Picaya

The scientific generic name Pikaia is given by the name of Mount Pika Peak ^[1] located in Canada on the border of Alberta and British Columbia . The name of the mountain can be translated as Pischehovaya peak.

In 1909, the American paleontologist C. D. Walcott discovered numerous remains of fossil organisms in the Central Cambrian deposits of British Columbia (Canada) in the Burgess shales . Many of them belonged to soft-body skeletal forms. During the excavations following this discovery, a large amount of paleontological material was collected. Among the samples found were “fingerprints” of pikaya, which in fact were thin carbon and aluminosilicate films on the surface of a rock ^{[2] [3]} .

In 1911, C. D. Walcott published a scientific description of pikaya. This animal was singled out by him in an independent family Pikaidae with a single genus and species and assigned to polychaete worms - polychaetes ^[1] .

At the end of the 1970s, British paleontologist S. Conway Morris discovered a chord- like formation in pikaya, as well as muscle segments - myotomas characteristic of chordates , so most experts began to classify pikaya as chordates ^[3] . In 2012, S. Conway Morris, together with the Canadian paleontologist J.-B. Karon has published new data on the anatomy of pikaya. As a result of studying all 114 samples discovered by that time, these experts proposed a new interpretation of the body structure of this animal ^{[2] [3]} .

Appearance

The body length of adults reached from 1.5 to 6 cm, on average it was 4 cm. The height of the body was from 7 to 16% of its length, on average, about 11% ^[2] .

The body of the pikaya is elongated, rounded in the front part and compressed laterally in the remaining parts. A narrow ridge resembling a fin stretched along the ventral side. Another ridge, although not so high, was located on the dorsal side ^{[2] [3]} .

The head was very small, divided into two rounded lobes, on which there was a pair of tentacles . There is reason to believe that these tentacles were resilient, since after the death of the animal they usually remained straight, judging by the preserved prints. Most likely, they performed the function of the organs of smell and touch. At the base of the head on its lower side was a mouth opening . No organs similar to the eyes were found ^{[2] [3]} .

Behind the head on both sides of the pharynx there were 9 pairs of branching appendages, which are interpreted by researchers as external gills . Near the bases of these appendages, small rounded spots are sometimes visible, possibly corresponding to gill slits ^{[2] [3]} .

Anatomy

The muscles are divided into many transverse segments - myotomes , which number up to hundreds. Myomers have sigmoid outlines. The borders between myotomes are clearly visible on fossil remains, since the carbon content is increased in the corresponding places of the prints. Apparently, myotomas were separated by layers of dense connective tissue, which decomposed more slowly than the muscles of the animal ^{[2] [3]} . Segmentation extends to the ventral fin ^[2] .

Inside the body on the dorsal side there is a dense longitudinal cord, which retains a voluminous, convex shape even in fossil remains. It starts behind the throat and stretches to the very end of the body. Previously, it was considered by researchers as a chord . However, its thickness is too large for the chord. In addition, it is located too high, leaving no room for the neural tube , which should be placed higher than the chord. The previous interpretation was revised in 2012 by S. Conway Morris and J.-B. Karon. In their work, this cord is called the spinal organ. The origin and functions of this body are still unclear ^{[2] [3]} .

Chord and neural tube , as interpreted by S. Conway Morris and J.-B. Karon, located below the spinal organ ^{[2] [3]} .

The digestive system opens with a mouth opening at the base of the head. In the front of the intestine there is an extension - the pharynx. Anus , according to the reconstruction of S. Conway Morris and J.-B. Karon, located at the posterior end of the body ^[2] .

Another thin longitudinal strand is located on the ventral side of the body. S. Conway Morris and J.-B. Karon interpreted it as a blood vessel . In some specimens, a similar vessel was also found on the dorsal side ^[2] .

Probably, the animal could actively swim, curving the body in a wave-like fashion, in the first place - its flattened back ^[3] .

Most likely, pikaya was a filter and filtered water suspended in the pores of its pharynx ^[3] .

• Scientists have identified the oldest ancestor of vertebrates (Rus.) . Lenta.ru (March 6, 2012). Date of appeal September 24, 2017.
• Pikaia gracilens (description and images) (eng.) . Royal Ontario Museum . Date of treatment April 24, 2012. Archived May 15, 2012.