Pikaia gracilens is an extinct, worm-like animal from the Cambrian Period that lived over 500 million years ago. First discovered in the famous Burgess Shale fossil deposits, this small creature inhabited ancient seas during a dynamic period of evolutionary history. Its simple appearance belied its eventual placement in the story of life.
Unearthing Pikaia in the Burgess Shale
The story of Pikaia begins in the Burgess Shale, a geological formation in the Canadian Rockies. This site is a Lagerstätte, a fossil deposit with extraordinary preservation that offers a glimpse into the ecosystems of the Middle Cambrian, 505 million years ago. The fine-grained mudstone of this ancient marine environment captured soft tissues, providing a unique window into this era.
Paleontologist Charles Doolittle Walcott first discovered the fossils of Pikaia gracilens here in 1911. Walcott initially classified Pikaia as a type of polychaete worm due to its segmented appearance. For decades, these fossils were largely overlooked until a re-examination in the 1970s revealed their true nature.
Anatomy of an Ancient Ancestor
The physical form of Pikaia was slender, eel-like, and flattened from side to side, measuring less than two inches long. Its simple body had a small, poorly defined head with a pair of short tentacles. With no evidence of eyes, these tentacles likely served a sensory function, helping the animal navigate its environment.
The preservation of the Burgess Shale fossils reveals internal structures. Running along the animal’s back was a flexible, rod-like structure identified as a notochord, a supportive feature that defines a major animal group. Accompanying the notochord were V-shaped blocks of muscle known as myomeres, which provided the propulsive force for swimming.
Life in the Cambrian Seas
Based on its anatomy, Pikaia was likely a free-swimming animal, propelling itself with eel-like, side-to-side undulations. This movement was generated by its myomeres contracting against the stiffened notochord. While capable of swimming above the seafloor, it may have also spent time on the deep ocean bottom.
The feeding habits of Pikaia are inferred from its simple anatomy. Lacking a complex jaw, it was not an active predator. The presence of mud in its gut indicates it was likely a deposit feeder, consuming organic particles from sediment, or possibly a filter-feeder that ingested microorganisms from the water.
The Evolutionary Leap to Vertebrates
The anatomical features of Pikaia are significant for understanding the history of life. The presence of a notochord is the primary characteristic that defines the phylum Chordata, the group to which all vertebrates belong. This discovery repositioned Pikaia from a worm to one of the earliest chordates known from the fossil record. Its existence demonstrates that the foundational body plan for our phylum appeared during the Cambrian.
The notochord is a precursor to the vertebral column, or backbone, that defines vertebrates like fish, amphibians, reptiles, birds, and mammals. While Pikaia is not a direct ancestor of modern vertebrates, it represents an early offshoot of the lineage that would eventually lead to them. It embodies the basic anatomical toolkit from which more complex vertebrate structures evolved.
The segmented myomeres and notochord provided Pikaia with a simple method for swimming. In later descendants, this system was elaborated upon, leading to the diverse forms of locomotion seen in fish and all subsequent vertebrates. Pikaia serves as a fossil link, connecting our own existence to this creature from the ancient Cambrian seas.