Nectocaris

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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 {{#invoke:Lang|lang}}, {{#invoke:Lang|lang}}, meaning "swimmer" and {{#invoke:Lang|lang}}, {{#invoke:Lang|lang}}, "shrimp"). Two morphs are known: a small morph, about an inch long, and a large morph, anatomically identical but around four times longer.<ref name="Smith2013c">Template:Cite journal</ref>

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.<ref name=":0">Template:Cite journal</ref><ref name=":1" /> Their affinities to any animal group beyond Bilateria are uncertain, though they have been suggested to be members of Lophotrochozoa.<ref name=":1" />

The closely related Ordovician taxon Nectocotis is a second genus, closely resembling Nectocaris, but suggested to have had an internal skeletal element.<ref name=Smith2019>Template:Cite journal</ref>

AnatomyEdit

Template:Css Image Crop Nectocaris had a flattened, kite-shaped body with a fleshy fin running along the length of each side.<ref name=SmithAndCaron2010/> 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.<ref name=SmithAndCaron2010/> The funnel often gets wider away from the head.<ref name="SmithAndCaron2010" /> The funnel has been suggested to represent an eversible (able to be turned inside out) pharynx.<ref name=":1" /> Internally, a long cavity runs along the body axis, which is suggested to represent the digestive tract.<ref name=":1" /> 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.<ref name="Smith2013c"/> The fins also show dark blocks, with fine striations superimposed over them. These striations often stand in high relief above the rock surface itself.<ref name="Smith2013c"/>

DiversityEdit

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.<ref name="Smith2013c"/>

EcologyEdit

File:Nectocaris pteryx.jpg

Life restoration

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,<ref name="SmithAndCaron2010" /><ref name="Smith2013c"/> though this has been questioned by other authors.<ref name="Runnegar2011" /> 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).<ref name="Smith2013c"/> They are thought to have been freely-swimming nektonic organisms,<ref name=":1" /> that were either scavengers or predators on soft-bodied animals, using their tentacles to manipulate food items.<ref name="SmithAndCaron2010" />

AffinityEdit

The affinity of Nectocaris is controversial.<ref name=Runnegar2011/><ref name=":1" /> 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,<ref name="SmithAndCaron2010" /> 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.<ref name="Smith2013c"/> 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,<ref name=":0" /><ref name=":1" /><ref name="Runnegar2011" /> and can only be confidently placed as members of Bilateria.<ref name=":1">Template:Cite journal</ref>

History of studyEdit

USNM PAL 198667 Nectocaris pteryx Image 08.jpg

Holotype of Nectocaris, an incomplete specimen that gave rise to erroneous reconstructions
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.<ref name=SCM1976>Template:Cite journal</ref>

Because the genus was originally known from a single, incomplete specimen and with no counterpart,<ref name="WonderfulLife">Template:Cite book</ref> 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.<ref name=SCM1976/><ref name="Waggoner1996">Template:Cite journal</ref> Its fins were very unlike those of arthropods.<ref name=SCM1976/>

Working from photographs, the Italian palaeontologist Alberto Simonetta believed he could classify Nectocaris within the chordates.<ref name=Simonetta1988>Template:Cite journal</ref> He focussed mainly on the tail and fin morphology, interpreting Conway Morris's 'gut' as a notochord – a distinctive chordate feature.<ref name=Simonetta1988/>

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<ref name=SCM1989>Template:Cite journal</ref> 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.<ref name="SmithAndCaron2010">Template:Cite journal</ref>

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.<ref name="Mazurek2011">Template:Cite journal</ref><ref name="Runnegar2011">Template:Cite journal</ref><ref name=":1" /><ref name=":0" />

VetustovermisEdit

Vetustovermis (from Latin: "very old worm")<ref name=Glaessner1979>Template:Cite journal</ref> 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.<ref name="Smith2013c"/>

The original description of Vetustovermis hedged its bets regarding classification, but tentatively highlighted some similarities with the annelid worms.<ref name=Glaessner1979/> It was later considered an arthropod,<ref name=Luo1999>Template:Cite book</ref><ref>Template:Cite news</ref> 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.<ref name="SmithAndCaron2010"/>

Early press reports misspelled the genus name as Vetustodermis.

PetaliliumEdit

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

Fossil of Petalilium

Petalilium (sometimes misspelled Petalium)<ref name="Chen2005">Template:Cite journal</ref> is an enigmatic genus of Cambrian organism known from the Haikou area,<ref name=Steiner2005>Template:Cite journal</ref> from the Maotianshan mudstone member of the Chengjiang biota.<ref name='Han2006'>Template:Cite journal</ref> The taxon is a junior synonym of Nectocaris pteryx.<ref name="Smith2013c"/>

Fossils of PetaliliumTemplate:Efn 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,<ref name=Luo1999/> and a ctenophore affinity has been suggested,<ref>Template:Cite book</ref> neither interpretation is supported by any compelling evidence.<ref name='Hu2007'>Template:Cite journal</ref>

Some of the characters observed in Chen et al.'s (2005) study<ref name="Chen2005"/> suggested that Petalilium may be related to Nectocaris.<ref name="SmithAndCaron2010"/>