PTEROSAURI, I RETTILI VOLANTI

Daniele Tona – 10 gennaio 2011

Mi permetto di scrivere quattro righe di presentazione a questo che più che un articolo, è un’utilissima guida per entrare nel mondo degli Pterosauri ed uscirne soddisfatti e pieni di nuovi  spunti. Chi erano, come erano fatti, cosa mangiavano, come volavano e come si muovevano a terra…

Daniele Tona ha colpito ancora ed ecco, tutto quello che volevate sapere su questi meravigliosi rettili volanti! (S.R.)

 

daniele tona
Daniele Tona

Durante l’era Mesozoica, prima che (come trattato in un precedente articolo) un gruppo di Dinosauri spiccasse il volo evolvendosi negli uccelli moderni, i cieli del pianeta erano popolati da eseri viventi tanto bizzarri quanto straordinarii: erano gli Pterosauri, il cui nome tradotto dal greco significa letteralmente “lucertole volanti”.

Prima degli Pterosauri, altri gruppi di rettili avevano già evoluto più volte e in modo indipendente strutture per il volo, ma esse erano semplici espansioni della cassa toracica che consentivano unicamente un volo planato di tipo passivo. Gli Pterosauri sono stati il primo dei tre gruppi di vertebrati noti (gli Pterosauri stessi, gli uccelli ed i mammiferi Chirotteri, ossia i pipistrelli) a modificare radicalmente il loro corpo onde sviluppare un volo attivo.

Fossili di Pterosauri sono noti fin dalla fine del Settecento, quando il naturalista Cosimo Collini descrisse dei resti rinvenuti nelle rocce calcaree di Solnhofen (le stesse da cui proviene Archaeopteryx) appartenuti a una forma di vita che identificò come un animale acquatico che usava le lunghe zampe anteriori come pinne. Fu solo ai primi dell’Ottocento che Georges Cuvier riconobbe la vera natura dell’animale, che fu così battezzato Pterodactylus (“dito alato”). Nel corso dei due secoli successivi sono state scoperte all’incirca un centinaio di specie di Pterosauri, straordinariamente diverse in termini di dimensioni, aspetto e nicchia ecologica. Per via del loro scheletro delicato i resti di Pterosauro si conservano raramente, e spesso sono frammentari; in taluni casi, però, i fossili si sono conservati eccezionalmente bene, ed è stato possibile determinare elementi come la copertura cutanea, il profilo delle ali o quello del corpo.

 

Ma quali sono questi adattamenti anatomici che hanno permesso agli Pterosauri di decollare per colonizzare i cieli del Mesozoico? Il primo, e più evidente, sono senz’altro gli arti anteriori trasformati in ali. La mano degli Pterosauri era infatti composta dalle prime tre dita (che in alcune delle forme più recenti, come Nyctosaurus, spariscono del tutto) e dal quarto dito, incredibilmente allungato per sostenere la membrana alare principale, chiamata brachiopatagio. All’altezza del polso era presente un osso esclusivo degli Pterosauri, lo pteroide, che sosteneva una seconda membrana più piccola detta propatagio ed estesa dal polso alla spalla; non si ancora da cosa derivi lo pteroide, che a seconda delle ipotesi è visto come un carpale modificato, il metacarpo del primo dito o un osso di neoformazione.

La peculiarità della membrana alare stava nella sua struttura interna: lungi dall’essere un semplice lembo di pelle come quello dei pipistrelli, era costituita da Leggi tutto “PTEROSAURI, I RETTILI VOLANTI”

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