Paleo Profiles: Pikaia

When we think of important fossil discoveries we might think of Sue, the near-complete Tyrannosaurus, the Archaeopteryx, whose feathers provided a 'missing link' between dinosaurs and modern birds, or the Tiktaalik, which we looked at last time in Paleo Profiles. However, there is another, and is so important that it might help shed light on the evolution of chordates, back-boned animals. This evolutionary question was the Pikaia. First appearing in the fossil record over 500 million years ago this tiny creature has become one of the most discussed and controversial fossils in the study of evolution.

Discovery and Fossils

The first Pikaia fossil was found in Alberta, Canada by geographer and palaeontologist Charles Walcott. Alberta and British Columbia are well known in palaeontological circles for the abundance of fossils dating from the Cambrian period - the earliest part of the 'Phanerozoic Eon', a time lasting from over 500 million years ago to now where plant and animal life as abundant. Especially with the Burgess Shale in British Columbia, has allowed palaeontologists to understand the abundance and diversity of animal life which emerged millions of years ago. Finding the little fossil near the Pika Peak in Alberta, Walcott named the fossil Pikaia in honour of the mountain. However, due to the regularly segmented body of the animal, he classified it as a worm. This was not particularly an unusual find - Walcott himself would discover a vast array of worm species from Canada. Nearby Burgess Shale would allow palaeontologists to find a plethora of Cambrian fossils, including the Pikaia. Today, we have over 100 Pikaia fossils with most coming from the Burgess Shale. In 1979, Simon Conway Morris, who specialised in the Cambrian, went back and looked at the Pikaia fossils at hand. He did this again in 2012 as more Pikaia fossils were unearthed. Paleontologists often look back on discovered fossils to find things which might have been missed, or to relate them to new findings. Conway Morris found that Pikaia was not a worm, instead he argued it was a very primitive chordate. Quite possibly, Pikaia was a stem chordate - the ancestor to every back-boned animal to exist since then.

Biology

Conway Morris's and Caron's reconstruction

Compared to later vertebrates, the Pikaia was a very simple chordate. With segmented bodies and two tentacles on the head it strongly resembled a worm. It was also very small at just 38 millimetres in length, it could easily fit on your fingernail. Within this tiny body there was a relatively complex system for respiration and digestion, so much so, it could potentially question when diversity of life exploded. Traditionally, the Middle Cambrian has been described as the 'Cambrian Explosion' for the diversity of life which emerged, so the complexity of Pikaia at such a period could indicate that this occurred even earlier. Palaeontologists have compared the Pikaia to an animal still in existence today - the lancelets (shown below).

Lancelets, like Branchiostoma above, bear a striking resemblance to Pikaia - a streamlined respiratory system and filter-feeding on plankton and zooplankton. Quite possibly it may have been somewhat see-through just like the lancelet. During the Middle Cambrian the earliest fish, or what would evolve to become the earliest fish, have been preserved so palaeontologists could see their organs - Haikouichthys from China is a good example of this. Thurston Lacalli has further analysed the segments of the Pikaia's body and found that they would likely have been a fairly slow swimmer, a bit like a hagfish. There are still controversies around where Pikaia fit into the evolution of chordates. Simon Conway Morris has argued that it was a stem chordate, so Pikaia could have been one of several species which served as the ancestor to the chordates. However, it is not certain as Pikaia could instead be a close relative of the stem chordates.

When and Where

Pikaia has only been discovered in Canada, with most of them being discovered in the Burgess Shale in British Columbia. The Burgess Shale has allowed palaeontologists to uncover a wide range of plant and animal life from the Cambrian period - roughly 514 million years ago. The world of the Cambrian was incredibly different from the world of today. There was one continent, clustered around the southern hemisphere, which made the planet's climate much colder; the Earth was further recovering from the 'Snowball Earth'. As a result, the Cambrian was cold. Oxygen content was two-thirds the level than it is today, and the levels of carbon dioxide was seven times the level than it was before the Industrial Revolution. The ozone layer is believed to have only came into existence around 600 million years ago, so by the Middle Cambrian it had started shielding the planet's surface from the sun's radiation. This meant that the surface was dangerous for life, but it was safer in the seas, (which covered most of the planet). The Cambrian, as mentioned earlier, saw the 'Cambrian Explosion' where the diversity of life exploded.

Pikaia's Habitat

A Burgess Shale reconstruction by Carel Brest van Kempen

The Burgess Shale of the Middle Cambrian resembled that of a modern coral reef. The large amount of soft-bodied animals and plants to be fossilised at the Burgess Shale indicates that it had muddy grounds. Quite possibly, a big reason why the animals and plants died at Burgess Shale was because of mud slides which buried the life underneath. Due to this, palaeontologists have managed to unearth such a wide range of life, and find well-preserved fossils, such as the Pikaia fossils. The Cambrian was home to a very bizarre group of animal life. Periodically through the planet's history there are explosions of diverse life adapted to very specific ecological niches, however, extinction events often wipe out the variety of life so the truly unique species vanish without descendants. In Burgess Shale we see the first jellyfish, hard-bodied arthropods, worms, trilobites, and sponges. However, alongside Pikaia were truly weird animals. Among them included Hallucigenia, a tentacled worm with spikes, Wiwaxia, a soft-bodied mollusc with spikes, and Anomalocaris, a prawn-like predator designated the world's first 'superpredator'. One, Opabinia, caused laughter when it was first revealed for its jaws on the end of tentacles and five eyes. Quite possibly, Pikaia could have been preyed upon by Opabinia and Anomalocaris.

The sources I have used are as follows:

-'Pikaia', Prehistoric-Wildlife.com, [Accessed 09/04/2020]

-Simon Conway Morris and Jean-Bernard Caron, 'Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia', Biological Reviews, 87, (2012), 480-512

-Jon Mallat and Nicholas Holland, 'Pikaia gracilens Walcott: Stem Chordate, or Already Specialized in the Cambrian?', Journal of Experimental Zoology, 320:4, (2013), 247-271

-Thurston Lacalli, 'The Middle Cambrian fossil Pikaia and the evolution of chordate swimming', EvoDevo, 3:12, (2012)

Thank you for reading. For other Paleo Profiles we have a list here. For future blog updates please see our Facebook or catch me on Twitter @LewisTwiby.