MOTH locality
The Man-on-the-hill locality is a fossil site in the Northwest Territories of Canada renowned for its incredibly well-preserved Early Devonian fish fossils. Discovered in the Mackenzie Mountains in the 1960s, MOTH accumulated greater prestige in the late 20th century, with many fossil fish species only known from this one site. The fauna consists of both jawed fish and jawless fish. The geology of MOTH reconstructs the area as a calm marine environment with mixed sediment sources along the western coast of Laurussia.
History
In the mid-1960s, geological mappers from the Geological Survey of Canada discovered well-preserved fish fossils on a steep mountainside in the Mackenzie Mountains, about northeast from the mining town of Tungsten, Northwest Territories. At the time, the site was known as GSC locality 69014. By the late 1970s, it had gained renown among Canadian paleoichthyologists, initiating a long list of new species discovered at the site.The University of Alberta Laboratory for Vertebrate Paleontology, which catalogues the site as UALVP locality 129, handled most subsequent collecting efforts. UA paleontologists Brian D.E. Chatterton and Mark V.H. Wilson led expeditions in 1983, 1990, 1996, 1998, and 2013, greatly increasing the volume of fossils recovered from the site. A nearby rock landmark, resembling a man sitting on the ridge, inspired a persistent nickname for the site: Man-on-the-hill. The main fossiliferous section of MOTH is an Early Devonian horizon at the level of 180 meters, and fossils are also found on the talus slope of the mountainside. Though MOTH is the most productive fish site in the Mackenzie Mountains, it is not alone: well-preserved Silurian fish are also known from lower layers of the site, strata in the vicinity of Avalanche Lake, and elsewhere in the range.
Geology
MOTH corresponds to a transitional zone between the clastic shales of the Road River Formation and the shelf carbonates of the Delorme Group. An earliest Devonian age is established by its fossil fauna, with distinctive Lochkovian fish such as Waengsjoeaspis, Canadapteraspis, Romundina, Altholepis, Polymerolepis, and Seretolepis. Brachiopod and conodont fossils also agree with this age estimate. At the time, the area would have been the tropical western continental shelf of Laurussia, fractured into many smaller basins and platforms by rifting along the Cordilleran front.The vertebrate-bearing layers of MOTH are mainly composed of finely layered light grey argillaceous limestone and dark grey silty calcareous shale. Fine grains of dolomite, calcite, and quartz are the predominant minerals in the sediment, reflecting a mixture of carbonate and siliciclastic inputs, including sharp grains sourced from airborne dust. The layers are flat-lying and undisturbed apart from subtle bioturbation, indicating perpetually calm waters. Older studies advocated for a shallow-water environment, based on the assumption that its fish and invertebrates preferred coastal or freshwater habitats. A 2005 overview instead supported a more offshore environment, below storm wave base. In any case, the presence of pyrite indicates that bottom waters were anoxic, allowing for high-fidelity fossil preservation. The exact cause of death is unknown for the fauna, as there is no evidence for turbidites or seasonal water column disruptions. MOTH can be considered a lagerstätte.