Introduction

Pikaia gracilens, an ancient and primitive chordate creature, is documented from the Middle Cambrian Burgess Shale in British Columbia. Initially classified as an annelid by Charles Doolittle Walcott in 1911, it was later identified as a chordate by Harry B. Whittington and Simon Conway Morris in 1979. This designation has made it one of the most renowned early chordate fossils and earned it the title of "earliest described Cambrian chordate."

Its existence is believed to have spanned the later phase of the Cambrian explosion. Over a hundred specimens have been unearthed since its original discovery.

Physical description

Pikaia gracilens possesses a body structure reminiscent of a lancelet and likely moved in a manner akin to an eel. The presence of a notochord and myomeres (segmented blocks of skeletal muscles) running the entire length of its body signifies its chordate characteristics. The primitive aspect of Pikaia is reflected in its body covering, a cuticle, which is typical of invertebrates and some protochordates.

Pikaia exhibits a body resembling that of a lancelet, tapering at both ends, and lacking a well-defined head. Its average length measures about 1.5 inches (3.8 cm). The largest individuals recorded by Walcott reached up to 5 cm (2.0 in) in length.

The creature features a pair of prominent, antennae-like tentacles on its head, akin to those seen in invertebrates like snails. These tentacles are attached in a manner that creates a two-lobed structure at the head. They may be comparable to the tentacles found in present-day hagfish, a jawless chordate. It was an active and free swimmer.

Pikaia possesses a small, circular mouth suitable for consuming small food particles in a single bite. Along the underside of the head, immediately after the mouth, there are a series of short appendages. However, the exact purpose or nature of these appendages remains uncertain.

The pharynx of Pikaia is associated with six pairs of slits, each adorned with tiny filaments. These filaments might have served as a respiratory apparatus. This feature sets Pikaia apart from modern lancelets, which possess distinct pharyngeal gill slits on either side of the pharynx, primarily used for filter feeding.