Petalilium is an enigmatic genus of Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 organism of uncertain affinity known from the Haikou area, from the Maoshoiatan mudstone member of the Chengjiang biota.

Morphology

Fossils of Petalilium(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

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

 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 affinity. The upper part of the body, interpreted as a mantle

Mantle (mollusc)

The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

, 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.

Classification

Whilst it was originally described as a phyllocarid

Phyllocarida

Phyllocarida is a subclass of crustaceans, comprising the extant order Leptostraca and the extinct orders Hymenostraca and Archaeostraca....

, and a ctenophore

Ctenophore

The Ctenophora are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to in size...

 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 mollusks, a view supported by its similarity to the putative early cephalopod

Cephalopod

A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

 Nectocaris

Nectocaris

Nectocaris pteryx is a species of possible cephalopod or arthropod affinity, known from the Middle Cambrian Burgess Shale.Nectocaris was a free-swimming, predatory or scavenging organism, possibly occupying a niche similar to the arrow worms...

.

See also

• Cambrian explosion
  Cambrian explosion
  The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
  
  
• Chengjiang biota
  • List of Chengjiang Biota species by phylum