Lytoceratida

Triassic through the End Cretaceous

The Lytoceratida was perhaps the most widespread and long-lived (as an order) of the NeoAmmonoids. Despite their long fossil record, their general morphology changed very little over time.  It seems they had found the perfect formula for Mesozoic living.

There were two superfamilies in the Lytoceratid order:

• Lytocerataceae
• Most abundant during the Jurassic
• Tetragonitaceae
• Most abundant during the Cretaceous

Diversity, Geographic Range and Habitat

• Widespread throughout the Jurassic and Cretaceous periods
• Despite their longevity and numbers, their morphology was quite static and underwent very little change
• Found in a wide range of environments
• Deep water limestones around western Mediterranean are especially rich in Lytoceratida (as well as Phylloceratida)
• This order appears to have been quite adapted to deep water since many fossil specimens are found in these deposits

Characteristics

Shell Structure

• More evolute than their close relatives the Phylloceratida

• Lytocerataceae
• Evolute
• Serpenticone styled shell
• Some species had loosely coiled whorls
• The Pictetia's shell was so loosely coiled that none of the whorls were in contact with the others.
• Tetragonitaceae
• Regular involute shells
• Similar to other superfamily in most other characteristics

Siphuncle

• Ventral

Septal Neck

• Prochoanitic (points backwards, towards the older chambers)
• Septa (chambers) are closely packed, possibly indicating that they could have better withstood deep-water pressure

Suture Lines

• Complex suture lines with a distinctive division in lobes and saddles
• The most primitive species in quinquelobate (CHANGE THIS TO A NUMBER)

Ornamentation

• Both Superfamilies
  • Simple ribs periodically flared into thicker collars
  • Numerous constrictions covered the shell
  • Late Cretaceous species had either smooth shells with no ornamentation or were only weakly ribbed
    • This change was counter to the Mesozoic trend of more ornamentation

Sexual Dimorphism

Does not occur frequently amont the various species but displays as a small difference in size between the male and female conch, with the male being slightly smaller than the female.

Ancestry and Descendants

• Ancestor:  It is thought by many that the Lytoceratida derived from the Phylloceratida.
• Descendants:  Ancyloceratida whose loosely coiled shells provide amazing specimens in the fossil record

Ammonitella

• Size varied from .3 to 1 mm in diameter
  • Much smaller than the PaleoAmmonoids but equal or slightly larger than the MesoAmmonoids
• Amonitella of both superfamilies were the same
• Prosiphon was short and curved
• Shell displayed small tubercles