HETEROMORPH AMMONITES

Heteromorph Ammonites, Sowerby 1837

Ammonite shells have been collected by people for millennia. These ammonites are known from the Late Triassic and the middle Jurassic but were most abundant and creative in their form in the Cretaceous (Wiedmann, 1969).

The beautiful plate you see here showing two ammonites is from Sowerby (1837) and is one of the very first scientifically accurate studies of heteromorph ammonites. We see similar species to the heteromorphs above in the Nanaimo Group of Vancouver Island, British Columbia.

During medieval times they were believed to be snakes that had been turned into stone and were sold to people going on pilgrimages. They have been found in archaeological sites in many parts of the world. We find them in archaeological remains spanning human history, across cultures and civilizations.

Pictet's Paleontology Second Edition, 1853-57

Ammonites are prized for their scientific and aesthetic value and have been used as building materials, jewelry, amulets, charms to aid in the hunt, religious totems amongst other things. The original discus used by the ancient Greeks in their Olympics was a fossilized ammonite.

The beautiful plate to the left shows some of the heteromorph ammonites from Pictet's Paleontology in its second edition (1853-57). Some of the figures are copied from Astier or d'Orbigny works, not included in the first edition.

Ammonites are pleasing to the eye, usually taking on a planispiral form— although there are some helically spiralled and fully crazy spiralled forms — known as heteromorphs.

The most interesting of all the heteromorphs are the Cretaceous heteromorph ammonites of the suborder Ancyloceratina. The juveniles of this suborder played with every possible variation in their shell shape from regular planispiral and orthoconic to torticonic, hamitoconic and gyroconic.

The adults uncoiled the last whorl of their shell forming the characteristic U-shaped recurved body chamber you see below. A curious form as the aperture faces upward. Ammonites of the suborder Ancyloceratina may have developed a stationary brooding phase that could have several ecological advantages over free-swimming monomorphic ammonites.

Ancyloceras matheronianus

Alexander Arkipkin wrote a great paper on them in the Journal of Molluscan Studies back in 2014 — "Getting hooked: the role of a U-shaped body chamber in the shell of adult heteromorph ammonites."

His focus is on the morphological features of the adult shell in heteromorph ammonites of the suborder Ancyloceratina but does useful comparisons to a vast number of heteromorph ammonites in collections at the Department of Earth Sciences of the Natural History Museum, London.

Heteromorphs come in a variety of shapes and sizes. They must have intrigued and mystified those who were first to find them as they do not have an intuitive shape at all for a marine predator. Do give Arkipkin's paper a read and bask in the wonder of the amazing forms of these ancient cephalopods.

Reference: Getting hooked: the role of a U-shaped body chamber in the shell of adult heteromorph ammonites. Journal of Molluscan Studies, Volume 80, Issue 4, November 2014, Pages 354–364, https://doi.org/10.1093/mollus/eyu019 https://academic.oup.com/mollus/article/80/4/354/1021718

Image 3-5: Ancyloceras matheronianus. A. General view. B. Inner part of vertical shaft with resolved ribs (RR). C. Inner part of U-shaped adult living chamber with less resolved ribs (WR). Abbreviations as in Figure 2. Scale bar = 1 cm.