
Fallen Boulder Reveals 313M-Year-Old Fossil Footprints at Grand Canyon - palo3
https://www.npr.org/2020/08/21/904943006/fallen-boulder-reveals-313-million-year-old-fossil-footprints-at-grand-canyon
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has2k1
An interesting note is, the footprints where formed on the supercontinent
Pangea.

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sradman
The paper _Early adaptation to eolian sand dunes by basal amniotes is
documented in two Pennsylvanian Grand Canyon trackways_ [1]:

> The narrow width of both trackways indicates that both trackmakers had
> relatively small femoral abduction angles and correspondingly relatively
> erect postures. They represent the earliest known occurrence of dunefield-
> dwelling amniotes―either basal reptiles or basal synapsids...

This geological formation was from the Moscovian age (315.2 to 307 Ma) or the
Kasimovian age (307 to 303.7 Ma) in the Pennsylvanian period.

[1]
[https://journals.plos.org/plosone/article?id=10.1371/journal...](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0237636)

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thdrdt
Can someone explain how these tracks were preserved on a sand dune?

As far as I know a sand dune moves constantly. I can't imagine tracks would
last a day.

Looking at the tracks it looks more like they were made in mud and later
covered by sand dunes?

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rement
The authors of the paper actually describe their hypothesis for how it
occurred [0]. Basically perfect conditions were required to preserve them.

In short the first tracks were made in dry sand, eroded a little, moistened by
fog, dew, or light rain, and then covered by dry sand again. The second tracks
were made through the dry sand into the moist layer which was then crusted
over with calcite and covered. Only the toe markings from the second tracks
were preserved.

[0]
[https://journals.plos.org/plosone/article?id=10.1371/journal...](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0237636#sec014)

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austincheney
The presence of a layer composed of a compressed calcium based salt suggests
earth that eventually became covered by ocean. Calcium deposits make for
fantastic preservatives.

See caliche and Llano Estecado as examples.

* [https://en.m.wikipedia.org/wiki/Caliche](https://en.m.wikipedia.org/wiki/Caliche)

* [https://en.m.wikipedia.org/wiki/Llano_Estacado](https://en.m.wikipedia.org/wiki/Llano_Estacado)

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simonebrunozzi
This is the important bit:

> among the oldest tracks on Earth of shelled-egg-laying animals, such as
> reptiles, and the earliest evidence of vertebrate animals walking in sand
> dunes

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dutch3000
how are they dating something that isn’t there? are they dating the sand?

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rement
In this case they are estimating the ages based upon the known ages of the
geologic formations surrounding the layers of sand the tracks were found in
[0][1].

[0]
[https://journals.plos.org/plosone/article?id=10.1371/journal...](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0237636#sec016)
[1] [https://www.geotech.hr/en/how-to-determine-the-age-of-a-
rock...](https://www.geotech.hr/en/how-to-determine-the-age-of-a-rock/)

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Yajirobe
How did they determine the age?

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rement
There is an entire branch of geology dedicated to answering that question
called stratigraphy [0]. It's hard to pinpoint exact dates but we can get a
pretty good estimate (313 million +/\- half a million is pretty good).

The authors describe how they determined the date in their paper [1]. In this
case they know the relative age of the formation the tracks were found in
(Manakacha Formation) because we know the age of the formations around it. The
ages of the surrounding formations are estimated by the known ages of the
fauna (other fossilized organisms) found in those formations (319 Ma below and
312 Ma above).

[0] [https://www.geotech.hr/en/how-to-determine-the-age-of-a-
rock...](https://www.geotech.hr/en/how-to-determine-the-age-of-a-rock/) [1]
[https://journals.plos.org/plosone/article?id=10.1371/journal...](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0237636#sec016)

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locustsandhoney
Erroneous dating.

"The intelligent layman has long suspected circular reasoning in the use of
rocks to date fossils and fossils to date rocks. The geologist has never
bothered to think of a good reply, feeling the explanations are not worth the
trouble as long as the work brings results (American Journal of Science
276:51)." –J. E. O’Rourke

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gus_massa
For not circular methods, take a look at
[https://en.wikipedia.org/wiki/Radiometric_dating#Accuracy_of...](https://en.wikipedia.org/wiki/Radiometric_dating#Accuracy_of_radiometric_dating)

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Yajirobe
From the article: 'after an organism has been dead for 60,000 years, so little
carbon-14 is left that accurate dating cannot be established.'

~~~
gus_massa
Yes, carbon-14 is only useful for "recent" samples. The next section of the
article discuss other radioactive materials that have a longer half life, and
are useful longer ranges.

