basal ring and may be present
on the apical structure, are oriented radially, with respect
to the centre of the silicoflagellate cell. Spines are linear,or nearly so, and in non-aberrant skeletons are always ofsimilar design on a given specimen.
Pikes attach to the abbasal or interior surfaces of thebasal sides and point in some abapical direction. Pikes
on an individual specimen are often similar, but may differ, and terminations are generally rounded, rather thanpointed, as on the distal ends of spines. Spines and pikes
also appear to serve separate functions. The purpose of thespines is uncertain; van Tol et al. (2012) suggested thatthey protect against predation, but Sargeant et al. (1987;
see also McCartney et al., 2010a) suggested that spines
might retard settling out of the photic zone. Takahashi etal. (2009) observed that the spine length can vary withenvironmental factors. Pikes, on the other hand, appear
to hold together two members of a double skeleton (alsoknown as doublets; Moestrup & Thomsen, 1990; McCartney et al., 2014a, 2015a).
Although pikes occur in most silicoflagellate genera,a number of species in each genus lack pikes. We believe
the paired skeletons of a doublet that lacks pikes would

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Figure 1: Descriptive terminology for silicoflagellate skeletal morphology: (a) Corbisema apiculata (Lemmermann) in apical view, (b) C. apiculata(Lemmermann) in abapical view, (c) Dictyocha sp. with four basal sides in oblique apical view

be connected at the abbasal surfaces of basal elements
and spines, and held together by organic material; such a
doublet is known as the corner-aligned configuration (McCartney et al., 2015a, b). A Corbisema doublet without
pikes and with corners aligned was illustrated by Bachmann (1970, pl.1, fig.17). Modern Dictyocha and Stephanocha (previously known as Distephanus, see Jordan &McCartney, 2015) are also corner-aligned, but have pikes
that are rotated away from the strut attachments, and proceed into the centre of the opposing skeleton in fairly close
proximity to the strut of the paired skeleton (McCartney etal., 2014a).
Recent documentation of Corbisema double skeletonsin the Cretaceous (McCartney et al., 2010b) and Eocene(McCartney et al., 2015a, b) shows two distinct configurations of doublets in the fossil record. Most known fossil doublets, and all of the modern examples, have basal
corners and spines of paired skeletons aligned. The known
fossil skeletons of Corbisema hastata (Lemmermann)
Frenguelli have one of the paired skeletons rotated 180˚ on
the long axis, so that corners are aligned with the opposite
basal sides of the other, in a Star-of-David configuration(McCartney et al., 2010b, 2015a, b).
Both double skeleton configurations, the corner-aligned and Star-of-David, are known among Palaeogene
silicoflagellates. The corner-aligned configuration has notbeen observed in the Cretaceous, with the only known
Corbisema double skeleton of this age being in the Starof-David configuration (McCartney et al., 2010b). In the
Cenozoic, Corbisema of the Star-of-David configuration
has been reported by McCartney et al. (2015a - Paleocene
and Eocene; 2015b - Eocene), and corner-aligned double
skeletons of this genus have been observed by Bachmann(1970 - Oligocene), Bukry (1987 - Eocene), Dumitrică
(1974 - Miocene) and McCartney et al. (2015a - Eocene).
Little is known about silicoflagellate doublets and there isinadequate information to enable identification of the doublet configuration through examination of an individual
skeleton. In this study, we suggest that the shape and orientation of the pike may be helpful in such determinations.
2. Materials and methods
Specimens examined in this study came from the following sources:(1) Lower Eocene material from Mors, Denmark, curated
in the Friedrich Hustedt Collection at the Alfred Wegener
Institut für Polar- und Meeresforschung, Bremerhaven,
Germany (accession number E1758). No exact samplingMorphological variation...Eocene Corbisema

site or level are available for this sample, however, thediatom and silicoflagellate assemblages strongly suggestthat it comes from one of the Fur Formation outcrops ofnorthern Denmark (e.g. Pedersen, 2008);(2) Kamichev or Kamischev, presumed to be Kamyshlov,
Sverdlovsk District, Russia, Lower Eocene (Oreshkina etal., 2004). A small subsample of this material was received
from the Natural History Museum, London. It represents
part of the original materials from this locality distributedin Europe during the early 20th Century (see e.g. Ross &Sims, 1985);(3) Lomonosov Ridge in the central Arctic Ocean was
drilled during Integrated Ocean Drilling Program (IODP)Leg 302 (also known as the Arctic Coring Expedition ACEX). The sample examined here (IODP 302-4A-11X2W, 63-64cm) is from the Middle Eocene (Stickley et al.,2008; Onodera & Takahashi, 2009).
The precise localities for the Mors and Kamyshlov
samples are unknown, and detailed studies of the silicoflagellates are not available (but see Perch-Nielsen, 1976,
for the Fur Formation, and Locker & Martini, 1987, for
Kamyshlov). Because these are isolated samples that havenot been studied in sufficient detail to enable confident
determination of species, and since the emphasis of thispaper is on pike diversity rather than taxonomy, species
identification is tentative.
3. Pikes of Eocene Corbisema
Pikes of Corbisema from the Lower Eocene Mors material show considerable variability. Several taxa present, including two-, three- and four-sided specimens, lack pikes.
Corbisema without pikes are generally of small size, buthave long basal spines (Pl.1, figs 1, 2). Corbisema and several Dictyocha doublets, observed from the Mors sample(McCartney et al., 2015a), and known Naviculopsis specimens (see McCartney et al., 2015a), suggest these taxa
formed corner-aligned double skeletons.
Only some of the Corbisema possess pikes, but these
display considerable diversity, and sometimes vary evenon one individual. Pikes may be linear in shape and point
laterally or abapically towards the centre of the cell (Pl.1,
fig.3), may be oriented perpendicular to the basal plane(Pl.1, figs 4-6), or may curve in various directions (Pl.1,
figs 6, 7). Pike size also varies considerably, from small(Pl.1, figs 3, 9, 10) to quite prominent (Pl.1, fig.11). Distal terminations are rounded and may be slightly bulbous.

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In some specimens, one or more sides bear a small pike,
whereas other sides lack a pike.
Pikes in most Corbisema specimens are located abapically, below the strut attachment, but may be rotated fromthat position. In any discussion of a rotation direction withrespect to the basal corners, we present the rotation froman apical view perspective. For those pikes that are rotated from the position of the strut attachment, nearly allobserved in this study show a dextral rotation (Pl.1, figs
9, 10; Pl.2, figs 3, 8, 12 (right specimen)). Rotation may
be slight or pronounced, and not necessarily consistentfrom one pike attachment to the next. Some specimensshow both rotated and unrotated pikes (Pl.2, fig.1). PerchNielsen (1975, pl.2, fig.16) illustrated a similar specimen
from the Cretaceous.
Many Corbisema skeletons observed from abapicalview have pikes that trend abapically in a position thatwould likely interfere with pikes where the paired skeletons are corner-aligned (Pl.1, figs 3-5; Pl.2, figs 2-4). A
single double skeleton of Corbisema in the Star-of-David
configuration was observed in the ACEX material (McCartney et al., 2015b) that has prominent pikes that curve
into the area of the apical portal of the opposing skeleton.
Doublets of Corbisema with similar prominent pikes aremost likely also of the Star-of-David configuration (seeMcCartney et al., 2010b, 2015a, b for photographs ofdouble skeletons).
4. Pikes with unusual terminations
We observed specimens with pikes of unusual L- or Tshaped terminations, abundant in the Mors material, butalso present in the ACEX and Kamyshlov samples, and illustrate these in Plate 2. These terminations are fragile andseldom preserved on all pikes, so we are unable to determine whether the occurrence is consistent. Usually, thesepikes are ~10µm long and narrow, and trend in a direction
perpendicular to the basal plane. Pikes with L-shaped terminations (Pl.2, figs 3-11, 13) turn at nearly right-angles
away from the cell centre. The distal portion of the pikeappears to be generally parallel to the associated strut, if
seen in apical or abapical view.
Pikes with T-shaped terminations (Pl.2, figs 14, 15) intersect a more-or-less perpendicular component that proceeds laterally in both directions. As with the L-shaped terminations, the distal portion is aligned with the associatedstrut. In some cases, the distal component is foot-shaped,

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Plate 1
SEM images of Corbisema showing basal structures in abapical view, except 4 (apical view) and 6 (lateral view)
All specimens from Mors

C. triacantha (Ehrenberg) Hanna

C. apiculata with short pikes pointing to centre of cell

C. apiculata (Lemmermann) Hanna with pikes perpendicular to basal plane (similar specimen illustrated in Lemmermann, 1901, fig.20)

C. apiculata with two pikes curved away from centre of cell (8 = detail of pike and surface ornamentation illustrated in 7)

Corbisema sp. with pikes slightly rotated

C. hastata (Lemmermann) Frenguelli

Morphological variation...Eocene Corbisema

181

Plate 2
SEM images of Corbisema showing pikes with unusual terminations (e.g. L-shaped in 13; T-shaped in 14, 15; sabaton-like in 16) in abapical view, except 9, 16 (oblique abapical) and 10, 11 (apical view)
1-8, 10, 11, 13-15 from Mors; 9 from Kamyshlov; 12, 16 from ACEX

one pike offset
from strut
attachment

Corbisema sp.

C. apiculata (insets = pike close-up)

C. hastata

brokenspecimen

C. hastata (insets = pike close-ups)

C. apiculata (inset = pike close-up)

one complete,
one broken
specimen

Corbisema sp.

C. apiculata

unusual pike terminations
on left specimen

Corbisema sp.

C. apiculata

Corbisema sp.

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with a shorter heel and longer foot. Observed specimens
with foot-shaped termination consistently show the longer
portion pointing away from the cell centre (Pl.2, figs 9,
12, 13). Distal pikes on specimens from ACEX material
may have sabaton-like terminations (see McCartney et al.,2011a, b, 2014b). These terminations occur at the ends oflong and linear pikes, with the termination pointing abapically (Pl.2, fig.16).
5. Discussion
In this study, specimens of Corbisema with basal rings ofsmall diameter generally lack pikes and have a flat basal
ring and long basal spines. Such a flat abbasal plane would
increase the surface contacts between paired skeletons in acorner-aligned configuration. Double skeleton specimensof four-sided Dictyocha have been observed in the Morsmaterial, in the corner-aligned configuration (McCartney
et al., 2015a). These have a planar abbasal surface, andthe presence of long spines strengthens the hypothesis that
Corbisema without pikes likely formed double skeletonsof the corner-aligned configuration. A double skeleton ofCorbisema that is believed to lack pikes has been observedby McCartney et al. (2015a) and is similar to a specimen
illustrated by Bachmann (1970).
McCartney et al. (2015b) illustrated a Corbisema doublet from ACEX in the Star-of-David configuration thathas prominent pikes that curve towards the middle of
the apical portals. This specimen is documented with 14
SEM photographs in the supplemental materials includedin McCartney et al. (2015a). Similarity in size and shapeof the Mors and ACEX specimens presented here to thisACEX doublet specimen suggests that these skeletons areprobably also associated with Star-of-David doublets (seeMcCartney et al., 2015a).
Corbisema skeletons with slightly rotated pikes generally have a medium-sized (~40µm; see Pl.1, figs 9, 10), flat
basal ring; the spines are also of medium size. Consistentrotation in a dextral direction, when seen in apical view,
could provide a means for two identical skeletons to forma corner-aligned doublet, without the pikes interferingwith one another. While the dextrally rotated pikes, andthe occurrence of a flat basal plane, may suggest a corneraligned double skeleton, the evidence is not sufficient tomake a strong prediction.
Skeletons of Corbisema with unusual terminations are
enigmatic. Several explanations for these terminal features

are plausible. In general, the length and position of thecentrally-located pikes often suggest that a Star-of-David
doublet is probable, since the paired pikes of a corneraligned double skeleton would appear to occupy the same
space. If the doublet is in the Star-of-David configuration,
the pikes may extend into the region of the large portals
on the adjacent skeleton, and thus might help support thecell boundary in that portion of the apical cell-surface.
Alternatively, the long pikes and cross-components may
provide attachment points for organic material that would
hold the paired skeletons together. A third explanation
could be that sabaton-like terminations, which are typicalof some Cretaceous species that lack a basal ring, may be
the result of a reversion to ancestral skeletal morphologies
during times of environmental stress, as proposed by Guex
(2006) and discussed by McCartney et al. (2010a).
6. Conclusions
Pikes have been overlooked features of silicoflagellate
skeletons. Several new observations on Corbisema pikestructure are made: 1) some small skeletons may possess
flat basal rings and lack pikes; 2) some skeletons haveprominent curved pikes; 3) pikes may rotate slightly in a
dextral direction, if seen in apical view, although the pikelocation may not be consistent on all basal sides; 4) pikes
may deviate from the general trend to form a sharp angle
in the distal direction; and 5) pikes may bear sabaton-liketerminations.
Pikes of Corbisema show considerable diversity ofshape and size in a number of Eocene samples. Somepikes can show a consistent rotation away from the strut
attachment, or have unusual distal terminations. The shapeand placement of the pikes on individual skeletons may
have potential use in determining doublet configuration.
Acknowledgements

We thank the Integrated Ocean Drilling Program for supplying the ACEX material, Friedel Hinz (the Hustedt Collection,
Bremerhaven) for sending the Mors material, and Patricia Sims
(the Natural History Museum, London) for kindly sharing asmall subsample of the Kamyshlov material. We are also grateful to Elisa Malinverno and David M. Harwood, who reviewed
the manuscript and made useful comments.

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