Catshark egg capsules from a Late Eocene deep−water
methane−seep deposit in western Washington State, USA
STEFFEN KIEL, JÖRN PECKMANN, and KLAUS SIMON
Kiel, S., Peckmann, J., and Simon, K. 2013. Catshark egg capsules from a Late Eocene deep−water methane−seep deposit
in western Washington State, USA. Acta Palaeontologica Polonica 58 (1): 77–84.
Fossil catshark egg capsules, Scyliorhinotheca goederti gen. et sp. nov., are reported from a Late Eocene deep−water
methane−seep calcareous deposit in western Washington State, USA. The capsules are preserved three−dimensionally and
some show mineralized remnants of the ribbed capsule wall consisting of small globular crystals that are embedded in a
microsparitic matrix. The globules are calcitic, but a strontium content of 2400–3000 ppm suggests that they were origi−
nally aragonitic. The carbonate enclosing the egg capsules, and the capsule wall itself, show 13C values as low as
−36.5‰, suggesting that formation was induced by the anaerobic oxidation of methane and hence in an anoxic environ−
ment. We put forward the following scenario for the mineralization of the capsule wall: (i) the collagenous capsules expe−
rienced a sudden change from oxic to anoxic conditions favouring an increase of alkalinity; (ii) this led to the precipitation
of aragonitic globules within the collagenous capsule wall; (iii) subsequently the remaining capsule wall was mineralized
by calcite or aragonite; (iv) finally the aragonitic parts of the wall recrystallized to calcite. The unusual globular habit of
the early carbonate precipitates apparently represents a taphonomic feature, resulting from mineralization mediated by an
organic matrix. Taphonomic processes, however, are at best contributed to an increase of alkalinity, which was mostly
driven by methane oxidation at the ancient seep site.
Key words: Elasmobranchia, Scyliorhinidae, taphonomy, exceptional preservation, collagen, Late Eocene, Washing−
ton State, USA.
Steffen Kiel [skiel@uni−goettingen.de], Georg−August Universität Göttingen, Geowissenschaftliches Zentrum, Abteilung
Geobiologie and Courant Research Center Geobiology, Goldschmidtstr. 3, 37077 Göttingen, Germany;
Jörn Peckmann [joern.peckmann@univie.ac.at], Universität Wien, Erdwissenschaftliches Zentrum, Department für
Geodynamik und Sedimentologie, Althanstr. 14, 1090 Wien, Austria;
Klaus Simon [ksimon@gwdg.de], Georg−August Universität Göttingen, Geowissenschaftliches Zentrum, Abteilung
Geochemie, Goldschmidtstr. 1, 37077 Göttingen, Germany.
Received 13 July 2011, accepted 7 November 2011, available online 8 November 2011.
Copyright © 2013 S. Kiel et al. This is an open−access article distributed under the terms of the Creative Commons Attri−
bution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original au−
thor and source are credited.
Introduction
Methane seeps in the deep sea are sites on the seafloor where
methane−rich fluids are expelled. Such sites are often colo−
nized by lush faunal communities dominated by animals liv−
ing in symbiosis with chemotrophic bacteria, which take ad−
vantage of the abundance of methane and hydrogen sulfide at
these sites. This high biomass on the otherwise nutrient−poor
deep−sea floor attracts various fishes that appear to have the
ability to cope with the elevated levels of hydrogen sulfide at
these sites. The most common fishes at methane seeps are
zoarcids, macrourids, and occasionally hagfish, eels, morids,
and a few other groups (Ohta and Laubier 1987; Hashimoto
et al. 1989; MacAvoy et al. 2002; Turnipseed et al. 2004;
Sellanes et al. 2008). Reports of sharks from modern meth−
ane seeps are restricted, to the best of our knowledge, to the
two species Centroscyllium granulatum (Dalatiidae) and
Halaelurus canescens (Scyliorhinidae) at seep sites in
740–870 m water depth northwest of Concepción Bay, Chile
(Sellanes et al. 2008). In situ observations and stable isotope
and fatty acid studies indicate that most of these fishes feed,
at last to some extent, on chemosymbiotic animals, although
this is unclear in case of the sharks (MacAvoy et al. 2002,
2003; Sellanes et al. 2008).
Living catsharks often attach their egg capsules to erect
structures in the water column, such as gorgonians, octocorals,
hydroids, and even derelict fishing gear (Able and Flescher
1991; Etnoyer and Warrenchuk 2007). Recently, catshark egg
capsules were found associated with worm tubes and carbon−
ates at a deep−water methane seep site in the eastern Mediter−
ranean where the worm tube thickets might provide a substrate
for protection and ventilation (Treude et al. 2011). At a Late
Eocene methane−seep deposit in western Washington, USA,
large numbers of similar egg capsules were found associated
http://dx.doi.org/10.4202/app.2011.0077Acta Palaeontol. Pol. 58 (1): 77–84, 2013
with worm tubes, as well as hexactinellid sponges (Treude et
al. 2011), showing that catsharks have been using such envi−
ronments as nurseries for at least 35 million years. Predators
have a significant role in ecosystem functioning and maintain−
ing diversity. By serving as nurseries for predatory deep−ma−
rine catsharks and other elasmobranch taxa, methane seeps are
important components of deep−sea ecosystems and should not
be considered as only extreme and exceptional habitats; their
presence or absence is likely to influence faunal diversity
along continental slopes (Treude et al. 2011: 179).
Elasmobranch egg capsules are rare in the fossil record.
Perhaps most common among them are the Paleozoic and Me−
sozoic spirally coiled capsules of the Palaeoxyris group found
mainly in freshwater deposits (Fischer and Kogan 2008).
There are occasional occurrences of chimaeroid egg capsules
(Chimaerotheca) ranging back to the Triassic (Brown 1946;
Bock 1949). Skate egg capsules are rare, with a few specimens
from the Oligocene of central Europe, named Rajitheca (Stei−
ninger 1966). All these records represent internal or external
molds, many are flattened, and a capsule wall has, to the best
of our knowledge, never been reported. The scope of the pres−
ent contribution is to (i) name the fossil egg capsules found in
the Bear River methane−seep deposit, (ii) provide a detailed
description of the capsule and its mineralized wall, and (iii)
discuss the fossilization process.
Institutional abbreviations.—GMUG, Geowissenschaftli−
ches Museum der Universität Göttingen, Germany; SMF,
Senckenberg Museum, Frankfurt, Germany; USNM, United
States National Museum of Natural History, Washington
DC, USA; UWBM, University of Washington, Burke Mu−
seum, Seattle, USA.
Other abbreviations.—EDX, energy−dispersive X−ray spec−
troscopy; LA−ICPMS, laser ablation−inductively coupled
plasma mass spectroscopy.
Geological setting
The Bear River limestone is an isolated carbonate body
found within uplifted deep−water siltstone beds mapped as
“Siltstone of Cliff Point” (Wells 1989). It has long been inter−
preted as an ancient seep deposit based on its geological set−
ting and the macrofaunal association (Goedert and Squires
1990; Goedert and Benham 2003; Kiel 2010a). The low 13C
values of the carbonate reported here provide unequivocal
evidence for its origin at a methane−seep site. The host sedi−
ments were deposited in the Cascadia accretionary wedge
during the Eocene when subduction of the Juan de Fuca plate
under the North American continent started, and their exhu−
mation started in the Miocene (Brandon and Vance 1992;
Stewart and Brandon 2004). Foraminiferal associations indi−
cate that the Bear River seep deposit formed in water depths
of 500–2000 m (Goedert and Squires 1990; Kiel 2010b). The
macrofaunal association is dominated by the mussel Bathy−
modiolus willapaensis, worm tubes are common, large sole−
myid, lucinid, and vesicomyid bivalves occur throughout the
deposit, as well as numerous small gastropods (Goedert and
Squires 1990; Squires and Goedert 1991; Squires 1995;
Goedert and Benham 2003; Kiel 2006, 2010a). A peculiar
feature of this seep deposit is the abundance of uncrushed
specimens of the hexactinellid sponge Aphrocallistes poly−
tretos (Rigby and Jenkins 1983) that are intermingled with
mussels, worm tubes, and the egg capsules reported here.
Material and methods
Thin sections of 60 μm thickness were prepared from cross
sections of a specimen with the ribbed capsule wall pre−
served. They were studied using a Zeiss Axioplan optical
stereomicroscope (lamb: Visitron HBO 50; filter: BP 365,
FT 395, LP 397), and a Zeiss 510 Meta confocal laser scan−
ning microscope (LSM). The LSM was run with an excita−
tion of 488 nm, emissions in the range of 490–704 nm were
measured with spectral detection using a Lambda detector.
Samples for scanning electron microscope (SEM) imag−
ing and EDX analysis were mechanically removed from the
capsule wall, etched for ca. 10 seconds in 10% acetic acid,
mounted on an aluminium sample holder, and coated with
14 nm of platinum. Images were taken with a LEO 1530
SEM at 3.8 KV, EDX analysis were done with an attached
INCAx−act analyzer at 15.7 KV.
Samples for stable carbon and oxygen isotope analysis
were extracted from the polished surfaces of the same rock
samples from which thin sections were prepared, using a
hand−held microdrill. Samples from outside the capsule wall
were drilled from in between and up to a few millimeters away
from the ribs; those from inside the capsule were drilled only
from the micrite, not from any of the microsparitic or cement
phases. Carbonate powders were reacted with 100% phospho−
ric acid at 75C using a Kiel III online carbonate preparation
line connected to a ThermoFinnigan 252 mass spectrometer.
All values are reported in per mil relative to V−PDB by assign−
ing a 13C value of +1.95‰ and a 18O value of −2.20‰ to
NBS19. Reproducibility was checked by replicate analysis of
laboratory standards and is better than ±0.05‰.
The mineralogy of the capsule wall was investigated by
Raman spectroscopy. Spectra were recorded using a Horiba
Jobin Yvon LabRam−HR 800 UV micro−Raman spectrome−
ter with a focal length of 800 mm. As excitation wavelength
the 488 nm line of an Argon Ion Laser (Melles Griot IMA
106020B0S) with a laser power of 20 mW was used. The la−
ser beam was dispersed by a 600 l/mm grating on a CCD de−
tector with 1024×256 pixels, yielding a spectral dispersion of
0.43 cm−1. An Olympus BX41 microscope equipped with an
Olympus MPlane 100× objective with a numerical aperture
of 0.9 focused the laser light onto the sample. With these set−
tings, the focal point of the laser on the sample had a diameter
of ca. 1 μm. The confocal hole diameter was set to 100 μm.
The acquisition time was 15 seconds for wave numbers in the
range of 100–2000 cm−1. For calibration of the spectrometer
78 ACTA PALAEONTOLOGICA POLONICA 58 (1), 2013
a silicon standard with a major peak at 520.4 cm−1 was used.
All spectra were recorded and processed using LabSpecTM
version 5.19.17 (Jobin−Yvon, Villeneuve d’Ascq, France).
Trace element analysis of the capsule wall was done by la−
ser ablation−inductively coupled plasma mass spectroscopy
(LA−ICPMS), using a PerkinElmer SCIEX ELAN DCR II
mass spectrometer coupled to a Lambda Physik Compex 110
Argon−Fluoride−laser with a wavelength of 193 nm. Samples
were placed in a low−volume sample chamber with a size of
5×3×0.5 cm (L×W×H) that was flushed with argon gas. Each
spot was measured for 10 seconds with a spot size of 10 μm.
Systematic palentology
The terminology follows Ebert et al. (2006).
Genus Scyliorhinotheca nov.
Type species: Scyliorhinotheca goederti gen. et sp. nov. described be−
low.
Etymology: From Scyliorhinidae, the catshark family that presumably
produced those egg capsules; and from Latin theca, envelope, capsule.
Diagnosis.—Egg capsules generally having the elongated,
http://dx.doi.org/10.4202/app.2011.0077
KIEL ET AL.—EOCENE CATSHARK EGG CAPSULES IN METHANE−SEEP DEPOSIT 79
A B
C
D
E
F
G
10 mm
10 mm
10 mm
10 mm
10 mm10 mm
10 mm
Fig. 1. The Late Eocene Bear River egg capsule Scyliorhinotheca goederti gen. et sp. nov. in different states of preservation. A. Steinkern showing the anterior
and posterior ends (USNM 544321). B. Posterior half of a steinkern, showing the tapering end with a horn (USNM 544322). C. Deformed steinkern with spin−
dle−like shape (USNM 544323). D. Steinkern with faint wrinkles on the surface, perhaps due to shrinkage (USNM 544324). E. Two specimens: the lower,
main specimen shows the anterior constriction and the flattened anterior end (left) and remnants of the ribbed capsule wall (right); the upper specimen (arrow) is
just a cast of a ribbed capsule surface; note relics of worm tubes on the right side of the block (USNM 544325) (image from Treude et al. 2011). F. Steinkern
showing indentation on lower left (USNM 544326). G. Cast (external mold) showing ribbing on capsule surface (USNM 544327).
inflated fusiform shape of those of living catsharks (Scyli−
orhinidae), with or without longitudinal ribs.
Scyliorhinotheca goederti gen. et sp. nov.
Figs. 1–3.
Etymology: For James L. Goedert, who discovered these fossils.
Holotype: Almost complete internal mold USNM 544321 (Fig. 1A).
Paratypes: Internal and external molds; all figured specimens are de−
posited in the USNM (numbers 544322 to 544328); additional speci−
mens are deposited at SMF (P9796 to P9798), UWBM (95285 to 95287)
and at GMUG (GZG.IF.11020).
Type locality: The Bear River seep deposit, southwestern Washington
State, USA (4619.943 N, 12355.964 W). This is LACM locality num−
ber 5802 and UWBM locality number C1459.
Type horizon: Siltstone of Cliff Point, latest Eocene (nannofossil zone
CP15b of Okada and Bukry 1980), based on the presence of Isthmo−
lithus recurvus).
Diagnosis.—The egg capsules have an inflated fusiform
shape and a slightly constricted waist; the maximum dimen−
sions are: length = 50 mm, width = 15 mm, thickness = 11
mm. The surface shows at least 12 rough ridges and two thin
secondary ridges. The lateral edges extend to narrow lateral
flanges along the entire length of the egg case. The anterior
border is slightly concave, with two well−developed respira−
tory fissures but apparently no horns; the posterior end is ta−
pering after a slight constriction, and the horns are short and
pointing at, or twisting around, each other.
Discussion.—We have seen no tendrils on either end of the
egg capsules, but this may be a preservational artefact. The
most similar egg capsules produced by modern catsharks are
those of the scylorhinid genus Apristurus: many have a simi−
larly tapering posterior end, and some show coarse ribs on the
surface (Sato et al. 1999; Iglésias et al. 2002, 2004; Flammang
et al. 2007). Fossil teeth attributed to the genus Apristurus are
known from the Late Eocene of European Tethys (Adnet et al.
2008), which agrees with the stratigraphic age of the egg cap−
sules reported here. The only other extant scylorhinid with a
fossil record ranging into the Eocene, including the Pacific
Northwest of the United States, is Scyliorhinus (Welton 1972;
Adnet et al. 2008); although the egg capsules of this genus re−
semble those of Apristurus in general shape (Rusaouën et al.
1976; Ebert et al. 2006), they are apparently smooth (cf.
Treude et al. 2011). Similar in outline are the egg capsules of
two species of the genus Schroederichthys (S. tenuis and S.
maculatus), but on their surfaces the egg capsules show only
tortuous longitudinal striations (Springer 1966; Gomes and de
Carvalho 1995) instead of strong ribs as in the Bear River
specimens.
The capsule wall
In cross section, the capsule wall is approximately 100 μm
thick; individual ribs are up to 700 μm high and between 100
and 150 μm wide at their narrowest point; they are often bent
sideways (Fig. 3A, B). The capsule wall consists of a micro−
sparitic matrix enclosing abundant and roughly evenly distrib−
uted globules having a diameter of 15–20 μm (Fig. 3C, D).
The matrix and the globules are both made of calcite, indicated
by peaks at the Raman bands at 280, 710, and 1084 m−1 (Fig. 4;
Urmos et al. 1991). The tips of the ribs are often made of
sparry calcite (Fig. 3C, D). Under crossed nicols, the matrix
shows extinction in patches, some of the globules also show
extinction, some do not (Fig. 3F). Confocal laser scanning mi−
croscopy shows that the globules have a much stronger auto−
fluorescence than the matrix (Fig. 3E). LA−ICPMS analysis of
the globules (N = 3) showed a Sr content of 2400–3000 ppm,
measurements on the background matrix (N = 2) showed a Sr
content of 1300–2700 ppm. The small size of the globules and
their close proximity to each other within the wall matrix
meant that LA−ICPMS measurements of the globules were
compromised by certain amounts of wall matrix material, and
vice versa (Fig. 5). It is thus likely that the actual Sr contents of
both globules and wall matrix are more distinct from each
other than we were able to document. Small pyrite framboids
80 ACTA PALAEONTOLOGICA POLONICA 58 (1), 2013
5 mm
10 mm
Fig. 2. Silicon rubber casts of external molds of the Late Eocene Bear River
egg capsule Scyliorhinotheca goederti gen. et sp. nov. A. USNM 544327
(the same specimen as in Fig. 1). B. USNM 544328.
Table 1. Stable carbon and oxygen isotope values of the capsule wall and
the carbonate associated with it; all values are relative to V−PDB.

13C 18O
Outside capsule −35.1 −6.6
−35.2 −6.0
−36.3 −5.8
Capsule wall −36.0 −4.4
−36.5 −6.3
Inside capsule −34.9 −5.0
−35.1 −5.0
−35.1 −4.9
no larger than 5 μm diameter (most are < 1 μm diameter) are
dispersed throughout the wall matrix (Fig. 6), as revealed by
SEM imaging and EDX analysis of etched surfaces of the cap−
sule wall. The Raman spectra (Fig. 4) of the wall matrix show
a small peak at 375 m−1, which also indicates pyrite (Mernagh
and Trudu 1993). This peak was not seen in the spectra of the
globules (Fig. 4).
Preservation and occurrence
All specimens are preserved in three dimensions, and several
show indentations that most likely originated after hatching of
the young shark (Fig. 1F). Many capsules are only preserved
as internal molds (steinkerns; Fig. 1), some are external molds
http://dx.doi.org/10.4202/app.2011.0077
KIEL ET AL.—EOCENE CATSHARK EGG CAPSULES IN METHANE−SEEP DEPOSIT 81
A B
C D
E F
500 µm 500 µm
200 µm 200 µm
50 µm50 µm
Fig. 3. Thin section micrographs of the capsule wall of the Late Eocene Bear River egg capsule Scyliorhinotheca goederti goederti gen. et sp. nov.
A, B. Two sections of a capsule wall with several ribs, dark, micritic seep carbonate on the outside, and several diagenetic carbonate phases in the interior;
plane−polarized light. C, D. Close−up on the middle rib in A, rotated ca. 90 clockwise. Note that the capsule wall consists of microsparite, enclosing globu−
lar calcite crystals; plane−polarized light (C) and UV light (D). E. Detail of the capsule wall; laser scanning microscope image generated by linear unmixing
using the spectra of the globules versus that of the wall matrix. F. Detail of capsule wall, showing extinction of some of the globules; crossed−polarized light.
showing the ribbing of the capsule wall (Figs. 1, 2), and only
in a few specimens is the capsules wall preserved in the miner−
alized form described herein. The egg capsules are enclosed
in, and filled with, micritic, microsparitic, and peloidal car−
bonate as well as carbonate cement that show 13C values as
low as −36.3‰, the wall itself revealed values of −36.5 and
−36.0‰ (Table 1). The capsules are always associated with
worm tubes (Fig. 1E) and three dimensionally preserved spec−
imens of the hexactinellid sponge Aphrocallistes polytretos,
and often also with the mussel Bathymodiolus willapaensis
(cf. Squires and Goedert 1991; Kiel 2006).
Discussion
Shark egg capsules are virtually unknown from the marine
fossil record (Fischer and Kogan 2008). Freshly laid catshark
egg capsules are made of collagen (Rusaouën et al. 1976),
a substance that is quickly hydrolyzed when exposed to water.
Thus, the calcification of the capsule walls requires special
conditions. We suggest the following scenario for the genesis
of the calcitic capsule wall: (i) the egg capsules experienced a
sudden change from an oxic to an anoxic environment; (ii)
aragonitic globules precipitated within the collagenous cap−
sule wall, likely favoured by the activity of bacteria degrading
the organic wall; (iii) subsequently the remaining wall matrix
was mineralized by aragonite or calcite; (iv) finally the ara−
gonitic parts of the wall recrystallized to calcite. This scenario
is based on the following lines of evidence:
Environment.—When catshark egg capsules at modern
methane−seep sites are laid into tubeworm thickets they de−
velop in a fully oxic environment (Treude et al. 2011). At the
Bear River site, the very low 13C values of the capsule wall
and the carbonate surrounding it, as well as the finely dis−
persed pyrite framboids show that at least part of the carbon−
ate formed due to the anaerobic oxidation of methane (cf.
Peckmann and Thiel 2004) and thus in an anoxic environ−
ment. Because collagen is quickly hydrolyzed in marine en−
vironments, we assume that a sudden change from an oxic to
an anoxic environment facilitated the three dimensional pres−
ervation of the egg capsules and their walls.
Shape and mineralogy of the globules.—The growth of cal−
cium carbonate globules like those documented here is com−
monly related to bacterial activity and the bacterial decay of or−
ganic matter (e.g., Buczynski and Chafetz 1991; Briggs and
Kear 1993), but crystal habit is clearly not an unequivocal cri−
terion for this kind of mineralization (e.g., Fernández−Díaz et
al. 1996). The low 13C values of the capsule wall including
globules suggest that wall fossilization (i.e., replacement of
collagen by carbonate minerals) was fostered by an increase of
alkalinity induced by anaerobic oxidation of methane (cf.
Peckmann and Thiel 2004). Although the globular crystal
habit—a habit untypical for methane−derived carbonate miner−
als of seep limestone—most likely reflects the taphonomical
alteration of the organic wall, the decay of the wall was appar−
ently not responsible for the generation of an oversaturation
with respect to carbonate minerals. Even so carbonate precipi−
tation was probably mediated by an organic matrix provided by
the decomposing wall, the decay of the collagenous wall did
not contribute significantly to alkalinity increase based on the
very similar carbon isotopic compositions of the wall and the
adjacent methane−derived carbonate phases; shark collagen
typically reveals higher 13C values of approximately −15‰ to
82 ACTA PALAEONTOLOGICA POLONICA 58 (1), 2013
Globule
Wall matrix
R
el
at
iv
e
in
te
ns
ity
R
el
at
iv
e
in
te
ns
ity
Raman shift (cm )-1
1084
280
710
375
250 500 750 1000 1250 1500 1750 2000
Fig. 4. Raman spectra of globules and microsparitic matrix of the capsule wall.
outside
capsule wall
inside
Fig. 5. Thin section micrograph of the capsules wall showing three of the
craters produced during laser ablation (arrows).
−13‰ (MacNeil et al. 2005). An originally aragonitic mineral−
ogy of the globules is suggested by their high Sr content
(2400–3000 ppm); such values are common for aragonite, but
calcite or dolomite precipitated in marine environments typi−
cally have lower Sr contents (Tucker and Wright 1990; Comp−
ton 1992). Aragonite cement formed at methane seeps in the
Black Sea exhibits Sr contents as high as 9500 ppm, the associ−
ated high−Mg−calcite revealed values of approximately 1200
ppm (Peckmann et al. 2001). Similarly, aragonite cement of
seep carbonates from the Niger Delta showed Sr contents of up
to 12 000 ppm, much higher than the contents of 100 ppm to
1000 ppm found for other carbonate mineralogies including
calcite and dolomite (Bayon et al. 2007). Savard et al. (1996)
reported as much as 11 000 ppm Sr in aragonite cement of Cre−
taceous seep limestone rocks. In many ancient seep deposits,
aragonite is recrystallized to calcite (Buggisch and Krumm
2005; Peckmann et al. 2007). Fibrous calcite cements with an
inferred aragonite precursor of Paleozoic seep limestone rocks
revealed Sr contents as high as approximately 20 000 ppm
(Iberg deposit; Buggisch and Krumm 2005), 13 000 ppm
(Tentes Mound; Buggisch and Krumm 2005), and 13 000 ppm
(Dzieduszyckia deposit; Peckmann et al. 2007), whereas the
micrite (i.e., microcrystalline calcite) of ancient seep deposits
typically exhibits contents in the range of a few hundred ppm to
little more than 1000 ppm (e.g., Campbell et al. 2002).
Wallmatrix.—Mineralization of the wall matrix subsequent
to the precipitation of the globules is suggested by the regular
shape of the globules. The growth of such globules within an
already existing calcium carbonate wall seems impossible.
Based on the Sr content of the wall matrix, ranging from
1300–2700 ppm, it is difficult to decide whether this carbon−
ate was primarily aragonite or a mixture of aragonite and cal−
cite. The very negative 13C values of the capsule wall (glob−
ules plus matrix) and the pyrite framboids finely dispersed
throughout the wall indicate that fossilization occurred under
anoxic conditions. The Bear River capsule walls share a
common fossilization mechanism with the tubes of seep−
dwelling vestimentiferan worms (Haas et al. 2009) in that
fossilization was driven by carbonate precipitation induced
by the anaerobic oxidation of methane. Unlike the fossiliza−
tion of the organic−walled tubes of vestimentiferans, how−
ever, where individual layers of the organic wall function as
templates for carbonate cement growth, the egg capsule
walls were permineralized by authigenic carbonate, resulting
in three−dimensional preservation.
Conclusions
To the best of our knowledge, the catshark egg capsules re−
ported here are the first fossil shark egg capsules from a fully
marine environment. The shape of the fossil egg capsules
closely resembles that of egg capsules produced by modern
catsharks (family Scyliorhinidae), therefore we erect the new
genus and species, Scyliorhinotheca goederti. Fossilization of
the capsule wall was favoured by anaerobic oxidation of meth−
ane—the biogeochemical key process at seeps—inducing re−
placement of the organic wall by carbonate minerals. The
crystal habit of the first replacive carbonate phase, however,
reflects taphonomical processes within the decaying wall.
Acknowledgements
We thank Jim Goedert (Wauna, Washington State, USA) for providing
the specimens and for discussion, Gernot Arp for help with the confocal
laser microscope, Axel Hackmann for preparation of thin sections,
Nadine Schäfer for her time and effort with Raman spectroscopy,
Gerhard Hundertmark for photography, Dorothea Hause−Reitner for
assistance with SEM imaging and EDX analysis (all University of
Göttingen, Germany), Michael Joachimski (University of Erlangen,
Germany) for stable isotope analysis, Jan Fischer (Bergakademie Frei−
berg, Germany) for discussion of, and literature on, fossil elasmobranch
egg capsules, as well as Jürgen Kriwet (University of Vienna, Austria)
and an anonymous reviewer for their efforts to improve the manuscript.
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