The Madagascan Nautiloids are found in a fossil layer that has unearthed some of the best Cretaceous mineralised specimens yet to be discovered. The mineral content of the fossil bed some millions of years ago has caused these wonderful effects of colourful inclusions of several semi-precious minerals, renowned as of a 'gemmy' quality. Quartz in the form of calcites and jaspers have infilled the internal phragmacone's (shells), of some fossil nautilus. The septa (chamber wall divisions), are picked out with lighter and darker coloured minerals, the chambers exhibit patterns in dark and lighter caramel colours which delight the onlooker today.
The Nautiloids are an enigmatic group of cephalopods, extant (living), genus types, the Nautilus pompilius and Nautilus umbilicalis survive today in the S.W.Pacific ocean. With ancestral lineage in the late Permian to Ordovician periods, since evolving Nautiloids have suffered extinctions in the Devonian, Carboniferous and Cretaceous. In fact, the Nautiloids seem in a state of contraction for much of this varied history. Seemingly in decline as the Ammonites were increasing successfully and in variety, a direct reflection of the ecology of the prehistoric marine environment in relation to both cephalopod branches.
The Ammonites eventually becoming extinct in the Maastrichtian age, just before the mass KT extinction event. It is thought that the Nautiloids survived probably due to their benthic ecology. Laying small batch eggs often, at depth or on the sea floor with more frequency, throughout their life, thus protecting the eggs from most major climatic changes. Whereas the Ammonites laid one batch of numerous eggs at the end of life and attached these to marine planktonic environments, close to the surface which made them more susceptible to any major planetary climatic change.
Nautili are nektobenthic carnivores and pelagic in habit. The umbilicus of this shell of the genus is involute and globose. Cenoceras described by Hyatt C.1884, Pseudocenoceras by Sparth C.1927.
Both the Ammonites and Nautili evolved from cephalopod beginnings in the Cambrian period. The Ammonoid group eventually dominating the prehistoric marine environment, being free swimmers above the seabed, before the arrival of vertebrate fishes to the marine scene. Nautiloids having early success were later in the Cretaceous period seemingly in decline. The mass Cretaceous extinction, the KT boundary event, which is the marker for the extinction of dinosaurs and many other life forms gave the tenacious Nautili a chance to regain a secure foothold under the oceans.
Genus: Cenoceras intermedius, Hyatt in 1884 (Sowerby), Nautiloid, Nautiloidea.
Age: Triassic, Carnian age 228- 235 mya to Mid Jurassic, Callovian age 166 - 164 mya
Location: Ambohimanga Rova, N.E. Antananarivo, Madagascar. Southern Indian ocean.
Cenoceras Nautiloid measurement.
Width: 5.5 cm
Approximate weight: 0,070 g