In his 1992 monograph “The Ammonite family Hildoceratidea in the Lower Jurassic of Britain”, HOWARTH intoduced the new genus Cleviceras, named after the northern England area Cleveland (which by the way literally means “cliff land” (Wikipedia)). It was not that this was an ammonite genus that was newly established because new specimen had been found, it was done to recognize that an understanding about the phylogeny, the history of the evolution of that genus, had been reached, that made it necessary to separate it from the previous genus Harpoceras.
Previously, Cleviceras had been under the genus Harpoceras mainly for morphological reasons : The sickle-shaped style of ribbing is very similar to Harpoceras falciferum, the type species of Harpoceras, allthough there are differences in detail.
But it was then recognized that these ammonite genera belong to parallel evolutionary lineages with different ancestors :
Cleviceras : Tiltoniceras – Eleganticeras – Cleviceras exaratum – Cleviceras elegans
Harpoceras : Protogrammoceras ? – Harpoceras serpentinum – Harpoceras falciferum
In short words : Cleviceras is no longer considered to be the ancestor of Harpoceras falciferum, so it needs to be a different genus.
Another solution for this problem would be to put the Cleviceras species under Eleganticeras, which was put forward by GABILLY in 1976 and BÉCAUD in 2006.
HOWARTH still favoured the new genus to be able to generically divide the ammonites due to the big change in ribbing between Eleganticeras and Cleviceras,
and continued the use of the genus Cleviceras in the 2013 part L, Vol 3B, chapter 4 of the Treatise on Invertebrate Palaeontology, so we´ll stick to that as well…
For Cleviceras there are two Yorkshire species :
Cleviceras exaratum (YOUNG & BIRD, 1828)
Cleviceras elegans (SOWERBY, 1815)
Both exhibit a strong dimorphism, the microconch is considerably smaller than the macroconch, with an adult macroconch reaching up to 4 to 5 times the size of an adult microconch at maximum observed sizes. Largest macroconches can get close to 200 mm in size, while largest microconches have been found between 50 and 60 mm.
An adult shell can be recognized by approximation of the last sutures before the body chamber : growth of the shell slowed down and the last few chambers
are smaller than the previous chambers, i.e. the sutures are closer to each other.
I “found” this macroconch of Cleviceras exaratum in my “vault”, a large metal cabinet in my cellar, that houses all the “to be prepped” specimen,
after reading about the fact that large macroconches are rare and remembered I had actually found a somewhat crushed one some time ago…
Both species have a strong hollow keel, which is floored on the phragmocone, but not floored on body chamber, which easily explains why the keel most easily breaks off on the chambered part of the shell : this additional bit of shell creates a predetermined breaking point between the phragmocone and the keel.
The difference between Cleviceras exaratum and Cleviceras elegans is easily explained when you look at the umbilical walls :
The umbilical walls of Cleviceras exaratum are vertical or even undercut, while the umbilical walls of Cleviceras elegans are beveled.
In the Jet rock, ammonites of both species are often hollow and filled with natural oil – just like the one I tried to re-prep for this blog post :
It is a 12 cm macroconch of Cleviceras exaratum, the body chamber cut open by a lobster, found in 1995. I knew there was oil in this one,
for it had already coloured the label it lay on brown, but I hoped after almost 20 years the oil would be gone…
When I tried to prep the other side, a crack opened up, a piece of the shell fell off and the hollow shell presented itself –
I could literally pour about 20 ml of liquid oil out of this ammonite – what you see on this picture on the kitchen towel is just what dribbled out afterwards !
Needless to say, continuation of preparation will be difficult…