Sip back as we travel across the Atlantic Ocean to the north east coast of Canada and the country’s largest French speaking province of Quebec and Parc national de Miguasha on the Chaleur Bay on the Gaspé Peninsula. In 1999 the Miguasha National Park was designated a natural UNESCO World Heritage Site because of its fossiliferous exposure of Escuminac Formation along the northern edge of the Gaspé Peninsula. Dated from the Frasnian Age of the Late Devonian Epoch the Escuminac Formation whose name is derived from the indigenous Mi'kmaq term for "here are small fruits". The Escuminac Formation is locally known as the René-Bureau Cliffs and is described as a konservat lagerstätten for its discoveries of well preserved fish including Sarcopterygii or lobe-finned fish. Known as the Miguasha biota this fossil record captures the transition of fish-to-tetrapod's and explains one of the most important evolutionary events in the history of life - when air-breathing four-legged vertebrates first walked on land.
The Gaspé Peninsula looks out east to the Gulf of St Lawrence and is a rugged landscape at over 240 Kilometres long and is roughly a 7-8 hour scenic drive alongside the St Lawrence River or less that a 2 hour flight from Quebec or a little longer from Montreal to the city of Gaspé and Michel-Pouliot Gaspé Airport.
The Gaspé Peninsula is possibly known more the Chic-Choc or Shickshock Mountains, the iconic Percé Rock of the Percé UNESCO Global Geopark that overlooks the peninsula or as place of sanctuary for wildlife especially sea birds.
The Gaspé Peninsula has a rich geodiversity and is directly linked to the formation of the Appalachian Mountains when the North American, African, and European continental plates collided creating a convergent boundary forcing the uplift of seafloors dating back to the Devonian Period known as the “Age of Fishes”. These large accumulations of sea sediments in this part of Canada form the Gaspé Basin.
The Gaspé Peninsula has also been affected by glaciation over the last million years. Since the ice retreated the land has experienced post-glacial rebound or glacial isostatic adjustment where the Gaspé Peninsula rose sufficiently above sea level to expose the fossiliferous rock formations including the Escuminac Formation.
The Escuminac Formation is representative of a shallow water environment and consists mainly of a sequence of fine-grained sedimentary rocks of sand and clay about 120 meters in depth.
Formed in a low-energy nearshore brackish estuary the rock layers exhibit several tell-tale features about the environment at the time. The layers of the Escuminac Formation show signs of ripples, flute casts formed by turbulent vortices that erode a series of depression in sediment and is later filled in with sand and tool marks or impressions made upon a clay sediment by the impact of waterborne debris.
The Escuminac Formation also exhibits the presence of turbidites created from short-live events such as earthquake, a collapsing slope or flood. When these happened sediment was locally dumped smothering vertebrates. Turbidites are good locations to find fossils and are normally encased in a concretion formed by the natural process of diagenesis. Diagenesis happens when the organic matter from the fish and other organisms is being broken down by bacteria and the relatively low temperature and pressure alters the texture and mineralogy of the original rock.
The fossil site at Miguasha was discovered 1842 by Abraham Gesner a New Brunswick geologist but it was not until 1892 and the discovery of the extinct lobe fish called Eusthenopteron foordi that the connection between body shape and body structure was determined to be similar to that of the first tetrapod's.
In 1938 British palaeontologist Thomas Stanley Westoll was first to describe the fossilised remains of a partial skull roof of an Elpistostege (meaning "hope from a skull roof") an advanced lobe-finned fish. Then in 2010 a complete specimen of the Elpistostege was discovered at the Miguasha National Park by Professor Richard Cloutier of the University of Quebec and colleagues. These discoveries confirmed that Elpistostege was an extinct genus of tetrapodomorph fish.
Thousands of fish specimens have been collected at Miguasha accounting for over 20 species across 10 different vertebrate biological groups including the Anaspida (meaning "shieldless ones"), Osteostraci (meaning "bony shells"), Placodermi (meaning “plate skin”), Acanthodians, Actinopterygii or ray-finned fish, Actinistians or coelacanths, Dipnoi or Lungfish, Osteolepidida and Elpistostegalia.
The most abundant of the species are Bothriolepis canadensis (Placodermi), Triazeugacanthus affinis (Acanthodians), Scaumenacia curta (Dipnoi) and Eusthenopteron foordi (Osteolepidida).
The Escuminac Formation can be regarded as konservat-lagerstätten whose laminated beds with exceptional fossilised preservation of both hard and soft tissue as well as bones of complete fish. The state of the Miguasha biota stands alongside the other fossil sites around the world such as the Cambrian Burgess Shale of Canada, the Ediacaran Doushantuo Formation of Guizhuo Province in China; the Carboniferous Mazon Creek in north east Illinois USA; Mississippian-aged fossils Bear Gulch of Montana USA, the Eocene Messel Shale of Germany, and the Cretaceous Jehol biotas of Liaoning Province in China.
The transition from fish to tetrapod marks one of the most significant evolutionary milestones as vertebrates moved from watery environments on to the land. This was achieved by several anatomical adaptations.
The fins of the Sarcopterygii developed a bony structure that prefigured the two sets of paired limbs of tetrapod’s and the evolution of wrists. These bones would evolve to become more robust and capable of bearing the weight of the body. Early tetrapod's developed a choanae or pair of openings connected to a respiratory system where the swim bladder of the Sarcopterygii was replaced by lungs to allow them to breath air. The need for controlling buoyancy was no longer requirement. These anatomical adaptations were the start of many more to come that would allow Tetrapod's to move, support themselves and survive on land.
The Escuminac Formation provides a window into a critical phase of Tetrapod evolution and the transition from their closest lobe-finned fish relatives the sarcopterygians.
Comments