AMMONITES: INDEX FOSSILS

Ammonites were prolific breeders that evolved rapidly. If you could cast a fishing line into our ancient seas, it is likely that you would hook an ammonite, not a fish. They were prolific back in the day, living (and sometimes dying) in schools in oceans around the globe.

We find their fossilized remains (and plenty of them) in sedimentary rock from all over the world. In some cases, we find rock beds where we can see evidence of a new species that evolved, lived and died out in such a short time span that we can walk through time, following the course of evolution using ammonites as a window into the past. For this reason, they make excellent index fossils.

An index fossil is a species that allows us to link a particular rock formation, layered in time with a particular species or genus found there. Generally, deeper is older, so we use the sedimentary layers rock to match up to specific geologic time periods, rather the way we use tree-rings to date trees. Photo: Dan Bowen, 2020 Tucson Gem and Mineral Show.

HOLCOPHYLLOCERAS MEDITERRANEUM

There is tremendously robust suturing on this lovely ammonite, Holcophylloceras mediterraneum, (Neumayr 1871) from Late Jurassic (Oxfordian) deposits near Sokoja, Madagasgar.

They were a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids — octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species.

Ammonites first appeared about 240 million years ago, though they descended from straight-shelled cephalopods called bacrites that date back to the Devonian, about 415 million years ago, and the last species vanished in the Cretaceous–Paleogene extinction event.

The shells had many chambers divided by walls called septa. The chambers were connected by a tube called a siphuncle which allowed for the control of buoyancy with the hollow inner chambers of the shell acting as air tanks to help them float. We can see the edges of this specimen's shell where it would have continued out to the last chamber, the body chamber, where the ammonite lived. Picture a squid or octopus, now add a shell and a ton of water.

ANDROGYNOCERAS OF YORKSHIRE

A stunning example of the ammonite Androgynoceras from the Yorkshire Coast, England.

The Geology of Yorkshire in northern England shows a very close relationship between the major topographical areas and the geological period in which their rocks were formed. The rocks of the Pennine chain of hills in the west are of Carboniferous origin whilst those of the central vale is Permo-Triassic.

The North York Moors in the north-east of the county are Jurassic in age while the Yorkshire Wolds to the southeast are Cretaceous chalk uplands.

The plain of Holderness and the Humberhead levels both owe their present form to the Quaternary ice ages. The strata become gradually younger from west to east. Much of Yorkshire presents heavily glaciated scenery as few places escaped the direct or indirect impact of the great ice sheets as they first advanced and then retreated during the last ice age. This beauty is in the collection of the deeply awesome Harry Tabiner.

MIDDLE TRIASSIC HUMBOLDT RANGE

Looking out over the Middle Triassic exposures of the Humboldt Mountain Range.

These hills were the site of the 1905 Expedition of the University of California’s Department of Geology in Berkeley funded by the beautiful and bold, Annie Alexander, the women to whom the UCMP owes both its collection and existence. Annie brought together a paleontological crew to explore these localities and kept an expedition journal of their trip which is now on display at the University of California Museum of Paleontology at Berkeley.

Annie's interest was the ichthyosaurs and she was well pleased with the results. They dodged rattlesnakes and tarantulas, finding many new specimens as they opened up new quarries in the hills of the Humboldt Range of Nevada.

Ichthyosaurs range from quite small, just a foot or two, to well over twenty-six metres in length and resembled both modern fish and dolphins. The specimens from Nevada are especially large and well-preserved. They hail from a time, some 217 million years ago, when Nevada, and parts of the western USA, was covered by an ancient ocean that would one day become our Pacific Ocean. Many ichthyosaur specimens have come out of Nevada. So many, in fact, that they named it their State Fossil back in 1977.

Fossil fragments and complete specimens of these marine reptiles have been collected in the Blue Lias near Lyme Regis and the Black Ven Marls. More recently, specimens have been collected from the higher succession near Seatown. Paddy Howe, Lyme Regis Museum geologist, found a rather nice Ichthyosaurus breviceps skull a few years back. A landslip in 2008 unveiled some ribs poking out of the Church cliffs and a bit of digging revealed the ninth fossil skull ever found of a breviceps, with teeth and paddles to boot.

Specimens have since been found in Europe in Belgium, England, Germany, Switzerland and in Indonesia. Many tremendously well-preserved specimens come from the limestone quarries in Holzmaden, southern Germany.