AMMONITES
Ammonites are common & conspicuous fossils in
Mesozoic marine sedimentary rocks. Ammonites are an extinct group of
cephalopods - they’re basically squids in coiled shells. The living chambered
nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a
different group.
Ammonites get their name from the coiled shell shape
being reminiscent of a ram’s horn. The ancient Egyptian god Amun
(“Ammon” in Greek) was often depicted with a ram’s head &
horns. Pliny’s Natural History, book 37, written in the 70s
A.D., refers to these fossils as “Hammonis cornu” (the horn of
Ammon), and mentions that people living in northeastern Africa perceived them
as sacred. Pliny also indicates that ammonites were often pyritized.
Agricola
described ammonites using Pliny's term “Ammonis cornu” in his 1546
book De Natura Fossilium (see title page
above).
Conrad
Gesner also called these fossils “cornu Ammonis” in his 1565
books De Rerum Fossilium and De Omni Rerum Fossilium (see title pages
above).
Gesner was
the first to include illustrations of ammonite fossils (see below, from Gesner,
1565, ff. 164r & 167v).
Dactylioceras commune (Sowerby, 1815) (5.7 cm at its widest), slightly
pyritized, cracked out from a concretion
Stratigraphy & Age & Locality: Whitby Formation, Toarcian Stage, upper Lower
Jurassic; Yorkshire coast, England.
William
Camden briefly mentions ammonites in his 1607 book Britannia. Ammonites
occur in rounded rocks at Huntly Nabb, near Whitby, Yorkshire, England. Camden describes them: “In quibus
effractis inueniuntur serpentes saxei suis spiris reuoluti, sed qui plerique
capitibus destituti.” (loosely translated from Latin: “Broken
specimens have stone serpents inside, coiled up, but generally lacking
heads.”)
[click here
for an early English translation of the entire Yorkshire section of
Camden’s book, by Philemon Holland]
Ammonites from the Whitby area of England have inspired
interesting legends & folklore. St. Hilda (614-680 A.D.), an early
abbess of Whitby, is said to have cleared the area of snakes by cutting off
their heads & throwing them over the cliffs. Whitby locals have been
carving snake heads on genuine ammonite fossils for centuries. These
“snakestones” have been valued as lucky charms and were perceived
to have medical value at one time. Many rocks, fossils, and minerals were
long ago considered by superstitious minds to be curatives.
Snakestone (5.0 cm across) - a Dactylioceras commune ammonite with carved
snake head, from the Yorkshire coast of England.
Dichotomosphinctes sp. (a.k.a. Perisphinctes
(Dichotomosphinctes) sp.) [ID correct?] (5.3 cm across) - commercial
specimen attributed to the Oxfordian Stage (lower Upper Jurassic) of
Moronodova, western Madagascar.
Cleoniceras madagascariense (5.4 cm across) with preserved shell
iridescence. This is a commercial specimen attributed to the Albian Stage
(uppermost Lower Cretaceous) of Mahajanga, northwestern Madagascar.
Quenstedtoceras lamberti Sowerby, 1819 from the uppermost Callovian Stage (uppermost Middle
Jurassic) at the Dubki Quarry near Saratov, southwestern Russia.
Above left: 3.1 cm across
Above right: 3.8 cm across
Eopachydiscus marcianus (Shumard, 1854) - ammonites could attain large body sizes. Some
fossils are so large that they cannot be picked up by one person. Here
are a couple Eopachydiscus specimens that are getting to be a bit
big. These are from the Duck Creek Formation (upper Albian Stage,
mid-Cretaceous) of Spring Creek, Cook County, Texas, USA.
(Bill Van Deventer private collection, displayed at
Coral Caverns, off Cavern Street in the small town of Manns Choice, southern
Pennsylvania, USA.