''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 ...

s, albeit this has been questioned because it seems to possess most of the characteristics attributed to craniates. It lacked fins and it had a weakly developed

cranium The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

, 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 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 ...

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

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...

. 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, livi ...

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 jaws 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 ar ...

. 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 verteb ...

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'', 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 Craniates 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, 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'' 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 ...

, who divided themselves into two main directions: jawless fishes like conodonts leading to the extant cyclostomes, and jawless fishes like the Cephalaspidomorph who developed armors and jaws to become the gnathostomes.

References

External links

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