Pikaia
Pikaia | |
---|---|
Life reconstruction of Pikaia gracilens | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Family: | †Pikaiidae |
Genus: | †Pikaia Walcott, 1911 |
Species: | †P. gracilens
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Binomial name | |
†Pikaia gracilens Walcott, 1911
|
Pikaia gracilens is an extinct,
The body structure resembles that of the lancelet and it swam perhaps much like an eel. A notochord and myomeres (segmented blocks of skeletal muscles) span the entire length of the body, and are considered the defining signatures of chordate characters. Its primitive nature is indicated by the body covering, a cuticle, which is characteristic of invertebrates and some protochordates.
The exact phylogenetic position is unclear. Proposed affinities include those of
Discovery
The fossils of Pikaia gracilens was discovered by
University of Cambridge palaeontologist Harry B. Whittington and his student Simon Conway Morris re-examined the Burgess Shale fauna and noted the anatomical details of Pikaia for the first time. The fossil specimens bears features of notochord and muscle blocks that are fundamental structures of chordates, and not of annelids. In 1977, Conway Morris presented a paper that indicated the possible chordate position, without further explanation.[16][17] He and Whittington were convinced that the animal was obviously a chordate, as they wrote in Scientific American in 1979:
Finally, we find among the Burgess Shale fauna one of the earliest-known invertebrate representatives of our own conspicuous corner of the animal kingdom: the chordate phylum... The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P. gracilens.[18]
Conway Morris formally placed P. gracilens among the chordates in a paper in the Annual Review of Ecology and Systematics that same year.[19] However, he provided no structural analyses such as using microscopes to confirm the chordate features. The comparative description only earned a "putative" chordate status.[20] The fossil's chordate nature was received sceptically for several decades.[21][22][23] Only in 2012, when detailed analysis was reported by Conway Morris and Jean-Bernard Caron, that the chordate position became generally accepted.[12][24]
The fossils are found only in a restricted series of horizons in the strata exposed on Fossil Ridge, close to the Yoho National Park. From the same location, other fish-like animal fossils named Metaspriggina were discovered in 1993. Conway Morris identified the animals as another Cambrian chordate.[25] The fossil specimens are preserved in the Smithsonian Institution and the Royal Ontario Museum.[3]
Description
Pikaia has a lancelet-like body, tapering at both ends, laterally flat and lacked a well-defined head. It measures an average of about 1+1⁄2 inches (3.8 cm) in length. Walcott recorded the longest individuals as 5 cm (2.0 in) in length.[15] Pikaia has a pair of large, antenna-like tentacles on its head that resembles those of invertebrates such as snails.[23] The attachment of the tentacles makes a two-lobed structure of the head.[26] The tentacles may be comparable to those in the present-day hagfish, a jawless chordate.[27] It has a small circular mouth that could be used to eat small food particles in a single bite. There are a series of short appendages on either side of the underside of the head just after the mouth, and their exact nature or function is unknown. The pharynx is associated with six pairs of slits with tiny filaments that could be used for respiratory apparatus.[28] In these ways, it differs from the modern lancelets, which have distinct pharyngeal gill slits on either sides of the pharynx and are used for filter feeding.[29]
A major primitive structure of Pikaia is a
Pikaia was an active and free swimmer.[15] 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.[27] Pikaia was probably a slow swimmer, since it lacked the fast-twitch fibers that are associated with rapid swimming in modern chordates.[27]
Reinterpretations
Walcott's original summary of the description of Pikaia reads:
Body elongate, slender, and tapering at each end. It is formed of many segments that are defined by strong annular shiny lines. Head small with two large eyes and two tentacles... Back of the head the first five segments carry short parapodia that appear to be divided into two parts. The enteric canal extends from end to end without change in character... This was one of the active, free-swimming annelids that suggest the Nephthydidae of the Polychaeta.
digestive tract, it runs along with a stiff rod that resembles a notochord. They reported in 1979: "Although Pikaia differs from Amphioxus, in several important respects, the conclusion is that it is not a worm but a chordate appears inescapable."[18]Conway Morris was convinced that the longitudinal rod was a notochord and the segments were muscle blocks that he concluded that Pikaia "is a primitive chordate rather than a polychaete. The earliest fish scales are Upper Cambrian, and Pikaia may not be far removed from the ancestral fish."[19] In 1982, he added further description in his Atlas of the Burgess Shale that Pikaia had one or more fins, but did not specify where they were present.[12]
Pikaia was not popularly known as a chordate fossil or as an ancient chordate until 1989.
Wonderful Life: The Burgess Shale and the Nature of History: "Pikaia is not an annelid worm. It is a chordate, a member of our own phylum—in fact, the first recorded member of our immediate ancestry."[35] From this remark Pikaia became generally recognised as a chordate and ancestor of vertebrates.[24][36][37]In 1993, Conway Morris came up with another possible chordate feature. He identified structures that looked like gill slits but gave a cautious remark: "[They] may have been present, but are hard to identify with certainty in the compressed material available.[26] The tiny pores on the side of the pharynx are normally gill slits in living chordates.[38] He also noticed that Pikaia is similar to Amphioxus in most general aspects, with major difference in its notochord not reaching the anterior end.[26]
Not all palaeontologists were convinced of the chordate designation without better analysis.
Another component of Pikaia fossils that constrains the animal to be accepted as a chordate is its distinct invertebrate character; its preservational mode suggests that it had
stem group. Its anatomy closely resembles the modern creature Branchiostoma.[43]A fossil species
Emu Bay shale of Kangaroo Island in South Australia, had been debated as among the oldest annelids, or at least other invertebrate groups.[44][45] Polish palaeontologist Jerzy Dzik in his formal description in 2003 notes that it "closely resembles the slightly geologically younger Pikaia" in having smooth cuticle as well as muscular segmentation, and projections on its backside (ventral chaetae) that look like Pikaia's tentacles. He concluded:In fact, there is little evidence for chordate affinities of Pikaia. Its relationship with Myoscolex [as annelid in his proposition] appears a much better solution. Both were initially identified as polychaetes and this line of inference perhaps deserves confrontation with more recent evidence than that available to the authors who proposed these genera.[46]
Comprehensive description
The first comprehensive description of Pikaia was published by Conway Morris and
The head is
One of the most unusual body parts is a series of appendages just posterior to the tentacles. Walcott had called the appendages parapodia, as a kind of body protrusions that aid locomotion in snails,[48] and mentioned five parapodia in each individual.[15] He was even puzzled by the absence on the major part of the body.[15] Other specimens have up to nine such appendages, and could not be parapodia. Fins are present as an expansion of the body on the dorsal and ventral sides. They are not present in many specimens indicating that they are delicate membranes and were lost during fossilisation.[3]
The backside of Pikaia fossils show a hollow tubular structure that extends throughout most of the body length, but not the anterior region. It is easily noticeable as a highly light-reflective portion and is known as the dorsal organ. Once described as the notochord,[18][19] its nature is not yet fully resolved and could be a storage organ. The true notochord, along with a nerve chord, is a fine lateral line that runs just beneath the thick dorsal organ.[3]
The main chordate character is a series of myomeres that extends from the anterior to the posterior region. On average, there are 100 such myomeres in each individual. The muscle segments are not simply "annular shiny lines" as Walcott described,[15] but are in concentric bends in the form of V-shaped chevron. The myomeres at the anterior end as simpler in appearance and show circular arrangement. Conway Morris and Caron concluded:
Whilst the possibility that Pikaia is simply convergent on the chordates cannot be dismissed, we prefer to build a scenario that regards Pikaia as the most stem-ward of the chordates with links to the phylogenetically controversial yunnanozoans. This hypothesis has implications for the evolution of the myomeres, notochord and gills.[3]
Evolutionary importance
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
The presence of cuticle, one of the principal characters of higher invertebrates,
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.[12]
Development of the head
The first sign of head development, cephalization, is seen in chordates such as Pikaia and Branchiostoma. It is thought that development of a head structure resulted from a long body shape, a swimming habit, and a mouth at the end that came into contact with the environment first, as the animal swam forward. The search for food required ways of continually testing what lay ahead so it is thought that anatomical structures for seeing, feeling, and smelling developed around the mouth. The information these structures gathered was processed by a swelling of the nerve cord (efflorescence) – the precursor of the brain. Altogether, these front-end structures formed the distinct part of the vertebrate body known as the head.[33]
Evolutionary interpretation
Once thought to be closely related to the ancestor of all vertebrates, Pikaia has received particular attention among the multitude of animal fossils found in the famous Burgess Shale and other Cambrian fauna. In 1979, Whittington and Conway Morris first explained the evolutionary importance of Pikaia. Realising the fossil to be that of a chordate in the Cambrian rocks, chordates could have originated much earlier than expected, as they commented: "The superb preservation of this Middle Cambrian organism [Pikaia] makes it a landmark history of the phylum [Chordata] to which all vertebrates, including man, belong."[18] It is for this knowledge Pikaia as an old chordate that it is often misleadingly and falsely[5] attributed to as an ancestor of all vertebrates,[6] or the oldest fish,[7] or the oldest ancestor of humans.[10][11]
Before Pikaia and other Cambrian chordates were fully appreciated, it was generally believed that the first chordates appeared much later, such as in Ordovician (484–443
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.[33] 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.
Gould's interpretation and evolutionary contingency
Gould, in his presidential address of the Paleontological Society on 27 October 1988, cited Pikaia to explain the trends of evolutionary changes:
Wind back life's tape to the Burgess (first erasing what actually came after), let it play again, and this time a quite different cast may emerge. If the cast lacked Pikaia, the first chordate, we might not be here—and the world would be no worse... Let us thank our lucky stars for the survival of Pikaia.[53]
He elaborated the same idea in "An epilogue on Pikaia" in his book Wonderful Life "to save the best for the last," in which he made a statement:
Pikaia is the missing and final link in our story of contingency—the direct connection between Burgess decimation and eventual human evolution... Wind the tape of life back to Burgess times, and let it play again. If Pikaia does not survive in the replay, we are wiped out of future history—all of us, from shark to robin to orangutan... And so, if you wish to ask the question of the age—why do humans exist?—a major part of the answer, touching those aspects of the issue that science can treat at all, must be: because Pikaia survived the Burgess decimation.[52]
This interpretation that the chances of evolutionary products are unpredictable is known as evolutionary contingency.[54][55] Gould, from this statement, is regarded as "the most famous proponent" of the concept.[56] His idea has inspired many research involving enolutionary contingency from palaeontology to molecular biology.[57][58][59] He used Pikaia among the Cambrian animals as an epitome of contingent event in the entire evolution of life; if Pikaia had not existed, the rest of chordate animals might not have evolved, thus completely changing the diversity of life as we know. According to him, contingency is a major factor that drives large-scale evolution (macroevolution) and dictates that evolution has no inevitable destiny or outcome.[60] However, as Gould explained, "The bad news is that we can't possibly perform the experiment."[59]
Ecology
Pikaia is suggested to have been an active swimming organism that swam close to the seafloor (nektobenthic) using side to side undulations of its flattened posterior for propulsion. The anterior appendages are unlikely to have been used in feeding, and may have had a respiratory function. Pikaia is suggested to have fed on small particles of organic matter.[3]
See also
Notes
- hemichordates, which have a notochord-like structure only at the early stage of their lives.
References
- ISBN 978-0-226-40319-9.
- ^ ISSN 2076-3263.
- ^ S2CID 27671780.
- ^ "Pikaia gracilens" Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011.
- ^ PMID 15522451.
- ^ a b SciTechDaily (2012-03-06). "Pikaia Gracilens Is a Relative of but not the Genetic Ancestor to All Vertebrates". SciTechDaily. Retrieved 2022-09-23.
- ^ a b "An Early Chordate that Lived 513 Million Years Ago". Discover Magazine. Retrieved 2022-09-23.
- ^ "Worm-like creature could be humans' oldest ancestor". 2012-03-05. Retrieved 2022-09-23.
- ^ "Human's oldest ancestor found". University of Cambridge. 2012-03-06. Retrieved 2022-09-23.
- ^ a b "Worm-like creature could be a relative of the earliest vertebrate". 2012-03-05. Retrieved 2022-09-23.
- ^ a b "Early Vertibrate Relative found". University of Cambridge. 2012-03-06. Retrieved 2022-09-23.
- ^ PMID 23606659.
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- ISBN 978-0-8137-3024-0.
- ^ a b c d e f g h Walcott, Charles D. (1911). "Cambrian Geology and Paleontology II: No.5--Middle Cambrian Annelids" (PDF). Smithsonian Miscellaneous Collections. 57 (5): 109–144.
- ^ Simon, Conway Morris (1977). "Aspects of the Burgess Shale fauna, with particular reference to the non-arthropod component". Journal of Paleontology. 51 (Suppl 2): 7–8.
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Further reading
- Bishop, A., Woolley, A. and Hamilton, W. (1999) Minerals, Rocks and Fossils. London: Phillip's
- Conway Morris, Simon. 1998. The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford University Press, New York, New York.
- Norman, D. (1994) Prehistoric Life: the Rise of the Vertebrates, London: Boxtree
- Sheldon, P., Palmer D., Spicer, B. (2001). Fossils and the History of Life. Aberystwyth: Cambrian Printers/The Open University. p. 41-42.
External links
- "Pikaia gracilens". Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011. Archived from the original on 2020-11-12.
- La evolución de las especies: ¿por qué sobrevivió Pikaia? (Spanish)
- Fossils of the Burgess Shale - Middle Cambrian
- Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia