In the Yorkshire lias another relatively rare family of ammonites is the Lytoceratidae – finding a Lytoceras always makes a day a great collection day, the evolute shell shape reminiscent of a “horn of plenty” and the complex suture make the ammonites very appealing.
Lytoceras is a rather “conservative” genus that remained relatively unchanged from the lower lias up to the upper cretaceous, its rarity in Yorkshire having something to do with the water depth : Lytoceras is considered a deep water genus that only occasionally strayed into the relatively shallow waters of the Cleveland basin.
Lytoceras seems to get more common during toarcian times, this may have to do with a temporary high in water levels that also allowed tethyan species like Frechiella to migrate in.
All of the liassic species that occur in Yorkshire have a more or less round whorl section, and have more or less evolute shells where the whorls hardly touch each other. The ribs on the shell generally show a typical fimbriation – a fringing of the ribs that leads to a characteristic shell pattern. A feature of the shell that is rarely preserved due to its fragilty (both during times of embedding and during preparation…) are the so-called flares – ribs that have developed into thin collar-like extensions of the shell.
Finding a Lytoceras with preserved flares is a real highlight – prepping it a real challenge, for the thin calcite flares almost break if you look at them – preparation is only possible in softer matrices where low impact prep methods (e.g. air abrasion) can be utilized.
One thing I did not know before reading newer literature about the topic (HOFFMANN 2010) for this post was that a pronounced sexual dimorphism has been
recognized in the Lytoceratidae, with relevant macroconch / microconch pairs so far published being
Lytoceras cornucopia / Lytoceras (Trachylytoceras) annulosum
Lytoceras ceratophagum / Lytoceras (Trachylytoceras) nitidum
Lytoceras fimbriatum (SOWERBY)
(luridum, maculatum, stokesi subzones)
Lytoceras cornucopia (YOUNG & BIRD)
Perilytoceras jurense (ZIETEN)
Lytoceras ceratophagum (QUENSTEDT)
Lytoceras sublineatum (OPPEL)
(bifrons, variabilis zones)
Lytoceras (Trachylytoceras) nitidum
there is no further record of them in later literature.
Both OPPEL and WRIGHT had “Ammonites gubernator” as a synonym for Lytoceras jurense (now Perilytoceras jurense), which is assumed here as well.
Due to the fragmentary nature of this specimen, this is not followed any further.
Trachylytoceras fasciatum is considered by HOWARTH as a potential synonym of Trachylytoceras nitidum, now Lytoceras (Trachylytoceras) nitidum.