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A new shell-bearing organism from the Cambrian Spence Shale of Utah
Julien Kimmig
a,
*, Paul A. Selden
a,b,c
a
Biodiversity Institute, University of Kansas, Lawrence, Kansas 66045, USA
b
Department of Geology and Paleontological Institute, University of Kansas, Lawrence, Kansas 66045, USA
c
Natural History Museum, London, SW7 5BD, UK
Received 26 March 2020; received in revised form 5 May 2020; accepted 13 May 2020
Available online xxx
Abstract
A new shell-bearing organism with preserved soft tissue, Armilimax pauljamisoni n. gen. n. sp., is reported from the middle Cambrian (Miaolingian:
Wuliuan) Miners Hollow locality of the Spence Shale of northern Utah. The described organism is known from a single articulated specimen and pre-
serves a prominent shell, a slug-like body, as well as a U-shaped digestive tract. Its overall appearance is similar to halkieriids, but it does not preserve
sclerites. The possible affinities of the new taxon and potential reasons for the presence of a U-shaped gut are discussed. Armilimax pauljamisoni is the
first shell-bearing animal of its kind from the Great Basin and extends the diversity of body plans in the Spence Shale Fossil-Lagerst€
atte.
Ó2020 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved.
Keywords: Burgess Shale-type preservation; Lophotrochozoa; Halkieria; Sipuncula; Great Basin; Lagerst€
atte
1. Introduction
The Great Basin of the western USA preserves some of the
most important Cambrian-aged Burgess Shale-type deposits in
the world, preserving assemblages from Series 2 to the upper
Miaolingian. Five of the most diverse and productive Cam-
brian Fossil-Lagerst€
atten occur in Utah: the Spence Shale in
northeastern Utah, and the two Wheeler Lagerst€
atten (House
Range and Drum Mountains), the Marjum Lagerst€
atte and the
Weeks Lagerst€
atte of western Utah (Robison and Babcock,
2011;Robison et al., 2015;Foster and Gaines, 2016;Lerosey-
Aubril et al., 2018,in press;Kimmig et al., 2019;Kimmig, in
press). The Spence Shale is the oldest (Miaolingian: Wuliuan)
of the Cambrian Lagerst€
atten in Utah and preserves a diverse
soft-bodied and biomineralized fauna, including several prob-
lematic species: e.g., Banffia episoma Conway Morris and Sel-
den in Conway Morris et al., 2015a;Eldonia ludwigi Walcott,
1911;Siphusauctum lloydguntheri Kimmig, Strotz and Lieber-
man, 2017 (Conway Morris et al., 2015a,2015b;Robison
et al., 2015;Kimmig et al., 2017,2019).
Vermiform taxa are present in the Spence Shale, including
Utahscolex ratcliffei Robison, 1969;Ottoia prolifica Walcott,
1911;Selkirkia spencei Resser, 1939;andS.cf.columbia
Walcott, 1911 (Robison et al., 2015;Kimmig et al., 2019;
Whitaker et al., in press), all of which represent priapulids, but
are not common. More prevailing are unidentified vermiform
remains which often cannot be attributed to any phylum (e.g.,
Broce and Schiffbauer, 2017). Shell-bearing organisms present in
the Spence Shale include several brachiopod genera, hyoliths, as
well as the mollusks Latouchella arguta Resser, 1939 and Skee-
mella radians Babcock and Robison, 1988 (Kimmig et al., 2019).
Halkieriids, Cambrian vermiform fossils covered by sclerites and
bearing at least one shell (Conway Morris and Peel, 1995;
Conway Morris and Caron, 2007;Zhao et al., 2017), have not
been described from the Spence Shale, nor any of the other Utah
Lagerst€
atten, but are known from the slightly younger Burgess
Shale (Conway Morris and Caron, 2007).
Here, we document and describe Armilimax pauljamisoni n.
gen. n. sp., a new shell-bearing, slug-like organism of
unknown affinity from the Spence Shale and discuss possible
taxonomic affinities of the animal.
2. Geological setting
The middle Cambrian (Miaolingian: Wuliuan) Spence
Shale Member of the Langston Formation of northeastern
* Corresponding author.
E-mail address: jkimmig@ku.edu (J. Kimmig).
https://doi.org/10.1016/j.palwor.2020.05.003
1871-174X/Ó2020 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved.
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Utah and southeastern Idaho, ranges in age from the Albertella
to Glossopleura Biozones (Liddell et al., 1997;Robison and
Babcock, 2011;Kimmig et al., 2019). The Spence Shale was
deposited on the passive western margin of Laurentia. Trace
fossils and geochemical analyses indicate that dynamic redox
conditions occurred during deposition (Garson et al., 2012;
Kloss et al., 2015;Kimmig and Strotz, 2017;Hammersburg
et al., 2018).
The Spence Shale is a dark grey to brown calcareous shale
with interlayered lime mudstone units, about 50 to 65 m thick
in the study area of the Wellsville Mountains, north of Brig-
ham City, Utah (Liddell et al., 1997;Kimmig et al., 2019). It
overlies the Naomi Peak Limestone Member and is itself over-
lain by the High Creek Limestone Member (Maxey, 1958;Lid-
dell et al., 1997;Kimmig et al., 2019). The Langston
Formation lies above the Precambrian/Cambrian Geertsen
Canyon Quartzite of the Brigham Group, a pale grey to white
quartzite with scattered pebbles that, at the top, contains per-
sistent Skolithos traces and a few trilobite tracks (Crittenden
et al., 1971). No body fossils have been recovered from the
Geertsen Canyon Quartzite.
The specimen described here was found at the Miners Hol-
low locality, which is situated on the west flank of the Wells-
ville Mountains north of Brigham City, UT (Section 14, T10N,
R02W; 41.6023˚N, 112.0334˚W; Fig. 1A, B). The specimen
was recovered from the third parasequence (or carbonate
cycle), approximately 3336 m above the base of the Spence
Shale (Fig. 1C) as measured by Julien Kimmig and described
by Liddell et al. (1997).
3. Preservation
An energy dispersive X-ray spectroscopy (EDS) analysis of
part and counterpart (Figs. S1S3) identified carbon through-
out, but no consistent film, likely due to the diagenetic alter-
ation of the rock, as well as surface weathering. Spectral maps
indicated the following variations in percentage by weight for
different detectable elements throughout the specimen (9 data
points): O, 40.543.9%; Si, 19.127.4%; Al, 10.313.2%; C,
0.08.6%; Fe, 0.86.9%; K, 3.65.8%; N, 2.413.9%; Ca,
1.43.6%; Mg, 1.01.9%; S, 0.00.9%; Ti, 0.01.8%; P,
0.00.6%. The fossil has an enhanced signature of C, Ca, and
Mg in the shell, indicating an original calcium carbonate com-
position. Carbon is found throughout the specimen, showing
the preservation of at least part of the specimen as carbon film.
There is indication of phosphatization of the shell (up to
»0.2 Wt%, Figs. S1, S3), and some minor phosphatization in
the specimen (up to »0.6 Wt%, Fig. S3), but it was not a major
part of the fossilization process. Overall, the preservation of
the specimen appears to be comparable to other fossils from
the Spence Shale. The preservation is similar to vermiform
fossils described in recent studies of fossils from the same
locality (Broce and Schiffbauer, 2017;Whitaker et al., in
press). In addition, diagenetic processes altered the original
calcium carbonate of the shell and parts of the other body to
dolomite, as has been reported from palaeoscolecids from the
Spence Shale (Whitaker et al., in press).
The preservation of Armilimax pauljamisoni suggests that
the animal was likely decomposing on the ocean floor for a
short period of time, likely less than a week, based on decay
experiments performed with modern priapulids (Sansom,
2016), before being buried. This is suggested by the partial
decomposition of the gut. Laboratory-based decay experimen-
tation of the modern priapulid Priapulus caudatus Lamarck,
1816 showed that labile, non-cuticular features, such as the
gut, begin decaying immediately following death, and are lost
in under a week of decay (Sansom, 2016). The part of the spec-
imen opposite to the shell was covered by sediment when it
was split, and is only preserved on the part, after preparation.
4. Material and methods
The specimen was collected by Paul G. Jamison of Logan,
Utah. It was photographed under ethanol with cross-polarized
light using a Canon 5D MkII camera equipped with 100 mm
macro lens using DSLR Assistant (www.kaasoft.com). The
focus stacks were merged and the contrast, colour, and bright-
ness adjusted using Affinity Photo (www.serif.com). Close-
ups of the shell were taken under ethanol with a Leica DMS
300 Digital Microscope, unless otherwise specified; the con-
trast, colour, and brightness of images were adjusted using
Adobe Photoshop (www.adobe.com).
Elemental mapping utilizing EDS was conducted at the
University of Kansas Microscopy and Analytical Imaging Lab-
oratory using an Oxford Instruments 80 mm
2
x-Max silicon
drift detector (SDD), mounted on a FEI Versa 3D Dual Beam.
Analyses used a horizontal field width of 2.39 mm, a low
accelerating voltage of 10 kV, a spot size of 4.5 mm, and a
1000 mm opening (no aperture). The EDS maps were collected
at a pixel resolution of 512 £512 with a total of 18 passes.
The specimen described in this paper is housed in the Divi-
sion of Invertebrate Paleontology, Biodiversity Institute, Uni-
versity of Kansas (KUMIP 490943).
5. Systematic palaeontology
Phylum, Class, Order, Family Uncertain
Genus Armilimax n. gen.
Etymology: From the Latin ‘armis’ and ‘limax’, meaning
‘armour’ and ‘slug’ respectively, in reference to the slug-like
shape and the presence of a shell.
Type and only known species: Armilimax pauljamisoni n. sp.
Diagnosis: Same as species.
Remarks: Armilimax n. gen. is not assigned to a phylum at
this point. A detailed discussion on its affinities is provided
below.
Armilimax pauljamisoni n. sp.
(Figs. 25)
Etymology: After the collector of the specimen, Paul Jamison.
Holotype: KUMIP 490943, part and counterpart.
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Type locality: Miners Hollow, Wellsville Mountains, Box
Elder County, Utah; Section 14, T10N, R02W; 41.6023˚N,
112.0334˚W.
Type horizon: Langston Formation, Spence Shale Member,
cycle 3; Cambrian (Miaolingian: Wuliuan), Glossopleura trilo-
bite zone.
Diagnosis: Elongated, unsegmented, slug-like organism bear-
ing conical subquadrate carbonate shell at one end with umbo
and concentric growth lines, U-shaped or coiled digestive tract
bearing fine annuli.
Description: Elongated, unsegmented slug-like body in dorsal
view. 30.1 mm long, 4.95.6 mm wide (Fig. 2). Symmetrical,
conical, subquadrate shell, with rounded edges 2.4 mm wide,
2.9 mm long, preserved at one end, with umbo (Figs. 2,4).
The shell shows concentric growth rings (Fig. 4). It is partially
surrounded by soft tissue and there is soft tissue preserved in
the broken parts of the shell (Fig. 4A, B). A suboval, poorly
preserved structure is preserved at the other end (Fig. 3). This
structure is 2.4 mm wide, 2.9 mm long, with no relief, 4.3 mm
from the non-shell bearing end of the specimen. Isolated
Fig. 1. (A) Map of the western USA showing the location of the Spence Shale. (B) Geological map (based on the USGS state maps for Google Earth Pro) of north-
ern Utah and southern Idaho showing the location of Miners Hollow (star), and other major Spence Shale localities in northern Utah and southern Idaho (modified
from Kimmig et al., 2019); AC, Antimony Canyon; BF, Blacksmith Fork; EC, Emigration Canyon; HC, Hansen Canyon; HCR, High Creek; MH, Miners Hollow;
ON, Oneida Narrows; PP, Promontory Point; SG, Spence Gulch. (C) Stratigraphic column and approximate horizon from which the specimen was collected, cour-
tesy of Anna Whitaker; MC, Miners Hollow cycle.
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Fig. 2. Holotype of Armilimax pauljamisoni n. gen. n. sp. (KUMIP 490943) from the Spence Shale, middle Cambrian, Miners Hollow, Utah, in dorsal view. (A)
Part after preparation. (B) Interpretative drawing of the part showing the gut, pyrite replacement, and the shell. (C) Counterpart. (D) Interpretative drawing of the
counterpart showing the gut, pyrite replacement, and the shell. (E) Possible reconstruction of the animal (created by Laura Mohr). Scale bars are 5 mm.
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Fig. 3. Close-ups of the non-shell bearing end of the part of the holotype of Armilimax pauljamisoni n. gen. n. sp. (KUMIP 490943) from the Spence Shale, middle
Cambrian, Miners Hollow, Utah, in dorsal view, showing differences in appearance based on light and microscope. (A) Close-up taken with Leica M205C stereo-
microscope, under diluted alcohol, with low angle light. (B) Interpretative drawing of (A), showing dark lines forming an unidentified suboval shape. (C) Close-up
taken with Olympus SZX16 microscope under diluted alcohol, with low angle light. (D) Interpretative drawing of (C), showing gut and an unidentified suboval
shape. (E) Close-up taken with Leica DMS 300 microscope under diluted alcohol, with low angle light. (F) Interpretative drawing of (E), showing gut andan
unidentified suboval shape. Scale bars are 2 mm.
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semicircular pyrite structures are preserved but vary in size
and do not form a consistent pattern (Figs. 2,5B), suggesting
they are of diagenetic origin.
The darker structure at the non-shell bearing end of the speci-
men is interpreted as the digestive tract (Figs. 2,5A). The pre-
served structure is U-shaped and bears fine annuli (Fig. 5A).
Remarks: The interpretation of posterior versus anterior is dif-
ficult due in part to the location of the shell and the shape of
the digestive tract. Both the halkieriid Orthrozanclus Conway
Morris and Caron, 2007 and the suggested halkieriid relative
Calvapilosa Vinther, Parry, Briggs and Van Roy, 2017 pre-
serve one shell and, in both taxa, it is located at the anterior
end (Conway Morris and Caron, 2007;Vinther et al., 2017;
Zhao et al., 2017). Additionally, the umbonal region is at the
anterior tip in Orthrozanclus,asitisinA.pauljamisoni in this
interpretation. In Calvapilosa the apex (or mucro) is also more
to the anterior (Vinther et al., 2017). But, if the gut is actually
U-shaped, it would indicate that the shell is at the posterior
end of the animal, similar to a snail. We further discuss the gut
shape below and presently leave the interpretation of posterior
and anterior open.
On the opposite end of the shell (Fig. 3), the preservation is
poor but a semicircular structure can be seen based on lighting.
It does not show any relief and might represent part of the gut
or possibly the feeding apparatus. If this is, indeed, the feeding
apparatus of the animal, it would suggest that this is the ante-
rior end. There are two additional structures which show some
three-dimensionality on the right of the semicircular structure
(Fig. 3A, B), which are visible in low-angle light. It is unclear
if these structures represent parts of the animal or not. There is
no evidence that A.pauljamisoni did have three types of scler-
ites, as present in halkieriids.
Stratigraphic and geographic range: Type locality and hori-
zon only.
6. Discussion
6.1. Gut
There are several possibilities why the gut of Armilimax
pauljamisoni is or appears U-shaped: (1) The gut is actually
U-shaped as it is in other mollusks, including gastropods and
Fig. 4. Holotype of Armilimax pauljamisoni n. gen. n. sp. (KUMIP 490943) from the Spence Shale, middle Cambrian, Miners Hollow, Utah, in dorsal view. (A)
Close-up of the shell in the part. (B) Close-up of the shell in the counterpart. (C) Elemental maps showing the major elements composing the of the shell on the
counterpart. ST indicates soft tissue. Scale bars are 0.5 mm in (A, B).
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cephalopods (Budd and Jackson, 2016). This would be reason-
able to assume, as the specimen likely did not decompose for
an extended period of time. An argument against the gut being
U-shaped is that these kinds of digestive tracts are often associ-
ated with sessile animals, e.g., brachiopods and stalked filter
feeders (O’Brien and Caron, 2012), in the Cambrian. (2) The
preserved gut represents only part of the digestive tract and the
coiled section is preserved. This is observed in some mollusks,
including chitons and monoplacophorans (Greenfield, 1972;
Haszprunar and Schaefer, 1996). The argument in favor of this
is that the shell is at the anterior end, as it is suggested for hal-
kieriids with one shell (Conway Morris and Caron, 2007;Zhao
et al., 2017). (3) The gut was originally straight and reached
from the anterior to the posterior end, but the decomposition
of the specimen is advanced, and the gut became loose and
curled up post mortem. The detachment and movement of the
digestive tract in worms has been shown in taphonomic experi-
ments (e.g., Monge-N
ajera and Hou, 2002) and might have hap-
pened in our specimen. (4) The U-shaped structure is not the gut
but actually, represents the visceral mass surrounding the intes-
tines. This would indicate that the gut was straight and likely con-
nected to the shell, which would be at the anterior end.
This interpretation would suggest that the internal anatomy of
Armilimax would be similar to the halkieriid Orthrozanclus elon-
gate Zhao and Smith in Zhao et al., 2017. An argument against
this is, the gut structures in fossils from the Spence Shale are usu-
ally preserved as the darkest structures in the fossil based on the
taphonomic pathways in the deposit (Briggs et al., 2008;Conway
Morris et al., 2015a,2015b;Kimmig et al., 2017;Whitaker et al.,
in press) and it is unlikely that this would not be the case in Armi-
limax.
To clarify the shape of the gut and if it is really U-shaped, par-
tially coiled up, or straight, more specimens will be needed in the
future. As we are dealing with a single specimen, it is difficult to
assess the interpretation of the U-shaped or coiled gut, but the
presence of it suggests that curved or coiled guts might have been
common in non-sessile animals in the Cambrian.
6.2. Biological affinities
There are few Cambrian organisms known that bear a shell
and have a slug-like body. The closest animals that have both
these features are halkieriids, Cambrian vermiform fossils cov-
ered by sclerites and bearing at least one shell (Conway Morris
and Peel, 1995;Conway Morris and Caron, 2007;Zhao et al.,
2017). Complete specimens are known from several Cambrian
deposits, including the Sirius Passet Lagerst€
atte (Conway
Morris and Peel, 1990,1995), the Burgess Shale (Conway
Morris and Caron, 2007), and the Chengjiang Lagerst€
atten
(Zhao et al., 2017). While Armilimax pauljamisoni shares the
presence of a shell and the elongated body with these animals,
there is no indication that A.pauljamisoni had a scleritome
consisting of three distinct sclerite types, which all other
described halkieriids share. In addition, A.pauljamisoni has a
coiled/U-shaped gut, while Halkieria Poulsen, 1967 and
Orthrozanclus Conway Morris and Caron, 2007 both have
straight digestive tracts (Conway Morris and Peel, 1995;
Conway Morris and Caron, 2007;Zhao et al., 2017).
Other animals that Armilimax pauljamisoni shares similari-
ties with are peanut worms (Sipuncula). These enigmatic ani-
mals have been placed in the annelids but differ from most
annelids in that they have an unsegmented body and do not
bear chaetae (e.g., Kristof and Maiorova, 2016). Modern sipu-
culans have a U-shaped gut, and some modern sipunculans
secret a horny or calcified cuticular plate: the anal shield
(Gibbs and Cutler, 1987;Kristof and Maiorova, 2016). Unfor-
tunately, these animals have a sparse fossil record (Huang
et al., 2004;Muir and Botting, 2007), but at least two Cam-
brian species have been reported from the Chengjiang biota
(Huang et al., 2004). Similar to A.pauljamisoni,Archaeogol-
fingia caudata Huang, Chen, Vannier and Salinas, 2004 and
Cambrosipunculus tentaculatus Huang, Chen, Vannier and
Salinas, 2004, have a U-shaped gut and an elongated unseg-
mented body. However, the gut in both species extends from
the mouth at the anterior end of the body to the posterior end,
curves and terminates about halfway through the body. In con-
trast, the gut in A.pauljamisoni curves about halfway through
the body. Additionally, neither A.caudata nor C.tentaculatus
bear shells.
Fig. 5. Holotype of Armilimax pauljamisoni n. gen. n. sp. (KUMIP 490943)
from the Spence Shale, middle Cambrian, Miners Hollow, Utah, in dorsal
view. (A) Close-up of the digestive tract, taken with cross-polarized lights,
showing fine annulations (indicated by arrows) and some pyrite in the counter-
part. (B) Close-up of some pyrite replacement. Scale bars are 2 mm in (A) and
0.5 mm in (B).
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Other Cambrian animals preserving U-shaped or twisted
guts and a calcified shell are hyoliths (Devaere et al., 2014;
Sun et al., 2016;Moysiuk et al., 2017), but they do not have an
elongated soft-bodied trunk and the shells are conical. While
they have been considered to be closely related to sipunculans
at one point (e.g., Sun et al., 2016), they are now considered
stem-group lophophorates (Moysiuk et al., 2017).
Recently, specimens of the gastropod Pelagiella exigua
Resser and Howell, 1938 with chaetae have been described
from the lower Cambrian (Stage 4) Kinzers Formation of
Pennsylvania (Thomas et al., in press). But they do not pre-
serve a slug-like body, the shell is coiled, and Armilimax paul-
jamisoni does not preserve chaetae. Due to this it is unlikely
that A.pauljamisoni represents a Cambrian gastropod. Armili-
max pauljamisoni shell also differs in shape from Latouchella
arguta Resser, 1939 and Scenella radians Babcock and Robi-
son, 1988, the only known mollusk shells from the Spence
Shale.
It is unlikely the animal is a brachiopod, as the shell does
not correspond to any brachiopod shells found in the Spence
Shale and the amount of soft tissue is uncharacteristically large
compared to the size of the shell. It is also unlikely that the
specimen represents a digested or regurgitated shell in a bur-
row, as the preserved gut suggest that what is preserved is an
animal. Additionally, the burrows found in the Spence Shale
usually preserve elongated pellets (Kimmig and Strotz, 2017).
Acknowledgements
We thank Paul Jamison for collecting and donating the
studied specimen. We thank Russell Bicknell, an anony-
mous reviewer and associate editor Xing-Liang Zhang for
their comments on the manuscript and the editor Si-Wei
Chen for the handling of the manuscript. We also thank
Timothy Topper for discussion of the specimen, Anna Whi-
taker for assistance with photography, Laura Mohr for the
reconstruction of the specimen and Ana Mendoza-Maya for
assistance with the SEM work. Sara Kimmig and Bruce
Lieberman are thanked for comments on a previous version
of the manuscript. This research was supported by a Pale-
ontological Society Arthur James Boucot Research Grant
to Julien Kimmig.
Supplementary materials
Supplementary material associated with this article can be
found in the online version at doi:10.1016/j.palwor.2020.05.003.
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