Guida alla descrizione e riconoscimento di invertebrati e tracce fossili (parte 8) Echinodermi

Stefano Rossignoli – 8 novembre 2013

Parte 1 – Introduzione. Come iniziare

Parte 2 –  Porifera, Archeociata, Stromatoporoidea

Parte 3 – Coelenterata (Coralli)

Parte 4 – Molluschi

Parte 5 – Le strutture geopetali

Parte 6 – Molluschi 2 – Gasteropodi

Parte 7 – Brachiopodi

Non ho ancora concluso (e chissà quando ci riuscirò?!!!) la settima parte del manuale/guida alla descrizione degli invertebrati fossili dedicata ai Brachiopodi ma eccomi qui almeno a introdurre l’argomento “Echinodermi” con qualche notizia generica che sia utile alla loro descrizione.

Il Taxon degli Echinodermi è davvero vasto. Se ne conoscono molti generi e specie attuali, pensate ad esempio alle Stelle Marine, ai Ricci, ai cetrioli e ai gigli di mare… Scientificamente hanno nomi diversi ma per ora li raggrupperemo tutti nel Taxon “Echinodermata”.

E’ certamente nel passato che gli Echinodermi mostrano il periodo di massimo splendore, soprattutto durante l’era primaria, nel Paleozoico.

E’ scoraggiante cercare di essere esaurienti in un articolo divulgativo ma ci proverò, un passo alla volta…

Primo: Gli Echinodermi attuali e con buona certezza quelli del passato sono tutti marini.

Questo non servirà molto a descriverli ma sarà utile nel “Passo successivo”, la ricostruzione paleoambientale, ovvero a utilizzare i fossili per comprendere come fosse un determinato ambiente nel passato…

Secondo: Il loro scheletro detto “Teca”, che è ciò che si conserva allo stato fossile, è composto da piastre di calcite.

Piastre di crinoide - calcare di Moltrasio - Monte Generoso
Piastre di crinoide – calcare di Moltrasio – Monte Generoso

E’ un mesoscheletro. Meso vuole dire che sta in mezzo, ovvero che contiene ed è ricoperto da parti molli che, normalmente, non si conservano allo stato fossile.

Gli Artropodi ad esempio hanno un esoscheletro che forma il loro strato più esterno.

Le piastre di calcite degli Echinodermi, in origine sono porose e, durante la fossilizzazione i pori tendono a riempirsi di altra calcite che segue il reticolo cristallino non distinguendosi dal minerale originale (quindi mostrando anche le stesse proprietà ottiche).

Terzo: La loro simmetria è pentamera, fino a diventare bilaterale e questa sarà una delle prime caratteristiche da elencare nella descrizione di un esemplare.

Quarto: Tutti presentano l’apparato ambulacrale, la bocca e l’ano e la loro forma, posizione, nonché la posizione delle piastre calcitiche che li delimitano saranno basilari nella descrizione.

Per quanto riguarda altre forme e strutture le tratterò, tempo permettendo, man mano che si presenteranno nei vari taxa…

 

crinoide-completo-monte-generoso-calcare-moltrasio
Un Crinoide piuttosto completo trovato sul Calcare di Moltrasio nel Giurassico inferiore del Monte Generoso

Le notizie principali per la descrizione saranno sempre quelle: la forma, la simmetria, i caratteri particolari che, come sempre e ancor più questa volta richiederanno una conoscenza approfondita dell’anatomia di questi animali.

E prima di tutto dovremo conoscere gli organismi attuali, poi potremo addentrarci nell’anatomia dei fossili.

Sottolineo ancora una volta come la forma e i caratteri particolari degli echinodermi spesso dipendano da localizzazione e caratteristiche delle piastre calcitiche di cui dovremo conoscere anche il nome!!!

 

La guida sarà in aggiornamento fino a che la riterrò soddisfacente.

A presto con i prossimi taxa.

Stefano Rossignoli 09 novembre 2013

 

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