An enigmatic univalve macromollusc from the lower Cambrian (Series 2, Stage 3) Heatherdale Shale, South Australia

Abstract

Fossils have been reported sporadically throughout the lower Cambrian Heatherdale Shale (Normanville Group), Fleurieu Peninsula, South Australia, but there has been little taxonomic documentation of faunas. Problematic cap-shaped fossils from the lower member of the formation originally attributed to Helcionella are herein formally described as a new genus and species, Tribelopoma amygdala gen. et sp. nov. The asymmetric valves with accretionary growth and probable calcium carbonate composition support a molluscan affinity, and comparisons with other enigmatic cap-shaped taxa suggest a possible helcionelloid affinity. Taphonomic processes related to early diagenesis provide evidence of a rigid shell, prone to brittle fracture during compaction.

Full text

An enigmatic univalve macromollusc from the lower Cambrian
(Series 2, Stage 3) Heatherdale Shale, South Australia
SARAH M. JACQUET, JAMES B. JAGO & GLENN A. BROCK
THE TAXONOMIC afnities of many lower Cambrian
‘cap-shaped’ fossils have been debated in the literature
for decades, and whilst some taxa have achieved almost
universal consensus regarding their taxonomic status, the
afnities of others remain elusive. Most often, cap-shaped
fossils are aligned with the univalved molluscs (Peel
2003), but there is considerable variation in size, shape and
morphology; ranging along a spectrum from cyrtoconic
forms with a degree of coiling, to simpler, generally low,
depressed discoid forms. Cap-shaped fossils, including
problematic forms, are a small part of the overall diversity
of the Cambrian Evolutionary Fauna (Sepkoski 1981;
Alroy 2010).
The Heatherdale Shale crops out on Fleurieu Peninsula,
South Australia (Fig. 1) and has a fauna sporadically
distributed throughout the unit (Daily 1963; Foster et al.
1985; Jago et al. 2006). Reported (but undescribed) taxa
include sponges, hyoliths, brachiopods, gastropods, and
non-biomineralised bivalved arthropods (Isoxys) (Abele
& McGowran 1959; Daily 1963; Jago et al. 1984, 2006;
Jenkins et al. 2002; Paterson et al. 2010). Presently, the
only formal taxonomic description of fossils includes rare
conocoryphid trilobites (Atops) (Jago et al. 1984; Jenkins
& Hasenohr 1989; Jell et al. 1992) and organic-walled
microfossils (Sphaerocongregus) (Foster et al. 1985) from
the upper member of the Heatherdale Shale.
Univalved molluscs have been noted since the
establishment of the Heatherdale Shale as a formal
stratigraphic unit (Abele & McGowran 1959, p. 310-311).
Abele & McGowran (1959) reported two species; the older
species derived from a lower carbonate-rich member, in
a creek east of Main South Road (Fig. 1, Loc. 6). This
taxon, originally referred to Helcionella Grabau & Shimer
1909, is particularly abundant in weathered material, and
is preserved primarily as internal and external moulds with
an iron oxide coating similar to that of hyoliths within
the same beds (Abele & McGowran 1959). The authors
tentatively attributed the younger species recovered from
the upper member, north of Carrickalinga Head, to Scenella
Billings 1872 based on the attened shape of the specimen,
concentric ornament and eccentric apex. Neither species
has ever been illustrated, but their occurrences have been
repeatedly mentioned in the literature (see Daily 1963, p.
583, 589, g. 1; Jago et al. 1984, p. 207, 2006; Foster et al.
1985, p. 261; Jenkins et al. 2002, g. 1).
The aim of this paper is to provide the rst systematic
description of a macroscopic mollusc from the lower
Cambrian (Series 2, Stage 3) Heatherdale Shale. A new
genus and species, Tribelopoma amygdala gen. et sp.
nov. is described in detail, and the taxon almost certainly
corresponds to the rst univalved molluscs reported from
the lower member of the Heatherdale Shale by Abele &
McGowran (1959). Tribelopoma amygdala gen. et sp. nov.
is unusual in possessing an asymmetrical shell outline that
differentiates it from many other early Cambrian univalve
molluscan taxa, prompting detailed discussion of its
possible afnities.
GEOLOGY AND LOCALITIES
Lower Cambrian strata of the Normanville Group cropping
out along the eastern coastline of Fleurieu Peninsula in
the eastern Stansbury Basin comprise an approximately
1 km thick succession of siliciclastics and carbonates,
which unconformably overlie the Ediacaran ABC Range
Quartzite (Gravestock et al. 2001). The Heatherdale Shale
is the youngest unit in the Normanville Group, resting with
apparent conformity above the Fork Tree Limestone and
disconformably below the Carrickalinga Head Formation,
the basal unit of the Kanmantoo Group (Jago et al. 1994,
2006; Gravestock et al. 2001). The Heatherdale Shale
was originally subdivided into two informal members: a
JACQUET, S. M., JAGO, J. B. & BROCK. G. A., 2016:05:23. An enigmatic univalve macromollusc from the lower Cambrian
(Series 2, Stage 3) Heatherdale Shale, South Australia. Australasian Palaeontological Memoirs 49, 21-30. ISSN 2205-8877.
Fossils have been reported sporadically throughout the lower Cambrian Heatherdale Shale (Normanville Group), Fleurieu
Peninsula, South Australia, but there has been little taxonomic documentation of faunas. Problematic cap-shaped fossils
from the lower member of the formation originally attributed to Helcionella are herein formally described as a new genus
and species, Tribelopoma amygdala gen. et sp. nov. The asymmetric valves with accretionary growth and probable calcium
carbonate composition support a molluscan afnity, and comparisons with other enigmatic cap-shaped taxa suggest a possible
helcionelloid afnity. Taphonomic processes related to early diagenesis provide evidence of a rigid shell, prone to brittle
fracture during compaction.
S. M. Jacquet (sarah.jacquet@mq.edu.au), G. A. Brock (glenn.brock@mq.edu.au), Department of Biological Sciences, Faculty
of Science and Engineering, Macquarie University, Sydney, NSW, 2109, Australia; J. B. Jago (Jim.Jago@unisa.edu.au) School
of Natural and Built Environments, University of South Australia, Mawson Lakes, South Australia, 5095, Australia. Received
1 October 2015.
Keywords: Mollusca, cap-shaped, Fleurieu Peninsula, Normanville Group, Helcionelloida, taphonomy.

AP Memoir 49 (2016)
22
lower member characterised by light-coloured calcareous
shales with intermittent layers of limestone nodules,
and an upper member consisting of grey to black pyritic
shales and siltstones with minimal to no carbonate present
(Abele & McGowran 1959). The contact between the two
members was reported by Abele & McGowran (1959) to
coincide with the last horizon with large carbonate nodules.
However, subsequent mapping indicated the nature of this
boundary varies considerably both in terms of lithology and
lateral extent (Daily 1963; Jago et al. 1994). There appears
to be considerable lateral lithological variations within
the Heatherdale Shale, although it is yet to be studied in
detail. The Heatherdale Shale was deposited below storm
wave-base in a deep marine to basinal setting, as part of
a transgressive succession following the shallow water
carbonate platform sediments of the Fork Tree Limestone
(Jago et al. 1994, 2006; Gravestock & Gatehouse 1995;
Gravestock et al. 2001).
The macromolluscan material described herein is
derived from numerous spot localities (see co-ordinates
in Fig. 1 caption) within the lower part of the Heatherdale
Shale: Locality 1, between 50–100 m above the base of
the Heatherdale Shale, Mount Terrible Gully; Locality 2,
southeast side of the water pipeline in Mount Terrible Gully;
Locality 3, a road-cut outcrop approximately 20 m above
the base of the Heatherdale Shale, near Pipeline Prospect;
Locality 4, the southern bank of Myponga River, about 350
m southeast of Myponga Beach; Locality 5, Rocky Gully,
1.5 km southwest from Myponga Beach, approximately
65 m above the base of the Heatherdale Shale; Locality
6, originally collected by Abele & McGowran, 1 km SW
of Loc. 2. Some specimens in the collections of the South
Australian Museum, probably collected by Dr B. Daily, in
the vicinity of Sellick Hill lack precise locality details (e.g.
Fig. 3E-H, L).
All localities occur stratigraphically below a
radiometrically dated tuff layer (approximately 300 m above
base of the Heatherdale Shale), which was recalculated
Figure 1. Regional geology of the Normanville Group on the eastern side of the Fleurieu Peninsula and corresponding spot localities;
Loc. 1, co-ordinates: 35°20’55.25’’ S; 138°27’12.5’’ E; Loc. 2, co-ordinates: 35°20’54.82” S; 138°27’10.56” E; Loc. 3, co-ordinates:
35°21’20.08” S; 138°26’22.99” E; Loc. 4, co-ordinates: 35°22’34” S; 138°23’14” E; Loc. 5 co-ordinates: 35°22’52.8’’ S; 138°22’20.6’’
E; Loc. 6, estimated co-ordinates: 35°21’10.84” S; 138°26’36.45” E.
Gulf
S t V incen t
PALAEO Z OIC
CAMBRI A N
Upper Member
Lower Member
Sellick Hill Formation
Wangkonda Formation
Mount Terrible Formation
Marinoan
Fork Tree Limestone
Heatherdale
Shale
PROTERO Z OIC
Cenozoic sediments
Watercourse
Road
Fault
Geological
Boundary
Carrickalinga Head Formation
N
BLACK
HILL
35° 20' S
138°25'E
35° 25' S
138°25'E
35° 25' S
35° 20' S
MAIN
SOUTH
ROAD
F
F
F
SELLICKS
BEACH
138°27'E
ROAD
GULF VIEW
SELLICKS
HILL
Myponga
Beach
Myponga
River
4km
0 21 3
OLD
SELLICKS HILL
ROAD
F
SELLICKS BEACH ROAD
MYPONGA
ROAD
BEACH
SAMPSON
ROAD
RESERVOIR
ROAD
Mt. Terrible Gully
STUDY
AREA
Adelaide
ADELAIDE
GEOSYNCLINE
0500km
SOUTH AUSTRALIA
F
F
?
Sample site
SELLICKS BEACH ROAD
Loc. 5
Loc. 6
Loc. 4
Loc. 2
Loc. 1
Loc. 3

AP Memoir 49 (2016) 23
from 526±4 (Cooper et al. 1992) to 522±2.0 Ma using the
SHRIMP method (Jenkins et al. 2002). Recent unpublished
TIMS dates suggest an even younger age in the upper part
of Cambrian Stage 3 (Jagodzinski, pers. comm. to GAB,
2014).
MATERIALS AND METHODS
Macromollusc specimens were collected from outcrop
as crack-out material consisting of internal and external
moulds. Isolated specimens were prepared using a
pneumatic micro-drill to remove surrounding matrix.
For macrophotography, specimens were either colour
imaged or inked black and coated in Ammonium Chloride.
Images were taken stepwise at different focal planes using
a Hasselblad H4D-31 with a HC Macro 120 mm II lens
and additional 13 mm, 26 mm and 52 mm extensions.
The images were then stacked together using image
software Phocus and Adobe Photoshop CS6. Fine-scale
measurements were made using public imaging software
package ImageJ (1.47v, available online at http://imagej.
nih.gov/ij, National Institutes of Health, USA).
Scanning electron microscope energy dispersive X-ray
spectrometry (SEM-EDS) analysis was completed using a
JEOL JSM–6480 LA SEM with EX-943000 SDD EDS (10
kV). Regional map analysis was completed for localised
elemental information, distribution and intensity.
SYSTEMATIC PALAEONTOLOGY
Morphological terminology follows that of Vendrasco et
al. (2011) and Jacquet & Brock (2015), with general terms
dened in Figure 2. Two main regions of the shell are
recognised including the supra-apical margin (in front of
the apex), the sub-apical margin (behind the apex) (Fig.
2A-B). The maximum distance across the shell is the length,
while orthogonal to that is the width. The midline runs
subparallel to the long axis and intersects the median point
on the transverse axis (Fig. 2A-B). The supra-apical surface
comprises the shell in front of the transverse line through
the apex (=apical transverse axis), and the remaining area
behind this line (usually the shortest eld) represents the
sub-apical surface (Fig. 2B).
All gured material has been assigned SAM P numbers
and is housed in the Palaeontological collections of the
South Australian Museum, Adelaide.
Phylum MOLLUSCA Cuvier 1797
Class HELCIONELLOIDA Peel 1991
?Order HELCIONELLIDA Geyer 1994
Discussion. The new taxon, Tribelopoma amygdala gen. et
sp. nov., displays characters comparable to various fossil
forms possessing a cap-shaped morphology, including
helcionelloids, stem brachiopods, problematic opercula,
stenothecoids and, to a lesser extent, cnidarians. Whilst it
is clear that the shells of T. amygdala gen. et sp. nov. have
undergone variable degrees of post-burial compression and
deformation, the repeated almond-shaped outline and shell
asymmetry suggest some variation is not taphonomic, but
intrinsically biological.
Bilateral symmetry is a common trait of many cap-
shaped taxa. For the Order Helcionelloida Peel 1991, the
vast majority of cap-shaped to patelliform shells exhibit
clear bilateral symmetry, though some asymmetrical coiled
and cyrtoconic forms do exist. The eccentric position of
the apex and likely possession of a rigid calcium carbonate
shell (indicated by brittle fracture patterns in the moulds)
does broadly support either an helcionelloid or tergomyan
afnity (Peel 1991). Internal moulds of Tribelopoma
amygdala lack surcial features such as paired muscle
Figure 2. Schematic diagrams of Tribelopoma amygdala gen. et sp. nov and descriptive terminology. Approximate position of apex
indicated by circle and cross, scale bars = 2 mm. A. SAM P49836, B. SAM P49832.
S
u
p
r
a
-
a
p
i
c
a
l
m
a
r
g
i
n
S
u
b
-
a
p
i
c
a
l
m
a
r
g
i
n
A B
Long axis (max. length)
Supra-apical
surface
Sub-apical
surface
S
u
p
r
a
-
a
p
i
c
a
l
m
a
r
g
i
n
S
u
b
-
a
p
i
c
a
l
m
a
r
g
i
n
Midline
Transverse
axis (max.
width)
Apical
transverse
axis

AP Memoir 49 (2016)
24
scars that would otherwise ally this taxon more closely
with the tergomyans. The featureless internal surface, thin
shell and clear convexity of the valve further distinguishes
Tribelopoma gen. nov. from other bilaterally symmetrical
disc-shaped forms such as the mobergellans (Bengtson
1968; Conway Morris & Chapman 1997; Skovsted 2003).
In addition to the millimetric size and convex to concave
lateral prole of mobergellans, they possess distinct
radiating ridges and paired shallow depressions on the
internal surface of the phosphatic shell (see Skovsted
2003), features which are absent on the material described
herein. The stem brachiopod Estoniadiscus Peel 2003
represents yet another morphological variant of bilaterally
symmetrical cap-shaped fossil. However, there is minimal
morphological commonality between these taxa, and
Tribelopoma amygdala lacks any indication of the
setigerous canals that perforate the shell in Estoniadiscus
(Peel 2003, 2015). Based on the relatively limited array
of morphological traits in the new genus, it is likely to
represent an aberrant asymmetrical and patelliform member
of the Helcionelloida.
Another possibility worthy of consideration is a
stenothecoid afnity for Tribelopoma. Stenothecoids are
members of the Class Stenothecoida Yochelson 1969, but
have been previously aligned with crustaceans (Resser
1938), bivalves (Poulsen 1932; Sytchev 1960; Robison
1964; see Yochelson 1969), brachiopods (Radugin 1937),
both univalved and possible bivalved monoplacophorans
(Runnegar & Pojeta 1974) and even attributed to their own
phylum (Rozov 1968). Their skeleton consists of bivalved,
asymmetrical and inequivalved shells, with the apex
situated marginally or slightly overhanging the adapical
margin (Yochelson 1969, gs 1A-C, 2A-B, 3A-C). The
asymmetrical outline, differential accretionary growth habit
and irregular spacing of concentric rugae in stenothecoids is
supercially similar to Tribelopoma amygdala. However, it
is difcult to reconcile a bivalved form with the new genus
since no evidence of conjoined valves is present in the latter.
Internal moulds of Tribelopoma amygdala display very few
features (with the exception of the corrugations, an internal
expression of the concentric rugae) and there is no evidence
to suggest possession of possible hinge teeth and sockets
(Robison 1964; but see Yochelson 1969). Furthermore,
the internal structures of stenothecoids are highly variable,
ranging from the parallel lateral and marginal grooves in
Stenothecoides Resser 1938 (Rasetti 1954, pl. 12, gs 1-2;
Yochelson 1969, g. 4) to the distinct herring bone pattern
produced by ridges extending from a median carina in
Bagenovia Horný 1957 (Horný 1957, pl. 3, gs 1-6; Koneva
1976, gs 1b-c, e-f, 2d; Aksarina & Pelman 1978, pl. 15,
gs 7-9). Whilst the external morphology and growth habit
of stenothecoids bears some close similarities to the new
genus, the absence of key internal features in Tribelopoma
makes such an afnity unlikely.
Asymmetrical shell morphologies are common
in scleritome-bearing taxa possessing paired sclerite
morphotypes. For instance, the scleritome of Trachyplax
Larsson, Peel & Högström 2009, from the lower Cambrian
(Series 2, Stage 4) of Greenland, possesses at least eight
calcareous sclerite types. Amongst these morphs, ve are
recognised as opposing symmetry pairs (B-D, G-H), all of
which have an asymmetrical form (Larsson et al. 2009).
Although the higher level taxonomy of this group remains
problematic, a possible multiplacophoran afnity cannot be
completely ruled out for Tribelopoma; however, the shells
lack obvious insertion or attachment areas, as well as obvious
surfaces where the shells might imbricate. In addition, the
asymmetrical shell outline typical of Tribelopoma, exhibits
a spectrum of intraspecic variability.
There are a number of depressed discoidal forms in
the fossil record, especially those with regular concentric
ornament that have been referred to the Cnidaria (see Bell
et al. 2001, table 1; Young & Hagadorn 2010, table 1).
Extant porpitoids [e.g. Velella velella (Linnaeus 1758) and
Porpita porpita (Linnaeus 1758)] possess a chitinous oat
(pneumatophore) that provides a rigid but exible internal
skeleton (Fryer & Stanley 2004). Fossil representatives
of this group are characterised by bilateral symmetry, an
elongate to circular outline, subcentral apex, and have both
radial and concentric rings (Norris 1989; Bell et al. 2001). In
contrast, Tribelopoma has clear topography (despite having
a compressed prole), an asymmetrical shell, and variable
width and relief of concentric rugae. Though some radial
sculpture is present on some specimens of Tribelopoma
(Fig. 3C, K), the close spacing and bifurcation of the
concentric ornament is dissimilar to the ordered distribution
of ribs and associated partitioning of pneumatocysts (air-
lled chambers) in extant porpitoids (Bell et al. 2001; Fryer
& Stanley 2004).
It is worth mentioning briey that a number of discoidal
fossils are found among the Ediacaran biota. Convex discs
with concentric ridges and grooves include forms such as
Tirasiana (Palij 1976), Nemiana (Palij 1976), Cyclomedusa
(Sprigg 1947) and Ediacaria (Sprigg 1947), though the
taxonomic status of these forms currently remains unclear
with suggestions they represent junior synonyms of
Aspidella (Gehling et al. 2000; but see MacGabhann 2007).
The youngest, albeit tentatively assigned, representatives of
Tirasiana have been found in the Stage 2 (Terreneuvian) of
South China. However, the remaining genera are conned
to the Ediacaran (Yang et al. 2014). The generally accepted
view that most Ediacaran discoidal fossils represent
holdfasts to frondose taxa (Gehling et al. 2000; Tarhan et
al. 2015; Burzynski & Narbonne 2015) is morphologically
inconsistent with the asymmetrical outline and probable
hard shell of Tribelopoma amygdala, and thus precludes
any afnity with these soft-bodied taxa.
Whilst the mix of otherwise limited character states
and taphonomic overprint impedes condent higher level
taxonomic assignment, a molluscan afnity is the most
Figure 3. Tribelopoma amygdala gen. et sp. nov., all scale bars = 2 mm. A. Paratype (SAM P4983) internal mould from SE side of
pipeline, Mount Terrible Gully (Loc. 2), apical view. B. Paratype, SAM P49832, internal mould, apical view. C-D. Holotype (SAM
P525224), internal mould, from SE side of pipeline, Mount Terrible Gully (Loc. 2); C. apical view, D. oblique-lateral view. E-F.
Topotypes, internal moulds from the Sellick Hill region (collected by Dr B. Daily); E. SAM P525220, oblique-apical view, F. SAM
P525222, partial internal mould of lateral surface, lateral view. G-H. Paratype (SAM P49833) internal mould, G. oblique-lateral view,
H. apical view. I. Topotype (SAM P525217), external mould from the Sellick Hill region (collected by Dr B. Daily), apertural view. J.
Topotype (SAM P49581), external (latex) mould from Pipeline Prospect (Loc. 3), apical view. K. Topotype (SAM P19948), internal
mould from, oblique-apical. L. Topotype, SAM P525215, external mould from the Sellick Hill region (collected by Dr B. Daily),
oblique-apertural view. White arrows indicate location of brittle fractures.

AP Memoir 49 (2016) 25

AP Memoir 49 (2016)
26
parsimonious based on available evidence.
Tribelopoma gen. nov.
Type species. Tribelopoma amygdala sp. nov.
Etymology. Greek: tribelo-, “three-pointed”; in reference
to the three main curved portions forming the asymmetrical
shell outline and poma, ‘lid’, for the low, simple relief of
the cap-shaped shell.
Diagnosis. Asymmetrical shells with broadly almond-
shaped outline ranging from subcircular to elongate ovoid.
Apex having eccentric position; varying from sub-central
to offset from shell midline. Sub-apical margin commonly
with strongest curvature to one side or other of the shell
midline. First order external ornament consists of tightly
and irregularly spaced concentric angular grooved rugae
which sometimes bifurcate; second order external ornament
of ne radial lirae.
Discussion. From a morphological and stratigraphic
perspective, Tribelopoma amygdala gen. et sp. nov.
conforms with the material originally reported as
Helcionella by Abele & McGowran (1959, p. 310) from
the lower member of the Heatherdale Shale. This form
was later collected by Dr B. Daily in 1962 (SAM P49830-
49836) from the southeast side of the water pipeline in
Mount Terrible Gully (Fig. 1, Loc. 2). However, the shell
bears minimal resemblance to the genus Helcionella Grabau
& Shimer 1909, which is distinguished by its bilaterally
symmetrical shell, subquadrate to elliptical apertural
outline and regularly spaced rounded to stepped concentric
rugae (Knight 1941, pl. 3, g. 1; Jacquet & Brock 2015).
The degree of shell coiling in Tribelopoma is unclear due to
various degrees of post-depositional compression, although
some specimens (Fig. 3C, E, J) display a possible bluntly
pointed apex with no obvious coiling. This suggests that
if coiling was present, it was restricted to earlier growth
stages.
Despite the asymmetrical shell, Tribelopoma amygdala
shares some morphological traits with helcionelloid genera
Totoralia Tortello & Sabattini 2011 and Minastirithella
Jacquet & Brock 2015, in possessing angular rugae and
radial lirae. However, the new genus can be distinguished
by its greater number of concentric rugae, which are
more closely spaced, producing sharp ridges and narrow
grooves (Tortello & Sabattini 2011, g. 4a-h; Conway
Morris & Peel 2013, g. 1.1-1.5; Jacquet & Brock 2015,
g. 9a-q). Scenella Billings 1872 (Knight 1941, pl. 2, g.
5) also possesses regular radial lirae, but is clearly distinct
from Tribelopoma in lacking high-amplitude concentric
ornament.
Based on external morphology alone, Tribelopoma
is supercially comparable to genera attributed to the
stenothecoids, with a close resemblance, in particular,
to the genus Stenothecoides. The new genus shares an
elongate asymmetrical shell, subcircular to elongate
ovoid apertural outline and also a similar growth habit.
The lateral margins in Stenothecoides similarly vary in
curvature. Concentric ornament is also irregularly spaced
and undulose with bifurcation evident on the supra-apical
surface. However, Stenothecoides can be distinguished by
its well developed, median ridge extending between the
apex and the supra-apical margin (Yochelson 1969, gs
2A-B, 3A, C). Tribelopoma lacks evidence of such a ridge,
but the supra-apical surface is more strongly convex than
the rest of the shell. The sub-apical surfaces of Tribelopoma
and Stenothecoides are quite distinct. Stenothecoides has
an apex very close to the sub-apical margin which is very
strongly curved, or even pointed. In some specimens, the
apex overhangs the margin; in Tribelopoma, the sub-apical
margin is broadly rounded, with the sub-apical surface
comprising nearly one third of the total shell surface. Apart
from the possession of an asymmetrical shell there is little
similarity between Tribelopoma and the other stenothecoid
genera (see Aksarina & Pelman 1978).
Tribelopoma amygdala sp. nov. (Figs 3-4)
Etymology. Greek; amygdale - ‘almond’. This refers to the
shape of the apertural outline and the burnt brown colour of
the iron oxide coating of many specimens.
Material. Holotype SAM P525224 (Fig. 2C-D). Paratypes
SAM P49831-P49836 and SAM P525225. 28 specimens
in total.
Type locality. Lower member of the Heatherdale Shale
(Cambrian Series 2, Stage 3), southeast side of water
pipeline, Mount Terrible Gully, Fleurieu Peninsula, South
Australia (35°20’54.82” S; 138°27’10.56” E).
Diagnosis. As for genus.
Description. Shell asymmetrical, low, almond-shaped
outline ranging from subcircular to elongate ovoid with
narrowest curvature at supra-apical margin. Shells range
from 4.5–13.2 mm maximum length (mean = 9.91 mm; n
= 17) and 2.9–9.9 mm maximum width (mean = 7.43 mm;
n = 15); mean width:length ratio of 0.72 (n = 14). Apex
located eccentrically to sub-centrally, offset from shell
midline, level with the widest part of the shell. Sub-apical
margin more broadly convex than supra-apical margin;
greatest curvature to one side or other of the shell midline.
First order external ornament consists of 10–22 (mean
15) concentric, irregularly spaced, commonly bifurcating,
grooved angular rugae. Individual rugae from 124–317 µm
in width. Second order external ornament consists of ne
radial lirae in some specimens.
Discussion. The asymmetry of the almond-shaped shell
outline in Tribelopoma amygdala (Figs 2A-C, 3A)
limits comparison to most other Cambrian macroscropic
helcionelloid taxa which tend to exhibit strong bilateral
symmetry. Despite this, numerous comparable traits are
present in bilaterally symmetrical patelliform to cap-
shaped fossils. Within east Gondwana, the macromolluscan
taxon Minastirithella silivreni Jacquet & Brock 2015 from
the Wirrapowie Limestone, Chace Range, in the Arrowie
Basin is perhaps most comparable and is of approximately
equivalent age and size (Jacquet & Brock 2015, g. 2).
This species exhibits distinct angular grooved rst order
concentric ornament that, particularly in compressed
specimens, can appear as sharp, narrow concentric ridges
(Jacquet & Brock 2015, g. 9E-F). However, the number
and spacing of concentric rugae differ considerably, with
a maximum of 12 rugae in M. silivreni and up to 22 (mean
15) in T. amygdala. Despite completely attened specimens
of M. silivreni indicating they have experienced similar
taphonomic processes, they lack the slight asymmetry in
outline characteristic of T. amygdala. Both taxa illustrate
similar compressional brittle fractures in the shell and
displacement of an otherwise eccentric apex (compare Fig.
3B,C, E, H, J with Jacquet & Brock 2015, g. 9I-J).

AP Memoir 49 (2016) 27
Other comparable taxa include Scenella antiqua
Kiaer 1916 from the lower Cambrian (Series 2, Stage 3)
Sparagmite Formation (Holmia fauna), Mjøsen in Norway
which is of comparable size (9.5–15.2 mm in length) and
has very ne, tightly but irregularly spaced concentric
rugae (Kiaer 1916, pl. 2, g. 1a-c). Faint radial lirae are
also present, but S. antiqua can be distinguished by its
subcircular outline (Kiaer 1916, pl. 2, g. 1a-c). In the
smaller paratypes of Marocella morenensis Yochelson &
Gil Cid 1984 from the lower Cambrian (Series 2, Stage 4)
Láncara Formation and within the Zafra-Alanis syncline
in Spain, the diagnostic sub-quadrate partitioning of the
shell is often not preserved (Yochelson & Gil Cid 1984,
gs 3A-E, 4A-E). Consequently, the shells appear as
simple elongate to ovoid forms, similar to T. amygdala,
although the regular spacing of the rounded concentric
rugae distinguishes M. morenensis from the new taxon
(compare Fig. 2A, C, F, I with Yochelson & Gil Cid 1984,
gs 3A-E, 4A-E). Specimens of Helcionella tchernyshevae
Vostokova 1962 from the lower Cambrian (Series 2, Stage
3) Usa Formation, Kiya River, Russia, possesses ne and
irregularly spaced concentric ornament similar to that in
T. amygdala (Pospelov et al. 1995, gs 8a-b). However,
H. tchernyshevae has a moderately high lateral prole
(H:L ratio of 0.52) compared to the low prole in the
new species, as well as an elliptical apertural outline with
subparallel lateral margins.
Although any afnity with the enigmatic Tirasiana?
disciformis? (Yang et al. 2014) from the Terreneuvian
Stage 2 Taozichong Formation, Guizhou in South China
seems unlikely, supercial similarities with Tribelopoma
are present. The discoid forms are typically subcircular
in outline, with concentric ridges both uniformly and
irregularly distributed. However, the width of the ridges is
greater in T? disciformis? (0.3–1 mm) than in T. amygdala
sp. nov. (124–317 µm) (compare Fig. 3A-C, H-J with Yang
et al. 2014, gs 3.1 – 6, 4.5-6). In addition, some specimens
of T? disciformis? exhibit a subcentral hemispherical bulge
(Yang et al. 2014, gs 3.3, 5.1), which differs from the
simple blunt apex in T. amygdala (Fig. 3C-E, J).
Taphonomy. Specimens of Tribelopoma amygdala are
preserved as internal and external moulds with varying
degrees of post-burial compaction. Most have an iron
oxide coating distinguishing them from the surrounding
buff-coloured matrix (Fig. 4A, C). Elemental mapping of
SAM P19948 shows a clear boundary between the margin
of T. amygdala and the rock matrix in terms of the iron
concentration (Fig. 4B, Fe map), with deposits distributed
mainly along the face of concentric rugae or in the grooves
between rugae. The iron oxide coating of the fossils is not
Figure 4. Taphonomic signatures associated with
specimens of Tribelopoma amygdala gen. et sp. nov. and
energy dispersive X-ray spectroscopic elemental maps. A.
SAM P19948 from the Sellick Hill region (collected by Dr
B. Daily), apical view, showing brittle fractures (arrows)
and region of elemental mapping (rectangle), scale bar = 2
mm. B. SEM backscatter image of region on SAM P19948
analysed (red dotted line indicates division between matrix
[left] and shell [right]) and EDS maps with the relative
abundance of elements (brightness of colours indicated
greater abundance), scale bar = 500 µm. C. Cluster of
four shells of T. amygdala gen. et sp. nov. (SAM P49834-
P498346, and SAM P525225) from the SE side of pipeline,
Mount Terrible Gully (Loc. 2), scale bar = 4 mm.

AP Memoir 49 (2016)
28
dissimilar to that associated with the soft-bodied fauna from
the Emu Bay Shale of Kangaroo Island, South Australia
(Cambrian Series 2, Stage 4) (Paterson et al. 2010, 2011,
2012; García-Bellido et al. 2013) and the Maotianshan
Shales of Chengjiang, China (Gabbott et al. 2004; Zhu et
al. 2005).
Most specimens are preserved as complete shells,
reecting a low energy, basinal and dysaerobic to anaerobic
environment for the Heatherdale Shale (Jenkins & Hasenohr
1989; Gravestock et al. 2001; Jago et al. 2006). Previously,
specimens have been reported to appear in great abundance
on bedding planes, possibly the result of accumulation and
sorting (Abele & McGowran 1958). However, the majority
of the collections reported here are represented by isolated
shells. One exception is a small slab recovered from the
Mount Terrible Gully (Loc. 2) in the collections of the South
Australian Museum (SAM P49834-P49836 and P525225),
which has four individuals with each apparently randomly
overlapping a small portion of an adjacent shell (Fig. 4C).
These specimens lack brittle compression fractures, have
the same positive topographic relief (internal moulds),
but are oriented in different directions. There is minimal
evidence of transport or damage of these shells which
might indicate they were deposited close to life position.
The main taphonomic features observed in the current
material are the result of post-depositional compaction.
Brittle radiating fractures occur frequently in the steinkerns
of T. amygdala (Figs. 3B-C, E, H, J, 4A), most with minimal
to no displacement or overlap of broken fragments. Where
preserved, the apical region exhibits two main taphonomic
features: (1) a circular portion of the shell including the
apical region collapsed into the main cavity (Fig. 3E, G-H),
and (2) the apex is effaced and skewed to one side (Fig. 3C,
D, J). The pattern of fracturing in the rst case was most
likely caused by a near empty cavity retained beneath the
shell post-burial and subsequent collapse due to overburden
pressures of accumulating sediment (Zuschin et al. 2003).
In the second case, the direction of accumulated pressure
may have caused the apical region to be offset from the
median plane, during minor structural shearing. These
features provide evidence of a rigid biomineralised shell in
T. amygdala gen. et sp. nov. rather than a chitinous or non-
biomineralised structure supporting a molluscan, rather
than a cnidarian afnity for the taxon.
ACKNOWLEDGEMENTS
We would like to thank Jim and Linda Stacey for access to
the lower Heatherdale Shale on their property at Myponga
Beach; D. García-Bellido, J.G. Gehling, J.R. Paterson,
J. Holmes and S. Doyle, the late B. Daily and various
students (University of Adelaide) over the years for their
eld assistance and collections; D. Birch and N. Vella for
assistance with SEM-EDS analysis (Microscopy Unit,
Macquarie University); and Dean Oliver for drafting the
nal version of Fig. 1. Mary-Anne Binnie facilitated access
and subsequent loan of specimens housed in the collections
of the South Australian Museum. This work has been
supported by funding from Australian Research Council
Discovery Project #129104251 (to GAB). We are grateful
to J. Peel and A. Kouchinsky for constructive reviews that
improved the submitted manuscript.
REFERENCES
Abele, C. & McGowrAn, B., 1959. The geology of the Cambrian
south of Adelaide (Sellick Hill to Yankalilla). Transactions of
the Royal Society of South Australia 82, 301-320.
AksArinA, n.A. & PelMAn, Y.l., 1978. Cambrian brachiopods
and bivalve molluscs of Siberia. Trudy Instituta Geologii i
Geoziki 362, 180 p.
AlroY, J. 2010. The shifting balance of diversity among major
marine animal groups. Science 329, 1191-1194.
bell, c., AnGseesinG, J. & Townsend, M. 2001. A chondrophorine
(medusoid hydrozoan) from the Lower Cretaceous of Chile.
Palaeontology 44, 1011-1023.
benGTson, S. 1968. The problematic genus Mobergella from the
Lower Cambrian of the Baltic area. Lethaia 1, 325-351.
BillinGs, E., 1872. On some fossils from the Primordial rocks of
Newfoundland. Canadian Naturalist 6, 465-479.
burzYnski, G. & nArbonne, G.M., 2015. The discs of Avalon:
Relating discoid fossils to frondose organisms in the
Ediacaran of Newfoundland, Canada. Palaeogeography,
Palaeoclimatology, Palaeoecology 434, 34-45.
conwAY Morris, S. & chAPMAn, A.J., 1997. Mobergellans from
the Lower Cambrian of Mongolia, Sweden, and the United
States: molluscs or opercula of incertae sedis? Journal of
Paleontology 71, 968-984.
conwAY Morris, S. & Peel, J.S., 2013. A New Helcionelloid
Mollusk from the Middle Cambrian Burgess Shale, Canada.
Journal of Paleontology 87, 1067-1070.
cooPer, J.A., Jenkins, r.J.F., coMPsTon, w. & williAMs, i.s.,
1992. Ion-probe zircon dating of a mid-Early Cambrian tuff in
South Australia. Journal of the Geological Society 149, 185-
192.
cuvier, G., 1797. Tableau Élémentaire de l’Histoire Naturelle des
Animaux. Baudoin, Paris, 710 p.
dAilY, B., 1963. The fossilferous Cambrian succession on Fleurieu
Peninsula, South Australia. Records of the South Australian
Museum 14, 579 -601.
FosTer, c., cernovskis, A. & o’brien, G., 1985. Organic-walled
microfossils from the Early Cambrian of South Australia.
Alcheringa 9, 259-268.
FrYer, G. & sTAnleY Jr, G.d., 2004. A Silurian porpitoid
hydrozoan from Cumbria, England, and a note on porpitoid
relationships. Palaeontology 47, 1109-1119.
GAbboTT, s.e., XiAn-GuAnG, h., norrY, M.J. & siveTer,
d.J., 2004. Preservation of Early Cambrian animals of the
Chengjiang biota. Geology 32, 901-904.
GArcíA-bellido, d.c., PATerson, J.r. & edGecoMbe, G.d., 2013.
Cambrian palaeoscolecids (Cycloneuralia) from Gondwana
and reappraisal of species assigned to Palaeoscolex. Gondwana
Research 24, 780-795.
GehlinG, J.G., nArbonne, G.M. & Anderson, M.M., 2000. The
rst named Ediacaran body fossil, Aspidella terranovica.
Palaeontology 43, 427-456.
GeYer, G., 1994. Middle Cambrian molluscs from Idaho and early
conchiferan evolution. New York State Museum Bulletin 481,
69-86.
GrAbAu, A.w. & shiMer, h.w., 1909. North American Index
Fossils: Invertebrates. A. G. Seiler & Company, New York,
909 p.
GrAvesTock, d.i., AleXAnder, e.M., deMidenko, Yu.e., esAkove,
n.v., holMer, l.e., JAGo, J.b., lin, T.r., MelnikovA, l.M.,
PArkhAev, P.Yu., rozAnov, A.Yu., ushATinskAYA, G.T.,
zAnG, w.l.., zheGAllo, e.A. & zhurAvlev, A.Yu., 2001.
The Cambrian biostratigraphy of the Stansbury Basin, South
Australia. Transactions of the Palaeontological Institute 282,
344 p.
GrAvesTock, d.i. & GATehouse, c.G., 1995. Stansbury Basin.
5-19 in Drexel, J.F. & Preiss, W.V. (eds). The geology of South
Australia. Mines and Energy South Australia Bulletin 54.
horný, R., 1957. Problematic molluscs (? Amphineura) from the

AP Memoir 49 (2016) 29
lower Cambrian of south and east Sibiria (USSR). Sborník
Ustředního Ustavu Geologického 23, 397-432.
JAcqueT, s.M. & brock, G.A., 2015. Lower Cambrian
helcionelloid macromolluscs from South Australia. Gondwana
Research, doi:10.1016/j.gr.2015.06.012.
JAGo, J.b., dAilY, b., von der borch, c., cernovskis, A. &
sAunders, n., 1984. First reported trilobites from the Lower
Cambrian Normanville Group, Fleurieu Peninsula, South
Australia. Transactions of the Royal Society of South Australia
108, 207-211.
JAGo, J.b, dYson, i. & GATehouse, c., 1994. The nature of the
sequence boundary between the Normanville and Kanmantoo
Groups on Fleurieu Peninsula, South Australia. Australian
Journal of Earth Sciences 41, 445-453.
JAGo, J.b., GATehouse, c.G., AleXAnder, e.M. & cooPer, b.J.,
2006. Cambrian of Fleurieu Peninsula. 13-20 in Jago, J.B.
& Zang, W.L. (eds). South Australia 2006. XI International
Conference of the Cambrian Stage Subdivision Working
Group. Geological Society of Australia, South Australian
Division, Adelaide.
Jell, P., JAGo, J. & GehlinG, J., 1992. A new conocoryphid
trilobite from the Lower Cambrian of the Flinders Ranges,
South Australia. Alcheringa 16, 189-200.
Jenkins, r., cooPer, J. & coMPsTon, w., 2002. Age and
biostratigraphy of Early Cambrian tuffs from SE Australia and
southern China. Journal of the Geological Society 159, 645-
658.
Jenkins, r. & hAsenohr, P., 1989. Trilobites and their trails in a
black shale: Early Cambrian of the Fleurieu Peninsula, South
Australia. Transactions of the Royal Society of South Australia
113 , 195-203.
kiAer, J.A., 1916. The Lower Cambrian Holmia fauna at Tømten
in Norway. Videnskapsselskapets Skrifter. I. Matematisk -
naturvitenskapelig Klasse 10, 140.
kniGhT, J.B., 1941. Paleozoic Gastropod Genotypes. Geological
Society of America Special Paper 32, 510 p.
konevA, S.P., 1976. New members of the class Stenothecoida from
the Lower Cambrian of Central Kazakhstan. Paleontological
Journal 1976, 230-233.
lArsson, c.M., Peel, J.s. & höGsTröM, A.e.s., 2009. Trachyplax
arctica, a new multiplated problematic fossil from the Lower
Cambrian of North Greenland. Acta Palaeontologica Polonica
54, 513-523.
linnAeus, C., 1758. Systema Naturae per regna tria naturae:
secundum classes, ordines, genera, species, cum characteribus,
differentiis, synonymis, locis. Editio decima, reformata.
Laurentius Salvius, Stockholm, 824 p.
MAcGAbhAnn, B.A., 2007. Discoidal fossils of the Ediacaran
biota: a review of current understanding. Geological Society,
London, Special Publications 286, 297-313.
norris, R.D., 1989. Cnidarian taphonomy and afnities of the
Ediacara biota. Lethaia 22, 381-393.
PAliJ, V.M., 1976. Ostatki besskeletnoi fauny i sleedy
zhiznedeyatelnosti iz otlozheniy verkhnego dokebriya I
nizhnego kembriya Podolii. 63-77. In Ryabenko, Y.A. (ed.)
Paleontologiya i Stratigraya Verkhnego Dokembriya i
Nizhnego Paleozoya Yugo–Zapada Vostochno–Evropeiskoi
Platformy. Naukova Dumka, Kiev.
PATerson, J.r., edGecoMbe, G.d., GArcíA-bellido, d.c., JAGo,
J.b. & GehlinG, J.G., 2010. Nektaspid arthropods from
the lower Cambrian Emu Bay Shale Lagerstätte, South
Australia, with a reassessment of lamellipedian relationships.
Palaeontology 53, 377-402.
PATerson, J.R., GArcíA-bellido, D.C. & edGecoMbe, G.D., 2012.
New artiopodan arthropods from the early Cambrian Emu Bay
Shale Konservat-Lagerstätte of South Australia. Journal of
Paleontology 86, 340-357.
PATerson, J.r., GArcíA-bellido, d.c., lee, M.s., brock, G.A.,
JAGo, J.b. & edGecoMbe, G.d., 2011. Acute vision in the giant
Cambrian predator Anomalocaris and the origin of compound
eyes. Nature 480, 237-240.
Peel, J.S., 1988. Molluscs of the Holm Dal Formation (late
Middle Cambrian), central North Greenland. Meddelelser om
Grønland Geoscience 20, 145-168.
Peel, J.S., 1991. Functional morphology of the Class
Helcionelloida nov., and the early evolution of the Mollusca.
157-177 in Simonetta, A.M. & Conway Morris, S. (eds), The
early evolution of Metazoa and the signicance of problematic
taxa.Cambridge University Press, Cambridge.
Peel, J.S., 2003. A problematic cap-shaped metazoan from the
Lower Cambrian of Estonia. GFF 125, 157-161.
Peel, J.S., 2015. Failed predation, commensalism and parasitism
on lower Cambrian linguliformean brachiopods. Alcheringa:
An Australasian Journal of Palaeontology 39, 149-163.
PosPelov, A., PelMAn, Y.l., zhurAvlevA, i., luchininA, v.,
kuzneTsovA, v., esAkovA, n., erMAk, v. & AksArinA, n.,
1995. Biostratigraphy of the Kiya River section. Annales de
Paléontologie 81, 169-246.
Poulsen, C., 1932. The Lower Cambrian faunas of East Greenland.
Meddelelser om Grønland 87, 66.
rAduGin, K.V., 1937. On the relations of the Cambrian and
Precambrian in the Gornaya Shoria. Problems in Soviet
Geology 7, 295-317.
rAseTTi, F., 1954. Internal shell structures in the Middle Cambrian
gastropod Scenella and the problematic genus Stenothecoides.
Journal of Paleontology 28, 59-66.
resser, C.E., 1938. Fourth contribution to nomenclature of
Cambrian fossils. Smithsonian Miscellaneous Collections 97,
1-43.
robison, R.A., 1964. Late Middle Cambrian faunas from Western
Utah. Journal of Paleontology 38, 510-566.
rozov, S.N., 1984. Morphology, terminology and systematic
afnity of stenothecoids. Trudy Institut Geologii Geoziki 597,
117-133.
runneGAr, b. & PoJeTA Jr, J., 1974. Molluscan phylogeny: The
paleontological viewpoint. Science 186, 311-317.
sePkoski, J.J., Jr., 1981. A factor analytic description of the
Phanerozoic marine fossil record. Paleobiology 7, 36-53.
skovsTed, C.B., 2003. Mobergellans (problematica) from
the Cambrian of Greenland, Siberia and Kazakhstan.
Paläontologische Zeitschrift 77, 429-443.
sPriGG, R.C., 1947. Early Cambrian (?) jellyshes from the
Flinders Ranges, South Australia. Transactions of the Royal
Society of South Australia 71, 212-224.
sYTchev, L.A., 1960. Phylum Mollusca: Mollusks. Class
Lamellibranchiata: Pelecypods. 253-256 in Halna, L.L. (ed.),
Paleozoic Biostratigraphy of Sayano-Altai Mountain Region,
1. Trudy SNIIGGIMS 19.
TArhAn, l.G., droser, M.l., GehlinG, J.G. & dzAuGis, M.P.,
2015. Taphonomy and morphology of the Ediacara form genus
Aspidella. Precambrian Research 257, 124-136.
TorTello, M.F. & sAbATTini, n.M., 2011. Totoralia, a new
conical-shaped mollusk from the middle Cambrian of western
Argentina. Geologica Acta 9, 175-185.
vendrAsco, M.J., kouchinskY, A.v., PorTer, s.M. & FernAndez,
c.z., 2011. Phylogeny and escalation in Mellopegma and other
Cambrian molluscs. Palaeontologia Electronica 14, 1-44.
vosTokovA, V.A., 1962. Kembriyskie gastropody Sibirskoy
platformy i Taimyra. Statei po paleontologii i biostratigrai
28, 51-74.

AP Memoir 49 (2016)
30
YAnG, X., zhAo, Y., wu, w., sun, z., zhenG, h. & zhu, Y.,
2014. Afnities and taphonomy of a Cambrian discoid from
Guizhou, South China. Journal of Paleontology 88, 339-347.
Yochelson, E.L., 1969. Stenothecoida, a proposed new class of
Cambrian Mollusca. Lethaia 2, 49-62.
Yochelson, E.L. & Gil cid, D., 1984. Reevaluation of the
systematic position of Scenella. Lethaia 17, 331-340.
YounG, G.A. & hAGAdorn, J.w., 2010. The fossil record of
cnidarian medusae. Palaeoworld 19, 212-221.
zhu, M., bAbcock, l.e. & sTeiner, M., 2005. Fossilization
modes in the Chengjiang Lagerstätte (Cambrian of China):
testing the roles of organic preservation and diagenetic
alteration in exceptional preservation. Palaeogeography,
Palaeoclimatology, Palaeoecology 220, 31-46.
zuschin, M., sTAchowiTsch, M. & sTAnTon Jr, r.J., 2003. Patterns
and processes of shell fragmentation in modern and ancient
marine environments. Earth-Science Reviews 63, 33-82.