The discovery of over 100 fossils of a tiny extinct creature called Metaspriggina from Cambrian age strata, between 500 and 515 million years old and exposed in a number of localities across North America, is helping to clarify our understanding of early fish evolution.
Professor Simon Conway Morris of the Department of Earth Sciences in the University of Cambridge and Jean-Bernard Caron, Curator of Palaeontology in the Royal Ontario Museum, Toronto, Canada have just published a detailed description of this 60 mm long, soft-bodied fish. The new information about Metaspriggina has come from 42 especially well preserved specimens from a locality called Marble Canyon in Kootenay National Park, British Columbia, which is a World Heritage Site. The Cambrian strata here have recently yielded a rich and well preserved fossil ‘Burgess Shale’ type biota like that of the nearby classic Burgess Shale localities of Yoho National Park.
The Burgess Shale is famous for its diverse fossil fauna and the preservation of their soft tissues, which reveals a great deal about the biology of early marine organisms with complex body forms. It is only in recent decades that fossils have been found which suggest that fish had already evolved so early in Cambrian times.
The new find by Conway Morris and Caron preserves several of the fundamental characters, which define early fish long before they acquired bony skeletons and jaws and are not normally preserved in the fossil record. The fossils clearly show that Metaspriggina was an elongate, laterally flattened animal animals, pointed at both ends but with a blunter and slightly bilobed head with forward pointing and bulging eyes and longer more pointed tail.
The fossil remains preserve a notochord and evidence of primitive vertebrae, which provide an axial stiffening to its body and is surrounded by serially repeated W-shaped muscle blocks whose S-shaped contractions allowed the animal to swim. The front of the animal has paired camera-like eyes (each with a lens) and paired nasal sacs for sensing the surrounding environment, as well as a possible cranium. And, there is a paired array of external gills, supported by structures known as branchial bars, for obtaining oxygen and food particles from the water. The elongate body also has a tail, which continues beyond the anus. Some of the specimens even preserve traces of soft tissues such as blood vessels, the gut, liver and possibly the heart.
The possession of these undoubted vertebrate features places Metaspriggina alongside other early soft-bodied jawless vertebrates from China, such as Myllokunmingia and Haikouichthys, and shows just how cosmopolitan these tiny creatures were by mid Cambrian times. The features seen in these fossils confirm those long predicted for ancestral vertebrates. They are distinctly more advanced than living cephalochordates, such as the lancelet, which are the most primitive survivors of early vertebrate evolution. Interestingly, the external position of the gills relative to the supporting branchial structure reflects that seen in the jawed fish, rather than the surviving jawless fish, which have internal gill arrangements.
Discoveries of early fish of early to mid Cambrian age, such as Metaspriggina from North America along with Myllokunmingia and Haikouichthys from China have revolutionized ideas about early fish evolution and have confirmed what has been predicted for over a hundred years for the acquisition of a succession of essential chordate and then jawless vertebrate features from branchial bars, the notochord, paired eyes and w-shaped muscle blocks.
To see some of the wonderful and remarkably preserved fossils from the Burgess Shale and find out more about this period in the history of life visit the Sedgwick Museum, where they are on display.
For further details visit http://www.rom.on.ca/en/blog/metaspriggina and http://www.burgess-shale.rom.on.ca/