''Metaspriggina'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

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 ...

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 ...

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 ...

and 44 specimens found in 2012 at the

Marble Canyon Marble Canyon is the section of the Colorado River canyon in northern Arizona from Lee's Ferry to the confluence with the Little Colorado River, which marks the beginning of the Grand Canyon. Lee's Ferry is a common launching point for rive ...

bed in

Kootenay National Park Kootenay National Park is a national park of Canada located in southeastern British Columbia. The park consists of of the Canadian Rockies, including parts of the Kootenay and Park mountain ranges, the Kootenay River and the entirety of the V ...

Whilst named after the Ediacaran organism ''

Spriggina ''Spriggina'' is a genus of early bilaterian 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 fo ...

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

, possibly transitional between

cephalochordate A cephalochordate (from Greek: κεφαλή ''kephalé'', "head" and χορδή ''khordé'', "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called lancelets. Cephalochordates possess 5 synapomorphies, or pri ...

s and the earliest

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

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

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 m ...

s. 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. Fin ...

s and it had a weakly developed cranium, but it did possess two well-developed upward-facing eyes with

nostril A nostril (or naris , plural ''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 turbi ...

s behind them. ''Metaspriggina'' also possessed a notochord, along with seven pairs of pharyngeal bars, possibly made of cartilage. 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 Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...

ns". The largest specimens are in length. Originally believed to be free-swimming but occasionally found on the sea floor, 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 jaw is any opposable articulated structure 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 and serv ...

s of vertebrates. The second pair evolved to form the

hyoid arch The pharyngeal arches, also known as visceral arches'','' are structures seen in the embryonic development of vertebrates that are recognisable precursors for many structures. In fish, the arches are known as the branchial arches, or gill arc ...

. In vertebrates this supports the jaws and the hyoid bone 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 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 Simon Conway ...

''. 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 me ...

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

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

Agnatha Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclosto ...

) 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 m ...

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 (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...

, 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 ''Haikouella'' is an agnathan chordate from the Lower Cambrian Maotianshan Shales of Chengjiang County in Yunnan Province, China. An analysis in 2015 placed ''Haikouella'' as a junior synonym of ''Yunnanozoon,'' another Maotianshan shale Cambrian ...

'' 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, partic ...

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

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

leading to the extant

cyclostomes Cyclostome is a biological term (from the Greek for "round mouth") used in a few different senses: * for the taxon Cyclostomi, which comprises the extant jawless fishes: the hagfish (Myxini) and the lampreys (Petromyzontidae). This was thought fo ...

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

gnathostomes Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...

References

External links

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