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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 νηκτόν, nekton]], meaning "swimmer" and καρίς, karis, "shrimp"). Two morphs are known: a small morph, about an inch long, and a large morph, anatomically identical but around four times longer.
Nectocaridids had controversial affinities before 2025. Some authors have suggested that they represent the earliest known Cephalopod. However, their morphology is strongly dissimilar to confirmed early cephalopods, and thus their affinities to cephalopods and even to Mollusca more broadly are rejected by many authors. Their affinities to any animal group beyond Bilateria were long considered uncertain, until a 2025 study, which interpreted them as relatives of modern Chaetognatha (arrow worms).
Because the genus was originally known from a single, incomplete specimen and with no Counter slab, Conway Morris was unable to deduce its affinity. It had some features which were reminiscent of , 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 . 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 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.
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.
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, 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 Vetusto dermis.
Fossils of Petalilium 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 muscle 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 Phyllocarida, and a ctenophore 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.
The 2025 study supported the synonymy of Nectocaris and Petalilium, but it was considered that this was a separate species ( N. latus) rather than N. pteryx.
Vetustovermis
Vetustovermis (from Latin: "very old worm") 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.
Petalilium
Petalilium (sometimes misspelled Petalium) is an enigmatic genus of Cambrian organism known from the Haikou area, from the Maotianshan mudstone member of the Chengjiang biota. The taxon is a junior synonym of Nectocaris pteryx.
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