Pikaia
| Pikaia | ||||||||||||
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Life reconstruction of Pikaia gracilens |
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| Temporal occurrence | ||||||||||||
| Middle Cambrian | ||||||||||||
| 525 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Pikaia | ||||||||||||
| Walcott , 1911 | ||||||||||||
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Pikaia is an extinct member of the Chordate family , possibly belonging to the skullless group (Cephalochordata). So far, only the type P. gracilens has been assigned to the genus.
The first part of the two-part binomial of Pikaia gracilens denotes the genus . Their name is derived from Mount Pika in the vicinity of the site. Since only one species has been found so far, the genus is monotypical .
Fossil finds
The fossil remains of the only known species P. gracilens come from the 525 million year old Central Cambrian layers of the Burgess slate , a world-famous fossil deposit in the Canadian Rocky Mountains. Almost all the finds go back to a location of a few meters in thickness, which also provided other fossils in very good condition. Heavily curved individuals are interpreted to mean that they were still alive at the time of embedding and trying to free themselves. The entire fossil community is now interpreted as a benthic community that has been buried by sudden, submarine mud flows. A total of 114 specimens of Pikaia have been found from the classic excavations by Walcott and subsequent excavations on behalf of the Royal Ontario Museum .
Pikaia gracilens was by Charles Doolittle Walcott discovered and from him in 1911 described . He placed the find in the group of annelids (Annelida). In 1971, the paleontologist Simon Conway Morris recognized the resemblance of the four to five centimeters long fish-shaped fossil to the skullless, a sub- strain of the Chordaten.
description
Pikaia was an elongated, laterally flattened organism with a body length of 1.5 to 6 centimeters, on average about 4 centimeters. The height of the animal reaches about 10 percent of its length, the width is estimated to be about half the height, based on curved or twisted embedded organisms. The fact that all individuals are embedded in a lateral position is a strong indication of a laterally flattened body; this points more to a freely swimming (pelagial) organism (probably near the bottom) than to a (benthic) organism living on the ocean floor down. Presumably the front end was more cylindrical, the animal flattened more and more towards the rear end. The body outline when viewed from the side was spindle-shaped, with a rather rounded front end and a long, pointed rear end. The dorsal side had a continuous, undivided fin edge. There is no evidence of a tail-fin-like formation at the rear end, which is shown in some earlier reconstruction drawings.
At the front end of the animal, a very small head area can be seen that barely reached 1.5 percent of the length of the animal and is little set off from the rest of the body. The head area was somewhat enlarged and, when viewed from above, was slightly bilobed. At the front end of the head there were two tentacles, slanting forward, possibly gills. The mouth opening can be seen in a few specimens on the ventral side of the head section. There is no evidence of the jaw or mouthparts.
The body sections behind the head area carried a number of other appendages on each side towards the abdomen (ventrolateral), the number of which, due to preservation, is not always recognizable, mostly five, at most nine of them. Each appendix consisted of a sturdy stem, attached to a series of thorny appendages, possibly the support structures of a soft tissue structure throughout life. The attachments were of comparable size.
The most striking feature of the fossil is a series of segment- like sections lying one behind the other (these led Walcott to erroneously interpret the animal as an annelid). Today they are considered to be the more stable, connective tissue sheath of a musculature divided into serial myomers . The suspected myomers did not have a straight, but an S-shaped curved shape. Also very noticeable on almost all fossils is a continuous, highly reflective longitudinal structure on the back line over the entire longitudinal axis of the animal. This is interpreted as a continuous dorsal organ. Presumably, however, it did not correspond directly to the chorda dorsalis of a recent lancet fish . This can probably be seen on a few fossils, in the form of a further structure divided lengthways into sections or segments directly on the abdomen of the dorsal organ. A parallel structure seen near the anterior end of some specimens could have been the encasing of a neural tube. Sometimes a simple, continuous intestinal tube can be seen on the abdomen. Although this is usually more difficult to see in the rear, it is concluded that it is a terminal anus. Another longitudinal organ near the abdomen is interpreted as a large blood vessel.
interpretation
Due to the serial structure, interpreted as myomers of a musculature and the dorsal organ with evidence of a notochord, the fossil has some typical features of the ( synapomorphies ) in the physique of the chordates. This assumption was already expressed directly after the first description in a letter from Charles Schuchert to the first descriptor Walcott (who was not a trained zoologist himself). Since its reinterpretation by Simon Conway Morris in 1971, it has mostly been understood as an ancient representative of the Cephalochordata , as an ancestor of the recent lancetfish with a similar body shape. The problem with the interpretation is, for example, the nature of the dorsal organ that in the new description by Conway Morris and Caron 2012 is no longer itself a possible notochord. A similar dorsal organ is found in the Yunnanozoon fossil (which is problematic in its assignment) from the Chinese Chengjiang fauna community . The paired appendages could be homologous formations to the gill arches of the vertebrates according to their location and structure .
Tribal position
Pikaia is not anatomically close to the early chorus dates from which later the first cranial animals (vertebrates), the jawless ones , emerged; rather, its clearly derived features indicate a long-term independent evolution . This leads to a separation between skullless chordates and vertebrates at a point in time well before the time of Pikaia . From the Chinese province of Yunnan , the 530 million year old fossils of two genera, Myllokunmingia and Haikouichthys , were described in 1999, which were classified as early vertebrates based on their anatomical features. The hypothesis can therefore also be substantiated by finds, because the oldest remains of fossil vertebrates found so far are older than those of Pikaia , which means that the genus is ruled out as an ancestor of the skull animals. The position of Pikaia as a primal representative of the chordates is quite well secured and has only rarely been contested. However, it is far more uncertain whether the species belongs in a clade with the recent lancet fish.
literature
- Simon Conway Morris & Jean-Bernard Caron (2012): Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia. Biological Reviews 87 (2): 480-512. doi : 10.1111 / j.1469-185X.2012.00220.x .
- Robert L. Carroll: Paleontology and Evolution of the Vertebrates. Georg Thieme Verlag, Stuttgart 1993, ISBN 3-13-774401-6 .
