Absolute dating fossils
She works for a company contracted by Los Angeles transportation officials to keep palaeontologists on hand as workers extend a subway line to the city's west side."They're making sure that they're recovering every single fossil that could possibly show up," Ms Leger said of her team of monitors."They call me anytime things are large and we need to lead an excavation."Since work on the extension began in 2014, fossilised remains have routinely turned up from creatures that roamed the grasslands and forests that covered the region during the last Ice Age, about 10,000 years ago.
California's stringent environmental laws require scientists to be on hand at certain construction sites.
Palaeontologists have staffed all LA subway digs beginning in the 1990s, when work started on the city's inaugural line, said Dave Sotero, spokesman for the Los Angeles County Metropolitan Transportation Authority.
We show uncertainty in fossil-dating propagates to divergence-time estimates, yielding estimates that are older and less precise than those based on traditional node calibration.
Ultimately, node and tip calibrations are not mutually incompatible and may be integrated to achieve more accurate and precise evolutionary timescales.
When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.
Paleontologists still commonly use biostratigraphy to date fossils, often in combination with paleomagnetism and tephrochronology.A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.
But the discovery that still makes Ms Leger shake her head in disbelief came about a year ago, shortly after construction began on the project's second phase.Fossil species of known age establish calibration directly, and their phylogenetic uncertainty is accommodated through the co-estimation of time and topology.However, challenges remain, including a dearth of effective models of morphological evolution, rate correlation, the non-random nature of missing characters in fossil data, and, most importantly, accommodating uncertainty in fossil age.Workers at a San Diego construction site found fossils including parts of a mammoth and a grey whale and multiple layers of ancient seashells.Last year, crews working on a development near Boston's seaport uncovered a 15-metre wooden boat possibly dating as far back as the late 18th century.Ms Leger is a palaeontologist who digs for fossils in the middle of a city rather than an open plain or desert.