A pipiscid-like fossil from the Lower Cambrian of south China

Abstract

Exceptional fossil preservation is critical to our understanding of early metazoan evolution. A key source of information is the Burgess Shale-type faunas. Fossils from these deposits provide important insights into metazoan phylogeny, notably that of stem-group protostomes,,, and related topics such as trophic specialization. Metazoan relationships are also being significantly reappraised in terms of molecular-based phylogenies,, but integration of these data with palaeontological systematics is not straightforward,. Moreover, molecular phylogenies are silent concerning the anatomies of stem-groups and the functional transitions that underpin the origin of different body plans,. Some hitherto enigmatic fossils possess unique character-state combinations that, although they can be shoe-horned into extinct phyla, may be more profitably interpreted as defining major stem-groups,. Here we describe a possible pipiscid, a metazoan previously known only from the Upper Carboniferous,, from the Lower Cambrian of south China. Pipiscids are currently interpreted as being agnathan chordates, but this discovery from the Chengjiang fossil-Lagerstätte indicates that the assignment of pipiscids to the Agnatha deserves to be reconsidered.

Full text

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particulates on the background concentration of CCN over the
ocean could also have an effect on regional and global radiative
budgets. To estimate this effect, we apply the fuel-based emission
factors for particulate matter reported by Lloyds26 to generate total
annual particulate-matter emissions for ships (0.85 Tg yr
-1
). This is
then globally distributed according to the method of Corbett and
Fischbeck2,3 to give us the emission rate of particulate matter (E
x,y
)
for any grid cell (x,y). We can then estimate the change in the CCN
number, DCCN
x,y
, assuming steady state for the CCN concentration
and using
DCCNx;yEx;y
vdep
fmDp
pNDp
p1
where v
dep
is the average CCN removal velocity (wet and dry), f
m
is
the CCN mass fraction of the emitted particulate matter, Nis the
number density of CCN (number per mass larger than Dp
p) emitted
by ships, and Dp
pis minimum diameter at which these particles
activate to form cloud drops. Based on the particulate-matter size-
distribution measurements of Lyyranen et al.27, for large engines
operating at full power, and assuming a Dp
pof 0.1 mm, we estimate f
m
to be 0.6 (CCN per g particulate matter emitted) and Nto be 10
14
CCN per g of CCN. In addition, we assume a CCN lifetime of three
days (vdep 0:4cms21). Assuming as a ®rst-order approximation
that the change in cloud droplet number is equal to DCCN for the
low CCN concentrations of the marine environment (100 cm
-3
), the
potential effect of ships on annual average cloud albedo and
radiative forcing can be estimated28. To avoid problems with the
nonlinear response of cloud droplet number to CCN concentration,
and to keep our radiative forcing estimate conservative, we only
consider ship particulate-matter emissions where continental in¯u-
ence is small. This is determined using our annual average model
predictions for NSS-sulphate (with ships) and a threshold of
0.2 p.p.b.v. NSS-sulphate.
Using this method, we estimate the change in global radiative
forcing due to cloud effects from ship particulate-matter emissions
to be -0.11 W m
-2
. This value is 14% of the IPCC estimate for 1990
global indirect forcing from all anthropogenic sulphate29. The
predicted average change in radiative forcing due to ships for the
Northern Hemisphere is -0.16 W m
-2
and for the Southern Hemi-
sphere is -0.06 W m
-2
. The direct effect of ship sulphur emissions is
expected to be less dramatic28. The sensitivity of our estimate to
various uncertain input parameters is shown in Table 1. As can be
seen, reasonable variation in individual input parameters can affect
this ®rst-order global estimate by at least a factor of 2.
Our results suggest that the emissions of sulphur and particulate
matter from the international shipping industry need to be con-
sidered in the study of marine and coastal atmospheres. Because
ship emissions, as a source of background sulphur, have been
neglected in the past, many observational studies of the marine
atmosphere need to be re-evaluated, particularly those in the
remote oceans of the Northern Hemisphere. M
Received 19 February; accepted 5 July 1999.
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dimethyl sul®de oxidation in the marine boundary layer. Geophys. Res. Lett. 19, 1125±1127 (1992).
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DMS in the marine boundary layer. Atmos. Environ. 28, 2431±2449 (1994).
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oxidation kinetics. J. Atmos. Chem. 21, 187±221 (1995).
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Lagrangian B1: A moving column approach. J. Geophys. Res. 103, 16433 ±16455 (1998).
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Channel (Marine Management (Holdings) Ltd, London, 1994).
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Emission in the South Coast Air Basin: Assessment of the Proposed Federal Implementation Plan Ship Fee
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Acknowledgements. This work was supported by the US NSF and the NOAAOf®ce of Global Programs.
Correspondence and requests for materials should be addressed to S.N.P. (e-mail: spyros@andrew.
cmu.edu).
A pipiscid-like fossil
from the Lower Cambrian
of south China
D. Shu*, S. Conway Morris², X-L. Zhang*, L. Chen*,Y.Li*
& J. Han*
*Department of Geology, Northwest University, Xi'an, 710069,
People's Republic of China
²Department of Earth Sciences, University of Cambridge, Downing Street,
Cambridge CB2 3EQ, UK
.........................................................................................................................
Exceptional fossil preservation is critical to our understanding of
early metazoan evolution. A key source of information is the
Burgess Shale-type faunas1±5. Fossils from these deposits provide
important insights into metazoan phylogeny, notably that of
stem-group protostomes2,3,6, and related topics such as trophic
specialization7. Metazoan relationships are also being signi®-
cantly reappraised in terms of molecular-based phylogenies8,9,
but integration of these data with palaeontological systematics is
not straightforward10,11. Moreover, molecular phylogenies are
silent concerning the anatomies of stem-groups and the func-
tional transitions that underpin the origin of different body
plans2,6. Some hitherto enigmatic fossils possess unique charac-
ter± state combinations that, although they can be shoe-horned
into extinct phyla12, may be more pro®tably interpreted as de®n-
ing major stem-groups2,3. Here we describe a possible pipiscid, a
metazoan previously known only from the Upper Carboni-
ferous13,14, from the Lower Cambrian of south China. Pipiscids

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are currently interpreted as being agnathan chordates13± 15, but
this discovery from the Chengjiang fossil-Lagersta
Ètte indicates
that the assignment of pipiscids to the Agnatha deserves to be
reconsidered.
Phylum Uncertain
Xidazoon Shu, Conway Morris & Zhang gen. nov.
Xidazoon stephanus Shu, Conway Morris & Zhang sp. nov.
Etymology. Genus name an abbreviation of Chinese name for
Northwest University at Xi'an. Species name stephanos (Greek)
for crown.
Holotype. Early Life Institute, Northwest University, Xi'an. ELI-
0000194.
Stratigraphy and locality. Qiongzhusi (Chiungchussu) Formation,
Yu'anshan member (Eoredlichia Zone); Lower Cambrian. Specimen
collected from Haikou, Kunming, located about 50 km west of
Chengjiang.
Diagnosis. Body with two-fold division, reminiscent of Banf®a but
anterior section more in¯ated and possessing prominent mouth
circlet. Anterior section with faint transverse divisions towards
front, otherwise smooth. Mouth de®ned by circlet of about 25
plates, divided into inner and outer regions, otherwise unarmed.
Circlet similar to plated mouth of Pipiscius, although in the latter
taxon the plates are more cuticularized and inner circlet folded into
pharynx. Posterior section tapering towards front and back, seg-
mented with cuticularized region of about six segments succeeded
anteriorly by about three less well-de®ned segments. Posterior
section similar to arthropodan metameres, but lacking evidence
of appendages. Cuticular segments also reminiscent of posterior
region in Yunnanozoon, but in latter taxon segments are ventrally
incomplete. Short terminal spines at posterior tip. Alimentary canal
with terminal openings, anterior region possibly expanded and
rectum with ?dilator muscles.
Figure 1 The Cambrian fossil Xidazoon stephanus, new species and Carbonif-
erous ?agnathan Pipiscius zangerli.a, Entire specimen and (to lower left)
incomplete individual of Xidazoon (compare to Fig. 2); b, detail of posterior section
showing segmental divisions, gut trace, ?dilator muscles and posterior spines
(right-hand side); c, detail of feeding apparatus of complete specimen; d, detail
of anterior and incomplete feeding apparatus of second specimen. e, Entire
specimen of holotype of Pipiscius, part (PF 8345); f, detail of feeding apparatus of
part; g, detail of feeding apparatus of counterpart. Scale bars: 10mm (a,b,e),
5mm(c,d) and 2mm (f,g).

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Description. Xidazoon stephanus, new genus and species, is known
from two, or possibly three, specimens on a single slab (Figs 1a, 2).
The most complete specimen is about 8.5 cm long and a second
individual shows details of the anterior (Fig. 1d). The body
comprises two main regions. The anterior section is moderately
in¯ated, and the prominent circlet of the presumed anterior is
interpreted as a feeding apparatus surrounding a voluminous
mouth (Fig. 1c). The apparatus itself consists of plate-like struc-
tures, transversely folded to de®ne inner and outer circlets. The
edges of the inner circlet of plates are ridged (Fig. 1c), but they do
not bear teeth or other extensions. In the second specimen (Fig. 1d)
the plates appear to be separated adorally by narrower recessed
areas. These may represent ¯exible inter-plate membranes. The
anterior of the second specimen is incomplete, and that of the ®rst is
too crushed to give more than an estimate of the total number of
plates. The better-preserved half-circumference displays about 13
plates, and an allocation of typical plate width around the circum-
ference (,45 mm) gives a total of about 25. The mouth is gaping,
but apart from the circlet of plates lacks evidence of jaws or other
associated structures. Behind the feeding apparatus the anterior
region bears faint, widely separated transverse divisions that may be
segmental.
The posterior section tapers in either direction from an expanded
central zone. It consists of about six well-de®ned segments (Figs 1b,
2), and in the anterior direction there is a series of more faint
transverse annulations. The surface appears to have been lightly
cuticularized. The segment boundaries vary from tightly adpressed
to separated, indicating originally relatively wide and ¯exible inter-
segmental membranes. At the posterior tip there are two or three
spinose projections (Figs 1b, 2).
Little is known of the internal anatomy. A gut trace is present in
the mid and posterior sections, and near the terminal anus diver-
ging strands may represent dilator muscles (Figs 1a, b, 2). Towards
the anterior of the visible gut trace it appears to expand, and in the
anterior section it may have been voluminous.
Preservation. The style and quality of preservation is similar to
other Chengjiang taxa, such as Yunnanozoon16±19. Thus, the extent of
decay appears to be limited. Features, notably the circlet of plates
and the posterior segmentation, seem to be original rather than
post-mortem artefacts.
Ecology. The ecology of Xidazoon is problematic. It was presumably
benthic, with the anterior circlet periodically contracting to ingest
detritus. The in¯ated nature of the anterior section in the most
complete specimen might be because of sediment ingestion. An
alternative possibility is that the anterior organ acted as a sucker for
lodgement on prey or hard substrates.
Discussion. Comparisons between Xidazoon and various extant
metazoan groups, such as the sipunculans and the much smaller
cycliophorans, are not convincing. Similarly, among the diverse
Burgess Shale-type assemblages, no exact counterpart to Xidazoon
has been recognized. There are some similarities to the otherwise
enigmatic Banf®a confusa5, which consists of a segmented unit,
apparently posterior to an elongate and smooth section, but this
taxon lacks evidence for the prominent feeding apparatus of
Xidazoon. The better-known anomalocaridids20 have a prominent
circular feeding apparatus and a bipartite body with segmented
posterior section. There are, however, many differences. The feeding
apparatus occurs in a variety of forms3,5,20, but none is particularly
similar to Xidazoon. Other characteristic features of the anomalo-
caridids, notably the anterior giant appendages and lateral lobes,
have no parallel in this new fossil.
The anterior circlet of Xidazoon is, however, similar to the
otherwise unique feeding apparatus of the putative agnathan
Pipiscius zangerli (Fig. 1e), a rare species from the 300-Myr-old
Mazon Creek fossil-Lagersta
Ètte (Upper Carboniferous) of Illinois.
The original description13 is convoluted, but in essence the feeding
apparatus is composed of two circles of sclerotized plates. The inner
series (`collar lamellae' of ref. 13) total 23, a number with no
apparent parallel in other metazoan organ systems. The outer circlet
is also cited13 as consisting of 23 plates. There is, however, a hitherto
Figure 2 Camera-lucida drawing of slab containing the two (or possibly three) specimens of the Cambrian fossil Xidazoon stephanus, new species.

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unrecognized duplication on the leading anterior plate, so that the
total number of plates is effectively 24. This duplication de®nes a
line of bilateral symmetry in the apparatus. The plates are separated
by narrow clefts (`vanes' of ref. 13) that presumably accommodated
shape changes associated with feeding. The principal similarities
between the anterior apparatus of Pipiscius and Xidazoon are the
double nature of the circlet with direct continuity between the inner
and outer plates, the similar number of plates and evidence for
articulatory zones (Fig. 1d) that seem to be comparable to the
`vanes' (Fig. 1f). The apparatus, however, are not identical. In
Pipiscius the outer plates have a more complex structure, housing
triangular insets. These latter units may have accommodated move-
ment of the apparatus, possibly necessitated by a more pronounced
sclerotization. Deep pits associated with the `vanes', and possibly
employed for muscle insertions13, are not evident in Xidazoon.
Finally, the inner circle (`collar') of Pipiscius is directed inwards,
whereas in Xidazoon it appears to be more rim-like.
Concerning the possible connection between Xidazoon and
Pipiscius, there seems to be three alternative evolutionary scenarios.
First, the annular feeding apparatus is simply an example of
convergence. Among the many suctorial and other biological
attachment structures similarities can be shown, for example,
with the attachment organ of the ectoparasitic ciliate Trichodina
pediculus21 and the arm suckers of the octopus22, although no
phylogenetic connection with Xidazoon can be seriously enter-
tained. Notwithstanding the bi-annular arrangement of about 25
plates, the few similarities that otherwise exist between Xidazoon
and Pipiscius make convergence a reasonable option. Second,
Xidazoon and Pipiscius are related, but the assignment of the latter
taxon to the agnathans13,14 is erroneous: together they would
represent a new major Palaeozoic clade of as yet unknown af®nities.
In this sense it would be comparable to such enigmatic groups as the
typhloesids23 and tullimonstrids24.
The third proposal is that Xidazoon is a precursor to the
agnathans, including Pipiscius. This presupposes the homology of
the circular feeding apparatus in the two taxa, and that certain
features (such as ®n-rays and possible myotomes) of Pipiscius are
indicative of a chordate relationship. In this scenario Xidazoon
would potentially provide new insights into the organization of
stem-group deuterostomes. A link may also exist with the coeval
Yunnanozoon16± 19. This Chengjiang taxon displays putative gill slits,
and the cuticular segmentation has some similarities with Xidazoon.
One reconstruction18 of Yunnanozoon also depicts a circum-oral set
of plates. The bipartite nature of Xidazoon is more strongly devel-
oped than in Yunnanozoon, but the almost arthropod-like segmen-
ted posterior section could provide an intriguing phylogenetic link
with the protostomes25. Continuing investigations of Lower Cam-
brian fossil-Lagersta
Ètten may yield relatives of Xidazoon that will
help to resolve the controversial status of these fossils in the context
of metazoan phylogeny. M
Received 19 November 1998; accepted 2 July 1999
1. Conway Morris, S. The Crucible of Creation: The Burgess Shale and the Rise of Animals (Oxford Univ.
Press, Oxford, 1998).
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772±776 (1999).
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helminthes. Science 283, 1919± 1923 (1999).
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Acknowledgements. We thank the National Foundation of Natural Sciences of China, Minister of Science
and Technologyof China, Royal Society, National Geographic Society, and St John's College, Cambridge
for support. Access to Pipiscius was facilitated by P. Crane (Field Museum, Chicago), and M. P. Smith and
P. Donoghue (University of Birmingham). Technical assistance by S. J. Last and D. R. Simons is
acknowledged, as are critical comments by P. Janvier, D. B. Norman and S. Jensen.
Correspondence and requests for materials should be addressed to D.S. (e-mail: dgshu@sein.sxgb.
com.cn).
Environmental controls on the
geographic distribution
of zooplankton diversity
Scott Rutherford*, Steven D'Hondt*& Warren Prell²
*Graduate School of Oceanography, University of Rhode Island, Narragansett,
Rhode Island 02882, USA
²Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
.........................................................................................................................
Proposed explanations for the geographic distribution of zoo-
plankton diversity include control of diversity by geographic
variation in: physical and chemical properties of the near-surface
ocean1±3; the surface area of biotic provinces4; energy availability5;
rates of evolution and extinction6; and primary productivity7.
None of these explanations has been quantitatively tested on a
basin-wide scale. Here we used assemblages of planktic foraminifera
from surface sediments to test these hypotheses. Our analysis
shows that sea-surface temperature measured by satellite8
explains nearly 90% of the geographic variation in planktic
foraminiferal diversity throughout the Atlantic Ocean. Tempera-
tures at depths of 50, 100 and 150 m (ref. 9) are highly correlated to
sea-surface temperature and explain the diversity pattern nearly
as well. These ®ndings indicate that geographic variation in
zooplankton diversity may be directly controlled by the physical
structure of the near-surface ocean. Furthermore, our results
show that planktic foraminiferal diversity does not strictly
adhere to the model of continually decreasing diversity from
equator to pole. Instead, planktic foraminiferal diversity peaks
in the middle latitudes in all oceans.
We used the Brown University Foraminiferal Data Base (BFD, 33
species and 6 subspecies .150 mm; 1,252 samples) to document the
global pattern of planktic foraminiferal diversity and to evaluate the
long-standing hypothesis of a latitudinal diversity gradient. Weused
planktic foraminifera for this study because all of the living species
are known and are included in the BFD, unlike other plankton
groups (such as radiolaria or copepods), which have hundreds of
species and greater taxonomic uncertainty. Our results clearly show
that, throughout the world ocean, planktic foraminiferal diversity
peaks in middle latitudes, is lowest at high latitudes and is inter-