Nectocaris

{{Speciesbox

| genus = Nectocaris

| species = pteryx

| fossil_range = {{Fossil range|Cambrian Stage 3|Wuliuan}}

| image = Nectocaris pteryx (ROM 60079).jpg

| image_caption = Specimen ROM 60079

| parent_authority = Conway Morris, 1976

| authority = Conway Morris, 1976

}}

Nectocaris is a genus of squid-like animal of controversial affinities known from the Cambrian period. The initial fossils were described from the Burgess Shale of Canada. Other similar remains possibly referrable to the genus are known from the Emu Bay Shale of Australia and Chengjiang Biota of China.

Nectocaris was a free-swimming, predatory or scavenging organism. This lifestyle is reflected in its binomial name: Nectocaris means "swimming shrimp" (from the Ancient Greek {{lang|el|νηκτόν}}, {{lang|el|nekton}}, meaning "swimmer" and {{lang|el|καρίς}}, {{lang|el|karis}}, "shrimp"). Two morphs are known: a small morph, about an inch long, and a large morph, anatomically identical but around four times longer.{{cite journal |last=Smith |first=M.R. |year=2013 |title=Nectocaridid ecology, diversity and affinity: Early origin of a cephalopod-like body plan |journal=Paleobiology |volume=39 |issue=2 |pages=291–321 |doi=10.1666/12029 |bibcode=2013Pbio...39..297S |s2cid=85744624}}

Nectocaridids have controversial affinities. Some authors have suggested that they represent the earliest known cephalopods. However, their morphology is strongly dissimilar to confirmed early cephalopods, and thus their affinities to cephalopods and even to molluscs more broadly are rejected by most authors.{{Cite journal |last1=Pohle |first1=Alexander |last2=Kröger |first2=Björn |last3=Warnock |first3=Rachel C. M. |last4=King |first4=Andy H. |last5=Evans |first5=David H. |last6=Aubrechtová |first6=Martina |last7=Cichowolski |first7=Marcela |last8=Fang |first8=Xiang |last9=Klug |first9=Christian |date=December 2022 |title=Early cephalopod evolution clarified through Bayesian phylogenetic inference |journal=BMC Biology |language=en |volume=20 |issue=1 |pages=88 |doi=10.1186/s12915-022-01284-5 |issn=1741-7007 |pmc=9008929 |pmid=35421982 |doi-access=free }} Their affinities to any animal group beyond Bilateria are uncertain, though they have been suggested to be members of Lophotrochozoa.

The closely related Ordovician taxon Nectocotis is a second genus, closely resembling Nectocaris, but suggested to have had an internal skeletal element.{{cite journal |last=Smith |first=Martin R. |year=2019 |title=An Ordovician nectocaridid hints at an endocochleate origin of Cephalopoda |journal=Journal of Paleontology |pages=64–69 |volume=94 |doi=10.1017/jpa.2019.57 |s2cid=201208912 |url=http://dro.dur.ac.uk/28457/1/28457.pdf}}

Anatomy

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|Description = Nectocaris pteryx from the Burgess Shale; funnel is visible folded to left of specimen. Image from Smith (2013).{{cite journal | doi=10.1666/12029 | title=Nectocaridid ecology, diversity, and affinity: Early origin of a cephalopod-like body plan | date=2013 | last1=Smith | first1=Martin R. | journal=Paleobiology | volume=39 | issue=2 | pages=297–321 | bibcode=2013Pbio...39..297S | s2cid=85744624 }}

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Nectocaris had a flattened, kite-shaped body with a fleshy fin running along the length of each side. The small head had two stalked eyes, a single pair of tentacles, and a flexible funnel-shaped structure opening out to the underside of the body. The funnel often gets wider away from the head. The funnel has been suggested to represent an eversible (able to be turned inside out) pharynx. Internally, a long cavity runs along the body axis, which is suggested to represent the digestive tract. The body contains a pair of gills; the gills comprise blades emerging from a zig-zag axis. Muscle blocks surrounded the axial cavity, and are now preserved as dark blocks in the lateral body. The fins also show dark blocks, with fine striations superimposed over them. These striations often stand in high relief above the rock surface itself.

Diversity

Although Nectocaris is known from Canada, China and Australia, in rocks spanning some 20 million years, there does not seem to be much diversity; size excepted, all specimens are anatomically very similar. Historically, three genera have been erected for nectocaridid taxa from different localities, but these 'species' – Petalilium latus and Vetustovermis planus – likely belong to the same genus or even the same species as N. pteryx. Within N. pteryx, there seem to be two discrete morphs, one large (~10 cm in length), one small (~3 cm long). These perhaps represent separate male and female forms.

Ecology

File:Nectocaris pteryx.jpg

The unusual shape of the nectocaridid funnel has led to its interpretation as an eversible proboscis. Martin R. Smith and Jean-Bernard Caron have suggested that it was used for jet propulsion, though this has been questioned by other authors. The eyes of Nectocaris would have had a similar visual acuity to modern Nautilus (if they lacked a lens) or squid (if they did not). They are thought to have been freely-swimming nektonic organisms, that were either scavengers or predators on soft-bodied animals, using their tentacles to manipulate food items.

Affinity

The affinity of Nectocaris is controversial. Martin R. Smith and Jean-Bernard Caron have suggested that nectocaridids represent early cephalopods. In a 2010 publication in Nature, they suggested that the ancestor of modern cephalopods and nectocaridids probably lacked a mineralised shell, while Smith in a later 2013 publication suggested that it may be more plausible that nectocaridids had instead lost a mineralised shell and developed a morphology convergent on modern coleoids. However, other authors contend that the morphology of nectocaridids is contrary to what is known about cephalopod and mollusc evolution, and they cannot be accommodated within these groups, and can only be confidently placed as members of Bilateria.{{Cite journal |last1=Kröger |first1=Björn |last2=Vinther |first2=Jakob |last3=Fuchs |first3=Dirk |date=August 2011 |title=Cephalopod origin and evolution: A congruent picture emerging from fossils, development and molecules |url=https://onlinelibrary.wiley.com/doi/10.1002/bies.201100001 |journal=BioEssays |language=en |volume=33 |issue=8 |pages=602–613 |doi=10.1002/bies.201100001|pmid=21681989 |s2cid=2767810 |url-access=subscription }}

History of study

File:USNM PAL 198667 Nectocaris pteryx Image 08.jpg|Holotype of Nectocaris, an incomplete specimen that gave rise to erroneous reconstructions

File:Nectocaris.jpg|Original (and obsolete) reconstruction based on a single, incompletely preserved, lateral specimen (left). The author of this reconstruction, based on the material then available, considered Nectocaris to bear arthropod and chordate-like features

Nectocaris has a long and convoluted history of study. Charles Doolittle Walcott, the discoverer of the Burgess Shale, had photographed the one specimen he had collected in the 1910s, but never had time to investigate it further. As such, it was not until 1976 that Nectocaris was formally described, by Simon Conway Morris.{{cite journal |author=Conway Morris, S. |year=1976 |title=Nectocaris pteryx, a new organism from the Middle Cambrian Burgess Shale of British Columbia |volume=12 |pages=703–713 |journal=Neues Jahrbuch für Geologie und Paläontologie, Monatshefte|author-link=Simon Conway Morris }}

Because the genus was originally known from a single, incomplete specimen and with no counterpart,{{cite book |author=Gould, S.J. |year=1989 |title=Wonderful Life: The Burgess Shale and the Nature of History |publisher=Hutchison Radius |isbn=978-0-09-174271-3 |bibcode=1989wlbs.book.....G |author-link=Stephen Jay Gould }} Conway Morris was unable to deduce its affinity. It had some features which were reminiscent of arthropods, but these could well have been convergently derived.{{cite journal |last=Waggoner |first=B.M. |year=1996 |title=Phylogenetic hypotheses of the relationships of Arthropods to Precambrian and Cambrian problematic fossil taxa |journal=Systematic Biology |volume=45 |issue=2 |pages=190–222 |jstor=2413615 |doi=10.2307/2413615|doi-access=free }} Its fins were very unlike those of arthropods.

Working from photographs, the Italian palaeontologist Alberto Simonetta believed he could classify Nectocaris within the chordates.{{cite journal |author=Simonetta, A.M. |year=1988 |title=Is Nectocaris pteryx a chordate? |journal=Bollettino di Zoologia |volume=55 |issue=1–2 |pages=63–68 |doi=10.1080/11250008809386601|doi-access=free }} He focussed mainly on the tail and fin morphology, interpreting Conway Morris's 'gut' as a notochord – a distinctive chordate feature.

The classification of Nectocaris was revisited in 2010, when Martin Smith and Jean-Bernard Caron described 91 additional specimens, many of them better preserved than the type. These allowed them to reinterpret Nectocaris as a primitive cephalopod, with only 2 tentacles instead of the 8 or 10 limbs of modern cephalopods. The structure previous researchers had identified as an oval carapace or shield behind the eyes{{cite journal |last=Conway Morris |first=Simon |year=1989 |title=Burgess Shale Faunas and the Cambrian Explosion |journal=Science |volume=246 |issue=4928 |pages=339–346 |bibcode=1989Sci...246..339C |pmid=17747916 |doi=10.1126/science.246.4928.339|s2cid=10491968 }} was suggested to be a soft funnel, similar to the ones used for propulsion by modern cephalopods. The interpretation would push back the origin of cephalopods by at least 30 million years, much closer to the first appearance of complex animals, in the Cambrian explosion, and implied that – against the widespread expectation – cephalopods evolved from non-mineralized ancestors.{{cite journal |last1=Smith |first1=M.R. |last2=Caron |first2=J.B. |year=2010 |title=Primitive soft-bodied cephalopods from the Cambrian |url= https://durham-repository.worktribe.com/output/1430329|journal=Nature |volume=465 |issue=7297 |pages=469–472 |bibcode=2010Natur.465..469S |doi=10.1038/nature09068 |pmid=20505727 |s2cid=4421029 |hdl-access=free |hdl=1807/32368 |df=dmy-all}}

Later independent analyses questioned the cephalopod interpretation, stating that it did not square with the established theory of cephalopod evolution, and that nectocaridids should be considered incertae sedis among Bilateria.{{cite journal |last1=Mazurek |first1=D. |last2=Zatoń |first2=M. |year=2011 |title=Is Nectocaris pteryx a cephalopod? |journal=Lethaia |volume=44 |issue=1 |pages=2–4 |doi=10.1111/j.1502-3931.2010.00253.x|bibcode=2011Letha..44....2M }}{{cite journal |last=Runnegar |first=B. |year=2011 |title=Once again: Is Nectocaris pteryx a stem-group cephalopod? |journal=Lethaia |volume=44 |issue=4 |page=373 |doi=10.1111/j.1502-3931.2011.00296.x|bibcode=2011Letha..44..373R }}

=''Vetustovermis''=

Vetustovermis (from Latin: "very old worm"){{cite journal |last=Glaessner |first=M.F. |year=1979 |title=Lower Cambrian Crustacea and annelid worms from Kangaroo Island, South Australia |journal=Alcheringa: An Australasian Journal of Palaeontology |volume=3 |issue=1 |pages=21–29 |doi=10.1080/03115517908565437|bibcode=1979Alch....3...21G }} is a soft-bodied middle Cambrian animal, known from a single reported fossil specimen from the South Australian Emu Bay shale. It is probably a junior synonym of Nectocaris pteryx.

The original description of Vetustovermis hedged its bets regarding classification, but tentatively highlighted some similarities with the annelid worms. It was later considered an arthropod,{{cite book |author1=Luo, H.-L. |author2=Hu, S.-X. |author3=Chen, L.-Z. |publisher=Yunnan Science & Technology Press |place=Kunming, China |year=1999 |title=Early Cambrian Chengjiang fauna from Kunming region, China}}{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/4156544.stm |title=Strange fossil defies grouping |publisher=BBC News}} and in 2010 Smith and Caron, agreeing that Petalilium was at least a close relative of Vetustovermis (but that treating it as a synonym was premature, given the poor preservation of the Vetustovermis type), placed it with Nectocaris in the clade Nectocarididae.

Early press reports misspelled the genus name as Vetustodermis.

=''Petalilium''=

File:Petalilium HK-MF-00001A 098.g2.jpg

Petalilium (sometimes misspelled Petalium){{cite journal |last1=Chen |first1=J.Y. |last2=Huang |first2=D.Y. |last3=Bottjer |first3=D.J. |year=2005 |title=An Early Cambrian problematic fossil: Vetustovermis and its possible affinities |journal=Proceedings of the Royal Society B |volume=272 |issue=1576 |pages=2003–2007 |pmid=16191609 |doi=10.1098/rspb.2005.3159 |pmc=1559895}} is an enigmatic genus of Cambrian organism known from the Haikou area,{{cite journal |last1=Steiner |first1=M. |last2=Zhu |first2=M. |last3=Zhao |first3=Y. |last4=Erdtmann |first4=B. |year=2005 |title=Lower Cambrian Burgess Shale-type fossil associations of South China |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=220 |issue=1–2 |pages=129–152 |bibcode=2005PPP...220..129S |doi=10.1016/j.palaeo.2003.06.001}} from the Maotianshan mudstone member of the Chengjiang biota.{{cite journal |last1=Han |first1=J. |last2=Shu |first2=D. |last3=Zhang |first3=Z. |last4=Liu |first4=J. |last5=Zhang |first5=X. |last6=Yao |first6=Y. |year=2006 |title=Preliminary notes on soft-bodied fossil concentrations from the Early Cambrian Chengjiang deposits |journal=Chinese Science Bulletin |volume=51 |issue=20 |page=2482 |doi=10.1007/s11434-005-2151-0 |bibcode=2006ChSBu..51.2482H|s2cid=129162009 }} The taxon is a junior synonym of Nectocaris pteryx.

Fossils of Petalilium{{efn|Petalilium was originally described as Vetustovermis by Chen et al. (2005), but recognised as Petalilium by Smith & Caron (2010).}} show a dorsoventrally flattened body, usually 5 to 6 centimetres, but ranging from 1.5 to 10 cm. It has an ovate trunk region and a large muscular foot, and a head with stalked eyes and a pair of long tentacles. The trunk region possesses about 50 soft, flexible, transverse bars, lateral serialised structures of unknown function. The upper part of the body, interpreted as a mantle, is covered with a random array of spines on the back, while gills project underneath. A complete, tubular gut runs the length of the body.

Whilst it was originally described as a phyllocarid, and a ctenophore affinity has been suggested,{{cite book |author1=Chen, L.Z. |author2=Luo, H.L. |author3=Hu, S.X. |author4=Yin, J.Y. |author5=Jiang, Z.W. |author6=Wu, Z.L. |author7=Li, F. |author8=Chen, A.L. |year=2002 |title=Early Cambrian Chengjiang Fauna in Eastern Yunnan, China |publisher=Yunnan Science and Technology Press |place=Kunming |pages=199 |language=Chinese, English}} neither interpretation is supported by any compelling evidence.{{cite journal |last1=Hu |first1=S. |last2=Steiner |first2= M. |last3=Zhu |first3=M. |last4=Erdtmann |first4=B.D. |last5=Luo |first5=H. |last6=Chen |first6=L. |last7=Weber |first7=B. |year=2007 |title=Diverse pelagic predators from the Chengjiang Lagerstätte and the establishment of modern-style pelagic ecosystems in the early Cambrian |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=254 |issue=1–2 |pages=307–316 |doi=10.1016/j.palaeo.2007.03.044 |bibcode=2007PPP...254..307H}}

Some of the characters observed in Chen et al.'s (2005) study suggested that Petalilium may be related to Nectocaris.