''Metaspriggina'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

initially known from two specimens in the

Middle Cambrian Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (di ...

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 fos ...

and 44 specimens found in 2012 at the Marble Canyon bed in Kootenay National Park. Metaspriggina NT small

Whilst named after the Ediacaran organism ''

Spriggina ''Spriggina'' is a genus of early animals whose relationship to living animals is unclear. Fossils of ''Spriggina'' are known from the late Ediacaran period in what is now South Australia. ''Spriggina floundersi'' is the official fossil emblem o ...

'', later work has shown the two to be unrelated. ''Metaspriggina'' is considered to represent a primitive

, possibly transitional between

cephalochordate The lancelets ( ), also known as amphioxi (: amphioxus ), consist of 32 described species of somewhat fish-like benthic filter feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae. Lancelets div ...

s and the earliest

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s, albeit this has been questioned because it seems to possess most of the characteristics attributed to craniates. It lacked

fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. F ...

s and had a weakly developed

cranium The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

, but it did possess two well-developed, upward-facing

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

s with

nostril A nostril (or naris , : nares ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbinates ...

s behind them. ''Metaspriggina'' also possessed a

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

, along with seven pairs of pharyngeal bars, possibly made of

cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...

. Surprisingly, they were not formed from a singular bone, but they were formed of multiple separate pairs of bones, along with first two of them that were enlarged compared to the others and that seemed to not support any gills, all of these characteristics suggesting a "distant link to

gnathostomata Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant vertebrates, including all living bony fishes (both ray-finned ...

ns". The largest specimens are in length. Originally believed to be free-swimming but occasionally found on the

sea floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

, the fossils from Marble Canyon showing the presence of eyes and their placement suggests it lived as a filter-feeder swimming above the sea floor. The exceptional preservation at Marble Canyon also preserved muscle detail, showing that the animal moved with a side-to-side swimming motion. In ''Metaspriggina'', the myomeral configuration has an additional ventral chevron, and a clear dorsal bend which defines a W-shaped arrangement that is directly comparable to fish. The discovery of pharyngeal bars (gill bars) makes ''Metaspriggina'' the oldest known animal to have this feature. The first pair of pharyngeal bars later evolved to form the upper and lower

jaw The jaws are a pair of opposable articulated structures at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth ...

s of vertebrates. The second pair evolved to form the

hyoid arch The pharyngeal arches, also known as visceral arches'','' are transient structures seen in the embryonic development of humans and other vertebrates, that are recognisable precursors for many structures. In fish, the arches support the gills a ...

. In vertebrates this supports the jaws and the

hyoid bone The hyoid-bone (lingual-bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid-cartilage. At rest, it lies between the base of the mandible and the third cervical verte ...

anchors the base of the tongue. The discovery of ''Metaspriggina'' makes the origins of gnathostomatans a little more confusing, as it was roughly contemporary with ''

Pikaia ''Pikaia gracilens'' is an extinct, primitive chordate marine animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Sim ...

''. As ''Pikaia'' did not have gill bars, unlike ''

Branchiostoma ''Branchiostoma'' is one of the few living genera of lancelets ( order Amphioxiformes). It is the type genus of family Branchiostomatidae. These small vaguely eel- or snake-like animals are close relatives of vertebrates. The scientific name m ...

'', there are two possible explanations for this. One is that the

Chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

phylum split in four before ''Metaspriggina'' lived, with ''Metaspriggina'' and the other craniates (both gnathostomatans and

Agnatha Agnatha (; ) or jawless fish is a paraphyletic infraphylum of animals in the subphylum Vertebrata of the phylum Chordata, characterized by the lack of jaws. The group consists of both extant taxon, living (Cyclostomi, cyclostomes such as hagfish ...

) grouped with ''Branchiostoma'' and the cephalochordates, and ''Pikaia'' out on a side branch. ''Metaspriggina'' is here a direct ancestor of all gnathostomatans, with the Agnatha the most closely related group. In this explanation, ''Pikaia'' is not a close relative of

Craniate A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the mu ...

s at all, nor of cephalochordates, but something even more primitive, and the defining feature of the craniate-cephalochordate group is their gill bars. The other explanation is that ''Metaspriggina'' was the ancestor of all gnathostomatans, again closely related to the Agnatha to form the Chordata. However, instead of being a very primitive relative, ''Pikaia'' was the ancestor of all cephalochordates, and the gill bars evolved convergently in them somewhere between ''Pikaia'' and ''Branchiostoma.'' This might explain why ''Branchiostoma'' has such a different number of gill bars to chordates (or at least their embryos). Considering that

conodonts Conodonts, are an extinct group of marine jawless vertebrates belonging to the Class (biology), class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning "cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known ...

, the teeth elements of a type of extinct fish belonging to the Agnatha, are already found in Cambrian stage 2 (521-529 MA BP), some 20 million years before the Burgess shale, this latter explanation does not stand. The split between Agnatha and other chordates must therefore have happened earlier, during Cambrian stage 1 or even Ediacaran times. Phylogenetic analysis suggests that ''Metaspriggiidae'' are "stem-vertebrates" along with ''

Haikouella ''Yunnanozoon lividum'' (Yunnan + Greek ζῷον ''zôion'' (animal), with species name Latin ''lividum''; (lead-coloured), referring to preserved colour of specimens) is an extinct species of bilaterian animal from the Lower Cambrian Chengj ...

'' and the Myllokunmingiids leading to the

crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, parti ...

vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

, who divided themselves into two main directions: jawless fishes like

leading to the

Cyclostomi Cyclostomi, often referred to as Cyclostomata , from Ancient Greek κύκλος (kúklos), meaning "circle", and στόμα (stóma), meaning "mouth", is a superclass of vertebrates that comprises the living jawless fish classes: the lamp ...

, and jawless fishes like the Cephalaspidomorph who developed armors and jaws to become the

gnathostomes Gnathostomata (; from Ancient Greek: (') 'jaw' + (') 'mouth') are jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant vertebrates, including all living bony fishes (both ray-finned a ...

References

External links

* * {{Taxonbar, from=Q140492 Cambrian chordates Burgess Shale fossils Prehistoric jawless fish genera Cambrian genus extinctions

See also

References

External links