University:
Boston University
Course:
MET HI 215 | Special Topics in History
Academic year:

2020
Views:

107

Pages:

20
Author:

Lorena West

What is a fossil? Motivations The concept of fossil helps to delineate the disciplinary turf of paleontology. Major developments in the field have sometimes involved rethinking the meaning of fossils. A thought experiment Imagine a planet (“Afossilia”) just like ours, but without any fossils … Suppose that God has removed all the fossils from the rocks. What exactly would the Afossilians be missing? Main claims 1. The concept of a fossil is hopelessly muddled and confused. We don’t really know what fossils are. 2. But this conceptual confusion doesn’t matter much for current paleontological practice, which focuses on inferring processes from patterns in large sets of data. Sources of confusion 1. Some pseudofossils are really trace fossils. 2. “Fossil” does not pick out a natural kind. 3. It’s even tough to identify necessary conditions for being a fossil. Sources of confusion 4. The need for an arbitrary age cut-off. 5. Literal vs. metaphorical uses of “fossil” are difficult to distinguish. 6. Puzzling cases: e.g. Ambergris 7. The problem of multiple casting & molding The $10,370 coprolite “This truly spectacular specimen is possibly the longest example of coprolite ever to be offered at auction. It boasts a wonderfully even, pale brown-yellow coloring and terrifically detailed texture to the heavily botryoidal surface across the whole of its immense length. The passer of this remarkable object is unknown, but it is nonetheless a highly evocative specimen of unprecedented size, presented in four sections, each with a heavy black marble custom base, an eye-watering 40 inches in length overall.” The Wilkes Formation, Toledo, Washington, USA Who did it? Miocene four-horned deer Or coprolite jewelry Fossils vs. Pseudofossils “Excrement-shaped masses of siderite and limonite have been reported from clay-rich sedimentary rocks that range in age from Late Permian to Holocene. These objects have been widely accepted as being coprolites, but the ferruginous composition, absence of internal inclusions, and scarcity of associated vertebrate remains suggest that they may instead be pseudofossils created by mechanical deformation of plastic sediment.” Fossils vs. Pseudofossils “A new hypothesis for the origin of these so-called coprolites is proposed, relying on organic-rich sediment, but inorganic processes. In this scenario, organic rich silt and clay was forcefully injected into hollow wood fragments and through knotholes, creating the characteristic features formerly cited as conclusive evidence for the fecal origin of these objects.” “I’d make a thin section to see if there was any residue in there from the last meal.” “I’ve done it with enough of them, and I haven’t found a dang thing in there. I have written more than a few letters to fossil supply houses and asked them to stop selling them as coprolites” Some pseudofossils, such as these dendrites, have inorganic origins Dinocochlea: The “Terrible snail”? First thought to be an internal cast of a gastropod shell, found near Hastings, UK, in 1921, 2.2m long. Later dismissed as a pseudofossil: an unusual concretion in lower Cretacous sediments. Or is Dinocochlea a concretion that formed around a burrow created by a worm? Authentic pseudofossils? Can a pseudofossil also be a fossil? “Fossil” does not pick out a natural kind Permineralization Casting & molding Compression and carbonization Even trace fossils (ichnofossils) do not comprise a natural kind Fossil steroids, a biomarker for sponges, from 635 mya (chemofossils) Fossilized burrows from Chaco Canyon, NM Banded iron formation—a trace fossil? Compare: Jade Jadeite Nephrite “Jade” picks out a heterogeneous group of minerals. “Fossil” picks out the products of heterogeneous historical processes.