Pikaia is an extinct animal known from the Middle Cambrian fossil found near Mount Pika in the Burgess Shale of British Columbia. It was discovered by Charles Walcott and was first described by him in 1911. Based on the obvious and regular segmentation of the body, Walcott classified it as a polychaete worm. It resembles a living chordate commonly known as the lancelet and perhaps swam much like an eel.
During his re-examination of the Burgess Shale fauna in 1979, paleontologist Simon Conway Morris placed P. gracilens in the chordates, making it perhaps the oldest known ancestor of modern vertebrates, because it seemed to have a very primitive, proto-notochord. Further, the status of Pikaia as a chordate is not universally accepted; its preservational mode suggests that it had cuticle, which is uncharacteristic of the vertebrates;[1] further, its tentacles are unknown from other vertebrate lineages.[1] The presence of earlier vertebrates in the Chengjiang, including Haikouicthys and Myllokunmingia, appears to show that cuticle is not necessary for preservation, overruling the taphonomic argument,[2] but the presence of tentacles is still intriguing, and the organism cannot be conclusively assigned even to the vertebrate stem group.[3]
Averaging about 1½ inches (5 cm) in length, Pikaia swam above the sea floor using its body and an expanded tail fin. Pikaia may have filtered particles from the water as it swam along.Template:Fact Its "tentacles" may be comparable to those in the present-day hagfish, a jawless chordate.Template:Fact Only 60 specimens have been found to date.
Description[]
Pikaia was a primitive chordate that lacked a well-defined head and averaged about 1+1⁄2 inches (38 mm) in length. Once thought to be closely related to the ancestor of all vertebrates, it has for that reason received particular attention among the multitude of animal fossils found in the famous Burgess Shale in the mountains of British Columbia, Canada. Pikaia had a pair of large, antenna-like tentacles on its head, and a series of short appendages, which may be linked to gill slits, on either side of its head. In these ways, it differs from the modern lancelet. The "tentacles" on its head may be comparable to those in the present-day hagfish, a jawless chordate.
Until recently, there was no comprehensive account of Pikaia's anatomy. However, Conway Morris and Caron (2012, see further reading) have now published an exhaustive description based on all 114 of the known fossils. They found some new and unexpected characteristics and considered most of these to be primitive features of the first chordate animals. On the basis of these findings, they built a new scenario for chordate evolution. Subsequently, Mallatt and Holland (2013, see further reading) summarized the entire history of Pikaia research, considered Conway Morris and Caron's descriptions, and concluded that many of the newly recognized characters are unique specializations (already divergent in the Cambrian) not helpful for establishing Pikaia as a basal chordate.
When alive, Pikaia was a compressed, leaf-shaped animal. It swam by throwing its body into a series of S-shaped, zigzag curves, similar to the movement of eels. Fish inherited the same swimming movement, but they generally have stiffer backbones. Pikaia had a pair of large head tentacles and a series of short appendages, which may be linked to gill slits, on either side of its head. In these ways, it differs from the living lancelet.
Although primitive, Pikaia shows the essential prerequisites for vertebrates. When alive, Pikaia was a compressed, leaf-shaped animal with an expanded tail fin; the flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail. It likely swam by throwing its body into a series of S-shaped, zigzag curves, similar to the movement of eels; fish inherited the same swimming movement, but they generally have stiffer backbones. These adaptations may have allowed Pikaia to filter particles from the water as it swam along. Pikaia was probably a slow swimmer, since it lacked the fast-twitch fibers that are associated with rapid swimming in modern chordates.
Conway Morris and Caron (2012) published an exhaustive description based on all 114 of the known fossil specimens; they discovered new and unexpected characteristics[which?] that they recognized as primitive features of the first chordate animals. On the basis of these findings, they constructed a new scenario for chordate evolution. Subsequently, Mallatt and Holland reconsidered Conway Morris and Caron's description, and concluded that many of the newly recognized characters are unique, already-divergent specializations that would not be helpful for establishing Pikaia as a basal chordate.
Discovery[]

Fossil of Pikaia gracilens (Syntype USNM PAL 57628)
In 1911, Charles Walcott and colleagues were digging in the Burgess Shale deposit in British Columbia, when they came across the fossil of a previously unrecognized creature. Walcott named it Pikaia gracilens, after the area "Pikaia's Peak" where the creature was discovered. At first, no one could tell exactly what genus it belonged in, but Walcott placed as a polychaete worm, since it greatly resembles a modern-day lancelet worm.
However, reexamination in 1979 by Simon Conway Norris made this creature more significant than previously thought. Norris discovered a very primitive, proto-notochord, which led him to believe Pikaia was in fact a chordate, and the earliest known ancestor to all vertebrates. This isn't universially accpeted, though, because its tentacles and cuticle aren't like anything found in modern vertebrates. The notochord is very intriguing, though, and whether or not Pikaia is a vertebrate ancestor or not is a very controversial debate in the paleontological field. About 114 specimens of this strange vertebrate have been found. The discovery of Pikaia also has lead to a controversy as to how diverse life was back in even the Pre-Cambrian period. [4]
Averaging about 1 1⁄2 inches (3.8 cm) in length, Pikaia swam above the sea floor using its body and an expanded tail fin. Pikaia may have filtered particles from the water as it swam along.[citation needed] Its "tentacles" may be comparable to those in the present-day hagfish, a jawless chordate.[citation needed] Only 60 specimens have been found to date.
Evolution[]
Much debate on whether Pikaia is a vertebrate ancestor, its worm-like appearance notwithstanding, exists in scientific circles. It looks like a worm that has been flattened sideways (lateral compression). The fossils compressed within the Burgess Shale show chordate features such as traces of an elongate notochord, dorsal nerve cord, and blocks of muscles (myotomes) down either side of the body – all critical features for the evolution of the vertebrates.
The notochord, a flexible rod-like structure that runs along the back of the animal, lengthens and stiffens the body so that it can be flexed from side to side by the muscle blocks for swimming. In the fish and all subsequent vertebrates, the notochord forms the backbone (or vertebral column). The backbone strengthens the body, supports strut-like limbs, and protects the vital dorsal nerve cord, while at the same time allowing the body to bend.
A Pikaia lookalike, the lancelet Branchiostoma, still exists today. With a notochord and paired muscle blocks, the lancelet and Pikaia belong to the chordate group of animals from which the vertebrates descended. Molecular studies have refuted earlier hypotheses that lancelets might be the closest living relative to the vertebrates, instead favoring tunicates in this position; other extant and fossil groups, such as acorn worms and graptolites, are more primitive.
The presence of a creature as complex as Pikaia some 530 million years ago reinforces the controversial view that the diversification of life must have extended back well before Cambrian times – perhaps deep into the Precambrian. The discovery of Metaspriggina, a primitive fish of similar age which seems to have already started to develop a jaw and the presence of conodonts, the teeth of an extinct type of fish belonging to the Agnatha some 20 MA before Pikaia, does provide support for that view.
Appearance[]
Pikaia is an elongated creature with a small anterior region bearing the head. The flattened body is divided into a series of segmented myomeres or muscle bands by S-shaped lines that lie on either side of the backbone, which runs the entire length of the body. Out from the head grows two equal lobes and a pair of short and slender tentacle-like structures but no evidence of eyes. Twelve pairs of small, short, pointed, gill-like structures grow on either side of the midline on the ventral side, and a ventral fin seems to be present on the posterior. Their gut is narrow, and their anus ends the body.
Behavior[]
Pikaia gracilens likely swam near the sea floor by undulating its body like a snake. They were deposit feeders, meaning they obtain nutrients from organic matter in the sediments.
Institutions[]
Most fossils are held in trust for Parks Canada at the Royal Ontario Museum, while the remainder are found in the Smithsonian Museum of Natural History in Washington, DC.
Paleobiology[]
Pikaia is a very peculiar-looking creature. At first glance, it'd appear to resemble a lancelet worm, but closer examination revealed that in fact had a notochord, which is a dorsal nerve chord that goes all the way down its back. This has lead many scientists to believe that this is the blue-print for a backbone, and this would make Pikaia the earliest known ancestor of all modern vertebrates, from fish to mammals, birds, reptiles, and amphibians. It had a flat body, making it look almost like a species of flatworm. Its tail ended in an almost fin-like structure, and probably allowed it to swim much like an eel. [5] Pikaia also had strange, tentacle-like appendages on its "head", and most scientists believe that this eventually lead to gills and fins in fish. Its body also has faint rows of segmented muscle blocks that go all the way down its body. It likely lived in the deep oceans during the Cambrian period and was constantly on the look-out for predators such as Anomalocaris. [6][7]
JPInstitute.com Description[]
An important little creature in the history of life on Earth, Pikaia is the first known living thing with a backbone. Having this feature, it is possibly the ancestor of all vertebrate life on Earth, including humans. It was only about two inches long and lived in the sea 550 million years ago. It looked like a small fish without fins. Not too much is known about this creature. It is known from a very fossil rich formation of shale rocks in Canada that are called the Burgess Shale.
References[]
- ↑ 1.0 1.1 Template:Citation
- ↑ Template:Citation
- ↑ Template:Citation
- ↑ http://bioteaching.wordpress.com/2012/10/16/pikaia-one-of-the-earliest-chordates/
- ↑ http://paleobiology.si.edu/burgess/pikaia.html
- ↑ http://dinosaurs.about.com/od/tetrapodsandamphibians/p/pikaia.htm
- ↑ http://burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=101&ref=i&