Houcaris gen. nov. from the early Cambrian (Stage 3) Chengjiang Lagerstätte expanded the palaeogeographical distribution of tamisiocaridids (Panarthropoda: Radiodonta)

Abstract

Radiodonts were cosmopolitan and diverse stem-euarthropods that have been generally regarded as the apex Cambrian predators. Four major groups have been distinguished including tamisiocaridids, primarily based on the endite features of the frontal appendages. Anomalocaris saron Hou, Bergström and Ahlberg, 1995, one of the most well-known radiodonts in the Chengjiang Lagerstätte, is generally treated as a member of the Family Anomalocarididae. New anatomical evidence reported here, allied with the data of microcomputed tomography (CT) shows that the endites in A. saron are paired, much longer than the height of associated podomeres, and furnished with multiple slender distal auxiliary spines. These new observations allow us to reassign A. saron to a new genus, Houcaris gen. nov., and strongly support its tamisiocaridid affinities rather than anomalocaridid as previously suggested. Houcaris saron thus represents the first tamisiocaridid species known from South China, as well as the oldest tamisiocaridid in the fossil record (Cambrian Stage 3). Our occurrence data, coupled with other distribution of tamisiocaridids, demonstrates that this group is restricted to the early Cambrian (Series 2), and occur across South China, Laurentia and eastern Gondwana within tropics/subtropics belt, indicating a possible climatic control on their distribution. Moreover, these tamisiocaridid records documented in several Konservat Lagerstätten suggest an ecological preference to shallow water environment with well-oxygenated sea bottom conditions.

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Vol.:(0123456789)
1 3
PalZ
https://doi.org/10.1007/s12542-020-00545-4
RESEARCH PAPER
Houcaris gen. nov. fromtheearly Cambrian (Stage 3) Chengjiang
Lagerstätte expanded thepalaeogeographical distribution
oftamisiocaridids (Panarthropoda: Radiodonta)
YuWu1· DongjingFu1· JiaxinMa1· WeiliangLin1· AoSun1· XingliangZhang1
Received: 5 August 2020 / Accepted: 28 December 2020
© Paläontologische Gesellschaft 2021
Abstract
Radiodonts were cosmopolitan and diverse stem-euarthropods that have been generally regarded as the apex Cambrian
predators. Four major groups have been distinguished including tamisiocaridids, primarily based on the endite features of
the frontal appendages. Anomalocaris saron Hou, Bergström and Ahlberg, 1995, one of the most well-known radiodonts in
the Chengjiang Lagerstätte, is generally treated as a member of the Family Anomalocarididae. New anatomical evidence
reported here, allied with the data of microcomputed tomography (CT) shows that the endites in A. saron are paired, much
longer than the height of associated podomeres, and furnished with multiple slender distal auxiliary spines. These new
observations allow us to reassign A. saron to a new genus, Houcaris gen. nov., and strongly support its tamisiocaridid affini-
ties rather than anomalocaridid as previously suggested. Houcaris saron, thus, represents the first tamisiocaridid species
known from South China, as well as the oldest tamisiocaridid in the fossil record (Cambrian Stage 3). Our occurrence data,
coupled with other distribution of tamisiocaridids, demonstrate that this group is restricted to the early Cambrian (Series 2),
and occur across South China, Laurentia and eastern Gondwana within tropics/subtropics belt, indicating a possible climatic
control on their distribution. Moreover, these tamisiocaridid records documented in several Konservat Lagerstätten suggest
an ecological preference to shallow water environment with well-oxygenated sea bottom conditions.
Keywords Radiodonta· Tamisiocaridid· Houcaris gen. nov.· Biogeography· Chengjiang biota· Computed tomography
Introduction
Radiodonta Collins, 1996 is a clade of stem arthropods
(sensu Ortega-Hernandez 2016) that have generally been
regarded as large apex predators of the Cambrian and Ordo-
vician Periods (e.g. Briggs 1994; Chen etal. 1994; Whit-
tington and Briggs 1985; Daley and Budd 2010; Paterson
etal. 2011;Daley etal. 2013a, b; Daley and Edgecombe
2014; Van Roy etal. 2015; Liu etal. 2018). These animals
are typified by paired grasping frontal appendages with a
series of morphologically diverse endites, radial mouthparts,
prominent dorsolateral compound eyes on stalks, a series of
flexible lateral flaps along its dorsoventrally flattened body
and tail fans. Frontal appendages are the most commonly
preserved element of radiodonts owing to their sclerotised
nature, and thus, these structures played a key role in under-
standing radiodonts taxonomy and ecology. Primarily on
the basis of the organisation of frontal appendages, four
major groups have been distinguished, including the fami-
lies Anomalocarididae Raymond, 1935, Amplectobeluidae
Pates etal.2019b, Tamisiocarididae Pates and Daley, 2019
and Hurdiidae Lerosey-Aubril and Pates, 2018. As one of
the four families of radiodonts, the Family Tamisiocaridi-
dae is characterised by a pair of frontal appendages bearing
elongate and thin endites with numerous auxiliary spines.
To date, tamisiocaridids contains only Tamisiocaris borealis
Daley and Peel, 2010, Anomalocaris briggsi Nedin, 1995,
and possible Tamisiocaris aff. borealis (Pates and Daley
2019).
Recent phylogenetic analyses exploring radiodont inter-
relationships have produced incompatibility with the tra-
ditional taxonomy of the genus Anomalocaris, with A.
Handling editor: Mike Reich.
* Dongjing Fu
djfu@nwu.edu.cn
1 State Key Laboratory ofContinental Dynamics andShaanxi
Key Laboratory ofEarly Life andEnvironments, Department
ofGeology, Northwest University, Xi’an710069, China

Y. Wu etal.
1 3
kunmingensis retrieved within the Amplectobeluidae, and
A. briggsi within the Tamisiocarididae, leaving Anomalo-
caris to be polyphyletic (Van Roy etal. 2015; Liu etal. 2018;
Lerosey-Aubril and Pates 2018). Anomalocaris saron Hou,
Bergström and Ahlberg, 1995, a classic radiodont species
from the well-known Chengjiang Lagerstätte (Cambrian
Series2, Stage 3), was originally described 25years ago
(Hou etal. 1995). Several researchers have illustrated frontal
appendage material of A. saron, nevertheless, new anatomi-
cal features have yet been well studied (Cong etal. 2018;
Guo etal. 2018; Pates etal. 2019b), and then no more data
have been involved in the subsequent phylogenetic analysis
of Lerosey-Aubril and Pates (2018). Therefore this taxon
is still generally treated as a member of the Family Anom-
alocarididae (e.g., Van Roy etal. 2015; Liu etal. 2018;
Pates etal. 2019b). In this contribution, we have studied
new material of this species, drawing on microcomputed
tomography (CT) study, to elucidate morphological details
of frontal appendage that forces the assignment of A. saron
to tamisiocaridids rather than anomalocaridids. Our work
herein also adds to spatio-temporal distributional data on
tamisiocaridid radiodonts and evaluate palaeogeographic
distribution patterns and habitat preferences of this group.
Materials andmethods
The studied appendage specimens documented here were
collected from seven localities (Ercai, Erjie, Jianshan,
Sanjiezi, Mafang and Haoyicun) of the Cambrian (Stage
3; locally Qiongzhusian) Yu’anshan Member of the Chi-
ungchussu Formation in eastern Yunnan Province, China,
which falls within the Eoredlichia–Wutingaspis Trilobite
Zone (Hou etal. 2017; Zhang etal. 2017). The material is
deposited in the Shaanxi Key Laboratory of Early Life and
Environments (LELE) and Department of Geology, North-
west University (NWU), Xi’an, China.
All specimens here were gathered from split slabs of
mudstones, and a few specimens were further prepared
with fine needles under high magnification using stereomi-
croscopes. Fossils were photographed with a Canon EOS
5D Mark II digital camera and images processed in Adobe
Photoshop CS 6. Camera lucida drawings were made
using a Zeiss Discovery V12 microscope and prepared
with Corel Draw X4. Measurements were measured from
specimen digital photographs using ImageJ freeware (https
://image j.nih.gov/ij/index .html). X-ray micro-computed
tomography (CT) was used to reveal ventral structures
concealed within the rock matrix (e.g., Chen etal. 2019;
Zhai etal. 2019a, b, c; Liu etal. 2020). Specimen scanning
was performed by Zeiss X-radia 520 Versa for MF-002.
Each scan generated a set of radiographs saved as TIFF
stacks which were further processed with the DRAGON-
FLY 4.1 software (http://www.theob jects .com). The 3D
models rendered in Dragonfly were screen-captured as
images in the figures.
Terminology and abbreviations. As listed in Table1, the
morphological terms used in the description of radiodont
frontal appendages vary considerably. For accuracy and sim-
plicity, we here give a new description scheme for radiodont
frontal appendages.
Radiodonts frontal appendages consist of a series of
podomeres bearing a diverse morphology of endites along
their length and can be separated into two major regions:
the ‘base’ and ‘claw’. The anatomical term ‘base’ follows
Maddocks (2000), which is employed herein to designate the
proximal enlarged peduncular portion of frontal appendages
bearing no endite or reduced endite. And the term ‘claw’ fol-
lows Haug etal. (2012), which refers to the region distal to
the ‘base’ bearing relatively large and sophisticated endite.
The previously used terms ‘shaft’ (Hou etal. 1995; Cong
etal. 2018) and ‘distal articulated region’ (defined by Cong
etal. 2018) were not preferred over ‘base’ and ‘claw’ as they
have various meanings and are generally applied to tools,
not organs (Lerosey-Aubril and Pates 2018). The bound-
ary between the ‘base’ and ‘claw’ of the frontal append-
age can often be identified by the presence of an angle on
the dorsal surface of the frontal appendage, and the mor-
phology and position of endites (Pates etal. 2019a). And
thus, the ‘base podomere’ and ‘claw podomere’ refer to the
podomeres of base and claw respectively. Significantly, the
term ‘endite’ (defined by Boxshall 2004) initially refers to
the inner spinose or setose inner lobe of Arthropoda postan-
tennulary limb, we here extend this term to the radiodont
frontal appendages.
Table 1 List of morphological terms of radiodont frontal appendages used in previous works and this study
Briggs (1979) Hou etal. (1995) Haug etal. (2012) Cong etal. (2018) Liu etal. (2018) Pates etal. (2019a) This study
Appendage
podomeres
Shaft Peduncle Shaft Appendage
podomeres
Shaft Base
Appendage
podomeres
Appendage
podomeres
Claw Distal articulated
region
Appendage
podomeres
Distal articulated
region
Claw
Ventral spine Endite Spine Endite Endite Endite Endite

Houcaris gen. nov. from the early Cambrian (Stage 3)
1 3
The other descriptive terminology mainly follows that of
Cong etal. (2018) and Pates etal. (2019a). ‘Auxiliary spine’
refers to the small spine located on the lateral margins of endite.
‘Terminal spines’ are spines at the distal end of the appendage.
Cp1–X refers to claw podomeres 1 to X, Bp1–3 refers to base
podomeres 1–3, and the numbering reflects a proximal–distal
axis. And En1–EnX refer to endites on Cp1–CpX.
The terminology essentially follows Briggs (1979) for its
orientation. Proximal (toward the base of the frontal append-
age) and distal (away from the base of frontal appendage) are
used here in their customary sense to designate directions
within the front appendage. Thus, the ‘distal auxiliary spine’
and ‘proximal auxiliary spine’ refer to the auxiliary spine
occurring on the distal and proximal margin of an endite,
respectively. The height of podomere refers to the distance
between its dorsal and ventral margins, and its length refers
to the distance between its boundaries with the preceding
and following podomeres.
Systematic palaeontology
Superphylum Panarthropoda Nielsen, 1995
Phylum (stem-group) Euarthropoda Lankester, 1904
Order Radiodonta Collins, 1996
Family Tamisiocarididae Pates and Daley, 2019
Type genus. Tamisiocaris Daley and Peel, 2010; from the
early Cambrian (Series 2, Stage 3) Sirius Passet Fauna of
North Greenland.
Revised diagnosis. Radiodonts with frontal appendages
bearing paired thin endites much longer than the height of
the associated podomeres; endites on proximal and inter-
mediate claw podomeres bearing multiple slender auxiliary
spines (modified from Pates and Daley 2019).
Remarks. In original description, the appendage endites of
the Family Tamisiocarididae are subequal rather than alter-
nating in length, which are well represented by Tamisiocaris
and Anomalocaris briggsi. Our present material indicates
that the appendage endites of tamisiocaridids could also
alternate long/short on odd/even numbered claw podomeres.
Compared to the arrangement of endites, the elongate
endites, slender and dense auxiliary spines are what really
matters to the filter-feeding of tamisiocarids. And thus, the
presence of the subequal endites is no longer treated as the
diagnostic character of the Tamisiocarididae. The family
hitherto comprises Houcaris gen. nov., Tamisiocaris Daley
and Peel, 2010, and Anomalocaris briggsi Nedin, 1995.
Genus Houcaris nov.
Etymology. Named after Professor Xianguang Hou for his
contributions to early research on the Chengjiang biota and
Chengjiang radiodonts; and ‘caris’ meaning ‘crab’, a com-
monly used suffix for marine euarthropods.
Type species. Houcaris saron (Hou etal., 1995); from the
Chengjiang biota (Series 2, Stage 3) of eastern Yunnan,
South China.
Diagnosis. Tamisiocaridids with paired elongate, distally
tapered frontal appendages consisting three base podomeres
and 13 claw podomeres; claw podomeres in proximal and
intermediate region bear paired elongate endite altering
long/short on odd/even numbered claw podomeres, each of
which processes multiple distal auxiliary spines; En1 stout
and slightly curved; claw podomeres are separated by trian-
gular flexible arthrodial membrane.
Remarks. Anomalocaris magnabasis (previously Anomalo-
caris cf. saron) from the Pioche and Carrara Formations of
Nevada, USA (Pates etal. 2019b; see also Lieberman 2003)
is reassigned to the new genus Houcaris as it also bears the
long and thin endites with fine distal auxiliary spines and
the same number of claw podomeres as seen in H. saron
(see discussion below). The two-part structure of endites is
recognised in H. magnabasis by Pates etal. (2019b). How-
ever, the argumentation of presence of two-part structure
merely based on the different colouration and morphological
differences in different preservation stage, and the authors
did not give further exhaustive description or key anatomi-
cal information of the endites of this feature (e.g. articu-
lation between endite base and the distal spine or simple
boundary). Moreover, the same research concerning the two-
part structure in other radiodont families (tamisiocaridids,
amplectobeluids and hurdiids) remains scanty. Therefore,
we here consider that the two-part structure in Anomalocaris
and Houcaris magnabasis as a taphonomic artefact rather
than a morphological characteristic. And thus, this character
does not appear to be of systematic value and is excluded
from the diagnosis of this genus.
Occurrence. Stage 3 (Chengjiang, China) and Stage 4
(the Pioche and Carrara Formations, USA) of Cambrian
unnamed Series 2.
Assigned species. Houcaris saron (Hou, Bergström and Ahl-
berg, 1995) and Houcaris magnabasis (Pates, Daley, Edge-
combe, Cong and Lieberman, 2019b).

Y. Wu etal.
1 3

Houcaris gen. nov. from the early Cambrian (Stage 3)
1 3
Houcaris saron (Hou, Bergström and Ahlberg, 1995)
Figures1, 2, 3
1995 Anomalocaris saron sp. nov. Hou etal.: 166–167,
figs.2a–f; fig.3b, c; fig.4.
1996 Anomalocaris saron—Chen etal.: 197–198, fig.266.
2017 Anomalocaris saron—Hou etal.: 154–155, fig.19.1.
2018 Anomalocaris saron—Cong etal.: fig.5c, f.
2018 Anomalocaris saron—Guo etal.: fig.2f.
2019b Anomalocaris saron—Pates etal.: fig.12A.
Material examined. Holotype: EC-0036AB. Paratype:
SJZ-400AB. Other specimens: EC-007, JS-0048AB,
JS-0187AB, JS-0763AB, JS-1073AB, EJ-1906AB, EJ-
1911AB, EC-2296AB, MF-002, MF-1136AB, MF-1257AB,
MF-1136AB, MF-1257AB, HY-1795AB, HY-1807, HY-
1795AB, HY-1807, SJZ-261AB, SJZ-400AB, SJZ-534AB,
SJZ-537AB, SJZ-581AB, SJZ-261AB, SJZ-400AB, SJZ-
534AB, SJZ-537AB, SJZ-581AB.
Emended diagnosis. Houcaris with elongate distally taper-
ing frontal appendage consisting of 16 podomeres, including
three base podomeres and 13 claw podomeres; Cp2–Cp8
nearly square, Cp9–Cp12 rectangular and much longer than
height; En2–En8 at least one and a half times as long as
the height of the associated podomeres, and bear 5 distal
auxiliary spines and 2 proximal auxiliary spines; auxiliary
spines near the base part of endite are shorter; En1 stout and
ventro-distally curved; two smaller setules projected from
distal ventral surface of each claw podomere; Cp10–Cp13
project paired elongate dorsal spines arching forward; Cp13
also bearing one terminal spine and a pair of secondary dor-
sal spines (Modified from Hou etal. 1995).
Description. The frontal appendages are elongate, slender
and moderately narrowing distally. Frontal appendages range
in length from 1.7cm to at least 12cm (Figs.1, 2), as meas-
ured along the outwardly convex dorsal margin from the
proximal margin of proximal base to the distal extremity of
the appendage. The frontal appendages are almost incom-
pletely preserved in lateral aspect. Rare complete specimens
are known, which consist of a three-segmented base and a
13-segmented claw (Fig.2i). The base podomeres are both
higher and longer than the claw podomeres. The base is
angled at 127–145 degree to the claw on the dorsal surface
(Figs.1, 2). In SJZ-261, one tiny endite projects from the
ventral region of Bp1 (be in Fig.2g). The proximal claw
podomeres (Cp2–Cp8) length/height ratio ranges from 0.8
to 1; whereas in distal claw podomeres (Cp9–Cp12), the
length/height ratio more than 1.5. The proximal margin of
each podomeres, apart from the first (outwardly convex), was
straight and inclined at about 80° to the dorsal margin of the
appendage. The result of CT shows the endites are paired per
claw podomere (Fig.2d, e). En1 are stout, ventrally curved
flanked by two relative large auxiliary spines (Fig.2f–g). In
Cp2–Cp12, elongate thin endites differ in the way they pro-
ject on podomeres and in their orientation relative to them.
On Cp2–Cp7 of the holotype, the endites project proximally
from the proximal half of the podomere at an angle of c.
75–85° (relative to the long axis of the appendage; Figs.1,
2). On the contrary, En8–En12 project distally from the dis-
tal half of the podomere at a decreasingly low angle relative
to the ventral margins (these angles might be distorted dur-
ing preservation). The endites are quite elongate and at least
one and a half times as long as the height of the associated
podomeres (Figs.1a–e, 2a–h). Endites alternate long/short
on odd/even-numbered claw podomeres, reducing distally in
length (Figs.1a, b, d, e, 2a, b, h, i). The number of auxiliary
spines on distal edge of endites is variable and increases dis-
tally along the appendage, and the proximal edge of endites
bears two auxiliary spines. En2–En8 bear 5 distal auxiliary
spines; whereas, En9–En12, only bear one distal auxiliary
spines (Figs.1c, 2a, b). En2–En12 bear 2 proximal auxiliary
spines, which projected from the half length of the endites.
No evidence that these single auxiliary spines are in pair
has been observed. Apparently, some variation in the posi-
tion of the proximal auxiliary spines, but in the proximal
claw podomeres they tend to protrudes from about the mid-
length of the endite; whereas, they project from near the base
of the endite in the more distal claw podomeres. In some
specimens, two small closely pitched setules project from
the distal ventral surface of claw podomeres, ranging from
1 to 2.5mm in length (see in Figs.1c–d, 2a–e, g, h). Cp12
bears one simple endite without auxiliary spines (Fig.2a,
c). In SJZ-261, a series of small protuberances present at
the base of endites on the podomere’s ventral surface (white
arrows in Fig.2f). In JS-1073, Cp12 bears a pair of dorsal
spines (Fig.2j). The dorsal spines arch forwards and extend
generally parallel to the dorsal margin of the podomere. The
distal end of the appendage is ventrally curved and narrow,
Cp13 (most distal claw podomere) also bears one terminal
spines and a pair of secondary dorsal spines (sds and ts in
Cp13 in Fig.2j).
Fig. 1 Houcaris saron (Hou, Bergström and Ahlberg, 1995) from
the Chengjiang biota, South China (Series 2, Stage 3). Frontal
appendages in lateral aspect. a and d part and countpart of holotype,
EC-0036, nearly complete appendage, showing the elongate and thin
endites furnished with multiple distal auxiliary spines. b and e cam-
era lucida drawing of a and D. c close-up of En4 (boxed in a), show-
ing the five distal auxiliary spines (solid black arrows) on the elon-
gate endite and two small setules on the distal ventral surface of claw
podomere. Am arthrodial membrane, Bp base podomere, Cp claw
podomere, das distal auxiliary spine, pas proximal auxiliary spine, se
setules. All scale bars represent 10mm
◂

Y. Wu etal.
1 3
Fig. 2 Houcaris saron (Hou, Bergström and Ahlberg, 1995) from the
Chengjiang biota, South China (Series 2, Stage 3). Frontal append-
ages in lateral aspect. a part of MF-002 appendage showing elongate
and thin endites on proximal and middle claw podomeres. b close-
up of MF-002 part (boxed in a), showing two small setules originate
from distal ventral surface close to En6. c camera lucida drawing of
a. d Three-dimensional computer models derived from a micro-CT
scan of MF-002, showing the paired endites per claw podomeres.
e white box e in d showing small setules and the detail features of
endites. f, g part of SJZ-261 appendage with camera lucida drawing
of f showing proximal part of appendage. White solid arrows indicat-
ing the small protuberances near the base of endites on the ventral
surface. h SJZ-400, nearly complete appendage, showing the elongate
endites. i JS-1073, nearly complete appendage, showing three base
podomeres and 13 claw podomeres. j close-up of JS-1073 (boxed in
i), showing paired dorsal spines on Cp12 and paired secondary dor-
sal spines on distalmost claw podomere. As auxiliary spines, be base
podomere endite, EnL left endite, EnR right endite, sds secondary
dorsal spine. Scale bars represent: 5mm (a, c–i), 1mm (b, j)

Houcaris gen. nov. from the early Cambrian (Stage 3)
1 3
Remarks: The new Chengjiang material illustrated herein
allows us to provide more detail anatomical information.
Despite several recent studies have illustrated Houcaris
saron appendage material (e.g. Cong etal. 2018; Guo etal.
2018; Pates etal. 2019b), no diagnosis and formal descrip-
tion concerning this taxa have yet been provided after its
original description (Hou etal. 1995), in which the append-
age endites only bear two pair of auxiliary spines, whereas
the present material shows that the majority of endites bear
five slender distal auxiliary spines and two proximal auxil-
iary spines. The previously illustrated specimen shows that
En1 also bears at least five distal auxiliary spines (Guo etal.
2018: fig.2f, g), which is obscure in our new specimens. Our
observation also further unequivocally excludes the possibil-
ity of the reconstruction of one proximal base podomere and
fifteen claw podomeres (Chen etal. 2004; Lerosey-Aubril
etal. 2014: fig.3) and two base podomeres and 14 claw
podomeres (Hou etal. 1995, 2017). Moreover, the specimen
illustrated by Chen etal. (1994) was previously treated as H.
saron (Hou etal. 1995). We here consider this specimen is
not H. saron owing to its less auxiliary spines and relative
short endites (Chen etal. 1994: fig.1b, c). Microcomputed
tomographic (micro-CT) scanning provides solid evidence
to substantiate the presence of paired endites in H. saron
for the first time (Fig.2d, e), and further reconfirms that
the presence of paired endites remains a consistent trait of
the Tamisiocarididae and indeed a characteristic feature of
Radiodonta (Pates etal. 2019a).
Occurrence. Cambrian Series 2, Stage 3, Yu’anshan Mem-
ber, Chiungchussu Formation, Eoredlichia–Wutingaspis
trilobite biozone (Jianshan, Mafang, Ercai, Erjie, Sanjiezi,
Haoyicun), eastern Yunnan, South China.
Discussion
Houcaris gen. nov. asamember ofTamisiocarididae
Houcaris saron (Hou, Bergström and Ahlberg, 1995) (pre-
viously Anomalocaris saron) is unequivocally identified as
Tamisiocarididae as presently defined (see also original defi-
nition of this family in Pates and Daley 2019) by the diag-
nostic features of frontal appendage, including the multiple
podomeres, elongate and tapering outline, paired elongate
endites that much longer than the height of the associated
podomeres and multiple slender distal auxiliary spines.
H. saron can be easily distinguished from the members
of Amplectobeluidae and Hurdiidae, since the former was
characterised by hypertrophied endite and simple spine-like
endites devoid of auxiliary spines, and the latter by blade-
like endites.
This species has been traditionally considered as a repre-
sentative of the Family Anomalocarididae (e.g. Hou etal.
1995; Cong etal. 2018; Guo etal. 2018; Pates etal. 2019b).
However, the presence of longer endites with multiple aux-
iliary spines of this taxon is extremely differs from the rep-
resentatives of Anomalocarididae, such as Anomalocaris
canadensis Whiteaves, 1982, Anomalocaris cf. canadensis
from Emu Bay Shale (Daley etal. 2013b) and Anomalocaris
Fig. 3 Artistic reconstruction of the frontal appendage of Houcaris
saron, showing a series podomeres bearing paired elongate and thin
endites furnished with multiple distal auxiliary spines. Drawing by
Daowen Lv, copyright Shaanxi Key Laboratory of Early Life and
Environments; used with permission. Not to scale

Y. Wu etal.
1 3
aff. canadensis from the Weeks Formation of USA (Lerosey-
Aubril etal. 2014). Although the characteristic alteration
of long and short endites in anomalocaridids and amplec-
tobeluids is also present in Houcaris, the difference in
length between adjacent long and short endites in proxi-
mal podomeres is quite unconspicuous in H. saron. And the
minute difference in endite length would not prevent their
appendages from forming a basket shape when they are bent
and pulled toward mouth for filter-feeding. Whereas, in typi-
cal anomalocaridids and amplectobeluids, the long endites
are apparently longer than the adjacent short endites, such
as Anomalocaris canadensis (Daley and Edgecombe 2014:
figs.12.7, 13.4), Anomalocaris pennsylvanica (see Pates and
Daley 2019: fig.3, fig.6a), Amplectobelua symbrachiata
(see Cong etal. 2017: fig.2d). Despite Houcaris bears simi-
larity with anomalocaridids in the arrangement of endites,
the substantially morphological differences of endites (e.g.
length, arrangement and morphology of auxiliary spines)
prompt the exclusion of Houcaris from Anomalocarididae.
And the alteration of long and short endites is no longer
restricted to anomalocarids and amplectobeluids.
Anomalocaris briggsi has constantly been retrieved within
the Tamisiocarididae in recent phylogenetic analyses
(Vinther etal. 2014; Van Roy etal. 2015; Lerosey-Aubril
and Pates 2018; Liu etal. 2018; Moysiuk and Caron 2019).
Compared to other radiodont taxa, the frontal appendage
of Houcaris bears closer resemblance to that of A. briggsi,
Fig. 4 Comparative sketches of tamisiocaridid radiodont frontal
appendages. a Houcaris saron (this study). b Houcaris magnaba-
sis (redrawn from Pates etal. 2019b). c Tamisiocaris borealis Daley
and Peel, 2010 (redrawn from Pates and Daley 2019). d Tamisiocaris
aff. borealis (redrawn from Pates and Daley 2019). e ‘Anomalocaris’
briggsi Nedin, 1995 (redrawn from Daley et al. 2013b). Compare
to H. magnabasis, H. saron the ratio of height/length of each claw
podomeres is much lower, and the length of endites relative to claw
podomere height is higher. Further, H. saron does not well display
the two-part structure of endites as seen in H. magnabasis. Compared
to ‘Anomalocaris’ briggsi, Houcaris possesses less and more robust
auxiliary spines. Houcaris also can be differentiated from Tamisio-
caris not only by the less podomeres, but also by the endites being
shorter relative to podomere height, and by the much less auxiliary
spines

Houcaris gen. nov. from the early Cambrian (Stage 3)
1 3
since both of them have slender and distally tapering mor-
phology, and the number of claw podomeres in H. saron
(13; Figs.1, 3) is close to that in A. briggsi (14; Fig.4e).
More importantly, the length of endites relative to podomere
height in Houcaris (endite length:podomere height ratio
is c. 1.5) is quite comparable to that in A. briggsi (endite
length:podomere height ratio is c. 1.6; Fig.4e; see also
Daley etal. 2013b: figs.1, 2). These similarities between
H. saron and A. briggsi support the tamisiocaridid affinities
of Houcaris.
More recently, Pates etal. (2019b) reinterpreted Anom-
alocaris cf. saron Lieberman (2003) as Anomalocaris
magnabasis from the Pioche and Carrara Formation of
Nevada, USA. In the light of the anatomical features of
the frontal appendages in A. magnabasis, such as long and
thin endites with fine distal auxiliary spines and the same
number of claw podomeres as seen in Houcaris saron, we
here reinterpret this American species as Houcaris magna-
basis within the Family Tamisiocarididae. Compared to H.
saron, the ratio of height/length of each claw podomeres
in H. magnabasis is much higher, and the length of endites
Fig. 5 Distribution of the Family Tamisiocarididae during the Cam-
brian. a, Stratigraphical distribution, showing tamisiocaridids are
restrict to Cambrian Series 2 (the early Cambrian). b, Palaeobiogeo-
graphical distribution, indicating tamisiocaridids occur across South
China, Laurentia and eastern Gondwana within a relatively narrow
tropical belt. Palaeocontinental reconstructions during the early Cam-
brian time redrawn, modified and simplified by Torsvik and Cocks
(2013: fig. 2.7). Each taxon indicated by specific number. Numbers
in a correspond to those plotted in b. References for different tamisio-
caridid radiodont taxa: 1 = Daley and Peel (2010) and Vinther et al.
(2014); 2 = Pates and Daley (2019); 3 = Nedin (1995) and Daley etal.
(2013b); 4 = Hou et al. (1995) and this study; 5 = Lieberman (2003)
and Pates etal. (2019b). Ca the Carrara Formation, Cj the Chengji-
ang biota, EBS Emu Bay Shale, K the Kinzers Formation, P the
Pioche Formation, SP the Sirius Passet biota

Y. Wu etal.
1 3
relative to claw podomere height is lower. Further, H.
magnabasis apparently displays the two-part structure of
endites (Pates etal. 2019b). Therefore, H. saron from the
Chengjiang biota extends the paleogeographical range of
this genus outside of Laurentia for the first time, and also
constitutes the oldest occurrence for this genus (Cambrian
Stage 3). The occurrence also suggests a palaeobiogeo-
graphical connection between soft-bodied faunas from the
early Cambrian of geographically disparate South China and
western Laurentia.
In the context of the Family Tamisiocarididae, Houcaris
gen. nov. also resembles Tamisiocaris borealis and Anom-
alocaris briggsi in having paired elongate endites with
multiple slender auxiliary spines. Moreover, in Houcaris,
the endite in the distal podomere is miniaturised as seen
in A. briggsi (Daley etal. 2013b: figs.1, 2). In Houcaris,
however, the endites are alternating long and short, whereas
for T. borealis and A. briggsi, the endites are subequal in
length. Furthermore, the auxiliary spines in Houcaris are
much less than those in T. borealis and A. briggsi (Table2;
Daley etal. 2013b; Vinther etal. 2014; see also Lerosey-
Aubril and Pates 2018). Houcaris also can be differentiated
from Tamisiocaris (Vinther etal. 2014) not only by the less
podomeres, but also by the endites being shorter relative to
podomere height.
Spatio‑temporal distribution oftamisiocaridids
In addition to the newly erected genus herein, the Family
Tamisiocarididae by far contains Houcaris gen. nov. and
Tamisiocaris Daley and Peel, 2010, as well as Anomalocaris
briggsi Nedin, 1995. The biogeographical and temporal dis-
tribution of Tamisiocarididae is summarised in Fig.5.
The record of tamisiocaridids is restricted to the early
Cambrian (Series 2), occurring across South China (east-
ern Yunnan), Laurentia (USA and Greenland) and eastern
Gondwana (Australia) (Fig.4). Houcaris saron from the
Chengjiang biota and Tamisiocaris borealis Daley and Peel,
(2010) from the Sirius Passet fauna of North Greenland (see
also Vinther etal. 2014) represent the earliest fossil record
of tamisiocaridids, suggesting the emergence of tamisioca-
ridids in Cambrian Stage 3. After this early first appear-
ance, the tamisiocaridid fossil record continued sparse and
occurred in the west margin of Laurentia (Nevada, Penn-
sylvania) and eastern Gondwana (South Australia) at Stage
4, including Anomalocaris briggsi from Emu Bay Shale
of Australia (Pararaia janeae Zone; see also Daley etal.
2013b), Houcaris magnabasis from the Pioche Formation
(Eokochaspis nodosa zone and Nephrolenellus multinodus
zone) and Carrara Formation (Nephrolenellus multinodus
zone) of USA (Pates etal. 2019b), and possible Tamisiocaris
aff. borealis from the Kinzer Formation of Pennsylvania,
USA (Pates and Daley 2019). The occurrence of Houcaris in
geographically disparate South China and Laurentia during
the early Cambrian may reflect their relative strong capabili-
ties for dispersal in the water column, despite the lacking
of anatomical evidences (e.g. eyes, body flaps and tail fan).
Ultimately, this group virtually disappeared from the mid-
dle Cambrian rock record, which seems likely to represent
a true evolutionary absence, since the fossil record of other
radiodont families, are relative abundant in the depositions
of middle Cambrian, such as the Burgess Shale (e.g. Daley
and Budd 2010), Spence Shale (e.g. Briggs etal. 2008;
Table 2 Detailed comparison of frontal appendage characters of selected tamisiocaridid species
Y presence, N absence, U unknown, No number, Bp base podomere, Cp claw podomere, Ba base, Cl claw, En endite, aux auxiliary spine, d.a.s.
distal auxiliary spine, tm triangular membrane, dist distal, prox proximal
? indicates that thenumber of claw podomeres remains uncertain owing to the partial preservation
Houcaris saron Houcaris magnabasis Anomalocaris briggsi Tamisiocaris
borealis
No. of Bp 3 2 1 U
No. of Cp 13 13 14 ?18
Angle between Ba and Cl Y N Y N
Height:width ratio in proximal Cp 1.3:1 2.5:1 3:1 1.6:1
EnL/CpH on odd numbered Cp 1.7:1 1.4:1 1.6:1 2.5:1
Numerous aux Dist Dist Dist and prox Dist and prox
Max. no. of d.a.s 5 5 10 30
Rows of endites 2 2 2 2
Tm between Cp Y Y Y Y
En alternate long/short Y Y N N
Stout En1 Y Y N N
References This study Pates etal. (2019b) Daley etal. (2013b) Vinther etal. (2014)

Houcaris gen. nov. from the early Cambrian (Stage 3)
1 3
Daley etal. 2013a) and Wheeler Formation (e.g. Briggs
etal. 2008; Lerosey-Aubril etal. 2020). The demise of tami-
siocaridids at the end of Cambrian Stage 4, may have influ-
enced by factors that caused the mass extinction event of
archaeocyathids and redlichiid/olenellid trilobites (ROECE;
see Zhu etal. 2006) at the early–middle Cambrian boundary
(EMC), such as volcanically (Kurtz etal. 2003; Hough etal.
2006; Jourdan etal. 2014) or eustatically (e.g. Li etal. 2017;
Chang etal. 2017, 2019) associated expanded anoxia, as
well as the oligotrophic environment caused by aggravated
N loss as well as enhanced P input (Chang etal. 2019).
From a palaeogeographic point of view, tamisiocaridids
are present on South China, Laurentia (USA and Green-
land) and eastern Gondwana (Australia). This distribution
restricted to subtropical to tropical belt (Fig.4b) when plot-
ted on world maps based on palaeomagnetic data (Torsvik
and Cocks 2013), in contrast to Hurdiidae, which invade the
higher-latitude areas of the southern hemisphere (Baltica
and Avalonia) (Daley and Legg 2015; Pates etal. 2020).
Their latitudinal preference for the tropics/subtropics sug-
gests that tamisiocaridids may be warm-water animals that
controlled by changes in sea temperatures and climate zones.
Depositional environments
Tamisiocaridids in Konservat Lagerstätten provide further
evidence to suggest that tamisiocaridids have an ecological
preference to the shallow and well-oxygenated environment
(Table3). Houcaris magnabasis occurs from inner- (the
Pioche Formation, Nevada, USA; Pates etal. 2019b) to the
middle-shelf (the Carrara Formation, Nevada, USA; Pates
etal. 2019b) environment (Webster etal. 2008). H. saron
is preserved in the Chengjiang biota, which is generally
considered to have been deposited in shallow outer-shelf
settings (e.g. Hu 2005; Zhang etal. 2008; Hou etal. 2017)
with the fossils having been subjected to minimal transport
(Hou etal. 2004; Zhang and Hou 2007; Hou etal. 2017).
The presence of Anomalocaris briggsi Nedin, 1995 in the
Emu Bay Shale (Daley etal. 2013b) indicates that tami-
siocaridids could also survive in the nearshore, inner-shelf
shallow water environments. It seems to be that Tamisiocaris
represents an exception, with T. borealis being deposited
at the outer edge of the relict platform (Sirius Passet suc-
cession of Buen Formation; Daley and Peel 2010; Vinther
etal. 2014), and Tamisiocaris aff. borealis at a low energy
deep environment with intermittent, pulsed sedimentation
(Fine Pelitic Facies of the Emigsville Member of the Kinzers
Formation; Pates and Daley 2019). However, the sediment
from both two deposits was considered to be transported
from further shallow inboard or elsewhere (Skinner 2005;
Harper etal. 2019). It indicates that the organisms, including
Tamisiocaris in the depositional sites may be entombed far
away from their living environment. That is, Tamisiocaris
also likely to survive in shallow water environments and
probably the photic zone. Actually, tamisiocaridids have
generally been considered as filter-feeders, and then the low
oxygen deep marine settings would be unfavourable to pri-
mary producers and zooplanktons survival.
Conclusions
The detailed study of new material of Anomalocaris saron
from the Chengjiang biota leads a better understanding of
the appendage anatomy and sheds new light on the taxon-
omy of this important lower stem-group euarthropod. The
paired thin endites of this species are elongate, and bear
multiple slender auxiliary spines along its distal margin.
This character prompts a reassignment of A. saron to a new
genus, Houcaris gen. nov., and supports its tamisiocaridid
affinities. Houcaris saron gen. nov., together with Tamisio-
caris borealis from the Sirius Passet fauna of North Green-
land, represents the earliest fossil record of tamisiocaridids
around the world (Cambrian Stage 3). From the distribution
Table 3 The distribution of tamisiocaridid radiodonts from Konservat Lagerstätten representing different sedimentary environments
Fm Formation, Gu Guzhangian, Wu Wuliuan, H Houcaris, T Tamisiocaris
Biota Location Stage Environment Taxa References
Chengjiang Yunnan, South China 3 Offshore, outer part of broad clastic shelf;
below storm wave base
HThis study;
Hou etal. (1995)
Sirius Passet biota North Greenland 3 Shelf-slope break; below storm wave base TDaley and Peel (2010);
Vinther etal. (2014)
Pioche Fm Nevada, USA 4 Inner shelf HLieberman (2003);
Pates etal. (2019b)
Carrara Fm Nevada, USA 4 Middle shelf HPates etal. (2019b)
Kinzers Fm Pennsylvania, USA 4 Seaward of a carbonate shelf TPates and Daley (2019)
Emu Bay Shale Australia 4 Nearshore clastic setting; tectonic sub-basin Anomalo-
caris
briggsi
Daley etal. (2013b)

Y. Wu etal.
1 3
data of tamisiocaridids, this group is widespread longitudi-
nally but occurs within the tropical/subtropical regions dur-
ing the early Cambrian. This latitudinal preference implies
that tamisiocaridids may be warm-water animals that are
controlled by changes in sea temperatures and climate zones.
The discovery of tamisiocaridids in the Chengjiang biota
also provides support for the idea that these animals appear
to adapt to shallow water environments. The description
of biogeographic patterns and habitat preferences of other
radiodont groups (i.e. anomalocaridids, amplectobeluids and
hurdiids) is currently in preparation.
Acknowledgements We are grateful to Meirong Cheng, Cong Liu, Shu
Chai and Juanping Zhai at the Shaanxi Key Laboratory of Early Life
and Environments for joining in the fieldwork and the technical assis-
tance. We deeply appreciate the editor-in-chief Mike Reich (SNSB-
BSPG Munich) and one anonymous reviewer for their thoughtful and
constructive comments which greatly improved this manuscript. We
also would like to thank Hao Yun for discussion. Special thanks go
to Daowen Lv and Xi Liu (Northwest University Museum) for their
contribution to the reconstruction artwork, and Jie Sun and Yifei Sun
for scanning the fossils. This research was supported by the Natural
Science Foundation of China (41930319, 41772011, 41720104002,
41890844, 41621003), the Strategic Priority Research Program of
the Chinese Academy of Sciences (XDB26000000), and 111 Project
(D17013).
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