Nel primo articolo abbiamo parlato della scomparsa dei dinosauri 65 milioni di anni fa, quando un meteorite colpì la terra causando la morte di quasi l’80% delle creature terrestri (leggi l’articolo). Vittime dell’evento di estinzione furono anche le ammoniti (Figura 1), molluschi simili ai calamari e ai polipi di cui si conservano innumerevoli fossili. Questi fossili non contengono nulla dell’animale originario, nemmeno la sua conchiglia. Una volta sepolto sul fondo del mare, le parti molli dell’animale si sono decomposte lasciando la conchiglia vuota.
Nel corso di millenni, diversi minerali e sedimenti hanno riempito la conchiglia, riprodotto tutti i dettagli interni, formando un calco naturale della conchiglia (Figura 3) . Nella maggior parte dei casi, anche il guscio si decompone con il tempo. In rarissimi casi, in presenza di particolari condizioni di preservazione, gli strati piu’ interni del guscio rimangono. Sono fatti di madreperla e vibrano dei colori dell’arcobaleno a seconda dell’angolo da cui li si guarda (Fugura 4). Campioni di questi rari fossili sono esposti in una mostra temporanea presso il Museo di Storia Naturale di Macerata.
Ammonites are extinct relatives of the squid and octopus. Similarly, they had a soft body, tentacles to grab their prey and jaws and teeth to eat them (Figure 1 shows what we think was their appearance). Their food consisted of larval snails and crustacean, the so called plankton. Unlike their modern relatives they had a shell, mostly coiled, made up of several segments that the animal itself built while growing. The body occupied the largest and last segments of the shell. The smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas, much like a submarine. Ammonites appeared about 400 million years ago, experienced alternate fortunes, flourished in the Jurassic and Cretaceous before disappearing around 65 million years ago, together with the dinosaurs (Figure 2). Most probably, the loss of plankton, severely hit at the Cretaceous-Tertiary boundary, contributed to the extinction of ammonites. People erroneously believe that the ammonite’s fossils that they see in museums are the remnants of the shell of the animal. What instead they are watching at is the cast of the original shell (Figure 3). When the animal dies, it sinks to the bottom of the sea. If no predators eats it, it will get buried underneath other sediments that will compact around it. The soft parts decompose quicker leaving the empty shell.
The empty shell acts now like the play-doh molds children play with. Sediments fill up the empty space producing a faithful copy of the interior of the shell. Within million years the hard material of the shell decomposes too, leaving now the cast of the shell embedded in the rock. This cast can be found, extracted from the surrounding rock and exposed in museums worldwide. The Museum of Natural History in Macerata has a rich collection of ammonite’s fossils. At this time until March 18 th, a new collection is on exhibit, comprising iridescent ammonite fossils (Figure 4). They are rare fossils that still conserve part of the shell, at least the most internal layers, made of what is called nacre or mother-of- pearl. This material displays different colors when seen from different angles. Their beauty is astonishing. So, go and visit the Museum, talk to the curators, let them show off the new collection, facts and curiosities about it. And please, do not forget to tell them: CM Junior sent me!
Glossary
Ammonites = ammoniti
extinct = estinto
relatives = imparentati
squid = calamari
octopus = polipo
prey = preda/prede
jaws = mascelle
snails = lumache
shell = conchiglia
coiled = avvolta a spirale
walled off = chiuso da un setto
buoyancy = galleggiamento
flourished = prosperarono
hit = colpito
remnants = cio’ che e’ rimasto
cast = calco
sinks = affonda
buried= sotterrato
underneath = sotto
play-doh molds = formine Dido’
faithful = accurata
embedded = chiuso dentro
on exhibit = esposta al pubblico
iridescent = iridescente, che assume colori diversi a seconda dell’angolo da cui si guarda o dalla provenienza della luce
nacre or mother-of- pearl = madreperla
astonishing = sorprendente, incredibile
curators = curatori
show off = mettere in mostra
