Ontogeny of the Redlichiid Trilobite Metaredlichia cylindrica from the Lower Cambrian (Stage 3) of South China

Abstract

A number of immature and mature exoskeletons allow the first detailed description of the ontogeny of the early Cambrian redlichiid trilobite Metaredlichia cylindrica, from black shale of the Shuijingtuo Formation in Hubei Province, South China. The material includes numerous complete protaspides, within which two stages can be differentiated according to the appearance of a shallow furrow that separates the protopygidial area from the protocranidium. Also, identification of the subsequent ontogenetic stages, including meraspides and holaspides, depends on isolated cranidia that display prominent morphological changes such as the contraction of frontal glabellar lobe, appearance of the fourth pair of glabellar furrows, and modification of the facial suture from proparian to opisthoparian. Incorporating the whole ontogenetic sequence allows us not only to trace the developmental trends of various structures with growth, but also to assign the protaspides to their adults correctly, particularly with the help of meraspid specimens.

Full text

ONTOGENY OF THE REDLICHIID TRILOBITE METAREDLICHIA
CYLINDRICA FROM THE LOWER CAMBRIAN (STAGE 3) OF
SOUTH CHINA
TAO DAI AND XINGLIANG ZHANG
State Key Laboratory for Continental Dynamics, Early Life Institute and Department of Geology, Northwest University,
Xian, 710069, China, ,xzhang69@nwu.edu.cn.
ABSTRACT—A number of immature and mature exoskeletons allow the first detailed description of the ontogeny of
the early Cambrian redlichiid trilobite Metaredlichia cylindrica, from black shale of the Shuijingtuo Formation in
Hubei Province, South China. The material includes numerous complete protaspides, within which two stages can
be differentiated according to the appearance of a shallow furrow that separates the protopygidial area from the
protocranidium. Also, identification of the subsequent ontogenetic stages, including meraspides and holaspides,
depends on isolated cranidia that display prominent morphological changes such as the contraction of frontal
glabellar lobe, appearance of the fourth pair of glabellar furrows, and modification of the facial suture from
proparian to opisthoparian. Incorporating the whole ontogenetic sequence allows us not only to trace the
developmental trends of various structures with growth, but also to assign the protaspides to their adults correctly,
particularly with the help of meraspid specimens.
INTRODUCTION
THE CAMBRIAN redlichiid trilobite Metaredlichia Lu, 1950
is known only from the lower Cambrian (Stage 3) of
South China. The type species of Metaredlichia was first
described by Zhang (1953) as Redlichia cylindrica based on
two incomplete cranidia from the lower Cambrian Shuijingtuo
Formation in Changyang, Hubei Province, China. Subse-
quently, the genus was supplemented by the additional species,
‘Metaredlichia sp. 1’ and ‘Metaredlichia sp. 2’ (Zhang et al.,
1980, p. 137, 138, pl. 29, figs. 6, 7), each on the basis of one
distorted specimen. To our knowledge, ontogenetic stages
within the subfamily Metaredlichiinae are unknown, with the
exception of probable protaspid specimens described as ‘genus
and species indeterminate 1’ by Zhang and Pratt (1999).
It is common to use larval characters in trilobite systematics
and cladistic analysis at all taxonomic levels (e.g., Whittington,
1957; Fortey and Owens 1975; Edgecombe et al., 1988; Fortey
and Chatterton, 1988; Speyer and Chatterton, 1989; Fortey,
1990, 2001; Chatterton et al., 1994, 1999; Chatterton and Speyer,
1997; Lee and Chatterton, 2003, 2007; Dai and Zhang, 2012). By
placing considerable emphasis on protaspid morphology,
morphological information revealed by other developmental
stages may not have been fully utilized (Park and Choi, 2011).
Other ontogenetic phases, particularly the meraspid stage, should
be considered when using ontogenetic data for trilobite
classification. It is possible that analysis of the entire growth
series will answer unresolved questions in trilobite systematics. In
this study, new material from the lower Cambrian Shuijingtuo
Formation (Qiongzhusian, Cambrian Stage 3, Series 2) in
Yichang and Changyang, Hubei Province, including the larval
stages, offers an opportunity to explore the postembryonic
development of the redlichiid trilobite M. cylindrica.
MATERIAL
Numerous calcareous specimens, preserved as external and
internal molds in black shales, were collected from two
outcrops in the lower part of the Shuijingtuo Formation in
Yichang and Changyang of Hubei Province, South China:
Wangjiaping, 19 km northwest of Yichang, and Dingjiaping,
5.5 km northwest of Changyang (see Dai and Zhang, 2011,
text-fig. 1). Due to secondary diagenetic deformation, only 27
protaspides, five meraspid cranidia and seven holaspid
cranidia were sufficiently preserved for detailed study.
Measurements of sagittal length of the protaspides were
made from anterior to posterior exoskeletal margins, exclud-
ing the length of posterior fixigenal spines, and those of
transverse width exclude the length of anterior marginal
spines. Sagittal length of the meraspid cranidia was measured
from anterior margin to occipital ring, excluding the length of
fixigenal spines.
The described and figured specimens from this study are
housed in the collection of the Geological Department of
Northwest University, Xi’an, China (NWUYTX 20201–20218).
SYSTEMATIC PALEONTOLOGY
Terminology.—Morphological terms and abbreviations
largely follow Whittington and Kelly (1997), Edgecombe et al.
(1988) and Lee and Chatterton (1996, 1997). Anaprotaspis and
metaprotaspis (Beecher, 1895; Chatterton and Speyer, 1997) are
used as substages of the protaspid, before and after a pygidial
portion can be distinguished from the protocranidium, usually
by the appearance of a shallow furrow behind the head. These
substages can be recognized among protaspides of M. cylindrica.
In addition, some abbreviations are used in the description
below: excl.5excluding; exs.5exsagittal; incl.5including; L5
length; sag.5sagittal; tr.5transverse; and W5width.
Order REDLICHIIDA Richter, 1932
Suborder REDLICHIINA Richter, 1932
Superfamily REDLICHIOIDEA Poulsen, 1927
Family REDLICHIIDAE Poulsen, 1927
Subfamily METAREDLICHIINAE Zhang and Lin, 1980
Genus METAREDLICHIA Lu, 1950
Type species.—Redlichia cylindrica Zhang, 1953 from the
lower Cambrian Shuijingtuo Formation in Changyang, Hubei
Province, China.
METAREDLICHIA CYLINDRICA Zhang, 1953
Figures 1–4
1953 Redlichia cylindrica ZHANG, p. 126, pl. 4, figs. 5–7.
1965 Metaredlichia cylindrica ZHANG,LUETAL., p. 67, pl. 9,
figs. 8–10; ZHANG ET AL., 1980, p. 136, pl. 29, figs. 1–5.
Journal of Paleontology, 86(4), 2012, p. 646–651
Copyright ’2012, The Paleontological Society
0022-3360/12/0086-0646$03.00
646

FIGURE 1—Protaspides of Metaredlichia cylindrica (Zhang, 1953) from Hubei, South China. 1,8–10, from Dingjiaping, Changyang; 2–7, from
Wangjiaping, Yichang. 1–5, anaprotaspid stage: 1, 2, cluster 1, NWUYTX 20201–20202, 396; 3105; 3–5, cluster 2, NWUYTX 20203–20205, 368; 365;
365; 6–11, metaprotaspid stage: 6, 7, substage 1, NWUYTX 20206–20207, 378; 365; 8–10, substage 2, NWUYTX 20208–202010, 355; 365; 365; 11,
close-up of the protopygidium of protaspis NWUYTX 20210, showing the marginal and protopygidial spines (arrowed).
DAI AND ZHANG—CAMBRIAN REDLICHIID TRILOBITE ONTOGENY 647

1999 ?genus and species indeterminate 1, ZHANG AND
PRATT, p. 123, figs. 3.3, 3.4, 6.1–6.10.
PROTASPID PERIOD
Figures 1, 4.1–4.3
Twenty-seven protaspides of M. cylindrica were investigat-
ed, 0.55–0.96 mm long and 0.54–1.10 mm wide, most lacking
librigenae, within which two stages are recognized: anapro-
taspides and metaprotaspides (Figs. 1, 4.1–4.3).
Anaprotaspid stage.—Nine protaspides can be assigned to
this stage, of which five are figured (Figs. 1.1–1.5, 4.1). Shield
sub-circular in outline, gently to moderately convex trans-
versely and longitudinally. Anterior margin curved; anterior
border narrow, slightly widening (sag., exs.) abaxially;
anterior border furrow shallow. Axis sub-cylindrical, defined
by shallow axial furrow, length (sag.) 78–88%length of shield;
divided into 5 axial segments by four transverse furrows,
anterior four as protoglabellar lobes (L1–L4), fifth as occipital
ring (LO). Sagittal furrow extends forward from SO to
anterior margin of L4, halving the four glabellar lobes into
paired lobes. Protoglabellar lobes widening slightly forward
from L1 to L3 and strongly in L4; L4 sub-trapeziform,
strongly expanding forward, with anterior margin reaching
anterior border furrow, approximately twice length (sag.) and
width (tr.) of L1–L3. Protoglabellar furrows shallow, SO–S2
straight, S3 angled slightly forward. LO narrower than L1
(sag., tr.), width (tr.) 39–46%width of L4; posterior margin
curved backward. Pair of fossulae impressed at junction of L4
and eye ridge. Eye ridge weakly to moderately developed,
curved posterolaterally, forming a continuation of palpebral
lobe, with posterior tip situated opposite L2. Anterior sections
of facial suture convergent forward, posterior sections steeply
directed posterolaterally. Fixigena convex prominently, widest
(tr.) between posterior tip of palpebral lobe and L2.
Indentation defined by facial suture represents extremely
narrow librigena, anterolaterally situated, in front of the
anterior fixigenal spines. Three pairs of marginal spines
present: anterior pair short and straight, located at about
mid-exoskeleton length, 49–59%of sagittal exoskeletal length
from anterior, pointing slightly posterolaterally; middle pair
short, pointing posterolaterally, situated midway between
anterior and posterior pairs, closer to anterior pair; posterior
pair longer, about twice as long as others, pointing backward.
Lateral border furrow shallow and wide. Posterolateral
margin moderately straight, extending posteromedially, form-
ing pair of posterior spines at the end.
Length and width measurements were taken for a total of 27
protaspid exoskeletons (Fig. 2), of which two size clusters can
be observed in anaprotaspides. The cluster of smaller size is
referred to cluster 1, 0.50–0.54 mm long and 0.52–0.60 mm
wide (Fig. 1.1, 1.2); the larger one is cluster 2, 0.70–0.81 mm
long and 0.72–0.82 mm wide (Fig. 1.3–1.5). Morphological
changes between the two clusters are very subtle. Other than
the size variation, it seems that the middle marginal spines are
not apparent or preserved in the protaspides of cluster 1.
Metaprotaspid stage.—Eighteen specimens (Figs. 1.6–1.11,
4.2, 4.3), 0.73–0.96 mm long and 0.78–1.10 mm wide, which
possess nearly similar morphology with the preceding stage,
are characterized by the appearance of a shallow furrow that
separates protopygidial area from protocranidium. This phase
can be further differentiated into two substages according to
the number of axial segments in the protopygidia.
Substage 1 (Figs. 1.6, 1.7, 4.2) with shield sub-circular in
outline, 0.73–0.78 mm long and 0.78–0.82 mm wide. Posterior
tip of palpebral lobes situated opposite S1 or L1. Proto-
pygidium small, 17–19%of shield length (sag.), with one axial
segment defined by shallow axial furrow; a pair of marginal
spines probably developed along posterior margin. Shallow
furrow extends from LO laterally and then curved backward,
separating the protopygidium from the protocranidium.
Substage 2 (Figs. 1.8–1.11, 4.3) with shield ranged from
0.82–0.96 mm long and 0.84–1.10 mm wide. Protopygidium
with two axial rings occupies 22–30%of shield length (sag.);
two pairs of protopygidial marginal spines (Fig. 1.10, 1.11)
located between posterior fixigenal spines, posterior pair
shorter than anterior one. Interpleural furrow between the
two ribs undeveloped. Posterior margin slightly curved.
MERASPID PERIOD
Figures 3.1–3.3, 4.4, 4.5
Five meraspid cranidia were investigated and can be
subdivided into two stages, 0.75 to 1.16 mm in length, of
which three specimens are figured (Fig. 3.1–3.3).
Meraspid stage 1.—Cranidium (Figs. 3.1, 4.4) is similar to
the metaprotaspid protocranidia and probably represents a
degree 0 meraspis. Cranidium sub-pentagonal in outline.
Frontal glabellar lobe extends forward, reaching anterior
border. Proparian facial suture short, anterior sections slightly
convergent forward, with copposite LA; posterior sections
diverging posterolaterally, with eopposite S1. Three fixigenal
spine pairs still preserved, anterior and middle pair contracted
and minute; posterior pair pointing backward. Eye ridge
slightly curved posterolaterally; palpebral lobe with posterior
tip situated opposite S1. Pair of fossulae in each side of LA
weakly impressed. Posterior border furrow weakly defined.
Meraspid stage 2.—Cranidium sub-trapeziform in outline,
0.83 to 1.16 mm long (Figs. 3.2, 3.3, 4.5). Glabella wider (tr.)
in L1, narrowest in L2 and L3; S3 slightly angled projecting
forward. Occipital ring wider than L1 (tr.). Eye ridge and
palpebral lobe more distinct, with posterior tip closer to
posterior cranidial border. Anterior and middle spine pairs
absent, posterior pair points backward. Fossulae between the
eye ridge and LA absent. Posterior border narrow (exs.),
extending laterally to fixigenal spine; posterior border furrow
well impressed, conjoined with lateral border furrow.
FIGURE 2—Scatter diagram of length versus width of protaspides of
Metaredlichia cylindrica (Zhang, 1953) from Yichang and Changyang,
Hubei, South China.
648 JOURNAL OF PALEONTOLOGY, V. 86, NO. 4, 2012

HOLASPID PERIOD
Figures 3.4–3.8, 4.6, 4.7
Seven holaspid cranidia, 1.23 to 1.61 mm long, can be
subdivided into two stages, according to the appearance of the
fourth pair of glabellar furrows (S4), of which three are
assigned to stage 1 and four are assigned to stage 2.
Holaspid stage 1.—Cranidium (Figs. 3.4, 3.5, 4.6) sub-
trapezoidal in outline. Anterior border moderately wide (sag.);
anterior border furrow shallow. Glabella wide and convex,
rising above fixigenae; LA broadly rounded with anterior
margin reaching anterior border furrow; S1 transverse, curved
backward, S2 and S3 probably discontinuous. Occipital ring
wider and longer (tr., sag.) than L1, with occipital spine
located posteromedially. Fixigena narrow (tr.). Eye ridge and
palpebral lobe curved with posterior tip close to glabella and
posterior border. Facial suture opisthoparian, anterior sec-
tions divergent forward; posterior sections slightly diverging
posterolaterally then cutting posterior border. Fixigenal spine
absent. Posterior border narrow and convex (exs.), slightly
expanding abaxially to intergenal angle, then tapering to distal
end of border.
Holaspid stage 2.—Cranidium (Figs. 3.6–3.8, 4.7) is char-
acterized by the appearance of the fourth pair of glabellar
furrows, short and obscure, projecting anteromedially; ante-
rior border wide (sag.) and flattened; preglabellar field narrow
FIGURE 3—Meraspides and holaspides of Metaredlichia cylindrica (Zhang, 1953) from Hubei, South China. 1–6, from Dingjiaping, Changyang; 7,8,
from Wangjiaping, Yichang. 1–3, meraspid period: 1, stage 1, NWUYTX 20211, 360; 2,3, stage 2, NWUYTX 20212–20213, 353; 335; 4–8, holaspid
period; 4, 5, stage 1, NWUYTX 20214–20215, 335; 340; 6–8, stage 2, NWUYTX 20216–202118, 330; 338; 342.
DAI AND ZHANG—CAMBRIAN REDLICHIID TRILOBITE ONTOGENY 649

(sag.); anterior sections of facial suture strongly divergent
forward, posterior sections longer, almost transverse to
posterior border, then intersecting posterior border. Eye ridge
and palpebral lobe curved distinctly laterally with posterior tip
closer to glabella.
DISCUSSION
The main morphological changes during ontogeny include:
1) anterior cranidial border becomes wider (sag.) and more
flattened; 2) glabella with L1 wider and LA contracting
gradually until holaspid period; 3) frontal glabellar lobe
reaching anterior cranidial border until holaspid stage 1, with
preglabellar field appearing in holaspid stage 2; 4) sagittal
glabellar furrow in protaspides is lost in later ontogeny (see
also Zhang and Pratt, 1999, figs. 5, 6.7–6.10; 5) occipital node
(or spine?) of the holaspid period (Fig. 3.6) might be also
present in meraspides; 6) eye ridge and palpebral lobe extend
posterolaterally in protaspides but become curved laterally in
holaspides, with posterior tip moved backward and closer to
glabella and posterior border; 7) facial suture from proparian
in meraspides to opisthoparian in holaspides, with anterior
sections from slightly convergent in protaspid and meraspid
period to divergent in holaspid phase; 8) two distinct pits,
which were probably associated with attachment of the
hypostome, are retained until meraspid stage 1 and then
disappear; 9) increase in the L/W ratio of cranidium; 10)
fixigena becomes progressively narrower (tr.) and fixigenal
spines are retained through the meraspid stage but are absent
in holaspides; and 11) retention of the anterior and middle
fixigenal spine pairs of protaspides in the meraspid stage 1 of
M. cylindrica (Fig. 3.1), as in early meraspides of other
trilobite groups, including Tesselacauda depressa,Kawina
sexapugia,Rossaspis pliomeris,Protopliomerella contracta
(Lee and Chatterton, 1997) and Porterfieldia acava (Edge-
combe et al., 2005).
In addition, the traces of a fourth pair glabellar furrows (S4)
are recognized in holaspid stage 2 of M. cylindrica, seemingly
dividing the glabella into five pairs of lobes. However, one of
the striking characteristics during the segment development of
trilobites is that of cephalic stability and trunk variability in
the segment numbers. The expression of individual segments
is least clear in the cephalon, but the number of segments in
this region is generally considered almost invariant during
ontogeny, as Hughes (2003, p. 187) argued that ‘‘the serial
homology of the posterior segments is generally accepted on
the grounds of similarity, but that of S4 is less secure: where
present this furrow is short, weakly incised, and is commonly
spaced and oriented differently from those furrows succeeding
it’’. Consequently, although the morphology on the cephalic
region of M. cylindrica changed markedly during ontogeny, it
is considered that the number of segments defined in this
region generally remained constant throughout all ontogenetic
phases and the appearance of the fourth pair of glabellar
furrows (S4) in late holaspid stage may not correspond to
addition of new segment in the cephalic region.
Zhang and Pratt (1999) described two protaspid stages
(Stage 0 and Stage 2) of ‘genus and species indeterminate 1’
from the lower Cambrian Shuigoukou Formation in Xichuan,
Henan Province, China, which might belong to M. cylindrica
of this study. Of the 40 shields (e.g., Zhang and Pratt, 1999,
figs. 3.3, 3.4, 6.1–6.10), 24 are early protaspides with no axial
ring behind the occipital ring, i.e., ‘stage 0 (P0)’ (Zhang and
Pratt, 1999, figs 3.3, 3.4, 6.1–6.6), which were subdivided into
three early protaspid instars (P0a, P0b and P0c) based on size,
development of sagittal glabellar furrow, and addition of the
third pair of fixigenal spines between anterior and posterior
fixigenal spines. In these morphological variations, the
addition of the middle spine pair, which was observed only
in ‘Stage 0c’, might be the most prominent morphological
marker subdividing the protaspid ‘stage 0’. Accordingly, it is
most likely that the specimens designated as ‘P0a’ (Zhang and
Pratt, 1999, figs. 3.3, 6.1, 6.2) and ‘P0b’ (Zhang and Pratt,
1999, figs. 3.4, 6.3, 6.4) with anterior and posterior fixigenal
spine pairs are equivalent to the cluster 1 of anaprotaspides in
this paper; the specimens attributed to the ‘P0c’ (Zhang and
Pratt, 1999, figs. 6.5, 6.6) correspond to the cluster 2 of
anaprotaspides, both of which possess mid-fixigenal spines
and longer posterior spine pair. In addition, 16 larger shields
FIGURE 4—Reconstruction in dorsal views of ontogenetic series of Metaredlichia cylindrica (Zhang, 1953). 1–3, protaspid period: 1, anaprotaspid
stage; 2, metaprotaspid stage 1; 3, metaprotaspid stage 2; 4, meraspid period stage 1; 5, meraspid period stage 2; 7, holaspid period stage 1; 8, holaspid
period stage 2. Scale51 mm.
650 JOURNAL OF PALEONTOLOGY, V. 86, NO. 4, 2012

(Zhang and Pratt, 1999, figs. 6.7–6.10) assigned to ‘protaspid
stage 2 (P2)’ due to their protopygidia bearing two axial rings
and two pairs of protomarginal spines are comparable to the
metaprotaspid stage 2 described here.
ACKNOWLEDGMENTS
We are grateful to B. Pratt, S. Westrop, J. Stewart
Hollingsworth and R. Gozalo for their constructive and
insightful suggestions. Financial supports by the Natural
Science Foundation of China (NSFC, Grants: 40872004 and
40925005), the Major Basic Research Project of the Ministry
of Science and Technology of China (Grant: 2006CB806400),
‘‘Sanqin Scholarship’’ project of the Shaanxi Authority and
NWU Doctorate Dissertation of Excellence Funds (10YYB01)
are greatly acknowledged.
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