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Thecate stem medusozoans (Cnidaria) from the early Cambrian
Chengjiang biota
by HANZHI QU, KEXIN LI and QIANG OU*
Early Life Evolution Laboratory, State Key Laboratory of Biogeology & Environmental Geology, & School of Earth Sciences & Resources, China University of
Geosciences, Beijing 100086, China; ouqiang@cugb.edu.cn
*Corresponding author
Typescript received 13 May 2022; accepted in revised form 6 October 2022
Abstract: Cnidarians are phylogenetically located near the
base of the ‘tree of animals’, and their early evolution had a
profound impact on the rise of bilaterians. However, the early
diversity and phylogeny of this ‘lowly’ metazoan clade has
hitherto been enigmatic. Fortunately, cnidarian fossils from
the early Cambrian could provide key insights into their evolu-
tionary history. Here, based on a scrutiny of the purported
hyolith Burithes yunnanensis Hou et al. from the early Cam-
brian Chengjiang biota in South China, we reveal that this spe-
cies shows characters distinct from those typical of hyoliths,
not least a funnel-shaped gastrovascular system with a single
opening, a whorl of tentacles surrounding the mouth, and the
lack of an operculum. These characters suggest a great
deviation from the original definition of the genus Burithes,
and a closer affinity with cnidarians. We therefore reassign the
material to a new genus: Palaeoconotuba. Bayesian inference of
phylogeny based on new anatomical traits identifies a new
clade, including Palaeoconotuba and Cambrorhytium, as a stem
group of sessile medusozoan cnidarians that are united by the
synapomorphies of developing an organic conical theca and a
funnel-like gastrovascular system. This study unveils a stem
lineage of medusozoans that evolved a lifelong conical theca in
the early Cambrian.
Key words: Cambrian, Cnidaria, Medusozoa, Burithes,
Palaeoconotuba,Cambrorhytium.
CNIDARIANS, consisting of Anthozoa and Medusozoa,
form a typical diploblastic metazoan lineage that diverged
very early in the history of metazoan evolution metaphor-
ically known as the ‘tree of animals’ (Han et al.2020).
The body of cnidarians can be divided into ectoderm and
endoderm, with the mesoglea sandwiched between them
(Mendoza-Becerril et al.2016). Cnidarians originated
before the Cambrian explosion, probably in the Ediacaran
or the Cryogenian, and the crown group may have
appeared in the Neoproterozoic (Chen et al.2002; Van
Iten et al.2014). Therefore, their cryptic early evolution
is probably pivotal in elucidating the origin of the bilate-
rians. Cnidarians are among the earliest metazoans that
underwent biomineralization to produce an exoskeleton,
thus leaving a wealth of fossil records. However, there are
hardly any well-accepted cnidarian fossils in the Precam-
brian. Nevertheless, possible candidates have been
reported from all over the world, such as fossils from the
Lantian biota in South China, represented by Lantianella
and Piyuania (Yuan et al.2011; Wan et al.2016), the for-
mer of which was also interpreted as a kind of conulariid
(Van Iten et al.2013); Haootia quadriformis, an Ediacaran
fossil found in Newfoundland, which was positioned as a
stem cnidarian based on its muscle tissue though lacking
gastric cavities, canals and mesenteries (Liu et al.2014);
Vendoconularia triradiata, a conulariid-like fossil from
Russia showing unique three-fold symmetrical pattern
(Ivantsov & Fedonkin 2010); recently, the oldest docu-
mented conulariid, Paraconularia ediacra, is reported
from the Ediacaran Tamengo Formation in Brazil (Leme
et al.2022). Fossils from the Cambrian Lagerst€
atten show
convincing evidence of cnidarian affinity. The Kuan-
chaunpu biota (c. 535 Ma) from South China yielded
diverse cnidarian candidates, including pentamerous
medusoid embryos (Han et al.2013), sea anemone-like
fossils (Han et al.2010) and thecate medusozoans repre-
sented by Olivooides characterized by a conical peridermal
theca and resolved as a stem-group cubozoan (Han
et al.2016a). The Chengjiang biota (c. 520 Ma) yielded a
few fossil cnidarians which cast new light on the early
history of this phylum, including Xianguangia sinica
with suspension-feeding tentacles (Ou et al.2017); Yun-
nanoascus haikouensis with rhopalia (Han et al.2016b),
and Nailiana elegans, a stem anthozoan exhibiting evi-
dence of macrophagous predation (Ou et al.2022). How-
ever, thecate medusozoans, which are prevalent in the
Kuanchaunpu biota, have yet to be reported from the
Chengjiang biota.
©2023 The Palaeontological Association. doi: 10.1111/pala.12636 1
[Palaeontology, 2023, e12636]
The Chengjiang biota yields diverse hyoliths, which are
extinct benthic invertebrates with biomineralized conch
and operculum that can be divided by their morphology
into orders Hyolithida and Orthothecida (Mart
ı Mus &
Bergstr€
om 2005; Liu et al.2020a). The conch is bilaterally
symmetrical, characterized by a flattened ventral side with
an outward extension (ligula) on the aperture (Mart
ı Mus
& Bergstr€
om 2005). Hyolithida developed a pair of helens
that is characteristic of this order (Liu et al.2020a).
Helens are probably derived from the clavicles on the
operculum, and formed through an ontogenetic process
(Skovsted et al.2020). The exact phylogenetic position of
hyoliths is still under discussion; it has been assigned to:
stem-group lophotrochozoans (Liu et al.2020a),
lophophorates (Moysiuk et al.2018), a sister-group of
sipunculans (Sun et al.2016), stem-group brachiopods
(Sun et al.2018) and mollusca (Butterfield 2003). Inter-
estingly, Cambrovitus balangensis from the Cambrian Kaili
biota (c. 508 Ma) was formerly considered as a hyolith,
but was later thought to have a closer relationship with
cnidarians, and is more likely to represent a medusozoan
polyp (Zhu et al.2000).
Here, in our scrutiny of the purported hyolith, Burithes
yunnanensis Hou et al., 1999, in the Chengjiang biota, we
reveal that this species has some features that are dis-
tinctly different from those typical of Burithes and other
hyoliths, but are reminiscent of some polyps in cnidarians
and can also be found in Cambrorhytium. Therefore, we
propose that ‘Burithes yunnanensis’ is more closely affili-
ated with cnidarians. Our phylogenetic analysis using
Bayesian inference suggests that ‘B. yunnanensis’is
resolved as a stem-group medusozoan. This study sub-
stantiates the hypothesis that medusozoans evolved a ses-
sile stem clade with a lifelong conical theca in the early
Cambrian.
MATERIAL AND METHOD
All studied specimens (n = 32) of ‘Burithes yunnanensis’
were collected from the Yu’anshan Member of the Heil-
inpu Formation, (Cambrian Series 2 Stage 3) of Sanjiezi
Section in Kunyang city, Yunnan province. Specimens
are deposited in the Early Life Evolution Laboratory,
State Key Laboratory of Biogeology and Environmental
Geology, China University of Geosciences (ELEL), Nan-
jing Institute of Geology and Palaeontology, Chinese
Academy of Sciences (NIGP), and Yunnan Institute of
Geological Science (YDKS). Energy dispersive x-ray
spectrometry (EDS) analysis was performed at the Insti-
tute of Earth Sciences, and x-ray fluorescence spectro-
meter (XRF) analysis was conducted at the State Key
Laboratory of Geological Process and Mineral Resources,
China University of Geosciences. Photos were taken
using a Canon EOS 5D camera under sunlight. Measure-
ments are supported by ImageJ and TpsDig2. Morpho-
metric analyses were carried out using TpsDig2 (Guo
et al.2020), TpsRelw and PAST (Hammer et al.2001).
Phylogenetic analysis was conducted with Mrbayes v3.2
using Bayesian inference (Huelsenbeck & Ronquist 2001).
SYSTEMATIC PALAEONTOLOGY
Phylum CNIDARIA Verrill, 1865
Subphylum MEDUSOZOA Peterson, 1979
Class, Order, Family uncertain
Genus PALAEOCONOTUBA nov.
Figures 1,S1,S2
LSID. https://zoobank.org/NomenclaturalActs/8692C82A-
0E84-4E09-9A8E-C51958C04C05
Derivation of name. Palaeo derived from Greek palaio-,
ancient; conotuba means ‘conical theca’, in reference to
the conical theca they possessed. Gender feminine.
Diagnosis. Solitary, polyp-like animal enclosed by a theca
with a whorl of tentacles surrounding the aperture in
gross morphology. Theca conical, aperture margin
straight, lacking transverse or longitudinal growth lines or
other ornamentations. Operculum absent. No differentia-
tion of venter and dorsum. Conical theca tapers from
aperture to apex where its wall converges at an acute
angle. Apex rounded without holdfast. Tentacles finger-
like, without pinnules and cilia. Internal gastrovascular
system funnel-shaped, including a capacious gastric cavity
and a slim, band-like vascular canal. No opening at the
end of the vascular canal.
Type species. Burithes yunnanensis Hou et al., 1999
(Fig. S7D).
Remarks. Perhaps because of the poor preservation of the
holotype of Burithes yunnanensis where only the conical
theca was preserved with no records of internal anatomi-
cal features, this genus was originally synonymized with
the genus Burithes. However, if ‘Burithes yunnanensis’
turns out to be a hyolith, its external morphology and
internal structures should resemble those typical of hyo-
liths. Our study revealed that they differ distinctly (see
Discussion). Hence, we find it necessary to establish the
new genus Palaeoconotuba.
2PALAEONTOLOGY
Palaeoconotuba yunnanensis comb. nov.
Figures 1H–N,S1A–I,S2D–F
1999 Burithes yunnanensis Hou et al., p. 85, fig. 112.
?1999 Glossolites magnus Luo et al., p. 95, fig. 39, pl. 26,
figs 3–5.
2004 Burithes yunnanensis; Chen, p. 204, fig. 309.
2006 Burithes yunnanensis; Han et al., fig. 1.
2010 Burithes yunnanensis; Zhao et al., table S1.
2010 Burithes magnus Zhao et al., table S1.
2014 ‘Burithes’yunnanensis Steiner et al., p. 117.
?2016 Glossolites magnus Luo et al., table 2.
LSID. https://zoobank.org/NomenclaturalActs/A9462ED5-
63F5-4255-B4AC-2F5509F97AC5
Holotype. NIGP-115438, Burithes yunnanensis Hou
et al., 1999 (Fig. S7D).
Diagnosis (emended). A species of Palaeoconotuba possess-
ing a short theca and a wide aperture (Text S1), with
width/length ratio of c. 0.5–0.55 and an apical angle of c.
28°–45°(Fig. S4).
Description. The theca is conical, length varies from c.19
to 30 mm. The aperture is straight, and its width varies
from c. 10 to 15 mm. The conical theca wall converges at
an acute angle at the apex. The apex is rounded, no hold-
fast is observed. The conical theca is thin and flexible;
therefore wrinkles appear on the surface which is strongly
flattened (Figs 1H, I,2G, H; Fig. S1I). The soft parts are
mainly a crown of tentacles and a funnel-shaped gas-
trovascular system (Fig. 1C, H, K; Fig. S3). In the oral
part, finger-like tentacles project upwards from the mar-
gin of the aperture probably in a radial pattern, measur-
ing about c.2–3 mm in total length. Tentacles are poorly
preserved, only show an indistinct area (Fig. S1A, B). The
crown of tentacles surrounds the mouth opening that
leads to the gastrovascular system inside the polyp. The
gastric cavity is capacious and conical, which is filled with
sediments that formed a triangular hyporelief on the sur-
face in some specimens (Fig. 1H, J, L; Fig. S1E, I). Its
width is narrower than the aperture at the oral region
and sharply tapers downwards at one third of the length
of the theca (Fig. 1H, J, K) into a straight vascular canal
at the basal part that extends to its end with no opening
at its bottom (Fig. 1H, I, K, M, N; Figs S1A, C–E, G, S5).
Locality & horizon. Cambrian Series 2 Stage 3 (c.
520 Ma), biozone Eoredlichia–Wutingaspis, Yu’anshan
Member, Heilinpu (previously Chiungchussi) Formation.
The only known fossil site hitherto is in Yunnan
Province, South China.
Palaeoconotuba elongata sp. nov.
Figures 1A–G,S1J–M,S2A–C
2017 Burithes yunnanensis Hou et al., p. 106, fig. 16.3.
2021 Burithes yunnanensis; Yang et al., extended data fig. 4k
(suppl. material).
LSID. https://zoobank.org/NomenclaturalActs/
8D4AF9DA-0CFE-4BE4-9183-905A16679B72
Derivation of name. Latin elongata means elongate, in ref-
erence to the elongated theca of this species.
Holotype. YKLP-13856 (Fig. 1F).
Diagnosis. A species of Palaeoconotuba possessing an
elongated theca and a narrow aperture (Text S1) with
wide/length ratio of c. 0.37–0.45 and an apical angle
c.24°–30°(Fig. S4).
Description. The theca is conical, with length varying
from c. 24 to 40 mm. The aperture margin in general is
straight, despite few specimens slightly curved (Fig. 1B;
Fig. S3A–C), with apertural width varying from c.9to
14 mm. The conical theca converges at an acute angle at
the rounded apex without an attachment disc. The wall
of conical theca is flexible, with wrinkles seen on the sur-
face due to preservation (Fig. S1M). The soft body con-
sists of a gastric cavity with a whorl of tentacles in the
oral part and a slender vascular canal in the basal part
that collectively exhibits the shape of a funnel. The tenta-
cles, which are finger-like, project upwards from the mar-
gin of the aperture probably in a radial pattern, with at
least 3–5 tentacles discernable in exceptionally preserved
specimens (Fig. 1C, D, F, G). Each tentacle measures
about c.2–3 mm in total length, c. 1 mm in width at the
proximal end, and tapers to its distal end. Tentacles have
no secondary pinnules and cilia. An inferred mouth
opening surrounded by the tentacles leads to the capa-
cious gastric cavity. The gastric cavity is in a conical
shape (Fig. 1C; Figs S3,S6B, G), and in some specimens
it is filled with sediment (Fig. S1L). The vascular canal
connects the gastric cavity upwards and extends down-
wards at the bottom without a basal opening (Fig. 1C, E;
Fig. S6).
Locality & horizon. Cambrian Series 2 Stage 3 (c.
520 Ma), biozone Eoredlichia–Wutingaspis, Yu’anshan
Member, Heilinpu (previously Chiungchussi) Formation.
The only known fossil site hitherto is in Yunnan Pro-
vince, South China.
QU ET AL.: CAMBRIAN THECATE MEDUSOZOANS 3
4PALAEONTOLOGY
PRESERVATION AND TAPHONOMY
All of our specimens are compressed as flattened body
fossils, with the long axis being parallel to the bed surface.
The theca shows no signs of brittle compression, but only
flexible wrinkles on the surface showing signs of crumple
(Figs 1H, I,2G, H; Fig. S1L, M, I). The EDS and XRF
analyses (Figs S5,S6) suggest that the theca has under-
gone pyritization during its preservation, weathering to a
brown oxide in the later stages. Tentacles have also been
pyritized, and only a few specimens preserve the tentacles.
Some poorly preserved specimens have only a blurry
brown area in which individual tentacles cannot be iden-
tified (Fig. S1A); others with perfect preservation have
tentacles with excellent morphology and every one of
them can be distinguished easily (Fig. 1C, F). The gastric
cavity is often filled with sediment during the preserva-
tion process, forming an inner core (Fig. 1J, L; Fig. S1L).
The vascular canal can either be filled with dark sediment
or have undergone pyritization and oxidation, thus show-
ing a bright band on the EDS and XRF analyses (Figs S5,
S6).
DISCUSSION
‘Burithes yunnanensis’ is neither a species of Burithes nor
a hyolith
Palaeoconotuba spp. were originally assigned to Burithes
as species ‘Burithes yunnanensis’ (Hou et al.1999,2017;
Chen 2004), although some researchers noted differences
from hyoliths, such as the strongly flattened (Hou
et al.1999,2017), soft and flexible (Chen 2004) ‘conch’,
on which we observed wrinkles.
However, the original definition (Missar
zevskij 1969;
Qian & Xiao 1995; Fig. S7A–C) of the genus Burithes
includes: (1) length varying from a few millimetres to a
few centimetres; (2) conch slightly curved in the ventral–
dorsal plane, with sharp apical angle varying from 20°to
25°; (3) dorsal side flattened, with one or two pairs of
muscle scars near the lateral margins; (4) lateral margin
of the aperture has a pair of distinct sinuses; (5) shape of
ventral section semi-elliptical to arch-shaped; (6) transi-
tion from the dorsal side to the ventral side is sharp,
which makes the lateral edges of the shell often keel-like;
(7) ornamentation is in the form of growth lines and no
longitudinal ornaments were observed.
Besides the descriptions above, some other species in
Burithes also have the following features (Peel et al.2020):
(1) ventral side of the aperture has a ligula that in length
measures about one-sixth of the conch’s length, and in
width measures about half of its length; (2) ventral sur-
face has periodic, co-marginal growth lines with occa-
sional halts; (3) the ventral side has Y-shaped cracks that
extend from the ligula margin down the median line,
probably a taphonomic artefact. Also, Malinky
et al.(2004) noticed some similarities between specimens
from the Dyeran of northeastern Greenland and Burithes,
such as: the orthoconic conch; small apical angle; flat or
slightly raised venter; short and rounded ligula at anterior
edge.
But these features are inconsistent with our observa-
tion: (1) the apex of ‘Burithes yunnanensis’ is not sharp
and acute, but rather rounded, which contradicts the defi-
nition; (2) all the specimens of ‘B. yunnanensis’ are
flattened with no three-dimensional features left or dor-
sal–ventral differentiation, therefore the sharp transition
between the dorsum and venter in this species remains
doubtful; (3) there is no evidence showing that ‘B. yunna-
nensis’ has a ligula as the aperture is straight, with slight
curve at most in some specimens, thus the lateral sinuses
and the helens also are not observed therein; (4) no
growth lines were found on the conch; (5) the composi-
tion of the ‘conch’ is different from other hyoliths. As a
result of rapid sediment deposition (Zhao et al.2012),
conches of other hyoliths (e.g. Triplicatella opimus) of the
Chengjiang biota are either preserved three-dimensionally
(Fig. 2A, E) or with brittle cracks (Fig. 2A, B, D, F); but
the ‘conch’ of ‘B. yunnanensis’ is strongly flattened with
neither of these two features, only wrinkles on the surface
(Fig. 2G, H), which suggests that the ‘conch’ is thin and
flexible. Moreover, the EDS and XRF analyses (Figs S6,
S7) show that the ‘conch’ is enriched in iron (a result of
pyritization during the taphonomic process and later oxi-
dation), which is common in organic structures like
FIG. 1. A–G, Palaeoconotuba elongata gen. et sp. nov.: A, ELEL-EJ08595; B, ELEL-SJ102072; C–E, ELEL-SJ102087: C, showing
tentacles, gastric cavity and vascular canal; D–E, close-ups of rectangles in C, showing tentacles (D) and vascular canal (E); F–G,
YKLP-13856 (courtesy of Peiyun Cong), holotype of P. elongata, showing tentacles (white rectangle); G, close-up of rectangle in F.
H–N, Palaeoconotuba yunnanensis comb. nov.: H–I, ELEL-SJ080936A, showing gastric cavity and vascular canal; I, close-up of the
rectangle in H, showing vascular canal; J, ELEL-SJ102080B, showing the gastric cavity; K, ELEL-SJ102086A, showing gastric cavity and
vascular canal; L, ELEL-SJ102064, showing gastric cavity; M, NIGP-176308 (courtesy of Fangchen Zhao), showing vascular canal;
N, ELEL-SJ102083, showing vascular canal. Vascular canal marked by white arrows, gastric cavity marked by yellow arrows, and tenta-
cles marked by green arrows. Scale bars indicate: 1 cm (A–C, F, H, J–L, N); 2 mm (D); 1 mm (E, I); 3 mm (G); 5 mm (M).
QU ET AL.: CAMBRIAN THECATE MEDUSOZOANS 5
muscle tissue (Liu et al.2020a) but rare in aragonitic
conches. Therefore, we propose that the ‘conch’ of
‘B. yunnanensis’ is organic and ductile, and probably
composed of chitin.
In addition, ‘Burithes yunnanensis’ also has traits that
are inconsistent with hyoliths. Hyolithida generally pos-
sess a conch, a folded operculum and a pair of helens.
The conch has a subtriangular cross section, a flat ventral
side, and dorsal side with a central longitudinal groove
flanked each side by a longitudinal ridge, a protruding
ventral shelf (ligula) and a pair of lateral sinuses at the
aperture (Liu et al.2020b). Orthothecida have a variable
cross section of the conch and a flat operculum, with no
helens (Liu et al.2020a). As mentioned above, ‘B. yunna-
nensis’ has a conical organic theca, sharing no similarities
with the conch of the hyoliths. This suggests that ‘B. yun-
nanensis’ is also morphologically different from them.
It is well documented that hyoliths bear a U-shaped
digestive tract that was stable in the early Palaeozoic (Liu
et al.2021), and many intensive studies regarding the guts
have been published; Orthothecida have a straight anal
tube and a sinuously folded, or even chevron-like gut
FIG. 2. Comparison of Triplicatella opimus and Palaeoconotuba gen. nov. A–F, ELEL-EJ080156-A, T. opimus, showing three-
dimensionally preserved or brittlely cracked conch and operculum; C–F, close-ups of white rectangles in A and B, showing: C, opercu-
lum, with white arrow suggesting cylindrical central mass; D, F, conch with brittle fractures; yellow arrows marking the cracks;
E, three-dimensionally preserved conch, with yellow arrows marking the ridges. G–H, ELEL-SJ102084, P. yunnannensis, showing wrin-
kles on the conch; H, close-up of the white rectangle in G, with green arrows marking wrinkles. Scale bars represent: 1 cm (A, B, G);
2 mm (C); 3 mm (D, F, H).
6PALAEONTOLOGY
(Kruse 2010; Devaere et al.2014; Berg-Madsen et al.2018;
Liu et al.2021), probably a result of ontogeny: the digestive
tract is in a simple U-shape first but becomes folded sinu-
ously afterwards (Devaere et al.2014), which is a trait of
filter feeding animals (Liu et al.2020b). Hyolithida have a
simple U-shaped digestive tract (Devaere et al.2014; Berg-
Madsen et al.2018,Fig.S8G), suggesting a feature for ben-
thic animals (Liu et al.2020b). If ‘Burithes yunnanensis’isa
species of the Order Hyolithida (Hou et al.1999,2017;
Chen 2004), it should possess a U-shaped gut. However,
our specimens preserved with the gastrovascular system
exhibit a gastric cavity in the upper part that sharply tapers
into a straight vascular canal, which continues extending
downwards to its end with no opening to the outside
(Fig. 1C, H, K, M, N). Also, the EDS and XRF analyses
(Figs S5,S6) indicate that the vascular canal has also
undergone pyritization, thus presenting a bright band with
no trend of turning up on the figure. All of this evidence
suggests that ‘B. yunnanensis’ has a gastrovascular system
with only one opening, which is difficult to reconcile with
the interpretation of it being a hyolith.
The tentacles of hyoliths are rarely reported due to
poor preservation. The hyoliths found in the Burgess
shale are preserved with lophophore-like, retractable feed-
ing apparatus, which is reminiscent of those of some bra-
chiopod larvae (Moysiuk et al.2018); however, Liu
et al.(2020a) disagreed with the interpretation of the
lophophore because the tentacles only protrude at the
central anterior part of the operculum rather than all
around the mouth (Fig. S8A–F), and they suggested that
the tentacles are likely to be connected to a cylindrical
central mass (pharynx, Fig. 2C; Fig. S8A–F) attached to
the operculum so that the tentacles and the pharynx
remain in the same position relative to the operculum
even if the latter is isolated (Fig. 2A, C; Fig. S8A, C, E),
and the pharynx then reaches outside the operculum to
form a neck that probably serves as a connection to the
conch (Fig. S8F). However, only few specimens of ‘Bur-
ithes yunnanensis’ show tentacles, and these are preserved
around the margin of the ‘conch’ aperture without any
evidence of the presence of the abovementioned tissues
(Fig. 1C, D, F, G; Fig. S1A, B). Hence, we interpret these
finger-like tentacles as projecting directly from the aper-
ture of the ‘conch’. This feature does not correspond to
the structure seen in hyoliths either, nor is there any evi-
dence for the presence of an operculum in our specimens.
It is a perplexing problem that the tentacles were pre-
served alone given that the tentacles and the operculum
are often preserved as a whole in hyoliths. Should there
be a cylindrical central mass and a neck in ‘B. yunnanen-
sis’, as illustrated by Liu et al.(2020a), the operculum
would not be easily lost and would be preserved as well,
which means it is very likely that ‘B. yunnanensis’ does
not possess an operculum at all.
Taken together, these traits clearly set ‘Burithes yunna-
nensis’ apart from hyoliths. We reason that they probably
suggest a radial symmetrical pattern since there is no
robust bilateral evidence, and hence the tentacles are most
likely to surround the central mouth as a whorl. We
therefore argue that the genus ‘Burithes’ reported from
the Chengjiang biota is invalid and reassign it to a new
genus, Palaeoconotuba.
Palaeoconotuba spp. are stem-group medusozoans
We have shown above that Palaeoconotuba has a polypoid
body, a funnel-shaped gastric cavity and a straight vascu-
lar canal with one single opening, an organic conical
theca, a whorl of tentacles that surround the mouth, and
a putative radial symmetry. These simple anatomical
characters imply that this genus is most likely to have a
polypoid cnidarian affinity rather than with hyoliths that
have a U-shaped gut and an aragonitic conch. Further-
more, we propose that Palaeoconotuba is more closely
affiliated with the Medusozoa than the Anthozoa within
the cnidarians, based on the following arguments:
1. The composition of the exoskeleton. Exoskeletons are
common in cnidarians (Han et al.2016c) where most
of the Anthozoa have a calcareous exoskeleton com-
posed of calcite or aragonite (Drake et al.2020).
Though Ceriantharia has a non-calcareous exoskele-
ton, its exoskeleton is mostly made up by the pty-
chocyst secreting a mucus net or filaments and
catching nearby sediment particles, thus it is not
completely organic (Daly et al.2007; Stampar
et al.2015). Medusozoan polyps have exoskeletons
that are mainly corneous (chitin-protein). The polyps
in Cubozoa and Scyphozoa can be roughly divided
into three types: stephanoscyphistomae, scyphistomae
and cubopolyps, the first type of which is completely
enclosed by a peridermal theca (Straehler-Pohl 2017).
As demonstrated above, we propose that Palaeocono-
tuba has an organic, non-calcareous, conical exoskele-
ton, which is close to stephanoscyphistomae in
composition and morphology.
2. The morphological differentiation of the polyp. The
column of anthozoan polyps is nearly consistent in
width from top to bottom, whereas there is a sharp
change in medusozoan polyps which in general is
divided into a calyx and a stalk. Mesenteries remain
in polyps of some modern medusozoans, but occur
only in the aboral portion, and fuse orally and trans-
versely into an expanded structure called the claus-
trum (well developed in Staurozoa and Cubozoa),
whereas other lamellar structures (e.g. gonad lamellae
and interradial mesenteries) are produced in all direc-
tions (Han et al.2013). The claustrum is absent in
QU ET AL.: CAMBRIAN THECATE MEDUSOZOANS 7
Hydrozoa due to the loss of mesenteries. This differ-
entiation is retained in polyps of the early Cambrian
(Fortunian) sessile medusozoans which were sur-
rounded by a conical periderm (Han et al.2016d).
Palaeoconotuba is also characterized by a conical peri-
derm that accommodates a capacious gastric cavity at
the aboral region and a succeeding slender vascular
canal towards the aboral end. This suite of morpho-
logical traits suggests the presence of a calyx and a
stalk, which is comparable to modern medusozoan
polyps.
3. Internal anatomical features. Most of the meduso-
zoans produce pelagic medusae from polyps by stro-
bilation (Straehler-Pohl & Jarms 2005). The internal
anatomical characters of the polyp change during
development. Palaeoconotuba shows some similarities
with polyps of the Staurozoa and Scyphozoa: the gas-
tric cavity of Palaeoconotuba might correspond to that
in the calyx, and the straight vascular tract might cor-
respond to the stalk canal in the stalk of the Stauro-
zoa (Kikinger & Salvini-Plawen 1995, text-fig. 4) and
these can also be matched in Discomedusae (Scypho-
zoa) polyps (Helm 2018, text-fig. 2B) that may repre-
sent a stage in the ontogenetic process. Also,
Palaeoconotuba spp. possess no holdfast at the bottom
as its theca is aborally closed, which is comparable to
Olivooides (Han et al.2016a). The lack of a holdfast
is common in Olivooides and Quadrapyrgites from the
Kuanchuanpu Biota, and has been proposed as an
ancestral feature in medusozoans (Dzik et al.2017).
Other tubular fossils, Sphenothallus Hall, 1847,Byronia
Matthew, 1899, and Cambrovitus Mao et al., 1992, and
conulariids, have been proposed as having a medusozoan
affinity. Sphenothallus is characterized by an elongated
tube composed of finely laminated apatitic or organic
lamellae that tapers to a holdfast at the apex with two
longitudinal thickenings located opposite one another at
the wide diameter of the aperture (Van Iten et al.1992;
Li et al.2004); Byronia possesses a slender organically or
apatitically laminated theca with a minute attachment
disc. The outer surface of the theca has transverse ridges
that intersect across the longitudinal striae (Zhu
et al.2000); Cambrovitus was previously assigned to a
hyolith, but the occurrence of the holdfast at the apex
ruled out this possibility. Its theca is very slender, with
numerous transverse striae on the surface (Zhu
et al.2000). Our specimens have a conical theca in gross
morphology, but lack the longitudinal thickenings; the
conical theca is flattened without any longitudinal ridges
that are indicative of such a structure undergoing tapho-
nomic process, which is incomparable to Sphenothallus;in
addition, Palaeoconotuba has no ornamentation on the
external surface of the theca, which together with the
absence of a holdfast, rules out an interpretation as these
medusozoans.
Conulariids are most likely to be a group of thecate
scyphozoans, with a geological range from the Ediacaran
to the Triassic (Van Iten et al.2006). In general, conulari-
ids possess a tetramerous radial symmetry and a pyrami-
dal theca with transverse ribs (Leme et al.2022).
Palaeoconotuba bears some similarity with conulariids,
including the overall morphology in lateral view and lon-
gitudinal thecal grooves occurring on a few specimens
reminiscent of the corner sulcus of conulariids. However,
we propose that Palaeoconotuba is not their Cambrian
representative. The dubious longitudinal thecal grooves
observed in some specimens (Fig. 1A, Fig. S1H, L, M) are
taphonomic artefacts rather than a biological structure in
that: (1) these grooves do not consistantly occur in all
specimens; (2) the grooves can usually be seen in the api-
cal part near the lateral margin of the theca (Fig. 1A, K,
M; Fig. S1H, M) but hardly seen towards the oral por-
tion, differing from the corner sulcus in conulariids that
extends from the apex to the aperture; and (3) more sig-
nificantly, the groove in some specimens shows signs of
bending (Fig. 1K; Fig. S1L, M), which further indicates a
taphonomic artefact that we previously interpreted as
wrinkles rather than the sulcus characteristic of conulari-
ids. Moreover, none of the specimens of Palaeoconotuba
show transverse ribs or longitudinal midlines on the
theca, which adds difficulty if Palaeoconotuba is inter-
preted as a conulariid. In addition, the conulariid perid-
erm, usually composed of alternately organic and
biomineral lamellae (Ford et al.2016), is more prone to
breakage, while Palaeoconotuba is most likely to have pos-
sessed an organic, flexible theca.
Taken together, we propose that Palaeoconotuba spp.
are medusozoan cnidarians. Their life cycle probably
involved only planula and polyp stages, lacking a medusa
stage. As the only known co-occurring pelagic meduso-
zoan from the Chengjiang biota, Yunnanoascus haikouen-
sis (Han et al.2016b), had a body size of c. 1 cm, which
is obviously smaller compared to specimens of Palaeo-
conotuba spp. whose body size varies from 2 to 4 cm.
Extant medusozoans, such as scyphozoans and cubozoans,
have rather minute polyps and ephyrae (up to a few mil-
limetres) during strobilation (Helm 2018); their medusa
stage occupies most of their life therefore the former is
reduced. Should Palaeoconotuba spp. reproduce pelagic
medusae through strobilation regardless of being mono-
disc or polydisc, its ephyra would be at least the same size
of the polyp. Moreover, it has been proposed that the
ancestor of medusozoans might have possessed only plan-
ula and polyp stages (Han et al.2020) and the medusa
stage was derived subsequently, which is another support
for the absence of medusa stage in our specimens
8PALAEONTOLOGY
combined with the result of phylogenetic analysis. Our
phylogenetic analysis was based on 115 characters and 42
taxa under Bayesian inference (see Text S2 file for detail)
and suggests that Palaeoconotuba was a stem-lineage of
medusozoan cnidarians (Fig. 3; Fig. S9) which also
accommodates Cambrorhytium,Sphenothallus,Byronia
and Cambrovitus. This result implies that the most recent
common ancestor of medusozoans is a diploblastic, ses-
sile, polypoid animal possessing a gastrovascular system
with one opening and a whorl of tentacles. Their life cycle
probably involved only the planula and polyp stages. We
suggest that the conical theca might be an ancestral trait,
as seen in coronate medusozoans with an organic conical
theca during the polyp stage (Jarms et al.2002a,2002b;
Helm 2018). In brief, the phylogeny indicates that medu-
sozoans once developed a stem lineage with a lifelong
conical theca, whose living habit is prominently different
from that of the descendants of the crown group, thus
further expanding our knowledge of medusozoans.
Palaeoconotuba is closely related to Cambrorhytium
Cambrorhytium Conway Morris & Robison, 1988, is also a
controversial genus, which was formerly assigned to Cni-
daria or Priapulida (Conway Morris & Robison 1988). The
first specimen of Cambrorhytium was reported from the
middle Cambrian of British Columbia where the authors
compared the possible fibrous tentacle-like structures at
the aperture of the specimen to those in cnidarians,
proposing that it should be assigned to cnidarians (Conway
Morris & Robison 1988). Later, Cambrorhytium gracilis
and C. minor reported from the Three Gorges Area of
South China exhibited a basal disc, a straight vascular tract,
and probably evidence of budding (Chang et al.2018),
which further supports their cnidarian affinity.
Archotuba conoidalis reported from the Chengjiang
biota is a junior synonym of Cambrorhytium elongata
(Steiner et al.2003), which is often found adhered to the
shells of trilobites and brachiopods (Fig. S10) (Dornbos
et al.2005; Lei et al.2014). This species was also formerly
considered as priapulid, but later assigned to Cnidaria
(Hou et al.2017) for the well-preserved C. elongata has a
polypoid shape, a funnel-shaped gastric cavity
(Fig. S10A), a straight vascular canal (Fig. S10), and the
apex of the theca is sharp with scarce ornamentation,
traits shared with other cnidarians (Lei et al.2014).
We reason that Palaeoconotuba and Cambrorhytium are
closely affiliated in that: (1) they both possess a polypoid
form and an organic theca; (2) their tentacles that sur-
round the mouth are morphologically alike (Conway
Morris & Robison 1988, text-fig. 12 3a–3b; Fig. 1F, G);
and (3) in view of internal morphology, they both have a
straight vascular canal, some of our specimens also
exhibit a funnel-shaped gastric cavity that connects the
vascular canal, which is similar to that seen in Cam-
brorhytium (Conway Morris & Robison 1988, text-fig. 12
1a–1b; Fig. S10). As suggested by our phylogenetic analy-
sis, Cambrorhytium and Palaeoconotuba represent a stem
lineage of medusozoans that are united by the synapo-
morphy of developing a conical peridermal theca.
Early evolution of medusozoans
The Terreneuvian Fortunian Stage in South China hosts
considerable sedentary thecate medusozoans, such as
Hexangulaconlariidae, Carinachitidae, Olivooidae (Han
et al.2020) and Obelia (Shao et al.2015). The size of
these fossils is on a millimetre scale. In terms of the
exoskeleton, these fossils commonly possessed a tower-
shaped, calcareous or organic conical theca (e.g. Olivooi-
dae) with transverse ridges that represent the trend of
up-growing (Han et al.2016a), they also had a narrow
and folded aperture that the tentacles may not have been
able to reach out of entirely for hunting for food. The
medusozoan fossils from the Cambrian Stage 2 show
signs of lateral expanding in order to protect the soft tis-
sue inside (Han et al.2020). We reason that Palaeocono-
tuba has taken after the trait of the thecate medusozoans,
however, its body size is at a centimetre scale with no
transverse ridges, the aperture is wide so that there is
enough space for the tentacles to reach out. These charac-
ters suggest that during the Cambrian Age 3, the thecate
medusozoans represented by Palaeoconotuba occupied a
more stable niche and thrived comparing to their prede-
cessors in the Terreneuvian. Also, the increase in individ-
ual size corresponds to the Cope’s rule, which points out
the tendency for size increase over time (Novack-
Gottshall & Lanier 2008). The appearance of Palaeocono-
tuba in the Chengjiang biota reveals that thecate meduso-
zoans extend from the Fortunian Age (as represented by
diversified meiofaunal medusozoans from the Kuan-
chuanpu biota) to at least Age 3 of the Cambrian, as
exemplified by Palaeoconotuba in the Chengjiang biota.
The phylogenetic analysis suggests that Palaeoconotuba
is positioned as a thecate stem medusozoan, hence we
further reason that Palaeoconotuba may have also inher-
ited the sessile ecology of their ancestors from the Fortu-
nian Stage. The apex end of Palaeoconotuba spp. shows
no sign of attachment to the exoskeleton of other organ-
isms; hence they may have inserted the posterior portion
of their theca into the soft sediments on the sea floor, as
can be seen in the living Ceriantharia. Comparing the
tentacles of Palaeoconotuba to those of Xianguangia (Ou
et al.2017) and Nailiana (Ou et al.2022), we find that
the former has no branches or pinnules, which are also
absent in Nailiana, hence suggesting that the former was
QU ET AL.: CAMBRIAN THECATE MEDUSOZOANS 9
FIG. 3. Summary of the phylogenetic relationships produced by Bayesian analysis based on 115 characters and 37 taxa under
Mkv + Γmodel (see Fig. S9 and Text S2 for details). Numbers at nodes indicate posterior probabilities. Scale bar represents 0.03
expected changes per site. Palaeoconotuba is resolved as a stem-group medusozoan. Animal silhouettes are from PhyloPic (http://
phylopic.org/: we thank Michelle Site (Chaetognatha); Hans Hillewaert and T. Michael Keesey (starfish); both CC BY 3.0).
10 PALAEONTOLOGY
not a suspension feeder but rather more likely a predator
with smooth tentacles laden with cnidocytes (Fig. 4).
Our study sheds light the early evolution of meduso-
zoan cnidarians: early Cambrian medusozoans are charac-
terized by sedentary polypoid predecessors enveloped in a
conical theca in gross morphology, but showing compara-
ble anatomical features with modern medusozoan polyps
(Han et al.2016a). As far back as at least 518 Ma, one
branch evolved a larger body and a pelagic medusozoan,
as exemplified by Yunnanoascus haikouensis (Han
et al.2016b), losing their conical theca in adaptation to
the swimming habit (Han et al.2016c).
CONCLUSION
We herein conclude that the ‘Burithes yunnanensis’ in the
Chengjiang biota has some traits that clearly set it apart
from the hyoliths, but share a close relationship with the
cnidarians. Hence, we revise the genus ‘Burithes’ in the
Chengjiang biota into Palaeoconotuba gen. nov., and fur-
ther establish a new species, Palaeoconotuba elongata. The
Bayesian inference based on characters suggests that it
represents a stem-group medusozoan, which is also the
first thecate medusa reported from the Chengjiang biota.
The appearance of Palaeoconotuba bridged the gap of a
cryptic evolution that the medusozoans have once devel-
oped a group of a benthic predatory clade with a lifelong
conical theca in the early Cambrian, from the Fortunian
Age to Age 3, they gradually took a stable niche and
thrived at that time. By contrast, most of the extant medu-
sozoans possess a peridermal theca at their polyp stage, but
the exoskeleton is abandoned when they proceed to the
stage of swimming medusozoans. Although it may have
been the inconvenience of having a theca that lead to the
extinction of this clade, these thecate medusozoans in the
early Cambrian have broadened our knowledge and shed
light on the scale and pattern of the early evolution in this
phylum during the Cambrian explosion.
Acknowledgements. We appreciate invaluable comments from
Jian Han (Northwest University) and Heyo Van Iten (Hanover
College). We thank Christian Skovsted, John Malinky, Ben Yang
and Junfeng Guo for providing the original literature regarding
Burithes; Fangchen Zhao, Peiyun Cong, Xiaoya Ma, Derek Siv-
eter and Meirong Cheng for providing images of Palaeoconotuba
and Cambrorhytium; Zhifei Zhang for providing images of Trip-
licatella; Jialin Zhao, Chonghan Yu, Jie Yang and the State Key
Laboratory of Geological Process and Mineral Resources for the
assistance in experimental work; Weize Shi and Y€
uping Shi for
providing artistic reconstruction and the translation of Russian
literature. This research was supported by grants from the
National Natural Science Foundation of China (41972009), the
Chinese ‘111’ project (B20011), the Alexander von Humboldt
Foundation (Q.O.: CHN 1164230 HFST-P), and the College Stu-
dents’ Innovative Entrepreneurial Training Plan Program of
China University of Geosciences (K.L.: 202011415408).
Author contributions. Qiang Ou conceived the research. Hanzhi
Qu, Kexin Li, and Qiang Ou wrote the paper. Qiang Ou col-
lected most of the fossil specimens. Hanzhi Qu and Kexin Li
conducted the phylogenetic and morphometric analyses and pro-
duced the figures. All authors analysed and interpreted the data,
discussed the conclusions, and approved the final manuscript.
DATA ARCHIVING STATEMENT
This published work and the nomenclatural acts it contains,
have been registered in ZooBank: https://zoobank.org/
References/138C4907-61C4-4761-9112-C0434BF9C535. Data for
FIG. 4. Ecological reconstruction of Palaeoconotuba gen. nov. (left foreground) and Cambrorhytium (right background). Artwork by
Weize Shi.
QU ET AL.:CAMBRIANTHECATEMEDUSOZOANS 11
this study are available in MorphoBank: http://morphobank.org/
permalink/?P4406.
Editor. Nadia Santodomingo
SUPPORTING INFORMATION
Additional Supporting Information can be found online (https://
doi.org/10.1111/pala.12636):
Fig. S1. Palaeoconotuba spp. from the early Cambrian of
South China. A–I, P. yunnanensis comb. nov.: A–C, ELEL-
SJ102066. Showing the tentacles, capacious gastric cavity and
slim vascular canal; D, NIGP-CJ4234, showing vascular canal; E,
ELEL-SJ102086B, counterpart of ELEL-SJ102086A (Fig. 1K),
showing gastric cavity and vascular canal; F, ELEL-SJ08-1449A;
G, ELEL-SJ102065, showing remains of vascular canal; H, ELEL-
SJ102067; I, ELEL-SJ102088-1, showing triangular gastric cavity.
J–M, P. elongata gen. et sp. nov.: J, ELEL-SJ102075, showing vas-
cular canal; K, ELEL-SJ102077; L, ELEL-SJ102081, showing vas-
cular canal; M, ELEL-SJ102082B, showing vascular canal.
Fig. S2. Palaeoconotuba spp. from the early Cambrian of
South China. A–C, P. yunnanensis comb. nov.: A, ELEL-
SJ102068; B, ELEL-SJ102071; C, ELEL-EJ08-1705. D–F, P. elongata
gen. et sp. nov.: D, ELEL-SJ102076; E, ELEL-SJ102074; F, ELEL-
SJ102073.
Fig. S3. Line drawings of Palaeoconotuba specimens ELEL-
SJ080936A and ELEL-SJ102086A, showing boundaries between
the internal structure of the polyp and the conical theca.
Fig. S4. The width/length ratio and the apical angle of P. yun-
nanensis and P. elongata.
Fig. S5. Energy dispersive x-ray spectrometry (EDS) analysis
of Palaeoconotuba yunnanensis comb. nov. (ELEL-SJ102086B,
Fig. S1B).
Fig. S6. X-ray rluorescence spectrometer (XRF) analysis of
Palaeoconotuba elongata (ELEL-SJ102087A and SJ102087B, part
and counterpart).
Fig. S7. Type species of Burithes (Burithes distortus) (GIN
3593/203, 3593/67; Institute of Geology, Russian Academy of
Sciences) and holotype specimen of Burithes yunnanensis
(NIGP115438).
Fig. S8. The tentacles of Triplicatella opimus and the digestive
tract of ‘Linevitus’malongensis.
Fig. S9. Phylogenetic placement of Palaeoconotuba within the
metazoan tree inferred from Bayesian inference analyses in
MrBayes v3.2 under Mkv + Γmodel.
Fig. S10. Cambrorhytium elongata.
Fig. S11. Morphological analyses of Palaeoconotuba gen. nov.
A, results of principal component analysis. B, landmarks and
sliding semi-landmarks in TpsDig2. C–D, the average shape of
P. yunnanensis (C) and P. elongata (D) respectively, obtained
from the centroid of each domain. E, results of relative warp
analysis, showing different shapes at different end points respec-
tively.
Text S1. Morphometric analyses.
Text S2. Phylogenetic analysis, including a description of the
coding characters.
Table S1. Examined specimens of Palaeoconotuba spp. from
the Chengjiang biota.
Table S2. Data matrix (see also Qu et al.2023).
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