#sphenodiscus

Opalized fossils are the result of organic materials such as bone or shell being replaced by opal, which is an hydrated amorphous silica that diffracts light in a characteristic manner. Many opalized fossils, and most opals generally, seem to come from Australia. I’m not a geologist or paleontologist, so I will let an expert describe further: “The process of mineralisation of bone and shell for preservation in the fossil record is dependant on the chemical and physical conditions during diagenesis, particularly the composition of the mineralizing fluid. Fundamentally these processes can be considered as mineral replacement reactions, where the biomineral is replaced by another mineral such as quartz, opal or calcite.

[…] The processes leading to the formation of precious opal and the opalisation of fossils in the Australian opal deposits are poorly understood. […] Precious and potch opal are also found replacing, shells, wood and less frequently, the bones and teeth of marine reptiles (Barnes et al. 1992). The skeletons of Cretaceous plesiosaurs and ichthyosaurs that have been partially or completely opalized have been found at Andamooka, Coober Pedy and White Cliffs (Kear 2003).” 

- Opalisation of fossil bone and wood: clues to the formation of precious opal. B Pewkliang, A Pring, J Brugger - Regolith, CRC LEME, Australia, 2004 Basically, these fossils can be both gorgeous and paleontologically valuable! Photos: Opalized petrified wood - PlanetOpal

Opalized teeth and bones - Gemological Institute Of America

Opalized belemnite (cephalopod) - Wikimedia

Opalized bivalves -  Gemological Institute Of America

Opalized fish teeth -  Gemological Institute Of America

Opalized plesiosaur vertebra - James St. John

Opalized bivalves - Wikimedia

Opalized belemnite (cephalopod) - Wikimedia

Opalized bivalve - James St. John 

Sphenodiscus

Sphenodiscus плаває поряд з Pharmacicnthys у пізньому крейдяному морі.

Sphenodiscus — вимерлий рід акантоцератоподібних амонітів. Рід був знайдений на багатьох континентах і, як вважають, був широко розповсюдженим під час маастрихтського етапу пізньої крейди. Це був один з останніх амоноїдів, що еволюціонував до зникнення всього підкласу в палеоцені, котре сталося одразу після крейдово-палеогенового вимирання.

Повний текст на сайті "Вимерлий світ":

Ammonite fossil | ©National Museum Wales 

Iridescence Sphenodiscus lenticularis Ammonite

Phylum Mollusca, Class Cephalopoda, Subclass Ammonoidea, Order Ammonitida

Sphenodiscus lenticularis, Fox Hills Formation, South Dakota Late Cretaceous (~ 70 mya)  This specimen displays the extraordinarily intricate suture patterns that are characteristic of this species. Also evident is part of the beautifully iridescent nacreous layer. A laterally flattened ammonite, this animal is thought to have been a relatively fast swimmer.