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''Nectocaris'' is a genus of squid-like animal of controversial affinities known from the
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
period. The initial fossils were described from the
Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fo ...
of Canada. Other similar remains possibly referrable to the genus are known from the
Emu Bay Shale The Emu Bay Shale is a geological formation in Emu Bay, South Australia, containing a major Konservat-Lagerstätte (fossil beds with soft tissue preservation). It is one of two in the world containing Redlichiidan trilobites. The Emu Bay Shale ...
of Australia and
Chengjiang Biota The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...
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 Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...
, ', meaning "swimmer" and , ', "shrimp"; , ', means "wing"). Two morphs are known: a small morph, about an inch long, and a large morph, anatomically identical but around four times longer. Nectocaridids have controversial affinities. Some authors have suggested that they represent the earliest known
cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...
. However, their morphology is strongly dissimilar to confirmed early cephalopods, and thus their affinities to cephalopods and even to
molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...
more broadly are rejected by most authors. Their affinities to any animal group beyond
Bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
are uncertain, though they have been suggested to be members of
Lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, bra ...
. The closely related Ordovician taxon '' Nectocotis'' is a second genus, closely resembling ''Nectocaris'', but suggested to have had an internal skeletal element.


Anatomy

''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 In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mainl ...
, 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 The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
. Internally, a long cavity runs along the body axis, which is suggested to represent the digestive tract. The body contains a pair of
gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s; 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

The unusual shape of the nectocaridid funnel has led to its interpretation as an eversible
proboscis A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular mouthparts used for feeding and sucking. In vertebrates, a proboscis is an elong ...
. Martin R Smith has suggested that it was used
jet propulsion Jet propulsion is the propulsion of an object in one direction, produced by ejecting a jet of fluid in the opposite direction. By Newton's third law, the moving body is propelled in the opposite direction to the jet. Reaction engines operatin ...
, though this has been rejected by other authors. The eyes of ''Nectocaris'' would have had a similar visual acuity to modern ''
Nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in ...
'' (if they lacked a lens) or
squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...
(if they did not). They are thought to have been freely-swimming
nektonic Nekton or necton (from the ) refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms t ...
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 has suggested that nectocaridids represent highly
derived Derive may refer to: *Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguation ...
early cephalopods that lost their shell and developed a morphology convergent on modern coleoids. However, most 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. It has been suggested that a "more likely hypothesis is that ''Nectocaris'' represents an independent lineage within the
Lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, bra ...
, which developed a mode of life remarkably similar to cephalopods."


History of study

''Nectocaris'' has a long and convoluted history of study.
Charles Doolittle Walcott Charles Doolittle Walcott (March 31, 1850February 9, 1927) was an American paleontologist, administrator of the Smithsonian Institution from 1907 to 1927, and director of the United States Geological Survey. Wonderful Life (book) by Stephen Jay G ...
, 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 Simon Conway Morris (born 1951) is an English palaeontologist, evolutionary biologist, and astrobiologist known for his study of the fossils of the Burgess Shale and the Cambrian explosion. The results of these discoveries were celebrated in ...
. Because the genus was originally known from a single, incomplete specimen and with no counterpart, Conway Morris was unable to deduce its affinity. It had some features which were reminiscent of
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s, but these could well have been convergently derived. Its fins were very unlike those of arthropods. Working from photographs, the Italian palaeontologist Alberto Simonetta believed he could classify ''Nectocaris'' within the
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
s. He focussed mainly on the tail and fin morphology, interpreting Conway Morris's 'gut' as a
notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consi ...
– 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 A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
, with only 2 tentacles instead of the 8 or 10 limbs of modern cephalopods. The structure previous researchers had identified as an oval
carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...
or shield behind the eyes 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 The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil record. ...
, and implied that – against the widespread expectation – cephalopods evolved from non-mineralized ancestors. 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 ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...
'' among
Bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
.


''Vetustovermis''

''Vetustovermis'' (from Latin: "very old worm") is a soft-bodied middle Cambrian
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
, known from a single reported fossil specimen from the South Australian
Emu Bay shale The Emu Bay Shale is a geological formation in Emu Bay, South Australia, containing a major Konservat-Lagerstätte (fossil beds with soft tissue preservation). It is one of two in the world containing Redlichiidan trilobites. The 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 The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...
worms. It was later considered an
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
, 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 A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
Nectocarididae ''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 ...
. Early press reports misspelled the genus name as ''Vetustodermis''.


''Petalilium''

''Petalilium'' (sometimes misspelled ''Petalium'') is an enigmatic genus of
Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
organism known from the Haikou area, from the Maoshoiatan mudstone member of the
Chengjiang biota The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...
. The taxon is a junior synonym of ''Nectocaris pteryx''. Fossils of ''Petalilium'' show a
dorsoventrally Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
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 Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
foot, and a head with stalked eyes and a pair of long tentacles. The trunk region possesses about 50 soft, flexible,
transverse Transverse may refer to: *Transverse engine, an engine in which the crankshaft is oriented side-to-side relative to the wheels of the vehicle * Transverse flute, a flute that is held horizontally * Transverse force (or ''Euler force''), the tange ...
bars, lateral serialised structures of unknown function. The upper part of the body, interpreted as a
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
, 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 Phyllocarida is a subclass of crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipe ...
, and a
ctenophore Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), an ...
affinity has been suggested, neither interpretation is supported by any compelling evidence. Some of the characters observed in Chen ''et al''.'s (2005) study suggested that ''Petalilium'' may be related to ''Nectocaris''.


See also

*
Cambrian explosion The Cambrian explosion, Cambrian radiation, Cambrian diversification, or the Biological Big Bang refers to an interval of time approximately in the Cambrian Period when practically all major animal phyla started appearing in the fossil record. ...
*
Chengjiang biota The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...
**
List of Chengjiang Biota species by phylum This is a list of fossils found at Maotianshan Shales, whose most famous assemblage of organisms are referred to as the Chengjiang biota. Phylum Arthropoda 80 species, not counting Dinocarida, Nektaspida and Trilobita * '' Acanthomerid ...


Footnotes


References


Further reading

* – 3D animations are available and a more detailed consideration of ''Nectocaris'' * – Brian Switek discusses the taxonomy and history of ''Nectocaris'' in his blog * –
BBC News BBC News is an operational business division of the British Broadcasting Corporation (BBC) responsible for the gathering and broadcasting of news and current affairs in the UK and around the world. The department is the world's largest broadc ...
coverage of the Smith & Caron's (2010) re-description * – a blog article supporting Mazurek & Zatoń's (2011) view *


External links

* {{Taxonbar, from=Q132992 Nectocarididae Burgess Shale animals Cambrian Series 2 first appearances Miaolingian extinctions Maotianshan shales fossils Prehistoric cephalopod genera Prehistoric invertebrates of Oceania Controversial taxa Fossil taxa described in 1976 Emu Bay Shale Cambrian genus extinctions