Lytoceras or Visitors from the deep

Lytoceras ceratophagum (16 cm) and half a Cleviceras exaratum (9 cm), both pyritized

Lytoceras ceratophagum (16 cm) and half a Cleviceras exaratum (9 cm), both pyritized

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

I am aware of the following liassic members of the Lytoceratidae family from Yorkshire :

Lytoceras fimbriatum (SOWERBY) 

(luridum, maculatum, stokesi subzones)

Lytoceras fimbriatum (10.5 cm) from the maculatum subzone, with preserved flares

Lytoceras fimbriatum (10.5 cm) from the maculatum subzone, with preserved flares

This is a specimen from the maculatum subzone, which is relatively rare. L. fimbriatum is more common in the luridum subzone, but rarely well-preserved.

Lytoceras cornucopia (YOUNG & BIRD)

(bifrons zone)

Lytoceras cornucopia (8 cm)

Lytoceras cornucopia (8 cm)

This specimen has no shell on the inner whorls, thus showing the beautiful suture and constrictions on the innermost whorls.
The firmbriate ribbing on the outer whorl is just visible.

Perilytoceras jurense (ZIETEN)

(thourarsense zone)

Perilytoceras jurense (4 cm)

Perilytoceras jurense (4 cm)

Perilytoceras jurense (syn. Lytoceras jurense) has an oval whorl section.
 

Lytoceras ceratophagum (QUENSTEDT)

(falcifer zone)

 see title picture
Lytoceras ceratophagum and Lytoceras cornucopia are very similar, L. ceratophagum has more radial ribbing whereas L. cornucopia has slightly rursiradiate ribbing.

Lytoceras sublineatum (OPPEL)

(bifrons, variabilis zones)

Lytoceras sublineatum (5 cm)

Lytoceras sublineatum (5 cm)

Lytoceras sublineatum differs from the other species in having a more compressed whorl section.

Lytoceras (Trachylytoceras) nitidum 

(falcifer zone)

Lytoceras (Trachylytoceras) nitidum (3 cm)

Lytoceras (Trachylytoceras) nitidum (3 cm)

This is the tiny microconch of the macroconch Lytoceras ceratophagum.
The Whitby museum type specimen list also mention the three following :
Pachylytoceras gubernator
Pachylytoceras ? peregrinum
Trachylytoceras fasciatum
Allthough HOWARTH mentions the first two in his “The Yorkshire Type Ammonites And Nautiloids of Young and Bird, Phillips and Martin Simpson” paper as holotypes,
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.
BUCKMAN pictures the Whitby specimen of SIMPSON´s Ammonites peregrinum as Alocolytoceras peregrinus, but this is not mentioned later, either.
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.
AndyS
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2 Comments

  1. s tymon

     /  January 21, 2013

    Yet another brilliant and very interesting read Andy, another one of my favourites, the very rare Lytoceras…Thank you!

    Reply

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